K&B 7.5 Pro problem

[Craig Murphy]

Having a weird problem with a K&B 7.5 Pro (Water Cooled one) I've recently rebuild, it was a full rebuild with bearings, new rod, piston & sleeve (that was a nightmare to get hold of), all new seals, etc so it basically a new motor. It has had about a litre of fuel though it, and has only been run a couple of times over the last year or so. The carb is stock and it has a OS remote needle, I use K&B 1L plugs (have also tried HP's and O’Donnell Blues), 25% Nitro fuel, the hull is a Shaman setup as per Carl's book.

Anyway the problem is when you pull on full throttle the boat starts to slow down like it's starving for fuel (not a lean condition were they speed up first), back off to half throttle and boat goes fine and will run around like that all day. Acceleration is good to full throttle, and the boat does go quite well even at half throttle.

I've stripped down the entire fuel system a couple of times but have yet to find the source of the problem, tank is holding pressure, no blockages, tested all the flow points with a flow meter and all seems good. I'm sure it's something simple I've just missed so if anybody has any ideas it would be great as it's starting to do my head in...

Cheers,

 

[TUNLBOTR]

Having a weird problem with a K&B 7.5 Pro (Water Cooled one) I've recently rebuild, it was a full rebuild with bearings, new rod, piston & sleeve (that was a nightmare to get hold of), all new seals, etc so it basically a new motor. It has had about a litre of fuel though it, and has only been run a couple of times over the last year or so. The carb is stock and it has a OS remote needle, I use K&B 1L plugs (have also tried HP's and O’Donnell Blues), 25% Nitro fuel, the hull is a Shaman setup as per Carl's book.

Anyway the problem is when you pull on full throttle the boat starts to slow down like it's starving for fuel (not a lean condition were they speed up first), back off to half throttle and boat goes fine and will run around like that all day. Acceleration is good to full throttle, and the boat does go quite well even at half throttle.

I've stripped down the entire fuel system a couple of times but have yet to find the source of the problem, tank is holding pressure, no blockages, tested all the flow points with a flow meter and all seems good. I'm sure it's something simple I've just missed so if anybody has any ideas it would be great as it's starting to do my head in...

Cheers,

g'Day Murph,

Sounds like the low speed disc in the carb is cocked out of place and is closing needle flow during opening, and cracking back full at half throttle.Take off the carb and rotate the barrel open, put a piece of fuel line on carb fitting and blow in it, then close barrel and see where the flow is stopping at.... Just a guess.

Cheers back at ya,

Randy

 

[julianconde]

or the bearing in the front are failing that could be another possibility

 

[Jim Mamaril]

Having a weird problem with a K&B 7.5 Pro (Water Cooled one) I've recently rebuild, it was a full rebuild with bearings, new rod, piston & sleeve (that was a nightmare to get hold of), all new seals, etc so it basically a new motor. It has had about a litre of fuel though it, and has only been run a couple of times over the last year or so. The carb is stock and it has a OS remote needle, I use K&B 1L plugs (have also tried HP's and O’Donnell Blues), 25% Nitro fuel, the hull is a Shaman setup as per Carl's book.

Anyway the problem is when you pull on full throttle the boat starts to slow down like it's starving for fuel (not a lean condition were they speed up first), back off to half throttle and boat goes fine and will run around like that all day. Acceleration is good to full throttle, and the boat does go quite well even at half throttle.

I've stripped down the entire fuel system a couple of times but have yet to find the source of the problem, tank is holding pressure, no blockages, tested all the flow points with a flow meter and all seems good. I'm sure it's something simple I've just missed so if anybody has any ideas it would be great as it's starting to do my head in...

Cheers,

Check to see if you are rotating the carb barrel the correct direction,and that the spraybar openings are pointed towards the inside when full open. I always set mine up to pull to open with the arm at about 8 Oclock. If all that is correct then I agree that the low speed disc is rotated to far. Hope that helps.

Jim

 

[Mike Larson]

The previous guys are on target, I think.... while the disc, I do not like, also..... Your flow meter isnt showing it choking out on full open?...

Knowledge of the 45's and bigger having a draw problem, had me switching my fuel tank tubes for the 5/32 size to help the motor get it there.... i've set a few tunnels up with the hopper actually above the carb, causing headaches of flooding but better availabilty to the motor, catch 22, but, simply pinching the line, versus a leanout on top end seemed the better option, until you forget .... Flowmeter should tell you the tale, too... good luck.... Mike

 

[Craig Murphy]

Thanks for the reply guys will check the carb out tonight....

 

[Harry Moore]

It may be a long shot but I was reading the owners guide for the K&B 7.5 OB and it recommends the long reach glow plug P/N 7311for 25% not the 1L ; and then another article about glow plugs http://www.fubarhill.com/GLOW.htm and especially http://www.scootworks.com/rdrc/gloplugs.html that kind-of explains the problem you are having. Cheers

 

[camaroboy383]

It may be a long shot but I was reading the owners guide for the K&B 7.5 OB and it recommends the long reach glow plug P/N 7311for 25% not the 1L ; and then another article about glow plugs http://www.fubarhill.com/GLOW.htm and especially http://www.scootworks.com/rdrc/gloplugs.html that kind-of explains the problem you are having. Cheers

actually,,,,,,,,,,, the #7311 is the 1L......just figured i would clarify that for you...

alden

 

[Craig Murphy]

I have tried both plugs in the motor neither made any difference to the problem, but normally it runs the HP's (7300) in the winter and 1L's (7311) in the summer...

Anyway while I have been too busy this week with work to have a good look at it, it does look like the carb problem described in the posts above, will have some time over the weekend to check it out and report back.

 

[Craig Murphy]

Pulled the carb down over the weekend, all looked ok except for some fine hair "type" material around the hole to the spray bar... :blink: could be the problem but didn't seem to be restricting flow. Anyway all clean now and after pulling it apart gives me a better unstanding of how it works... hmm I can see why some people don't like them.. simple I guess :unsure:

One thing I did notice was that after reading the manual it notes the carb should open clockwise, mine was the opposite, counter-clockwise... not sure if that was contributing to the problem or not but changed the linkage setup and reversed the servo movement just in case.

So hopefully one or both of those are the cause of the problem will know in a few weeks when I can get back out on the water.

Also I double checked the throttle movement on the flow meter before taking the carb apart looking for the restriction some of you noted I should have seen reaching full throttle, and all was nice and smooth, starting at about 120 at idle though to about 40 on a Boris meter....

Thanks again for everyone's advice..

 

[Jim Mamaril]

Pulled the carb down over the weekend, all looked ok except for some fine hair "type" material around the hole to the spray bar... :blink: could be the problem but didn't seem to be restricting flow. Anyway all clean now and after pulling it apart gives me a better unstanding of how it works... hmm I can see why some people don't like them.. simple I guess :unsure:

One thing I did notice was that after reading the manual it notes the carb should open clockwise, mine was the opposite, counter-clockwise... not sure if that was contributing to the problem or not but changed the linkage setup and reversed the servo movement just in case.

So hopefully one or both of those are the cause of the problem will know in a few weeks when I can get back out on the water.

Also I double checked the throttle movement on the flow meter before taking the carb apart looking for the restriction some of you noted I should have seen reaching full throttle, and all was nice and smooth, starting at about 120 at idle though to about 40 on a Boris meter....

Thanks again for everyone's advice..

Murph, I think you will find that your problem will go away, with it rotating clock wise it was closing off at full throttle.

Jim