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My dual carbs "stumble" when I accelerate. Fixes??
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Skulptorchaz
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PostPosted: Tue Jul 30, 2019 8:19 am    Post subject: My dual carbs "stumble" when I accelerate. Fixes?? Reply with quote

I mentioned this in my other thread but I thought I'd try a new thread as well.
I have EMPI 40's (I know but, it's what I have) on my 1915 with 110 cam and 1.25 rockers.
When I hit the gas, it has a slight dead spot and then comes on like a scalded dog. I was told it could be the pump, and I could adjust that. (I'm not sure how) or I may need a larger jet. My jets now are - Main 115, air 220, idle 57.
I am assuming it is the carbs, as I understand they have a tendency to do this but, I guess it could be something else.
Thanks gents.
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MrGoodtunes
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PostPosted: Tue Jul 30, 2019 9:51 am    Post subject: Re: My duel carbs "stumble" when I accelerate. Fixes?? Reply with quote

You probably mean whenEVER you accelerate. But if it's only when you take off from idel in 1st gear, then try opening up your idel circuit volume adjusting screws. (Do EMPIs have this? My Brosol 32s do.) It the stumble happens any time you accelerate, then check your accelerator pumps. Mine have 3 notches for adjusting spring tension to get more or less squirt. You've probably already mess'd with timing and dwell; if you have a different distributor available, give it a try.
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PostPosted: Tue Jul 30, 2019 4:36 pm    Post subject: Re: My dual carbs "stumble" when I accelerate. Fixes?? Reply with quote

I don't have a lot of experiance with this but here is my set up for comparison.

1911cc engine

Dual 40 Webers (not Empis)

47 idles

31 vents

127 mains

205 airs

14* initial timing

Very mild street cam

32 mpg at the moment and I have just a very very slight hesitation off idle if you are in first gear and accelerating very gently. Plant it medium or hard and it's not there.

Your main jets will have no influence on this at all but idle adjustment, timing and cam duration will.
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Dale M.
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PostPosted: Tue Jul 30, 2019 7:13 pm    Post subject: Re: My dual carbs "stumble" when I accelerate. Fixes?? Reply with quote

What is your distribuitor...

Dale
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PostPosted: Tue Jul 30, 2019 10:14 pm    Post subject: Re: My dual carbs "stumble" when I accelerate. Fixes?? Reply with quote

You got some reading to do:
https://www.thesamba.com/vw/forum/viewtopic.php?t=572107

In my experience hesitation things to check with dual carbs:
operating temperature (cold engine hesitates due to fuel not atomizing but dripping)
vacuum leaks (my HPMX's main throttle shaft leaked and could not be fixed)
distributor (like Dale said, what is it?) timing, advance
idle jets
accelerator pumps (I added a few turns to the pull bar nut on my HPMX's to get an earlier shot of gas)
balance of linkage (different than idle vacuum balance)
mixture screws adjusted poorly (not even, lean idle)
clogged jets/progression holes for fuel

Eliminate one thing at a time or you'll never solve it completely.
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PostPosted: Wed Jul 31, 2019 5:31 am    Post subject: Re: My dual carbs "stumble" when I accelerate. Fixes?? Reply with quote

I'd start with the distributor as Dale mentioned. My bet is you have a 009 which are well known to have a stumble off idle. The next would be to crack the idle circuits open just a smidge.
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Skulptorchaz
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PostPosted: Wed Jul 31, 2019 2:42 pm    Post subject: Re: My dual carbs "stumble" when I accelerate. Fixes?? Reply with quote

THANKS GENTS!!!
I will admit, I am like most in that I like to "discuss" my problem at the time as I think it helps me understand better when someone can answer the questions "I have" but, having said that, THANK YOU YDBd! That is a great thread to refer to. I just breezed it but it looks to have a bunch of info I can glean before I fire off questions.

I can do that tonite Letterman but, forgive my inexperience but exactly how do you "crack the idle circuits open just a smidge"? thanks.

Dale, my guess is 009. It's mechanical advance.

Thanks OPRN.

I'll see what I come up with tonite.
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PostPosted: Wed Jul 31, 2019 2:59 pm    Post subject: Re: My dual carbs "stumble" when I accelerate. Fixes?? Reply with quote

It could also be a linkage problem and both carbs are not opening up exactly the same. The linkage must be perfect for correct idle to 1/4 throttle. After that it's not so much a problem.
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PostPosted: Wed Jul 31, 2019 3:40 pm    Post subject: Re: My dual carbs "stumble" when I accelerate. Fixes?? Reply with quote

Thanks Dave. I got that bit covered. I also have them synced. I gotta look a bit deeper now.
'preciate it!
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PostPosted: Wed Jul 31, 2019 5:31 pm    Post subject: Re: My dual carbs "stumble" when I accelerate. Fixes?? Reply with quote

Smile Just turn the idle screws a 1/8 turn or so - might not even need that much. Your distributor will have a plate on the side with the model number - get that for us.
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PostPosted: Wed Jul 31, 2019 6:45 pm    Post subject: Re: My dual carbs "stumble" when I accelerate. Fixes?? Reply with quote

OK, I can mess a little with the mixture screws but they should be pretty close. I was thinking you might have been talking about something else. (???) Sorry. I am interested in tweaking the accelerator pump a bit as friend of mine (who has been racing VW's for 40+ years) told me over the phone that it might be getting air a bit faster than the pump was squirting fuel. (???) He said that can cause it too. I can only "assume" that means screwing the nuts "in" towards the engine...... but with the spring pushing on it, I don't know.

The numbers are stamped on the dist, housing and end in 009.
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PostPosted: Wed Jul 31, 2019 7:59 pm    Post subject: Re: My dual carbs "stumble" when I accelerate. Fixes?? Reply with quote

Your jetting looks off. I would try 52/130-135/210

Is the stumble transitioning from 2500-2800 or when you goose it?
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PostPosted: Wed Jul 31, 2019 9:26 pm    Post subject: Re: My dual carbs "stumble" when I accelerate. Fixes?? Reply with quote

It has a “slight hesitation” or some say “dead spot” when you very first hit the throttle, then it comes on strong. It doesn’t necessarily have to be from a stop. I was wanting to start with the easy stuff first - adjusting, etc.
I have swapped jets a while back and it helped but, it’s just not there yet.
Thanks!
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PostPosted: Wed Jul 31, 2019 9:33 pm    Post subject: Re: My dual carbs "stumble" when I accelerate. Fixes?? Reply with quote

The threads "showing" on your accelerator pumps should be 1/4" as a start. Adjust them there and try ......
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PostPosted: Wed Jul 31, 2019 9:47 pm    Post subject: Re: My dual carbs "stumble" when I accelerate. Fixes?? Reply with quote

[quote="Letterman7"]I'd start with the distributor as Dale mentioned. My bet is you have a 009 which are well known to have a stumble off idle./quote]

I'm going to reemphasize this.

You may tinker with the carbs all you want but your distributor does not have a smooth advance curve off idle. The remedy is to get a better distributor.
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PostPosted: Thu Aug 01, 2019 11:11 am    Post subject: Re: My dual carbs "stumble" when I accelerate. Fixes?? Reply with quote

Got it. But before I go buying a new (or new to me) distributor, I want to make sure everything is adjusted and tweaked correctly.
BTW, can a 009 be made to work correctly? Can lubing it up or (???) help?
I will keep my eye out for another distributor. I'd be interested in what people think are good ones.
Thanks YBDB.
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PostPosted: Thu Aug 01, 2019 6:50 pm    Post subject: Re: My dual carbs "stumble" when I accelerate. Fixes?? Reply with quote

I assume the carbs are the Kad clones. Worked with Kads a lot, so making that assumption. Typically with a mechanical, I find that they like the timing around 32 max. And I tend to set up that accel pump for a little stronger shot. If you are seeing black smoke when you hit it, then, you have gone too far and its too fat.
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PostPosted: Thu Aug 01, 2019 7:34 pm    Post subject: Re: My dual carbs "stumble" when I accelerate. Fixes?? Reply with quote

Distributor. The 009 was made for idle and 3000 - 3500 RPM, for industrial engine applications like saw mills, air compressors, fire engines where the engine runs at a steady state. It was not made to transition the engine up and down in RPM.

Some people get them, (009) to work decently by compensating in other areas like opening up the squirters, over advancing the idle timing, restricting air bleeds, over enriching idle circuits and a few other tricks. But it is a compromise, which leaves other performance areas on the table. Each engine is different, and if you want to spend time experimenting and buying extra squirters, jets and so forth to find where the engine wants to use the 009, it can be done. If not, get a Distributor made for variable RPM and close the book on the stumble.
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PostPosted: Sun Aug 04, 2019 7:00 am    Post subject: Re: My dual carbs "stumble" when I accelerate. Fixes?? Reply with quote

I've been playing with jets on my 1904 as I too had a stumble. I've ended up with. 55/140/f11/190.

Have you eaten it your a good run and pulled a plug to see how it looks? With 55/135/190 my plugs were very light tan, bordering white. Stepping up to 140 got rid of the stumble and better colour

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PostPosted: Fri Aug 09, 2019 5:22 pm    Post subject: Re: My dual carbs "stumble" when I accelerate. Fixes?? Reply with quote

If you do replace your 009 distributor with a vacuum advance type make sure you have a vacuum source for the vacuum line. Some dual carb setups do not have this provision without modification.
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