Stumped by a timing problem

[MAC71]

Originally Posted by 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.

The carb is just a 1850 so no screw in air bleeds or IFR's. I did try a eddelbrock and no difference.
I just re checked my timing.
12 initial
35 full no vacuum advance
and using a vacuum pump I discovered that the vacuum advance adds 12 degrees, but using my dial back light full with vacuum advance looks to be around 44 degrees. I checked initial using a normal light and it matches the other light at 12 initial.

[MAC71]

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.

did it before I took the engine apart. Cam was good. lifters back in same holes.

[MAC71]

Thank you everyone for the help.

I had used a dial indicator on the #1 piston to check the timing marks. It was about 2deg off but I marked the correct location. I will recheck this weekend with a piston stop.
Talking to my friend that was with me and he reminded me that it was doing this off idle with the points distributor when I hooked up the vacuum advance. The timing chain was so warn out that I was able to take the chain off with out taking the cam gear off so that's what I thought it was, and maybe it was and this is now a different issue.

A little bit more detail of the problem, with the vacuum advance connect it runs fine until cursing at around 55-60 + mph. That starts around 2200 rpm range. it will run about 2-3 miles and then it will start popping/back fire really badly threw the carb and will not stop until I slow down. No pinging. here is the kicker, if I advance the initial timing to about 24 degrees which is the max I can an start and run it down the highway it does not back fire but will ping.
Other than this it runs really well.

[MAC71]

Also the engines stamping is CTW which is a 1970 chevelle/camaro 396 350HP. PO did not know this and only new it was a big block. Did not fully degree the came to see what it is but it is stock or really small as it has a good idle at 700 RPM and 18 in of vacuum.

[tranz am]

The fact your chain was so loose might be part of the problem.
When engines are rebuilt, sometimes they are line bored which effectively removes material from the crank journals and moves the centerline of the crank upward reducing the deck height. If your chain was that loose it makes me wonder if it was indeed linebored. If it was you will need a shorter timing chain. Ask me how i know about this? Just went through this on my 454 build. The new stock length chain was just too damn loose. I was able to catch it before hand but sometimes we forget to check small things. Mine required a .010 short chain.

[tranz am]

I think your stumping a lot of people with the fact you can add more initial and the problem goes away. Thats a very odd problem.
So just to clarify, so it doesnt do it until a couple min after crusing 2200rpm @ 55-60 mph?

[CarterD2015]

A shade tree method for fixing your timing problem would be to rev the engine to where the problem occurs. The vacuum advance should be connected. Then adjust the distributor by hand until the engine smooths out.

[67 cst swb]

At this point I would replace the Distributor. It seems like the only thing that was consistent from Old engine to New engine.
My first guess was the vacuum advance was bad, allowing too much advance with vacuum line connected OR possibly too light of spring an the Mechanical advance within the distributor.
But my thoughts are with the distributor.

[tranz am]

I thought of another issue that could be affected buy vacuum advance. Pull off the cap and look inside at the top of the cap. Ive seen issues when the screws that hold down the coil penetrate into the cap. If this happens you will get missfires and popping. Worth a look.

[RichardJ]

For you last two guys, he has said at least twice that it backfired with the old points distributor.

Quick question. Where is the PCV valve connected? That connection should be to manifold vacuum which is usually below the Holley bowl. Can't see in your picture if there is a fresh air intake on the other valve cover for the PCV system.

Have you checked for low fuel pressure from weak pump or restricted fuel line or tank sock? Non-vented gas cap? Sustained cruise speed would cause the carb to go lean, but changing carbs, I would have expected the problem to show differently.

You are sure it is backfire and not after-fire or exhaust backfire? The delay before it starts popping could be explained by heat buildup in a warped or cracked exhaust manifold.

I'm looking at these areas because you have new cam gears, checked valve timing, two carbs, two distributors, but same fuel pump, same fuel supply, gas cap and same exhaust system.

[tranz am]

I know said he used the points dizzy but worth a look in there anyways.
Its hard to diagnose this problem without being with the truck while its doing this.
im just trying to help him make some simple checks to eliminate the possibilities.
You have a goid point weather its a true backfire or explosion after the exhaust valve.

Originally Posted by RichardJ

For you last two guys, he has said at least twice that it backfired with the old points distributor.

Quick question. Where is the PCV valve connected? That connection should be to manifold vacuum which is usually below the Holley bowl. Can't see in your picture if there is a fresh air intake on the other valve cover for the PCV system.

Have you checked for low fuel pressure from weak pump or restricted fuel line or tank sock? Non-vented gas cap? Sustained cruise speed would cause the carb to go lean, but changing carbs, I would have expected the problem to show differently.

You are sure it is backfire and not after-fire or exhaust backfire? The delay before it starts popping could be explained by heat buildup in a warped or cracked exhaust manifold.

I'm looking at these areas because you have new cam gears, checked valve timing, two carbs, two distributors, but same fuel pump, same fuel supply, gas cap and same exhaust system.

[tranz am]

he could be having whats called spark scatter in there. That will create havoc it in the combustion chambers.

the points style distributors were more prone to this issue because of the smaller diameter cap. this is one of the reasons why GM designed the HEI to have such a large cap.
The fact that the vacuum can is amplifying this problem is what im looking at here and things that are effected by the added timing at high manifold vacuum.

[no1udknow]

here's a question, It seems to me I remember reading somewhere that some of the big blocks had a different distributor, is it possible it is the wrong one? is it points or hei? if it is hei, wrong module maybe? maybe a crossed spark plug wire?

also, is one of your valves floating and keeping open so it is backfiring through the carb? maybe pull the valve covers, turn the engine, first check to see that each rocker has a little lash, next check the stem height when the valve should be closed.

[snipescastle2]

I know this has probably been said before, but I had a friend who's distributer shaft bushings were so worn, at higher RPM's, it would do exactly like your describing, I'm wondering it the Vacuum advance can is side loading the distributer shaft as the vacuum is applied......can you see if there are any unusual wear marks on the inside of the distributer cap? just a hunch.......
Ben

[tranz am]

One other thing. That carb is way to big for a stock cam 396. A 700-750 cfm is a much beter choice.

Originally Posted by MAC71

The carb is just a 1850 so no screw in air bleeds or IFR's. I did try a eddelbrock and no difference.
I just re checked my timing.
12 initial
35 full no vacuum advance
and using a vacuum pump I discovered that the vacuum advance adds 12 degrees, but using my dial back light full with vacuum advance looks to be around 44 degrees. I checked initial using a normal light and it matches the other light at 12 initial.

[67 cst swb]

Originally Posted by tranz am

One other thing. That carb is way to big for a stock cam 396. A 700-750 cfm is a much beter choice.

1850 is the model number. Its 600 cfm.
https://www.holley.com/products/fuel.../parts/0-1850S

[tranz am]

I see I thought he meant 850. Now 600 thats a very small carb for that motor.

Originally Posted by 67 cst swb

1850 is the model number. Its 600 cfm.
https://www.holley.com/products/fuel.../parts/0-1850S

[67 cst swb]

Originally Posted by tranz am

I see I thought he meant 850. Now 600 thats a very small carb for that motor.

If its a daily driver it should be fine. Its a little small, but, as long as its not a full throttle application (and he already owns it) it will do fine.

[Churchill]

plug gap?

[Ozzy2013]

Well do the fuel pressure test.Do the leak down test .Do the compression test.These done will eliminate a lot of guess work otherwise you are chasing your tail.

[MAC71]

Some one stated check inside the cap, we'll I noticed the wire lug for 2 different cylinders were really lose so I replaced the 3 month old cap with a old cap. It idled 200 rpm faster and I had to turn out the idle mixture screws to smother it out, and manifold vacuum is higher! Around 20 in at 700 Ron in neutral.
I reconnected the vacuum advance and will test it in the morning headed to work.
Also fuel pressure at the carb is 6 psi, checked tonight.

[MAC71]

Will do a compression test this weekend. Plug gap is .045

If the cap fixes the problem than maybe the point distributor issue was the timing chain.

Will update tomorrow

[MAC71]

Originally Posted by tranz am

I see I thought he meant 850. Now 600 thats a very small carb for that motor.

I don't think it's to small. Let's say the VE is .85 which is optimistic with stock heads and cam and max RPM is 5500 I would only need 540 cfm. Also the 600 cfm carb compared to say a750 cfm carb will have better response and better low rpm torque and most likely more over all average power.

And I already have it.

In high school I had a 400 small block in a 71 c10 with a 600 cfm carb that ran 12.9 1/4 mile. I tried a 750 and dropped to a 13.1 but gained 2 mph.

Now I only take my corvette to the tracks.

Again thank you all

[mechanicalman]

Originally Posted by slomotion

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

Good call.

[MAC71]

After the cap replacement the backfire is gone but it has a lean surge now but that's a jet change.
So before the rebuild with the points it had to have been the timing chain but the bad distributor cap looks to be the problem after the rebuild.

Thank you all for the help I would have never thought the cap