Not accelate at 5th gear
 

My small block 363 with Quick fuel carb SS750 is good between 1st and 4th gear.
It has good response and stable AFR around 12.2~12.5.
But I am always frustrated at the time I fully accelerate at 5th gear in the racing track.
It reach around 130~140mile, AFR is around 10.0~10.5.It almost not accelerate smoothly. Until 4th gear it has been fine.
I guess it is short of air at the high speed range like over 130miles.
AFR show me "Too rich".
But my carb is 750cfm ,it's a little too big one for 363.(363/2*6500/1728*0.9=614)

I have little knowledges about carb setting.(and English,because I'm Japanese)
What should I do ?

Please someone give me any advice.

Jet setting is kept original from the factory.

Superformance Mk3, 6500miles ,I love driving at racing tracks.

Show us your air cleaner.

Hajime - just out of curiosity, what is your 5th gear ratio and your final drive ratio? That is probably the root of the issue. In many of the current 5 speed aftermarket transmissions like the Tremec TKO, 5th gear is a highway cruising gear and not meant for acceleration. Perhaps you need to downshift to 4th when stronger acceleration is required.

yep..,,,

Your AFR seems low ..it should be 13......if it's toovlean it ain't gonna make power ...might burn a piston though..

Thank you for all reply.
My air cleaner is an oval type.I exchanged it with new one,It didn't improve the issue.
5th gear ratio is 0.68,and Final drive ratio is 3.444.
I drove at the racing track so I needed to upshift to 5th gear.I could't downshift to 4th.
"AFR is low". I also think so.Yesterday I changed main jets to small ones.I'll take care not for braking engine.

https://youtu.be/1jj0ltsJqVI

The air-fuel ratio problem does need to be solved. But here is some more to think about.
My car: 1135 Kg Cobra replica with 6.7 liter Ford Windsor engine, Holley carburetor, many modifications. RPM limited to 6000. 5-speed Richmond transmission with 0.77 overdrive.
On a fast track I hit 6000 RPM in 4th gear at 125 MPH, and shift to 5th gear at 5000 RPM. It accelerates well to 135 MPH but slower to 140 MPH. I think it would take 2.5 - 3 kilometers of straight road to reach 150 MPH. Aerodynamic drag on my Cobra gets very high above 130 MPH.

If shifting to 5th gear in your car drops the RPM enough to lower your horsepower to at or below the power needed for your speed, you will not accelerate. At any speed, the power available from your engine must exceed the drag on your car for it to accelerate faster. If you can safely go to a higher RPM in 4th gear before shifting to 5th, you might have enough engine RPM in 5th to accelerate more.

Thank you for reply,Tommy.
I really understand your thinking.
It's my driving movies in the racing track.
https://youtu.be/1jj0ltsJqVI
I hit 5800 RPM in 4th gear at 120MPH,and shift to 5th gear. It accelerate to 130MPH before the straight end but surely it was slower.
I think I need to be more courage to keep accelerating.

Your problem might be the aerodynamics.
I don't think these cars were designed using a wind tunnel... ;)
Perhaps adding a front spoiler and blocking some of the opening for the radiator might help some.

Hummm.... sounds like the reason that Shelby moved on to the Daytona design...?

Which brings us back to the fundamental Carroll Shelby principle of how to build a car to go faster: More Power!

Hi , i had the same problem with my old tremec the .68 5th gear is to tall to accelerate, you need to change your gearbox to a TKO with the .82 5th gear . Cheers

As several have said, your 5th gear ratio is too deep. It needs to be raised to .82. Depending on which transmission you have it can be done with gears or a gear and an output shaft change. Might be easiest for you to acquire another transmission with the road race ratios. 5th needs to be .82

As these guys have stated, it's what I did, and you won't believe the difference. Lots of sources, here's one.
TKO600RR
New Tremec TKO600RR 5-Speed for Ford, 26-spline input

Agree with the drive ratio discussion. You should decide what is your main purpose for the cobra. If you are mostly racing, you should get the 0.82 5th gear and make your differential gear match your fastest desired speed in 5th gear. If you are going to be mostly on the road/highway, keep the 0.65 5th gear and live with the drop in power in 5th. If you are currently hitting redline in 4th and can't accelerate in 5th, carb tuning or gear changes (transmission or differential) is all you can do to help the poor acceleration in 5th.

But 10:1 ratio is also way too rich.

What is your AFR at cruise (1800 rpm on a flat road)? This is most likely your transition slot tuning.

What is your AFR at 3000 rpm on a flat road? This would be the primary main circuit.

Wat is your AFR at WOT? This would be the primary mains in combination with the secondary mains.

What happened when you changed your main jets? How much of a change did you make? My guess is that if you were at 12-12.5 at cruise and went to 10:1 at WOT you were rich all around and changing just the primary main jets would not help much. Likely you are still rich at WOT and likely normal to lean on mild acceleration.

If you can answer the above questions, we can try to help get your carb working more efficiently. It may not pull much harder at 140mph with your 5th gear being 0.68, but you will get much better gas mileage. :)