Need help with engine problem

[drdave69]

My ZZ502 is still not running very well after a few attempts to fix it. I drained the fuel and had the carb rebuilt (by Eric, thanks) and installed new: plugs, plug wires, distributor cap/rotor and new coil (HEI). The engine still doesn't want to start and run very well. It will NOT idle below 1000 rpms. I checked the timing and it appears to be correct since there is only a single indicator line on the timing marker and it was on it at 1000 rpms. I checked the intake and carb with starting fluid to see if there is a vacuum leak and did not notice any increase in rpm.

Is there something else I can look at? Could the timing chain be stretched? Is there a valve train issue?

[earntaz]

Have you tried a 12V jumper wire from the battery to the HEI? This will bypass all possible connection issues in the ignition.
Have you changed the valvetrain at all? If so, is the valve adjustment too tight?
Is the harmonic balancer stock? If timing cannot be set correctly -- engine will be hard to start will not idle. Possibly remove all the plugs and check true TDC at #1.

TAZ

[seventieshow]

"a single indicator line on the timing marker"...Is the line TDC, 8 degrees, 12 degrees? ...should not be TDC at 1000 RPM. Or any RPM. Probably more like 10 degrees at 1000 RPM.

"is the valve adjustment too tight?" Sort of sounds like this problem. As TAZ said, confirm TDC and timing marks and check valve adjustment.

[earntaz]

After reading Don's remarks above -- something else just occurred to me. Check the advance mech in the distributor -- does it rotate freely? Sometimes the shaft binds up in an HEI ... they are good for that! TAZ

[drdave69]

I will try a 12v jumper to the hei after I double check tdc. As I stated, there is no degree indicators on the balancer or indicator. Just a single line. The engine is as it came from GM and I have not changed or adjusted anything. It just all of a sudden started running this bad after a new clutch and exhaust (went from 3" to 2-1/2" side pipes). I checked the advance mechanism in the hei and it is not binding. From what I have read the timing should be 10 deg and I assume that is what the single line indicator is for. I do not know what the valve adjustment specs are for this engine.

[seventieshow]

You could try driving it with the exhaust opened up, see if it makes a difference.

[seventieshow]

With this additional information, I would leave the valve train be. I get the impression it has a vacuum canister on the distributor but they (Chevrolet) want it plugged off and not used.
Are the 2.5 side pipes a full 2.5, I've always heard the factory pipes were restrictive.
Since you are there anyway, look and make sure nothing got pinched by the bell housing after the new clutch.

[Lynn]

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: drdave69</div><div class="ubbcode-body"> From what I have read the timing should be 10 deg and I assume that is what the single line indicator is for. I do not know what the valve adjustment specs are for this engine. </div></div>

Other than Studebakers, I have never seen an engine with a single line that was anything other than to indicate tdc. Can someone with 502 experience shed some light on this? Did you get a spec sheet with the engine telling you where timing is set? Surely said spec sheet would tell you what the single line referenced. I am afraid you have zero initial advance. Guaranteed to run like it is tied to a tree. Do you have the distributor specs? Find out how much total advance (in crank degrees) is built into the distributor (not counting vac) and set the initial to give you a TOTAL OF 36. That should be good for starters. So... if you find the distributor has 28 degrees total cent. advance built in, set initial to 8 degrees. If it has 26 degrees built in, set initial to 10 degrees. Is this a GM crate motor? If so, I have a hard time believing you didn't get a spec sheet.

Is there a scale of any sort (i.e. one line on the damper, but a scale on the timing cover)? If not, and if you don't have a &quot;dial back&quot; timing light, you will need to approximate.

[seventieshow]

This link has a PDF that tells basic timing information.

http://www.chevrolet.com/performance/crate-engines/big-block-zz-502-deluxe.html

[drdave69]

Thanks for all of the suggestions.
First off, I bought this car and engine combo as is and I was told that the engine was installed around 10 years ago. No paperwork was included. I have printed the install/engine specs from the GM Performance site for reference.

I plan to check where TDC is in relation to the timing mark when I get back to it. I am not well versed on how to use a dial back timing light so that may be above my pay grade.

If you think the exhaust is restricting it that much to cause this issue I can install the 3&quot; pipes back onto it (I had already planned to do this). I have already pulled them out of the attic to swap them out. I will also check for any pinched wires, etc. when I look into the exhaust.

One additional note: when I started it the other day it did not want to stay running until warm and it stalled when I backed it out. It was difficult to get it started again and puffed a little fire back thru the carb before I finally got it started after pressing the throttle down.

[Lynn]

I would make sure it is timed properly before swapping out parts.... even if you are going to swap the exhaust eventually.

I can promise you that you can use a dial back timing light. Once you see it hooked up and figure it out, you will see how simple it is. You really don't need a dial back light. Assuming you have an 8 inch damper, and assuming the single mark is Zero, 10 decrees advanced will be 1.44 inches from the single mark.

[SuperNovaSS]

It sounds like the car was running fine before the exhaust and clutch job. Is that right? If that is the case, I would back track from there. Was the distributor moved or removed when the clutch was done? Ground not hooked back up? Pinched wire as mentioned earlier? Battery not disconnected during welding which would have fried the module? Ignition modules are cheap and an easy swap. I wouldn't start thinking about internal issues like valve clearance if the car was running well before the exhaust and clutch job. If the distributor was not messed with during the job, I would not be worried about ignition timing.


Jason

[drdave69]

Once I get the time to get back at it I will check TDC. As far as I know the distributor was not messed with when the clutch was changed and there has been no welding done on the car. I will also check for any pinched wires. I know that timing and vacuum leaks can cause the fire in the carb so I am leaning toward a timing issue since I could not detect a vacuum leak.

[drdave69]

I looked for any pinched wires and did not find any. I took the distributor cap off and removed the #1 plug. Bumped the engine around and the rotor correctly points to the #1 plug on compression stroke. I then examined the dampener again and noticed faint lines and numbers on it. Couldn't see them previously with the crusty surface so I sanded it a little and marked the 10 deg BTDC with white paint. It was aligned at about the 12 deg mark so I put everything back together and checked it with the timing light after letting it warm up (wouldn't stay running otherwise). I adjusted it to the 10 deg. Using the electronic timing light I was able to switch it RPM and noticed that it was idling at around 750 rpm but the tach was at 1000 rpm. I assume the tach is off. It will still &quot;run on&quot; when I turn it off and still isn't running very well.

My next plan is to put it on my lift and change the oil (it is due) then swap the 3&quot; side pipes back onto it.

[VintageMusclecar]

Dave;

Have you made any adjustments to the carb yet?--where are the idle mixture adjustment screws currently set at?

Have you put a vacuum gauge on it? If so, how much vacuum is there at idle?

Eric

[SuperNovaSS]

Is the tach still a factory mechanical tach or is it electronic? If it is electronic, I would try disconnecting it. Also, I believe you said this was GM HEI. If so, as I said before, I would swap the module or at least have it tested. Modules, coils, and pickups for HEI go out from time to time and can cause the issues you are describing.


Jason

[drdave69]

Eric, I adjusted the mixture screws to achieve the highest vacuum at around 13 - 14 inches. I printed an instruction sheet on how to set the choke and fast idle cam. I'll get to that the next time I work on it.

I put the 3&quot; side pipes back on it and drove it a little while this evening. While it will idle when warmed up it still doesn't want to when cooled down a little and it still has a little &quot;run on/dieseling&quot; when turned off. I may play with the timing in 2 deg increments to see if it helps any.

Jason, the tach is the original (I assume) and it appears to be mechanical. The 502 has the factory HEI. I have replaced the coil but not the module(s) in the body. Is there a way to test it/them? As you stated would a bad module(s) cause the poor running characteristics or would it cease to run at all?

Thanks for all the help guys. Hopefully I can figure this out without having to find somewhere to take it.

[napa68]

The module controls dwell. Bad modules, while often just stop running, can cause a variety of other issues. They also too, can be heat related (heat soak)

Tim

[SuperNovaSS]

Many auto parts stores have factory ignition module testers. I used to work at NAPA and we had one. Yes, a bad module could definitely cause the issues you are describing and the symptoms can vary with temperature and humidity. The GM HEI one is very common and inexpensive. Any auto parts store should have one. Install is very quick and easy.


Jason

[drdave69]

Made a little progress over the weekend. Based upon some instructions located on the internet I checked the hei coil and it checks out as ok. I then focused on the carb since it doesn't want to start when cold. I adjusted the electric choke a little and noticed that the fast idle cam was not functioning. I adjusted it and now it starts and idles high when cold as it should. Seems to run better now that these adjustments have been made and it has the 3&quot; side pipes back on it.