Author Topic: Engine misses and loses power  (Read 1994 times)

Online berger

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Re: Engine misses and loses power
« Reply #60 on: 27.02. 2020 15:14 »
hi jim chin up mate I feel for you, I can't add to the advice given which is all good so plod on get some cloths ready lift the barrels shove cloths in and see what you find, then have some black&tans *beer*

Online Triton Thrasher

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Re: Engine misses and loses power
« Reply #61 on: 27.02. 2020 16:58 »
It’s common for stupid engineers to bore old cylinders to an insufficient piston clearance,  causing seizures when it gets near full temperature.

Online RDfella

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Re: Engine misses and loses power
« Reply #62 on: 27.02. 2020 18:22 »
If the bores aren’t perpendicular to the deck or base flange than either the machinist has been insufficiently careful or his equipment is getting past it – which, given the lack of boring done these days, is fairly likely. I can’t actually think of an engine that gets bored using the base flange. In fact, as that would require setting up on parallels for engines with protruding cylinders, it’s more likely to introduce errors that locating on the deck. Had this very problem with my own A10. It was + .020” but when I bored it to + .040” one cylinder ‘ran out’ near the bottom. Clearly the previous bore was not perp and would explain why the piston had picked up. Damned nuisance, as I had a set of original Hepolite ready. Had to go to + .060” and a set of JP pistons which, like the ‘box’ they came in, looked rather cheap. Except they weren’t.
Not sure about TT’S comment re ‘stupid engineers’. With today’s pistons of unknown provenance or material, who really knows what the clearance should be? Gone are the days of the Hepolite catalogue with all the information pertaining to their pistons that an engineer could ever need.
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

Online Superflash

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Re: Engine misses and loses power
« Reply #63 on: 27.02. 2020 22:10 »
Speaking of which, rang my engineer to see how it was going with the rebore of my cylinders.  I got a set of 040 overs from JP pistons in SA. They recommended clearances way tighter than the factory specs. Anyway after discussing the issue and in light of the chat here, i asked him to give them another thou. What with global warming that'll be enough to stop them nipping up hopefully.  Cheers
68 Trophy
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Offline BSA_54A10

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Re: Engine misses and loses power
« Reply #64 on: 28.02. 2020 20:59 »
Speaking of which, rang my engineer to see how it was going with the rebore of my cylinders.  I got a set of 040 overs from JP pistons in SA. They recommended clearances way tighter than the factory specs. Anyway after discussing the issue and in light of the chat here, i asked him to give them another thou. What with global warming that'll be enough to stop them nipping up hopefully.  Cheers

DO NOT USE THE SUPPLIED RING THEY WILL TAKE THOUSANDS OF MILES TO BED IN AS THEY ARE TOO HARD
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Trevor

Online Superflash

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Re: Engine misses and loses power
« Reply #65 on: 28.02. 2020 21:45 »
Thanks for the heads up. Are there other ones you would recommend that fit the JP pistons? He hasn't started on the rebore yet so maybe I should tell him to hold off till I sort some new ones out? Cheers
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Offline BSA_54A10

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Re: Engine misses and loses power
« Reply #66 on: 29.02. 2020 06:18 »
Go ahead with the pistons, they are fine I have used near 100 of them.
Toss the rings on ebay for the "nothing is too cheap to put on my BSA crowd " and get some standard rings from almost anybody.
Trojan keep Hastings
The new asian Hepolites are all over the web
When i get a order ready for Total Seal  I buy theirs because I really like them , but I have to get some one in the USA to order them because the pirates who have the retail rights down here make Ned Kelly look honest.
They charge about 3 to 4 times the USA price then add a $ 15 to $ 40 freight charge plus put GST on the lot.
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Trevor

Online Triton Thrasher

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Re: Engine misses and loses power
« Reply #67 on: 29.02. 2020 12:14 »
They recommended clearances way tighter than the factory specs.

That doesn’t sound very clever.

Offline Clive54bsa

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Re: Engine misses and loses power
« Reply #68 on: 01.03. 2020 20:25 »
Hi BigJim, I'm following this post intently as I've had the same experience on my '54 Golden Flash. It will run all day perfectly at 55 mph, but as soon as I push it up to 60-65, within about 30-60 seconds I'll notice a miss, which will quickly grow to a cough and finally it backfires. However when I pull over it idles perfectly and I can immediately ride away as long as I don't go over 55 (slower speeds if in a lower gear so I think it's around 3700-4000 rpm). I've filled the petrol tank and drained it into another and it flows perfectly, no tank vacuum, fuel is getting to the 276 carb, checked tappet clearances, timing is spot on. Ran it in the dark to check for any visible electrical arcing.
I changed the carb to a 375 monobloc and no change. I have run the bike for about 8 years like this, as 55 mph is just fine for me, but something is obviously  wrong. I have always noticed that if I applied the choke it would delay the symptoms for a while. So just recently I played with the mixture settings firstly installing a larger main jet, then I raised the needle 1 notch at a time until I ran out of notches and "voila" it didn't miss at all. I ran for about 10 miles at about 70 mph and it ran fine, however when I got home I pulled the plugs (L82C) and they were very sooty, so continuing to ride at those settings would certainly foul the plugs eventually.
A friend of mine who'd I term as an expert of British twins believes it's probably the magneto is not able to create enough juice to fire the plugs at high speed, and richening the mixture aids in the ability to fire at speed. I did have the mag rebuilt a few years back, but I supposed it could be that, I tried every thing else, I think.


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Online chaterlea25

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Re: Engine misses and loses power
« Reply #69 on: 01.03. 2020 21:30 »
Hi Clive
That sounds like the mag slipring tracking, I have come across similar symptoms before
Have you tried cleaning it?
There were poor quality sliprings on the market some years ago

John
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Online Slymo

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Re: Engine misses and loses power
« Reply #70 on: 02.03. 2020 00:49 »
For Clive I would suggest a plug chop. Take it to the speed where it coughs and backfires and hit the kill switch. Pull the plugs and check the colour. If they are black it might be a excessive main jet size.
NZ

Offline Swarfcut

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Re: Engine misses and loses power
« Reply #71 on: 02.03. 2020 08:27 »
 Daft as it sounds, the maximum speed at which the points can open and close will act as a speed limiter. Check that the magneto points pivot is free and lightly lubricated. Any binding here will delay the rate at which the points close, and also check that the spring has not weakened over time. This is a bit subjective, or confirm by substitution. Easy fix, worth a look, as is the cleanliness of the slip ring, and the condition of the carbon brushes....pick ups, armature (next to nameplate, under the brass screw) and on the back of the points plate (early brass type......later steel does not have this)  Later steel type will missfire and run erratically if points spring touches the cam ring. Try in the dark with magneto cover off if you have the later points type.

 Swarfy.

Online BigJim

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Re: Engine misses and loses power
« Reply #72 on: 10.03. 2020 21:12 »
Ok, for anyone still following this a quick update. All expletives have been removed and a sedative of the black and tan variety has been ingested. On a positive note am choosing to ignore the vertical scoring on the cylinder wall at present. This will cause upset i'm sure but hey ho.
So have crack on original head around a holding down bolt so chose to buy and refurbish a replacement. Spent lots and it has created more problems than oily beggar. Have buggered about with alternative valves but ended up with no compression and lots of distress and cursing! The original oily one is currently fitted and will be tested at some point tomorrow when back from work.
The main reason for this post is to show gratitude for advice given re fitting the rockerbox. Have had to become a bit of an expert on this recent journey. Doesn't mean that it always goes well but generally much better. I have probably fitted it 10 times in the last week!
The big tip is the bending of the comb to sit flat on the top fin. Then the clamping with that welders clamp thingy. Massive increase in the chances of success. Can add that removal of the 2 rear holding down studs and the top inspection cover stud really help.
More to follow, may even post pictures of the scoring in the barrel for general distress.
Also big thanks for support and advice already given.
 *bash* *bright idea* *pull hair out* *beer* *beer* *beer*
Jamie,  Supporter of Distinguished Gentleman's Ride

Online mikeb

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Re: Engine misses and loses power
« Reply #73 on: 11.03. 2020 04:11 »
perseverance and black & tans - the true secrets of BSA ownership!

btw have you tried the rubber band trick on the rocker box for aligning pushrods, as per:
http://bsa-info.nz/wp-content/uploads/rockers-rubberbands.jpg

I'm following this thread with interest - I'm sure you will get there in the end
New Zealand
'61 Super Rocket  - '47 B33 -  '18 Triumph Street Triple RS

Online BigJim

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Re: Engine misses and loses power
« Reply #74 on: 13.03. 2020 22:15 »
Yes to rubber bands.
Have refitted old head and generally been on my knees wondering if my life could be better spent doing something/anything else. Then on the first test run a whine/whistle appeared coming from the left exhaust rockerbox area. This was the final straw and i returned home in quite a funk. Bike was angrily parked and an alternative fast reliable beggar was wheeled out and taken for a lunatic blast. When passing the A10 mention was made of the grinder and sledgehammer with associated dark mutterings. Very dark indeed, genuine talk of divorce after many years of ownership.
In a calmer state i tightened the oil rocker feed bolts which were a little loose and thinking this may have been the noise off i went. Guinness may not be a performance enhancer after all. Initial results were promising and i thought i had cracked it. The whine/whistle returned but possibly to a lesser degree. At the moment to maintain my sanity i am saying it's exhaust escaping around the pipe. Am planning to go to the autojumble at Shepton Mallet tomorrow to meet up with some friends. This will be a proper test, kill or cure!
The reason for this post originally was regarding the newly fitted head to which i will return at some point. Am still hoping that after great expense it can be fixed. suspect valve guides not suitably reamed. Being of a suspicious nature am also wondering if it is even the same head that i originally handed over. The stove enamelling is so thick that anything could be underneath it! Should have marked the combustion chamber. That could be copied of course.
To conclude; the beast runs, is still in my ownership, have not resorted to violence.
 *countdown* *pull hair out* *bash* *beer* *beer* *beer*

Jamie,  Supporter of Distinguished Gentleman's Ride