Topic: Non-SOHC question for tuners

[01Thomas]

I have a 1981 Kawa GPz 1100 motor fitted with a 1170cc wiseco kit and some Andrews cams which cannot be identified other than they are labelled 'Andrews'. It is fitted with Mikuni VM33 smoothbores.

The bike starts very easily and makes good power at low revs, and I haven't taken it past 3500 rpm yet, so lets leave the higher rev band out of it for now.

The problem is that the damn thing won't idle. I can screw in the throttle stop screw until it idles, and when I blip the throttle the revs hang up high at about 4000 rpm. so I can't rely on the throttle stop screw to give me an idle.

I can get it to idle nicely and steadily at about 1100 rpm by manipulating the throttle with my hand, but this is not right and an inconvenience. Also when I do this a fuel mist develops in front of the carb intakes, so there is some funny air-flow reversion stuff happening.

This leads me to the conclusion that the cams are causing this poor idle.  Assuming that I'm right in this regard, should I be looking at the intake or the exhaust? (Logic tells me the intake cam needs attention... but logic has been known to be wrong before).

Taking this train of thought a bit further, if it is the intake cam, should I try advancing it or retarding it? Advancing it would make the valves open earlier in the cycle, but also close earlier in the cycle.

These smoothbores are the third set of carb fitted to this bike and with all these carbs the engine exhibits the same symptoms, so I believe the carbs are not the source of the problem.

Any ideas, anyone?

regards
Thomas

[turboguzzi]

assuming you checked everything like sync, air leaks, played with air screws, etc, could have something to do with the ign box kicking in advance too early, would simply swap for another one to make sure.

but dont expect 33 SBs to idle that good in any case, not really made for that.

[joesmotos]

Couple of things..those old 33 smoothbores were sorta finicky...i recall how they would get air leaks through the top covers of the carbs(the cap on top you remove to synchronize them).l..,air leaks between the rubber manifolds and the head were common...you might want to get a can of brake cleaner or carb cleaner and spray a light mist at the tops or the manifolds to see if theyre leaking,you will hear it and smell it in the exhaust if so..smells pretty bad.

As far as cam timing goes,you need to know where the cams are set right now...with the valve cover off you can verify stock timing just by looking at the marks on the sprockets i do believe..with the engine at top dead center..

Another thing,the stock timing advancer springs may be weak and not pulling the advancer back...

I would do the spray thing first just to see if it changes anything,checking the advancer is easy too,you can grab it and twist it a little and see if it returns smoothly..

[scottly]

when I do this a fuel mist develops in front of the carb intakes, so there is some funny air-flow reversion stuff happening.

Any ideas, anyone?

regards
Thomas

The fuel mist suggests that there is still pressure in the cylinder when the intake valve opens. Possible causes might be early exhaust valve closing, early intake valve opening, or even a plugged exhaust. My FT 500 thumper ran quite a bit better after blowing a large mud-dauber nest out of the exhaust.

[01Thomas]

Thank you for all your help. I'll look at the carb top sealing issue first and work my way down through the other suggestions. I do realise that a steady idle was not what these carbs were designed for and that I may well have to be satisfied with a bit of a ragged idle, but it's got to be better than no idle.

thanks again
Thomas