Poor cold starting & below par performance

[s7paul]

Hi all,

Ever since I got my bike on the road I've never been entirely happy with a couple of things that my original 500 (back in 1979) didn't suffer with.

The first problem is cold starting, where it takes lots of cranking on the starter to stutter into life, and when it does you have to juggle the throttle & choke for a while before it runs cleanly (initially sounds like it's not running on all 4).

The second problem is overall performance, where it feels less powerful than I think it should (it's certainly not as fast as my old one), and feels a bit "woolly" when you snap the throttle open - a bit like if you were to try to accelerate hard while the choke was still partially on.

Once the bike is warmed-up, it starts on the button instantly, and ticks over just fine.

The bike has Dyna coils, and electronic ignition (it's had two different systems fitted, and ran the same with both).  The carbs have just been fully stripped, ultrasonically cleaned, and re-assembled using all new jets & seals.  Again, this doesn't seem to have made any difference.

The valves clearances are spot-on, as is the timing, and I'm starting to run out of ideas, so any advice would be much appreciated.

Cheers,

Paul

[tom400f]

Have you tried a compression test?

[LesterPiglet]

Any air leaks around the carbs/airbox? Have you checked the plug colour? Is the air filter clean? Have the carbs been synchronised after cleaning?

[s7paul]

Not done a compression test yet as a) I don't have a tester and b) the engine has only done a few thousand since rebore, new pistons and rings.  However, I think it's probably time to invest in a tester, just to make sure.

I don't think there are any air leaks, but good point; I'll check.

Carbs are synchronised (using vacuum gauges) and the air filter is new.  Haven't checked the plugs in the last few months, so I'll do it tomorrow.

Thanks for the suggestions.

[s7paul]

So, following a bit more investigation, having run the bike with the fuel tap switched off until it stalled (in readiness for a compression test), the plugs look like this:



Having now done a compression test, and wished I'd spent the £20 for a tester some time ago, the results are:

1  178
2  152
3  160
4  140

After a quick squirt of oil down the plugs holes, No. 2 went up to 165, and No. 4 went up to 150.

I also double checked the highest & lowest once the engine had cooled down, and No. 1 was still hitting 170, with No. 4 down to 135.

I guess the biggest concern is the large variation between cylinders (as I assume anything over about 140 should be good enough).  But would this alone be causing my problems?

I've also noticed that if the bike has been running one day, it's easier to start the next day, whereas if it's been stood for a couple of weeks, it's a lot harder to start. 

[tom400f]

Not sure myself whether much can be made of the plug colour when the engine has been allowed to starve of fuel, will leave others to comment on that.

Amongst the work you mention you don't say whether the valves have been lapped in with grinding paste. There wouldn't seem to be anything else left that could account for the variation?

[matthewmosse]

Has it been over reved post rebuild or similar? - more likely the tappets need adjustment or valves lapping in but I've seen the top rings smash their way completely out of the top of th piston and the engine still ran well like that - in that case there were 2 clues that all was not well - the bike was hard to start once warm and leaked oil - probably the oil leaks had more to do with past rebuild quality.

[s7paul]

I'll have to do a proper plug chop when I get the chance; didn't have time today.

The valves were lapped in, with a nice even contact band.  Valve clearances are correct, and it hasn't been over-revved.  No oil leakage, and starts fine when warm.

It's clearly going to have to come apart, as I'm not happy with such a wide variation in compression.  If I can get 170+ on one cylinder, I'd like the other 3 to be just as good.  I think I'll just try to optimise it as it is, to get through the best of the weather, then strip it down later in the year.

[matthewmosse]

If your lucky a bit of gentle use will allow something that isn't quite bedded in free off or a sticky ring to unstick. I've had engines stop leaking (and not because they ran out of oil!) and even up compression readings after a bit of regular light use - supposedly low milage usa inport engine in my 500/4 being one.

[s7paul]

Now that I've got the compression tester I can check periodically to see if there are any changes.

Thanks all for the comments/advice.