Uncontrollable carbs

[the-chauffeur]

Hey gang

After w-a-y too much work, rework and re-re-work, I think it's safe to say the 750 K2 is nearly finished.  Hooray.  I've just swapped the head for that of a K3/4 which is much smarter than the one that was on there, and it fires up and runs very well.  With a list as long as your arm of new parts, vapour blasting and rebuild work, it's now lookin' pretty good.  It's even quite quiet, which I'm sure has something to do with new chains and bearings throughout.  

Anyways, one remaining thing I've never been able to sort out is the carbs.  What I mean by that is that when it's running at idle, either the engine is struggling to stay ticking over or it's racing - there's no middle ground.  It's almost as though the thread on the idle screw is too coarse, because no matter where it's adjusted to, the idle's either way too low, or far too high.  Off idle (i.e. under power) it's fine - and very responsive - but calm idle is proving elusive.

The carbs have been entirely stripped out, vapour blasted, ultrasonically cleaned and all of the jets and other internals have been replaced with new bits.  I've also synched them and made sure the all the linkages/gates (inc. choke) are matched to eachother.  And yes, the cables are free and have slack in them.  So they now look lovely (almost as good as a set of SL350 carbs I've just redone); shame they don't seem to be controllable.

Just in case you were wondering, this is pretty much what I start with:

 . . . and once I've soldered up the holes in the floats (!) and overhauled them, the carbs end up looking like this:

But back to the 750 carbs, I'm trying to work out whether the idle I'm stuck with is how the 750's are generally, or whether this is something particular to mine - and if it is mine, why?

What do we think?

[UK Pete]

Neil my K1 can be tempermental at tick over and when everything gets really hot it does what yours does, my F2 is worse as it runs so lean that you have to get it really hot before it sets up properly
I have a decent set of carb balancers you can borrow if you like, no doubt you have checked the obvious things like air leaks especially round the carb rubbers, also check your advace mechanism on your timing
Pete

[the-chauffeur]

Thanks as always Pete.

Good point about leaks - some of the (very few) bits I haven't yet replaced during the various rebuilds are the carb-to-engine rubbers.  I've done the carb-to-airbox rubbers, but not the ones forward because I guess I thought they were so thick, leaks were unlikely.

However, I'm also having more and more difficulty mounting the carbs in such a way that they sit parallel to the engine head without coming into contact with the fuel tap.  Are the numbers on top of the rubbers 'sposed to face directly upwards or be offset in some way? 

Whatever, I've been wondering whether the rubbers may have shrunk (or something) - they've certainly hardened quite a bit over time.  Since that might have something to do with the mounting/funky idle, I've just ordered a new set from Silvers.  But it's interesting to hear you have similar issues with other years.

One other thing - one of the guys at 21st Moto (local to me) has asked if they can borrow a couple of my bikes to put on display when they have an open evening in a few weeks time.  I thought that was pretty cool . . .
 

[Bryanj]

Make sure you use the narrow Honda clips on the rubbers NOT wide jubilee type as they don't work, also dont try and get a tickover below 1100, it wont be reliable mate

[Spitfire]

I fitted new carb/engine rubbers to mine and the difference with the original ones is amazing, there was a thread either here or on the US site on efforts to soften them again but I can't remember what the outcome was.
This also applies to tyres, the tyres on my F1 were fairly new 10 years before I started rebuilding it but they felt about as flexible as wood compared to the new tyres that I fitted.

Cheers

Den

[the-chauffeur]

Cheers guys.

Bryan - thanks for the pointer on the idle speed.  From what you've said, I think I may have been aiming a little too low.  I'll wait for the new rubbers to arrive and then have another go at setting it up.

 

[K2-K6]

As the other posts, the engine to carb rubbers seem to get veeerrrry hard over time and particularly the end that fits to the carbs ends up being very hard to deform with the clip to enable you to get a good seal there.

I've always felt that below about 950 (indicated) revs it's gets very uncertain for real stability and you can chase it forever and never be happy. As Bryan says, 1100 is a good compromise without chasing your tail.

I guess when you get the carbs down to flowing that slow then component tolerance may exceed what you are trying to balance.

Also the points switching at slow speeds gets more "fuzzy" to give you more variance there. Maybe worth trying new condensers if they are sparking (run it in the dark at low rpm to check for sparking at the contacts) as the timing will shuffle about a small amount as a result, unless you've got electronic.