Stumped...idle missing on 4 cylinders

T

Tracy72RN

Member
Well I've posted a few times on getting our '74 Scout II tuned up and running right.

It's a 345 4 spd with Holley 23xx carb.

I've done everything I could think of plus what was recommended to me here. It's been running about 95% well...having only a slight studder with off idle acceleration. My wife has been driving it back and forth to work for 3 weeks no with no trouble. I drive it a couple times a week to get the kids to and from school and to make sure it's running ok.

I got in it today to go get the kids from school and when I released the choke to idle it was idling rough...it usually doesn't idle rough even when cold. Also by the end of our 1/4 mile road/driveway to the main road it's warmed up enough but it remained idling rough as if missing. No noise...just missing. Once you revved it past what I would guess is 1500 rpm +- it runs fine. It has power...and cruises just fine with no misses...very smooth just as it has been for several weeks now.... But when you stop and it goes to idle it's rough again.

Dwell is at 32*. I have tried setting it from 22-32 and it doesn't change much...but it runs/drives the best at 32*.

All plugs look good except #8 which is dark...no crusties...no moisture...just dark. The other look near new (they all are actually).

Timing it 6* btdc. Runs best here...I've run it from 0*-10*...6* is best.

I've messed with the idle mix screws...no change other than to run worse as it should when they aren't set right.

I pulled each individual plug wire off the dist cap while it was running. When cyls 8, 3, 5, and 7 were pulled...nothing changed...so it seems it's missing on those 4 cylinders... At idle only. I've checked and rechecked and rechecked those 4 cylinders connections both at the plugs and the cap...nothing makes it better.

It has a new ballast resistor as well as coil condensor, pints etc.

Again...just yesterday this thing was running fine...even the weather today is identical to yesterday.

It sure looks like fuel injection is going to be under our Christmas tree!! :icon_mrgreen:

any help is appreciated.

Thanks!

Jason
 
Sounds like a fuel delivery issue that's most prevalent at idle and transition to cruise. The engine is starving for fuel. Once you open the throttle up, you transition away from the idle circuit to the primary circuit which is not obstructed, and all cylinders are then able to fire on a consistent basis. If you were only hitting on 4 out of 8 at cruise and hard acceleration, that would be rough, gutless and sluggish. So what I think you've got is a partially plugged idle circuit. Something you can try is to remove both idle mix screws and blast some carb cleaner into the holes. You can even chase it with a quick blast of compressed air. Then reset your mix screws as per normal. See if that helps. If it does, that means you have junk in your tank or fuel lines that is being loosened by the lovely ethanol blend fuel you're burning. Which begs the question, what does your current fuel filtration system consist of?
 
It just has an inline fuel filter which is what I would guess is similar to stock.

It's been replaced recently as well...the po did put in a new fuel tank around 1-2 years ago but I know the lines May indeed have some trash in them.

I will try the idle mix screws and see if air etc in there will clear something and then reset them.

I know those cylinders are (or should be) getting spark since when I remove the plug wires at the cap...it sparks as it should (man I don't want to touch it) and then as I replace them it sparks as it should (I got lucky...I didn't touch it! Lol )

thanks for the input...as soon as the sun's up here I will be doing this suggestion.
 
1-8-4-3-6-5-7-2

the firing order above and the red are the misfiring ones... I think the 7 should be 2 thus mimicking a blocked idle screw circuit on one barrel... Remember that the factory dual plane intake manifold is divided so that each carburetor barrel feeds every other cylinder in the firing order... The basic name for this type of manifold is a "180 degree" manifold...

Take a close look at the idle circuit that feeds the missing cylinders and I think you'll have it solved..... Blast it out like
sb74 described or yank the carb and give it a good cleaning.
 
It was the fuel filter. Despite it being new...it disentigrated (sp).

I pulled the idle screws out and ran carb cleaner in them...nothing. Ran carb cleaner down the carb while running...nothing. Then I removed the idle screws again and blasted air through...it idled well for about 10 seconds and then began to miss again.

Pulled off the fuel filter and whatever filter media was inside...wasn't any more. It had come apart badly enough to where the whole inside was rattling around...even the internal metal part(s).

Cleaned everything out...put in a new fuel filter (and an air filter) and viola...idling like a champ. I just hope I got all the trash out so it doesn't act up on my wife.

Thanks guys!!
 
Glad you solved it. Corngaz is the devil. It got hungry and consumed your filter like a fat girl at an all-you-can-eat buffet.
 
scoutboy74 said:
glad you solved it. Corngaz is the devil. It got hungry and consumed your filter like a fat girl at an all-you-can-eat buffet.
Click to expand...

Yumm yumm! Lol. Sad thing is I can get non ethanol gas but it's 20 minutes away and costs a dollar more per gallon. We use it in all of our small engines (lawnmower, weedeater etc) but can't afford to run it in our dd.

Ethanol sucks!! :)
 
Yeah, way too spendy for a road rig. About the best thing you can do is treat with marine stabyl (blue not red).
 
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