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Hi ! i'm trying to resolve a problem on my 62' A65

single Monobloc carb, Boyer digital (red box), dual Boyer ignition coil

It was runing nice, did 40 thousands miles with no big problem.

But suddenly it's running bad at start (coughing, need to rev it high, harsh sound) but after 5/10mn running (maintaining high revs to avoid stalling) it runs better, and after 5mn more it runs ok. Thought it was ignition problem, changed spark plugs, connectors, tried another coil, and another Boyer box i had, the problem stays exactly the same. Cleaned the carb (fully opened it and cleaned it all), same problem ....

Any idea ?


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Sounds like a pilot blockage or an air leak, is the slide worn badly? O- ring in the manifold perished? O- ring on the air screw can perish badly. I would try a different carb that is known to run well, old carbs at that kind of mileage can be problematic.

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Have you removed the points cover or changed the point cover screws lately ?

Try it without the point cover in place, there's nothing to lose.


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Why do you think the points cover would change something here ?


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The slide moves freely, i checked it when i cleaned the carb, and i clearly hear it moving up and down - i also checked the move before putting back the air filter, and it moved perfectly from full closed to full open and back with no blocking point ... i may search for an air leak, the main O ring is in place and looks good, but i didn't chek for the o ring on the air screw ....


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PEH, from what i see on schematics, there is no o ring or seal on the air screw on the Monobloc carb ....


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Originally Posted by Rokplo
Why do you think the points cover would change something here ?

Because a bike I know had a seemingly incurable running problem. It was eventually discovered by chance that it ran fine with the point cover removed. One of the cover screws was intermittently grounding out one of the points LOL


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Pilot air bleed clogged? Sounds like the Concentric problem if Monoblocs do the same thing….

Wow!! “ It was runing nice, did 40 thousands miles with no big problem” Is that possible ???? 😳

Last edited by KC in S.B.; 09/19/22 1:40 pm.

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Sure it's possible. Did 2 complete check, with engine strip down, no major problems found. Some minor worries time to time, but always found a way to repair and get back home. It's the first time i can't find how to fix it ....


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Originally Posted by Pureblood
Originally Posted by Rokplo
Why do you think the points cover would change something here ?

Because a bike I know had a seemingly incurable running problem. It was eventually discovered by chance that it ran fine with the point cover removed. One of the cover screws was intermittently grounding out one of the points LOL

Unless the screws were 3” long, the cover would have no effect at all on an A65.


Originally Posted by Rokplo
Hi ! i'm trying to resolve a problem on my 62' A65

single Monobloc carb, Boyer digital (red box), dual Boyer ignition coil

It was runing nice, did 40 thousands miles with no big problem.

But suddenly it's running bad at start (coughing, need to rev it high, harsh sound) but after 5/10mn running (maintaining high revs to avoid stalling) it runs better, and after 5mn more it runs ok. Thought it was ignition problem, changed spark plugs, connectors, tried another coil, and another Boyer box i had, the problem stays exactly the same. Cleaned the carb (fully opened it and cleaned it all), same problem ....

Any idea ?

You’re running a Boyer micro digital… first thing I would look at/charge or change is the battery.

Sounds like the charging system is topping the battery up the more it is running but maybe a cell has gone down or it has been stored a little while with a partially discharged battery.

Electronic ignitions are very susceptible to battery issues. Infact when having an electronic ignition, most “seemingly carb problems” are infact electrical problems… more often than not the battery.


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Originally Posted by Allan G
Unless the screws were 3” long, the cover would have no effect at all on an A65.

3/4" is enough on an A50


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Pureblood, in the OPs first post he states he has EI not points.Nothing out of ten for reading comprehension.

+1 for what Allan said, check your battery voltage, before starting, then when running. Sounds like a bad battery to me.
Equally possible , there may be a dodgy connection somewhere between battery and Boyer, a jumper wire will prove this one way or another.

Last edited by gavin eisler; 09/19/22 5:23 pm.

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sometimes the whole jet block in the carb or just the jets can come loose ... easy thing to check


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Forgot to say at the beginning : the battery is perfect and fully charged, no worries about this - anyway, the digital Boyer works perfectly at 10v (that’s why i put the digital Boyer, the black ones misfires and changes the advance under 12v)


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Two thoughts come to mind.....

1) compression, ....... busted ring?? ...... but then it would probably be hard to start, but perhaps not if on only one pot. Feel the exhaust, you will soon see if the issue / misfire is with both cylinders

2) Forget the battery, ... how many volts does the Boyer actually GET? (as suspected by Gavin)

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I got 13,4V on the battery, no problem here - i ordered a compression tester, i'll check the compressions when i get it (ring or valve problem ?) i'll check the voltage directly on the Boyer connectors, just in case of a faulty cable ...


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If I’ve learned anything this year it’s that voltage only tells you part of the battery story.

Much smarter people than me ( you know who you are) taught me about “load testing”. I don’t remember right now (nothing unusual for me) but I think I had 6 batteries in the shop that all showed good voltage. Purchased a load tester…..threw 3 of those batteries away.

Mr Healy says “You don’t need a new battery, you need a good battery

Unless you load test it……you don’t know what you have.

Damnit…….give the boy (me) a little knowledge and he starts acting like an expert!!! laughing


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I've had batteries going dead, the voltage was going down and the horn was low, as the lights .... and the bike was still running well with a little rev ... the actual battery is in perfect condition, fully charged, horn is loud and lights are bright ...

don't know about this "load test", sounds like a marketing invention for me ^_^


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Originally Posted by Rokplo
I've had batteries going dead, the voltage was going down and the horn was low, as the lights .... and the bike was still running well with a little rev

don't know about this "load test", sounds like a marketing invention for me ^_^

On a bike with a Boyer?

All a man can do is try to help…..can’t make someone actually listen. Yea, load testing a battery is simply a marketing invention

That said…..I had a Commando that the valve springs were bad/weak. Ran like a pig until it warmed up.

Last edited by Gordon Gray; 09/20/22 1:58 pm.

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As i said before, the Boyer digital (red box) works very well with low voltage. At 10v it works perfectly. Not as the old ones (black box) that begin to fail under 12,3 v ....


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Its not the voltage when not in use but its ability to supply a even current when under load. Look up Volts-Ohms-Amps

[Linked Image]

You could have a battery which works fine when you put a meter across it and the bike not running, but start using the bike and it runs poor. maybe it will get better as the bike runs on and the battery slowly accepts charge. (I have had this) with a high output alternator I have gotten away with it for some time but eventually the battery just says see-ya!

With mine, I fitted a capacitor in parallel to the battery for a period of time and that showed a huge improvement in the bikes running (it always ran ok before hand). In the end I found my problem lied with my regulator, despite charging the battery the bike ran like crap. I then fitted the MOSFET Trispark reg-rec and its ran really good with just that and the battery and didn't refit the capacitor.


You could also connect the white wire from the Boyer straight to the battery and add an aditional red wire from the battery to the +VE side of the (2nd if you have two fitted) coil which is where the red wire from the Boyer box also meets. - this will rule out any wiring gremlins (as per the funny picture).


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I already put a mosfet regulator (Schindengen) because all the electronic regulators we find are crap. When a battery gets poor when running, the voltage lowers ... in my case the voltage is around 14v when running, and 13V whith lights on ... we discuss a long time about battery charging, but it's obviously not the problem here ....


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With the engine not running switch on the headlight and leave it on for two minutes.
See if the headlight brightness reduces in that time.
If it does your battery is crap.
This is called a load test and doesn't cost you a cent (or even a euro).

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it doesn't reduce, as i said, my battery is fully operationnal, i can let the light on 15 minutes, it won't reduce .... But if anyone here is interrested in batteries problems, he can create a new topic about battery load tests ....


ps : for the record, i did this 2 mn test - at start, battery was 13.4V - after 2mn of bright light, it went to 13.2V - and the light was still bright ....

Last edited by Rokplo; 09/20/22 5:57 pm. Reason: more informations

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Good-- you have just conducted a battery load test.
Didn't feel like a slick marketing con trick did it?
Based on your result I would agree with you that the battery is not your problem.
Having established that your battery is good you now need to make sure that its volts get to where you want them to get to.
Check all the connections from the battery---both on the supply side and the earthing side.
Dont just measure volts-- tug on the connections while measuring volts--often the volts are expected to get through just one strand of wire.
Best of luck!

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Originally Posted by Rokplo
Hi ! i'm trying to resolve a problem on my 62' A65

It was runing nice, did 40 thousands miles with no big problem.

But suddenly it's running bad at start (coughing, need to rev it high, harsh sound) but after 5/10mn running (maintaining high revs to avoid stalling) it runs better, and after 5mn more it runs ok. .........
Have you checked valve clearances? If they are tight, that could be the trouble. Also, high mileage BSA's can carbon up the valve stems so the valves stick which could also cause your problems. It sounds like most everything else is ok from what I've read in the thread.

It makes me think of a Honda F4I 600 project bike I had some years back. According to the seller, it would only start sometimes and he had given up on it. It turns out it had negative valve clearance on several valves (Valves being held open). I corrected the clearances and the bike fired right up and purred like a kitten. I never heard it run with the tight valves but I imagine it would have sounded like a 4 cylinder version of your BSA until it warmed up.

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Stuart it's a possibility, i ordered a compression tester, it will tell me if compression is ok ...

if it is, no problem of valve or rings ...

if there is a compression problem, i'll check valve clearance, and if valve clearance is ok, i'll have to open the head to check what's going on here ....

Il also ordered gaskets for the carb, i'll put new ones everywhere i can, to avoid a potential air leak.

I also heard of valve getting a little bit stuck in the guide when cold, and become free with heat, because of a bit of dirt stuck somewhere ....


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Tridentman, if there was a wire problem, it wouldn't become good with heat (or it would be the first time i hear about this)


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Originally Posted by Rokplo
…..snip…..
I also heard of valve getting a little bit stuck in the guide when cold, and become free with heat, because of a bit of dirt stuck somewhere ....

It was along time ago but I had a Commando that would run like a pig until it warmed up……then was fine. I have no recollection of how I found out what the problem was but it turned out to be weak ( not broken) valve springs. Swapped out for new and the problem went away.

Just trying to help


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Originally Posted by Gordon Gray
Originally Posted by Rokplo
…..snip…..
I also heard of valve getting a little bit stuck in the guide when cold, and become free with heat, because of a bit of dirt stuck somewhere ....

It was along time ago but I had a Commando that would run like a pig until it warmed up……then was fine. I have no recollection of how I found out what the problem was but it turned out to be weak ( not broken) valve springs. Swapped out for new and the problem went away.

Just trying to help


Thanx, i'll have a look if i have to open the rockers cover ....


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Someone had to have told me about that being a possible problem…..honestly I wouldn’t have known to look at/check them.

I think it was one of those things where I actually got the new springs first figuring new springs wouldn’t be a bad idea.

The bike would start but REALLY stumble until warmed up then it ran okay.

I only paid $325 for it so it got a lot of worn out stuff replaced.

Good luck……keep at it, you’ll sort it


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This may have been covered disregard if it has.

How’s the carb slide, ever been sleeved.
Not sure if you checked vacuum leaks. I use WD 40 sprayed around the flange. If it revs you got a leak, be carful.
How’s the plugs looking on a cold run, before it smooths out. ie: crappy running may be lean do to sloppy slides or vacuum leak.

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Originally Posted by Richard Phillips
This may have been covered disregard if it has.

How’s the carb slide, ever been sleeved.
Not sure if you checked vacuum leaks. I use WD 40 sprayed around the flange. If it revs you got a leak, be carful.
How’s the plugs looking on a cold run, before it smooths out. ie: crappy running may be lean do to sloppy slides or vacuum leak.


Slide seems ok, il check for air leaks, i ordered a set of new seals and gaskets, i'll renew it all to be sure


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Carb clean, brake clean or easy start would show an increase in revs when sprayed around a leaking flange. WD40 isn’t very combustible and usually causes engine revs to falter.


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Have you felt the exhausts like I mentioned earlier?
Sometimes it is hard to tell if a problem is on both cylinders or just one.
You may not feel any difference, but if you do then you can eliminate a load of common factors, like carb, ignition etc.
Probably need two people.
Just spent a length of time messing with a Chinese copy of a 1990s Honda. Previous hackers had installed new carbs plus a ton of stuff, missing the fact it wasn't firing with any gusto if at all on one pot! Still haven't fixed the problem (and probably won't) but at least I know what it is!

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This morning, before going to work, i checked valve clearance. Left inlet valve had not enough clearance (0.10 mm instead fo 20). My carb is in pieces, waiting for new gaskets and seals, so i couldn't try yet. Hope it's only that ....

I'll receive my compression tester today, so i'll check compressions tonight


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I finally rebuilt the (cleaned) carb with the old gaskets .... exactly the same thing, no change at all .... i also put (again) new spark plugs, and checked it after 3 mn reving up : perfectly clean as new, no gas wet or oil on it

Last edited by Rokplo; 09/22/22 9:18 am. Reason: orthograph

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Just mesured compressions : 7bars/100 psi in each side, coherent with the 7.25 :1 compression ratio ... still thinking it’s a carb problem ...


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Quoting the WSM, "7.5:1 CR, Comp pressure, both plugs out, throttle open, warm motor, first kick 65-70, Average, 120 -125 psi.
If readings obtained are substantially below those quoted, the engine is need of attention to valves, and piston rings or may even require a rebore"


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Originally Posted by gavin eisler
Quoting the WSM, "7.5:1 CR, Comp pressure, both plugs out, throttle open, warm motor, first kick 65-70, Average, 120 -125 psi.
If readings obtained are substantially below those quoted, the engine is need of attention to valves, and piston rings or may even require a rebore"


I did mesurements on cold engine, first kick was 100 ....

Last edited by Rokplo; 09/23/22 9:31 am.

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Originally Posted by Rokplo
This morning, before going to work, i checked valve clearance. Left inlet valve had not enough clearance (0.10 mm instead fo 20). My carb is in pieces, waiting for new gaskets and seals, so i couldn't try yet. Hope it's only that ....

I'll receive my compression tester today, so i'll check compressions tonight

at 0.004" inlet clearance it will run like a pig until doing a high RPM. I tried tightening up my inlet valve clearances by a couple of thou once upon a time. completely altered how the bike ran, and it wasn't very tractable compared to standard settings.


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (now rebuilt)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

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Simple easy way to eliminate the possible wiring / connections. Attach a jumper direct from battery to the Boyer, try running. Then do the same with the other “side”, try running. (Not sure if you are set up + or - to frame)
BTW, This is how I found the loose lug rivet on the IGN switch was my issue. (Only jiggled when running)


Down to ‘69 T120R now a Tr6R tribute bike
‘70 TR6C “happy in the hills”
‘67 A65L numbers match, “best effort” from basket *
Gone:
‘66 A65L“in ‘95 getting back in the game”+ empty ‘67 Case&Frame *
‘69 A65L
‘68 A65L “red bike” basket, sold & made whole by BB member
‘68 A65F nice Tribute bike
‘65 A50L bitsa from spare parts, Son’s fun
‘62 A10 Spitfire
‘65 T120R sad case, saved by BB member
'65 XLCH “scratched THAT itch”……
‘93 K1100RS heavy metal (should be gone, still here…)
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Rokplo Offline OP
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Originally Posted by KC in S.B.
Simple easy way to eliminate the possible wiring / connections. Attach a jumper direct from battery to the Boyer, try running. Then do the same with the other “side”, try running. (Not sure if you are set up + or - to frame)
BTW, This is how I found the loose lug rivet on the IGN switch was my issue. (Only jiggled when running)


Yes, good idea, i'll make a very simplified wiring with just Boyer, battery and coils, it will eliminate the electrical possibility (or confirm ?)

Last edited by Rokplo; 09/23/22 2:57 pm.

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