[Pro57]

I have a brand new sbc with a 671 Weiand blower (also new), and it continues to surge from 800 to 1500 rpm.If I bring the idle up to about 1500rpm, the motor smooths out, I beleive because the secondary idle circuit kicks in. This motor used to idle fine with a different weiand blower (3 blade rotors) but the blower (also new) had a bearing rattle so I sent it back and got Weiand's new design returned to me. This model has 2 blade rotors (less efficient at low rpm). I have 2 new edelbrock 750cfm carbs that I richened up, but still no good. I'm told the surging is a lean mixture, caused either by a vacuum leak or carbs that are too lean. I've also checked for vacuum leaks and had the intake welded and gasket ported to felpro 1206. All intake surfaces where also milled flat again. Does anybody have any ideas where to go next?

[4 Jaw Chuck]

Sounds like a vacuum leak somewhere, use an unlit propane torch to locate the leak. Have you checked your blow off valve to see if it is leaking?

[Pro57]

Thanks for your feedback.
I've checked for leaks using starting fluid, but haven't found anything. And yes, one of the places I checked was the pop off plate. The only thing I can think of is that possibly the underside of the intake has a crack in it and it's sucking air from inside the valley. I just have trouble with that concept because the intake is brand new (never dropped or mistreated) and it idled fine with the other blower (3 bladed rotor). Any ideas on how to check for a vacuum leak inside the valley wihout removing the intake and checking with dye?

[4 Jaw Chuck]

Not an easy thing to check for with the engine assembled Pro57, one common problem I see is those little plastic alignment buttons on the intake gasket. When they break off in the intake or head the next gasket will not seal properly because the new plastic button gets squashed and a leak ensues, typically in the lifter valley. This might not apply to your engine/situation. I would imagine you have gone over the carb gaskets. I would try propane first before tearing anything down, because it is a gas it will find leaking gaskets better than starting fluid.

The other possibility I could think of is did the old blower "sneeze the motor" and maybe blow a gasket inside the carb? Just a thought.

These little problems can be a bugger to find.

[Pro57]

I'm using Felpro 1206 gaskets and they don't use plastic buttons so that's not the problem.
I'm not sure what you meen by "Sneeze the motor" but I'm running New Edelbrock 750 carbs that don't use a power valve and only have one gasket at the very top of the carb. I took them entirely apart and cleaned them out, but they where already spotless. I'm going to bring my carbs over to a freinds house that has another one of my running stands and try the carbs individually on his big block. That way if they run well, I can take them out of the picture as far as trouble shooting.
I've never used propane to look for leaks but will definitely give that a shot first.
Once I exhaust these options I think a teardown is the next step.
I will be calling a few more blower companies today for some other possibilities, but their usually no help. Usually guys like you are the best source for fixing these types of issues. I appreciate your help. Thanks.

[4 Jaw Chuck]

Pro 57, it helps to disconnect the fan or remove it for testing with propane but it is not entirely necessary. Have you checked the ignition? A quick check of the timing reaction to rpm will dispel any doubts about it's function. Be sure to check every plug with the timing light, I have seen weird things happen with plugs. This might sound weird but have you sniffed the exhaust? A quick whiff will tell you if the car is running rich or lean, does the hunting get worse with idle mixture adjustment or better?

The only other thing I can think of is are the throttles syncronized properly, sometimes with multiple carb setups a mismatch is throttle angle will cause a hunting at idle. Usually this happens only with large open plenum manifold like tunnel rams but it could happen with a blower engine. Is the power enrichment valve being opened possibly due to the more efficient blower design?

Just bouncing ideas off of you to see if we can isolate the problem.

[Pro57]

Actually, I was able to get a hold of a couple of Blower and Carb vendors yesterday and they had some suggestions; most centered on the speed (or lack of) at which the blower is spinning. They feel that the combination of a lumpy cam, Big breathing Dart heads, the 2 rotor blower design and the 10% underdrive pulleys supplied with the kit all point towards the blower spinning too slowly. They want me to reverse the pulleys and run the motor to see if the surging significantly reduces or stops. They said that the 2 rotor design at low rpm will create a more significant surge than the 3 rotor. None of the vendors felt that a vacuum leak existed because of the lack of glowing headers, overheating or lean condition spark plugs. I hope their right.
The carb company suggested the same as you with the synchronization as well as changing some of the rod springs. I eliminated the sync issue last week when I ran it with 1 carb; and got he same response.
The timing is right on manufacturer (Lunati) spec.
I'm going to try the pulley scenario first (hopefully tonight) and cross my fingers for a fix.
I'll let you know what I finally figure out.
Thanks
Again

[4 Jaw Chuck]

I never considered the pulley underdrive would cause such a problem, it makes sense though. Weird how a change from a 2 to a 3 rotor would make such a difference, I hope you have found the problem. I look forward to you finding the solution so the rest of us can learn.

I learn something new everyday, luck to you!