15 replies to this topic

[TheHippo]

Hi all,

 

998cc running twin HS2s.

 

I have set up the carbs as per SU instructions (i.e. set the jets to the bridge then down a turn or two, undo the idle screw then wind in one full turn etc) and it is idling very quickly, which I suppose it would before you start adjusting them.

The problem is that even when I back both screws off completely such that they aren't doing anything the idle is still too high (around 1400 - 1500).

 

I think that I need to lean the carbs off a bit, but when I've tried this before it doesn't significantly reduce the idle speed. The only thing I have found that does impact it is the timing (which obviously isn't a solution). The carbs have been serviced, and I'm fairly confident that the spindles aren't leaking.

 

Does anyone have any suggestions as to why I run out of adjustment on the idle screws before it gets close to a sensible idle speed?

 

Thanks,

 

Dan

TheHippo

[johnR]

I've been through all this and more with my twin HS2s recently. What needles are you using? Is the manifold running hot? (too lean)Does the inlet manifold have any ports/stud holes that aren't blocked off?

[RooBoonix]

Do you have a brake servo installed? If so have you checked the vacuum line isn't leaking?

Is the issue still there with the throttle cable slackened off/disconnected

Edited by RooBoonix, 26 June 2019 - 07:10 PM.

[TheHippo]

johnR - glad to hear it's not just me! I think I've currently got M needles in there, but it has been behaving exactly the same with 3 different needles. With regard to the manifold, what would constitute 'hot'? All holes on the manifold are blocked.

RooBoonix - no servo, but as above all holes are sealed. Yes, the issue persists whether the throttle and/or choke cables are connected or not.

Thanks.

[RooBoonix]

Are the throttle butterflies able to close fully?

[Arthy]

Are the throttle butterflies able to close fully?

+1 for this. When I rebuilt my twin HS4, I didn’t properly centre the buttery’s so they couldn’t fully close. Even the smallest amount makes a difference!

[johnR]

Are the needles the floating or fixed type? With the fixed they need to be centred by loosening the locknut that secures the jets to the carb so that the piston clonks down onto the bridge freely when you lift and drop it. By a hot exhaust manifold I mean really hot - mine burnt the new zinc plating off it almost immediately, but my inlet was taking air from two redundant stud holes I'd overlooked. Are the return springs all in place - I've got three on throttle return duty, one from each carb and one from the throttle cam all going to the heat shield plate link bar.
When you say you have the jet adjusters down a couple of turns - have you tried starting from a position of them being done right up so allowing the minimum fuel at idle?

Edited by johnR, 27 June 2019 - 11:13 AM.

[Ivor Badger]

Check the fast idle screws on the choke levers

 

Are there return springs on both butterflies?

Edited by Ivor Badger, 27 June 2019 - 12:27 PM.

[TheHippo]

The butterflies were closing fully when I rebuilt the carbs, but will double check.
Needles are fixed, but I have centred them and the pistons lift and fall nicely.
Crikey, no, the exhaust manifold isn't that hot.
Have got three return springs fitted as you describe and again they all appear to be functioning correctly.
I will try raising the jets again but as I said this hasn't helped in the past.
The idle screws on choke levers aren't contacting the cams at all so can't be at fault.

Thanks for the suggestions. Keep them coming...

[gazza82]

Do the pistons fall at the same rate? One dropping quicker could mean it rises more easily ..

[TheHippo]

Do the pistons fall at the same rate? One dropping quicker could mean it rises more easily ..

Thanks, but would this affect idle speed? Surely this would only be a problem under acceleration?

[gazza82]

I might if it is lifting slightly on idle. I had a nightmare trying to get dual SUs balanced on a Midget. Turned out one damper was worn and that allowed one piston to lift ... and in doing so increased the fuel.

Edited by gazza82, 27 June 2019 - 10:09 PM.

[Ivor Badger]

Have you got the throttle cable holding the butterflies open? The only thing that can give you fast idle is excess air into the engine. whether it is rich or lean will not make that much difference.

another thought, are the butterfly spindles worn?

[TheHippo]

OK, thanks gazza82, I'll check it out.
Ivor - I'm pretty sure the butterflies are closing properly but will make sure. I have blocked the ends of the spindles and it didn't help so don't think it's those.

Thanks all.

[johnR]

If reducing the fuel input doesn't change anything you and all the controls are backed off and pistons down you must have an air leak. You can test for this by spraying something like hairspray/easystart or WD40 around the carbs/inlet (maybe good practice to have fire extinguisher handy!) - an increase in revs will show the location of the leak (although when I tried this with WD40 it didn't show up my leaks) Mine was detected by a garage that had a leak detector - basically a smoke generator that puffs smoke down the carb mouths so you can see where if it comes out where it shouldn't.