TheSamba.com :: View topic - Tuning webers- air bypass screws

Thu Aug 10, 2006 1:35 pm

I have dual 40 DCNF's on my buggy and have a question regarding the air bypass screws. Most of the tuning guidelines I have seen on line are for dual IDF carbs, but I am assuming the synchronization process for the DCNF's is essentially the same.
Most of the onlines tuning guides say to close the air bypass screws first and then synchronize the carbs, but then I never see anything about how, when or why to open these screws back up let alone how to adjust them.

Can someone shed some light on the purpose of the air bypass screws and how/ when to adjust them?

Maybe I'm just not looking at the right tuning tips on how to do this ?
Thanks :?

Thu Aug 10, 2006 2:14 pm

Bypass screws are to synchronize both barrels on ONE carb. When set correctly, one will be closed, one will be open enough to make the two match. The one with the lower airspeed reading is the one you have to open the bypass on.

If the difference between them is less than half a point on the snail scale, it's really not a big deal. All it's gonna do is smooth the idle slightly.

Thu Aug 10, 2006 6:32 pm

mharney wrote: Bypass screws are to synchronize both barrels on ONE carb. When set correctly, one will be closed, one will be open enough to make the two match. The one with the lower airspeed reading is the one you have to open the bypass on.

If the difference between them is less than half a point on the snail scale, it's really not a big deal. All it's gonna do is smooth the idle slightly.

I have used the bypass screw for that purpose. I have used dual 40 DCNFs for over 18 years. As long as you keep them clean, you won't have any problems.

Bypass screw is for that one cylnder on the engine that always seems to lag a littlebehind the rest.

I would not discount a smooth idle, my buddy thought I had fuel injection when I finally settled down and used the idle bypass screw.

Make sure all the internals are up to par and the valve adjustments are in spec. Then set the carb sync and bypass screws.

Thu Aug 10, 2006 7:52 pm

It's a toss-up. Opening those has other effects as well, that are less desirable, dpending on your jetting and how much you open them.

Fri Aug 11, 2006 4:44 am

Thanks for the replies. I had no idea what they were for. Am I correct to assume that this would be the last adjustment made after everything is synch'd up and before the linkage is re-attached to the carbs?. If everything seems OK I should just leave them closed then?

I am having a problem with one cylinder( #2) running a little rough and will try to use this adjustment to even things out.

Fri Aug 11, 2006 7:36 am

After you run through roughly tuning the carbs, you use your synchrometer to find out if both barrels on each carb are the same. If they aren't, you use the bypass screws to adjust and make both on the one carb the same, and then go through the tuning process again. Don't use it to fix rough running. Use it to match airflow. Other things tune out roughness not caused by uneven barrels.

Fri Aug 11, 2006 8:11 am

Ok thanks for the clarification. I'll tune them again this weekend and let you know how I make out.

Fri Nov 26, 2010 5:34 pm

Ressurecting this from the dead:
On a single EMPI HPMX are the bypass screws necessary at all? (it's on an EMPI manifold, I assume it's merged). What do the bypass screws allow past? just air or air/fuel mixture?

I've foolishly agreed to make an attempt at excorcising some of the demons from an evil black hole of EMPI/GEX goodness right off the pages of Hot VW's, still in the middle of cleaning and checking jets and float heights as the best idle (if you could call it that) so far was with the bypass screws and idle mixture screws all closed. :?
I'm curious if I'd be better off closing the butterfly and using the bypass screws for speed or is the butterfly the better choice for idle flow? The manifold is as warm as it will get with the crappy header supplying the preheat tubes but it still runs like the mixture is dropping out of suspension before it hits the cylinders.

Fri Nov 26, 2010 5:54 pm

Do you have the drilled holes in the throttle plates?
The aim of the holes in the throttles and the bypass screws is to adjust so the throttle position is correct at idle.

My guess is, correct would be 1/2 of the first progression hole visible below the throttle. But you can play with it one way or another some if it works better that way.

Fri Nov 26, 2010 6:24 pm

busdaddy wrote: I've foolishly agreed to make an attempt at excorcising some of the demons from an evil black hole of EMPI/GEX goodness right off the pages of Hot VW's

So why drag the rest of us into this black hole, unless you just hate us all? And if you hate us all, why should we help you? :lol:

I'm guessing you have the throttles open so much you have no idle circuit engaged. If the intake is a plenum, this is likely. So to get the idle circuit engaged, you will probably have to either drill the plates, or open the air bypass screws to excess, and hear it wheeze.

Yeah, don't expect centermounts to have good response down low with any semblance of consistency or proper mixtures. It's the price folks pay when they won't pay the price. Manifold heat is key, and it just ain't there.

I'd recommend playing with the air bypass screws first.. open them a little at a time, and drop the idle speed screw a little at a time, and at some point the mixtures should start showing a response. When they do, you know you are headed into the idle range of the throttle position. Once you know this is going to work you can improve your odds of making it work well by putting holes in the throttle plates in a relative centroid of area on the side opposite the progression holes. Start SMALL, like 1/32".

If the intakes are not plenum, then you should be able to do it without drilling plate bleeds. One sure sign that you are needing to do SOMETHING is that the mixtures will not respond, and if your speed screw is screwed in a lot after touching the throttle arm. A lot means more than a couple turns.

Fri Nov 26, 2010 7:25 pm

mharney wrote: So why drag the rest of us into this black hole, unless you just hate us all? And if you hate us all, why should we help you? :lol:
Sorry to drag you all down to my level, I'm not sure what I've done to deserve this but it must have been bad :P Right now I've gotta deal with the cards on the table, hopefully this guy will see the light and buck up for something better one day, but right now I can't let it out of my hands the way it currently runs, it's giving VW's everywhere a bad name. Just finished replacing a chewed up EMPI cam gear in a GEX motor for the guy (second case split in 5 years apparently, summer driver only), even with a full flow filter it managed to spread that liquified gear into every part of the engine, I don't think he's ready for the news on his carb and distributor choice this week :wink:

Just as I suspected, not enough idle circuit, awesome (I guess).
I haven't drilled the plates yet but I'll have a look when I take it off for the full cleaning and float level check, I'll see what kind of manifold it is too. The jets were drilled by someone else 6-7 years ago and the driving response is not that bad (relatively speaking), I'll play with the bypass screws and plate stop screw to try to get it more on the idle circuit before I break out the drill and report back, may not get it done tomorrow though.

Thanks Guys.