View Full Version : Sucking sound when throttle revved quickly

10-16-2015, 2:20 PM
Well, after a couple weeks/months of somewhat decent riding I tore the old Trump down to find what turned out to be a cold solder joint. Me being an idiot, I fucked with all my settings for timing and carbs and now that I have it put back together it runs like shit, of course. Main problem is a sucking noise through the Mikunis when I rev the throttle at idle. The engine dies if I hold it there, but if I do it quick it will return to idle normally. And takeoff is horrible, I have to really rev it and let the clutch out slow or it'll stall. And it pops horribly when it gets into the middle/top of the rev range, no power at all. I'd say it was running too rich, but kickback makes me think fuel starvation? It usually starts in 1 or 2 kicks though and idles good.

I had it really running great this summer, but we should probably chalk that up to dumb luck...

I know valve adjustment is accurate but beyond that I am grasping at straws. Any general advice on mixture settings or fine tuning timing?

10-16-2015, 6:51 PM
Mikuni's have a problems with plugging the intermediate jets...
they are located down inside a tube right behind the main jet, unscrew them and clean all the itty-bitty little holes.

popping is an indication of a lean mixture.

10-25-2015, 12:48 PM
Hey, I once bought a mikuni for my 6T unit. I couldn't get the engine running right cleanend the carb a few times and tried a 100 set ups. The engine was having a lot of under pressure and oil spitting out the crank vent, ran like shit. Then one day I've haded and put the 930 amal back on, the engine runs great. But I'm still wondering what the probleme could be... Any idea (Torch) ?????

10-26-2015, 11:40 PM
kickback has nothing o do with the carbs, youre timing being too retarded will do that tho

10-29-2015, 6:18 AM
Those sucking sounds usually also means very lean running.

10-29-2015, 8:04 AM
kickback has nothing o do with the carbs, youre timing being too retarded will do that tho

I'll have to disagree with that....

as the mixture gets leaner.... the timing must retard or the mixture will pre ignite.
as the mixture gets more and more richer as the throttle comes on, then the timing can advance to the point the power explosion can happen in the right part of the stroke > ( typically around at 38-40 BTC depending on mixture...this is the advantage of an 'ignition auto advance' unit ).

so kickback is usually sign of either too lean of mixture or...or too advanced timing. (or some combo )

The start up mixture on a Mikuni uses a mixture enrichener fuel circuit instead of a air choke like an Amal,

so if it's still kicking back when the enrichener is on

then you likely have a plugged enrichener fuel passage in the carb.... or your initial timing is too advanced . (or an intake air leak )

> in my opinion ...<

the "proper" timing of any engine is dependent on a lot of things.... mixture and octane , compression, air density and engine temp. and rpm and lots of stuff.

The suggested book timing is just an engineers best guess of an best average timing setting... that will work a stock engine anywhere.

your actual best overall timing setting can vary a bit.

you could loosen up the screws on the timing plate, and just look for a spot that allows you to kick start it and then check and see how well it revs up. if it revs up ok (with out it being under load) ...

then.... with a timing gun see where it's fireing at on a degree wheel or the stator timing mark...if it's all wrong then you still have a mixture problem.