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directordanw

Chasing a vacuum leak

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Finally got the jammed idle screws out of their towers on my carbs, replaced them and refitted the whole lot with new o rings etc.

And now my carb synchroniser is reading a huge vacuum leak on the rear carb - the one which I've done all the work on. Both barrels. I've tried to track it down by ear, and with WD40 but no joy.

What have I done now??? Is there something I could've missed? Done wrong? Does anyone have any experience of this? Thanks as always!

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What have you replaced during the refurb? This would help narrowing things down.

Have the rubber carburretor to manifold mounts been replaced for example?

Edited by Fueltheburn

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Everything has been replaced - every o ring, washer, gasket and spring. The mechanic who finally removed the jammed idle jet had to be fairly brutal - took a dremel to the tower so he could get hold of the screw rather than drill it out. But even if he'd damaged it in the process, which I was prepared for, that would only affect the one choke. But both barrels are very low.

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Hello. Have you got any further with this fault? Don't wish to ask stupid questions, but have you closed the air bleed screws fully? Are the throttle plates fully closed ?

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Not stupid at all!  I have checked them - after forgetting for a while then it suddenly dawned on me.  But yes, butterflies and air bleeds both shut.  My next point of inspection is the hose servicing the brake servo.  It's the rear pair that are low, and that's where the hose attaches, so I'm wondering if there's a leak there.  Still hoping it'll be something straightforward.

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Did think the servo hose, but that connects to cylinder 4, so I'm wondering why cylinder 3 is low as well. When you put the carb back on did you set a gap between the carb face and manifold face (think it's about 1mm). It's primarily to stop the vibration of the engine frothing petrol in the carb and heat transfer, but if over it tightened I suppose it could squash the rubber "o" ring out of shape.

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I think the gaps are OK.  Been round them listening with a bit of tube, and with a squirt of WD40.  Can't find anything but I will double check those again at the weekend.  Maybe the whole lot needs to come off again to check the O rings.

Edited by directordanw

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How low are they compared to cylinders 1&2. I presume it must be running quite rough. Have you done a compression on the cylinders?

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A piece of tube in your ear and move the other end around the problem are can point out leaks very quickly. If the leak is occurring internally then you have another problem which the tube may not be able to locate.

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Tried the tube - no joy. 1&2 both read normal, 3&4 are way down - registering on the gauge at 'vacuum leak'. Tested the compression and it's a bit low - 125 across the board without oil. But it is comparable on all four.

And yes, it sounds rough!

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Sounds to me if the linkage is incorrectly set up between front and rear carb.  I would double check that.    You could of course temporarily disconnect and block any vacuum feeds to rule them out   

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Ok. I know the vacuum is low on 3&4, but are they equal. You could try bringing them all inline by using the balance screw on the linkage.

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Yes, 3 & 4 both read equally low, 1 & 2 sit very comfortably in the 'normal' with hardly a flicker of the needle.  I genuinely don't think there'd be enough thread on the balance screw to bring them in line.  I'd have to open the air bleed screws so far on 1 & 2 to give myself a chance that those two would be unworkable too.  I beginning to worry that the carb may have been knackered when the jammed idle screws were removed, but it just seems so unlikely that both chokes would be buggered.

 

Anyway next up is stripping the carb again, checking the floats, needles etc.  Could a lack of fuel cause low vacuum?  Obviously if the rear pair aren't firing properly then 1 & 2 are picking up all the slack driving the crank but would that effect how much the pistons suck?

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To answer your question no - Also, there are only a couple of places that stray air could get in to spoil the vacuum, just isolate them in turn.  TBH stripping the carb is unlikely to get to the root of the problem, i'm 99% sure it will be an external factor - as Jason suggests. You could easily rule out the linkage by blocking both venturi on the offending carb and see if the vac returns.   guess it could also be the cold start mechanism as that's shared and allows a significant amount of air to bypass.  Final guess would be mounting O Rings

 

Jason where abouts in Bolton are you?  I'm in Daisy Hill.

Edited by 910Esprit

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Hello 910esprit. I'm in horwich. Would be good meet up. Have you joined norlog? Meet every first Monday.

It really doesn't take much to balance the carbs on the linkage. Might be worth while seeing if it makes it better or worse an 1/8 of a turn brings them in line. Have both air bleed screws been changed? How are the seats in the carb, have they been damaged.

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Needles and seats were replaced when the crabs were rebuilt, I can't see how they could have got damaged having the idle screws removed.  Both the air screws are the original, both the idle screws are new - new springs, o rings and washers too.

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Is it hissing from the black plastic jet cover on top of the carb when the choke is off?

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I won't get a chance to check that till next weekend.  I must be able to track it down!

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