Tuning Double Pumper Carbattoters???

Pete · Post by **Pete** » Thu May 16, 13 9:51 pm

Hi there.
Just rebuildng (another) set of Double Pumper Holleys. Twin DPs on a Tunnel Ram.

It appears that the mechanical Secondaries open quite late. i.e. the Primaries are about 60% open before the secondaries start to move.

As the car will be launching on a transbrake at around 4000rpm there is no need to "Flash" the Convertor and therefore I think I need a bit of a different set up to my traditional approach.

I now have small a 30cc Pump shot on the Primaries to prevent over-fuelling , and I have a 50cc Pump shot on the Secondaries to avoid a bog.

Has anyone had practical experience of shortening the link between primary and secondary throttle shaft brackets to make the secondaries open earlier - and was it effective in practice???

Many thanks

Pete

Pete · Post by **Pete** » Mon May 20, 13 12:05 am

Guy · Post by **Guy** » Mon May 20, 13 12:20 am

I have never shortened the linkage Pete but have used 1 to 1 linkage with 50cc shot on the front and rear and set the rear cam to delay the shot on the secondarys in conjunction with big shooters to get rid of any flat spot. Then tune the front shooters to eliminate any off idle bogs but still overlap with the rears.

Pete · Post by **Pete** » Mon May 20, 13 7:42 am

Thanks Guy!!

Blue · Post by **Blue** » Mon May 20, 13 7:53 am

Normally with a brake you'd use a rev limiter set to your desired RPM and have the throttle wide open, so opening rates and pump shot don't come into it at all.

Pete · Post by **Pete** » Mon May 20, 13 8:27 am

Blue wrote:Normally with a brake you'd use a rev limiter set to your desired RPM and have the throttle wide open, so opening rates and pump shot don't come into it at all.

Agreed.
The bit I do not understand with that approach (no practical experience) is how the motor does not flood...

AllKiller · Post by **AllKiller** » Mon May 20, 13 8:42 am

[quote="PeteAgreed.
The bit I do not understand with that approach (no practical experience) is how the motor does not flood...[/quote]

Tuning

Blue · Post by **Blue** » Mon May 20, 13 10:26 am

It won't flood because even though it's wide open, the revs are being held so it will only draw the fuel it needs. In a set up like that I would imagine you'd only want enough pump shot to make it driveable around the pits and burnout.

Pete · Post by **Pete** » Mon May 20, 13 10:30 am

Exactly, which is why I have actually REDUCED pump shot, especially on the primaries as opposed to increasing them if your were to flash the convertor......