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View Full Version : 3.5 won't stay running



DoktorC
03-31-2009, 07:42 PM
I've got an OLD K&B 3.5 on a Vee bottom. It starts and runs fine...until I put it in the lake. Once under load it will only run for about 8 second at WOT and it's struggling the whole time. I've tried lean and rich and it runs longer rich but...it's REAL rich. Theres tonnes of fuel spewing out the exhaust...it just doesn't seem to get on pipe. Fuel, Tank and lines are all new. I finally broke a drive shaft so I had to stop fiddling with it...but the parts are here and it's about time to start playing agian....any advice??

Riverman
03-31-2009, 07:50 PM
Weak plug? Sometimes they don't seem to hold the heat well after you take the battery off. Do you have a spare to try?

DoktorC
03-31-2009, 07:53 PM
Weak plug? Sometimes they don't seem to hold the heat well after you take the battery off. Do you have a spare to try?

ya...switch that too. Runs forever on the bench but not under load. I'm just getting the package running again after sitting for a number of years. But it was running when it blew up lol. Has lots of compression...the only thing that I don't know is if this is the same prop. Maybe it can't pull this prop....

Mark Poole ModVP
04-01-2009, 04:30 PM
Motor......make sure the propshaft is not binding and that the flexshaft is not binding when installed. I have seen those old KB's that the flexshaft was about 1/16" too long. Tons of fuel out of exhaust means too rich. Make sure you are running pipe pressure line to the tank.

Boat set up......You might need to install a 1 oz. "hopper" (surge) tank if the tank is down low in the v bottom boat. I run hopper tanks on all my outboards. Make sure propshaft is within 1/8 inch of the keel or higher. Prop too deep will cause that. Try a octura X437 3 blade, good prop for a mono. Anything more than an X440 will probably be too much prop for that boat. Tunnels can turn bigger props. Take needle valve out of carb, spray engine tuner through passage. Use McCoy MC-9 glow plug.

One more thing...if it is a new enough engine (within last 10 years) and has the finger ported head button, make sure the 3 finger side is lined up with the intake side and the 4 finger side is lined up with the exhaust side.

Dave Strong
04-01-2009, 07:39 PM
I haven't done any R/C stuff for years, if your runnung a red carb with a pipe you will have the type of problems you talk about. I could never get the red carb to work well with pipes. Dave

DoktorC
04-01-2009, 08:21 PM
This baby is WAY old lol. Long before red carbs and pipes. It's even got the short skeg lol. I haven't used this package for a long time but I've cleaned and soaked everything. As far as the boat set up it used to run and run and run BUT I think it may have blown up (had a bearing failure) before I got a chance to try this new prop.

It's an Octura 220 which seems REALLY big compared to the X437...I'm really thinking that this is my issue. Too bad it took 3 weekends of wanting to kill the boat for it to dawn on me lol. I'll try a smaller prop first and if that doesn't solve it I'll try raising the tank...then I can install a hopper if I need.

Thanks for all the advice guys!!!

1BadAction
04-01-2009, 08:27 PM
I could never get my K&Bs running good on a KB carb. Perry carb and later an OS carb, excellent running. the 7.5cc KBs ran awesome, I had one that was a 1985 that was bone stock and it would drop off plane and idle around if I wanted it to. :D anyhow, I think alot of the 3.5 carbs had air leakage problems, thats why you hear of so many issues with them.

Dave Strong
04-01-2009, 08:48 PM
Glad to hear it wasn't just me, ever played with exhaust throtles ?
Dave

SuperBoat Steve
04-03-2009, 06:57 PM
make sure the head bolts are tight, and that you did not blow the gasket.
I have seen that many times on 3.5 . Just a thought!

Steve

RIVERRUMMER 70
05-01-2009, 12:04 PM
Glad to hear it wasn't just me, ever played with exhaust throtles ?
Dave

I HAVE, AND I CAN SAY THEY ARE LIKE NIGHT AND DAY TO CARB MOTORS....GO FROM 19,000 RPM TO 32,000 AND RESPONSE IS WILD TO SAY THE LEAST.... 3.5 K&B ON A DUMAS HOT SHOT IS OUT OF CONTROLL:reddevil:
I RUN 60% NITRO AND HAVE BEEN BREAKIN CRANKS EVERY HALF GALLON OR SO WITH THE SETUP I RUN.
I WOULD LIKE TO GET A 7.5 ON THE PLATE AND SEE WHAT I CAN DO TO IT ON MY HUGHY TUNNEL:reddevil: