2276 48idf jetting issues

[Jimbosmith]

Hello all.

2276
FK47 cam 10.5/1
1.4ratio rockers
1 3/4 4-1exhaust
DPR wedgemated crank
45-37.5 cb super pro heads
Duel 48idf carbs

Current jets
Idle 65
F2 tubes
150mains
200airs
40mm vents

Carbs stripped floats set to 10mm and new gaskets
AFR monitored via Fueltech 02 wideband

Engine pulls hard no stutters or coughs. No back fires I can hear (solid mounts and no sound deadening)
Mixture screws set roughly 1 1/4 turns out for best running.

At idle tuned by ear. 1k idle speed
0.85

3k constant, barely any throttle cruising. AFR 0.76

Seems to lean out when I go wot to a better 0.88-0.90

Any thoughts? Am I chasing numbers here. It pulls super hard and drives fine.

Potential things I’m thinking of trying 42-44mm vents 🤔

Video here https://youtu.be/_AB29ml3GVc

[67rustavenger]

[Jimbosmith]

Yes so translate to roughly 11-11.8 at cruise

5.5mm? Seems low. All the Weber books I have say 10-12mm. I don’t think I’d get 5mm.

[67rustavenger]

[Jimbosmith]

Ha no worries, would love a set of Ida but next step is EFI I think

[Jimbosmith]

Still the same if anyone has any ideas.

Took it to the drag strip and it really sooted up the exhaust. After 4 passes

[madmike]

Try F11 e- tubes and a 55 idle
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[67rustavenger]

Your cruise readings are rich. Lower the size of the idle jets, incrementally, if you have them.

It seems odd that you're getting sooted up exhaust with 150 main jets.

Curious, what ignition system are you running?
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[RandyV]

Surprising that you got sooted up after those passes...but....spark plug is the better tell. Exhaust could just be getting sooted up at any point and your AFR shows that, if anything you're leaning out up top. (plus excessive accelerator pump squirting can juice up exhaust)

As for jetting.....I'd move to at least the following....

60 idle
160 main
200 up to 230 air and you'd probably notice no difference at all unless you kept your eyeballs glued to AFR gauge here so worry less about air than idle & main.

Remember too......you can have high or low AFR at **ANY** idle jet depending on what you do with your mixture screws. Your idle jet doesn't determine your idle AFR...it only determines your lower rpm cruising attitude and somewhat...AFR. (-Conolly)

BTW, motor sounds great and if you start to get frustrated at ANY point playing with this crap....go back to where you feel it runs best and ignore the gage and forums for a bit...you can always come back to it....carb crap can make you crazy! (Don't ask me how I know)

[jpaull]

FYI, Because your car doesnt sputter, or do something horrible, it can still be horribly off. And Seat of the pants only tells you your lean enough to haul ass. Your soot might be from idle circuit, and might be crazy lean everywhere else.

Your combo should be needing alot more fuel then what your mains/airs can deliver as is.

Whats your 4k+RPM full throttle AFR?

I have a more mild combo then you, and it was 15.8:1 afr with your jetting anything 4k rpms and above. It felt fine. I had no idea it was horribly lean till i put a wideband on it.

I ended up with
1.65 air, and 170 main before the AFR got into range with enough fuel, and thats with 38 vents and F2's. (Same 60 idles)
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[email protected] MPH 1/4 Mile & 8.1 @ 83.7MPH in 1/8 Mile with Mild Type 1 VW Mag Case 2234cc commuter engine in stock weight bug w/only .491 total lift(CB2292 Cam), 42x37 heads, 48idf's, Street tires, Belt on, Mufflers, Pump gas, video of the run here: https://youtu.be/M3SPqMOKAOg

Transmission by MCMScott:
Rhino case, Klinkenberg 4.12, Superdiff, 002 mainshaft with 091 first idler. Weddle 1.48 Third & 1.14 Fourth.

[Jimbosmith]

Ignition im using

MSD 6AL2
Pertronix Billet flamethrower distributor
8.3mm plug leads

i have a whole host of jets, so i can try some more combos. I have had a play around. It Just felt weird going down on the jets given how much fuel it should be wanting in the first place hence why im questioning it.

I dont tend to monitor it too much when im just driving around but when i have glanced over i have seen it sometimes as rich as 10.73 AFR

i have a set of Custom 43mm Vents that i could try too but i havent got that far yet.

Here are the plugs after a drive.

[jbbugs]

I run a 2276 in my road race car. Was running an FK43, 48 IDFs with F2, 160 mains, 180 air, 60 idles, at sea level.
It sees a Lot of WOT, and midrange. The poor thing takes a serious beating every time we race.

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[Jimbosmith]

[jbbugs]

I’m running 40 mm vents.
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Patina my ass, that's a Rust Bucket!

[nsracing]

40mm venturis for street? And no stumble? So you must rev to 4500rpm just to get out of your smoke?

[John Connolly]

I ran custom 51IDFs from NS Racing with no venturis, it sounds like a Ferrari. No stumble, massive power.

FK47 isn't a street cam, FWIW.

Try 57.5 or 60 idles. Prob 150-160 mains, 180 airs (to begin with).

The other big mistake a lot of guys do is

plugs too cold, and/or too much plug gap. Don't go over .030" gap (.75mm), even with a high output ignition.

Run NGK 6 or 7s.
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[nsracing]

I remember - them carbs, John.

HEre they are w/ stainless steel custom plates i machined/ precision ground.

[/img]

[Jimbosmith]

Right, i gave up but im back to trying to sort this out.


Here is the current setup


Fuel pressure is checked and perfect

43mm Vents

F11 Tube

155 Mains

200 Airs

60 Idles



So from idle to WOT the engine pulls fine no stutters or flat spots
F11 tubes
Cruise AFR (5-10% throttle input) is reading 11.9
WOT AFR is reading 13.97

F2 tubes were
11.03 Cruise
13.38 WOT


above is with same jets only changed the tubes

Now i tried a 55 idle and a 180 Air on F11 Tubes and the Cruise Afr 12.79 came up and i think i needed a bit more mains as it was leaning out too much. However there was a big lean spot before the mains kicked in. You could power through it quickly but if you rolled the throttle on it hit that spot and struggled.

With the current setup listed above its great through all RPM but i just wish the cruise was a bit leaner. I have a set of 57idles on order. Any other suggestions?

Maybe worth looking at timing?

[Jimbosmith]

From a recent day at the drag strip. Can’t seem to catch a 12sec

[stevemariott]

My best ETs at the drag strip were with AFRs between 12.9-13.1 on the top end.
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