Frustration maximum: Webers

[jstein]

Hi guys - I'm hoping to get a sanity check, and maybe some advice.

My '79SC came to me a couple of years ago with Webers installed by the PO; it worked fine. Nothing spectacular, but idled well and had great throttle performance. It was a bit rough in the transition region (~3300 RPM) while on the track - but other than that seemed to be fine. I didn't know any better as this was my first experience with a 911 motor.

AFAIK this was a "standard" weber conversion.

Last winter the motor was rebuilt and higher compression P&C (domed Mahle's) running at about 9.5:1. DC-40 cams were also put in.

I've never been able to get the car idling and transitioning well since.

I've been playing with jets, attempting to tune, and generally making myself (more) nuts.

I'm currently on travel, and can't recall what configuration I have the carbs in currently - but I've upped both the idle and main jets to compensate for the more aggressive cams; still I have these issues. A sample:

"Idling" is inconsistent - popping, backfiring (probably exhaust leak at the muffler / SSI flange - combined with the rich mixture) and hard to maintain. Sometimes the idle is "solid" at 1200 RPM, sometimes it drops below 1000. All of this with no adjustments.

Off-throttle response is sometimes slow to return to idle.

Transition region is horrible; the whole car stutters when around 3K RPM and the pedal is floored.

Idle-mix adjust screws don't seem to have any effect; even with the throttle idle adjustment backed out almost completely.

So - some direct questions until I can post my current setup:

What would the experts recommend for jetting given my setup? 9.5:1 compression, DC-40 cams, 7degress advance.

Does the idle-mixture screw adjust fuel flow or air?

Does anyone know of a 911/Weber expert in the Chicagoland area I can pay for some help?

Thanks
--Josh

[berettafan]

i'll just toss in a thought while we wait for the experts: if your adjustment screws do nothing you probably have dirty passages and need to remove/clean/rebuild the carbs.

also, are you sure of your timing?

[Steve@Rennsport]

Josh:

First,...you need to find out exactly what venturies, jets and emulsion tubes you have for anyone to be able to help you,...

Second, you need to confirm that your fuel pressure is 3.5psi and that your float levels are perfect.

Once you have #1 (above), call me,.....

Idle mixture screws flow fuel but if your idle jets are wrong, turning those will not help much at all.

[RoninLB]

what I don't get is why is the formula vent exceeded for track cars, especially the 3.0?

afai know there's a limit on how much air a pump can pump.

differeent strokes for different folks!

[Brent Lineker]

Go see George Wetherered at Midwest Eurosport in Joliet at the autobahn raceway.Super place and will only do and charge what is neccesary.Ph.630-595-5577.Good luck,he got my Webers working perfectly.I drove cross country from Chicago to Vancouver averaged 85-90mph and 21 mpgs.

[Steve@Rennsport]

+ for George @ Midwest Eurosport.

[Facey]

mine (46mm webers) got grumpy in the past 6 months w/o any changes, the back 2 cyclinders on the left are now pulling more air then the rest of the engine....

however i did change headers + muffler, and even w/o muffler for a bit...

hard to get back...i'll admit to that, gonna try and find someone local to vancouver island... anyone??? i think i've met a guy who seems very capable...

that being said, my only issues are intiall 800rpm to o-n the floor, it almost feels like it bogs for a second, and then crusing @ 2200rpm off throttle is "poppy"....

...and to think... for the money could have had a year or two hold ricer which gets 30mpg and idle w/o any noise....

[70SWT]

If you can't figure it out in your garage with perhaps the help of a local club/fellow enthusiast buddy, taking 'er into a shop is the only solution. Carbs are simple in theory but Webers are so adjustable that it almost becomes a liability in the hands of those of us who don't work with them regularly.

There's a place called Rolf's in Evanston that supposedly does good work at a reasonable price; this is from a secondhand source, so YMMV. If you can find them online and feel them out over the phone, it may be an option.

[jpnovak]

Josh, I would first do as Steve Weiner said and verify the jetting and venturies you have in the car. While you have them apart give them a good thorough cleaning. This cleaning will include removing the accelerator pump covers and diaphrams and then using compressed air (>80psi) and carb cleaner to blow out and clean every passage and jet. Squirt each passage with carb cleaner, let soak and then blow using compressed air. Carb cleaner on its own will not remove debris.

The mention of no response from the idle mixture screws tells me that you might have clogged idle passage ways. Even if the car is lean you should be able to shut the car down by turning in the idle screws. This might also explain the transition performance since most webers transition to mains about 3-3.5K depending on jetting. Clogged passages could cause the idle circuit to go prematurely lean. It could also be that your idle jets are too small. We can't determine that without getting a good stable baseline to compare.

After the carbs are clean, assemble and start the car. For baseline settings I found that about 1.5 turns out on the air bleed screws and 2 turns out (both from full seat) on the idle screws should get the car running. Now do a quick balance for air and then idle mixture. Don't worry about fine tuning now. What happens when you turn the idle screws in? At idle you should make 1/4 turns (max) in at a time. STart with #1. after 3/4 turn the car should noticeably change idle tone. If you turn the idle screw all the way in and no change you either have very small jets or the circuit is still clogged. Go around and check all six throats this way.

you want to turn each cylinder in until the idle drops and then open back up to the point where the idle corrects. I usually need about 1/8 turn past this point to make sure that the transition point is smoother.

I found that my car does not idle well with the air corrector screws all the way in. It seems to bog down significantly at low revs unless I have them all out a turn and then balanced. I suspect this might be due to the vacuum inertia of moving fuel through the idle circuit.

Once its stable (hitting on all 6 throats) give each throttle shaft a wiggle. Do this on the front by the linkage bell crank and also in the rear by the springs. if you detect a change in idle speed the shafts may need to be rebushed. If no chance the carbs should be nice and healthy and you can go into a finer balance.

[RoninLB]

I know little of Webers but PorschePHD wrote that Weber air correction jets should be cleaned routinely.





And jp
" I found that my car does not idle well with the air corrector screws all the way in. It seems to bog down significantly at low revs unless I have them all out a turn and then balanced."


the bog means the acceleratior is in play, right.
Then the air screw turned in all the way means a richer idle, right.
How did you adjust your acceleration circuit. Did you measure flow or leaned it out till a stumble, then increased it till it accelerated clean?

I like the leaning method. I see higher EGTs that way also.

[Joeaksa]

My '72 had the same problem. Popping is a lean mixture and the rough transition to the higher RPM's can be clogged ports and idle jets. The idle system provides fuel through the transition sequence when the main jets take over.

Check your fuel filters and do what Steve and Beretta recommended. Yes, your knees will get tired but its part of the drill.

[T Bird]

Josh
Your car's symptoms suggest to me that your jetting is way off. The car ran tolerably well with the Webers before the extensive engine mods. Those mods have a drastic impact on the the size of the idle jets, main jets, air correction jets, and venturis, as well as the type of emulsion tube you will need to tune your Webers so that they can meet the requirements of your new motor across the rev band.

No response from the idle mixture screws, slow return to idle and stumbling on acceleration all suggest idle jets that are too small for this application. I don't think it's very likely that you have clogged passages, since you didn't have that problem before and there are 6 idle mixture screws here. How likely is it that all 6 passage ways got clogged at the same time?

As for your high speed stumbling, if the float levels are all set equal and your venturis are properly sized, your mains should kick in around 2500 RPM to 3000 RPM. I believe that the stumbling you are experiencing is due, most likely, to the wrong emulsion tubes, coupled with poorly matched main jets and air correction jets.

Of course, all of this assumes that you have checked everything else, like ignition timing, valve clearances, and filters. You have to eliminate those variables before you start to make adjustments to the Webers.

Unfortunately, I cannot give you recommendations on jets, venturis and emulsion tubes for your specific application. My experience in that regard is limited to 2.4 litre engines. I would suggest after you confirm that everything else is working properly, that you contact Richard Parr at PMO and carefully and completely explain to him the precise modifications that you have made to this engine. His experience is vast and hopefully, he will be able to make suggestions to you that will get you in the ballpark. He can also sell you all the parts and tools you'll need.

After you are done the above, then you are ready to begin the work of setting float levels, adjusting accelerator pumps, adjusting vacuum, setting up side-to-side balance, adjusting mixture, and finally, adjusting idle speed. About 5 years ago, I posted here some very complete instructions for doing these operations. You can search this forum for that post, or drop me a PM and I will send you a Word version.

Good luck.