jbbodz

Hey guys, i have just made an account to see if anyone can help me out with my rc boat.
I have a Prather lapcat I with a K&B 3.5cc outboard. I have installed the red carb, got the 13mm crank, after market conrod, i have ported the crankcase, port and polished the cylinder head and exhuast and yea. Im still at school so I can't affor to go out and buy heaps of expensive stuff. So i was wondering can anyone help me with evrything, ie, the needle valve position, outboard height on the transom, type of prop, and problems i will face with the outboard etc- ANYTHING. I know a fair bit about this stuff, i just never actually use anything i make, so abit of help would be good, cheers- can put up pics if wanted

ob nut

My Feedback: (1)

Props: prather 215, Octura x440, x438, x437/3, x440/3 and many others.

Prop shaft starting point would be even with the spoon bottoms. Place the boat on a flat surface and put a ruler (straight edge) and check the shaft position.
Boat running slow, pushing water: Raise the motor up.
Boat running WILD, on the edge: Lower the motor.
Boat nose up: Give prop shaft neg angle.
Boat nose down: Give prop shaft positive angle.

Get a gold carb over the red one. The red has too big a diameter, making needle adjustments hard. Good starting point on the needle is around 4 1/2 to 3 1/2 turns out. Start at 4 1/2 and work your way in.

CG should be around 28% to 33% forward from the spoon backs.

K&B 3.5's love running high nitro. If you are just playing around, 45% or lower will work fine. Racing might want higher nitro.

Problems running tunnels.. They are addictive! make sure to start the motor in the correct direction. If using a normal setup plane starter, the leads have to be reversed.

jbbodz

Hey thanks for the reply, sounds all very useful. I am after a long life of the outboard so i think i will go with 22% castor oil, 15% nitro and the rest ethanol/methanol watever it iscalled. I have two octura props, they are brass three bladers, one is a 437/3 and the other a x440/3 . could you tell me what these markings mean, i would image the /3 means 3 blades and the 440 and 437 have something to do with blade pitch? for now i think i am stuck with the red carb, so could you tell me how to tune it? I had an OS plane motor and i recal once it was running you would turn the needle in the lean the fuel mixture until maximum revs was reached, then you would turn it out about 3 clicks to ritchen the mixture. however i do notice the os engine had a needle in the carb so that the fuel mixture was constantly adjusted..the K&B doesn't. so what im asking is how do i tune this right ?? cheers , would you like to see some pictures?

ob nut

My Feedback: (1)

Not much on being a prop guru, can't help ya out there. But believe you are about right on pitch and such.

If you must keep the red carb, here's how to set the carb on the bench. When the motor idles and blip the throttle to full, how does the motor rev up?
Quickly - lean
Doesn't - rich
Take a few seconds to clear - just about right...


Pics.. Sure, post them.....

jbbodz

Thanks for the help; much appreciated. If anyone else can shed some light on this project feel free to help. I also have another two outboards, one is pretty standard, has a gold carb (By gold i think it is the bottom of the line carb, not the gold one on the best 3.5cc modle). I have done abit of work to it like after market con rod, port and polish etc. The other one is quite powerful and has a exhaust throttle fitted. i have never run it but it use to hold the water speed record for a 3.5 on the tunnel hull some time ago here in NZ. It is very modified. Could anyone tell me how to opperate this exhaust throttle model. Does it even idle? etc

Here is some pictures of the current setup. Have finished sanding it today(Not sanded in the pictures). guess i shall paint it..

ob nut

My Feedback: (1)

What type of exhaust throttle do you have on the motor? Pics... I've used both brass slider and Gardner XXL units. Prefer the XXL setup as it allows the pipe to be hooked right up.

For the most part, the ET's do not idle well.. They are for full open throttle use only.. Some say they waist fuel.. I used to love running them on my boats. Will have to get them out of the moth balls for next year.


K&B 3.5, 13mm, titanium rod, Keely head, Gardner ET and Garcia pipe. Don't have the Design Graphics lower anymore.

jbbodz

Wow that is quite impressive. Looks like a very modified outboard you have there . Here is the other two i have, one with an exhaust throttle, ported crankcase and sleave along with polished crank, aftermarket rod bla bla bla. Im sure it will go ok. However i would like to sell these two outboards so i can raise abit of money haha

ob nut

My Feedback: (1)

That gold carb ain't bad. I have one and there are no issues. The slider ET works ok. Basicly it's like the carb opening, but at the exhaust. Needle adjustment at the ventrui. Do you have any specific questions on how an ET works or how to hook it up?

jbbodz

yea i am just wondering how the ET system actually works. i see a carb allows air flow to be changed along with fuel flow but i do not understand how the exhaust throttle works? I would imagine that the ET changes the bak pressure and somehow this slowsthe engine revs ..?Is there an obvious amount of performance increase from it? Is it hard to use etc?
cheers, just wondering if it is worth keeping

ob nut

My Feedback: (1)

On a basic carb setup, there is a barrel (roll or slide) that opens allowing air to flow IN with the exhaust is fully open. On the ET, it's the opposite. The venturi (carb) is fully open allowing all the air in while the exhaust throttle closes/opens to restrict/allow the spent gases out. Now with an ET, the normal pipe pressure to the tank does not work. A crank pressure tap is required. Your's has this as there is a nipple on the mounting lugs.

Todays carbs are much much better than say 20+ years ago. Getting a proper needle setting is a bit harder on a ET. But it can be done. Give it a shot.. If not, then find a carb and see if you can tell a difference.

jbbodz

Ohh kay, thanks a lot for the help mate. This is all very useful infomation to me. I have one other question, slightly different and bit off topic,, i have just finished sanding the hull down and would like to paint it. I would like to do a nice job, but just use spray can. If i just paint the hull in gloss paint, do i need to spray it with some sort of gel coat?to make it look better? i will obviously be needing to use an enamel fuel resistant paint. cheers,

rcguy1411

the ET works like an on off switch they are designed to run wide open they don t like to idle you will get better high end performance with the ET as far as the paint i use duplicolour for the base coat and also use there high build primer then i use a fuel proof clearcoat i have had good results with classkote clear from the LHS i blan on doing a lapcat III this winter

rcguy1411

a pic on my last hull i sprayed with the steps i listed above hope it helps also a pic of my lapcat

jbbodz

ohk thanks for that. I will look into the painting. your boat looks good ! thanks for all the help, very useful (:

ob nut

My Feedback: (1)

Most spay can paints won't hold up to high nitro. Since I use only PPG 2021 clear coat, don't have problems with 65% nitro.

jbbodz

It is becoming extremly hard in New Zealand to acquire spray pain that can with stand nitro, RC stores don't even sell. I went to a pannel beater and they said for $150 NZD they could paint the boat. and that would be a real good paint job.

Have decided to go with this option, will add photos soon once the boat has finished being painte ;P

jbbodz

Hey guys, would someone be able to tell me what lapping is? from what i can gather it is some way of wearing the piston and sleave in so at high rpm the piston doesnt grab to the tapper of the barrel and break the conrod. is this lapping process some sort of running in? wearing of the sleave ? and i hear you can use wd40 or apparently toonthpaste!? could someone explain the process etc to me thanks

jbbodz

Have just finished assembling boat the other day. Had the hull professionally painted for $150 NZD which is about $100 USD . The paint job is perfect, just as good as a new cars. could anyone answer my previous question?

cheers