Carb tuning help

[scottyz]

I just broke in my new engine. Its s 383 small block chev with 9.7-1 comp ratio, XE274 comp cam flat tappet, 200cc procomp aluminum heads and a vic jr intake. Carb is a Holley 750 vac secondary 3310 model. The fuel pressure is set at 6 psi. After driving for a bit the secondaries will start to drip fuel into the carb at idle. I imagine it is doing this while driving also as the car seemed to load up when I slowed down for my first stop. Float level seems good. Primary jets are 72 right now. Engine idles at about 750-800 RPM with about 9 inches of vacuum. Plugs are super black. What power valve would you guys recommend and what do you think could be done about the fuel dripping into the secondaries? Are the jets a good size or should I go bigger/smaller? Thanks

[sporter]

In order to eliminate confusing primary (front bowl) issues from secondary issues, I usually disconnect the rear bowl and run it dry. This will allow you to tune the primary side without interference from the rear bowl issues. Primary jets are not too bad, anywhere from 68-74 will probably work. Verify vacuum at warm idle in gear. If its 9" then a 6.5 or 7.0 power valve will work. Double check you fuel level settings in both bowls. Too low is better than too high. If your idle screws don't seem to effect the idle vacuum you can either increase the idle air bleeds or drill 1/64 to 1/32 holes in the primary butterflies. You can also switch to Rhoads lifters and pickup a few inches of vacuum, which will help your idle and off idle performance.

[68Formula]

First of all, leave the power valve alone. You have a heathly cam. That and the fact that you are rich is causing a low vacuum problem. Address this before changing the power valve.

If messing with the float bowl doesn't help, do the following.

Remove the carburetor and verify that the transfer slots on the secondary side are not exposed. If the slot is exposed, there is a small screw on the base that can be adjusted.

Hopefully one of those fixes should work.

On the primary you should see 1/3 of the slot exposed at the most.
Now, if more than 1/3 of the primary slot is exposed or you read a vacuum level from the ported source at idle in neutral, you have a couple options.

One involves drilling the holes as Sporter mentioned. This is often done and recommended by many knowledgeable carburetor guys.

Personally, I like to do what can be undone first. And it's difficult to undrill a hole. So try this:
Your vacuum advance (hopefully you're running one) is probably hooked to ported vacuum (about mid-level). Disconnect the spark advance from the ported source and plug that port on the carburetor. Reconnect the spark advance to manifold vacuum. At this point your idle speed should increase. Turn the idle screw down until the idle reaches an acceptable level. Now connect a vacuum gage to the ported source. If you do not see vacuum then the transfer slot is no longer exposed. Which means you are not getting extra fuel.

Now you should be able to adjust your idle screws to get the highest vacuum reading. If not (and still completely rich) then it's possible your power valve is blown.

After you make these changes, if connected to ported vacuum I'd estimate at least 12" Hg full manifold vacuum should be possible.

If during driving you hear any pinging, then purchase an adjustable canister and follow directions to reduce the amount of vacuum advance that comes in.

[GMachineDartGT]

I don't agree with putting vacuum advance to a full time source. You should be running in the neighborhood of 15-17 degrees initial advance without the vacuum. Putting the vacuum adv on full time will also create a hot start problem, especially with a sb chevy. Do the right thing an curve the distributor properly. I would set total 36-38. I would leave the vacuum advance out of the picture for now. Your cam is small enough that you will be able to make use of it. Secondly, I would go to the plugs, and make sure they aren't too cold. Check the threads towards the base for discoloration. Make a full throttle pass as well, and see if the plugs clean up. You may have a couple of things going on here. As far as the carb goes, judging from your specs, the carb should work well out of the box. The power valve is not going to affect the idle. If the car has a crisp idle and can be stalled by bottoming out the air fuel screws your most likely in good shape. 1.5 - 2.0 turns out should do it. If it doesn't you have some clues. I have done lots of my tuning with an LM-1 air fuel meter which works wonders for tuning in conjunction with reading plugs.

[LowBuckX]

I don't agree with putting vacuum advance to a full time source. You should be running in the neighborhood of 15-17 degrees initial advance without the vacuum. Putting the vacuum adv on full time will also create a hot start problem, especially with a sb chevy.

I totaly disagree.
*********words of JOHNZ ****************
Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

TO read the whole thing click here.. Vacuum Advance 101 Long read but worth it.

[69HuggrrrPT]

Yes, definatly good reading in that link. JohnZ is the man!

[rockvillespeed]

i also disagree with not using full time vaccum.
this is a great way to pick up some idle quality and vaccum, and driveability.
also can help kill part-throttle ping under light loads.
it also houldn't create a hot start issue as there's no meaningful vacuum at cranking speeds, so shouldn't load down the starter.
i've not observed hot start issues relevant to fulltime vaccum sourcing, including one car with chronic hot start issues.
absolutely worth a try.

[Blue69Gen1]

scottyz,

I have the exact same problem with a holley 750 db. Secondaries dripping at idle, loading up and stalling under braking. Did everything listed above (except drill holes in the primary butterflies) and never could fix the problems. The only thing that worked was to screw the secondary float leveal all the way down to shut them off. Also changed the secondary needle and seat twice and the float once. I am ready to throw the carb away.

[gen3bu]

the only way to effectively and timely tune an engine is on a chassis dyno. we went thru a similar problem on a customers car recently. he brought in a 1969 camaro that was full of mail order engine parts (not well matched of coarse). the idle afr was 12.2 and wot was 14.3 and climbing with the same base line in the idle circuits (750 dp). after a lot of all the same adjustments above the car ran great and had excellent driveability. make sure that there is .010-.015 clearance on the accle pump arms

[cad]

http://www.bob2000.com/carb.htm search on "drip". There are also several links at the bottom of the article supporting his tips.