luddite

Small change, but still no start. Screwing the air screw in or out incrementally seems to make very little difference. Hmmmm. Back to the drawing board.

Yes, it's nicely sealed all around now, it wasn't before. I couldn't resist and have just refitted the carb and given it a try. It doesn't seem to have made a whole lot of difference, but as it's after 11:00pm here I can't say I tried all that hard. Sleep on it and same again tomorrow I think... Thanks for the help!

Ross, thanks for the reply. As you were typing the words, I was realising my floatbowl tangs were bent, and slight burrs around the screw holes were lifting the bowl away from the flange in places. Not sure if it was bent enough to lift the pilot fuel jet above the fuel level but it's damn flat now! It could be that when I cleaned the carb after the engine was in storage, I introduced a leak or air gap. The needle jet was very worn. I'd estimate around 75% increase in area. I did photos, but they're not yet available. Using a simple suck test to compare old needle/jet against new revealed I could hardly suck any air through the new parts, but with or without needle made little difference to the resistance using the old. Now that just has to make some difference! I should get a chance to build it back up tomorrow evening. It will not beat me!

Thanks Woody and Greg. I have to agree with both of you, I'd have thought that with the slide down, the needle jet has little or no influence so the bike should start on the pilot circuit alone. But it doesn't so I'm clutching at straws now! If I change it and it does nothing, I've eliminated it,if it does something, however slight, it may be a pointer and shows me that it's most likely a carb issue. I know about the pilot circuit on Amals; this one is very clean indeed, no blockages of any kind. The float floats at the design height, all jets are clear and unobstructed. The flanges have been trued on glass, including the rubber one. The points open at 3mm BTDC precisely. Good point about the condenser though, I've had issues with these before but they tend to give intermittent firing problems in my experience. Quick and cheap to change so I may as well eliminate any doubt by changing points and condenser. Mentioning the OKO (where did you get it, by the way) has reminded me I have a little-used Mk2 Amal from a later 250 somewhere in the shed. As far as I remember the bore is still 27mm on the later 250s (is this correct?) but I should change the jets to stay the same size as the Mk1 currently fitted to my 350. To start with at least. As has been said, it's a simple bike with a simple carb, but that's what's so damn frustrating! Grrr!

Hmmm. There weren't, I thought there should be, but chatting to the guys at Bultaco UK convinced me otherwise. I'm sure there were on my model 10 and I think so on my model 80. The witness marks on the seal holders would indicate that none have been there though. The O-ring issue wasn't as bad as I thought. The smaller crank one was there after all, just a lot thinner than it should have been. I changed the crank seals whilst I was in there - the ones I had in seemed very flimsy compared to the Bultaco UK items (they were autojumble specials). Anyway, back together and it still won't go! It does start on wide-open throttle and a hefty prime, holds high revs for a couple of seconds and dies. I'm happy that primary and secondary compression is OK, that the spark is healthy and at the right point, and that the carb pilot drillings are spotlessly clean, and that it is jetted correctly. So I gave Amal a call to chat about acceptable wear on the Mk1 concentric, and they are of the opinion that the wide throttle is necessary to balance an overfuel issue, which is entirely logical. The most likely cause of this is wear in the throttle slide bore causing the needle to wear the needle jet to the point where massive overfuelling is preventing the engine from running. So, to prove the theory I've bought new jets etc. and if that works I'll splash out on a new Amal. I'm waiting for the postman now :-) Still doesn't explain how it could work so well before I took the engine out, though, unless moving the engine around dislodged a blockage in the jet which was compensating.

Seal holders out - and the diagnosis is looking good. It would appear by their squished profile that I didn't actually change the big O rings and on the mag. side in particular they're not doing a lot of sealing. But in addition, I can find no trace of the RH crank O ring. Not one of my better jobs, must have been one of those early morning garage sessions when my son was a squalling baby and I'd had less than my usual twelve hours :-)

Couple of pics before it comes to pieces again. Not smart I know. A few 'wrong' bits I know. But once it's up and running it'll do the job I'm sure and I can improve it as I go (and it was very, very cheap ).

I have a horrible feeling you may have just hit the nail on the head - I can't actually remember putting new ones in, but I'm pretty sure I can remember picking old ones out. Thanks due again!

Thanks for the reply, Big John. Rev and die cycle was without me touching the throttle, which now I'm sitting here thinking about it must indicate air getting into the system from somewhere other than the carb inlet, or it wouldn't have the volume of air available to enable combustion to such a degree. Looks like I'll be pulling the engine out for a better look I think. But before that I'll give The Procedure a go. Thanks again.

I have recently dragged my last remaining Bultaco from the back of the shed where it has spent the last couple of years in disgrace. A few years back I decided to try sidecar trials, so got hold of a chair which was once fitted to a '74 model 125. I purchased a matching bike which was running absolutely perfectly, but had trashed lower frame rails and other frame damage so was ideal for the project. Mods duly done, the bike was reassembled, but wouldn't start. Crank seals I thought, so I whipped a new set in. Still no joy. To cut a long story short I stripped and reassembled the bike (I didn't split the cases though, or lift the barrel) till I was sick of the sight of it, and the best I could get it to do was to start, rev massively, die, rev massively, die, and repeat for a few cycles. New year, new start so out comes the 'Taco. Timing's perfect, big fat blue spark (one of the best I've seen in fact!) so that side's OK. The carb, an Amal 627, is clean and set to book; it's a bit worn but I've run with worse. The fuel flow is a tiny bit slow but is flowing to fill the chamber. If I prime directly into the head or the crankcase, the bike will catch but not run which tells me the top end is OK. Now bearing in mind I didn't split the cases at all, and I have changed the seals, could anyone please offer any suggestions as to what I might check next before I start taking the engine down? Thank you.