It's a mystery.

[Tim L]

Hi all,

I bought a 1973 XS2 last year to build a street tracker. I decided not to re-build the engine straight away as it ran
Now almost finished I have ridden it a few times and everything was great until the left hand side went a bit fluffy, around about the same time as there was an odd clunk, not too loud or worrying, from the engine. Not even a clunk really, what is less than a clunk but more than normal?
I went back to work overseas and a few months later came home, started her up. She ran for a minute, nicely on the right not so nicely on the left, then cut out.
I had ordered some later carbs, linked ones, off e bay so when they arrived I put them on and now she won't start. I have used a big old battery to turn her over on the electric start and there is compression on the exhaust side and suction through the carbs.
I put the old carbs back on and still no joy.
Today I have checked that there is a good spark and the points are opening at roughly the right time. I am pretty sure that fuel is getting through too, I can smell it although there isn't even a bang or a pop due to unburnt fuel in the cylinders which I would expect after turning it over for a minute or two.
Any ideas?
Has anyone ever known a cam belt to jump and thus affect timing?
Any pointers, clues or hints would be gratefully received.

Many thanks.

Tim.

 

[KevC]

There's never an easy answer Tim, start with the basics & work methodically through everything.
THIS should point you in the right direction.
Good Luck

 

[XS1961]

Firstly, Id change the cambelt for a proper chain
.Seriously though, I have known a chain to jump a tooth or two if the tensioner wasnt set-up correctly at re-assembly

 

[ady66]

If you're spinning the engine for a while the plugs should get pretty wet with fuel. If not suspect the carbs. Might also be worth trying a different set of plugs - they don't always behave the same in the chamber as in the open. I find the most trouble I have with my XS carbs is when they are left for a while without riding (too many bikes, not enough time!). If I know I'm going to park one for a while I generally run the carbs dry before it goes in the shed which seems to help. Re. the clunk - check the advance mechanism on RHS of camshaft in case something's come loose. Good luck.

 

[Tim L]

Not a mystery any more.
I tried 2 sets of carbs, cleaning them both first.
Checked timing.
Each time I used a spare spark plug to prove there was a good spark.
And there was my mistake. The spare was good but the ones in the bike weren't.
Brand new NGK B8ES, done about 15 miles. Both failed. Many valuable hours wasted on a wild goose chase!
New plugs, bike fires up easy.
Can't believe both plugs were duff after 15 miles.
Trouble is, I now need to re check everything as I also put new header pipes on and one side is very blue after 2 minutes, the other isn't. Either fuel problem or ignition timing. So that is the next job.

 

[twinsarehot]

That has happened to me. When I was trying to key in the carb settings, there was a point of no return in terms of fouled plugs.

 

[pumps]

Not a bad idea to carry a few extra plugs in your spares kit along with the proper tools to swap them out. I've done it and it works wonder for your mood when you can be off and running again.