Weber tuning on modified 258 - JeepForum.com
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post #1 of 57 Old 06-09-2017, 07:39 AM Thread Starter
stripperguy
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Weber tuning on modified 258

OK, Matt, here is a separate thread for my issues.

To consolidate from previous entries:

Gentlemen,
I recently bought an assortment of idle jets for my Weber 38 DGAS. I've never taken the time to properly tune the Weber, it runs quite well at all engine speeds except at idle.
I had existing idle jets sized 45's, and couldn't get my throttle plates closed enough and keep it running. The mixture screws seemed to have no effect.

Anyway, I bought 50, 55, and 60's for each side (they're different choke side to non choke side).
When the jets arrived today, I jumped up to the 60's, and I can maintain a 650 rpm idle with 1 turn in on the throttle plates and 2 turns out on the mixture screws.
I'm thinking I need even larger idle jets.

Does the jet size refer to the orifice diameter, in hundredths of a mm? So a 60 has an ID of 0.60 mm or .0236 inch?
If so, I can avoid buying more jets and just open up the other, too small jets.

For background info, I am an apprenticeship served toolmaker, with 40 years experience and a full toolroom at my disposal.
My 77 CJ5 has a 79 258 in it, with about 30,000 miles on a .030 over full rebuild. I have the already mentioned Weber 38DGAS, Clifford intake, Comp Cams 252H camshaft, and Hedman 6 into one headers with all 2 1/4 exhaust and some Flomaster muffler.
Oh, and at idle (yes the choke is fully disengaged) I have 2" vacuum at the "S" port.

So, after all my long winded BS...
Are the idle jet sizes just the ID in mm??
BTW, I have read about half of this thread, you guys really know your sh!!t.
------------------------------------------------------------------------------------
Thanks, Matt
I do currently have ported vacuum advance, a switch to manifold vacuum would be easy enough. I should then just plug the "S" port?
I appreciate all the collective wisdom here...I really enjoy learning the ins and outs of these Webers.

-------------------------------------------------------------------------------------

OK, here's where I'm at

Removed my carb adapter and drilled and tapped a 1/4 NPT for the vacuum advance.
Plugged the "S" port on my Weber 38DGAS
Verified no vacuum leaks (no EGR)
Verified manifold vacuum -- 19" at idle
Checked timing at idle with vacuum advance disconnected and plugged 10 degrees BTDC
Drilled a 3/64" hole in the trailing edge of each throttle plate
Tried a set of .65 mm idle jets -- couldn't get it to idle with screw less than 1 turn in, but it ran better with the mixture screws about one turn out.
Tried a set of .55 mm idle jets -- couldn't get it to idle with screw less than 1 turn in, mixture screws actually do something, ran best with them about 2 turns out.
"S" port vacuum reads 2" at idle
In both cases above, the idle was kinda lumpy and not smooth.

Also, I noticed that the idle speed increases quite a bit when I disconnect the vacuum advance and didn't yet have it plugged. It was very smooth, too.

Does that mean it needs more air flow??? Is that why some folks need holes in their throttle plates? Those two little 3/64" holes are not much area compared to that open "S" port...

------------------------------------------------------------------------------------

Quote:
Originally Posted by swatson454 View Post
What happens when you hook the vacuum advance back up to manifold vacuum and reset the mixture screws?


Shawn
Manifold vacuum hooked to advance
Reset the mixture screws
set idle screw at 1/2 turn in
.55 mm or .60 mm or .65 mm idle jets, will not idle.


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post #2 of 57 Old 06-09-2017, 10:36 AM Thread Starter
stripperguy
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OK,
I'm thinking back to when I was temporarily disconnecting the manifold vacuum from the distributor...
engine idled much faster and smoother.

Was that because of the increased air flow?
I already have a pair of 3/64" holes in my throttle plates, with virtually no effect.

My vacuum advance line has a 1/4" ID, for an area of 0.049 sq inches
Those two 3/64" throttle plate holes provide a total of 0.0032 sq inches...not much of an impact.
Does that mean that if I increase the throttle plate holes to 0.176" ID, I'll get a similar idle performance as when the vacuum line is disconnected??

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post #3 of 57 Old 06-09-2017, 11:23 AM Thread Starter
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So I was just out in the garage
Pulled the CJ out and warmed it up.
Blocked open the choke once it was warm
Temporarily disconnected my manifold vacuum advance hose, did not clock the line
Engine speed increased quite a bit, didn't check what it was
Reduced the idle speed screw until it wasn't even touching
Adjusted the mixture screws to 3 1/2 turns out
Idle jets are .50 mm
Engine ran smooth and a rock steady 650 rpm

So, before I commit to increasing the throttle plate holes, am I on the right track? Maybe increasing the throttle plate holes by the equivalent area of the vacuum hose is a little too much, the idle speed screw was/is at zero turns in.
It seems I will need to swap to .55 mm idle jets as well, to reduce the number of turns on the mixture screws.

Just looking for a sanity check here, I'd like to finish this up and finally put the CJ back on the road next week.

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post #4 of 57 Old 06-09-2017, 11:52 AM
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Quote:
Originally Posted by stripperguy View Post
I already have a pair of 3/64" holes in my throttle plates, with virtually no effect.

My vacuum advance line has a 1/4" ID, for an area of 0.049 sq inches
Those two 3/64" throttle plate holes provide a total of 0.0032 sq inches...not much of an impact.
Does that mean that if I increase the throttle plate holes to 0.176" ID, I'll get a similar idle performance as when the vacuum line is disconnected??
Thought I would jump back in here for a comment or two.


The .45mm idle jets are in a stock/bulk carburetor. For a 4.2 / 258 cid Jeep, the consensus is .50mm or .55mm. REDLINE and others ship their conversion kits with .55mm. (then tune to your engine)


The holes drilled into the throttle plates (3/64") are large, and this is to get your throttle plates closed up to the 1/2 turn in maximum on the speed screw... This idle speed screw setting equates to the leading edge of the throttle plate at or below the enriching holes. This procedure is to get you on the idle circuit, and then find the "Lean Best Idle" from the mixture screws. "Typically" these wind up around 1 1/2 turns out from lightly seated. Some tuners, who run a .50mm idle jet typically find the Lean Best Idle around 1 3/4 to 2 turns as Shawn has also stated. This is for a leaner idle circuit.


Some like ported vacuum advance and others like manifold vacuum advance.
Ported advance reduces hydro carbons and increase cylinder temperatures.
Manifold vacuum increase hydrocarbons, reduces cylinder temperatures, and increases idle speed through more initial timing, therefore reducing the "need" to increase the idle speed from the idle speed screw opening the throttle plate.


You need to get the idle speed screw at or below 1/2 turn in. "We"/you have cheated by drilling two "large" throttle plate holes 3/64" to increase the idle speed, yet reduce the idle speed screw setting. When the enriching holes are exposed and experiencing very rich idle, you are shutting off the idle circuit fuel by closing in the mixture screws.


You also stated you disconnected the manifold vacuum advance. I believe it was too rough an idle? This manifold vacuum advance should give you 100 - 150 RPM increase. Typically, the extra/additional vacuum advance is 8-15 (?) degrees added to your initial BTDC advance (10-12 BTDC). This is adjusted with a 1/8" Allen wrench turned counter clockwise 1/2 turn at a time REDUCING canister advance. (insert Allen wrench into canister "hose" connection)


There is a lot here to go over. Try one thing at a time, review your results, then move on to the next adjustment.


Let us know, there are many here who know a lot about this 38-DGES and tuning it.


UPTILLNOW
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post #5 of 57 Old 06-09-2017, 01:08 PM Thread Starter
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UTN,

I just now came back in from messing with it.
I gradually increased the throttle plate holes to .125".
And I swapped in .55 mm idle jets.
Now, with manifold vacuum to the distributor connected, I have control over the idle speed AND the mixture screws.
I have the idle set at 1/2 turn in, and the mixture screws at 2 turns out (this is best lean)
I read ZERO vacuum at the "S" port!! Yahoo!!!!!!
I can dial my idle to be anywhere from 800 rpm to 600 rpm. This is with my timing still at 10 degrees BTDC (vacuum disconnected) at 800 rpm or so.

So, I'm a happy guy. I appreciate all the guidance that you guys have given, and the search function was also invaluable.
I've had my engine in the state that it is (Weber 38DGAS, Clifford intake, Comp cam 252H, Hedman headers, .030 FULL rebuild) for about 15 years. It's always run strong, but was miserable to start after not running for a while. Now I know why...
Anyway, I just finished a too long SOA conversion and regear after 6 years of piddling. In that time, my gas tank got pretty sludgey (is that even a word?) and clogged up my Weber when I tried to breathe life into it again.
That clogged Weber led me to look closely at how and why the 38 Weber works so well, or not so well. The true test will be tomorrow, when I start it up after a day. Hopefully, my float bowl will not have siphoned dry by the too exposed progression holes.

Thanks again to all who helped me along the way.

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post #6 of 57 Old 06-09-2017, 01:19 PM
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Glad to hear you are making progress. You can lower the idle speed to 650 -700RPM, this adjustment should reduce the throttle plate opening.


WOW, 1/8" holes in both barrels. This is more than I have ever heard of, and you are using the speed screw to increase the idle speed. This is a classic moment in tuning Weber's for me.

You may want to go to the .60mm idle jets and thus bring the mixture screws in closer to 1 1/2 turns out. I just realized you have a Clifford manifold with the larger port/plenum and slower air speed/velocity .


How does it drive?


UTN
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post #7 of 57 Old 06-09-2017, 01:38 PM
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Nice work, Stripper.

I guess the lesson is a Clifford intake likes a lot more air than a stock intake.

And thanks for starting a new thread.

Matt
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post #8 of 57 Old 06-09-2017, 08:10 PM Thread Starter
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I noticed Hoover7's comments (https://www.jeepforum.com/forum/f8/he...-head-4016441/)
and he seemed to be experiencing the same conditions as me, and even had the same epiphany.
That's what is great about the internet, all the shared wisdom (disregarding the shared ignorance, of course)

UTN,
I might still switch to the .60 mm jets, but it's running quite well as is.
How does it drive? Pretty darn nice! It's always been strong, but now it isn't so stinky.
Back when I first rebuilt and modified the engine, the improvements were so great that they masked any lacking potential. Oh, and I can mash the throttle wide open at any engine speed without a hint of stumble or stutter.
So now the engine performs as well as the drivetrain and suspension.
Slightly off my own topic, but I had quite a few specific performance goals when I did my SOA. I'm very happy to report that I put a check in every box. I have 12 inches of travel front and rear, a supple ride, very stable steering, near perfect gearing (for my uses), and enough tire for where I like to wheel.
Anyway, I was in a rush to finish this phase of the project because MDB (My Darling Bride) and I sold our house and have yet to build a new one, or even find someplace else to live!
So my garage tinkering days will end soon, at least for the next year or so.

Tomorrow, I'll grab the dealer plate again from my SIL, and give the CJ a good workout.

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post #9 of 57 Old 06-10-2017, 06:11 AM
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Mr Stripper,
I have a fairly similar setup to you. I'll set out basics so you can gauge the points of departure...

Newly rebuilt 1985 258 0.30 over
Weber 38 [redline]
HEI dizzy on manifold vac
Clifford intake and header
Clifford 272 cam
Stock mech fuel pump
Malpassi FPR set at 2.5-3 psi after regular 3 way filter.
PCV/evap canister/no EGR
TF999 auto
Pulling 18" vac in N
12* BTDC

Ok, I had been chasing my tail a little with the 38 set up for a while. Never really cracked it but kept on trying to achieve running I was happy with. The "golden" combination eluded me. I was very grateful to find good advice in threads on here especially from UTN, Swatson and Matt but the one thing I resisted trying was drilling the plates. Well about a month ago I sucked it up and gave it a shot.

Started at 1.5 mm...nothing. 2.0 mm.. marginal. 2.5 mm bingo.. stuff started to work out and i am very close to happiness. I have not had a chance to measure s-port vac but i am now a little over 1/2 on the idle speed screw and am 1 turn out on mixture screws. Big performance difference. Currently AFR is sitting at 14.5 curb idle @650rpm when warmed up and that is where I had to leave it as I only get to work on the CJ every few of weeks. I did think at that time that drilling plates 3.00 mm would perhaps get me under 1/2 turn in idle screw.

I should add that this is on .60 idle jets. I have been through jetting from .45 to .65 and .60 just seems like the natural fit for my set up. idle Pick up is noticeably better than .55 and .65 was just a little too rich and subsequently ran out of turns in on the mixture screws to lean it out enough. Will have to recheck/verify that jetting now with more tuning and testing to come but I really think that around 3.00 mm holes look like the goods for our intakes. Keep you posted.

Congrats on your new-found performance.
Cheers
whollsee
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post #10 of 57 Old 06-10-2017, 07:54 AM
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I'm reminded of the beating I took years ago for mentioning holes in the throttle plates, lol.

I'm glad you guys had the courage to do it and you seem to have had a methodical approach. Well done, fellas.


Shawn

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post #11 of 57 Old 06-10-2017, 08:46 AM
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You could always solder the throttle plate hole and start over again.


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post #12 of 57 Old 06-10-2017, 09:56 AM Thread Starter
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Mr whollsee,
It seems you have a little bigger cam than I do...but yes, we both needed similar solutions.

swatson,
Don't take any chit from anyone...your wisdom comes through the computer screen loud and clear!

Mr Strenk,
That's exactly what my S-I-L said...

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post #13 of 57 Old 06-10-2017, 11:13 AM
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Quote:
Originally Posted by stripperguy View Post
Mr whollsee,
It seems you have a little bigger cam than I do...but yes, we both needed similar solutions.

swatson,
Don't take any chit from anyone...your wisdom comes through the computer screen loud and clear!

Mr Strenk,
That's exactly what my S-I-L said...
Thank you! What I've learned over the years is that we have more to learn... always. What you thought was set in stone five years ago just vaporized and you now have a new level of understanding.

I think what you guys have just demonstrated is how much manifold design determines the carb requirements (settings). Air stretches and squeezes and does all sorts of things that don't seem to make sense. Why would the air want to basically knock at the door of the bottom of the throttle plate and suck a decent mixture through the carb when it can just sit back in some small spot that has no velocity.

I have to quit. You guys can find a three-chapter write-up in Shawy's Excuses and Alibis Volume 4. It should be somewhere around the chapter regarding the pros and cons of bedding down your best friends wife. Where's Hutch when I need him?

And don't get me started on camshafts or "you need backpressure to make the engine run right"


Shawn
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post #14 of 57 Old 06-12-2017, 01:46 PM
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Quote:
Originally Posted by swatson454 View Post
I'm reminded of the beating I took years ago for mentioning holes in the throttle plates, lol.

I'm glad you guys had the courage to do it and you seem to have had a methodical approach. Well done, fellas.


Shawn
I think I remember the words that were used about drilling holes at that time "BLASTPHOMY"!


I haven't been opposed to drilling holes to close the throttle plates, just as a last resort. Then, in my conservative nature I never went more than 1/16" hole. With the camshafts, slow air velocity in the "Clifford" manifold, and what ever kind of fuel we are trying to burn now a days, it's really a guess as to how big a hole to drill. My personal conservative nature is smaller is better.


Shawn, you are an out of the box kinda guy and I like that. With some of your high performance knowledge, this seems to get allot more accomplished.


Flame suit on, fire extinguishers near by.
UPTILLNOW


EDIT: Two 1/8" holes = 1/4" diameter of air passing the already opened (1/2 turn in) throttle plate.... wow, that is all.
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post #15 of 57 Old 06-12-2017, 02:20 PM
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Quote:
Originally Posted by uptillnow View Post
...

EDIT: Two 1/8" holes = 1/4" diameter of air passing the already opened (1/2 turn in) throttle plate.... wow, that is all.
Two 1/8" Holes = 0.0245 sqin
One 1/4" Hole = 0.0490 sqin

just saying....
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