Jeep Enthusiast Forums banner

Searching for the golden tuning

39K views 683 replies 23 participants last post by  Matt1981CJ7 
#1 ·
I am beginning the saga of fine tuning my amc 304 for best performance. I will be working on both the carb and timing to make my tired old motor perform to its fullest.

As for equipment, I have a freshly rebuilt holley 2300 2bbl carb (similar in tuning to a motorcraft carb), an unknown brand HEI, an edelbrock performer intake, and unknown brand tube headers.

I will be using JeepHammer's "tuning amc v8s" instruction thread as my guide. I will list part numbers of all my tools, parts, and kits as I use them. I will post everything I do, even the embarrassing srew-ups.

This thread is for people who are interested in tuning their motors at home using good, old fashioned elbow grease, and for those looking for a laugh I'm sure.

Wish me luck, here I go! :2thumbsup:
 
#2 ·
The first step I'd recommend to anyone looking to REALLY dial in their carbs, is to at least take the thing apart and clean/inspect everything. Ideally, one would simply rebuild the carb whether it needed it or not, but the wallet is a different story. Mine was terribly sooty (pic 1), so I went ahead and rebuilt it. Also, my baseplate was leaking, so I bought a new baseplate assembly from ebay for $100.

Besides making sure the carb is in good working order, pulling the carb off of the manifold may expose a potential problem you didn't know you had (pic 2). Not only did I have a vacuum leak from this square bore to motorcraft spreadbore setup, but there was so much gasket and manifold interference that I figured I'd need either a motorcraft spreadbore carb, or a new manifold. I chose an edelbrock performer manifold (pic 3 and 4).
 

Attachments

#3 ·
I am interested in seeing how this develops THE OLD SCHOOL WAY. I also am surprised you didn't go with a small Holley four barrel. I think you would get better gas mileage and much more power.

Take care,
 
#4 ·
I hope you enjoy the read. :thumbsup:

Who knows, this exercise may eventually lead to a new carb by the time I'm done. In a perfect world, I'd dyno the jeep now, then after I get a full holley 2300 tune, and finally after I add a small, 4 bbl carb, but money is also a factor.

I may have to resort to girlfriend calculated 0-60 times :rofl:
 
#6 ·
Oxmarqt said:
Good luck and I hope to keep track of your progress.

Hopefully(for her sake), the Jeep is the only thing your girlfriend will be calculating 0-60 times for.....:laugh:

Paul
"those" times would be 60 and on... In minutes ;)

And she'd be too distracted anyway.
 
#9 ·
Dada,

Do you plan on messing with your advance curve at all? How about jetting the carb for your specific altitude? I found these two tasks to be key in getting a proper tune.

Here's a video of my 31-year old 258, that's never been torn apart, after I successfully tuned it.

Matt
Nice vid Matt :thumbsup:
That thing raps up nice and smooth!

I plan on digging pretty deep into this tune. I will start with a base tune (vacuum tune the idle mix screws, set base timing to 8*, set fuel height, etc). Then I will step it up and chart and tune both the centrifugal advance, and the main jets. Once I nail those, I will chart and tune the vacuum advance.

I probably wont screw around with really fine tuning (with cams and whatnot) the accel pump, or digging into the power valve channel sizes in the carb. Unless I have to that is.
 
#8 ·
This is what I will be doing this weekend. So I guess I hope you have to get everything done on Saturday during the day. That way I can read it Saturday night and do it Sunday. Cause I need as much help as I can get. Good Luck.
 
#10 ·
Matt,

Your video is awesome! I didn't even see the engine shake. Great job.

Take care,
 
#11 ·
1SASjeepster,

Thanks, man. The timing light and vac gauge were loosely duct taped to the rail. If you watch closely, there is a slight shimmy at about 3700 RPM, but it goes away quickly above 4000.

The ol' girl actually had a bit more throttle to go, but I didn't want to blow something up with video proof. :laugh:

My 258 has got almost 100K miles on her, and I think she knows she's about ready to get replaced by a 360. So she's trying to prove she's still worthy. ;)

Matt
 
#12 ·
Tools I will use for this project.

vacuum gauge:

I have two of these: a cheapy for the cab, and a nice one for tuning. Here are their summit part numbers

cheapy- http://www.summitracing.com/parts/WMR-W80594/
nice one- http://www.summitracing.com/parts/OTC-5613/

Timing light:

I read a few reviews on these, it seems you either need to buy a simple (on button only) unit or a $150 unit. I heard good things about this one, so I bought it.

http://www.summitracing.com/parts/ANM-CP7527/

Timing tape:

http://www.summitracing.com/parts/MSD-8985/

Vacuum pump:

I don't have one yet, but I will get one when I begin the vac advance testing
 
#13 ·
With a fresh new manifold, and a freshly rebuilt carb, I begin the quest. The very first thing I would do, if I hadn't done it already because of a bad missfire, would be pull the plugs and make sure they are in decent condition, and make sure they are meant for your rig, and make sure they are gaped appropriately for your ignition (.035" for stock points, .045" for HEI). My plugs were gaped at .045" on the driver side, and .035" on the passenger side. WTF? I cleaned them with a wire brush and brake cleaner, then gaped them all to .045". Also, now is a good time to replace your old plug wires, cap, and rotor/points if you even think they may need it. I had 8mm summit wire and a brand new cap and rotor on my rig.

These are the steps I followed to prepare for the base tune. I originally tuned the base timing and mix screw with the vacuum gauge just to get her going, but once I bought better tools, I redid all the tuning. I will start from the "redid" part since I don't recommend tuning the base advance with the vac gauge for reasons I will cover later.

before starting the motor.

Step one: set idle mix screws where they were before the manifold and rebuild. If you don't remember set them to 2 turns out. [PIC 1]
Step two: disconnect vac advance and plug it to prevent vacuum leaks [PIC 2 & 3]
Step three: hook up timing light to number one cylinder and battery. Number one is the front most cylinder on the driver side. Make sure to hook the induction clamp away from other wires and metal and as close to the plug as possible [PIC 4 & 5]
Step four: hook up vacuum gauge to manifold vacuum source. [PIC 6]
Step five: put some fuel in the float bowl. [PIC 7 & 8]

You are now ready to start the motor.

These are the events that unfolded when I started/attempted to start the motor.

Event one: "clilck" and nothing happened
Event two: try to crank again, and "click" no crank
Event three: SMOKE from the motor compartment
Event four: PANIC
Event five: notice the smoke came from the manual choke cable
Event six: assume the starter ground was now the choke cable and firewall
Event seven: trace ground lead from block to batter
Event eight: tug on negative lead, and it pops out of the battery terminal.
Event nine: buy new terminal
Event ten: rewire block lead to terminal
Event eleven: start motor, and it runs like crap (what I expected)

The motor started and barely ran, so i kicked up the idle adjust screw to make it idle faster, but the motor was running very badly. My vacuum gauge read a steady 12 in Hg. I thought to myself, "at least its steady, that means the motor is somewhat healthy." A vacuum gauge can tell you all kinds of diagnostic information besides tune. I forget what all the various needle behaviors mean, but look it up some time. Its great info to have.

The following steps are the ones I took to get a rough initial tune. At this point, I do not have a timing tape on the balancer. Aslo, my original screw setting was 3/4 turns out on both sides

Step one: turn passenger side mix screw out half a turn. the motor instantly began to run smoother and vac went up 2 in Hg.
Step two: turn passenger side mix screw out until vac stops climbing. set the screw at the point where vac stops climbing and round it to the nearest 1/4 turn (1/8 turns become hard to remember)
Step three: turn driver side mix screw out until vac stops climbing. This should put the screws at the same setting, BUT MAKE SURE THAT THE SCREWS ARE TUNED OUT THE SAME AMOUNT ON BOTH SIDES. At this point the idle was well over 1400 RMP and very smooth and nice. So I backed the idle adjust screw down to roughly 800 RPM. I had no tach at this point. I now have a sunpro super tach II from autozone mounted on the steering column (I will eventually have a full gauge set mounted indash)
Step four: Adjust base timing to 6*-8* advance using timing light. Originally I used the vac gauge to set base timing. To do this you simply advance the timing until it stops increasing vacuum, then retard timing until you decrease 1 in Hg from max. Upon buying the timing light, I discovered this put me at about 13* base timing (yikes). This is why I amended my steps to include a timing light. [PIC 9]
Step five: allow motor to warm up completely
Step six: Check fuel height in float bowl. I adjusted this height to be about 1/16" below the check hole since I intend the wheel this rig.
Step seven: readjust idle mix screws to max vac.

The base tune is complete!
 

Attachments

#14 ·
I'll admit that I have an advantage over the average joe when it comes to fuel mixture. I've been tuning dirt bike carbs for a while now, and I have developed that special "feel" for when my mixture is off. This Holley carb is WAY simpler than a Keihin FCR on a more modern 450f/250f. For example, the FCR has a pilot jet and mixture screw (idle and barely off idle circuit), jet needle profile and height (mid range circuit), and main jet (top end circuit).

I can give some pointers in detecting the quality of your fuel ratio by "feel". First, the better way to do it is with an O2 sensor and rich/lean meter or multimeter/voltmeter. The next best way to do it is with a plug test. Install new plugs, get up to "haul ***" speed (about 2500 RPM I'd guess) for a short while (1 minute is probably fine), push in clutch and kill motor, stop vehicle in neutral or with clutch in, check plugs, light tan is good, white is too lean, brown is too rich, black is TOO rich. I have no $$ for sensors and bung welding into the exhaust, and I have no long flat "haul ***" stretches where I can get to and perform this test.

Keep in mind this is all about main jet tuning

Pointer 1: Smell. Its easy to smell when a motor is running rich. With the top off, I can smell a small amount of exhaust fumes in the cab, and, if they are rich (smells like fuel), the main jets are too rich. If you don't smell fuel, the jets are either good or too lean. Make sure you don't confuse the rich smell with any fuel leaks you may have in the motor compartment. Non charcoal canister jeeps tend to smell like fuel all the time, so when you smell exhaust smell, pick it apart and determine if there is also a added fuel smell to it. Also, make sure you are only using the smell test at RPMs and loads that don't involve the powervalve, accel pump, or idle screws as these will all mislead you. So basically run this test at part throttle cruise (PTC) in the 2000 and up RPM range.

Pointer 2: Sound (assuming no catalytic converters). A backfire (while driving) usually means you are too lean, as do fluctuations in engine RPM for a steady throttle position. Alternatively, if you have an exhaust system you can hear off idle, you should also be able to hear it on decel and PTC. A quiet or silent exhaust pipe durning PTC or decel usually means too rich.

Pointer 3: Sight. This one is easy, black smoke is a sure sign of way too rich. Be sure that its not your power valve or accel pump making the extra rich smoke, those are separate issues. You want to make sure its not smoking during PTC or decel where there is no influence from other systems.

Pointer 4: Fuel mileage. Unless your power valve is always open (bad), or you are perpetually climbing a hill, or perpetually start-stop-start-stopping, your main jetting and timing curve make the largest impact on fuel economy. Without going into the timing curve yet, the main jetting is rich if you have poor economy. For example, I've been getting 8 MPG before attempting to make changes to the timing curve and main jets.

Pointer 5: Feel. If you have sluggish and smooth performance in RPM ranges from 1300 and up, your motor is likely rich. If you have poor and erratic and noisy performance in the same RPM range, your motor is likely lean.

Pointer 6: Temperature. A hot motor may indicate a lean setting.

The most difficult piece of information to obtain, and the piece I cant give you, is exactly what a great performing motor is like in all these categories. You have to tweak and tune up to and slightly beyond your best tune, to discover exactly where that mark lies.
 
#15 ·
Dadamany,

I read where you cleaned your spark plugs with brake cleaner. I know it would probably never happen in a spark plug situation at least not enough to get you BUT I will tell you this anyhow. Brake cleaner and high heat create PHOSGENE GAS. It is a very deadly chemical weapon that only one whiff of it is enough to destroy your lungs for life. One whiff, not one full breath, just one whiff. A welder used brake cleaner on some parts because he ran out of degreaser and didn't want to go to the parts store. He was in a greatly ventilated garage. He began and was out. He got to the hospital and he has been going weekly for breathing treatments because his little whiff destroyed most of his lungs. It is something to think about.

I understand the spark plug is in the cylinder and exhaust gases flow out through the exhaust pipes BUT there are instances in a mechanics life when the gas mixture backs up and goes out through the carburetor. Be careful.

Take care,
 
#16 ·
Dadamany,

I read where you cleaned your spark plugs with brake cleaner. I know it would probably never happen in a spark plug situation at least not enough to get you BUT I will tell you this anyhow. Brake cleaner and high heat create PHOSGENE GAS. It is a very deadly chemical weapon that only one whiff of it is enough to destroy your lungs for life. One whiff, not one full breath, just one whiff. A welder used brake cleaner on some parts because he ran out of degreaser and didn't want to go to the parts store. He was in a greatly ventilated garage. He began and was out. He got to the hospital and he has been going weekly for breathing treatments because his little whiff destroyed most of his lungs. It is something to think about.

I understand the spark plug is in the cylinder and exhaust gases flow out through the exhaust pipes BUT there are instances in a mechanics life when the gas mixture backs up and goes out through the carburetor. Be careful.

Take care,
YIKES!! I wish I knew that BEFORE I did it!! goodness! :fear:

If I ever write a book on tips for how to not kill yourself, that will most definitely be included.
 
#17 ·
Having said all of that jazz about main jets, I was running pretty quiet, sluggish, but smooth, so I pulled my old #80 main jets and swapped them with #79s (one step leaner). I didn't bother to test drive it (this was a few nights ago, I'm about 4 days behind), because I was so cocky about having the "feel" for mixture tuning. So, upon starting the jeep, two big splotches of black, wet, fuel coughed out of each exhaust pipe (running duels), and I knew something bad was happening, but I drove it anyway. I was blowing clouds of black smoke on the way to work, and had no power at all, but idle was just fine and dandy. I had a massive stumble off the line as well. It would start to go, stumble, go fine, then cough its brains out. At work, I did a little thinking, and realized I didn't reassemble my accel pump arm when I put the bowl on after changing the jets. Duh! over my lunch break, I headed down to the jeep, popped the hood. drained the bowl into a solo cup, pulled the bowl off, and fixed the arm, no problem. I went to grab the cup, and it had turned to a pile of gooey melted plastic. A lesson to everyone: red solo cups may do well with alcohol, but not gasoline!

I knew this would only fix the stumble off the line, and I was pretty sure that I messed up the main jets somehow. I figured either they were mislabeled, or one fell out, or some goofiness. I drove to lunch to test it out again, and the poor thing coughed and sputtered so bad, and the plugs loaded up so thick, that I was getting slower and slower. I knew I wasn't going to make it home later. I remembered it ran better with a base timing of 13* as set by the vac gauge, and I just so happen to have my vac gauge and a 9/16" ratchet, so I pulled over in the McDonald's parking lot, popped the hood, plugged the vac advance, and timed the base with the vac gauge like I did before. I then proceeded to enjoy a medium #13 (the thought of bad luck didn't even cross my mind until now) and drive home. I made it the 10 miles home... BARELY. I had my flashers on and I was doing 20 max in a low RPM to keep the mixture a little towards the idle/leaner side. She died 20 yards from the garage. I begged her to start, and I used my granny 1st to put her away.

I called work, told them what happened, and came back in with the truck to finish the day.

Later that night, I pulled the jets and compared them to my 80s. Sure enough the 79s were way way bigger... like a fuel firehose. I took another good hard look at the old jets, and I figured out that they were 60s :facepalm: I put them back in, retimed the base to 8* and put new NGKs in her. It was back in business after burning all the crap out of the pipes. :thumbsup:

The next day I bought 59s and 58s at summit.
 

Attachments

#18 ·
Okay, trying to play catch-up here. I put the 59 mains in last night, and also put the timing tape on and took a centrifugal advance curve.

I popped in the 59s first. Changing jets on the Holleys is very easy...

1: pull either bottom bolt to drain the bowl. Use some towels and a little fuel catch undet the bowl.
2: undo the other three bolts and pull off the bowl. The metering block (has idle screws in it) may come off as well. no big deal.
3: jets are visible at this point in the bowl side of the metering block. Take them out.
4: put new jets in. DONT PUT THEM IN SUPER TIGHT! Jets only need to be snug. They seem to tighten themselves down over time.
5: reassemble bowl making SURE TO PUT THE ACCEL PUMP ARM UNDER THE SPRING LOADED PUSHER BOLT this time.
6: fill bowl with fuel.
7: done.

After a short time, the fabric seals on the bowl bolts tend to tear up, use this part to fix that.

http://www.summitracing.com/parts/HLY-108-98-10/

or

http://www.summitracing.com/parts/MOR-65225/

Then I put the timing tape on. I used an electrical wire to gauge the circumference of the harmonic balancer, and then did some math to figure out the diameter of the harmonic balancer so I could figure out which tape to use. I then did some extra math to figure out how much influence the wire thickness had on the circumference measurement to double check I chose the right timing tape.

I'm curious how accurate the timing marks on the timing cover are on these motors. I'm sure I did the tape measurement right, and I'm sure I got the right tape on there, and I got the 0* on the tape and the balancer lined up, but, according to the tape, I was sitting at about 12* advance when I thought I was at 8* :/ Not sure what thats all about. At least with this tape, if it is slightly wrong in the end, I will still get a good idea what changing centrifugal timing elements does for the overall curve.

Charting the timing curve was nice and easy as well. All i did was set the GF in the cab while I adjusted the idle adjust screw to reach 200 RPM steps from 800 to 2600, which she verified from in the cab (the sunpro has 100 RPM gradients so that was easy), and the I wrote down the timing, for each step. I had the vac advance plugged off for this test so I could isolate exactly what the centrifugal was doing.

Then we went to sonic burger and enjoyed the added power and exhaust noise from the leaner jets :D
 

Attachments

#20 ·
Since I have a decent base tune, I decided to take a PTC (partial throttle cruise) vac reading to determine what my power valve should be. Cruising on a flat road at 55, my vac reading was 12 in Hg. Since I will be wheeling this rig, I decided to go with an 8.5 in Hg power valve. Jeephammers advice is to use a power valve 3 in Hg less than PTC for wheeling, but I went with 3.5. This helps those hills and starts a bunch! Especially considering the one in the Holley rebuild kit was a 4 in Hg!!!

I didn't get pics :facepalm: but the power valves come with gaskets, and they screw into the Venturi side of the metering block. Super simple.
 
#21 ·
So this is how today went: wakeup, ponder what to change with my tuning, get a call from my buddy that his bike broke down, hop in the truck, throw the bike in the truck, go to shop, deal with shop papers, go to breakfast, drop friend off, get a call about mom's surprise BBQ, pick little brother up for BBQ, drive keep for 1 hour with bro, hip in car, go to BBQ, day over.

Needless to say, today got totally waylaid. I had fun with the bro, and at the BBQ, but I was hoping to get pretty far today.
 
#22 ·
However, I was able to play with the centrifugal timing quite a bit yesterday, and now that I'm done preaching about base tuning, and jet tuning, here comes the fun stuff.

I bought an MSD HEI weight and spring kit [pic 1] and installed it

I had to remove the distributor cap. Under the cap is the rotor [pic 2]. Two little screws take the rotor off. Under the rotor is the spring and weigh setup [pic 3]. Next the springs come off. I used needle nose pliers for this BE GENTLE [pic 4]. With the springs removed, the weights slip off. Next snap the "E" clips off of the center plate (don't lose them!), and pull it off. All the parts should now be loose. [pic 5].

Reinstall the new parts in the opposite order. This is where I ran into problems. The MSD kit is supplied with three bushing sizes to fit the weights on the distributor. None of them fit my setup. I had to drill out the bushings to make them fit. I figured it was a loss at this point, but I went with it anyway. The kit came with three sizes of springs also, and I used the ones that most closely resembled the ones I had in before. There is a very significant quality difference in the MSD setup over the stock unit (most likely a summit brand unit) [pic 6]. [pic 7] shows the new kit installed. After finishing the install, I ran a new curve [pic 8]. This curve kicked in way late and had low maximum. Also, the RPMs shot up from 1800 to 2100 at the slightest change of the idle adjust screw. So I ditched that setup, pulled it all apart , and popped in one lighter spring an left one previous spring in and reran the timing curve [pic 9]. This curve came in earlier than the last, but still behind stock, but the max advance was still way too low. So I ditched the MSD setup all together :(

I went back to stock, and used one stock spring and one of the lighter MSD springs. Upon changing the center plate back to stock, I shot one of the tiny "E" clips across the face of the earth. That's why I call them Jesus pins. It's the only word I can muster when they shoot into the next dimension, or wherever they go. I was able to find one that was left over from my carb rebuild kit though [pic 10].

My final timing curve of the day was pretty good, the adv started nice and early, and went up to a pretty high amount [pic 10]. I hopped on the rig, and did some really hard takeoffs and 0-60s (with no timer) and had no detonation. I gained a TON of responsiveness and power from this new timing curve, and the jeep totally rocks now. I'm not sure I will drop to both light springs, I think this will start the curve climbing at around 800 RPM, and that may be too much. I will also plop in the #58 mains tomorrow or the next day.

I left my computer at work, so I apologize for no excel graphs, but the numbers should do for now.

Tomorrow, I will be scuba diving all day.
 

Attachments

#23 ·
em2mccue said:
This is what I will be doing this weekend. So I guess I hope you have to get everything done on Saturday during the day. That way I can read it Saturday night and do it Sunday. Cause I need as much help as I can get. Good Luck.
I hope what I did so far helps you out! I didn't get as far as I wanted, but right now my motor runs freaking good compared to last week!
 
#26 ·
You mentioned in an earlier post that you wanted or was going to research what a vacuum gauge can tell you. Here is what I use...

Take care,


When using the gauge, hook into a good port directly off of the intake manifold. Any competent engine should be running between 18-22 inches of mercury.

(1) The vacuum gauge can be used to check for a leak in the valve train, intake manifold or head gasket. With the engine at normal idle and at operating temperature, hook up the gauge. Normal operation would have the needle/ pointer holding steady and not fluctuating. If the needle holds steady, than drops an inch or more, and then returns again to normal, and repeats… this signals one or more engine valves are closing but not seating properly, thus creating a leak. Each time the valve doesn't seat properly, the pointer fluctuates. The fluctuation isn't much, maybe 1-3 inches of mercury.

(2) If the pointer drops sharply 5-10 inches from normal, and then returns… this signals a cylinder head gasket. This behavior will repeat itself. If the leak is between two adjacent cylinders, the leak will be more pronounced.

(3) If the pointer/ needle fluctuates constantly at a reading of 3-8 inches of mercury BELOW normal, It indicates intake system leakage.

(4) If you think you may have a "back pressure" issue, get the vehicle at normal operating temperature and slowly bring engine to 2,000 rpms. Close throttle quickly. If no back pressure is present, the pointer will jump past your normal reading and smoothly return to normal, just as fast. If it is slow or uneven return, there is an issue of back pressure. You can also test it another way… hold the engine at 2,000 rpms and watch needle. If it begins to decrease gradually instead of holding steady, there is an exhaust restriction somewhere.

(5) Continuous pointer movement an inch of so above or below normal indicates the need for further checking of the ignition system with electrical test equipment. You may have a weak ignition coil, high tension cable leaks, bad distributor cap, bad spark plug or incorrect spark plug gapping.

From "Petersen's Basic Tune up and Test Equipment."
 
#27 ·
86cj74.2L said:
Those recurve kits usually reduce total advance to 15 degrees instead of the factory 20. For big cams that need 18at idle to keep them happy. That way they still get their 34 to 36 total without too much timing on top or too little down low.
That makes a ton more sense now, thanks :)

Too bad summit didn't know enough to inform me of that. But at least the springs were useful.
 
#29 ·
1SASjeepster said:
You mentioned in an earlier post that you wanted or was going to research what a vacuum gauge can tell you. Here is what I use...

Take care,

When using the gauge, hook into a good port directly off of the intake manifold. Any competent engine should be running between 18-22 inches of mercury.

(1) The vacuum gauge can be used to check for a leak in the valve train, intake manifold or head gasket. With the engine at normal idle and at operating temperature, hook up the gauge. Normal operation would have the needle/ pointer holding steady and not fluctuating. If the needle holds steady, than drops an inch or more, and then returns again to normal, and repeats… this signals one or more engine valves are closing but not seating properly, thus creating a leak. Each time the valve doesn't seat properly, the pointer fluctuates. The fluctuation isn't much, maybe 1-3 inches of mercury.

(2) If the pointer drops sharply 5-10 inches from normal, and then returns… this signals a cylinder head gasket. This behavior will repeat itself. If the leak is between two adjacent cylinders, the leak will be more pronounced.

(3) If the pointer/ needle fluctuates constantly at a reading of 3-8 inches of mercury BELOW normal, It indicates intake system leakage.

(4) If you think you may have a "back pressure" issue, get the vehicle at normal operating temperature and slowly bring engine to 2,000 rpms. Close throttle quickly. If no back pressure is present, the pointer will jump past your normal reading and smoothly return to normal, just as fast. If it is slow or uneven return, there is an issue of back pressure. You can also test it another way… hold the engine at 2,000 rpms and watch needle. If it begins to decrease gradually instead of holding steady, there is an exhaust restriction somewhere.

(5) Continuous pointer movement an inch of so above or below normal indicates the need for further checking of the ignition system with electrical test equipment. You may have a weak ignition coil, high tension cable leaks, bad distributor cap, bad spark plug or incorrect spark plug gapping.

From "Petersen's Basic Tune up and Test Equipment."
This info is perfect for this thread. Thanks a bunch :thumbsup:
 
#30 ·
I don't know if I mentioned this earlier, but even though my new tuning rocks (not done with it yet!), I have a huge blip at 800 RPM that makes my takeoffs pretty lame. Its not a power valve thing, because when I'm wheeling and I put the RPM at 800 steady, it chugs and bumbles even on a flat surface, but as soon as I push the pedal, the accel pump and, depending on how hard I push, the power valve take it right out of the chug. I feel like it is my base timing advance.

Having said that, I did some research on timing at altitude (I live at 5000 ft) and found that you can advance the base timing quite a bit at this altitude. The rule I found, but I'm not sure I can really trust it yet, is you can increase timing 1* per 1000 ft. This would put my new base timing at 13*. And, what do you know, that is almost exactly where my vacuum gauge base tuning was set.

So, I RETRACT MY PREVIOUS STATEMENT ABOUT VACUUM TUNING THE BASE ADVANCE. I will reset my base timing using my vacuum gauge, then report what that value is. I will hopefully do this tonight.

I also ran across the subject of octane rating in tuning the vehicle's timing. I'm using 91 octane all the time, and that allows me to even further advance my timing, but I think I will tune the timing for 87 octane to keep the jeep safe. I don't need racecar tuning, I just need a really good/golden DD tune.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top