The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => Amal, Carburation, Fuel => Topic started by: muskrat on 16.02. 2025 05:48
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G'day Fellas.
I haven't been happy with the Cafe lately. Seemed to be a little weak on the left pot. Hasn't had much running since new top end 80 over *eek*. Bit hard to measure compression with a 23 engine sprocket and takes all my weight to kick it twice let alone 10 to do a proper test. Two identical carbs and identical spark on both. So something is wrong so I ripped the top end off. Tested the valves aren't leaking so the barrels were next. Surprise surprise, the oil ring on the right has a piece missing. Lucky it stayed in the groove, no damage to piston or bore. Still no reason why the left felt weak. The rings on both sides are identical gap *dunno*
Now to find a set of 72mm rings, probably end up wit Trihard T120 +40 if I can find a GOOD set. I won't use Hepolite or JP (most of them come from the same factory and distributed by Asia Pacific and put in their own packets). The hunt is on *pull hair out*
Cheers
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Good pick up Musky! I wonder if that happened on assembly? Did you use ring compressors? I must admit I'm not keen on them. When I put mine back together recently I popped the pistons and rings back into the barrels first, easing the rings in very gently with my finger nails and no pressure on the pistons to enter the bores. It was an easy job then to line the small ends up with the barrels supported and just push the wrist pins in with a bit of warming from a hot air gun on the pistons. Gotta make sure you put the base gasket on first though... *smile*
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Hi Musky,
Check the valve guides?
Maybe the left side is drawing air down the guide?
John
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G'day Ian.
I use poly pipe ring compressors that Ive used all my life. Only ever had a problem once with three piece oil rings. One "rail" got caught and ruined the bore. The rings were Hep's that I don't like, there is a chance it happened on assembly.
G'day John.
It could well be the guides. I'll pull the valves today and check. If I have to order new ones I'll convert the inlets to take seals.
Cheers
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G'day John.
OK I pulled the valves. All 4 are good (as the day installed), inlet no perceivable rock at full lift and only the barest on the exhaust.
Where to now *dunno*
If one side would give trouble I'd expect the right side as this was the side that had the dropped valve and head repair. https://www.a7a10.net/forum/index.php?topic=10630.msg79189#msg79189
Cheers
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G'day Fellas.
I inquired about a set of GPM +40 T120 rings at Steadfast Cycles in the US. The rings are $105AU but the freight to me is $115AU *eek*. I could get the same size parcel (3"x3"x2") from the UK for 1/4 of that. Looks like the new Trumpet doesn't want to trade with the world, either in (tariffs) or out (post office).
Will keep looking.
Cheers
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Hi Musky, I just had a similar experience with car vinyl roof material quote from the US, last year it was US$95/yard, they just requoted me US$125/yard plus postage of course!! *eek*
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It’s usually worth checking that both carb needles are clipped where you think they are.
At least I’ve found it worth checking!
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If it’s only “a little weak” on one cylinder, that doesn’t have to mean there’s a real diagnosable problem.
One of the good things about twin carburettors is the facility to tune the mixture on individual cylinders.
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Just to deviate a little bit: Mine is a single carb, and I have tried everything that I can think of to richen the weak mixture on the L/H/S cylinder without success. I even fitted a bias manifold spacer but it has made no difference.
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There used to be an adaptor you could get that was slightly wedge shaped, angling the carb, therefore angling the incoming charge towards the weak cylinder.
(Or did I dream that!)
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Yes, that's what I was talking about. It made asolutely no differencs at all
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Be interesting to try running a velocity stack in front of the carb ilo. an air filter? (Not advocating running without a filter by the way purely as an experiment!)
A mate with a twin carb A65 couldn't start his at all (twin 32mm Amal Premiers) and it may not have the mixture completely right as yet but doing this made a huge difference to the starting and the slow speed pick up. Seems to smooth the airflow through the carb and help the fuel to mix better at lower air flow velocities. What they were designed to do I guess!
Other than that you'd need to put the head, manifold, carby and filter housing etc on a flow bench to see what's happening.
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G'day Ian.
I don't run air filters on the Cafe, just short stacks with wire gauze. A flow bench is a good idea as I can't see any mechanical problems. The mob down in Windsor that did the head repair may have one. God knows how much that would cost (probably a weeks wage).
Cheers
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I found that adjusting the cam ring on the magneto so that both cylinders were running the same advance sorted (mostly) a weaker cylinder.
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G'day Ian.
I don't run air filters on the Cafe, just short stacks with wire gauze. A flow bench is a good idea as I can't see any mechanical problems. The mob down in Windsor that did the head repair may have one. God knows how much that would cost (probably a weeks wage).
Cheers
There's one at the Tafe here in Regency Park where they run various automotive courses. If you know the right people to ask access has been granted...Maybe there's a Tafe nearer you that might have similar? Thing is though if you weren't getting this previously then something has changed so has to be a failure or a wear out thing rather than basic set up problem doesn't it?
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G'day Yrti.
Both my BSA's run Boyer 12v electric ignition. Thanks for the thought.
G'day Ian.
I haven't really run her much since the rebuild. When doing 1st tune up I noticed it. Basically hands over the exhausts I noticed right was pushing back more than the left. No leaks in the pipes.
OK just ordered the rings from Steadfast, he said it would only be about $25US and will refund the excess postage. Nice to do business with real people rather than algorithms.
Now to find course stones for my hone.
Cheers
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Hi Musky,
I would have tried swopping the carbs before diving down theoretical rabbit holes *????*
John
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G'day John.
I wish I had done that first. Two identical carbs should be the same flow.
When I did a comp test the left seemed lower but that could have been me running out of puff.
Too late now, rings are on their way, I'll order hone stones today.
Another problem is the freshly painted f/glass tank has a few bubbles under the paint. I'll start a new thread for that.
Cheers
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Hi Musky,
I would have tried swopping the carbs before diving down theoretical rabbit holes *????*
John
I wonder when you would have found the broken oil scraper ring.
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G'day TT.
Yes that would have been further down the track when it escaped the groove destroying the maxed out bore. I have never re-sleeved a set that had already been sleeved.
Cheers
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I recall an article from the late 70's about A10s running rich on the right hand side, I believe they solved the problem with thicker gaskets between the carb and the head. Apparently it was a well know problem back then. How succesful this was is lost in the mists of time
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The solution???? was/is an angled bias gasket. In my personal experience it doesn't reolve the problem at all. I found it made asolutey no difference
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Hi Musky,
I would have tried swopping the carbs before diving down theoretical rabbit holes *????*
John
I'm choking on humble pie ::hh::
Tonight I thought bugger it. Top end back together with new rings. Fired her up and the same rough on the left. Swaped the carbs and now the right *ex* So it is a carb problem. I had run the carbs through the ultrasonic cleaner (left one 3 times) *pull hair out* Now to save up my beer tickets for a pair on Amals. I'll go with Mk1.5 concentrics as I don't want to make more manifolds for Mk2's. Bugga.
Cheers
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Hi Musky,
Sometimes there's no substitute for poking a strand from a wire brush through the carb drillings *eek* plus a long nozzle on a can of carb or brake cleaner to blast any remains out
John
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G'day John.
That's what I did to both before and after the bath in the ultrasonic. Those PWK carbs have a few very small holes and a weird 3 hole O ring. The smallest tip cleaner wont fit but a wire off the wire brush will and passed through all the holes. *pull hair out* The "bad" carb did respond a little bit to the choke. I'll buy another set of carb kits and persevere till I can afford new carbs ($1100AU).
Cheers
I'll now move this over to the carb section.
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G'day Fellas.
The two new 930 premiers arrived yesterday ($978). I pulled the PWK's off (anyone want them?).
I removed the choke slides and plugged the hole in the lids. Fitted carbs to my home made manifolds (had to buy longer UNF bolts), adjusted the cables so both slides disappeared at full lift at the same time (quick easy way to balance carbs), hooked up fuel lines to my auxiliary tank. OK here goes nothing. Tickle both carbs and she fired up 2nd kick *grins*. Sounds great blipping the throttle to 4 grand. The I noticed one carb was flooding *pull hair out*. The old problem with the 15 degree downdraft causing the float to hit the roof and flood but why just one (pair of L & R out of the same box). So today the carbs will be removed and put on my 15 degree jig to get the fuel height the same on both. I'll then put'em back on and set up my balance gauges to get'em spot on.
I'm itching to take her out for a blast, it's been too long.
Cheers
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G'day Fellas.
Two steps forward three steps back *ex*
Day didn't start well. Guess who forgot to turn the fuel off last night *????* *bash*. Lucky I only have a small auxiliary tank as it all ended up on top of the piston *pull hair out*. At least I know the rings are good as none got down into the sump. So after clearing that I dropped the offending bowl to find the float pin was stiff in the float so it wouldn't lift to shut the fuel off. Removed the two 150 degree banjos to fit single banjos for my fuel height gauge. One side was pretty much spot on, the other 1/4" too high *eek* The banjos I removed weren't good either, the two spigots that face each other (balance) hadn't been drilled right through. Not giving me much confidence after spending over 1,1/2 weeks pay. Another thing I noticed was both the pilot screws have tight spots when screwing them in & out.
Anyhow I got both fuel heights spot on and balanced them both on the cables and stop screws. Now adjusting the pilot screws with the help of my Colortune I find one side is 3/4 turn out and the other is 2,1/4 turns out *sick*
Tomorrow the carbs will be pulled off and stripped to see what else I can find *rant*
These two Premier carbs must have been a Friday arvo build *sick*
Cheers
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Musky nothing right anymore they just want the money, i have some old stock clear petrol tubing 5mm bore 9 mm OD it is good stuff that i've used for years . i am now running out because this modern petrol makes the tubing hard over a period of time. i just bought some new stock petrol tubing at 5mm bore and guess what , the OD is only 7.5 mm. i used to go to local outlets for this tubing and thought about trying to get some old stock but these places are now gone.
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G'day Fellas.
Still having trouble with the left carb flooding. I've tried some creative bending of the float tang to no avail. The poor bowl has been on and off 100 times *pull hair out*. I even treated the brass seat to a cotton bud and brasso in the drill to clean it up. Then I noticed something odd with the needle. It was sitting on the bench on a piece of paper and with my one eye I saw something strange. Got the magnifying glass out and saw the rubber tip had a bit sticking out *ex* pics attached. The new needles are three sided but I only had a four sided spare. It worked a lot better but it still floods after sitting with the fuel on for 15 minutes or so.
So much for Burlens blurb of testing every carb at the factory *ex*
"Not happy Jan" I'll send them another email.
Cheers
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G'day Fellas.
Thar's better, fired up 2nd kick and both sides equal. Still floods on the right if fuel is left on for 15 minutes but if I remember (yer right) to turn it foo when stopped.
Next job is bubbles in the new tank paint.
Ho Hum.
Cheers
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G'day Fellas.
Things that make you go hmmm. Bit the bullet and bought two new alloy float needles. Hang on, nothing like the ones in the new carbs! The new ones have 3 ridges, similar to the old 4 ridge ones. The ones in the carbs (see pic in last post) are triangular just like the ones in a Mk11 concentric. Ok put the new ones in and both carbs flooded in seconds *pull hair out*. Trimmed off the errant rubber off the original and put them back in. A lot better but one side does flood in 1/2 hour if the fuel is left on *rant*. Bowl will be removed again (the hard one to get to) and seat lapped again. Really getting over this, so much for new Premiers.
Time to crack a bottle of whisky!
Cheers
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I'm back.
Only took 1/4 bottle *eek*. Took a fair bit of lapping but I think the seat is good now. All back together but will have to wait till Sat arvo to test. Fingers crossed.
Cheers
Now for the rest of the bottle!!!