Needle jet behaviour has changed over time

[mikeb]

I'm no expert here Scott… I’ve re-read the whole thread and keep thinking it still does sound like fuel starvation even tho you’ve cleaned and changed gauze filters, fuel lines and checked the flow rate from the tank/taps. And even tho it doesn't happen at 70mph. One thing mentioned above has been a plug chop right when its died (either at 50 in 3rd or 60 in 4th). Could you do that? it would be good data to see how the plugs were looking at the point of failure - a roadside plug photo would be interesting to see.

[berger]

has anyone mentioned the tank cap vent, I was thrashing mine one day and it died, I found the tank vent rubber seal had moved a bit . I know mines a honda tank but it could be your tank vent acting up

[RDfella]

Yep, mentioned the tank vent several posts ago. As you say, it's often not thought of, but in this case it doesn't really seem to fit the problem.

[Scott and Jay]

Thanks, I can do plug chops, Mike. However, it is hairy stopping randomly on the motorways I have to do the test riding on. I look at the plugs when I come home and they have been light tan. John Saywell suggested it might be time to book it into a dyno session. He leant me a 3 and 4 slide (mine has been a 3 1/2), yesterday. I tried the 3 this morning. I put the needle back to position 2, because these settings are both "standard" for a 389/47 Monobloc. This showed none of the signs of richness, for slide tuning - that the Amal guide says to watch for (or of leanness - both at accelerating from idle). But the same symptom recurred, when I got to 60. It accelerated through to 65 fine, but when I slowed down to 60 the motor died and spluttered again (my markings show this is around 1/4 throttle) . Go down to 50 and it's fine.

I saw a thread that suggested the new Amal Premiers' needle jets are suspect  - is there anything in this? I suppose I could try raising the needle, successively, now - still with the no 3 slide. Sort of struggling a bit, now...

[ironhead]

Thanks, I can do plug chops, Mike. However, it is hairy stopping randomly on the motorways I have to do the test riding on. I look at the plugs when I come home and they have been light tan. John Saywell suggested it might be time to book it into a dyno session. He leant me a 3 and 4 slide (mine has been a 3 1/2), yesterday. I tried the 3 this morning. I put the needle back to position 2, because these settings are both "standard" for a 389/47 Monobloc. This showed none of the signs of richness, for slide tuning - that the Amal guide says to watch for (or of leanness - both at accelerating from idle). But the same symptom recurred, when I got to 60. It accelerated through to 65 fine, but when I slowed down to 60 the motor died and spluttered again (my markings show this is around 1/4 throttle) . Go down to 50 and it's fine.

I saw a thread that suggested the new Amal Premiers' needle jets are suspect  - is there anything in this? I suppose I could try raising the needle, successively, now - still with the no 3 slide. Sort of struggling a bit, now...

G'day S&J.
To isolate this problem, can you get hold of another carby ( borrow one?) to try? At least then you can narrow the problem area.

[Scott and Jay]

Thanks, Ironhead. I have my old 376 monobloc, which I could try - good point. First, I would like to eliminate the no. 4 slide - which our engineer also lent me. I have argued myself round to the opposite assumption, that it could be too rich (on my normal 3.5, and certainly on the 3) - when it goes back down to the top of the slide range (this is when it happens). It would be consistent with a gradual change-of-condition explanation, which I have been searching for - that being my air cleaner becoming more restrictive.
It will be in a weeks time, however. I need to recover from damaged ribs. I had to stop suddenly in a shopping car park to avoid a car that swerved in front of me. I braked suddenly, therefore - and toppled over. My A10 is undamaged. I was on my way to test the effect of removing the filter on top of the float chamber. I still did the test ride, which proved this made no difference. One more to eliminate..

[mikeb]

Bad luck on the ribs Scot. At least the a10 is ok.
I'd be swapping that carb out first to see if that makes a difference. Problem is it's all guesswork so far so to see if carb or mag would be actual data. Then if carb a plug chop would eliminate more guessing....
BTW 2 yrs back I caught my Ducati at the expenses of a rib. Saved the paint but ribs hurt

[Scott and Jay]

Next update - to the "saga", it's still there. It just takes me a while to get on to things. The no 4 slide didn't fix the problem. I put a whole new temporary, coil ignition in. The problem remained. I have just come back from a ride with my old 376 carb in. Same thing, at 60 mph. So, after all the other things I've ruled out (based on your suggestions, from the start), it's down to mechanical - possibly sticky valve. Engineer John will be given a bundle of components to check: head, rocker box, pushrods...

[mikeb]

that's bad news Scott. hope you get it sorted and I remain interested to hear what you end up discovering is the cause

[muskrat]

Bugga!

[Scott and Jay]

Mike and Muskrat, and all

Engineer John confirmed that neither exhaust valve, both of which looked white-hot, were seating and sliding properly. They would have been good when the k-liners were put in 4.5k miles ago, before the oil got baked on to them. I have had to order one new one because it was too pitted to reface and seat properly. He is going to:-
- hone (and maybe ream another 1/2 thou ") - the exhaust k-liners
- rebate them at the base, so they are shielded by the cast iron liners

Then they should be good for another 4k (maybe 40k) miles. The inlet valves were fine (only running 1.5k thou " clearance). I'm looking forward to it. I will report back after my first ride...

Thanks and regards

[Scott and Jay]

This final post on my long ordeal should wrap this up. Had a nice ride out with the combined clubs - to a cancer-fundraising meet (up at QE2 park, just past Paekakariki). My bike now holds 60 mph. nicely - whew!, and thanks to all...

[mikeb]

glad you got it sorted Scott, and thanks for updating us on the solution. you certainly get points for perseverance!

[richard boys]

i put some nice shiney chrome 1/4 turn petrol taps on my a10 they did not leak result !  but the action in turning was stiff took a while to tickle the carb ;this was down to fuel flow through the tap removing the tap and squinting up through in the open position it was mostly blocked, during assy the rubber had not been cleared when the hole was formed to allow fuel flow passage .i could start the bike but performance was restricted  until the dribble caught up with the needs of the engine .. i have a 57 road rocket with a 61 super rocket engine