Rough-idling 360, can't figure it out!

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MopaR&D

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This isn't in an A-body but it is an SBM, the 360 in my '72 D200 pickup is driving me nuts. It has a rough idle issue where it occasionally feels like it's down a couple cylinders, especially in gear (727 auto). I have replaced a lot of parts which needed replacing anyway and/or because I wanted more performance; Edelbrock 1406 on a Performer manifold in place of the stock 2-bbl setup, Pertronix Ignitor 2 kit to replace the stock points, new plugs (just replaced them with .035" gap, had some in with .045" and thought that might be too much but it made no difference), plug wires, cap, rotor. The cylinder heads were also replaced because they had burned/sunken exhaust valves; thought that might have caused the rough idle but again no dice. I did a compression test with the old heads and one cylinder was down pretty low from the others but it turned out to have a badly sunken exhaust valve.

I've tried all the tuning tricks; plenty of initial advance (18*), idle mixture and speed, doesn't change. No vacuum leaks either. I've noticed when the timing light is hooked up and the engine stumbles it looks like the spark drops momentarily. Could it be the original distributor? I'm running out of ideas. Other than the rough idle the truck runs fantastic which is why I'm so confused, as soon as you touch the throttle or you're going down the road it smooths out and feels fine.
 
Get a spray bottle of water and spray around at every gasket on the intake and check for vacuum leaks. Don't use anything flammable, that's dangerous and water will show it up just as quickly. Spray around the vacuum booster check valve and where the hose connects on both ends. Around the carburetor base. All around the intake gaskets on both sides. Anywhere there might be a vacuum leak. That's the number one cause of a rough idle.
 
Hook up a vacuum gauge and consider getting an AFR-gauge to see what is actually happening with your mixture.

You can pinch the pcv-hose closed as a quick check. Most engines will run a little smoother that way, but if your pcv-valve is shot it will create a larger vacuumleak.
 
If you have been running the carb for some time, and if it gets parked for a month here and there, i wouldnt hesitate to pull carb and give it a teardown and thorough cleaning.
Original distributor, could have worn bushings,causing shaft to dance around,playing havoc with timing.
 
1972 360 original engine ??

Those stock camshafts can start going flat over that much time. Lobe peaks start wearing off along with eating onto the lifter base causing a concave surface area.

New no zinc/low zinc oils are hard on the earlier camshafts.
 
My D150 was having similar issues. After trouble shooting I started replacing parts........I replaced the distributor, carb, coil, ignition box, plugs, wires, fuel pump, fuel filter.....after all of that (and to my embarrassment) it was a vacuum leak that I had missed WAY back when I started trouble shooting (near the master cylinder)....before I bought all the parts :\
 
Start with trouble shooting, is it fuel related, spark related, vacuum related, compression related, the list can go on? In order to try to help you diagnose the problem, we need just a "little" more information.
 
steady vacuum >20 in? concentrate on ignition of you say the timing light pulses are erratic. Id put the points back in and try that instead of the pertronix just to isolate that piece. I had an erratic idle on my 4 banger and it turns out the carb idle circuit was full of crap E10 California gas jello. felt just like a vacuum leak too.
 
Make sure you're getting fuel through both sides of your idle circuit. If plugged/restricted on one side, it can cause several cylinders to drop out at idle/low speed and then "come alive" when you get the rpms up enough to start feeding off of the main circuit, similar to what you described. With the Performer manifold, after the engine warms up, you'll feel the two center cylinders on one bank and two outer cylinders on the opposite bank being noticeably colder, as if they're not firing. If it's the centers on the driver's side and outers on the pass side that are cold, your pass side idle circuit is plugged/restricted. The opposite also applies. Either way, a carb teardown/cleaning is in order.
 
Does it clear up after ~1800 RPM? It took about 3 minutes of a very fast idle and 1/2 can of carb spray to unf*uck my carbs idle circuit. I jacked the idle up just enough to allow it to keep itself idling and then started spraying just enough to keep it running, it finally cleared itself like magic. I had no vacuum leaks. AFR meter will show unburned fuel as high O2 output.
 
1972 360 original engine ??

Those stock camshafts can start going flat over that much time. Lobe peaks start wearing off along with eating onto the lifter base causing a concave surface area.

New no zinc/low zinc oils are hard on the earlier camshafts.

killjoy. lol
 
Does it clear up after ~1800 RPM? It took about 3 minutes of a very fast idle and 1/2 can of carb spray to unf*uck my carbs idle circuit. I jacked the idle up just enough to allow it to keep itself idling and then started spraying just enough to keep it running, it finally cleared itself like magic. I had no vacuum leaks. AFR meter will show unburned fuel as high O2 output.

Doesn't def*uck sound better? LMAO
 
Buddy had me chasing timing for the longest time,wanted to replace ign control and refused to believe it was carb. Edelbrock 1406,i finally put my foot down and pulled carb apart. Bowl on one side was perfect, other side was green slime. I dont think i spent an hour on it, and didnt damage any gaskets. I like to put a thin film of grease on gaskets. May help seal and will come off without damage.
 
Lots of replies thanks guys... so first off, this rough idle issue was there even with the stock 2-bbl Holley which I rebuilt and cleaned before doing the 4-bbl conversion and it did nothing. This Edelbrock was used and had sat for some time so I went through it before putting it on the truck, it was in better shape than I expected. The truck also never sits more than 3 weeks or so and I usually add either Lucas ethanol fuel additive or occasionally fill up with ethanol-free gas. I'm not ruling out the carb but there are other places I'd rather look into first.

I'm going to try plugging off the brake booster vacuum port and spray water around the carb/ intake area to check for leaks this evening.

@George Jets if I had a cam lobe going bad wouldn't it be making noise? I didn't inspect them closely but when I had the intake off I looked over all the cam lobes and they all looked normal and consistent.

I'm thinking of also testing the electrical stuff, particularly the voltage at the coil when running I feel like there might be something funky going on there. This problem has existed since I first got the truck running with all the original parts and there are still some weird minor electrical gremlins I need to figure out.
 
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Look for Weak ignition. Check all grounds, power at coil.
At idle pull vac on the egr and see if the idle gets shitty, you know its working if it does.
Change the coil for one that's good., bypass the ballast and try it.
Octane boost helps old gas.
Vise grip close all vac lines and see.
New plugs?

Make sure idle mix screw are identically set!!
 
Lots of replies thanks guys... so first off, this rough idle issue was there even with the stock 2-bbl Holley which I rebuilt and cleaned before doing the 4-bbl conversion and it did nothing. This Edelbrock was used and had sat for some time so I went through it before putting it on the truck, it was in better shape than I expected. The truck also never sits more than 3 weeks or so and I usually add either Lucas ethanol fuel additive or occasionally fill up with ethanol-free gas. I'm not ruling out the carb but there are other places I'd rather look into first.

I'm going to try plugging off the brake booster vacuum port and spray water around the carb/ intake area to check for leaks this evening.

@George Jets if I had a cam lobe going bad wouldn't it be making noise? I didn't inspect them closely but when I had the intake off I looked over all the cam lobes and they all looked normal and consistent.

I'm thinking of also testing the electrical stuff, particularly the voltage at the coil when running I feel like there might be something funky going on there. This problem has existed since I first got the truck running with all the original parts and there are still some weird minor electrical gremlins I need to figure out.

A worn cam lobe will be like a misfire( a dead cylinder or running on 7 cylinders). if it is intake side,there isnt enough air fuel mixture to enter the combustion chamber to have adequate combustion. If exhaust side, i would think it may be the same or even a pop through the carb. Too much exhaust gas will exit when the intake valve opens. The cam can be checked by pulling valve covers, start or tun engine by hand, see which rocker is barely moving. Also hook up a vacuum gauge that can indicate if its valvetrain also, the needle may flutter if a valve is hanging up or a valve spring is broken etc. In auto tech school my driveability teacher taught us to "make sure the engine is mechanically sound before attempting to tune it up" basically if there is a valve /cam/ problem, youll never be able to smooth it out by tuning it up. I aslo remember EGR ssystems screwing up idle. yoi um ay wan tto check that ,make sure EGR valve isnt stuck open.
 
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Just a heads-up my truck doesn't have EGR so that's one less potential issue.

I'll post up a video of it running later today. If a cam lobe was bad it would be a consistent misfire on one cylinder right? This is more of a momentary thing where it idles mostly smooth for a few seconds then drops and "chugs" like it lost multiple cylinders, and the process repeats. I'll hook up my vacuum gauge and show that in the video too.
 
Just a heads-up my truck doesn't have EGR so that's one less potential issue.

I'll post up a video of it running later today. If a cam lobe was bad it would be a consistent misfire on one cylinder right? This is more of a momentary thing where it idles mostly smooth for a few seconds then drops and "chugs" like it lost multiple cylinders, and the process repeats. I'll hook up my vacuum gauge and show that in the video too.
How is the ignition module? is it grounded good? I remember grounding the module case to the intake in my 70 dart with a jumper wire.
 
Lots of replies thanks guys... so first off, this rough idle issue was there even with the stock 2-bbl Holley which I rebuilt and cleaned before doing the 4-bbl conversion and it did nothing. This Edelbrock was used and had sat for some time so I went through it before putting it on the truck, it was in better shape than I expected. The truck also never sits more than 3 weeks or so and I usually add either Lucas ethanol fuel additive or occasionally fill up with ethanol-free gas. I'm not ruling out the carb but there are other places I'd rather look into first.

I'm going to try plugging off the brake booster vacuum port and spray water around the carb/ intake area to check for leaks this evening.

@George Jets if I had a cam lobe going bad wouldn't it be making noise? I didn't inspect them closely but when I had the intake off I looked over all the cam lobes and they all looked normal and consistent.

I'm thinking of also testing the electrical stuff, particularly the voltage at the coil when running I feel like there might be something funky going on there. This problem has existed since I first got the truck running with all the original parts and there are still some weird minor electrical gremlins I need to figure out.

Normal and consistent on your quick inspection of the cam is good.

When the cams just start to go flat you start to lose valve lift, the hydraulic lifter will adjust up automatically to keep it from ticking.

The thing I have seen with these stock cams going flat is that it is just usually 1 or 2 lobes that go flat quicker than the others. So that causes an imbalance in the engine creating a rough idle.

I have a stock '76 360 2 bbl engine here with fresh heads that is going through the same thing, odd idle. After changing 3 different carbs and jumping through all the whoops to try to get it right, no joy.

Just was progressively getting worse, finally yanked it out. It is sitting on an engine stand here slated for tear down and a new 340 cam and lifters.

Then back together again, run it on the engine test run stand, break in the new cam properly. Should run like a million bucks after that, ready to go for whatever is coming next. Such a nice clean bore on this engine, so going to do new rods and mains and a new oil pump at the same time.

1006161643.jpg
 
How is the ignition module? is it grounded good? I remember grounding the module case to the intake in my 70 dart with a jumper wire.

Hmm I'm not sure I'll have to check that, it would make sense if there's a bad connection to the Pertronix module.
 
One last thing here MopaR&D,

I swapped out the suspect '76 360 engine with a different good one, using all of the same: distributor, brain box, all wiring, plug wires, cap and rotor, and carburetor.

My idle problem and odd running problem went away, idles like a dream and runs out nice now. So my problem was in the core of the engine.

Everything is leaning to a few cam lobes going flat on the '76 360. Those camshafts are the whole "Run Program" for the engines. When the cam starts to wear out the whole "Run Program" goes out the window.

Good Luck on your Truck !!
 
Here is a comparison of a couple of cams where the lobes are starting to wear flat.

1970 318 cam on the right

1974 Stock 318 cam on the left

Cam on the right has 2 lobes that are definitely wearing down.

'74 cam on the left also has some lobes just starting to wear down, but no where near as bad as the '70 on the right.

20200812_160410.jpg


20200812_160537(0).jpg
 
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