2x40IDF jetting help

[bcrazy]

Need some jetting advice for my engine.

Carbs are 2x40DIF with 36 mm venturis.
I am waiting for smaller venturis, but for now i need to jet it to run ok with the venturis i have.

Jetting is now
60 idle
160! main
F11
Air 200

Engine pulls fine to 3500, but then starts to stumble/misfire.
I went up on the mains from 150 to 160, and it helped some.

I think maybe the 36 mm venturis, are making jetting harder..


Engine:
Porsche 912 1720cc
Webcam 86a (290deg)
Stock ported heads 38x34
1.5/8 4-1


Advice needed on how to get it to run ok, until i get my 34 mm venturis. (or should i go for 32mm?)

[modok]

I never tried vents that big.
But if bigger mains actually did help, smaller airs will help too.
I'd get the 32 vents since the engine is pretty small cc.

[vwracerdave]

I'd get 32mm venturies.

Another part of the problem is that the 1 5/8" exhaust is too big for your engine.
_________________
2017 Street Comp Champion - Thunder Valley Raceway Park - Noble, OK
2010 Sportsman ET Champion - Mid-America Dragway - Arkansas City, KS
1997 Sportsman ET Champion - Thunder Valley Raceway Park - Noble ,OK

[tpnuckels]

28 vents are stock for the 40 idf.
I've got 4 if you want them.

[[email protected]]

your plugs are fouled from the 60 idles; get 50s in there pronto.
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[bcrazy]

[bcrazy]

New jetting today:

32.5 mm venturis
140 main
50 idle
200 air
F11

Runs great from 2-6.5k.
Not much power before 1500 rpm but i guest that is because of the cam.
Midrange from 3k is great.

[[email protected]]

Mains should be ~125-130
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[bcrazy]

Have now installed a wideband sensor.
WOT is around 12.5:1 from 2k-4k
Then after 4K it starts leaning out to 14:1 at 6.5K
And i can not get it to idle properly, seems to need down to 11:1 to get a smooth idle, but still it suddenly pops (like it's lean) and seems to stall, but sometimes pics up the revs again, sometimes stalls. The carbs are clean, and so is the fuel filter.

Jetting is the following:
Venturi 32.5
Idle 55
Main 130
F11
180 Air

The idle is what really annoys me. Idle can be rock steady at 1000 rpms for 20-30 seconds, then it suddenly pops in the carbs, and the engine stalls

If i try to adjust idle to about 13,5:1, i need to adjust the idle speed screws 2.5 turns in.

Is my cam to big for a decend idle???
Should i go down to 160 on the air?

[jfats808]

[bcrazy]

[modok]

How was it with the 140 main 200 air? Just curious really.

I wonder if the uneven idle could be you distributor. Advance moving around at idle? It ought to have something like 8-10 deg steady at idle speed. Worth checking if you have not allready.

[bcrazy]

Did not have the a/f meter then, but it seemed to run better at high revs.

My timing seems to be steady, altough i have not checked it for consistensy over time at idle. Will do that.

[pdub]

Vacuum leak? Exhaust leak?

[bcrazy]

Turned out my gas tanks was full of rust.
New cas tank in now.

Still need more advice

Jetting:
33.5 mm venturi
55 idle
130main
150 air
F11
Timing is set at 32-33 deg

I have gone down on the air from 200 to 150 because it leans out above 5k. I tried a 125 main and it leaned out even more on the top end, and part throttle was terrible, but wot from 2-4 was 12.5-13:1

Wideband readings.
1/4 throttle cruise between 2-4k spikes to 19.5:1(meter maxed out) and pops and spitters.
2/4 throttle is aorund 14:1

WOT
2-3K, around 12:1
3-4: 12.5-12.8:1
4-5K 13.5:1
5-6.5K: 13.7-14:1

My biggest problem is that it leans out and sputters at low throttle cruise.
I have tried more acc pump, but it only helps some, It will sputter at STEADY throttle 1/4 basically at whatever rpm's

And high rpm's goes lean, it was 16:1 with the 200 airs.

It does not idle so good either, needs 2 turns on the speed screws for 1000rpm's, but the mixture screws are set at 1.25 turns out.(this reads abouts 12:1), if i try to lean it out, it starts spitting and popping trough the carbs and will eventually stall)

It does however pull GOOD from 3.5K and up.

I'm thinking now to try some F7's and start over?

Any ideas?

Float are set as per instructions on this site

http://www.rescue912.com/spider/WeberIDF_Setup/

11, 32 mm

[modok]

First question that comes to mind for me is--
Was it lean at 1/4 throttle with the 140 mains?

[bcrazy]

[modok]

Ok, well here's what I'd do.
Go up to 135 main/or down to a 32 vent(you typed 33.5, typo?)
Solder closed the four lower holes of your f-11 tubes, this turns them into f-15 tubes, which lean out less at higher r's.
then try going back up to 180-200 air jet

Hopefully the f-15 tubes will allow you to use a larger air without leaning out the top end too much, should do the trick.

I should also mention that leaning out to 14 afr at 6000+ can be ok, some engines like this. In many cases an engine will want 13-1 at 3-4k rpm, then as revs rise lean out to 14-1 at redline. Works for me.

I do hope you get it tuned in, if all else fails I am sure 30 vents would make it possible, big vents are more trouble!

[[email protected]]

float should be 10mm closed, not 11.
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[74 Thing]

Re-set floats and double check fuel pressure and timing and valve adj and check for ex leaks and carbs synchd then start over with new readings.