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Kirkham Motorsports

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  • 1 Post By Bobcat
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  #1 (permalink)  
Old 10-12-2020, 06:41 PM
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Default Different richness left and right sides

Here is a weird one....

Quick fuel Q-750 - set equal on both sides. Donít remember the exact jets, restrictor, air bleeds, etc, but they are equal for each side of the carb.

Eddy Performer RPM intake (not Air-Gap).

Last week at the track I was noticing the car seemed a little down on power. Around the corners while holding a constant throttle (say 3000 rpm while not accelerating) it would stutter/miss. As soon as I got back into the throttle, the miss would resolve and it would accelerate (at about 80% of what Iíd expect).

I pulled a few plugs and noted that the plugs for the driverís side of the carb (plugs 1,4,6,7) seemed much richer than the passenger side of the carb (plugs 2,3,5,8).

Other than perhaps a dirty carb/plugged passageway, what else could be causing this? Am I stuck pulling the carb apart and soaking it in the ultrasonic carb cleaner? I have sprayed carb cleaner down all 8 air-bleeds without any change.
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Old 10-12-2020, 06:53 PM
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Sounds like you need jet extensions and race floats so that the main jets aren’t uncovered Or flooded during hard cornering, acceleration and braking. Fuel is sloshing around in the float bowl and can uncover the mains briefly or flood them with excess fuel during hard cornering and acceleration and braking.
Easy fix.
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Old 10-12-2020, 09:36 PM
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Road race floats if you like cornering hard.
I would also run a larger needle and seat, and slightly less fuel pressure.
Example, standard .110 seat and 7psi produces more foamy fuel than .120 or .130 with 4psi. A return style regulator will also have better control over fuel pressure at the bowls.
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Old 10-13-2020, 06:23 AM
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I have jet extensions (I know) and the appropriate floats (I think). I’m not cornering that hard - just holding a constant throttle in a moderate turn. Fuel pressure is set at 6 psi. I do have a return style fuel pressure regulator.

The fuel pressure and needle/seat combo wouldn’t make it lean on the driver’s side and rich on the passenger side, right?
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Old 10-13-2020, 06:30 AM
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Does that intake have an open plenum or milled divider? I have an old 427 corvette with mostly L88 pieces in the motor including the open plenum, dual plane L88 intake. They (GM) ran staggered jetting in it to better adjust the air/fuel mixture for differences in runner flow. Each barrel in my Holley has a different jet size. Not sure how common this might be in other motors - Seems to be dependent on the intake.

Or could be as simple as a partially clogged air bleed as you mentioned.
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Old 10-13-2020, 06:59 AM
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FWIW , I had the same ( I think ) issue on the track and on the street with my 4150 DP . Long story short ... at 2700 to 3000 rpm at light /partial throttle , you`re where the main circuit is just starting to come in and you`re still mostly on the TS and IFR ckt . What PV do you have ? Most QF carbs have a 6.5 PV .... go to a 9.5 or even a 10.5 pv and see what happens . My guess is that will cure the miss/bog as you roll into WOT. At that point , your vacuum ( as mine was ) probably is well above the 6.5 setting of the PV and you`re actually running lean because the PV hasn`t opened up yet . Don`t believe internet lore about selecting a PV by taking idle vacuum and dividing by half . Get your vacuum reading at cruise and light throttle with very little acceleration and then go under that by about 2 inches . It won`t make you rich at idle as the internet says since it is really not in the ckt at that point .
Also , check your TS setting on primary and secondary . You should have about .025" showing with the blades set for your idle speed .
You may have to play with your TS restrictor , but I`m betting that going to a 9.5 or 10.5 PV will cure it . That`s what I run on the street and track with no issues .
The best way to check your plugs ( my opinion ) is to cut the shell off with a die grinder so you can see the porcelain at the bottom . That`s where you need to read the plug .
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Old 10-13-2020, 07:03 AM
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Could be leaking intake manifold gaskets. With engine warmed up and at idle, close the idle mixture screws one at a time. If the engine does not die, or stumble badly, you are leaking the air past the intake gaskets. Can also check this by squirting carb cleaner along the intake gaskets at idle and listen for a change in idle. And, verify intake manifold bolt torques are to spec, I think 18-20 ft-lb.
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Old 10-13-2020, 08:56 AM
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The biggest issue is that unless the intake has been professionally balanced, each runner will flow highly differing amounts of air. You will almost always see different A/F ratios side to side unless the intake has been balanced.
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Old 10-13-2020, 07:27 PM
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Quote:
Originally Posted by Bobcat View Post
FWIW , I had the same ( I think ) issue on the track and on the street with my 4150 DP . Long story short ... at 2700 to 3000 rpm at light /partial throttle , you`re where the main circuit is just starting to come in and you`re still mostly on the TS and IFR ckt . What PV do you have ? Most QF carbs have a 6.5 PV .... go to a 9.5 or even a 10.5 pv and see what happens . My guess is that will cure the miss/bog as you roll into WOT. At that point , your vacuum ( as mine was ) probably is well above the 6.5 setting of the PV and you`re actually running lean because the PV hasn`t opened up yet . Don`t believe internet lore about selecting a PV by taking idle vacuum and dividing by half. Get your vacuum reading at cruise and light throttle with very little acceleration and then go under that by about 2 inches . It won`t make you rich at idle as the internet says since it is really not in the ckt at that point .
Also , check your TS setting on primary and secondary . You should have about .025" showing with the blades set for your idle speed .
You may have to play with your TS restrictor , but I`m betting that going to a 9.5 or 10.5 PV will cure it . That`s what I run on the street and track with no issues .
The best way to check your plugs ( my opinion ) is to cut the shell off with a die grinder so you can see the porcelain at the bottom . That`s where you need to read the plug .
I love others who believe this theory, and not the internet hogwash.
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Old 10-13-2020, 09:30 PM
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Maybe take off the carb, and take it to a dyno shop where they can place it on a running engine with dual plane intake w A/F sensors in every exhaust header, and see what the carb is doing in a neutral environment ?
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Old 10-14-2020, 11:32 AM
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The Performer RPM is a dual plane intake. I have a 1 inch open spacer between it and the carb. My PV is a 10.5 - it had a 4.5 when I got it. It idles at 14 inHg and cruises at 16-18.

I have my primary TS square. I use the secondaries to set the idle speed - about 950 rpm. There are small holes drilled in the secondary throttle plates to allow the TS to be where they need to be at idle. I do have TS restrictor, but don’t remember what their sizes are off the top of my head.

First thing I’m going to do is take the whole thing apart, clean everything, ultrasonic bath included, and re-assemble. Hopefully its just a plugged passageway.
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Old 10-14-2020, 12:38 PM
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Also could be an exhaust leak on one side.
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Old 10-15-2020, 01:40 AM
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Quote:
Originally Posted by t walgamuth View Post
Also could be an exhaust leak on one side.
No, this shows up as 2 plugs on one side and 2 on the other side as going rich, so related to carburetor fueling into one side of a dual plane manifold, cylinders 1,4,6 and 7.
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Old 10-15-2020, 05:17 AM
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ahah!
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Old 10-15-2020, 03:21 PM
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Open plenum and if you have a spacer an open spacer. Fuel float levels. Check jet sizes, mains and secondary may be mixed up. If it is down on power generally then its something bigger. My best guess.
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Old 10-16-2020, 09:22 AM
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Possible vacuum leak between the carb and the manifold. Two gaskets to leak with the spacer, plus the spacer itself could be cracked.

Pull it and inspect it. If there is no leak evident, reassemble with a different, known to be good, carb and see what happens. Any carb off of a similar cid engine that is tuned well to that engine should be close enough for all but WOT. If you have a wide band just make sure it doesn't go too lean. Fat is OK for a while.
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Old 10-17-2020, 05:48 PM
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I pulled the carb and didn't find anything wrong. Ran the plates thru the ultrasonic cleaner. Will put it together with new gaskets.
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Old 10-18-2020, 02:56 PM
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Quote:
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I pulled the carb and didn't find anything wrong. Ran the plates thru the ultrasonic cleaner. Will put it together with new gaskets.
What floats are fitted? If you have the stock black nitrophyl, then the road race floats will be more suitable.
https://www.holley.com/products/fuel...arts/16-103QFT
The Holley XP bowl also offers better slosh control compared to any other commercially available bowl.
And this issue can only occur with a dual plane intake, so possibly consider a single plane intake.
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Last edited by Gaz64; 10-18-2020 at 02:59 PM..
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Old 10-18-2020, 03:18 PM
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The primaries have a metal float without jet extensions. The secondary side has a black nitrile float with cutouts for the jet extensions.

I was wrong earlier. The carb came with a 4.5 power valve. I am currently running a 7.5, not a 10.5 like I mentioned earlier. My motor idles at 13 and cruises at 15-18. I'll bump that PV up a bit to see if it gets rid of the hesitation part - throttle in the corners.

I have the dual plane Performer RPM but have a 1" open spacer. I haven't really considered a single plane intake.
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