Hello im running a stock 26cc zenoah in an Aeromarine Avenger Pro.I cant seem to keep running in the water.It dies intermittently when releasing the throttle or when first putting it in the water (if im not flooring it).On the bench -it wont idle with the low side needle any HIGHER than 3/4 turn.What should the idle rpm be?I have tried a new plug and tried adjusting high, low and idle needles many different ways.Out of the water it seems to run well at 3/4 on low needle and 1.5 on high needle with good throttle response but seems to vibrate a bit.It has a wet pipe,a WT644 carb and I am running pre mixed 40:1 TRUFUEL (92 octane with syntehtic) that I got from OSH hardware.The boat has no clutch.Any ideas? Can it be the fuel? Thanks

Hi welcome to MGB

maybe the carb diaphram ? might have sticking needle/seat, maybe dirty ?
might have air leak? carb gaskets intale and carb ? seals ?

just a thought ... try simple stuff first

I had a strut bearing trying to seize that did the same thing a couple of years ago. Have you checked your drivetrain?

here this link from MGB may help you out take a look it's worth it
www.modelgasboats.com/Troubleshooting/

Welcome to MGB lot's of info on this site to help you out.

Regards
Luc

Thanks for the replies.I am waiting for a carb rebuild kit now.

Hi
Welcome to the forum also.

I run a stock Zen260PUM with the Walbro 644 pumper carb with no running issues.

Two things from your post that make your set up very different to mine are:
1 Your fuel/oil mix. I use 20:1 mix with semi synthetic oil. You mentioned 40:1.
2 My carb needle settings are way different to the ones you mentioned. My low needle is 1 +3/4 turns and high needle 1 + 1/8 turns.
My Zenoah carb manual suggested a leaner idle and richer high needle. However having switched to the settings mentioned some time ago, I noticed a VAST improvement in idling + starting of the engine combined with much better acceleration from idle to flat out. Also I noticed that with the richer idle needle the high needle seems less critical to adjust for top speed.
I thought I’d mention this in case it provides some extra food for thought.
Cheers
Craig

Thanks for the info Craig.I just finished rebuiling the 644 carb and the dang thing still wont idle unless I run it way lean -like 1 turn out on the low.I tried a spare wt257 carb and it does the same thing.I'm going to try a the different gas/oil ratio you suggested.When I back the low needle out past 1 turn in starts to smoke and idle rough like it is too rich.I tried raising the idle to compensate but that doesnt help.Could the ring be damaged?
Thanks,Cris

Hi Cris

If you tried new fuel and still the same issues I’m afraid I am at a loss here. I take it that the plug is set at the correct gap?

I assumed you were using a new engine not a used one. If it is a second hand engine and if the ring is damaged I would expect running issues.

Over to those with more experience than I, but do let us know the eventual outcome.

Cheers
Craig

Im going to take a look at the piston and ring.
Thanks,Cris

Cris

Don't forget there are some fab articles on this site including the Zenoah Teardown one:

www.modelgasboats.com/Zenoah_Teardown/