SBC Carbed 385CI Tunning Help

[SmallBCS10]

Well I stubble off of the line and after a burn out, and cleaning the system with a healthy rev It still seems to load up.

I have a holley DP 650 with some work done, Jets, Squirters, diaphram 30cc, Cams, Etc.

I have a Air/fuel gauge setup and it is really helpful but I can't get the truck to stop strubbling from foot brake. anything over 3k rpms there is no issue. I have been searching head to toe for a vacuum problem but none found and all gaskets have been changed again.

My next thoughts are to lock my distributor out so its at full advance from idle on. I run it at 31 advance and it hits full at about 3k. What do you guys think about locking it out? thoughts suggestions? HELP ME!! lol Its really making me angry.

[Firebirdjones]

Locking the distributor is feasable, but you'll need a timing start retard feature that is found on something like an MSD box. They knock the timing down to 20 degrees on crank so it's easier on the starter. Without it you create other problems for youself.
I only find it necessary to lock a distributor at full advance for a very very radical camshaft on an engine that isn't running any vacuum advance. And even this isn't very often.

If you are loading up there could be one of several issues. Very hard to diagnose here on the computer, if you were close by I could fix it easier. But I'll give you a few things to look at.

Metering blocks: make sure they are completely flat with a straight edge. Even new gaskets won't cure a warped meter block. You'll have an internal fuel leak, generally they load up at idle or won't idle well at all. Holley sold a kit years ago to fix a warped block (now discontinued) but it basically puts the metering block in a vice and gently bends it the other direction for better gasket seal.

Main body: While the metering blocks are off, check the main body surface with a straight edge. Generally years of removing float bowls will pull all four corners of the main body out, which leaves the center of the main body concaved, again affecting the gasket seal and causing internal leaks. The fix is to disassemble the carb, and hit the main body (front and back) with a medium flat file until you see an even file pattern accross the entire face. Clean and blow all passages before reassembly.

Power valve: Make sure it isn't blown. If you can't physically see a split in the diaphram,,,,stick it in your mouth and suck on it, and you should see the valve move back and forth without much effort. If it leaks at all it causes idle issues, usually loading up. Make sure there isn't a double gasket on it. Sometimes when they are removed the gasket splits in half, people don't catch it and install a new gasket on the power valve and reinstall, affectively stacking the gaskets and this causes a leak.

Also check your vacuum reading at idle while "IN" gear. Depending on camshaft, what ever that vacuum reading may be. Lets say for instance 7 inches. You'll need to drop the power valve down to a 5. Stock for your carb is 6.5. That would be to close to my example, and the valve would trickle open at idle causing a rich condition. Try to stay about 2 inches below your lowest idle reading.

Idle air bleeds: They are the outer most brass orifice in the top of the airhorn right behind the boosters. On your particular model carb, they are fixed and non replaceable. Check to make sure they are clear of obstruction. Any debri or carbon build up will richen the idle mixture. Check both the front and the back.

Some other things to help......

Base timing: I'd shoot for 16-18 degrees initial timing at idle, with vacuum advance unplugged (if you have it) maybe more with a larger cam depending on other variables like compression etc...
The higher base timing will help with any idle fluctuations you may have, and is especially helpfull with larger camshaft that produce low vacuum. A higher base timing setting will help the engine make a little more vacuum. And I would suggest this before you go testing for the proper power valve. Because your vacuum requirement "WILL" change once the timing is set with more initial.

From there shoot for about 18-20 centrifical advance which will give you a total of about 36-38. This will change depending on your setup. I would need more information like gearing, transmission, gas used, weight, compression, cast iron or aluminum head, camshaft overlap etc.....
Your 31 degree setting, (and I am assuming this is a total advance number you are giving) is a bit soft for a gen 1 sbc unless it has a modern cylinder head with an efficient combustion chamber,,,,or you have an old cast iron head with high compression trying to run pump gas, no rear gear, and a heavy car to tote around. Otherwise I'd bump that number up. Sneak up on 34-36 and see how it acts.

From there, if you run a vacuum advance, (no reason not to) First make sure it's plugged to "FULL TIME MANIFOLD VACUUM" and not hooked to a ported vacuum source (such as the vacuum nipple on the meter block. Full time vacuum will give you idle vacuum and some advance on top of your 16-18 initial setting, which also helps with radical camshafts and low vacuum. This will also help the car to run cooler in low speed idle situations.
This vacuum advance will only come into play at idle and low throttle applications. Once full throttle the vacuum is dropped out of the equation, this is exactly how it should work.
Hooking it to ported vacuum however (like the metering block) will work the opposite and is bad news. You have no vacuum at idle, the car will run warmer, and as you progressively add more throttle, more vacuum advance is added, and it never goes away at full throttle. So you wind up adding to your total advance numbers, which you "DON'T" want to do.

Once you have the vacuum hooked up to work properly, invest in an adjustable vacuum advance unit. You'll want to dial in 10-12 degrees of extra advance while at idle. Something you'll have to play with using a dial back timing light depending on how much idle vacuum you have, you can adjust the spring tension on the diaphram to dial in the addtional advance. You won't need much more than 10-12 degrees on top of your 16-18 initial. This also helps idle tremendously. Most cams that had a good thump by now are running fairly smooth when I'm done with them, produce good idle vacuum, and throttle response is very snappy and crisp.

Check these things and let me know. My bet is that since you mentioned the carb has been apart (not sure how many times) you probably have a warped meter block or warped main body by now causing you idle fits.