changing q-jet secondary rods

[Dave1695]

Thanks for the feed back gentlemen, sounds like the change from 57 to 44 rods should show something, either good or bad. Plus changes by percentages will help in the future. The small change that I made (57 to 55) was probably too small to mean anything. Plus DA that night was at 1100 feet on last run, probably wanted way more fuel. Thanks again, appreciate the advice.

[Alan Nyhus]

Originally Posted by Dave1695

Thanks for the feed back gentlemen, sounds like the change from 57 to 44 rods should show something, either good or bad. Plus changes by percentages will help in the future. The small change that I made (57 to 55) was probably too small to mean anything. Plus DA that night was at 1100 feet on last run, probably wanted way more fuel. Thanks again, appreciate the advice.

Dave, the tendency of QJets is to run rich as air flow increases (rpms going up). Secondary metering rod changes alone don't always fully address this. Also, the primary side can contribute close to 40% of the total fuel flow at W.O.T.

Hope this helps. -Al

[Daran Summerton]

With the small qjet float bowl, too avoid starvation try to run the leanest combo.

[James Perrone]

A question to the q jet experts.
Any of you own a FAST. Q jet car.?
All this formula On what secondary rods is kind of useless to tune a car
Trial and error is how you find what a car likes.
Computers in the car are overrated still gotta make runs
What you feel in the car and on the time slip is gospel

[Stan Weiss]

James,
Very True.

But when you roll into the track and the weather is nowhere near what you have run before it gives you a pretty good starting point.

In the end for me it is the time slip and reading the spark plugs (yes, low tech still works ).

Stan

[SS734]

With the Qjet it all depends on the size of Primary and Secondary bleeds.
Too small of a bleeds it will go dead Rich. Too big of Bleeds and you can't get a small enough tip.

[Dissident]

Originally Posted by Alan Nyhus

Dave, the tendency of QJets is to run rich as air flow increases (rpms going up). Secondary metering rod changes alone don't always fully address this. Also, the primary side can contribute close to 40% of the total fuel flow at W.O.T.

Hope this helps. -Al

Mr Nyhus and Others that might be interested in some facts from measurements,
1) Most 750 type QJs flow test to be from a very poor low of 164cfm to an average of 170-175cfm at 20.4"H2O test pressure.
2) Most 750 type QJs flow test at 660cfm to 675cfm on the secondaries at the same test pressure as above.
3) Doing some quick math to see how much that is in mass flow of air in lbs/hr at sea level, yields 798 lbs/hr on Primaries and 2622 lbs/hr on Secondaries.
4) Allowing for max power at somewhere near 12.5:1 A/F ratio would yield fuel use on primaries of 63.84lbs/hr (127.68Hp) and 209.76lbs fuel use on secondaries (419.52Hp) supplying a total of 547.2Hp

As you can see, that is 23.33% fuel on primary and 76.67% on the secondary. IF one sees that the primary is supplying the 40% as claimed in the quote, one can only assume the secondaries are not at WOT or something is categorically wrong with the carburetor or the fuel supply system tune.

Hope that helps.....No, I do not have a race car but have supplied assistance to make more than a few more than competitive.

Regards,
HB2
Dissident

[Jeff Stout]

I measured the diameter of butterflys on both sides and it worked out 20% primary 80% secondary. May not be exact science IDK

[Dissident]

Although that is a good observation, Mr. Stout,
The 800 type QJ have the same throttle plate and blades but flow considerably more on the primaries due to a larger venturi for those carburetors which produces an average of 207cfm to 209cfm. And the bias changes the percentage of fuel used, but that is yet another oddity of the QJ organized leaks. Makes for interesting work on the dyno long before heading to the track.

Regards to all that like this kind of stuff,
HB2
Dissident

[Alan Nyhus]

Originally Posted by Jeff Stout

I measured the diameter of butterflys on both sides and it worked out 20% primary 80% secondary. May not be exact science IDK

The would be close if you were comparing primary and secondary sides that had the same fuel metering structure (like a typical Holley 4150/4160 style carb).

Measuring throttle bores or primary/secondary air flow and extrapolating that to fuel flow doesn't take into consideration how a QJet triple venturi primaries meters fuel as compared to the the non-booster secondary side that has a simple discharge tube. On the primary side, even the idle circuit contributes to fuel flow as the primaries open.

As quirky as they are, I love 'em. -Al