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Old 11-05-2012, 02:20 PM
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Originally Posted by cobalt327 View Post
0.032"-0.035" max on the plug gap.

The way the PV is supposed to work is it will stay shut until the engine load causes the vacuum to drop to below the PV rating, at which point the PV opens and enrichens the air/fuel mixture to help overcome the load the engine is under.

So, check to see what the vacuum reading below the carb and above the blower is. If this reading is less than the number on the power valve, the power valve will be open and the engine will be overfueled. The engine doesn't need to be under a load for there to be fuel coming in from the power valve. If the vacuum at idle is lower than the power valve rating, there will be excess fuel, for example. This might be part of the reason you have a rough, 'lumpy' idle.

But since the carbs aren't boost referenced, the PVs should be shut- even when you want them open. That is, unless one or both PVs are ruptured from a backfire or old age. Newer Holley carbs have backfire blowout protection for the PV- if it's functioning properly. If not or if the carbs don't have blowout protection for the PV, then they could be damaged allowing fuel in when it isn't needed. If the wrong PV gasket is used, or two gaskets are accidentally used, there can be a fuel leak. Just things to look for when you take them down for cleaning.

If the initial ignition advance is too low, that will also account for a poor quality idle, and can cause the primary butterflies to be open too far. If the butterflies are open too far the engine will be pulling fuel in from the transition slot of the carb instead of running mainly on the idle circuit. If the timing is advanced at idle and the carb curb idle screw is backed out to close the primary butterflies, the idle mixture screws should become functional.

Sometimes the secondary butterflies need to be cracked open farther than "normal" to add bypass air to get the idle back to normal. Sometimes you even see where the primary throttle blades have been drilled w/small holes in them to add bypass air, although this is seldom really needed if the other methods (secondary bypass air and increasing the timing at idle) are used.

Something else to look for when you first pull the carbs is to see what the transition slot looks like. It should be between a "square" and a slot about 0.040" long. If this is too long, that's when the engine will be trying to idle on the transition circuit instead if the idle circuit. Photo below shows what I'm referring to.

If the carbs aren't boost referenced, I wouldn't want to run it on the dyno. The dyno loads the engine heavily, and w/o the PV adding fuel it can go lean. You have to be sure that the PV's were working when the blower is making boost and the vacuum drops.

This is a concern to me because of the small carbs. If you had a pair of 750 cfm carbs you might not need boost referencing at low boost levels- but this would need to be verified before loading the engine.

A 65 PV might open later than you need. It wouldn't surprise me for you to end up w/an 85, as an example.
cheers for the advise !!
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