R65LS - idle rising when ridden

[Rob Frankham]

In very small adjustments while the engine is running. Allow the engine to settle after each adjustment and occasionall boost the revs to 'clear it's throat'...

Rob

[Inveloveritas]

There was me thinking that would be the last stupid question.

How wrong I was.

Idle screws were about 1/4 turn out, now 1/2 turn. Would it have been lean at 1/4 ?

Than the idle screws. They are backed off full, they don’t even touch the plates. I’ve then wound them in so they are just touching the plates and it all seems great. I’ll try and take it on a run tomorrow and see if it rises again when really warm but the short run this arvo was all good.

Is that likely to be correct about the throttle screws ?

[barryh]

Idle screws were about 1/4 turn out, now 1/2 turn. Would it have been lean at 1/4 ?

Than the idle screws. They are backed off full, they don’t even touch the plates. I’ve then wound them in so they are just touching the plates and it all seems great. I’ll try and take it on a run tomorrow and see if it rises again when really warm but the short run this arvo was all good.

Is that likely to be correct about the throttle screws ?

The mixture screws at 1/2 turn out is about right and 1/4 turn would have been weak unless the fuel level in the bowls was very high.

The next bit may depend on whether the butterflies were removed when the carbs were overhauled.
The throttle stop screws - that is the ones that touch the plate shouldn't be backed off until they don't touch the plate or only just touch it. You would expect 1/2 to 1 turn in after touching the plate in order to open up the butterflies for idle. If that 1/2 to 1 turn isn't necessary to make the engine idle then it's a possible indication that the butterflies aren't installed correctly. They need to go the correct way around due to the edges being beveled to fit the venturi and they have to be centred on the spindle. The best test is with the piston out or raised out of the way, hold a carb up to the light with the butterflies closed and you should only barely see a very fine and even ring of light around the butterfly. It should be obvious if it isn't centered and too much light may mean the beveled edge is the wrong way around.

[Rob]

.

[Seth]

Many years ago I had a similar problem (faster idle when warm) and found spigot into the head was loose causing a vacuum leak. When the engine is hot, try rotating the carb on the spigot. It shouldn’t move. Just another thing to easily check.

[Rob Frankham]

Duane mentioned that these twins are really two singles linked to a common crankshaft. Not particularly relevant here, but it might help.

True enough but so is every twin... by the same token a Pratt and Whitney Wasp Major is 28 singles linked to a common crankpin. True, interesting but not very helpful...

Rob

[Inveloveritas]

Idle screws were about 1/4 turn out, now 1/2 turn. Would it have been lean at 1/4 ?

Than the idle screws. They are backed off full, they don’t even touch the plates. I’ve then wound them in so they are just touching the plates and it all seems great. I’ll try and take it on a run tomorrow and see if it rises again when really warm but the short run this arvo was all good.

Is that likely to be correct about the throttle screws ?

The mixture screws at 1/2 turn out is about right and 1/4 turn would have been weak unless the fuel level in the bowls was very high.

The next bit may depend on whether the butterflies were removed when the carbs were overhauled.
The throttle stop screws - that is the ones that touch the plate shouldn't be backed off until they don't touch the plate or only just touch it. You would expect 1/2 to 1 turn in after touching the plate in order to open up the butterflies for idle. If that 1/2 to 1 turn isn't necessary to make the engine idle then it's a possible indication that the butterflies aren't installed correctly. They need to go the correct way around due to the edges being beveled to fit the venturi and they have to be centred on the spindle. The best test is with the piston out or raised out of the way, hold a carb up to the light with the butterflies closed and you should only barely see a very fine and even ring of light around the butterfly. It should be obvious if it isn't centered and too much light may mean the beveled edge is the wrong way around.

Thanks Barry. I rebuilt the carbs myself, I used to work at Carburettor Exchange. The butterflies were spot on, no gap around and in the correct orientation. So, not doubting what you’re saying, I’m 100% confident that’s not the issue in this instance. Any other ideas ? Thanks.

[barryh]

Thanks Barry. I rebuilt the carbs myself, I used to work at Carburettor Exchange. The butterflies were spot on, no gap around and in the correct orientation. So, not doubting what you’re saying, I’m 100% confident that’s not the issue in this instance. Any other ideas ? Thanks.

Well back to first principles then.

If the butterflies are properly closed and the idle speed still rises,there has to be air getting in someplace else. A confirmation of that, if I've understood you correctly, is that the engine is idling with the throttle stops barely touching the plate which would mean the butterfly isn't even open to it's normal idle position. If that's the case air has to be getting in somewhere and bypassing the butterfly. The usual test for an air leak is to spray something flammable around the intake spigot and carb rubber to see if it has an impact on the idle speed.