High throttle miss

[Texasdoc]

Anyone have any suggestions on what changes to make? I have the original jets that came with it - 74 primary, 82 secondary (with extension tubes). 6.5 PV. I have the idle screws set where it idles about 13.5:1. I think it is very rich thru the transition circuit.

I've also noticed (and noted here before) that minimal changes in the idle screws make a huge difference in the AFR at idle. 1/16th of a turn of the idle screws will change the AFR by a whole point.

Primaries are set where the transfer slot is square (as tall as it is wide). I use the secondaries to set the idle speed (1000 rpm).

Frequently, the car seems like it doesn't want to return to idle. The RPMs will hang around 1200-1300 and eventually creep back down to 1000. I have two throttle return springs so I'm pretty sure it is returning to the stop screw.

Idle - 13.5
Off idle/cruise at 1500 rpm - 11.0
Light acceleration - 12.0-12.5
Medium acceleration - 13.0
WOT - starts rich 12.5, ends at 13.5

Letting off the throttle, it goes pure lean (gauge pegs at 17) then settles down to idle at 13.5. Going around a corner or heavy braking also goes lean. Fuel is 1/2 way up the sight windows on primaries and secondaries.

Thoughts?

[Gaz64]

What are the air bleed sizes?

I would increase the idle air bleeds by about 4, and decrease the high speed air by 2, and retest.

Gary

[Texasdoc]

Unfortunately, I don't know what size any of the bleeds or restrictors are. This is how it came from Brent. There were no instructions or parts list in the box.

How can I tell what size they are?

[Gaz64]

The jet size might be stamped on the outside of the jet.

You could be lucky and read some numbers in situ.

I know you have a Quick Fuel 750, but specifically what carburetor have you got?

Gary

[snapier]

This sounds very similar to a problem that I just had. My Superformance (SP01400) would break up when I would get on it but regular driving it was just fine. Then finally one day I went to start the car and no spark. Took two weeks to figure out that the magnetic pickup in my msd distributor and also at the same time my duraspark ignition module where bad at the same time.
Good luck...

[Texasdoc]

Originally Posted by Gaz64

The jet size might be stamped on the outside of the jet.

You could be lucky and read some numbers in situ.

I know you have a Quick Fuel 750, but specifically what carburetor have you got?

Gary

It is a custom Q-750 series from Brent. The jet sizes are 74 and 82 as stated.
The idle air bleeds are 70 (on the outside) and the high speed bleeds are 33 (closest to the squirter) for both primaries and secondaries.

[Gaz64]

Ok, first thing to do is give all the air bleed a spray of carb cleaner and compressed air to make sure and clean and clear.

And double check your air cleaner gasket fit etc.

What air filter are you using, size etc?

[Texasdoc]

Originally Posted by Gaz64

Ok, first thing to do is give all the air bleed a spray of carb cleaner and compressed air to make sure and clean and clear.

And double check your air cleaner gasket fit etc.

What air filter are you using, size etc?

Will do these things. Air filter is the normal cobra filter but I have a K&N element, not the standard paper element.

Update: I cleaned the K&N just to be sure. The guys over on racingfuelsystems suggest decreasing the high speed air bleeds from 33 to 30 or 28 to lean out the transition circuit.

They also suggest increasing the base advance and decreasing the total advance. Right now it is base 16, total 37, no vacuum advance. They suggest base as high as stable, total timing around 30 with vacuum advance connected to the ported manifold port.

I guess I could start a new topic over in the Holley tuning section since the high speed miss is gone...

[Gaz64]

I would reduce the size of the idle feed restrictions in the metering block.

Your mixture screws are ultra sensitive, which says the idle jets are too large.

Even getting it to idle correctly, and the transition goes rich says the IFRs are too large.

We aren't talking much here.

If they are say 31 now, drop to 29.

I can't see how making the high speed air bleed SMALLER will lean out the transition circuit.

The smaller HSAB will make the high end richer.

Gary

[Gaz64]

I'm not a member on the racingfuelsystems website, but I have read your thread on there.

I fail to see how tuners recommendation of reducing the high speed air bleed will lean your transition circuit.

You could take out the main jet, and the high speed speed altogether or block them off, and as long as airflow doesn't start the main circuit, no amount of change to the main circuit will change the idle/transition circuit metering.

If the current IFRs have no number, get hold of a pin gauge set to measure them.

Don't muck around with bits of wire.

If they are smaller than your new HSABs of 31, then they are probably 28s or 29s.

Your mixture screws at 3/4 turn, and rich above idle says the idle circuit is still too rich.

A larger idle air bleed in the early 70s may cure all of this off idle richness.

So to me, you need slightly larger IABs, and slightly smaller HSABs for the top end.

Gary

[Gaz64]

Your rear float level is too high causing the rear flooding condition.
The fuel comes out of the boosters not the bowl vent.
Think about what happens to the rear float bowl fuel level near the metering block while heavy braking.
I would also drop the fuel pressure down a bit.

[Texasdoc]

I have the bits on order. Planning on changing the IFRs when I get the bits. I think the recommendation to change the air bleeds was to stop it from going lean (relatively) at WOT. It starts at 12.5 but goes to 13.5 as the RPMs climb.

I'm not going to do the wire thing. Too much variability.

I hate that it takes five days from when I order to get parts from summit or jegs.

I'll change the IFRs first and see where we are.

[Gaz64]

Originally Posted by Texasdoc

I have the bits on order. Planning on changing the IFRs when I get the bits. I think the recommendation to change the air bleeds was to stop it from going lean (relatively) at WOT. It starts at 12.5 but goes to 13.5 as the RPMs climb.

I'm not going to do the wire thing. Too much variability.

I hate that it takes five days from when I order to get parts from summit or jegs.

I'll change the IFRs first and see where we are.

That's correct.

Tuner's words read about decreasing the HSAB to lean the transition circuit.

He writes very well, but I think he mucked up here.

Changing the HSAB affects the high end of the main circuit, irrelevant to the idle circuit.

Certainly will richen the top end up.

Gary

[jhv48]

If it's one of Brent's carbs, he has a guy build them especially for him. I think his name is Scott. When I had to make some adjustments to my carb, I called Scott and he talked me through it. Brent has his number.

[Texasdoc]

I've talked to Brent and Scott. They are both great guys and have been helpful. Scott was thinking I would be into the mains around 1500. He wanted me to decrease the main jets and subsequently increase the PVCRs to keep WOT rich enough. I've done these changes but it didn't lean out my cruise. He doesn't remember what IFRs he put in my carb, so I'm going to have to measure them.

I think we are dealing with different circuits in these threads. My big problem is that I make one change, watch what it does, decide on what change to make next, but then have to wait about a week to get the new parts from Summit. I make that change, see what it does, then have to wait another week. There aren't any hotrod shops nearby to go get parts.

I still recommend Brent (and Scott) for parts and recommendations. But you can dyno a carb on a test motor all you want - until you get it on your motor, in your car, under your weather conditions, you won't know how it will perform. I'm close to getting this running great. A few more changes and this will run like a "smashed cat".

[Texasdoc]

Drill bits arrived. I have .033 IFRs. Now I need to order smaller ones. I guess I can also order some low 70s air bleeds too, just to have in case the IFR change doesn't get it exactly where it needs to be.

[Gaz64]

The TSR goes into the main body and not the metering block.

Adding one into where the blue circle is, is just in series with the IFR for the mixture screw.

The transfer slot will still be fed by the IFR whether it is top or bottom.

[Texasdoc]

Right, the TSRs go into the body opposite where the blue circle is on the picture. I just didn't take a picture of the body.

[Gaz64]

Ok, good to see we are on the same page.

[Bryan Wilson]

I haven't seen a reply as to weather the rev limiter was checked. I have a real bad miss as the revlimiter kicks in.
Cheers,
Bryan