Surging Rough idle

The coupe with the 351 Cleveland I just picked up, when its idling it idles rough and raises the RPM off and on but sort of in a pattern, when I'm driving it seems like it surges between 40-45. Seems to have plenty of power otherwise, and runs smooth except in these situations mentioned above.

Today we replaced ALL the vacuum lines/hoses, points, cap and rotor, removed carb and made sure the carb isn't leaking air or gas checked the flatness of the carb base:, carb seems fine, except its a 2BBL. Replaced the voltage regulator, starter solenoid and fuel filter all with Motorcraft parts.

Not anything changed the issues mentioned. above.

Wednesday we are going to replace all the plugs and wires. We pulled a few of the spark plugs just to test there condition, the 3 we pulled were clean and gaped correctly.

Does anyone have a suggestion here or have you experienced this issue? What did you find fixed this or what suggestions might you guys have?

Were a little confused here. :huh:

Surging at 40-45 on cruise would mean you need to jet up 2 sizes and retest.

Surging at idle. Mine does it also. After messing around it was easier to just raise idle Rpms a little to smooth it out.

Thxs whats your thought on the rough idle?

Thxs whats your thought on the rough idle?

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Joe, this and a new carb should help with all 3 issues

surging

rough idle

pesky 2 V carb

every engine is different could be a cam swap, could be age of the engine internal wear. lower compression

my idle is rougher due to aftermarket cam. not much you can do about it as far as air/fuel mixture, i raised rpms to smooth it out some more. around 800-850 rpms. it still bounces a bit but at least my teeth fillings are not falling out.

i hate to say it but swapping a carb or intake for new ones doesn't do much and can lead to more problems if not done right.

when you have a original top end to the engine and you go to swap out the intake and carb you run into problems with throttle cable, transmission kick down, choke wiring, air cleaner clearance, Coil placement ignition wire routing... problems trickle down the line.

your carb might need more work it might need to be cleaned, you may have weak spark for some reason. it could also be running lean at idle and needs to be richened up slightly. lean idle usually causes the rough idle or bouncing the car does.

you could have a minor vacuum leak that would cause slight lean at idle. all you need is a crack in a vaccum plug or old hoses and that would cause a slight leak that would effect idle.

A stock carb is going to be very limiting on adjustments which is another reason people go aftermarket.

when you do the tune up it might solve the issue. the engine is sensitive so if something is slightly off you feel it.

if you have points ignition i would recommend swapping to electronic it helps.

on a modern car you are suppose to replace rubber hoses about every 5 years, but nobody does it, not even the dealers that are suppose to do it during 50k and 100k service you can imagine all the 40 year old rubber most cars have sitting under the hood.

not only hoses, but pcv and egr valves that may not of been changed or cleaned. plus different vacuum motors on the distributor and air cleaner that might be leaking or malfunctioning.

most people limit a tune up to caps, rotors, wires, plugs.

then maybe they flush out the carb or rebuild it with new gaskets.

but then they leave everything else and just assume its ok.

example i had a blown brake booster didn't even realize it and on top of that it turned out the booster check valve(reed valve) was also blown out.

then i had a bad vacuum canister it wasn't leaking but the reed check valve was also blown in it. it was causing a vacuum capacity loss.

basically the booster and the vacuum canister act like a reserve battery and store vacuum for when there is a dip from the engine vaccum and there is a compensation. in my case when my engine droped vacuum during acceleration my brakes lost power and my climate control system shutdown. that would in turn cause a engine misfire because the intake mix would go lean. all kinds of issues would crop up. in the second case it didn't have an actual leak but i did have a problem with the vaccum that drives the accessories and it was minor and caused problems.

when you complete the tune up see where you stand and you can decide to look around more or just leave it alone. also if you have a working pcv system i would inspect it, clean out or replace the pcv on the valve cover a sticking pcv can cause a slight vac leak at idle as well.

Check for a vacuum leak it can cause both conditions.

Sounds like a carb issue to me

Carb is on my short list probably going with a Holley 2300 500 CFM

Opinions? My mechanic is old school and he really thinks this is a great carb for the 351 Cleveland. Ive been told by others to go with a Carter AFB.. So confusing!! :huh:

Nope not doing the intake and 4bbl. This coupe is not a race car. I have a Shelby for that

Snug up on the intake manifold bolts.

Possible leaky intake gasket.

Can't hurt.

I'd stick with the 2V, too. Never had a (major) problem with them myself.

Timing gear set also, will cause intermittent idle issues as described...

Did you change the carb base gasket in order to eliminate the possibility of any vacuum leaks at that particular location?

Well here's one piece of many Ive added lately! The coupes going in for 50 series Flowmasters tomorrow and getting true dual exhaust instead of a single pipe.

I know the 50 series aren't really loud, but they will sound a ton better than what it has now. I also didn't want it insanely loud, this is a cruiser for me not a racer.

I'm hoping the 2300 Holley helps with a rough idle and surging!

A bad dizzy vacuum advance cause the stated problem.