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Author Topic: HELP engine keeps reving its nuts off  (Read 1807 times)

digger

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HELP engine keeps reving its nuts off
« on: September 02, 2010, 08:38:32 pm »

i have a cmb Evo 2000 - 90

i start it on the bank and it starts fine but after a few seconds it revs its nuts off

i have gone open and shut with the low end but it dose the same

i have also done the same with top end with no luck

i have about 8 to 10 inch of fuel line from tank to carb ( the shortest route i can go ) is this the prob ? i dont whant to put things in to peoples heads as to say yes this is the prob but i am trying to give as much info as poss

the tank is NOT pressurised

please can any one HELP !!!!!!!!

Regards Digger
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Tombsy

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Re: HELP engine keeps reving its nuts off
« Reply #1 on: September 03, 2010, 06:48:31 am »

I think it's from being too lean, could be needle valve is dirty or an air leak.
Could be a leak in your tank or fittings, a pinhole in the fuel line, crack in the engine casting, or a poor gasket.
My CMB .45 was acting goofy and it was a slit in the fuel line at the fitting.

You probably have but double check your throttle servo and linkage to be sure it's working properly.
We always run pressurized tanks at our altitude but I guess at sea level you don't always have too.
Good luck hope this helps.
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w3bby

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Re: HELP engine keeps reving its nuts off
« Reply #2 on: September 03, 2010, 08:06:19 am »

It sounds as if it is going lean, can't draw enough fuel. I would pressurise the tank as a first stage and go from there.

sheerline

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Re: HELP engine keeps reving its nuts off
« Reply #3 on: September 03, 2010, 09:42:43 am »

I'm not familiar with this this particular engine but it sounds like an air leak or possibly fuel frothing, this last introduces air into the fuel line .
Things to check....
1: Check the carb is seated and sealing correctly to the body of the motor.
2: look for air bubbles in the fuel line whilst the motor is running.
3: check for nicks in the fuel line.
If no air bubbles are present in the fuel line, check the point at which the line attaches to the carburreter as it could be nicked there where the air will be induced right at the point of entry.
A pressurised system would be preferrable on most setups.
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digger

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Re: HELP engine keeps reving its nuts off
« Reply #4 on: September 03, 2010, 06:48:49 pm »

thanks guys

i pressuirsed the tank and it looks like that has done the trick
fingers crossed

regards Digger
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BJ

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Re: HELP engine keeps reving its nuts off
« Reply #5 on: September 05, 2010, 03:59:39 pm »

thanks guys

i pressuirsed the tank and it looks like that has done the trick
fingers crossed


Below is the text of some advice that I was offered.....

In my experience, any attempt to treat CMB carbs like proper carb is
doomed to failure. A couple of rules of thumb:

1) Don't try and set the "low end needle" for a decent idle. If you
do, it will restrict the fuel flow higher up the range - causing
leanness in the midrange and/or top end. You really need to set the
low end rich, get the top end right - then adjust the low end needle
for reasonable low - high transition - preferably on the water.

2) They don't seem to like pipe pressure - especially a lot of it. If
you run full pipe pressure, you will struggle even more with the low
end setting. Or at least that was my experience when I first got the
boat (and one of the reasons that I ended up with a float chamber in it).

The CMB carb on my (non-CMB engine) 21 is out about three and a bit turns on
the high needle. The low needle is wedged with silicon tube around the
thread - so I can't easily screw it in but two-ish turns out should
be a reasonable start. Mine has some pipe pressure - but not much.
Keep meaning to put a float chamber in it - like my other boat.
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Martin (Admin)

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Re: HELP engine keeps reving its nuts off
« Reply #6 on: September 05, 2010, 05:51:09 pm »


I once had this problem (many moons ago) and it turned out to be the tank vent blocked.
 Once the engine started to drink the fuel, it created a partial vacuum in the tank which leaned
 out the fuel until the point where it revved up and starved itself to death....
....  bending the con rod 90o in the process!   >:-o
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