Backfire through carb.

[Pete Gill]

Hi All

I rebuilt my B31 (1949)  a while ago but haven't had chance to get out on it to test ride.
I've had it going on the main stand and have warmed it up and run it through the gears to make sure it selects and the clutch works okay.
I have done this 2 or 3 times this winter just to get the engine going and iron out any faults.
It's been running well up to now and I'm pleased with it.
Last time i did this i found the engine seemed to misfire and stop suddenly when running between 1000 to 2000 revs. It popped back through the carb.
It happened again shortly after when restarted. Ive not tried to restart since for fear of causing damage (although it will start easily).
The only adjustment i had made that day was to slightly open the air screw on the pilot jet as it was i think running a bit rich.
I have checked the timing and points as i thought it might have slipped but its spot on.
Carb is not loose on manifold.
Plug is black with thin film of soot but not oily or wet.
Can any of you advise?
My thoughts now include a sticking inlet valve? ( head rebuilt recently with new valves and guides)
Could it just be old fuel or a carburettor issue?
I have a new Amal monobloc set up standard running with a bell mouth.
Magneto has been overhauled and tested recently.

Pete

[LightningAndStarfire]

Does it have manual or automatic advance unit? Lack of lube on an automatic advance unit can cause backfire.

[Pete Gill]

It's a manual advance lever. Set up the be full advance when cable is tight, slackened off a bit to start up.

[Pete Gill]

UPDATE

Ive removed the tappet adjuster cover to check and found the exhaust pushrod unseated.
I am certain the adjustment was correct on assembly.
I'm thinking the valve stems may not have sufficient clearance in the guides and are sticking or perhaps a valve spring has failed. It'll take a while for me to get around to this so Ill update as soon as i can.

Has anyone had experience of similar?

Pete

[steve s]

Funny that it was ok, it could well be something in the carb, weak mixture /lack of fuel can cause backfireing. Check the simple things 1st would be my advice

[martinh10]

I recently replaced the valves and guides on my B31. On assembly I thought the valves were a really nice fit in the guides. The bike started and ran OK in the garage but one mile up the road on my first run it stopped abruptly. I pushed it home and on removing the tappet cover found it had thrown the exhaust pushrod. On removing the head it was evident that the exhaust valve, although now fully closed, had stuck fully open at some stage and the inlet valve had touched it.
I made a Go-No Go gauge for the exhaust guide and found it to be slightly undersize. According to BSA Service Sheet 702 it should be ".352/.353. I purchased an adjustable reamer and reamed it to suit the gauge.
I've not got the head back on yet (waiting for other parts) but I'm hoping it's sorted the problem.

[Pete Gill]

Thanks for the replies.
Martin it sounds like i have a similar problem. I'm on with another job at the moment so I'll get back in a couple of days when I've got the head off.
Pete

[Pete C]

I had exactly the same Problem as Martin 

[Pete Gill]

Ive taken off the head and found the exhaust valve seized in the guide.
I had to knock it out. It's straight with no evidence of impact on the piston but quite galled so i suspect it will need a new guide and maybe valve.

Have any of you with similar issues advice re types of guide to use?

Pete

[Pete C]

Presuming you are using cast iron guides, make sure you don't have stainless steel valve stems. The stem should be hard chromed or nitrided. This only applies to the exhaust valve.
Many valves are now made from stainless due to lead free fuel so you need to specify the finish when you buy valves.

[Pete Gill]

Thank you Peter I'll make sure i do that

Pete