Old K&B 3.5 inboard questions
02-17-2010, 05:17 PM
#1
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Old K&B 3.5 inboard questions
I have just built my first monohull with an old K&B 3.5 inboard engine and have a bit of trouble with the engine. It looks very similar to this one:
http://cgi.ebay.com/VINTAGE-K-B-INBO...item414d107559
except mine still has the stock "Barrel" exhaust off the front with the exhaust throttle.
The problem I am having is that fuel is spitting out the bottom of the venturi when bench testing (I have not tried the boat in the water yet) and also there seems to be exhaust coming from somewhere below the exhaust barrel. What could be going on? Is it just running too rich, or should expect weird things due to bench testing and not being run in the water?
Also, should I just go ahead and ditch the barrel exhaust?
thanks
http://cgi.ebay.com/VINTAGE-K-B-INBO...item414d107559
except mine still has the stock "Barrel" exhaust off the front with the exhaust throttle.
The problem I am having is that fuel is spitting out the bottom of the venturi when bench testing (I have not tried the boat in the water yet) and also there seems to be exhaust coming from somewhere below the exhaust barrel. What could be going on? Is it just running too rich, or should expect weird things due to bench testing and not being run in the water?
Also, should I just go ahead and ditch the barrel exhaust?
thanks
02-17-2010, 05:43 PM
#2
RE: Old K&B 3.5 inboard questions
Yes, ditch the exhaust throttle and send it to me.
Seriously, you don't want to get rid of the throttle because it's a cool piece of hardware and it should be able to work properly.
I think if you take the throttle off and check the mating surfaces for any flaws that would keep things from seating together nice and put it back together with some Permatex Copper Seal RTV (use sparingly! Just rub it in on both surfaces...don't leave any thickness) you should have good luck sealing the exhaust leak - unless the barrel is worn so much the exhaust is shooting out the end of the barrel seam, hitting the flange and flairing out.
The venturi problem with the fuel back-spitting can be caused by not pointing the spray hole straight in. Another thing is you are dealing with an exhaust throttle where it will naturally do that to some extent. You're trying to stop the flow of air at the end of the motor, instead of at the front and it will try and spit back through the motor when it gets the backpressure.
The spray hole should only be rotated side-to-side to aid in your mid-range mixture. Something like a low speed needle on a two-needle carb, so be sure to take off your venturi and needle assembly and take a good look where the orifice is pointing. It should be staring straight back at you through the bottom of the carb. Start with it there, and then if you have problems with your low and mid-range, you can start playing by rotating where it's pointing.
I've been through these same engines myself, so I'll be able to help you as much as I can.
Seriously, you don't want to get rid of the throttle because it's a cool piece of hardware and it should be able to work properly.
I think if you take the throttle off and check the mating surfaces for any flaws that would keep things from seating together nice and put it back together with some Permatex Copper Seal RTV (use sparingly! Just rub it in on both surfaces...don't leave any thickness) you should have good luck sealing the exhaust leak - unless the barrel is worn so much the exhaust is shooting out the end of the barrel seam, hitting the flange and flairing out.
The venturi problem with the fuel back-spitting can be caused by not pointing the spray hole straight in. Another thing is you are dealing with an exhaust throttle where it will naturally do that to some extent. You're trying to stop the flow of air at the end of the motor, instead of at the front and it will try and spit back through the motor when it gets the backpressure.
The spray hole should only be rotated side-to-side to aid in your mid-range mixture. Something like a low speed needle on a two-needle carb, so be sure to take off your venturi and needle assembly and take a good look where the orifice is pointing. It should be staring straight back at you through the bottom of the carb. Start with it there, and then if you have problems with your low and mid-range, you can start playing by rotating where it's pointing.
I've been through these same engines myself, so I'll be able to help you as much as I can.
