Weird Holley Carb Problem - Please Help

67GTFastback

Founding Member
Dec 14, 2001
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Calgary, Alberta
I have to Holley carbs, a 600 cfm C70F (67 GT 390 Carb) and a 735 cfm C90F (69 428 Carb). I have cleaned them both up, added a kit to them, and have different results. The 600 runs very well. However, when I put the 735 on my car, fuel spills out of the secondary chambers onto the secondary throttle plates. I have used the same fuel bowls to rule out float problems or anything to do with the fuel bowls, including adjusting the float level very low to avoid overspilling. I have rebuilt many Holleys but never seen this before. The car fires up without problem and has a high idle obviously, around 1200rpm or so. I have checked the gaskets and it is sealing properly around all of the venturis in the carb on the secondary metering block. The secondary throttle plates are closed properly so it isn't being drawn in on vacuum. It has 79 jets on that metering plate, 70 on the primary metering plate. Any ideas are greatly appreciated. Thanks in advance.
 
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I had a similar expierence with my road demon. the secondary bowl was spilling over, the only way we could get the float down was to open the idle jets to a very unnatural setting. we knew it wasn't right but it worked fine for two weeks.
then it spilled over again. we took apart the secondary float and needle assembly and found a very small metal flashing. put it back together and voila, it worked fine...i know you tried eliminating those, but go over it just in case. along with others advice.
 
Does the one carb have a secondary metering block and the other a secondary plate?I tried swapping my cathedral style center hung float bowls from a carb with jets on the secondary side on to one with the secondary metering plate.I found that the brass float would hit the metering plate and cause fuel to pour out of the boosters and vent.The only way to clear the plate was to get one of those plastic floats.
 
Both carbs have primary and secondary metering blocks with the stainless steel transfer tube between the two bowls. They are center hung floats. If it were too high fuel pressure, wouldn't that happen on the 600 cfm carb as well? If not, how do I correct it if that is what it is? Again, I have used the same fuel bowls for comparison. The metering blocks and core are the components I have switched out. Thanks for your help.
 
I had a similar problem with my 4160.. Mine was that the adjustment screw had begun to strip and metal was getting in the bowls and clogging areas and also causing the floats to stick.. yeah.. it flooded so bad that my car actually ignited..

Try tapping on the screw like D.Hearne suggested

Do you have a fuel pressure guage hooked up or can hook up?
I dont know the idle psi but I know full throttle is no more than 6 psi
my guess idle would be 4 or 5?
 
67GTFastback said:
Both carbs have primary and secondary metering blocks with the stainless steel transfer tube between the two bowls. They are center hung floats. If it were too high fuel pressure, wouldn't that happen on the 600 cfm carb as well? If not, how do I correct it if that is what it is? Again, I have used the same fuel bowls for comparison. The metering blocks and core are the components I have switched out. Thanks for your help.

by using the same bowls on each and only having problems with the 735,
leads me back to an internal gasket mismatch.
there are diferant gaskets for blocks with jets and without.
After rechecking float levels and still have an internal leak down the venturi's,
take a close look at the metering plate gaskets.

PB