Seems like I have to keep advancing my timing

[chrismoore701]

I have a brand new 350 with a mild cam timing set at 12 degrees. . Ran great for 400 miles. Started to die and giving my all sorts of trouble. Played with the carb a couple times. Distributer was tight. Finally adjusted timing to 12 degrees and she ran great for another 400 miles. Then started to die again. Then started to stutter while driving. Trying to adjust timing again. Seems like sweet spot is around 20-22. I removed the vacuum advance which makes no difference in the rpms. The advance is connected to the top port on the front of my holly. Any thoughts? Brand new destributer

[Dead Parrot]

Double check the suction gas lines from the tank(s) to the metal line on the frame. Same for from metal line to fuel pump. Chased an intermittent power loss on mine for several weeks before the Wiley Coyote light bulb came on. Suction line from dual tank valve to metal frame line looked like it could have been the original rubber line from 75.

Does a vacuum gauge show any vacuum on that carb port at idle? Having no RPM change when you disconnect the vacuum advance seems odd. Make sure the advance pot is good. Even new parts can be bad from the box.

[chrismoore701]

The only vac lines I have hooked up are the following

Brakes to intake
Trans. Teed to the brakes
Heater port. Lower port on front of carb
Advance upper port on front of carb

I will check the advance and check the advance port tonight

[chrismoore701]

Ok vacuum port has about 4 inches of vacuum at idle. I pulled vacuum on the advance and it holds. Does not start advancing until about 5-6 inches of vacuum. And thereby go

[Mr.Chevy4x4]

With a mild cam you should have a lot more than 4 inches at idle, more like 12+. First thing I would do is run all of your valves and make sure rocker adjustment is correct, and check for vacuum leaks.

Mike

[CSHADES]

Check your vacuum at a manifold port. Depending on cam specs it should have 16-18 inches of vacuum. It sounds like the cam timing is changing if you have to advance the timing. We used to advance the dist on cars that the timing chain jumped to make them run long enough to get them in the shop.

[chrismoore701]

But how is the cam timing changing?

[CSHADES]

The timing chain is the most common cause of the timing changing like you are describing. That is why I said you should check the vacuum reading at a manifold vacuum port not a ported vacuum port. if the vacuum is dropping from16-18 inches that usually means the cam timing is changing. Ignition timing usually doesn't change much over the life of the engine but what you have going is interesting. I see it is supposed to be a new engine, what is the story on it?

[chrismoore701]

I have 800 miles on it. Had it built, and the guy that built it, is standing behind it 100% I can take it over there and have him look at it, but two things, one, I want to fix it and two, I take it over there and we try one or two things and all seems better until I drive it a while. Just tough to schedule.

[CSHADES]

I would check the vacuum first and you can do that anytime, but I would be interested to know what it is at when it is running bad. If it is rebuilt I am assuming it had a new chain
put in at that time. But if it isn't getting enough oil to the chain or maybe a cheap chain? it will stretch.

[Dead Parrot]

If your ported vacuum is about 4 at idle and your advance pot starts to add in about 5, could be the cause of your issue. You are right at the magic point where the vacuum advance starts to come in at idle. A minor increase in ported vacuum causes a increase in advance,which causes a speed up of idle. You adjust the idle slower, which lowers the vacuum, the timing falls off, idle slows down more.

Move it to manifold vacuum and set your initial timing to 10btc with the advance pot disconnected. The initial 10 + the about 12 from the advance pot = 22 which you said seemed to be the sweet spot.

[chrismoore701]

Ok here is the latest. My dad has a 63 vette. 327. Same carb. Posted vacuum reads 0 at idle and 15 at the manifold. Timed at 18

Truck ran like crap on the way there. Kept dying. Tried to increase the timing to 26. Stemmed better until I drove it. Kept dying. Then I adjusted it by ear. Found whet I think may be the sweet spot. Turns out. I am at 32. Vacuum is now 0 at ported and 15 at the intake. I will be calling the builder this weekend and let him know what's up. What about hotter plugs?

[rich weyand]

Originally Posted by Dead Parrot

Move it to manifold vacuum and set your initial timing to 10btc with the advance pot disconnected. The initial 10 + the about 12 from the advance pot = 22 which you said seemed to be the sweet spot.

What he said. Connect the vacuum advance to manifold vacuum and set the timing to 10* BTDC.

Vacuum advance on ported vacuum is emissions nonsense.

[chrismoore701]

I will try that

[CSHADES]

So now you have bumped the timing up again? Something is going on it should have more than 15 inches of vacuum and you shouldn't have to keep advancing the timing to get to run right. Ported vacuum to the advance is not emissions nonsense, you want more advance as the rpms increase that is why you go with ported vacuum. Why would you want most of your advance in at idle? If that was the case you wouldn't even need a vacuum advance.

[chrismoore701]

Going back to the other options. It was mentioned to change the vacuum to intake. Remove it and set timing to 10. I can't keep the truck running at 10 degrees. As for the 15 inches. Dads vette has that as well at 15 inches and 0 on the ported for the advance and it runs great. Thinking something is letting go inside. Going to talk to builder this week. Seems like maybe the distributer gear or the internals of it are going bad

[Shugalou]

Any chance it could be the coil? Heats up and expands and won't run?

I'm tuned in to see what you find out.

[chrismoore701]

Not unless it heats up real fast. Took it out at 22 degrees yesterday. Cold and it puked before I got 1 block

[rich weyand]

Originally Posted by CSHADES

...you want more advance as the rpms increase that is why you go with ported vacuum.

This is not correct. Mechanical advance takes care of more advance as the rpms increase. Vacuum advance is to correct the timing for the thinner mixture at high manifold vacuum. Thinner mixtures burn slower. It is a matter of combustion chemistry.

Consider that ported vacuum *immediately* goes to full vacuum advance as soon as you crack the throttle. Why? What changed? Nothing.

Ported vacuum was part of emissions controls introduced on January 1, 1968. I know. I was there and working on cars when it changed. From 1938 through 1967, the Big Three exclusively used manifold vacuum for advance. The GTOs, the Camaros, the Mustangs, the Plymouth Max Wedge, the hemis, the 427 side-oiler, all those cars used manifold vacuum, as did all the tiny engines like the 195 straight-six Chevy and the 225 slant-six Chrysler.

It wasn't because they were stupid all those years and didn't know how engines worked. They changed to ported vacuum in 1968 to get a late spark at idle, so unburned charge would dump into the exhaust manifold, into which they pumped air with the A.I.R. pump in order to burn up pollutants that caused smog in stopped traffic in big cities. As soon as you wanted some power, the advance had to go all in, to use the full charge in the cylinder, so you got full vacuum advance as soon as you cracked the throttle.

Read this post:
http://67-72chevytrucks.com/vboard/s...d.php?t=689321

[rich weyand]

It could be that you are running way lean. A leaner mixture burns slower, and needs more advance. Could you have gotten a bad load of fuel, with crap in it? Or be getting debris from the inside of old fuel lines? Rubber fuel lines age-harden and should be replaced after 15 years or so. If the Holley is internally clogged, you could be running way lean and lots of advance is getting it limping along.

The correct timing should be between 12*-16* BTDC at idle with the vacuum advance disconnected. Then connect the vacuum advance to manifold vacuum.

To get it to idle at 12*-16* BTDC, you may need to richen the mixture, to speed the burning process up. Set the timing, connect the vacuum advance, and then adjust the mixture for best running, usually to highest vacuum at idle.

[rich weyand]

Here's another good article on manifold vs. ported vacuum advance.

http://www.hotrodders.com/forum/vacu...tml#post330637

[rich weyand]

I keep thinking about your problem. It could also be a vacuum leak. One thing you could try is disconnecting the brake booster and transmission lines and blocking the ports on the carb, just for testing at idle. If it clears up the problem, you have a leak in a line, or a bad brake booster or transmission vacuum modulator.

The reason I say a vacuum leak is it will leak air into the manifold, making the mixture leaner, which slows the burn, which will want more advance.

[chrismoore701]

Originally Posted by rich weyand

I keep thinking about your problem. It could also be a vacuum leak. One thing you could try is disconnecting the brake booster and transmission lines and blocking the ports on the carb, just for testing at idle. If it clears up the problem, you have a leak in a line, or a bad brake booster or transmission vacuum modulator.

The reason I say a vacuum leak is it will leak air into the manifold, making the mixture leaner, which slows the burn, which will want more advance.

I have done all the leak tests, I have rebuilt the carb, The fuel line is new. I had this problem before. it was timed at 12 degrees, ran great for 400 miles. Then started dying at stop lights. So I thought it was the carb, couldn't be anything else, as the distributor was tight. After exhausting the carb issue, I tried to advance the timing to 14. That did the trick. Now with 800 miles on the motor it happened again. Tried 16 degrees, then 22, then 26, then adjusted by ear and checked and it is now at 32.

No idea what is causing this, except the distributor coming apart.

Going to check the plugs tonight and call my builder.

[chrismoore701]

Took it to my builder tonight and he confirmed it is 32 degrees. And thinks it is the distributer. I pulled the cap and router off the springs are still on the weights and they appear to be working. He is coming over tomorrow to check it out more and maybe pull the distributer. I think it is either that or the chain. But I doubt that could be it as I think I would hear and see more problems. More tomorrow so tune in

[chrismoore701]

Have to postpone tonight. Jacked my back