H30/31 dies at stop lights

[busdaddy]

Lets have a little discussion on early carb theory.
I've been dealing with a H30/31 on a 1600 SP that was rebuilt by a reputable VW carb guru, the throttle shaft has no play and the float level is good, it's cleaner than new inside and the engine it's on has recently undergone a full reaseal and tune up, no vacuum leaks, timing issues, etc..., freshly cleaned fuel tank and new filter too, cutoff solenoid works as advertised.

On to the issue... it runs great, plenty of power and no hesitation (even with an 009) but when the idle mixture is set correctly (~3.5% Co) it wants to die when coming to a rapid stop, it won't quit just revving it and letting it return to idle and if you stop really slowly it's fine, but if you stop like you would in traffic and the light suddenly changes it quits, starts right back up but dammit! Richening the idle mixture lessens it's tendancy to quit but even with it set to ~7-9% Co it still makes the idiot lights flicker and occasionally stalls. I'm not satisfied with richening the mixture further and am contemplating the possibilty that it may need a different idle jet. It's currently running a pair of 55 idles (no electrics, just simple jets) and the large volume (speed) screw only requires about 1.5 turns to hit 900 RPM, so here's the question.... would swapping the rear (idle bypass system) jet to a leaner one improve this by requiring me to open the bypass screw more and increasing the flow? or should I go richer and choke it down more (still likely requiring an increase in bypass)?. Or should I be swapping the progression hole jet and leave the bypass alone?

I think VW didn't put too much thought into this particular issue as most of those carbs originally used the throttle positioner/altitude compenastor, decel valve system and if it had one I wouldn't even have to ask this, but since it doesn't and I'm very OCD about my tuning I want to get it just right.

Thoughts?
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[blue77bay]

i have the same set-up as you in my wifes daily beetle ,H30/31 sp1600,i use a svda but otherwise same, i have found a 130 main 50 idle and 65power jet with a 125z air corrector to work well ,i assume you are at sea level, i have also found the running 44-46 deg dwell helps,the book calls for 48-52 if my memory is close ,have you looked at your fuel pressure? i'm still thinking just looked at your location,dont get any more sea level than that!
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[busdaddy]

Sea level indeed, 32' ASL at the smog testing station to be exact. It had a 127.5 main and failed smog on the driving Co miserably, didn't make a bit of difference on the idle issue though

I haven't tested the fuel pressure but it pulls like crazy on long hills and otherwise runs great (love those single ports ), the idle issue seems more like a starvation issue opposed to a too rich condition so I've never explored the FP further. Dwell was in the mid 40's a few hundred miles ago, haven't checked it since.
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[old DKP driver]

busdaddy,
Is this the carburetor that has the (2) pilot jets that is similar to
the 30pict-3. rather than the electric cutoff valve.

what is the lowest CO reading you can get even with the poor
idle condition.
If i remember correctly it was common for the pilot jet orfice
to become egg shaped and not seat. making the mixture screw
limited when trying to adjust it.

just a thought.

Chris
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V.W.owner since 1967

[blue77bay]

when stopping abruptly ,what happens if you do a crash stop in neutral?
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A large hammer collection is like an instant problem solver
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[Tram]

Electromagnetic cutoff jet?
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[Amskeptic]

[busdaddy]

Old DKP, yes 2 separate idle jets but no cutoff on the progression circuit installed, just plain jets, cutoff solenoid on the bypass channel on the left of the carb as usual. I got the Co down to 2.4 by ear before I thought it was going to fail on Hc's due to a lean miss (no meter this time, smog test said 2.4), can't recall Hc's but they were still good. Mixture screw seems to adjust normally (I've dealt with the egg shaped orfice before and the changes are sometimes pretty dramatic for small movements).
Blue77bay, crash stops are what does it the worst, like all the fuel runs to the front of the bowl and leans it for a nanosecond.
Tram, no cutoff on the jets.
Colin, more airflow through the bypass is just what I was thinking, didn't consider a temporary retard to increase flow though, that's why I asked here the screw is in pretty far, it had a loose throttle plate that gave it an 800-900 RPM idle even with the screw all the way in, I haven't tried closing it fully since fixing that but I didn't have to adjust it much from my 2.5 turn initial setting after blowing out/reinstalling the carb.
It may be a few days before I can try your test as the bus is back with it's owner currently, I like where you're going with the test though and should be able to determine if it's lean or rich and how it relates to more or less flow soon.
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[420GOAT]

hey im not as smart as these guys but maybe clean the jets, or even clean the bowl. we are human and the rebuilder may have missed something.
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[420GOAT]

is the choke flopping around?
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[blue77bay]

[quote="busdaddy"
Blue77bay, crash stops are what does it the worst, like all the fuel runs to the front of the bowl and leans it for a nanosecond.
.[/quote]
so rolling along at say 60klm/h in neutral at idle its fine ,then hit the skids and it all goes away? then restarts a few seconds later without problems?
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I like hammers. Lots of hammers. Hammers and me go way back.
A large hammer collection is like an instant problem solver
Mark tucker wrote:,wack off as needed

[jeffsbugs]

I'm with Blue77,

Richen the float setting, then set the others to compensate, ie, more air, and mixture to get the best lean idle. To bad you need to get air care.

Other carbs have problems going up hill so you have to lean them....then when going down hill they suck.

Just a thought.

Jeff
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'73 Westfalia, rebuilt 1700, dual OG solex - 3500 miles and counting
'62 Baja

[Jody '71]

B-Daddy,

Assuming this 1600SP is in an early Bay up to '70, and using the 30-31, wouldn't you have to re-jet it for Bus application? I'm sure Andy would know. He has all of the charts, etc. Getting a 30-31 to work well with an 009 in a 1600SP Bus appears to be a confounding issue here. Are all of the vacuum ports capped off properly on the carb? Are there any vacuum leaks at all between the carb, manifold and at the mainifold to heads?
What fuel pressure does the 30-31 like with your 009 for early Bay Bus? Did you check out the pump's output pressure yet?
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'66 Beetle
2011 Hyundai Elantra Touring

[busdaddy]

[Amskeptic]

[jeffsbugs]

Hmm, ok, so what size jets are you running right now?

idle, power and main jet?

Jeff
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'73 Westfalia, rebuilt 1700, dual OG solex - 3500 miles and counting
'62 Baja

[Jody '71]

You may want to try removing the idle solenoid. Sometimes they get whacked out. But I'm sure you already checked its function. I don't have one on my 1600SP. But then again this is for my Beetle which has the late model German Solex 30-1. You can opt to use the solenoid or not. The solenoid is basically something that shuts off the fuel. I think.
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'66 Beetle
2011 Hyundai Elantra Touring

[busdaddy]

[jeffsbugs]

[busdaddy]