Weber 34 ich - Problems running rich

MFC · May 11, 2017

Ok Latest update, Had a look at the basics, Valvle clearances checked (number 1 exhaust was very loose .38mm) others no too bad. Had a look at the static timing which I thought was ok but it was out and have now advanced by about half an inch (maths for degrees). Also something has been bothering me, there is a pipe which must be part of the emmisions control sytem which goes from the rock cover cap to an inlet manifold connector under the carb (i think on the adapter plate) seems to be a permanent vacuum sucking air and bypassing the carb hmmmm would have thought that would make leaner????????
So confirmed no valve at either end so have blanked it off (might have been a later mod etc)
Any thoughts on the above - so all I have now is the rocker cover venting into the air intake elbow which sort of makes sense... or?????
Thanks again all those who have put ideas forward.
Have ordered service kit and float in any case!!!

JimS · May 11, 2017

I would also expect the rocker breather to go to the air intake not straight into the manifold, as you have it now, the other setup sounds odd.
I've just rebuilt a couple of dellortos on another project, I was amazed at the effect the old fuel had had on the inside, they were severely gummed up.
It would probably not hurt putting the carby on a bench and servicing / inspecting

MFC · May 11, 2017

JimS said:
I would also expect the rocker breather to go to the air intake not straight into the manifold, as you have it now, the other setup sounds odd.
I've just rebuilt a couple of dellortos on another project, I was amazed at the effect the old fuel had had on the inside, they were severely gummed up.
It would probably not hurt putting the carby on a bench and servicing / inspecting

Thanks for you comments, I also thought is was odd. have already had to bits but when the service kit comes will go through it all again!

puddlejumper · May 11, 2017

If your breather has been going into the carb bas plate then you would be sucking all sorts of crap into the engine. That outlet on the carb base plate is for the vacuum pipe from the servo. Have you been using a lot of oil by any chance??

MFC · May 11, 2017

puddlejumper said:
If your breather has been going into the carb bas plate then you would be sucking all sorts of crap into the engine. That outlet on the carb base plate is for the vacuum pipe from the servo. Have you been using a lot of oil by any chance??

No amazing oil consumption by always hard to say v leaks etc. Thanks for the clarification i thought it was odd, must be a later S3 manifold

puddlejumper · May 11, 2017

Could be that is what has been "sooting" your plugs though......

Tall Ratbag · May 12, 2017

puddlejumper said:
Could be that is what has been "sooting" your plugs though......

A few thoughts / possibilities;

- If there was a large manifold leak caused by the fitting on the adapter plate which you've since blocked, the throttle would have to be opened a long way to get the engine to tick over. This could cause the idle circuit of the carb not to be activated by the proximity of the throttle plate to the idle / progression drillings. This would explain the idle mixture screw not having an effect. When the carb is set up to allw the engine to tick over, how much clearance is there between the edge of the throttle valve and the barrel of the carb? On my engine, the throttle valve is all but fully closed, and would need a feeler blade to check the clearance. If the throttle is much further open than this, the idle circuit won't be working properly.

- One part of the carb which could give a rich mixture is the economy valve located in the upper cover. You mentioned that the drillings in the carb are all clear, but, for this valve to correctly weaken the mixture under part load, the passage from the base of the carb, through the top cover gasket and round to the back of the diaphragm must, as well as being clear, must be leak free. This would be difficult to check - I would use a mityvac. But if the top cover gasket is OK, and the economy diaphragm is OK, and the passages are clear then, it should be OK.

- I'm not sure of the threads for the main jet and the accelerator pump check valve are the same. Could these have been assembled the wrong way round at some point in the carb's history? The main jet should screw into the threads at an angle, in line with the emulsion tube and air corrector,, while the cheack valve is fitted vertically.

As per puddlejumpers idea, is the soot on the plugs dry, or wet?

MFC · May 13, 2017

OK the up

Tall Ratbag said:
A few thoughts / possibilities;

- If there was a large manifold leak caused by the fitting on the adapter plate which you've since blocked, the throttle would have to be opened a long way to get the engine to tick over. This could cause the idle circuit of the carb not to be activated by the proximity of the throttle plate to the idle / progression drillings. This would explain the idle mixture screw not having an effect. When the carb is set up to allw the engine to tick over, how much clearance is there between the edge of the throttle valve and the barrel of the carb? On my engine, the throttle valve is all but fully closed, and would need a feeler blade to check the clearance. If the throttle is much further open than this, the idle circuit won't be working properly.

- One part of the carb which could give a rich mixture is the economy valve located in the upper cover. You mentioned that the drillings in the carb are all clear, but, for this valve to correctly weaken the mixture under part load, the passage from the base of the carb, through the top cover gasket and round to the back of the diaphragm must, as well as being clear, must be leak free. This would be difficult to check - I would use a mityvac. But if the top cover gasket is OK, and the economy diaphragm is OK, and the passages are clear then, it should be OK.

- I'm not sure of the threads for the main jet and the accelerator pump check valve are the same. Could these have been assembled the wrong way round at some point in the carb's history? The main jet should screw into the threads at an angle, in line with the emulsion tube and air corrector,, while the cheack valve is fitted vertically.

As per puddlejumpers idea, is the soot on the plugs dry, or wet?

OK first, thanks for the very detailed response above. I have had all the carb to bits and checked all the diaphragms etc and blow through what I could, but had ordered a service kit and would change everything anyway as the carb is seven years old. So still waiting fr the service kit, so decided to look at you other points. I was 99% convinced that the idle screw was as per factory instructions ie no more than 2 turns our and had checked this many times over the last week. But today had another look, this tims air filter off and linkage off (weber instructions say to exercise the linkage to ensure at minimum which I had done). BUT with the linkage off I founf I could push the butterfly further back, and back until it was really closed, then two turns out.
Engine refused to start (of course) and with a bit of playing resetting mixture to two turns out etc etc got the thing ticking over. Still very sceptical took for the standard 5 minute round trip to ensure plugs were soot black etc, arrived back and all looked nearly normal!!!!!! unbelievable.
So it looks like when the weber was first put on, the servo connection that formed a bypass made the chap who fitted it turn up the idle screw, which over the years became the norm, having blanked the vacuum off, let the idle circuit do its thing it seems to work.
So a longer test is needed, but progress for the first time, so a big thank you for the lead on this one and the time you have taken (and all who have contributed) Thanks again and will report after longer test drive etc etc

Weber 34 ich - Problems running rich

MFC

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JimS

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MFC

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puddlejumper

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MFC

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puddlejumper

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Tall Ratbag

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MFC

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