Engine only runs at full throttle

[TheDutch]

Hot off the back of my fuel tank problem my CU engine is now stalling under idle or part load. After replacing the tank etc. the fuel filter would become clogged with dirt and possibly water, but despite clearing this out it is still not running right. I can just about get it started and, by giving it lots of throttle, keep it going, but anything less than 2/3 throttle seems to stop fuel getting into the engine.

I have replaced all the rubber in the engine bay, fuel filter and pump are good, spark is ok, and have disconnected idle stabilizing unit so I suspect something is up with the carbs and that they may be partially blocked. That is my hypothesis, as it'll still do maybe 60mph, but you just can't pull away slowly, and have to sit like a boy racer revving the engine at the lights!

Do the standard carbs have one jet for idle/low engine speeds and another for high speeds/open throttle? It looks like the idle mixture is controlled from the left hand carb, is there a needle in there that could be blocked?

Any ideas gratefully received - last night I set off for the ferry port only to return disappointed at 6:30 this morning, so I am losing more of my holiday with every day and need this sorted If the worst comes to the worst then I'll have to try and get it to a specialist; can anybody recommend a mechanic in the NW London / South Bucks area?

Cheers,

Neil

[geordie skydiver]

Racing throttle sounds like a air leak, I had a similar problem, I ended up swapping the twin carbs for a single weber 32/36.

I hope you sort it out soon.

[CovKid]

Give the carbs a good clean, check for wear in the throttle flaps (ie up and down movement), that chocke operates correctly, no pipes are off or frayed and makes sure everything is airtight where it should be. Carb repair kits would seem a sound idea but throttle wear is VERY likely if they've never really had any work done on them. Wear in throttle flaps will make idling unstable at best and impossible at worst - as will any air leaks on the inlet side of the combustion process.

Make sure no electrical wires have come off on the carbs - particularly magnetic cut off solenoids

[TheDutch]

The engine has done 59,000 miles - could carb butterfly wear be a possibility at such a low mileage?

I'll double check for air leaks but I don't think it's likely. All the solenoids click resoundingly with the ignition, though the 'idle enrichment' module does not function. The chokes work ok, and are completely off when warm.

I have taken a look at the spark plugs which look like it is running lean, so air leaks or carb/fuel supply problems seem likely. Still don't know about the different jets / blockages I could be suffering from though.

Neil

[corleone]

Check that all the throttle linkages are ok, had to race my engine to get it to the MOT station last week. Wouldn't idle at all. The linkage on the right hand carb had vibrated loose so the whole thing was badly unbalanced.

Worth a try.

[CovKid]

Carbs could be full of crud. Doesn't take more than ten mins to take off the top half and expose the float bowl. Not uncommon to find water in the bottom of them as well as other rubbish - filter or not.

[TheDutch]

Carbs could be full of crud. Doesn't take more than ten mins to take off the top half and expose the float bowl. Not uncommon to find water in the bottom of them as well as other rubbish - filter or not.

Did exactly this earlier on today! Was a fair amount of rubbish in there, injection needles were blocked and other crud removed. Top speed has now been rasied from 65mph to 76mph. The idling/part throttle problem still remains but is better.

My clean did not involve removing the main jets so I might go further and try them tomorrow. Am still in two minds about getting a rebuild kit.

Does anybody know a normal baseline setting for the idle speed screw? Mine stalls if screwed all the way in, and does not make a great deal of difference when unscrewed many turns, it just hisses a lot.

[cumbriankeith]

You can get the idle mixture screw out from the top of the left carb - it's right next to the idle air intake coming from the airbox (screw it in gently counting the turns before removing - it may be way out of whack).

You can also remove the basic idle screws from each carb in situ - they point outwards from next to the throttle spindles - basic setting is 2 turns out from the bottom. (screw them in gently counting the turns before removing - they may be way out of whack)
Take all three out, check/clean and replace - should only take 10 mins and you never know...

[toolsntat]

Just had this sort of problem occur and suspect mine is a fuel supply problem on tickover.
Will get down to having a look at it later but whats puzzling me is how and at what revs does the idle change because mine likewise runs ok when driving ??

[cumbriankeith]

"baseline" for idle mixture screw is in the 2 to 3 turns out from bottom - really needs setting up on a proper machine...

[TheDutch]

I have just finished re-cleaning the carbs as best I can without removing them from the engine, and now have a sweetnut tickyboo perfect sounding tickover.

I was able to remove the top halves from both carbs without damaging the gaskets, and took out everything I could get at to carb clean and blow through with a foot-pump as thoroughly as possible. By measuring the number of turns for any tuning screws I was able to put it all back the same.

Replaced all the ignition parts also, and now it seems to be 90% good (so far), and will hopefully enable me to get to Harwich sometime tonight ready for a ferry early tomorrow morning.

Any other hesitation I'm getting is most probably down to the timing/fuel adjustment, but I don't really have time to get it looked at and am happy with it as it is.

Questions and congratulatory outbursts welcome,

Neil

ps. toolsntat, if you haven't already, I'd suggest trying to give the carbs a semi-refurbish like I have. Cost me nothing and did the world of good. Likewise the ignition stuff, it doesn't really cost a great deal (£25ish from GSF) and it all helps.

[cumbriankeith]

Nice one - thanks for reporting the good news and enjoy the trip

[toolsntat]

Likewise Neil hope all goes well
As I`ve just replaced the leads etc. will have to look at the carbs as you have...
Cheers
Andy