Coil brand new. Fires up fine.
Thanks for your suggestions. Carbs are 689. Needles in notch 2 slides and block as shown. Unsure about main and pilot
Misc: Everything Else Starts, runs then slowly "chokes up"
- Thread starter doctired
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Greetings doctired, whilst l fully agree this could be an obscure fuel problem it is screaming condenser failure and as this can easily and cheaply be checked by placing an additional condenser on the low tension side of the coil. Just try as it is the simplest check before getting involved with other potential problems, might work and save a lot of frustration? Good luck, hope this helps.....
Thank you so much for the kind advice. I shall look at this carefully. However, the Distributor was fully rebuilt by Maughans with new parts and has done about 200 miles since, none of it under strain or over 60mph....
I agree that a new conenser would be the simplest way forward to see if it fixes the problem. I have a series D Rapide with monoblocs.376.
689 is a right hand version of a 389 fitted to BSA LIGHTNINGS. Series D s were fitted with 376 monobloc which are 1&1/16, 389 are 1&1/8 and were fitted to D black shadows and Black knights.
Looking at the Burlen web site, 689 s were main jetted with 270 mains which is the upper end for a series D shadow.
So even if the condenser corrects the issue, your carbs need a good looking into. If the intakes(carb mountings) havent been blended to match the carb diameter there will be a step at the rear of the carb and possibly the head which can cause fuel drop out and pooling of fuel.
Very kind of you to reply. Sounds a great avenue of investigation
Maughans rebuilt this engine to shadow spec. The carbs are a matched pair as far as I know. The stampings are correct.
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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...
I have used an old racing head with 1-3/16 opened port on a road bike while the normal head was being sorted ( or waiting for money to be sorted) so an 1-1/16 carb fitted. She went very well indeed in fact I am dammed if I can remember how long I used it certainly a year or so. I think the head is on my egli comet now
So many heads so little time
So many heads so little time
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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I think it is unlikely to be solely a carburettor problem, what is the likelihood of two carburettors, both operating at different heights developing a simultaneous fault at 20 miles into a run, you would at least expect a period of time when the bike would go onto one cylinder for a while, same as when you run out of fuel, I would definitely drop in a new condenser and maybe even a coil, but try the condenser first.
A rider at the Austrian leg of the recent International Rally had a breakdown on the road, when I arrived on the scene he had the situation under control with carbs off the bike, slides out, float bowl covers off, fuel all over the place, so convinced was he that the problem was fuel related, I could not do much to help, but as a last resort I offered to pick the bike up later in my van, but later on I bumped into him at the hotel, when I asked him how he eventually solved the problem he said he thought that the points heel was running a little dry on the distributor cam and the points gap had closed up to the point where the ignition would not function, such a simple fundamental problem, but so easy to overlook or ignore in the heat of the moment, and jump to the wrong conclusion, I am sure we have all acted similarly at some time or other.
A rider at the Austrian leg of the recent International Rally had a breakdown on the road, when I arrived on the scene he had the situation under control with carbs off the bike, slides out, float bowl covers off, fuel all over the place, so convinced was he that the problem was fuel related, I could not do much to help, but as a last resort I offered to pick the bike up later in my van, but later on I bumped into him at the hotel, when I asked him how he eventually solved the problem he said he thought that the points heel was running a little dry on the distributor cam and the points gap had closed up to the point where the ignition would not function, such a simple fundamental problem, but so easy to overlook or ignore in the heat of the moment, and jump to the wrong conclusion, I am sure we have all acted similarly at some time or other.
Going back to doctires original posting it seems to me that the most significant thing is that the bike runs well for twenty miles. That rules out anything to do with inlet ports, carb defects and so on. In twenty miles it will have used all the fuel in the fuel pipes etc and will have had to take fuel from above the taps. We are left with three possibilities, fuel supply, ignition or some unusual mechanical problem wherein something which starts off tight and well adjusted becomes loose and no longer doing what it should. It seems not to be restricted fuel flow, (eg blocked tank vent, dirty filters on the petrol taps etc). If it was a failing coil or condenser then that would be easily checked by testing to see what strength of spark there is immediately after limping home as compared with the spark when the bike has been sat overnight. Assuming the strength of the spark is satisfactory then it would seem impossible that something was slipping (eg bob weights, drive to distributor, etc) as then there would be nothing to get that mechanical fault to re-align itself when the bike was cooling down. If all these items have been checked then there has to be something mechanical which when it gets hot causes the fault. If you turn the engine over slowly as soon as you get home, with the fault, is there a noise present which is not there when the engine is cold. Is the compression the same with the fault as without? I'm thinking sticky valves, valve collars moving when hot and then going back to their correct position while cooling down.