The BSA A7-A10 Forum

Technical => A7 & A10 Engine => Topic started by: scotty on 20.03. 2019 20:29

Title: Backfire left cylinder
Post by: scotty on 20.03. 2019 20:29
Was feeling chuffed with myself after replacing head gasket and installing rocker cover without drama

After  few pre checks and pre lubing bike kicked over first time but back fires badly on the left exhaust

No prior history of this

Going to re check valve lash etc

Has anybody experienced this before?


Title: Re: Backfire left cylinder
Post by: bikerboy on 20.03. 2019 20:51
I think you might need to give a bit more info for any sensible suggestions.

Was it running ok beforehand?

Is it a misfire or a backfire in the exhaust, there is a big difference?

Is it when giving it throttle or when de-throttling? Is it when on tickover for that matter?

Was it as soon as it started or when it was actually being ridden?

What have you tried so far?
Title: Re: Backfire left cylinder
Post by: scotty on 20.03. 2019 21:21
Thanks for the reply Bikerboy

Was running ok before I had the head off but I parked her last October so this is the first time this year

The bike starts fine
The backfire/misfire happens at idle and at increased throttle
Unable to take it for a ride at the mo

After reading your post I’d say it’s a misfire

I’m wondering if I didn’t pay enough attention to the internal timing when I put the head back on
I’d better go back to timing basics 😳
Title: Re: Backfire left cylinder
Post by: Swarfcut on 20.03. 2019 21:53
Scotty.. This will be a simple fix, with a bit of luck.

 Check the pushrods are correctly located in the ends of the rocker cups.  Then reset the tappet clearances.   Easy to back off the adjusters, get the pushrod  on the back of the cam, then raise the rocker and makes sure the pushrod enters the cup. Mis- aligned   is how they get bent.

 It may be an ignition problem, try substituting the HT leads,  lead for lead, not swapping them side to side. Nothing you have done to the head will affect the ignition timing.

 Backfiring in the exhaust can also be a weak mixture, but more likely in both pipes. As you have had the top off, the pushrods are a good place to start.

Title: Re: Backfire left cylinder
Post by: WozzA on 21.03. 2019 00:27
Did you use exhaust gasket goo on the headers?  possible air leak ???
I had the same thing happen to mine...    Just my 2c worth.. 
Title: Re: Backfire left cylinder
Post by: bikerboy on 21.03. 2019 00:47
I agree with air leak being a possibility but thats normally more apparent on de-throttle.

Start with the simple things, swap the plugs and see if the misfire swaps cylinders.

Then obviously recheck the tappet clearances making sure that the threads on the adjusters are about equal in care you have trapped a push rod on the outside of the cup.

A compression check if you have a tester is always useful and only takes a few minutes

Is it low on fuel or having left it so long has the fuel turned into goo as it has a habit of doing?

Cleaning the carb after a long lay off seems mandatory now due to that damned ethanol, yes I know that should affect both cylinders.

I cant see why having the head off would affect the timing but one other question is did you clean the top of the pistons and if you did what with? It might well be that there is some cleaning product flying around in there and hitting the spark plug.

Are both plugs the same colour when you remove them?

All basic tests because I doubt its anything serious.

Oh and of course check your points, assuming its not electronic ignition of course
Title: Re: Backfire left cylinder
Post by: scotty on 21.03. 2019 02:22
Many thanks for all the great suggestions
I’m going to mull them over with a beer and have another crack at it tomorrow
Title: Re: Backfire left cylinder
Post by: BSA_54A10 on 21.03. 2019 07:08
Simple things first.
Swap the plugs & see if the misfire changes sides.