Racing Fuel Systems

Re: Lean out QF 850 cruise

2023-11-20T16:06:39+01:00

Mark suggested .081 as a starting point. We all have our own personal preference for cold start behavior. I personally don't mind tickling the throttle a little to to get the engine to operating temp to ensure a nice clean mixture once it warms up. That's what thermostats are for.

Statistics: Posted by treyrags — Mon Nov 20, 2023 4:06 pm

Re: Lean out QF 850 cruise

2023-11-19T13:46:01+01:00

For a true street car ( not a race car ) the IFR should sized for more than just consistent engine temps, unless you like a stalling engine while it is warming up and feathering the throttle while you back out of the garage , even with a choke. Car cools off somewhat when your somewhere and idles like shit until heat is back in it. Also I think the .081” TSR will be to large to make a difference to your cruise AFR. Easier to drill it larger than keep making new ones.

But what do I no . I tune street carbs , not race carbs.

Statistics: Posted by rgalajda — Sun Nov 19, 2023 1:46 pm

Re: Lean out QF 850 cruise

2023-11-13T05:27:51+01:00

.081 is a fine place to start. As far as idle jet and bleed sizes, it really depends on the engine its going on. I had to go to .039 on a 600+ HP Pontiac, same carb on my engine was fine with a .037. Vacuum at idle, timing curve, how the cam pulses, texture in the intake tract... lots of things will affect how the idle circuit needs to be tuned. Lean it until you get a stumble or surge in light throttle positions, give it just enough to run clean. Engine temps need to be consistent, gas has multiple chemicals that evaporate at different temps, pump gas can be really temperamental. Sometimes only a 10 degree change will have an effect. Large swings in ambient temps will change it as well, so you may need to make an idle bleed change from summer to winter.

Statistics: Posted by jmarkaudio — Mon Nov 13, 2023 5:27 am

Re: Lean out QF 850 cruise

2023-11-08T17:14:13+01:00

thanks! i just ordered 50 of each in all 3 sizes, will pull the carb off and get to work. should be fun! and i will feel a lot better about going on long drives if my AFR cleans up.

Statistics: Posted by i82much — Wed Nov 08, 2023 5:14 pm

Re: Lean out QF 850 cruise

2023-11-08T13:57:56+01:00

3/16 is the correct length. Usually an 8-32 for most main body Holley carbs. Since yours already has a pressed in restrictor you would have to remove that and measure the hole.
I use a normal tap that has been marked for correct depth so the set screw bottoms out flush. You will have to tap shallow, screw in the set screw and test for depth. .064" is what I ended up with in my Quick Fuel carb. I don't know if you were going to oder blank or predrilled set screws? I ordered mine from McMaster Carr in all 3 sizes 6-32 , 8-32, and 10-32, 3/16 length.

Statistics: Posted by rgalajda — Wed Nov 08, 2023 1:57 pm

Re: Lean out QF 850 cruise

2023-11-07T14:31:43+01:00

We might have a few more days when I can drive the car, once we get the first decent snowfall they put the deicer down and then I put it away. But not sure if I will get around to drilling and tapping it before then.

i am not entirely familiar with the procedure, but I'm guessing a # 21 drill bit and a 10/32 tap? Can I just use a bottoming tap? Seems small enough that I might not need to start with a plug or taper tap.

Also will these blanks work? Not sure if they are the correct length?

https://blp.com/cart/index.php?main_pag ... ts_id=1495

Statistics: Posted by i82much — Tue Nov 07, 2023 2:31 pm

Re: Lean out QF 850 cruise

2023-11-07T12:54:11+01:00

Statistics: Posted by rgalajda — Tue Nov 07, 2023 12:54 pm

Re: Lean out QF 850 cruise

2023-11-07T03:29:14+01:00

Statistics: Posted by i82much — Tue Nov 07, 2023 3:29 am

Re: Lean out QF 850 cruise

2023-10-13T06:10:32+01:00

Statistics: Posted by Right hand drive — Fri Oct 13, 2023 6:10 am

Re: Lean out QF 850 cruise

2023-10-13T06:04:00+01:00

Statistics: Posted by Right hand drive — Fri Oct 13, 2023 6:04 am

Re: Lean out QF 850 cruise

2023-10-13T00:12:14+01:00

Surprised by the error above in the coloured [ green ] description of the transfer slot operation. 'Wider T slot = more fuel'.
At idle & until the t/blades are opened far enough off idle to block the top of the T slot, the T slot above the blades acts as an air bleed. If The T slot is wider, more air is admitted to the T slot section above the t/blades, which will lean the mixture.
In general, a performance carb will have a short T slot & an emissions carb will have a long T slot, for the above reason.

Statistics: Posted by GTO Geoff — Fri Oct 13, 2023 12:12 am

Re: Lean out QF 850 cruise

2023-10-12T12:27:37+01:00

If the pressure in the main well, created by the combination of amplification from the booster and the restriction of the main jet, isn't lower than the pressure in the throttle bore, that suggests we can do away with boosters and even venturis altogether! They aren't generating a low enough pressure to be needed for anything and we can get the same result from a straight tube with a discharge hole in the side.

Statistics: Posted by cat herder — Thu Oct 12, 2023 12:27 pm

Re: Lean out QF 850 cruise

2023-10-12T11:03:32+01:00

Statistics: Posted by Right hand drive — Thu Oct 12, 2023 11:03 am

Re: Lean out QF 850 cruise

2023-10-12T09:52:30+01:00

Statistics: Posted by cat herder — Thu Oct 12, 2023 9:52 am

Re: Lean out QF 850 cruise

2023-10-12T07:36:00+01:00

Statistics: Posted by Right hand drive — Thu Oct 12, 2023 7:36 am