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Unread 08-18-2013, 10:18 PM   #1
Brittel78cj5
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Acceleration Issues on 258 Inline 6

I posted this in the Engine/driveline thread as well, can't find forum rules sorry if this is an over-post, just looking for answers!
Specs:

78CJ5 with 258 inline 6 with the following:
Offenhauser intake
holley 390cfm 4 barrel
K&N air filter (getting dirty)
split headers (they look like hedman hedders) into dual exhaust with magnaflow mufflers
MSD 6al ignition with team rush upgrade, 8mm msd cut to fit wires, msd blaster2 coil
fuel filter before carb (sometimes has air in it),
When purchased, was told "it's got a cam" but I'm unsure of it being much more than stock.
Stock (as far as I know) dana 30 front and AMC rear with alloy usa solid axles.
Unknown transmission and differential (4 speed manual, granny 1st, reverse is to the right and up). Differential is to the right of tranny shifter, 4H, 2H, N, 4L.
33x12.5R15 Tires

Cylinder compression, cold engine, wide open throttle, screw-in type guage.
1: 150psi
2: 155
3: 170
4: 145
5: 160
6: 160
18 to 18.75 inHG vacuum at idle, around 20 when held around 2,000rpm
30psi hot idle oil pressure, 60 driving



THE ISSUE

Runs perfectly when driven normally with light acceleration, and no popping or very little popping out the exhaust when letting off/downshifting to a stop. With hard acceleration, engine runs horribly. After a hard acceleration, letting off to slow back down for another run yields lots of cracking and popping. Some of this is my exhaust, but it does it far more than a normal slow-down (as in, a light 0-60 and then back down verses a quick 0-60 and back down, but downshifting in the same manner). When I start off on a hard acceleration, the engine stutters or doesn't do much, followed by a some-what loading up sound, and then it's off to the races. When I was timed to 8* it didn't do so much the loading up sound, was just really doggy and laggy (so I will most likely re-time, depending on recommendations).
2nd is by far the worst and 3rd gear isn't much better. 4th gear isn't as bad but doesn't seem like there is much power (possibly shifting early out of 3rd).
Reading as many forum post as I can,

I've narrowed it down to ignition, timing, or the carb (or most likely a combo of all three).

I started with ignition (after messing around with the fresh rebuilt carb for a while). I did the team rush upgrade and used blue rtv to seal up the bottom portion (let cure for a few hours in the South Dakota heat and wind before adding rotor and cap). Dielectric grease went on the cap and plug wire connections. I have 8mm cut to fit msd wires, blaster 2 coil, and an MSD 6al ignition controller (my what I have been missing! Love the quick starts and thanks to all who have posted about this upgrade!)

Holly 390cfm 4 barrel. I don't remember if it is the 4150 or the 4160, but the part number is 0-8007. It used the round bore gasket. My elevation is 1290ft with frequent trips to 1850ft. I stay within this elevation running regular or premium gas. I may do a trip to the black hills 3200ft to 4600ft but would be a once a year at most trip.
I rebuilt it this spring but didn't re-jet (assume stock) and the power valve I replaced was the same number as stock (around a 6.5 if memory is correct). I went from a .025 pump discharge nozzle (squirter) to .032, and that helped when I had the stock ignition. After the carb re-build the engine ran FAR better. I replaced the diaphragm for the secondaries and the spring with the lighter yellow. I haven't gotten to swapping that to the heavy black spring though and focusing on the primaries, as I ran out of time. I installed the quick-change vacuum secondary housing and have the spring kit so this won't be an issue.

Timing. When I'm timing, I don't see much of a difference after I hook up my vacuum advance to the distributor. I have it hooked up to the timed port on the side of the primary metering block (may have gotten a little carried away water proofing the distributor, will pop the cap and inspect nothing but dielectric grease is in the way of that arm). I'm currently at 5*BTDC. I'm in the process of getting a tac so I know what rpm I'm idling at (I may have a temporary one laying around somewhere). Before ignition upgrade I was around 700-800 rpm.

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Unread 08-18-2013, 11:45 PM   #2
LumpyGrits
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I would look into why #4 cyl has such a low comp.
LG
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Unread 08-19-2013, 07:45 AM   #3
swatson454
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I've never seen an engine run better at 5* than it did at 8* unless the balancer had slipped. Have you verified TDC?

Does it lope any with that cam?

Have you put a timing light on it to make sure it's advancing with RPM? Plotting your advance curve is probably a good idea.

How do the plugs look?


Shawn
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Unread 08-19-2013, 06:47 PM   #4
Brittel78cj5
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Plugs are new, the rest I'll have to answer this weekend. I'll also check #4's compression again.
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Unread 08-20-2013, 04:43 AM   #5
86cj74.2L
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You still have the original jetting the 390 came with?

I would say you need to rejet. The Holley is workable out of the box but far from optimum IMO.

If you search for 390 and jetting you should come up with someone with a similar setup to yours and what their primary and secondary jetting is.
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Unread 08-20-2013, 07:45 AM   #6
WindKnot
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Not knowing who installed your carburetor, here is a link to the Holley instruction sheets for it.

Very beneficial to re-read them: http://www.holley.com/data/Products/...8108-2rev2.pdf

Because of your elevation, your starting point for jets should be the factory set up.
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Unread 08-20-2013, 09:53 PM   #7
Brittel78cj5
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Quote:
Originally Posted by WindKnot View Post
Not knowing who installed your carburetor, here is a link to the Holley instruction sheets for it.

Very beneficial to re-read them: http://www.holley.com/data/Products/...8108-2rev2.pdf

Because of your elevation, your starting point for jets should be the factory set up.


Reading this got me to where I need to be. I took my vacuum advance off the timed port and put it on a full vacuum port on the bottom of the carb. I cant post pictures yet, but if you look at that manual, there is a vacuum hose going from the top of the carb to the bottom....well... The previous owner felt the need to cap the bottom and top (by that I mean put a screw in the vacuum hose). I pulled those off and put a new hose on. I also put my timing back to 8*, and with the vacuum advance plugged in, it's over 10, I'd guess around 15.

I put the black spring in the vacuum secondary and took it for a test. INSTANT RESPONSE! I went to tinkering with the secondary spring and settled on the brown (medium heavy). It very slightly has a weak point in throttle, but I'm going to adjust the floats again and all the other little things before going to the next heavy, the black. I don't think the black is the spring for me, but will find out when I have other things adjusted correctly. The brown felt stronger once it got over that short weak spot.
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Unread 08-21-2013, 03:20 AM   #8
86cj74.2L
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How does it feel accelerating at WOT. All those springs do is delay when the secondary's open.

If you have the 13r-18r advance head your timing should be in the low to mid 20's at idle with vacuum source on manifold.
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Unread 08-21-2013, 06:07 AM   #9
Brittel78cj5
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Quote:
Originally Posted by 86cj74.2L View Post
How does it feel accelerating at WOT.
"It very slightly has a weak point in throttle". Maybe not my best explanation, but it gets up and goes, and then for just a second, has not even hesitation or a bog, (acceleration) just slows and then picks back up again. It may be the secondaries opening just a second early (in which case I'd have to settle on the heaviest spring) or not getting enough fuel when they do open, which is why I need to check my float levels and the other small adjustments because the brown (second to most heavy spring in the kit) is VERY close to correct.

Quote:
Originally Posted by 86cj74.2L View Post
All those springs do is delay when the secondary's open.
I understand this. I had the 2nd to lightest spring in there, and it was opening too early, causing a lot of the "bog" I described in first post. I went to the most heavy as to some-what "disable" that feature until I had the primaries running correctly. I then was swapping them in and out to find out which one opened when I needed it to.

Quote:
Originally Posted by 86cj74.2L View Post
If you have the 13r-18r advance head your timing should be in the low to mid 20's at idle with vacuum source on manifold.
I'll have to look again, but it would be pretty close to that.
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Unread 04-28-2014, 07:13 AM   #10
Brittel78cj5
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After a while of tuning and messing around with the needles I'm at 21" vac and the timing jumps up to the 20s when I plug in the vac adv. I've got a new power valve coming in (8.5) to replace the 6.5? that came with the rebuild kit.

Normal driving is about 10" vac, with the "weak" point I was talking about being around the 6.5-9" area when wanting to accelerate.

Some how my timing made its way to about 10-12*. Putting that back to 8* sure made a power difference that about spooked me on the gravel roads.
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Unread 04-28-2014, 07:52 AM   #11
Matt1981CJ7
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Quote:
Originally Posted by Brittel78cj5 View Post
Plugs are new....
I think Shawn was asking what the plugs look like. They can tell you a lot about the tuning of your engine, especially mixture and timing.

If it's still popping on deceleration, that's almost always loose manifold bolts.

Overall, it sounds like your heading in the correct direction with your tuning.

Matt
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Unread 04-29-2014, 11:36 AM   #12
Brittel78cj5
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Yep! That post was 8 months ago I checked them out that weekend . I've tinkered with it a lot since then and just wanted to give an update so anyone else that does some searching and falls on this thread has a bit to go off of.

I did have a bit of black being blown out the exhaust and I found to tune the air bleed needles to get max engine vacuum. This helped out a lot with the excess fuel. Winter came by and so I was playing with my electric choke and now that it's getting nice out again and I've had all winter to search around for answers I'm thinking (hoping) the correct power valve is gonna be this 8.5. I'll make sure all my floats are correct again and that I have max vacuum, then I'll run it for a little while and check plugs again.

On another note I'm working on installing the OME YJ conversion, which will have a thread all it's own in due time. Currently fighting a broken bolt which I had about beat, then just finishing up the rear and done.
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