Lean, 1800-2300rpm (carburetor)

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ArizonaGuy
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Lean, 1800-2300rpm (carburetor)

Post by ArizonaGuy »

Fairly healthy little carbureted stroker, Quick Fuel 750 DP. Manual transmission street/strip car.
I’ve put a lot of time and work into tuning, but under low load cruising it goes lean (17-18:1) between 1800 and 2300rpm.
IFRs were .036, that was too fat so drilled new ones to .032

Would going up to .034 on the IFRs solve this without killing the idle quality?
Would you up the IFRs on both sides of the carb, or just the primary side?
Or is it just trial and error?
F-BIRD'88
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Re: Lean, 1800-2300rpm (carburetor)

Post by F-BIRD'88 »

Yes going up .002" on the pri IFR will help a lot.

But fine tuning the PIAB's will likely correct that too lean AFR drift at just off idle.. The PIAB size is just a hair too big. Make it a bit smaller.

I have been right where you are, on a 750 dp HP carb.
Slight correction of the PIAB size fixed it.

What diameter is the PIAB's now?
ArizonaGuy
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Re: Lean, 1800-2300rpm (carburetor)

Post by ArizonaGuy »

F-BIRD'88 wrote: Wed Sep 22, 2021 6:38 pm Yes going up .002" on the pri IFR will help a lot.

But fine tuning the PIAB's will likely correct that too lean AFR drift at just off idle.. The PIAB size is just a hair too big. Make it a bit smaller.

I have been right where you are, on a 750 dp HP carb.
Slight correction of the PIAB size fixed it.

What diameter is the PIAB's now?
IABs were .076. I know those are on the large side, but for my deal it made for real crisp off-idle throttle response (especially considering Vic Jr single plane).
I made some .071s for the primary side and left the .076 secondaries alone. If that doesn’t work, I’ll get into the IFRs.
I’ll post an update once I can T&T.

Thanks for your help
:)
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Re: Lean, 1800-2300rpm (carburetor)

Post by F-BIRD'88 »

The piab sze is what is causing the afr to shift too lean when opening the throttle up from idle
The off idle transition area. right at yiur 1800-2300 rpm range.

Change to a .062" piab.. or solder up and redrill with a .062" bit.. It will now shift rich on throttle opening vs too lean with.076.. Now step by step hone the hole in the piab jet bigger using the drill bit and test afr as you go.


I bet you end up around .065-..069"

You can do this using a wide band or narrow band O²/afr guage. Start at .062". work up by hand honing the jet hole.. You'll see the progress and get it right when the AFR shift stays in the 14:1+/- ratio range on the guage as you open the throttle.
Do the primary side first.

The IFR size is fine. its the PIAB size.
.062" = 1/16" drill bit.. pretty common.

Use the .076" IAB jet. Solder up and re drill 1/16" (.062")
This is your start point to dial in the IAB size.

I use common electronics repair solder and a propane torch to solder up the jet hole. re drilling (.062") and hand honing to size works great. You will get the afr shift much better than just swapping many many jets.
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Re: Lean, 1800-2300rpm (carburetor)

Post by F-BIRD'88 »

I trust that you are adjusting the curb idle mixture screws for best idle vacuum and NOT a AFR guage reading at idle.. This will surly result in a lean off idle AFR shift at 1800-2300 rpm.
stealth
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Re: Lean, 1800-2300rpm (carburetor)

Post by stealth »

Just curious... did you have any ill-effects from the "lean" light cruise AF ratio?

Unless I was having an issue with drivability, like lean misfire, I'd be liking that for light cruise.
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Re: Lean, 1800-2300rpm (carburetor)

Post by F-BIRD'88 »

stealth wrote: Thu Sep 23, 2021 11:11 am Just curious... did you have any ill-effects from the "lean" light cruise AF ratio?

Unless I was having an issue with drivability, like lean misfire, I'd be liking that for light cruise.
What he has got now is way too lean at that throttle point. It will tend to stumble and surge.. You don't save gas when this critical off idle throttle point is that lean.
You want it 14:1 ish +/- range. This is exactly what dialing in the IAB size calibrates.
You won't get this right by playing with the IFR itself.
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Re: Lean, 1800-2300rpm (carburetor)

Post by ArizonaGuy »

Ok, it’s just about dead-nuts perfect now :D

The .071 PIAB was a step in the right direction, just not quite far enough.
Since I already had .036 IFRs (up from .032) I decided to put those in the primary side just to see what it did.
Turns out that combination is damn near right on the number 8)
Low load cruise AFR now dances around the low-to-mid 14s, whereas before (17-18:1) it would buck and surge.

So what I have now is a primary side idle circuit that’s a bit richer than the secondary side.
In the past I’d always made idle circuit changes square; i.e. same calibrations for front and back.
Fattening up the primary side did the trick.

I ended up taking one size jet out of the primary side, so my WOT is still at 12.8-13.2:1.
Win-win-win.
Thanks again fellas
8)
Last edited by ArizonaGuy on Thu Sep 23, 2021 11:54 am, edited 3 times in total.
ArizonaGuy
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Re: Lean, 1800-2300rpm (carburetor)

Post by ArizonaGuy »

F-BIRD'88 wrote: Thu Sep 23, 2021 11:05 am I trust that you are adjusting the curb idle mixture screws for best idle vacuum and NOT a AFR guage reading at idle.. This will surly result in a lean off idle AFR shift at 1800-2300 rpm.
Yes indeed, vac gauge for idle mixtures
ArizonaGuy
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Re: Lean, 1800-2300rpm (carburetor)

Post by ArizonaGuy »

stealth wrote: Thu Sep 23, 2021 11:11 am Just curious... did you have any ill-effects from the "lean" light cruise AF ratio?

Unless I was having an issue with drivability, like lean misfire, I'd be liking that for light cruise.
Yes, it wasn’t happy at all as lean as it was. No misfires, but it would buck and surge pretty good.
I had been blipping the throttle a touch (accel shot) to keep it from doing that.
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