AMC V8 Idle issues

My engine (AMC 360) does not idle smoothly, it seems to hunt up and down a couple of 100 RPM (i.e. does not stay at one consistent speed) this happens more once warm. Could this be caused from a miss adjusted idle mixture?

Other than this the engine runs great

Standard 360 with no smog kit, running MC2100/manual choke and HEI dizzy conversion

Any ideas

 

Could be a small vac leak somewhere, but you say a HEI conversion. Is your vac advance can hooked up to manifold vac or ported vac on the carb? Manifold vac pulls full vac at idle. You need a vac can that will max out at 2" of vac lower than your engine makes at idle. If your engine only makes say 15" of vac at idle and it takes 18" to max out your can, the engine will hunt up and down a few hundred rpms. If you are connected to ported vac at the carb, I would look for vac leak somewhere, maybe carb gasket, cracked vac line, etc..

 

Dirty EGR valve pintle will cause a bad idle.
Worn carburetor body at the throttle shaft will cause a vatiable idle.

Have you adjusted the idle mixture? It's very easy.

 

Mine did that more when I was connected to ported vacuum. I also had a small leak under the carb gasket, once I fixed that and went to manifold vacuum the engine idles awesome. I also tuned my carb according to instructions like these and it helped...http://www.jeepsunlimited.com/forums/showthread.php?490525-MC2100-tuning-Vacuum-reading

One other thing, if you have a vacuum gauge it helps to know the potential cause of various readings. I found this site helpful...http://www.tuningmadeeasy.com/tuning/vacuum-gauge-the-wonder-tool/

I'm not sure if the next two are applicable with an HEI but here goes, they have more to do with distributor timing. http://www.ifsja.org/forums/vb/showthread.php?t=84586, also my recent thread here has some good info and suggestions in it as well...http://www.earlycj5.net/forums/showthread.php?106419-Tuning-the-360-still-has-spark-knock-at-highway-speeds-after-engine-is-hot

 

Thanks guys for the responses,

My dizzy vacuum point comes off the carb at the port under the choke unit, as for vacuum reading the answer is no, I don't have a gauge and haven't adjusted them apart from the initial 2 1/2 turns (or what ever the manual said, can't remember), I was about to buy one and that's why I was asking whether it was a possible cause. I don't think it's a leak as I sprayed carb cleaner all around the dizzy ports and gasket and it doesn't seem to make any diff. The carb was rebuilt a couple of months back with all new parts (rebuild kit)

Now the hunting is not up and down up and down its got a definite pause between the high and low RPM (maybe 3- 5 sec)

 

I would suggest checking out a few of the threads that I posted. Especially the one where it shows how to tune the idle mixture with a vacuum pump. Also, consider changing your vacuum source from ported (where it is now) to manifold. I know when I did that, tuned the idle mixture, and reset the advance to 10*BTC the idle was pretty smooth even though I have a mild cam in the engine.

FWIW, my idle screws were out on the order of 3-3 1/2 times when I finally made decent vacuum.

 

I agree with switching to manifold vac from ported. I would also get a vac gauge to see what your engine vac is at idle, it will help you tune the carb and vac can. The pulsing of the idle does sound like the can is hunting. When you switch to manifold vac and plug the vac can line in, your idle will go up quite a bit, so you will have to bring your idle down with the idle screw and readjust the carb. switching to manifold vac will also give the motor better response at lower rpms, lower engine temps, and some report better gas mileage. What is your base (initial) timing set to, 8*, 10*, 12*?

 

If memory recalls 14* sounds right, I will try running a manifold vac and see what happens, any idea on where on the manifold I should take this from. I will organise a vac gauge and check idle vac readings.

 

I ran a "T" off off the rear of the carb between the carb and PCV. That's where I found the best steady manifold vacuum.

 

14* initial, IMO is a little high. I would shoot for something around 10-12*, but if its been at 14 for sometime and ran fine, Id leave it alone for now.. Any luck using the manifold vac? If it is still hunting, I would try carb adjustment and see if it helps.

 

Haven't had a chance to look at it yet, will try over the weekend and will let you guys know how I get on

 

The only reason I keep thinking timing, is when you change to a diff distributor the timing curve changes as well, along with the advance offered by the vac advance can. Other than the timing Id look for a vac leak (which you have ruled out) and carb adjustment. Good luck and let us know.

 

Well had a play today, found that the vac advance gives me 25*, so to get a max of 32-36 meant pulling the initial back to 0. Seems to run good at that. Have to recheck the total again and could maybe advance a couple degrees more on initial. 25* seems a lot.

0* initial - vac disconnected and plugged.
25* at idle (750 RPM) with vac connected.


What you guys think

 

I'm running on of these dizzys
http://www.ebay.com/itm/AMC-JEEP-CJ...Parts_Accessories&hash=item51b1a94458&vxp=mtr
Question is, is 25* vac advance normal, can this be reduced

 

I'm running on of these dizzys
http://www.ebay.com/itm/AMC-JEEP-CJ...Parts_Accessories&hash=item51b1a94458&vxp=mtr
Question is, is 25* vac advance normal, can this be reduced

Please discount the final advance figure given above as I've made a mistake, but the real issue is the 25* vac advance I'm getting at idle on full vac. Is this too much and is there a way of reducing it. I'm going to redo the timing on Monday

 

That seems to be a lot of advance at idle, there must be a wrong measurement there. I have an 360 with about 9* of initial timing and end up around 32-36 with full advance. I am however running a full MSD setup.

 

Are you absolutely sure you have the DISTRIBUTOR vacuum advance hooked to ported vacuum? Having 25* advance indicates you most likely have it hooked up to manifold vacuum which is backwards. At idle manifold vacuum is high. As the throttle plates open, manifold vacuum drops which means vacuum advance will decrease.
With ported vacuum source at idle, vacuum is low (little vacuum advance at idle) and as the throttle plates are opened and engine vacuum increases, vacuum advance increases, which is what you want.


Sent from my iPhone

 

You check it with a vacuum gauge, not a vacuum pump. You may get a better idle (possibly) with advance hooked to manifold vacuum but this exactly opposite if what you want. See above post.

VACUUM ADVANCE NEEDS TO BE HOOKED TO PORTED VACUUM!
Not manifold vacuum. Otherwise the vacuum advance is working backward. Reducing vacuum advance as engine rpm increases is backward!


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Nickmil, brief history, I was running on ported vac as I knew no better however I've always had a rough inconsistent idle and others have suggested trying manifold vacuum to see if that cured the problem. Initially I ran the V8 at 14* but never checked my total, it just seemed ok there with no noticeable pinging. So naturally I tired the same when I switched over to manifold vacuum but I suddenly saw 39* of advance at idle. Now I knew this was too much so to compensate a retarded my initial until I got down around the 25* at idle. This meant reducing the initial to zero showing that the vac canister on the dizzy is giving me 25* of vac advance. Now I know if I ran up my engine (no vac) to check the total I would be no where near the required 32-36* of advance (initial + mechanical). Forgive me if I'm wrong but even returning back to ported would still see me running 25* of vac advance just not maybe at idle (just after) so my main question is how can I reduce this, or is it safe to run this amount of advance at idle knowing that as soon as RPM increases the vac will decrease thus reducing the vac advance reading. I'm planning on rechecking this on Monday at ensuring that I have my initial/mechanical advance in the 32-36* region and then see where my initial comes in. Oh and I'm using a SnapOn digital readout timing light so I know the readings are correct.

 

Nick, You use the vac pump directly on the advance can to see how much advance it will give, quick and easy without having to check timing figures and subtracting. There is a lot of controversy over ported or manifold. I have articles from a retired GM engeneer that designed these systems and insists that manifold vac is the way to go and that ported was nothing more than the manufactures poor attempt at emissions. He stated you will have better throttle response, better starting, better gas milage, cooler running engine and gave examples as to why. Manifold is my personal choice, but others like ported. Ok, You have 25* from the advance can and 0* initial. If you take off your dist cap and look at the advance cans arm, you can see there is a rod that rides in a slot, you need to limit the travel of that rod to reduce your vac advance. They sell limiting plate for a few buck or you can make your own, making your own is trial and error as to setting the right advance, the limiting plate you buy will have setting on it for the amount of * you want. You need to get the vac advance down to probably 10-15* and bring your initial up to around 10*. Then you will have 10* initial, plus probably around 24 from the dizzy, that puts you in the 32-36* range for a total(which you will see at WOT connected only to manifold vac) and the 32-36* total plus the 10-15* from the vac advance for cruise and accelerating from a stop, keeping you under the 52* total limit with advance..
Nick is correct in that this does work opposite of ported vac up to about 2800-3000 rpms which is about where the vac advance drops out because of manifold vac loss.