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604 Bogging at 0 - 1/2 throttle; Shed some light plz
Topic Started: Feb 16 2016, 07:15 PM (1,704 Views)
AlexCr30
Knight
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CCM is almost there now, had to fit a second hand carb to get it running anything near like it should. Now it seems to be struggling at anything below half throttle. What do i need to adjust to improve this?

Current set up is:
185main
40 pilot (tried a 33 and a 50 both had no improvement)
Needle second clip from top, third from bottom
50 idle I believe.
Air screw 2 1/2 turns out

Also noticed the bike idles high if the throttle is blipped and eventually falls to a steady idle.

Any help will be greatly appreciated.
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jonny h
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generally you have an air leak, carb to engine.

it could be the float valve too.
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AlexCr30
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jonny h
Feb 16 2016, 09:48 PM
generally you have an air leak, carb to engine.

it could be the float valve too.
Have already had a new rubber from carb to head, but will have to check again.
Thanks
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jonny h
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the rubber doesnt have to be perished, the gasket on the inlet manifold might be dead, just spray the whole area with WD and see if the engine note changes.

as you have a 185 main, i'm guessing you have holes in the air box and free flowing exhaust?
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AlexCr30
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jonny h
Feb 17 2016, 01:15 AM
the rubber doesnt have to be perished, the gasket on the inlet manifold might be dead, just spray the whole area with WD and see if the engine note changes.

as you have a 185 main, i'm guessing you have holes in the air box and free flowing exhaust?
Ah ok, will try that.
Yeh holes in the box and scorpion exhaust
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jonny h
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i would double check the float height too
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AlexCr30
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jonny h
Feb 17 2016, 10:29 AM
i would double check the float height too
This should be 17mm I believe?
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jonny h
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yes mate, especially for the Scorpion system
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AlexCr30
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jonny h
Feb 17 2016, 02:56 PM
yes mate, especially for the Scorpion system
Ok will check that, do I just measure the height of the forks (not sure of the name) where the floats sit on or do I measure from the base of the carb to the top of the floats?

Sorry for pestering but not done float heights before
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vinny900
Marquess
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+1 for Johny h
If you've tried 33 and 50 pilots and no improvement then your pilot circuit is not working properly. Most likely cause is an air leak making the mixture much too weak. That would also make the throttle hang.

With a 185 mains at higher throttle openings the extra fuel will hide the air leak to an extent. Re float height, if it was too high, then this would affect higher speeds as well, as more fuel would be needed than at low speeds. If it was set too low, it would be overfueled and the throttle would definitely not hang.
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jonny h
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jonny h
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i'll say it now, i tried with two different carbs for over a year to get my bike to run how it should, and it never did.
always had tick over problems and hanging too.
when my bikes back together it's getting a brand new carb not a 17yr old one.
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AlexCr30
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Cheers guys, will tear it down tomorrow and see what's what
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motomaniac
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While its open, check the floats arent leaking/filling with fuel, best to replace if they are, rather than try seal the leak...
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tommysdad
Earl
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jonny h
Feb 17 2016, 10:29 AM
i would double check the float height too
I would check the float needle valve also .I had the same problem and replaced the float valve about 15 quid. it's a bit fiddly/delicate so take your time in getting the float height correct. see the link for PHM rebuild guide.

http://advrider.com/index.php?threads/rebuilding-a-dellorto-phm-sd-carburetor.635497/
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lahauteur
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Some ideas for me here too as my 636 tends to hold revs when I pull up and then drop to a normal tickover after a several seconds.
2001 Rotax 640e Supermoto and 1998 350 Enfield Bullet.
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AlexCr30
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What's the standard carb for the 604? The one in mine is a VHSB38
Found there to be a k51 needle in there when I believe the standard should be a k32?
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RB636
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Potato Farmer
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Should be a phm40, but i don't rate them personally. seen so many play up.

Id go mikuni everytime, in fact im putting a vm36 on a 604 i got with a phm, because the phm is fucked and im not putting another one on.

My first phm was fine for ages, but once it started playing up, no amount of ultrasonic cleaning, replacing seals and checking everything over would sort it, a new phm did sort it, but for how long I don't know.
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Sportax Racing
Anything & Everything ROTAX
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We successfully dyno'd a PHM 40 with accelerator pump the other week on the 604's. The results were very very good - smooth instant linear power with no bogging or hanging revs. They will be advertised in the Sportax shop very soon.

They have the advantage of being new with new cables and jetted to perfection unlike a twenty year old randomly jetted worn out version with out the pump..
www.sportaxracing.co.uk
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Gonzo250
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Which slide are you using?

The one I'm running has a 60/5 I believe, which activates instantly, and only in the first 1/3rd of throttle movement.
It makes starting...tricky, to say the least. Considering I'm limited to kickstart only at present (too lazy to rebuild starter motor and a tired sprag) it's rather tiring.

I mentioned I was looking at a 50/4 in my other thread, I meant to say 50/1...
I drink too much, I swear too much, I ride too fast and I crash too often.

It is a universal impossibility for me to leave my tools alone and my bikes standard. 'It must go faster!!'
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