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Old 07-18-2008, 02:51 PM
oldbogie oldbogie is offline
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Quote:
Originally Posted by dh79
I'm doing some fine tuning of my carb. It's a 750 double pumper.

I just got a innovate wideband, and I gotta say it is awesome for carb tuning. Here's where the air/fuel mixture is at now:

Idle: 14.5
Cruise: 12.5
WOT: 13.8

The primary jets are 71, secondary jets 85

So I'd like to lean out the cruise and richen the wot. The carb has removable air bleeds, but I haven't experimented with air bleeds before. I figure the simple way to tune this carb would be to jet a bit smaller on the primaries and a bit bigger on the secondaries. Is it possible to have too much difference in jet size between the primaries and secondaries? And if so, would I be better off changing some of the air bleeds also/instead?

My targets af ratios are:
Idle: 14.5
Cruise: 14-15
WOT: 12.5-13
Do these seem reasonable?

thanks,
dh
Leaning cruise is simply putting in a smaller jet on the primary side, but this really is knowing where the throttle position is against the load. On the flat it's not uncommon to see cruise still on the transfer slots, this is dependent upon vehicle rolling resistance which is made up or bearing and tire resistance added to aerodynamic resistance. Topography also comes to play, it doesn't take too much of an upgrade to require more power which changes the throttle position. You can get somewhat of an idea of power being consumed/throttle position by using a vacuum gauge when testing. This is so much easier to do on a dyno than the street.

Getting into WOT is a more complicated deal as the secondary system is coming into use as well as the power enrichment circuit on the primary. As you already have seen the secondary jetting is much larger than the primary which rather covers the demand for a richer mixture at WOT from that end of the carb. However, along with that is the introduction of the primary system's power circuit. There are two elements to contend with in the power enrichment circuit; first is when or timing, this is managed by the contest between manifold vacuum and a spring in the power valve. The valves are stamped with a number on their side that indicates at what vacuum it turns on. A higher number brings the valve on sooner (with less throttle opening)and a lower number introduces the system later (with relatively more throttle opening). The second concern is amount of fuel which is managed by jets located behind the power valve in the metering block.

Leaning the primary main metering to bring the cruise to stoichiometricly correct 14.7 will also lean WOT. The simplest approach to richening WOT is to increase the secondary jet size. I wouldn't touch the primary's power jets as this is a one way street that if you get wrong is difficult to adequately fix. An old trick on Holleys is to install a primary metering block in the secondary which then provides the secondary with a power circuit. This allows you to back off the main jet sizing a little and still get the proper overall richness at WOT.

Bogie
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