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So I just had this 1968 428cj engine rebuilt 300 miles ago and have been struggling with a stumbling issue. I believe timing is good after checking three times. Pertronix - base timing is at 12 BTDC with vacuum off. Then vacuum is attached to manifold.
I'm running with the following; Holley 750 Vacuum Sec with 68 primaries and a 81 secondary plate. 7.5 PV with a 42 accel nozzle. Idle mixture was set at 900 RPM with best vacuum of ~ 15.25 Hg. Shortie Headers, manual trans, AP45 Autolite plugs.
Still has the break in oil with roller 270 Howard Cam.
BTW I checked for intake vacuum leaks three times using various fluids and there's no difference in idle.
Is this possibly a vacuum leak from the valley sucking oil in? Maybe the rings aren't seated? Poor valve guides or just too much fuel? Next step I'll check compression...
Note the engine was idling a Lot with these plugs but why the two dry ones? The plugs look awful and the #1 and #6 look the best. I can't tell if the others are full of oil or just running way to rich as they do smell like gasoline? What do you think is going on? Thanks in advance.
Last edited by ChaoticMN; Dec 4, 2023 at 07:50 PM.
Reason: corrected sec jet size
I've had a theory about this for a while. Actually tried to ask about it on an FE forum and the conversation went off on a tangent and never got anywhere on this topic. I'll give those guys time to forget and ask again in a different way some day.
I think these uneven plug readings are due to using an intake built for higher rpms. They flow great to feed a high revving engine but at idle or cruise the air and fuel charge moves "lazily". Fuel is heavier so it kinda piles up causing some cylinders to be leaner and some fatter. I believe this is why the edelbrock performer 390 makes more power at lower rpm than the performer rpm intake. At higher rpm's the performer rpm makes more power. It's about the ability to thoroughly mix the air and fuel and distribute it evenly at a given engine rpm.
What intake do you have? I'm guessing since you spent money on a roller cam that you also spent money on an aftermarket intake.
Might be able to improve with carb tuning. I can't help you there.
I was thinking the similarly and was wondering if this topic might come up. I decided to stay with stock iron 4v C80E-C intake and C4AE-G heads reworked to the CJ valve size.
I also am starting to think the carb is flooding intake upon hot shutdowns.
I also had some issues with the choke not fully going off for abou 200 miles, note this was noticed due to fast idle screw being on the last choke step. So this was also a contibuter to being run rich.
Note there is no noticeable blue smoke out the exhaust.
I plan to install new plugs and run them at about 2100 rpm and pull em for a proper reading. Will have to wait til spring as the roads here are now full of salt!
I also plan on installing a Progression blue tooth distributor and an AEM wide band kit for better tuning.
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