Non-starting Classic

[MikeBristol]

Hello all,
Was out on my Classic a few days ago, seemed pretty normal though perhaps not quite as smooth and even as usual. At next start attempt (full choke at first), it started and revved to about 3,000, but when the choke was pushed in a bit slowed down and faltered before stopping. Same at next attempt, then next time not starting at all. Has fresh fuel, and my spark tester shows sparks at both sides, though a bit weak on the left-hand side. No spitting banging or any other sign of life. It's a low mileage bike with excellent compression. I will change the plugs tomorrow but doubt this is the issue.
What to check next? How could choke or other carb adjustment have suddenly changed to produce this effect?
Not used to SU carbs, and not wanting to fiddle and make anything worse!
Anyone in Shropshire area fancy a look? Considering sending it to Andover for other work, but there's perhaps a lack of expertise there now?
Thanks for any ideas, Mike

[Dekave294]

I have no experience of SU carbs, yet the problem you describe sounds similar what happens to Keihin CV carbs om my KTM if not used regualarly?
Where the pilot air jets block because of modern fuel and the diaphragms/slides don't function properly. If left for up to 6 weeks the pilot fuel jets can block too.
Perhaps worth considering?

[johnbirchjar]

Hi Mike,
SU carbs are pretty stable once they are set up,so a bit more info please,has the bike stood for any length of time?try checking the diaframs on the fuel tap,(the dreaded E fuel just loves them,I did away with the vac:fuel tap on my Classic,re-placed it with a conventional "Gravity Tap" after fitting THREE diaframs in one year! )another check is take the plugs out,turn the engine over and listen for the chambers to "Chuff",the old memory isn't what it was so not quite sure if the "Chuffing"is good news or bad,doubtless someone will tell,J.B.

[Anthony Duffield]

Mike,

I have a Commander, which may be different from your Classic, if my bike had the same symptoms it would suggest to me lack of fuel.

You said:

seemed pretty normal though perhaps not quite as smooth and even as usual. At next start attempt (full choke at first), it started and revved to about 3,000, but when the choke was pushed in a bit slowed down and faltered before stopping. Same at next attempt, then next time not starting at all. Has fresh fuel, and my spark tester shows sparks at both sides,

It ran on full choke then stopped, then no starting at all. This suggests your were starting to run out of fuel, using full choke provided enough fuel to get it started then it would not start at all.

If you have a vacuum pipe to the fuel tap, pull it off at the engine end, disconnect the fuel line to the RH carb, suck on the vacuum pipe to check that you have fuel, if you do suspect a stuck float.

Good luck.

[MikeBristol]

Thanks Anthony. I'll check this out tomorrow and report back, Mike

[johnbirchjar]

The lack of fuel?hence my suggestion to check the fuel tap diafram,J.B.

[MikeBristol]

The vacuum pipe from the petrol tap disappears behind a cover which I can't yet see how to remove, so at the moment I can't check for fuel in the carbs. What's the best method to see if the tap itself is faulty, to eliminate that before going further? Mike

[johnbirchjar]

Hi Mike,if your Classic is as per all Classics, the panel is only held on by two deutz screws,then you can get at the bottom of the fuel pipe where it runs into the carb:(I think,the old memory aint what was)take it off and with a descent battery turning the engine over you will know if fuel is getting at least to the carbs,if not it's an odds on bet that the fuel tap diafram is knackered(unless the fuel filter is all bunged up)It's a simple job,and if this isn't the problem, then a process of elimination begins,J.B.

[MikeBristol]

I'll take a closer look!

[MikeBristol]

Had a few minutes today and checked that the petrol tap appears to be working correctly - fuel flows when engine cranked. So next to check the carb end. Is that possible without removing the panel mentioned above, which at the moment I haven't been able to shift? Can I just undo the unions at each side, held in place by retaining plates 50-0616?

[MikeBristol]

Should read retaining plates part no. 50-0617 !

[Joe]

Hello,
Yes, just loosen the screws and rotate the retaining plate out of the way and you will be able to pull the fuel feed unions off to see if you have fuel.
If you still have the original fuel filter fitted check the end hasn’t reduced in size due to modern fuel by pulling off the fuel hose and checking the internal diameter of the filter. If this has reduced you won’t have sufficient fuel for both carbs and the bike will only run on one chamber, the left hand side as this is supplied first with fuel.
Good luck,
Joe.

[MikeBristol]

Thanks Joe. I've removed the unions in turn and fuel flows to both sides when cranking. I don't see a fuel filter in the parts book, and there's no mention in the manual. Do all Classics have them? When I next get out to the shed I'll whip out the plugs and check for fuel there, Mike