Slight miss in my 69 Z28
 

At idle my car has a slight miss/stumble. This issue does not occur above idle or at acceleration. The car will idle at 900 and the tach will drop to 700 or so. The drop is almost undetectable by ear or feel. I hooked up another tach to the car to confirm that my tach was not going bad and had the same result. I suspected bad plug wires those checked out ok. I cleaned the distributor cap terminals. (I have not changed the cap yet, it looks ok). Spark plugs are good. I recently replaced a bad condenser. The points have not been replaced. I was thinking that there may be trash in the idle circuit of the carb since the problem is only at idle. Is this a possibility? Any ideas are appreciated.

Thanks

Lawrence

Re: Slight miss in my 69 Z28
 

First off -- has the carb ever sat without fuel? What I mean is, if the carb has fuel in it after rebuild and then the car sits for an extended period of time -- the fuel evaporates and the gaskets (cork) dry out and shrink. This will sometimes cause issues with idle. You have already said there may be contamination in the idle circuit -- could cause idle issues as well. This is just my perspective -- there are many experts here that may chime in. TAZ

Re: Slight miss in my 69 Z28
 

Lawrence
Can your condition be more of a surging idle up and down ? Do not rule out the vacuum advance can. You might even eliminate it for TS. Might have to temporarily adjust your idle up.
Is it a stock can. # on it ?

Mike

Re: Slight miss in my 69 Z28
 

Mike,

The idle does not act as if the vacuum hose was disconnected and then put back on the can. But that is a good call. I did not think of that. Yes it is a stock can. Can't recall the number off the top of my head, but I remember chasing the can down based on John Hinckley recommendation. I painted, and did a mild resto on this car in 2009 so the diaphragm could be shot. I will check that out thanks.

Lawrence

Re: Slight miss in my 69 Z28
 

Reach out to Eric at Vintage, the master will solve the issue...

Re: Slight miss in my 69 Z28
 

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: JRSully</div><div class="ubbcode-body">Reach out to Eric at Vintage, the master will solve the issue... </div></div>

Yes, he is the Yoda, or more precisely, the Obi-wan-Carb-nobi of tuning carbs. [img]<<GRAEMLIN_URL>>/grin.gif[/img]

Re: Slight miss in my 69 Z28
 

Yea Eric has helped in the past.

Re: Slight miss in my 69 Z28
 

Mike,
Pulled the vacuum line off the can last night. Cruised around and seemed to be ok. But when the car really warmed up I had the same problem. Maybe a little exagerated with no vacuum advance? Anyway next step is to pull the carb and poke around and look for issues.

Thanks

Lawrence

Re: Slight miss in my 69 Z28
 

Did you plug the vac source for the distributor when you disconnected it?

Re: Slight miss in my 69 Z28
 

hi, are you 100% sure not a plug. does it sound like a single cylinder miss? i know how simple this sounds but i have found many and i mean many times it being just a bad plug.

Re: Slight miss in my 69 Z28
 

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: mockingbird812</div><div class="ubbcode-body">Did you plug the vac source for the distributor when you disconnected it?</div></div>

Yes I plugged the vacuum source.

Re: Slight miss in my 69 Z28
 

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: starrider</div><div class="ubbcode-body">hi, are you 100% sure not a plug. does it sound like a single cylinder miss? i know how simple this sounds but i have found many and i mean many times it being just a bad plug.</div></div>

All the plugs are good. Checked them first.

Re: Slight miss in my 69 Z28
 

springs on the vacum advance wieghts..did jerry do your dist? had the same issue with a 67 Z..changed one of the springs and it cured the problem..advance was changing back and forth causing a change in idle speed..nagged me for a couple of weeks!

Re: Slight miss in my 69 Z28
 

You may be able to see that with a timing light ... check timing w/light and see if the TDC mark fluctuates ... should be steady. Check spark scatter by checking in total darkness -- raise hood and see if you have fire to ground from the plug wires. TAZ

Re: Slight miss in my 69 Z28
 

I rebuilt my carb, and still have the same problem. I am glad I rebuilt the carb because the bowls, and the metering blocks had white fine silt/sludge in the bottom of them. I am surprised the thing even ran. Before I rebuilt the carb and even now the engine does not idle good. When I try and set the idle at 1000 rpm it dances above and below 1,000. And when I plug and unplug the vacuum advance it does not respond as good as it should. So I think Mike's theory about the vacuum canister could bare fruit?? Before I do that I will try and test the canister. Stay tuned....

The thing runs like a scolded dog though once you get beyond 1500 rpm.

Re: Slight miss in my 69 Z28
 

I thought it may be a vacuum leak..acts like it but changed the brake booster hose and still have the problem. I took the distributor off and moved the springs and they snap back, but that does not mean one of the springs is not bad? No Jerry did not do the distributor. I had another guy do it some time ago. I am going to check the timing and see if the light bounces around. If the plug wires are bad wouldn't the car pop at high rpms? Car runs great at high rpm, the idle sucks.

Re: Slight miss in my 69 Z28
 

When you rebuilt the carb did you test or replace the power valve?

Re: Slight miss in my 69 Z28
 

Can you verify what your initial timing, total timing @ 3000 is? How much vacuum advance?

Any chance you could get it on an O-scope and examine the ignition system. You could also use a wide band O2 probe in the tailpipe and dial in the carb too.

The cylinder pressure at low speed is greater than in the upper R's and increased cylinder pressure = increased resistance in the ignition system. Would seem like it ignition related.

Re: Slight miss in my 69 Z28
 

If you hold your hand over the top of the carb and restrict air flow does it die out or does the idle pick up? If the idle raises you could have a vacumn leak.

Re: Slight miss in my 69 Z28
 

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: William</div><div class="ubbcode-body">When you rebuilt the carb did you test or replace the power valve?</div></div>

Yes replaced the primary, the secondary size in the kit was wrong per Eric Jackson so I reused the secondary power valve.

Re: Slight miss in my 69 Z28
 

Did a few tests today. And I found this to be odd. Had the idle at 900, vacuum line plugged, dial back timing light hooked up, I was only able to get 4* BTDC with the dial on the timing light set at zero. So I was unable to match up the line on the balancer with the 0 on the timing tab. I don't ever recall having this issue with the car. I went ahead and advanced the timing so I could get 4* on the timing light dial with the balancer zeroed out on the timing tab. Took the car for a drive and it spark knocked so obviously too far advanced. I set it back to where it was.
I had 33* total advance at 3000 rpm with no misses or anything weird. The engine purred at 3000 rpm. I also checked the total advance at 2800 and it again was 33*. I did not walk it down to see when the total advance came in.
When I had the gun on the tab at 900 rpm the engine would miss and the timing light would go dark instantly and then come back on. When the light came back on you could barely see that the timing mark was at like 6* or 7* and then come back up to 4* once it stabilized a little. After the 3000 rpm total timing test I set the curb idle back to 900 and the car idled terribly. The tack needle was bouncing all over the place. The other odd observation that I am noticing that very seldom happened in the past is that the car is dieseling when I turn the car off. I never had this issue before. Now about every third time or so when I turn the car off it diesels. This has got to be ignition issue but I am at a loss. I have not changed the points condenser, plug wires yet. I tested the plug wires and they seemed to be ok. I had a condenser go bad a few years ago but the symptoms where different. I guess I better change the points just to cross that off the list? Any suggestions are appreciated.

Lawrence

Re: Slight miss in my 69 Z28
 

How many degrees is the vacuum advance, and is hooked to manifold vacuum?

You are running high test pump gas, right?

Re: Slight miss in my 69 Z28
 

What is the dwell currently?


Jason

Re: Slight miss in my 69 Z28
 

Did you put a vacuum gauge on it and what did the vacuum gauge say or do{a vacuum gauge would show a broken valve spring}. Also a compression test would help you fine a low cylinder that would only show up as a miss at idle. I hope's this helps.

Re: Slight miss in my 69 Z28
 

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: pxtx</div><div class="ubbcode-body">How many degrees is the vacuum advance, and is hooked to manifold vacuum?

You are running high test pump gas, right?</div></div>

The vacuum canister starts moving the breaker plate at 5.5&quot; and stops at 9&quot;.

Yes premium fuel.

Re: Slight miss in my 69 Z28
 

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Plowman</div><div class="ubbcode-body">Did you put a vacuum gauge on it and what did the vacuum gauge say or do{a vacuum gauge would show a broken valve spring}. Also a compression test would help you fine a low cylinder that would only show up as a miss at idle. I hope's this helps.</div></div>
Tonight I am going to put a vacuum gauge on it and see what it does. I agree that could tell me alot. Should had already done that.

Lawrence

Re: Slight miss in my 69 Z28
 

Good info. When you say starts, is that because you are hooked to ported vacuum?

How many degrees of advance does the vac ad give you?

Re: Slight miss in my 69 Z28
 

I would check vacuum level at idle and compare that to power valve #. I've had to change a power valve because it was too low numerically and it was leaking and opening too soon. It idled poorly and loaded up, and stumbled during initial acceleration. Now the car runs perfect. I was convinced it was a distributor vacuum /timing issue and it wasn't.

Re: Slight miss in my 69 Z28
 

This ethanol crap doesn't help with our fuel systems. I started up the Z yesterday, planning on taking it down of the jack stands, and taking it out for a blast on such a beautiful day! Fuel leak at the accelerator pump, and the fuel pump. Shut it off, and put the cover back on.
Buddy

Re: Slight miss in my 69 Z28
 

Don't know if this has been mentioned. With the motor idling, take a can of spray carb cleaner and spray aound the base of carb and in the area of the intake gaskets. If the idle changes -- chances are there is a vacuum leak in that area. TAZ

Slight miss in my 69 Z28
 

Check and clean all connections including coil and battery also grounds, everywhere!
Try hot-wiring direct power to the coil also run it in the dark and look for jumping spark.
It may even be the Coil itself, they do goofy things sometimes although usually @ higher revs.
And distrib' shaft play can cause erratic gap too also act differently whether under load or not.
If you're still thinking vacuum leak, even though you've swapped the booster hose it may be best cork it off to test and discount booster itself as leaking too?
Keep trying and good luck!
[img]<<GRAEMLIN_URL>>/beers.gif[/img]
~ Pete

Re: Slight miss in my 69 Z28
 

I THINK I found the problem. I put new condenser and points in and went to start the car. Nothing, no spark at all. I set the gap on the points and kept messing around with the new points. Nothing again. I re-installed the old points still nothing. Saturday the car ran. So I took the ground wire loose from the base plate and moved it around trying to see if there was any damage. Could not see anything so screwed it back to the base plate. Car fired right up. The problem is still there but it was not as bad as before. So I have to assume that when I was removing installing the points I was creating an open situation with base plate ground wire? I assume I have a bad ground?
I also checked the engine vacuum....just now...engine luke warm and perhaps not the best connection to my vacuum gauge. I had a steady gauge reading of 13&quot; of vacuum. I think that is ok for a stock Z28? Will check again when the car is warm and I have the breather off so I can jam my fitting on the base plate of the carb.
Thanks for all the suggestions. Keep you posted on next steps with the distributor.

Re: Slight miss in my 69 Z28
 

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: paceme</div><div class="ubbcode-body">I would check vacuum level at idle and compare that to power valve #. I've had to change a power valve because it was too low numerically and it was leaking and opening too soon. It idled poorly and loaded up, and stumbled during initial acceleration. Now the car runs perfect. I was convinced it was a distributor vacuum /timing issue and it wasn't.</div></div>

Steve, vacuum level is 13&quot; at idle. Engine was luke warm and perhaps not the best connection on the base plate to the gauge hose. Maybe able to get a few more inches out of the reading. But I believe high enough and not an erratic needle reading. PV in the primary is 6.5 per Eric Jackson, and per the kit I just installed. However, I had to use the original 8.5&quot; secondary per Eric's original build because the kit came with a 10.5&quot;.

Thanks

Re: Slight miss in my 69 Z28
 

Ground wire on base plate,sound like the base plate is worn out,good one. Hope this help's you out.

Re: Slight miss in my 69 Z28
 

Ok guys I need someone a lot smarter than me to help me figure this out. This morning I went and checked the dwell since I had not done that yet. The reading was 85*!!! I could not even believe the car was running. I adjusted down to 30* and during the adjustment I saw a spark on the end of the screwdriver when I was adjusting the dwell. Don't recall ever getting a spark off an allen wrench or screwdriver while adjusting dwell? Anyway set the timing and dwell, car is just purring at 900 rpm with 12* static. I expect something is still wrong with the ground in the distributor but could that &quot;spark&quot; have been a discharged build up of unwanted voltage?

Re: Slight miss in my 69 Z28
 

I can't believe nobody brought up the dwell earlier [img]<<GRAEMLIN_URL>>/grin.gif[/img]. I'm glad to hear you found the problem.


Jason

Re: Slight miss in my 69 Z28
 

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: SuperNovaSS</div><div class="ubbcode-body">I can't believe nobody brought up the dwell earlier [img]<<GRAEMLIN_URL>>/grin.gif[/img]. I'm glad to hear you found the problem.


Jason</div></div>

I should had checked that earlier, but thought nah it isn't the dwell. Still does not really explain the breaker plate ground wire being a possible culprit. I am going to drive it and go from there.

Re: Slight miss in my 69 Z28
 

Still issues. Went for a ride and the engine acts like it is running out of timing at 4K rpm. I am going to put the new points in there so I have a base line.

Re: Slight miss in my 69 Z28
 

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: sixt9rsx33</div><div class="ubbcode-body">Still issues. Went for a ride and the engine acts like it is running out of timing at 4K rpm. I am going to put the new points in there so I have a base line. </div></div>

After that, don't forget to reset the dwell, and make sure the lead is attached to the distributor side (NEG) of the coil.
Then recheck your initial timing. A change of dwell will change that.

Verne

Re: Slight miss in my 69 Z28
 

I have new points, reset the timing, dwell, and put in a new coil. Also put the weakest springs in so the centrifugal timing comes in quick. The problem is still there but now at a higher RPM. The car dies at 4,500 RPM now. Idles great, pulls hard through the power band until 4500 rpm. It acts like the spark just goes away. I am going to check all the wires very close around the distributor. Any ideas let me know.