Carb queries

[Phil C]

Update and plea for advice: I put the carb back on the bike, put some fuel in the tank (same batch as before, so a couple of months or so old, but looked and smelt okay) and fired up the engine. It won't keep running unless I give it lots of revs. Plug looks a bit black but not too bad. Any ideas please? Trying to think what I might have done to cause it. I resoldered the wires on the alternator, but it couldn't be to do with that, could it? What could it be if not the carb? All advice/ thoughts would be much appreciated.  Phil





Jj

[AdrianS]

Does it start relatively easily?
If so and you need to "give it some revs" to keep it running, it does sound like a fuelling problem.
Have you tried screwing up the throttle speed screw to keep it running?
Failing that, try and get someone to lend you a carb to see if it confirms fuelling problems.

[Phil C]

Yes it starts fairly easily. I don't know anyone with a similar carb. I'll try altering the throttle speed screw - thanks.

Just a thought: couldn't be the plug breaking down, or the HT lead, could it?

Any other ideas very welcome!

Phil

[Phil C]

Help! I am absolutely flummoxed by this. My bike starts but unless I keep the revs high it immediately conks out. It won't idle.  As several of you have already said, it does seem like a fuel/carb issue (maybe connected with having the petrol tank lined recently?)But how can it be that when I squirt carb-cleaner in the AIR passageway it comes spurting out of BOTH the tiny holes in the carb bore, but when I squirt it in the FUEL passageway it only comes spurting out of ONE of them? How can that be? I'd be very grateful for any more advice to help me sort this out. Thanks again for your thoughts so far.   Phil 

[chaz]

Phil.
what bike?
what carb? numbers on the flange?
where are you?

cheapest option is plug and lead swap, one thing Ive tried is once running squirt easy start and see if it keeps running, a squirt when needed . this will rule out plug/lead  or carb.
failing that, answer the above , and I will see if Ive got something similar to try.

[Phil C]

So plug/lead breaking up could give those symptoms?

[AdrianS]

I would have thought that a plug/ lead failure would be noticed at high revs not tickover

[Pete A10]

You say the problem has started since you had the tank lined. Have you checked that you have full flow through the filter and tap on the tank and that there is no detritus/resin line partially blocking the flow. If there is a blockage it would explain why the engine starts but will not run (I.e fuel starvation).

[chaz]

presumably, tap was out when tank lined?
I trim around hole to get full flow, any thoughts on my offer?

[Phil C]

If I remember rightly the tap was out but here are photos of it, I think it's okay,  and a drill shank pushed up the hole in the tank goes freely into the tank. A drill bit slid through the tap goes through okay as well. So I think (!) that area is okay, although I don't know how quickly the fuel should pour from the tank when the tap is opened.I only had about a pint or do of petrol in the tank, but it was enough to flood the bowl when tickled.

Chaz, in answer to your questions, the bike is a 1965  B40F, the carb is a Concentric, on it is written 626   L301, and I'm in SW Devon.

If anyone has a similar carb on the bench and could see if carb-cleaner squirted into the FUEL passageway (threaded hole but no longer taking a jet, as the jet was repositioned I think about 1967 to be closer to the tiny holes and was pressed on place) see if it spurts out of BOTH tiny holes, that would be very helpful. On mine it only spurts out of one, but when squirted into the AIR passageway it spurts out of both.

I hope that makes sense and apologise for keeping repeating myself.

Phil

[Phil C]

I've got the carb off again. At the bench, I took the bowl banjo off, peeped into the float needle seating hole and - maybe ! - saw a partial blockage, so just a small hole remaining, as though there were a jet in there (which of course there isn't.)Took my eye away to look for my new can of air, looked back and the hole was clear (I know, couldn't make it up.😁) So either I was getting muddled with what I was looking at, or there might have been a blockage which  fell out. Unlikely I guess, given that there's a filter, but if there had been a blockage then I guess that might cause the bike to conk out rather than idle. Anyway, if the problem has gone when I put the carb back on then maybe that was it. But I think a blockage there would be very unlikely, given the filter is in place and looks fine.  Phil

[Phil C]

Anyway, please could anyone answer my query in my last but one post, about the two tiny holes?  Phil

[chaz]

will check my carbs tomorrow,

[Phil C]

Very kind of you, chaz. Look forward to hearing what you find.  Phil

[Stubaker58]

Hi Phil,
Reading through again have you had a go at your carb with eg ultrasonic cleaning?
I bought a small ultrasonic cleaner off EBay and it's just about big enough to fit a dismantled Monobloc. I still also go back to older methods like soaking in vinegar to get things clean too. It's a judgement call as to how long you leave things to marinade but it can be very effective at clearing out the tubes.
I know others resort to boiling their carbs in a washing powder solution!
A last resort is to drill out the pilot passageway as espoused by the Bushman.
Regards.