I was going through the new Chevy Hi Performance magaize today, and I saw a article, I have seen it a few other times pertaing to the same thing, was just wondering if anyone eles is experiencing the same problem.

The article goes like this, engines with wild cams the exhibit 6" or less often require a throttle blade setting to exceed the nornal position, which may uncover too much of the idel transfer slot, which creates an overly rich mixture, driveability problems, and often a off idel stumble.

I have had a rich problem for some time, I have had guys look at it, tune it, by guess and by golly, with vacumm gauges with tachs, but none have ever discussed this fix before. And like I said I have seen this fix before and will try it as soon as the cover cones off.

Here goes, To correct the condition, it may be necessary to drill a holes in the throttle blade, on the transfer slot side. It was recomended to start with a 0.040" hole and work your way up from there. This is suppose to let more air past the throttle blade , allowing you to close the idel speed screw to cover up more of the transfer slot , only the very top of the transfer slot should be exposed.

Like I said I have had many a garage mechanic work on this, I have gone through spark plugs by the box, if I don't idel, and just drive, but when I have to idel around, like at shows, or drive up windows for extened times, I can tell right away it starts fouling up and its all down hill from there.

Let me know,Thanks

Dave
Mr69vett

Need more info about your setup. Motor?Carb?Cam?

This is very common problem if you have modified your engine with a large sep. cam. Larger cams reduce the amount of vacuum below the base plate of the carb resulting in less air being moved into the engine resulting in more air being required. The fuel at idle most of the times is enough for the engine, there just isn't enough air to get the fuel into the engine to make it run. So what most people do is they open up the curb idel screw (on the throttle lever)to get more air in the engine. Problem is if you open then up to much you expose the idle transfer slot to vacuum and you start to pull fuel from the main well. When this happens the idle screws have no effect on engine performance. To tell if you have them to far open, all you have to do is screw all your idle screws in until the are seated lightly. If the engine continues to run, then the engine is getting fuel from somewhere else, most likely the idel transfer circuit. Now, if you do have a big cam and you are having this problem to haul off and drill your throttle blades because there is another adjustment that most people are not aware of that you should make before they result to drilling the throttle blades. Its call a secondary throttle blade opening screw. Most people are not aware of it and the only way you can adjust it is to remove the holley carb and turn it upside down.

To adjust for a engine with low vacuum (with the carb off then intake) first open up the primary curb idle screw to where its just below the transfer slots in the venturiis, then open up the rear throttle blades with the seondary throttle opening screw until its just below the idle slots. Now put the carb back on the engine. Start the engine and turn the curb idle screw until the correct idle rpm is acheived. If you can't idle it down to that rpm then you will need to pull the carb back off the engine and close the secondaries some, until that RPM is reached. Now adjust yoru idle screws until the highest manifold vacuum or engine rpm is acheived then go back and turn the curb idel to reset to correct engine rpm. Go back and forth between the curb idel screws and idle mixture screws until little or no change is noticed in engine vacuum or engine rpm.

If opening all the blades up to just below the transfer slots does not give you the rpm and return you idel mixture control to you then drilling of the throttle blades will be required.
One other thing to check that can cause your rich condition is manifold vacuum at idle with it in gear. If its not 12" or more, then you may need to lower your power valve rating. Drop it down to a #5. Larger cams cause the manifold vacuum to drop and if its too low it will not keep the power valve close and it will add extra fuel into the idle well and cause a rich condition. Also if its blown it will leak fuel through the valve into the powervalve well,through the power valve channel in the base plate, and cause the engine to run rich all the time. A good indicator of a blown power valve is to screw the idle mixture screws in and close the throttle blades down to the factory setting. If the engine runs, most likely its getting fuel from the power valve well. If the power valve is blown it will run rich all the time.

wow!

great response! One of the greatest benefits of the C3 registry is finding C3 tuning guru's!!!!

As many of the C3vr members have found, its one thing to build a great motor, its a whole 'nother story finding someone that can tune it to its full potential.

Its all about ignition and carbs!

Wow it was a long time since I wrote that post, and I just came back accross it. I agree with the GREAT response After Shark

Except I can say that I remedyed the problem..........., made a few changes to the motor again this spring, friends brother had a few parts laying around collecting dust, said 200 bucks and there yours, sounded like a good deal for a set of 64 CC heads ported polished, new valves, along with Peformer RPM, port matched to the intake, along with the notch in the center runner, for some reason he explained. I would need to provide the rockers, he reccomended full rollers, another 250, but then he also threw in a Performer RPM cam and lifters, Holley high volume mechanical fule pump, enough braided fuel line and AN fitting to make it look great, and also the help on the install. The only snafu we had was when we went to bolt the carb on the intake was Holley, not drilled for both, so it was either go buy a new intake or carb, seing as the intake was all bolted down, and I was having problems anyway with it, lets try a new one. Needles to say we played a few evening with it, got it very close right now, still have a slight off idel lag between 1500 and 2500, but am pretty happy, is running terrific and havent had to replace plugs and that was a step in the right direction.

Again thanks for the responce guys

Dave
Mr69vett

If your getting an off Idle lag (hesitation and then it takes off, not a bog) and its a Holley Spread bore then most likely you can correct the condition by going up on the squirter size until the hesitation disappears. I wouldn't touch the vacuum secondaries (if it has them) until you try the squirter size change.

I went from 66 main jet to 62 mains to cure my rich condition....this is on a 73 with a 350 and a Holley 600 cfm 4 bbl.....