I have just rebuilt my weber 45 and am having some trouble getting them to idle. I have the type of Webers with the air idle bypass valve.

The book says to leave the idle bypass valve closed to begin with when setting up. Wind the idle mixture skrews out by 11/4 turns and set the idle speed so it will run. Then balance the carbs after and adjust the idle mix.

I did this but found that I had to have the idle speed very high to get it to run. It was either high or die.

I read dave walkers book on engine management and he talks about this very problem and says it is due to you having to open the butterflies so much that they uncover the progression holes. At idle no progression holes should be even partially uncovered. I checked this by shining a torch down the venturi and removing the progression hole cover screw. This indeed seemed to be the case. I could see the edge of the butterfly over the first progression hole. Dave walker says that to stop this happening you must bleed enough air past the butterfly using the air idle bleed skrew to allow the car to idle without uncovering the progression holes.


I adjusted the idle speed skrew so that I could no longer see the edge of the butterfly and tried to start the car with the air bleed skrews one turn out. It started as before but when you come off the throttle it dies. I repeated this turning the skrews out further and further. I am about 6 turns out and the engine still dies.

Sorry for the long post. Does anyone know what I should try next? How much of the edge of the butterfly should you be able to see through the first progression hole (if any).

Thanks again

Rupert

Did you manage to synch the carbs when you had it idling?

are you sure that your fuel supply is good and that the fuel level is correct,what about ignition timing.I assume your was running ok before the strip down?Another thought could be large air leak.

You should be able to get them running well without the air bleed, or at least enough so the engine runs ok.

Expect an idle between 800 and 1000rpm depending on cam.

Big question is, what jets have you got fitted?

You may be better off starting with the idle screws 2 turns out, if your idle jets are too small then you wont be getting enough fuel through without the progression holes uncovered.

David

yep managed to get them more or less balanced.

They were running fine before they came off the car and ignition timing should be fine although i will check.

I managed to get them running a little better by winding out 2.5 turns as i read on another setup book.

it is spitting back alot so may be an air leak. it was not coming back down to idle very fast - i think this would poing to an air leak???

Thanks for the help

Rupert

If you get it running and then squirt wd-40 or easy start over the throttle spindles and misab gaskets etc. If it speeds up you have an air leak somewhere.

David

cool will try it. thanks

do you need to centre the misab plate or should it not be a problem?

Also how much should i compress the rubber washers?

Thanks once again flak monkey

Gap the rubber washer thingies to 2mm. You only need to use one cup under the nut, you dont need one on the carb side. Gap to 2mm between the cup and the carb body.

Misabs should centre up anyway.

David

Sorry but just to check i understand.

Only use one of the metal cup washers under the nut and not one on the otherside of the rubber bit (between the rubber and the carb body).

Thanks again monkey

quote:

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Originally posted by flak monkey
Gap the rubber washer thingies to 2mm. You only need to use one cup under the nut, you dont need one on the carb side. Gap to 2mm between the cup and the carb body.

Misabs should centre up anyway.

David

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Yep thats right. So you have: nut, washer, rubber, carb.

Thats the recommended weber fitment, quite why you get twice as many cups as you need is beyond me.

David

haha. great thanks - goodness you are fast!!

is this symptom i described with the slow falling revs usually caused by leakage of air then?

i guess that it is the increased vacuum in the manifold pulling air past the misab plates and thus slowing return to normal idle speed - am i right?? Will give it a go today.

Thanks

Rupert