44 IDF Tuning help please ..

[Sandeep]

Hi,

I'm having some trouble with my carbs ... engine is a stock 914 2.0 GA block with the FI cam, 050 dizzy, 44 IDF's and BAS header with dynomax 2 1/2 " Corvette muffler.

The carbs were brand new, out of the box and come with 50 idles, 135 mains, F11 tubes and 175 air.

My problem is the engine backfires at low revs but cleans it self up above 3000 rpm. I did some minor tuning last night and I found the engine runs alot better with the idle mixture screws turned out at 3 turns.

Does this mean I should get a bigger idle jet or can I run it this way ?

Thanks for any input.

Sandeep

[yetibone]

Sounds like the idle circuit is lean, mabye on one cylinder only. Make sure there's no vacuum leak at the base of the manifolds, or foreign debris in an idle jet, progression hole, or idle jet holder.
You could run bigger idle jets, probably not any bigger than .60mm, and turn the mixture screws in a couple of turns.

Yetibone

[petevw]

from the boxer autogruppe thread:

sandeep, take your car to steve at KNT. he is awesome syncing/setting up carbs. you wont be dissapointed.

for example,

on wed. i fired up my car. ran it in for 20 min. then he let it idle and reved a couple times to set up throttle bodies. in about 15 min he had it idling pretty good all by ear!!
i went to his shop the following day to get the airflow meter dual carb sync tool, to dial it in. got back home, threw it on the TB's and in the meter cylinder #1 was flowing just a little more than the rest. the other 3 were dead on.
again all done by ear!!
with my all cammed out, straight gear, flowmastered motor.

in my books he's the man to set up your carbs. he's done many 911's and vdubs.


later,
pete

[Wally]

Sandeep,
Like you, I aslo think your idles are too small. 55 is probably better/best. Assuming you have the standard 36mm venturi's, 135 mains also seem small. Maybe 150 and sligthly bigger air correction, like 190.
The 44IDF's in my view are a little overkill on a stock 2 liter anyway. You will have better torque below approx. 3500 rpm with 40IDF's with 32 mm venturi's.
Just my opnion though, I'am no expert, but you asked. I also believe the dyno, combined with a stack of jets, is the sheapest way to gain power.
Walter

[Sandeep]

Thanks for the replies !

Of course ! Steve at KnT .. Called him this morning to set something up.

Should have this thing solved by the weekend. I'll post the results.

Wally,

The 44 IDF's are great for this motor, I can lay scratch in 1st and a little chirp in 2nd.. and I can rev up to 5500 rpm and the motor is still pulling. If I was staying with the 2.0L I would have got the 40's but the plan is to go bigger next year with the engine combination.

Sandeep

[Wally]

Sandeep,
That sounds very good. I guess I have underestimated the versetility of the 44's !
Like to hear your results. Especially the jets and all. I'am curious what would work best for that engine.
Greetings,
Walter

[Ephry73]

I'm having the same problem with my 1.7 engine and 40IDF carbs with 32mm vents and 44IDF velocity stacks... I have 125 mains, 55 idles, and 200 A/C's... I will change to a bigger 135 main, leave the airs as is, and get the 57.5's on the idles, see how it runs. Air bleed srews all the way in....

i may have to get bigger airs as well? anyone?



Ephraim

[Sandeep]

I have 50 idles, thats the problem. I turned out the mixture screws to 4 turns (Yikes ! hope they don't fall out !) and it runs even better under 2500 rpm.

I have ordered 57.5 idles from AC.net (buggered up one 50 idle trying to enlarge the hole) :rolleyes:

I'll post results when I have them.

Sandeep

[Sandeep]

Got the 57.5 idle jets from ACN yesterday. I installed them and the engine runs ALOT better. No stumbling now and the idle mixture screws are at 1 1/2 turns from bottom.

I guess that solved the problem.

Sandeep

[Ephry73]

I went with a 140 main and still a little popping at idle. Will change to a 135 main and 57.5 idles, see how it runs.



Good to know your engine is running


E

[Ron Roberts]

How will changing the main have an effect on the idle?

Ron

[Ephry73]

nothing. what i meant to say was that the 140 is a bit on the rich side and l'm switching to a 137 for it better suits the engine. the 57.5 idle jet is what i needed anyways. Thanks for helping me clarify that.


E

[Ron Roberts]

I see. I know it might have sounded like a snooty question, but actually I had been wondering if maybe at WOT the mains were being used even at a low RPM. I even e-mailed John C on the matter because of something I read in his jetting article. Anyway I'm clear on it now.
Ron

[Ephry73]

John's the man when it comes to jetting. Now question about it. he's helped me out inmensely with my engines and with jetting. The jetting he recommended worked fine, but i'm not sure if it's a combination of the FAT system(exhaust) leaking or the tall velocity stacks and manifolds and the CR of the engine with the platinum plugs that was making it max out at 3/4 throttle. It just seemed to not go faster after 85MPH. As soon as I changed the jets to 140's it was going the same speed at 1/2 throttle. So go figure. As he himself said, it's not so much a science as it is trial and error with your individual engine. I will go back to the 125's when i put the copper plugs in, and see how that runs, but I know for a fact my idles are too small for the engine.


With the 32mm wents I have I should be aiming for a 137.5 main, so we'll see


E