K&B 3.5 inboard throttle exhaust

[chargerrc]

I have a K&B 3.5 with a CB titanium throttle exhaust on a 34" Youngblood mono hull, my question is where is best place to get tank pressure from the crankcase or the pipe? It is currently getting pressure from the pipe but engine is real finicky. Should I ditch the throttle exhaust and change to a carb? Also not very fast, it seems to run well in a small circle but once you straighten it out, the engine just bogs down. Is this engine to small for a 34" hull? Also aI see alot of people running a X440/3 with the .21 size engine I have tried this prop on my hull and does seem to be the fastest on the straight but terrible cavitation on launch. The engine also seems to be a little low on compreesion I am waiting on a new piston and sleeve right now. Also or there any good sites on throttle exhaust setups.

 

[MarkScott]

All the exhaust throttles I've run need crank pressure. Pipe pressure just doesn't cut it. With today's carbs, it might be better to switch to over. ET's can be messy -- leaving lots of unused fuel spitted out inside the hull. The ET's I've run have always been on outboards, so it's not an issue for me.

 

[chargerrc]

Thats what I thought. As for the pressure tap in the crankcase do i just replace one of the top end plate screws with a threaded pressure tap? If so, is there a particular pressure tap that I should use?

 

[MarkScott]

Here's a shot of my K&B 3.5 OB motor with a Gardner XXL exhaust throttle hooked up.

On the engine mounting lugs, you'll see a red fuel hose. That is hooked up to a Fox 4-40 Pressure Fitting.

 

[chargerrc]

Thanks OB Nut thats exactly what i needed

 

[NAMBA860]

to much boat and to little engine i run exhaust throttles on my engines and pipe psi works for me the other guy may be complaining about a mess becuase he is getting to much psi and pushing to much fuel.that yougblood is a fast boat but you will need something with twice as much power as that k&b has, it makes 1.3 h.p. is all. put a cmb long stroke in it and it will become a bullet. if money is an issue find a k&b 7.5 and put that in it i will run real good with that.

 

[Marty Davis]

Thats what I thought. As for the pressure tap in the crankcase do i just replace one of the top end plate screws with a threaded pressure tap? If so, is there a particular pressure tap that I should use?

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The pressure tap is VERY important. We always ran the black K&B pressure tap. It had a TINY hole in it.

What will happen is that if you use a tap that is too large, it will take too much crankcase volume out and the engine will not run well. BUT, if you run the very tiny tap, it will flood very easily, especially if you don't get the engine started on the first spin. It is an awesome power producing system, once you get it worked out.

Marty Davis

 

[MarkScott]

The Fox 4-40 that I use has a very small hole on it. I bought 5 at once and they were all the same size.

 

[chargerrc]

Well I just broke in my new K&B 3.5 today and the results were better than expected. After about 30 min of running rich, I started to lean it out a bit and man did it move. Im not sure how fast but semed to be around 50. I think a little more tuning, and adjustments and it will definately be in the 50's maybe faster. I have had this boat since June but today is the first time I was able to see it do what I knew it could. What a learning curve!! This is my first rc boat and everything needed to be rebuilt, but I finally got it. here is a link to some pics of the boat there is also some cool pics of some offroad truck racing. http://nitromaniacs.zoomshare.com

 

[Propjockey]

Interesting use of a school bus. :blink: