Weber carb problem....

DDS · 07-25-2003, 06:41 PM

My second issue of the evening....

I have a pair of Weber 42dcnf with 34 mm venturies on my 2056cc T4. .465" X 290 degree cam from FAT, ported, big valves, 9:1 CR.

The engine runs great but at idle the accelerator pump jets are continuously pulsing fuel.
Firmly holding the linkage doesn't stop it, though if I lift the pump's actuating lever from the cam on the throttle shaft, it stops.
I am using UAP/NAPA pump diaphragms rather than OE weber. I have compared the two and can't see that it would matter much.

Needless to say, this makes tuning a bit of a problem!

I have checked the little ball valve in the screw holding the A-pump jet in place and it works, and have swapped with another - also old but the ball was free, though I don't think that's the problem. (that check valve appears to have another purpose.)
I'm at the point of filing the cam where the accellerator pump lever sits at idle, but

Any ideas?

Many TIA , Dave.

SGB · SGB's Garage

Its been awhile, but on my 40 IDFs, there is a cam that actuates the accel pump. Yours is just too close. You may have to file a little off the cam or the cam follower. Look in a weber book. Best webber info I know of comes from CB (claude's buggies), look 'em up on the web...

skline · skline's Garage

http://www.cbperformance.com/

Ron Meier · 07-27-2003, 08:40 AM

The other thing to check is your float level and fuel pressure.

The accel pump, as I remember, comes directly off the fuel bowl. If the pump is putting greater than about 3-5 psig to the carbs, it will pressurize the float bowl and feed fuel on it's own, as the "little ball" is nothing more than a one way check valve.

You say the fuel pulses - in time to the pump or in time to the intake of the cylinder.

If your float level is high, it might cause a similiar problem.

Another thing to consider is the vacuum in the area of the discharge nozzles. If it is really high and the "spring" holding the ball into the seat is weak or bent, it could feed fuel on closed throttle.

Wish I had my Weber book handy!

Mark Henry · 07-27-2003, 10:47 AM

Dave, I’ve never had these issues with my DCNF’s so I don’t know exactly what to say about your problem. My Berg instructions say that the pump should be no more than 1 1/2 to 2 3/4 lbs. If using the cheap chrome regulator you must use one per carb, the red Holly is a better regulator.

If you want to scope out my set of Gene Berg 42’s your more than welcome.

DDS · 07-27-2003, 01:29 PM

Thanks for the input, this has been mystifying.
Mike (Pierce?) at pierce manifolds suggested that it may have something to do with the cam overlap, creating a vaccuum/pressure situation causing them to pulse - apparently this happens with certain cams and V8 applications (Shelby Cobras). I should check the firing order on those and see what happens to the opposite butterflies. Apparently the problem is sometimes solved by drilling a 1mm hole on the high side of each butterfly. Sometimes it can't be solved and must be 'tuned around'. Weird.
The fuel pressure has been variable because of a pump failure - maybe two. I have a VG holley regulator, not the little disk thing, and a liquid-filled gauge. I think the fuel pressure is OK, floats are OK and needle valves are OK. The pulses are definiely engine, not fuel pump related.

It is working for now - by switching the pin that provides the fulcrum for the pump to the 'other' position, giving the cam a bit less mechanical advantage on the pump. The jury is still out though as it this has been intermittent in the past.

Mark - it ought to beat your squareback now - when you drove it, it hadn't been properly timed plus the dizzy was loose... What size venturies are you running in your 42s?

Mark Henry · 07-27-2003, 05:25 PM

I knew it didn't have the pep it should.
My Square has a fair bit of zip, it's a 2007cc T1 little powerhouse. Although it's fast, it does run out of gear at about 70mph.

My cam has a fair of lump to it and I don't seem to have the problems you have. I would avoid modifying the carbs till all other avenues have been exhausted.

I think they have the 36mm venturies, 140 mains, 180 air and 55 idle jets. My beetle set also has 2" velocity stacks but the squareback doesn't.
Yes I have two sets.
The beetle set is off at the moment, so like I said you can check them out if you want. Sorry, I never got to copy a set of Berg instructions for you; I've just been too busy and forgot. I also have the float setting instructions as well. I might be down mid-week this week if you want to try to hook up. Unfortunately I will have my little kids in tow so I can't help you tune much.
Let me know, Mark

P.S. your teen is quite a cool ride, it will just take a bit of time to work all the bugs out of it, be patient.

DDS · 09-06-2003, 03:53 AM

For the sake of the archives....
Per the instructions from (Maharishi) Mike at Pierce manifolds, I drilled a 1mm hole in the high side of each butterfly disc, and lo and behold, the accell pump jets stopped pulsing at idle, and the induction became tuneable. Amazing. Add that to your list of arcan weber tuning trivia!

Dave