[rickster182]

My father recently bought a very nice El Camino SS with a new crate ZZ502. The motor has been in the car for about 9 months and has about 1500 miles on it. We've been tuning on it for the few weekends since we got it and can't get it to idle as smoothly as we think it should. It has good fire to all cylinders, good clean new plugs, timing is best at 9-10 degrees initial (anything above 11 initial will cause pinging under load). This motor seems to like manifold vacuum to the vacuum advance (idles rougher with ported vacuum) and we've checked for vacuum leaks. It makes about 11-12 inches of vacuum at idle (850 rpm). We've dialed the idle screws on the primaries with gauge attached till we found the best idle/vacuum at about 1 1/4 turns out. It is not a true double pumper so no secondary fuel mixture to adjust. I've tried opening the secondary blades in the back barrels a little to see if a little more airflow at idle was needed. Our primary throttle blades are in the correct position with the transfer slot just at the top of the blade at idle. I've set both the float levels to the bottom edge of the site screw threads. We've got a Holley red electric fuel pump that never starves the car under any load, no return line though I don't think the Holley red is supposed to need one. Carb is what appears to be a new Holley 850cfm 4150 so I'm assuming the power valve is the newer kind that won't blow out after a backfire, but maybe thats a bad assumption. The cam is a stock ZZ502 roller with 224/234 and a 110 lsa so, while we're not expecting dead smooth idle, that cam should be pretty mild, especially for a roller cam in a 500+ cube motor. Cam was installed straight up from the factory and has not been changed according to the previous owner. The motor runs great at any speed above 1000 rpm and has no apparent valvetrain adjustment problems. (have not done a compression test yet though) It just seems to have a near miss at idle that is different than the lope of a cam with a lot of overlap. It's more like a small miss every second or so. Any ideas on what the more likely suspects would be to try next are appreciated. I have a perfect working Holley 750cfm on my GTO and I can swap it on temporarily just to see if the carb is the problem. The fact that this 502 runs so well at all other rpm's keeps making me think it is likely the carb. Thanks for any ideas, Rick

 

[GRN69CHV]

The ZZ502 motors I have heard run have just a slight lope at idle. You need to check your distributor though. Should have a total of about 36 - 38 total timing. The HEI I am running only has about 18 of mech advance built in. I run 18 - 20 initial and ported vac advance with no detonation in my iron head 408CI motor.

 

[Jerry Briggs]

I agree the 9-10 degrees of timing is way off should be closer to the mid 30's. The 502 does lope just slightly at idle, but doesn't sound like a miss. Jb

 

[saturnstyl]

Timing was about 12-14 degrees with the timing bypass in place. The numbers are very small and hard to read on the balancer.
The intake vacuum at idle was 11 in HG. It fluctuated by 1 in at idle.
This is from a Ramjet 502, which internally is identical to your engine.

 

[TW]

That cam should pull at least 14-15" Hg at idle. My Crane cam with 226/236° at 0.050 idled with 14-15" @950-1000 rpm and liked 16° initial and 20 mechanical for 36° total. I also had a vacuum advance on full vacuum (added about 10° at idle). I would check total advance and advance at idle with the vacuum advance connected to see what you have then. I would then maybe back the valve adjustment off and readjust them to make sure they are not set tight (I'd go only a half turn). Something is not right. I would try the 750 carb also if everything else checks out.

 

[rickster182]

Thanks for the feedback. I've tried higher initial settings in the 12-16 range and found the idle cleaned up just a little, but I need to check my total advance at 3000 rpm to see what the mechanical is adding as I get pinging under hard acceleration at 11-12 or higher as it is now on 93 octane. The balancer that comes on these motors isn't the best for reading so I will mark it before I start. The spec sheet on this motor calls for an initial setting of 8 degrees which seemed pretty low but I wasn't sure since I spend most of my time on my Pontiac. I also wondered if the valves might be a little tight and holding one or two of them open a little, so if timing and the carb swap don't solve this one I'll go back and check the valves. I'm just trying to start with the simplest things and work my way down the list. Thanks for the help, Rick

 

[69-CHVL]

Your vac advance can is adding too much advance. Get the Crane adjustable unit. I would definitely run 16-18* initial and 36-38* total. Set the can to the minimun and work your way up till you ping, then back off.

That thing must idle horrible w/10* initial.

 

[rickster182]

I'll try the higher initial advance and the adjustable advance can. We agree it idles rough in the current state of tune! Its not like it dies when you drop it in gear, but it's just not idling as well as it should. Thanks, Rick

 

[Old fat bald guy]

rickster...... initial timing for the ZZ502 is 8-10 degree's with 34-36 total. They don't like manifold vacuum, ported is better. as I read your 1st post, i also was thinking valves because of the low vacuum. check the intake gasket between 2 & 4 and 3 & 5 for a crack / leak... trust me on this....
and no, the BBC's don't "time" like our "Good" poncho's do :thumbsup:

edit*** reading your last post, does this happen at idle IN gear or in park / neutral?

 

[GRN69CHV]

What are the specs for the factory ZZ502 crate motor carb - jets & PV's? Maybe he needs to start there?

 

[rickster182]

The idle is similar in gear or in park, a little rougher in gear with the lower rpm. I've tried the vacuum advance with both ported and manifold vacuum and the difference is significant. It is noticably rougher on the ported vacuum, but this may just be a symptom of another problem, possibly a valve or two that are too tight or a vacuum leak or a carb problem. I think the standard 502 carb jetting is something like 76/84 or so, power valve is a 6.5 or 8.5 and no power valve in the secondaries. It runs so well at all other rpms, I think the jetting is in the good range already but I may still swap carbs to the known good running unit just to try and eliminate the carb as the problem. I think a compression check is needed to help me see if I have valve adjustment prob. and I'll look close at the intake gaskets and try some starter fluid around it to see if I could have a leak there. I would love to find something simple like that and I'm sure it is something minor, just a matter of tracing it down. Thanks, Rick

 

[69-CHVL]

Rickster, it is something minor...not enough timing, Your vaccum will shoot up when you get around 16* initial and idle quality will be immensly better. GM recommends 4* initial for mine - it won't even run w/that. Anything less than 16* it will backfire right out through the carb. Salle Chevy has some good forums, and all their dyno testing shows about 16* initial and about 36* total for the aluminum-head crate motors. They aren't fans of the vac advance att all though, where I am.

Try increasing your timing a bunch, if it don't work, put it back. I would do this B4 going crazy w/carbs.

BTW, the 502 carb has 78F, 82R. Front PV is 4.5, rear is 3.5. That jetting is about as spot-on as you can get from my experience.

 

[rickster182]

Thanks Vince, I'll get back on how we come out trying different things. I hope the timing is all it is as that would be the easiest. I didn't realize this Holley 850 had a rear power valve! My father lives 30 minutes away so I only get to work on it for a few hours each Saturday and then it goes back to his house. He's having a lot of fun with it even with a little rough idle. Not many 79 year olds with ZZ502's and he is enjoying the car a lot. Thanks again for the help, Rick

 

[rickster182]

Well, we marked our balancer and reset the timing to 16 initial and checked the total at 3000 rpm. It was right at 36 so we feel like we have both settings about right now (I'm sure there is some reason the factory settings are published at 8 initial, but as ya'll said, that's definately not a good real world setting) I reset the fuel mixture screws on the primaries to max rpm/vacuum (just over 1 1/2 turns out) and for now we have capped the vacuum advance off till we get a chance to pick up a Crane adjustable advance unit. We were able to get the idle the smoothest it's been since we got the car. It now idle as close to smooth as we'll likley ever get a 502 to idle at 900 in park and it only drops to 750 in gear with the brake applied. This was a big improvement as the difference in idle rpm between park and in gear was 200-250 rpm before this change. It not only idles at 750 in gear, but the car doesn't act like it's surging or creeping at red lights and doesn't feel like it's going to die or sputter anymore. We will find a shop that carries the Crane adjustable advance can and set it up next weekend to see if we get even better results with it set on a low advance setting but we've reach the point now that we're pretty happy with the idle. Thanks for all the help, Rick

 

[69-CHVL]

Good to hear Rick. Sounds like you got it idling pretty good even w/no advance hooked up. My base timing is about 17-18* initial. When I hook up the advance, my base shoots to 38*, and idles pretty nice. My car doesn't idle that great IMO w/just 18* base - it needs that vac advace. When you get yours hooked up again, you will need to lower your idle speed and set the mixture screws again.

BTW, what distributor is in there - the factory GM one? Also, what did yoru vaccum guage end up showing?

 

[GRN69CHV]

The intial timing settings often cloud what is really going on. In the early smog era (early 70's), initial timing was on ly in the 6 -8 range, but the distributors had 28 mech built in. We always had to install a bushing to limit mechanical, then run more initial. Problem was, the real lean mixtures and high operating temps would cause run on (dieseling). It was the integretation of CPU's that allowed the factory to develop and install HEI systems that had the required 20* or so of mech. advance that we like for performance.

 

[rickster182]

The distributor is just the stock factory HEI unit. I did notice that it was all in by about 2200 rpm which is a little early so we may try some different springs after we get the vacuum advance set up. I'm now going to get to work on it a bit more today so we'll try reconnecting the current stock vacuum advance to the ported source again just to see how it runs. The motor seems to hunt just a bit with the vacuum advance hooked up so I'll keep trying different things till we find the best combination. It will probably need the adjustable unit to prevent it from advancing too much. The vacuum is close to 13 in. now at idle and that's plenty now for running the power brakes. The vacuum brake booster pump that the previous owner added is coming off and going on eBay soon. That thing was like having a woodpecker living in the front fender, even with headers! Thanks again for all the feedback, Rick

 

[69-CHVL]

It is possible that the springs in the dist. are too light causing your pinging, not the vac advance. I would think that if thats the 502 dist. that it would be setup w/the right curve. I am running a ZZ502 dist. myself and the weights are pretty light. You will have to just experiment some.

He is running 93 octane right?

 

[rickster182]

Yes, definately 93 octane premium only! Rick

 

[marketsolutions]

I have the ramjet 502, just installed in a 50 merc. Its really hard starting and when it starts it has a miss, popping thru the exhaust. Its rough at idle, and keeps stalling. I checked the timing, but not sure how to disconnect the advance. The firing order is correct. Anyone have any suggestions?