Inliners International Forums General Discussion Engines Idle Problems

Hey all,

Ive got a 1972 Nova with a 250 in it that has less than 200 miles on a new engine. I cant get this thing to idle smooth no matter what i do. I used to have a newer HEI distributor in the car, but suspecting that the distributor was the problem, i replaced it with the original points distributor, which i took apart, rebuilt, and gave it a new set of points. Started the car, still ran the same. The timing is set perfect to 4 degrees BTDC as the emissions control label says. I have also replaced all the spark plug wires. When it idles, it acts like it has a vacuum leak, like its missing on several cylinders. Im sure my problem isnt the engine, since it was rebuilt recently by a very reputable shop. Im beginning to suspect that the problem is with the manifold. The manifold is the original bolt-together piece, which I dont like, it used to have the seperated manifolds and it ran smooth. I have also replaced the carburetor, which had a bad accelerator pump, with a new remanufactured by holley 1bbl rochester monojet. I ahve eleminated everything that i cant hink of, and im in a hole. Can anyone tell me what the hell is wrong with this thing?

Thanks, Sam

You can check along the intake to head with some WD40 to chack for any leaks. What type of Cam did you put into this rebuilt motor. Stock Or nonstock? Idle For a stock shold be about 800 rpm some cams past the stock Or Rv type may need a little more idle RPMs. Also Double check The carb Idle mix and Set screw for Idle. Then after carb is set disconnect the Vac to Dist. and recheck timing. Hope this helps }[oooooo]


------------------
NovaMan/Twisted6/Larry

Well, i have no idea what the cam in this motor is, since i didnt rebuild it. The motor was rebuilt just before the car came into my hands. I suspect that the cam is stock.

The problem isnt that it doesnt idle, its that when it idles, its rough, the engine shakes back and forth, and it misses.

When i rev the engine up it smooths out perfectly. No shake, no missing, perfect. I have also replaced the fuel pump. The carbretor is set perfect, and the valves are set to specs aswell.

Also, my emissions sticker says that the idle is supposed to be set at 750 for automatic in drive.

[This message has been edited by samamorgan (edited 12-10-2003).]

Also, did anyone ever make a crossflow head design for straight sixes? (crosflow meaning that the intake is on the left and the exaust on the right)

Let it idle for a while then pull the plugs and compare them... might see a difference in 1 or 2 and point you in some direction. Might rule out a ignition issue. Check the plug color ...should be gray and not black. Sounds like it runs ok above idle so may not be ignition.

Clyde

Try pulling spark plug wires off one at a time. If the speed drops you know that cylinder is firing ok, If the speed doesn't change you know that is a cylinder with a problem. If two adjacent cylinders a the problem ones you might have a bad head gasket or those cylinders share a common intake runner that may be where your intake vacuum leak is. Or you have have a valve adjusted too tight and never closing. Lots of stuff to try.

------------------
Gael
37 chevy sedan, 261,t5
57 pickup
58 burb

Sam: Did you buy the car with this problem?

First I would recheck all fittings at carb, hoses for cracks, etc. Then do vacuum test for possible vacuum leaks. Next do a compression test(yes, I know its a rebuilt motor)just to make sure all cylinders are close. Fuel filter could be clogged or fuel pump bad.

Does your car have oxygen sensor or egr valve? They could be bad. You can check the sensor with a volt meter (should read .5 to 3 volts). If your running a catalytic convertor it could be plugged. You could have a cracked head. (vacuum test for it).

------------------
Stuck in the 50's
1951 Ply Concord truck
1954 Nash Rambler 2 dr
http://www.picturetrail.com/elplymino

One other thing I may have to add? Is I would also try and get ahold of who ever you bought if from and Try to find out How it was rebuilt. i would just recheck everything Like what has been said here. And only Other thing I could add at this time. If it wasn't for the fact that you say It also sounds as if it has a miss, And Not being able to hear it run , I may also have to lean alittle To???? maybe a not so stock cam????(if it wasn't for a miss fire that you are saying that you hear-- That could be some slim Odds But you just Never know????? AT What Point or RPM does it become smooth??
AND Have you thought about maybe Double checking Valve lash?? one a little to tight Or maybe to lose Just some thoughts ?? hope this helps }[oooooo]


------------------
NovaMan/Twisted6/Larry

All right, to answer all your questions:

I have checked and rechecked the valves, i have done the pull the wire off test, i have replaced the fuel pump, so it couldnt be the issue. I have checked the fuel filter (its one of those ones that are inside the carb where the fuel line connects to it) and the fuel filter is fine. I havent done a compression test, but thats a good idea, and ill try it out. I have done a spark plug clean and burn check several times.

The engine smooths out at about 1200 rpms, and on up it is perfectly smooth. I havent checked every fuel, vacuum, etc line in the car, but i checked all the important ones. The car has almost no emissions control things, besides a pcv valve coming out of the valve cover, this is a stock 72 nova coupe. I will ask the engine builder some questions about the engine, but i doubt hed remember, since its a commercial company. I got the car for my birthday form my mom, and the engine ran this way before it was rebuilt, so im 90% sure it cant be the engine.

Thanks for all your help.

Well, since we now know it did the same thing before the rebuild maybe you need to check/replace all vacuum hoses since they stayed on the car. Visual checks don't always reveal bad hoses. Hose is cheap.

Only the engine builder would know if you had a blown head gasket before the rebuild.
If so, the only way you would have another one is if the head/block is warped.
Likewise, the intake manifold was not touched during the rebuild, so it could be warped causing a vacuum leak. A compression check should be done. (back it up with a vacuum test). If you have two adjacent cylinders that read low, then you can do a leak down test on them to see what the problem is.

By the way, what does your oil look like?

Quick checks:
Berrimans B-12 Chemtool (spray can), give a little spray down the throat of carb while at idle, be careful this stuff is instant fire in a can. If motor smoothes out or speeds up, it's fuel related. Could be gas tank, fuel line, fuel filter etc etc. I had a new fuel filter come apart and send it's guts into the carb, what a mess. Some of this crap is made in China with cardboard.

Hook up a vacuum gage and pinch off each line including PCV, see what happens. BTW this is an invaluable tool, for setting timing adjusting carbs etc. Gotta have one.

In general dont assume anything, go through the complete tune up proceedures out of the manual, you'll find the problem.

I chassed one like this for weeks when I finally found the fuel filter guts inside the carb.

Thats a good idea about the chemtool. But doing that could also indicate a vacuum leak. I still havent done a compression test, but i will in about 5 minutes.

Ill go see how much a vacuum guage is, ive never even heard of one, but if it does what you say it does, it is an invaluable tool indeed.

Im about 99% sure its a warped intake manifold now, since i took both the intake and exaust manifolds off, and the manifold-to-head gasket isnt even crimped, and i had the manifolds tight as i could get them, im sure that the manifold is warped. But before i go buy $300 worth of manifold ill do some more checking. Thanks for the advice.

Also, ive had all the vacuum hoses to and from the carb blocked off at the same time, and the engine didnt change its characteristics, except for the vacuum advance.

The oil also looks good. Not gritty, black, etc. Looks like slightly dark new oil.

I havent done the 500 mile checkup yet, since this thing hasnt even gone 150 miles yet. But from the way the engine is osunding, i doubt there will be anythign in the filter.

[This message has been edited by samamorgan (edited 12-11-2003).]

Just did a compression test. Compression on all cylinders is perfect at 180. Definately not the head gasket.

Before you spend 300 on a intake Just have that one planed Right along with the exhaust That way you know ALL is Truely FLat and Seals Good. I'm sure that would be much cheaper to start with. }[oooooo]



------------------
NovaMan/Twisted6/Larry

I alteady bought an intake/exaust combo bot $100 from a junkyard. I needed one anyway since my exaust manifold had about 4 cracks in it. The exaust manifold is at the machine shop getting glass beaded right now, and after thats done, im going to take the intake down, bolt them together, and have them planed. I will tell you the outcome, and i thank you all for the help

Sam,check my post under "Holley 390" this may apply to your problem also---Just a thought---Paul

How many volts at the coil with ignition switch on? The HEI should have been 12 and when you changed back to points it should be 6.