accelerator pump problem Weber 34 DMSA

[squealingtires]

I've got a weber carb Model 34 DMSA on a '74 Fiat Spyder. I don't know jack about carbs but after looking through the rebuild manuals I was able to get the thing cleaned up and reassemble and now the engine runs.

When I punch the gas the engine hesitates then picks up steam. I narrorowed the problem down to the accelerator pump but can't figure out why its not working. The diaphragm is in good shape and the outlet passage is clear. The pump isn't drawing fuel in when the diaphragm expands (Pump cavity was bone dry when I removed the diaphragm). I read that there is a one way valve in the accelerator pump circuit--is this on the inlet or outlet side--in either case I've not found it yet? Other question is where is the inlet passage. There is a "accelerator pump blanking needle" installed in the upper left cavity..the right cavity is sealed. Is this where fuel comes in? The reassembly process just said to screw the needle in and yet i notice the brass piece has a slight taper to it...sorta like a needle valve????---should this be backed out???

Anyways if anybody has any insight on these Weber carbs let me know.

[manoa matt]

I'm not familiar with the 34 DMSA so my advise will be general.

Are the two round holes on the bottom of the pump housing clogged or is that some funky part of the casting? Have you tried using compressed air to blow out the passages? Maybe prod the passages with a needle or piece of wire to see if they are clogged.

This diagram is from a 32 ADFA, however most weber carbs use the same accelerator pump diaphram. I would'nt doubt it the circuit was the same too. http://www.flickr.com/photos/bigtimes_fiat/3844310488/ Click on the all sizes button to make it big enough to read. Or right click and save it to your computer.

Check that your accelerator pump jet is clear (#46) That little ball in there is also a check valve. I think the check valve you are refering to is labeled #47. The white tapered plug above #47 is a brass plug, you can see it if you remove the cover off the carb. Its pressed in and probably can't be removed. If that valve is clogged you will just have to soak it in cleaner and blow it out.

[squealingtires]

Are the two round holes on the bottom of the pump housing clogged or is that some funky part of the casting? Have you tried using compressed air to blow out the passages? Maybe prod the passages with a needle or piece of wire to see if they are clogged.

The holes are plugged with the casting material. I tried picking at them but nothing was moving so I quit. I've had everthing apart on this carb and blew every hole out I could find. I didn't take the brass plugs out of the side (external) could that be my problem. I didn't have a rebuild kit so I didn't want to remove those little boogers:-) If the fuel doesn't come in by way of the brass screw then it must come in through the exit. Maybe on the DMSA the ball valve acts on two paths. As the diaphragm suctions the ball closes and keeps air out then when the diaphragm pushes the ball blocks the path back to the well. I don't know... guess I'd better take it apart again. I'll post up better pictures tomorrow. Thanks for all the help so far. Making the new guy feel welcome



This diagram is from a 32 ADFA, however most weber carbs use the same accelerator pump diaphram. I would'nt doubt it the circuit was the same too. http://www.flickr.com/photos/bigtimes_fiat/3844310488/ Click on the all sizes button to make it big enough to read. Or right click and save it to your computer.

Check that your accelerator pump jet is clear (#46) That little ball in there is also a check valve. I think the check valve you are refering to is labeled #47. The white tapered plug above #47 is a brass plug, you can see it if you remove the cover off the carb. Its pressed in and probably can't be removed. If that valve is clogged you will just have to soak it in cleaner and blow it out

[swede]

squealingtires,

I have a 34 DMS and there is atleast two diffent diagram that fits mine so make sure that you have the right one, I will try to get technical information on both diagrams for you during next week.

[squealingtires]

Well, as it would turn out, I figured out the problem. The two upper openings were not as I had expected. I figured that since I couldn't push 150psi of air through the holes they must be permantly sealed off. I was wrong---with a thin wire, some carburetor cleaner, the compressed air, and a whole lot of patience, I was able to get the dried deposits out that were causing me problems...thats what 16 years in storage with gas in the system does. Thanks for all of the help...especially to manoa matt---the diagram really helped me out.


I do have another question about jetting. Like i said before I don't know much about carburetors. The two fuel jets in the bottom of the fuel bowl are 140 and 145 and the air jets above the emulsion tubes are 155 and 165. Is it correct to assume that the larger fuel and air jets belong with the secondary since it has a larger diameter?

[So Cal Mark]

usually the larger jets will be in the secondary

[squealingtires]

Thanks everybody...The Fiat 124 lives again!