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Jetting Update


brianstj

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I was having idle issues on my new motor running more compression/displacement/cam/exhaust with a 38 DGES Weber. I set the timing to 25 degrees BTDC @ 1500 RPM per recommendations, then I bought a jet kit from Redline. Since my carb settings were indicating really lean running, I went up 2 steps on te idel jet to start with - went from 45 to 55.

Now my idle mixture and speed setings are much closer to ideal, and the car is running MUCH smoother - especially below 2500 RPM. Before the car was pretty much useless below 2500 and I thought this was just life with the bigger 292 cam, but as it turns out it is possible to get a really smooth idle and good low end power with this setup.

I still need to do a little fine tuning and complete the whole jetting process, but so far things are starting to look much better already.

Brian

'72 2002

'91 325i

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Guest Anonymous
I was having idle issues on my new motor running more compression/displacement/cam/exhaust with a 38 DGES Weber. I set the timing to 25 degrees BTDC @ 1500 RPM per recommendations, then I bought a jet kit from Redline. Since my carb settings were indicating really lean running, I went up 2 steps on te idel jet to start with - went from 45 to 55.

Now my idle mixture and speed setings are much closer to ideal, and the car is running MUCH smoother - especially below 2500 RPM. Before the car was pretty much useless below 2500 and I thought this was just life with the bigger 292 cam, but as it turns out it is possible to get a really smooth idle and good low end power with this setup.

I still need to do a little fine tuning and complete the whole jetting process, but so far things are starting to look much better already.

You should have gone for 60's at a minimum and try reducing size of mains by 1/2 step.

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I live at 1800 ft., will see regular driving between 100-2500 ft. I am curious on the recommendation of a minimum 60 idle jet and reducing the size of the mains. What is this based on? I was actually expecting to have to increase the mains. What I am currently running is:

185 Air

142 Main

55 Idle

<2500 RPM operation is smooth and strong, idle is smooth, and the idle adjustments are close to the Redline baseline settings. I don't seem to be having any transition stutters and I am pulling pretty good in the upper RPMs - though I haven't flogged it too hard yet since the motor is pretty fresh. I am planning on giving it some hard pulls and playing around with the mains to see where I will end up.

I am still having an issue with the idle sticking at around 1500 when I come to a stop. I am still confident the linkage is closing fully, so I am thinking I need to make another pass at searching for vacuum leaks. My other suspicion is the distributor since I am just running what I had before and the history is unknown. Do these respond well to disssembly and cleaning? Any other suggestions where to look for issues?

Brian

'72 2002

'91 325i

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Further update -

So far with just the idle jets changed on the 38/38...drove the car to work today AND - there was no more "surging" at freeway speed, no more exhaust popping when coasting down hill, and no more dieseling when I shut the engine off.

Still some room for improvement - especially for the sticking idle - but things are getting better :)

Brian

'72 2002

'91 325i

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Thanks for the update.

Yeah, look at the mechanical advance under the breaker plate.

You have to really tear the dizzy apart to get to it.

When you do, you're looking for sticky weights, corroded springs,

worn pivots, etc.

If it's teh distributor, the timing light will 'jump' when the idle does drop below

1500... but without a real set of timing marks, it's really hard to tell...

HTH,

t

"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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Guest Anonymous

The 60 suggestion is likely for sea level operation. That's what I use too - at sea level.

As far as the other jetting if you are satisfied with the performance then maybe you have found the sweet spot. Unfortunately, the only way you will know for sure is to try larger mains 145 or so. But, as has been said so many times: "when in doubt, lean it out."

Your fast idle makes me wonder if you don't have a serious vacuum leak or an automatic choke that is maladjusted so the fast idle cam fails to disengage. I don't think there is a problem with your distributor, although it is a possibility. If the distributor advance were jammed so that your timing was "preadvanced," I think you would know it by all of the pinging that would ensure during even moderate acceleration.

Good luck!

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I threw in some 150 mains this morning and it seemed to feel a little bit stronger on the hard pulls. But now I have to back off playing with the car too much since I got popped for speeding this morning while trying out all these changes :( I guess I built the thing too damn fast... ;)

Brian

'72 2002

'91 325i

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Guest Anonymous
I threw in some 150 mains this morning ( I guess I built the thing too damn fast... ;)

Maybe you did not build it fast enough, otherwise they wouldn't have caught you.

Jetting, as you have discovered, is a combination of several things, including your seat-of-the-pants judgment. Consider pulling your plugs and reading them after the various jetting changes. They may tell you whether your jetting changes result in running richer or too lean. Bear in mind richer may warrant more timing and vice versa.

There is a happy medium - but some never find it.

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