Misc: Everything Else Starts, runs then slowly "chokes up"

T

timetraveller

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If the plugs sooted up on one run then would they not still be sooted up on the next unless they had been removed and cleaned?
 
John Cone

John Cone

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Has the nipple come off one of the choke slides and let the choke drop? Happened on my Prince before.
 
Gary Gittleson

Gary Gittleson

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I must agree with Norman (Timetraveller) here. I don't see how things like needle position, pooling of fuel in the intake, spark plug heat range, low compression, a dropped choke slide etc. could cause a bike to run fine for 20 miles before misbehaving, if that is indeed what is happening. Does it sometimes run longer? I think we need more information in order to help. So here are some questions.

Does the bike require a cool-down period before it will again run properly? Does it require some fiddling before it will? If so, what? Does it in fact ever truly run properly? Mr. Doctired, did you try applying the choke(s) when the problem occurs? If they even momentarily improve the situation, that would suggest a lean condition. Have you examined the plugs just afterwards? Once the problem starts, does it bog when asked for more power? Is it really "revs" that it doesn't want to give or is it bogging when the throttle is opened?

I have a little story to tell here. I once worked in a bike shop on Triumphs, BMWs and Yamahas. A guy came in with a Bonneville who said it bogged whenever he opened the throttle more than a little. I took it for a ride and it ran fine, so he took it home and returned the next day with the same problem. We went through that routine a few times until I finally watched him mount up. Before mounting, he lifted the seat and took off his deerskin gloves, parking them under the seat on top of the air intake. QED, as is said at the end of a mathematical proof.
 
doctired

doctired

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Old Bill said:
l think l am right in saying that D series bikes had two front heads so a pair of 376/9 would be the normal configuration, but, as this machine has a front and rear head a 689 ( no longer available from Burlen ) is an easier fit with regards to standard battery position. A 389 can be fitted to the rear head with an appropriate manifold ( voc spares ) and with a little ingenuity the battery can be shifted to allow space for the left side mount of the float chamber. Macvette is spot on about blending the manifolds, l have seen some awful alignment issues and most manifold adaptors are left as cast internally which is generally very rough! However, lets see what the condenser result is before things get really interesting!! Good luck...
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Thank you for that incisive response. The bike ran fairly well as regards carb and sparks prior to the rebuild. The bad news is that 5 years passed twixt the twain...
macvette said:
My engine was totally reconditioned by Maughans. It is shadow spec as far as compression ratio (7.3 to 1) is concerned but has other internal mods not relevant to combustion . Being a series D, as Old Bill says, it has two front heads. As returned to me, the ilnlet diam on the heads was/is 1&1/16" so it's fitted with 376 monoblocs. It runs 240 mains and 25 idle jets. My understanding is that most Rapides are rebuilt at 7.3 to 1 because that's the most commonly available piston these days.
At 200 miles, the engine has barely begun to bed down and needs to run slightly rich but not so rich that oil is washed off the bores so if the condenser improves the running, I would still check the carb settings.
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Thanks. That sounds reassuring...
 
doctired

doctired

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Gary Gittleson said:
I must agree with Norman (Timetraveller) here. I don't see how things like needle position, pooling of fuel in the intake, spark plug heat range, low compression, a dropped choke slide etc. could cause a bike to run fine for 20 miles before misbehaving, if that is indeed what is happening. Does it sometimes run longer? I think we need more information in order to help. So here are some questions.

Does the bike require a cool-down period before it will again run properly? Does it require some fiddling before it will? If so, what? Does it in fact ever truly run properly? Mr. Doctired, did you try applying the choke(s) when the problem occurs? If they even momentarily improve the situation, that would suggest a lean condition. Have you examined the plugs just afterwards? Once the problem starts, does it bog when asked for more power? Is it really "revs" that it doesn't want to give or is it bogging when the throttle is opened?

I have a little story to tell here. I once worked in a bike shop on Triumphs, BMWs and Yamahas. A guy came in with a Bonneville who said it bogged whenever he opened the throttle more than a little. I took it for a ride and it ran fine, so he took it home and returned the next day with the same problem. We went through that routine a few times until I finally watched him mount up. Before mounting, he lifted the seat and took off his deerskin gloves, parking them under the seat on top of the air intake. QED, as is said at the end of a mathematical proof.
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Thank you for your thought provoking, indeed heuristic, reply. I will have to wait a few weeks before I can check (Right hip replaced 3 vweeks ago) and will let you know. Cheers Doc
 
Martyn Goodwin

Martyn Goodwin

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Non-VOC Member
I would revisit the fuel tank cap. With it removed try blowing thru the hole in the top and feel for the breeze from the hole underneath (it NOT the same hole). If it is NOT free flowing run a drill bit thru BOTH holes - will do no harm to enlarge them slightly then repeat the test. If its still a problem try a different fuel tank cap. I had EXACTLY the same issue and this was the fix.

Just an afterthought - when riding is there ANYTHING on top of the fuel cap - like a tank bag - that could block the breather??
 
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