Acceleration Issues on 258 Inline 6 -
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post #1 of 6 Old 08-18-2013, 10:11 PM Thread Starter
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1978 CJ5 
Join Date: Aug 2013
Location: Eastern
Posts: 43
Acceleration Issues on 258 Inline 6

First post, hello to all!

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


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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post #2 of 6 Old 08-19-2013, 01:08 AM
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You sound very knowledgeable and I'm not asmuch as you are. Just curious if you've tried with the vacuum advance disconnected? Could the weights be stuck also?
Interesting thread with a lot of detail. I'm going to keep my eyes on it. Haha
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post #3 of 6 Old 08-19-2013, 07:22 AM
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1990 YJ Wrangler 
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Location: VA Beach
Posts: 7,685
If it starts haviong problems only after hard acceleration then you most probably have something that is caused by LOW manifold vacum. When you have throttle open, high load and ,relative to throttle position, low RPM you will have low vac.
Timing vac advance would be my first guess. Is your advance ported or manifold? If you are losing vac and the timing retards too much you will be way rich and it will pop through the exhaust.

If your timing is correct then look at the power valve. If it is defective or wrong for your setup then it will also flood under low vac.
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post #4 of 6 Old 08-19-2013, 06:51 PM Thread Starter
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1978 CJ5 
Join Date: Aug 2013
Location: Eastern
Posts: 43
I'll be able to work on it this weekend. I have this thread in another area and will update both as I go.
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post #5 of 6 Old 08-20-2013, 09:54 PM Thread Starter
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1978 CJ5 
Join Date: Aug 2013
Location: Eastern
Posts: 43
The answer for me was found in the other thread.

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:

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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post #6 of 6 Old 09-24-2013, 03:48 AM
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1997 TJ Wrangler 
Join Date: Jun 2011
Location: No longer Western KY
Posts: 111
It sounds like I am having the same issues, mine is not carbed however. It looks like I need to find a vacuum leak

"Tires are like boobs. They always look big until you have played with them for a while. Then you just want a bigger set!”
-"YEEP” from Virtual Jeep Club
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