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Carb Spitting back fuel

Puddle jumper

Well Known and Active Website User
Non-VOC Member
Hi Gang, Got my Vincent running today, had a coupe of oil leaks which I have now fixed, but I do have one problem, the carb for the front cylinder spits back fuel on tick over, I’m running 32mm Makuni s which are new so should be ok inside!! I wondered if this is a common problem for Vincents when they idle on the side stand, due to float level changing with the angle of the bike, or does one have any other ideas,
Cheers Paul
 

John Appleton

Well Known and Active Website User
VOC Member
Paul, you will no doubt recieve advice from lots of sources about incorrect valve timing, and obviously this is something to consider, but many years ago my 1949 rapide suffered from the same affliction. After several dismantling sessions I found that the position of ET 68 was incorrect, and even with ET69F diconnected from its linkage, it still held follower ET29 in the partially raised position thus restricting the exhaust valve travel and caused blowback through the carb, and I always had a left leg that smelled of petrol . Just another of those snippets that can have you thinking for weeks.
 

Puddle jumper

Well Known and Active Website User
Non-VOC Member
Thanks for that John, But I am not running any valve lifters and have removed whole the mechanism, so i think I can rule that out, do you think it is more likely to be valve related or due to a carb problem, i.e. float height jet size or air leak.
Thanks again Paul
 

John Appleton

Well Known and Active Website User
VOC Member
Paul , yep, you can rule out my ramblings! I think, on balance, that I would be checking valve timing if only because I have had a similar problem, but never had a carb cause it. There are no doubt other opinions about this, but you have to start somewhere eliminating possible causes and its fairly easy to get a fair idea if all is well without a major strip down. Just check that the overlap of the valves is equi spaced at 6degrees before TDC. This is also known as "equal lift" (ie. the closing exhaust valve has the same lift as the opening inlet). I have just added that for clarity, not as a comment on your knowlege. John

P.S. Pete has just reminded me that,for some strange reason, his original Comet will do this with BP5ES plugs fitted, BP6ES rectifies the problem.
 
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Puddle jumper

Well Known and Active Website User
Non-VOC Member
I will look at the valve timing but did double check it last week, I will also look at my plugs which from memory are BP5ES and 2 per head which might make the problem worse. Cheers Paul
 

BlackLightning998

Well Known and Active Website User
VOC Member
Hi Paul,

Lovely looking bike - I do hope that you get it running right - look forward to seeing it at an event. An article and some photos for the club magazine would be good - why/how you built it, etc etc - be an enjoyable read.

Regards
 

chasbmw

Website User
Non-VOC Member
Paul,
are you running withstandard ignition timing on the twin plugged engine?
I don't know about vincents but dual plgged airheard bmws typically work best with around 3 degrees Less advance at full revs. Static timing should be standard
Charles
 
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