Engine Question

[CharlieA]

Hello All, I am just a newbie at this and have a couple questions. I picked up a older Prather Lap Cat I with a K&B 3.5 OB on E-Bay. I also purchased a new fuel tank for the boat and cleaned up and lubed the engine/carb(gold)/lower unit, and got the engine started. It seems to run fine and I adjusted the needle so that when I give it throttle it would just start to gurgle rich then I closed the needle about 1/4 turn, I figured this would work and not be too lean. When I get the boat on the water with a full tank of gas it seems to run fine, but soon loses power and I will throttle back to keep it from dying. Then it will throttle up ok for just a short time and do the same thing. I am thinking this is a fuel draw problem? The tank is new and I don't see any air leaks anywhere, there really is not much room in the boat for raising the tank any. Any suggestions, would a fuel cell work better? Am I overlooking something ? Can I use some sort of fuel pump such as a Perry Pump? Yes the pressure valve is connected to the exhaust, in fact I have 2 engines and one of them has a tuned pipe that I tried also, but same thing happens. Any suggestions would be appreciated Thanks

 

[Rob Duck]

Sounds to me as the engines needle valve setting is too lean. They should be blubbering rich on the bench and will lean out when it hits the water as the load of the boat and prop will cause the engine to lean out. Also check the needle for any debris or obstructions in the carb by pulling the fuel line and flushing the carb with fuel or wd-40. Check what prop you are running. Octura x-437,440 are good starting points. Bottom of prop shaft even with the bottom of the sponsons. If the tank is lower than the center-line of the carb then you will experience a lean out effect as the fuel level lowers in the tank. You must compensate with a richer needle when the tank is full to get to the bottom of the tank. To set the needle for this set-up, only fill the tank with 2 oz.s of fuel and get it running as best as it will run (needle setting) and then fill the tank. It will run rich at first and then be right on at the end.

 

[MBrater]

I've got the same thing going on leaning out and quiting w/ my dumas hot shot 7.5 k&b pro. I,m not a racer, just run for fun! However when I adjust the needle w/ the 2 0z and then fill the tank and run its slobbering rich, I,m kinda disappoited it the power. Using 20% blue thunder fuel race formula w/ extra castor- 20 percent oil content. Ran three tanks of 20% Odonnell on the rich setting to break-in. Does it hurt the engine to quit lean if its not broke-in all the way? Am I expecting too much out of the 20% fuel? I,m using the k&b HP long reach glowplug, is there a better one for 20% fuel? It just seems like a big diference from way too rich when full to 1/4 tank -right on setting! thanks for the help guys! Mike

 

[Jerry Dunlap]

Re:Engine Question-MBrater's

Hi MBrater,

I'm a bit confused regarding how you set the needle-valve. It sounds like you are using a 2 oz. "hopper" tank and then a larger tank. You should set the needle-valve with both tanks full.

You didn't mention what prop you were running. The 7.5 Pro will definitely bog down if you are running too big a prop. With 20% fuel, I'd recommend an Octura X447.

You might want to consider trying a K&B 1L plug when running 20% fuel.

Jerry Dunlap

 

[Rob Duck]

Mbrater, Make sure the prop is the right size. As Jerry says, an x-447 is a good starting point. I run,as most competition guys do at least 40-50% nitro fuel. Make sure you have adequate oil in the fuel. In the pro engine I have found the sig 45% nitro,24% oil to be the best. The engine runs cool and makes gobs of power. I run an Andy Brown 1445 prop with 3.6" of pitch. He also makes a 1445 with 3.3" of pitch. Props also depend on your boat and set-up. The set-up of the fuel tank was to set the engine on the edge or as lean as it can go without quiting with the last 2 oz.s of fuel remaining in the tank as long as your not running a hopper system. With a hopper system you just fill the tanks and the hopper tank is the last tank to empty anyway and since its mounted at the same level as the carb you can set the needle just slightly on the rich side. With only 20% nitro you will have to go down a prop size from what the racers use. A 1445,even stock or a x-447 or a prather 220-225 should work well. Most fast boaters run the Octura 1450,x-450,x-447/3 or prather 230 props. Depends on altitude,boat and driving style. Again, my 2 cents canadian or 1 penny american....

 

[MBrater]

Jerry and Rob, thanks for the tips! Right now I'm running the pranther 230 ss balanced and sharpened, no hopper tank, guess I,ll try either some higher % fuel or maybe a 225 prop. thanks again, Mike

 

[alby]

I'm new to these engines but on my K&B 7.5 when I set the needle to the factory settings it's way to rich. When I peg the throttle it runs like crap but runs pretty good at half throttle and than I noticed when I was just about to run out of fuel it ran great. So I started leaning it out and it seems to run good. Being a nitro car and truck guy I am having trouble on the needle settings cause I like to watch my head temps, but this water cooled head is always ice cold. I would maybe lean it out a 1/8 turn at a time and see how it runs. If it dies when there is fuel in the tank than maybe you are to lean. Just my two cents.