Stumped by a timing problem

[MAC71]

I have a 69 C20 that has a 396 in it. When I got it in December it was all stock including the distributor and carburetor. The PO had the vacuum advance disconnected and I discovered that with it connected the engine back fires threw the carb. That was with base timing at 8 degrees, but did not check anything else as I pulled the engine.
So now its a rebuilt 396 .030 over, 9:1 compression edelbrock manifold, Holley carb, HEI distributor with a Summit racing CD ignition box, stock cam installed at 0 degrease, and it still does it.
I also checked to make sure the timing marks are correct and they are.
There is no pinging when this is taking place that I can hear and its not a loud truck.
I have tried various timing settings and it keeps doing it so I have to run with out the vacuum advance connected, but I want to sort this out for fuel mileage as I drive it every day.
Currently base timing 12 deg, full advance is 34 deg and the vacuum advance adds in 8 degrees to those numbers if connected. I also have tried both ported and manifold vacuum.
Just as a comparison I did a test drive with a different carb and no change.
the backfire is only cursing over around 2200 rpm or faster.

Any Ideas?

Post are no good with out photos right?

[MARKDTN]

Could the balancer have shifted? With all those new parts, something that was already there is where I would start looking.

[trac209]

Although a long shot and I think there would be some noises to indicate an issue I would also look into the valve train. Perhaps a broken valve spring that could be causing issue at rpms.

[MAC71]

Originally Posted by trac209

Although a long shot and I think there would be some noises to indicate an issue I would also look into the valve train. Perhaps a broken valve spring that could be causing issue at rpms.

Took the heads apart and valve springs were fine. Before I took the rebuild/engine dissemble I was thinking valve train also such as valve timing. When I took it apart the timing chain was really really warn out so I thought that was the problem, but all replaced in the rebuild. I installed cam at 0 and did degree it to double check.
With out the Vacuum advance connected it runs fine, no issues.

[MAC71]

Originally Posted by MARKDTN

Could the balancer have shifted? With all those new parts, something that was already there is where I would start looking.

I thought about the balancer at first, but when I checked where the cam was degreed to it was with the heads off so I also checked the balancer and marked it where 0 is which it was off but only by 2 degrees. I use silver paint markers after I check to run a line across the front of balancer to be able to see if it spun.

[trac209]

Originally Posted by MAC71

Took the heads apart and valve springs were fine. Before I took the rebuild/engine dissemble I was thinking valve train also such as valve timing. When I took it apart the timing chain was really really warn out so I thought that was the problem, but all replaced in the rebuild. I installed cam at 0 and did degree it to double check.
With out the Vacuum advance connected it runs fine, no issues.

Oh sorry I missed the only when vac is applied part. Have you tried to tune for best engine vacuum ? I would try that then check what timing it's at.

[RichardJ]

What trac 209 said.

You said you tried both ported and manifold vacuum for the vacuum advance. You didn't say you had to readjust the idle speed screw when you did that, but then you might have thought that didn't need mentioning.
If it didn't, manifold vacuum at idle should give full vacuum advance at idle and greatly increase idle rpm. Ported vacuum should have little or no vacuum at idle and not increase the idle rpm when connected after setting base timing.
Just asking.

[MAC71]

Originally Posted by RichardJ

What trac 209 said.

You said you tried both ported and manifold vacuum for the vacuum advance. You didn't say you had to readjust the idle speed screw when you did that, but then you might have thought that didn't need mentioning.
If it didn't, manifold vacuum at idle should give full vacuum advance at idle and greatly increase idle rpm. Ported vacuum should have little or no vacuum at idle and not increase the idle rpm when connected after setting base timing.
Just asking.

Yes adjustments are made to compensate.

So max full timing full throttle is usually 36 degrees give or take a few but cursing at part throttle you want more timing for fuel mileage which is going to be some where around 40-45, but for some reason this engine is back firing at cruse at that range. Don't know why, never had this happen before and have messed with lots of engines. Still like valve timing is off, but its not.

[slomotion]

Are you running hydraulic lifters? What's your oil pressure @ 2200 RPM? Just a SWAG, but could your lifters be pumping up holding the intake valves partially open? Can you hear any "thumps" in the exhaust pulses?

[trac209]

What I'm saying when I say tune for vacuum is set the timing and idle mixture for the highest engine vacuum reading on a gauge hooked to a full vac port on intake or carb. With a stock cam with fresh engine you should have anywhere from 18-20 hg on the gauge. Try that and see where the timing is on the light.

[MAC71]

Originally Posted by slomotion

Are you running hydraulic lifters? What's your oil pressure @ 2200 RPM? Just a SWAG, but could your lifters be pumping up holding the intake valves partially open? Can you hear any "thumps" in the exhaust pulses?

Hmmmm
So using stock gage and at idle oil pressure is dead center on the gage. Around 2200 it is just a hare left of the 3/4 mark that is to the right of centerline. before the rebuild the highest the gage would read was way left at the "1/4" mark. Cam and lifters were not changed as there was no visible wear and I used a mic to measure each loab and they were in tolerances.
I adjusted the valves at only 1/2 turn from 0 lash.
But again it runs just fine cursing with out the advance connected.

See why I stated Stumped.........

But the way I drive a 80 mile round trip where 90% of the drive is high way with a speed limit of 75, and 80 mph. I just do 70 but get 12 mph. Its hard to tell exactly my rpms on my tach but its around 2600-2800 rpm at 70.

[MAC71]

Originally Posted by trac209

What I'm saying when I say tune for vacuum is set the timing and idle mixture for the highest engine vacuum reading on a gauge hooked to a full vac port on intake or carb. With a stock cam with fresh engine you should have anywhere from 18-20 hg on the gauge. Try that and see where the timing is on the light.

Ok I see what your saying.

Now that I have thought about it I have not even checked what manifold vac is on this engine.

Something else, I live at 4500ft so usually engines want more timing here, this 396 not so much.

[slomotion]

At your altitude a lean carb could be contributing to this.......

[trac209]

Originally Posted by slomotion

At your altitude a lean carb could be contributing to this.......

This too^^^

[Churchill]

is it an adjustable vacuum can? If not maybe get one. Crane has a good recurve kit for HEI's. I have one on my truck and it's a good product.

Maybe it's getting too much air because of a faulty or improperly adjusted vacuum can, causing it to be lean and backfire. Idk, just a thought.

[MAC71]

Originally Posted by Churchill

is it an adjustable vacuum can? If not maybe get one. Crane has a good recurve kit for HEI's. I have one on my truck and it's a good product.

Maybe it's getting too much air because of a faulty or improperly adjusted vacuum can, causing it to be lean and backfire. Idk, just a thought.

Not sure, will have to check it was a left over one HEI from something else. It did do the same thing with the points distributor also.

So what your getting at is that if its adjustable bring it back from 8 degrees to maybe 4 and see what happens right?

[MAC71]

Originally Posted by slomotion

At your altitude a lean carb could be contributing to this.......

The Holley did it with over rich stock jutting and no difference jetted down 2 sizes which works for this altitude. Plugs a light tan.

[Churchill]

Well my adjustable can is adjusted by sticking an allen wrench through the hole and screwing the diaphraghm inside in or out. Idk what you have, but is most likely non adjustable unless someone put an adjustable one on there.

Also there's a little metal piece that came with my kit that looks like a lopsided oval disc with serrations on one side. it is screwed in right behind the vacuum can arm (seen when the dist. cap is off). It can be turned and blocks total vacuum. maybe there's something like that, idk. Just brainstorming!

[MAC71]

Originally Posted by Churchill

Well my adjustable can is adjusted by sticking an allen wrench through the hole and screwing the diaphraghm inside in or out. Idk what you have, but is most likely non adjustable unless someone put an adjustable one on there.

Also there's a little metal piece that came with my kit that looks like a lopsided oval disc with serrations on one side. it is screwed in right behind the vacuum can arm (seen when the dist. cap is off). It can be turned and blocks total vacuum. maybe there's something like that, idk. Just brainstorming!

its a non adjustable one.

[tranz am]

I think that what you are experiencing is an extremely lean condition in your carburator. The vacuum advance is just making it more noticable. At 2200 rpms you are not fully into your main circuit. I know you said that you richened up the carb but just changing jets does not change your idle circuit which is probably where this is taking place. Try adusting that by swaping out the air bleeds. Big blocks like about 35° total initial & mechanical and 8-12° vacume advance on manifold vacume.

[RPBurris]

Originally Posted by MAC71

Hmmmm
Cam and lifters were not changed as there was no visible wear and I used a mic to measure each loab and they were in tolerances.
I adjusted the valves at only 1/2 turn from 0 lash....
.

I am no expert engine builder but maybe a worn lobe on your cam? Did the lifters go back on the same lobes? I would suspect cam wear?? Again- just a weekend warrior's opinion.

[tranz am]

Your vacume advance is helping to solve this issue. I would check again that your ballencer is at true tdc when the piston is tdc. Put a cam degree wheel on the ballencer and stuff a piston stop in there. Your vacume advance is not the problem if it only pulls 8° of advance. . Its telling you something else is wrong. If your tdc is true, its not a valve trane issue, its not an ignition issue either if it only happens when vacuum advance is hooked up.

[tranz am]

Originally Posted by RPBurris

I am no expert engine builder but maybe a worn lobe on your cam? Did the lifters go back on the same lobes? I would suspect cam wear?? Again- just a weekend warrior's opinion.

That cant be the problem. It only happens with vacume advance hooked up.

[Ozzy2013]

Have you checked vacuum at idle and 2500?How about a leak down test to check the valves and lifters?Compression test?These are all often overlooked and are so easy to do and well worth it.If you have a known good carb and distributor maybe a buddy with one you can try .I would check fuel pressure at the carb at idle and 2000rpm you could be having fuel delivery problems .Cant say it enough when things don't add up get testing ,fuel ,air,fire.

[tranz am]

I think its one of these issues.

1. carb idle circuit needs adjusting.
2. your mecanical valve timing is off,
3. your timing mark or timing gun is off causing you to have more ignition timing than you think it has. When you hook up the vacuum can you are adding just enough timing that cumbustion happens early enouph it gets up past the intake valve.