jetting I really need some help

[crabman]

Hi Gents.
I am having a torrid time with the jetting on my Maico 490. I am running a 38mm flat side Mikuni carb and after a few minutes of riding the bike won't rev out. I am convinced I am running the wrong main, in fact to small. All help appreciated. Please if anybody has got a flat side carb at sea level working, please share the jetting specs. help help help. I want to race it this weekend. Gavin

[holeshot buddy]

jetting specs for a rm500 would be close and
a good starting point
they are 500cc and air cooled
run same carb
i will have a look for specs and get back to you
post up what specs are in carb now

cheers rusty

[Lozza]

The plug knows all the answers.

[evo550]

Yep,check the plug that's the key to jetting.
Is the "don't rev out" problem constant, or only after bike gets hot?
If it's a delayed reaction, I doubt it's jetting.
Have you taken the rag out of the airbox after the last wash?

[crabman]

thanks, it generally runs pretty well for a lap or two and then won't rev out , it almost sounds like it is suffocating. I bought the 38 mm flat side carb that was supposedly jetted for sea level. The main jet was a 300, ( plug is dry) i have since found out that a similar carb was sold to another South Africa and it was jetted also for a 490 at sea level with a 340 and 25 pilot jet.
I am starting to believe the problem may be elsewhere. All suggestions encouraged aother than the one that says burn it

[evo550]

Generally speaking
Rich jetting gives a black plug colour
Correct jetting gives a tan colour
Lean a Grey/white colour
To check if it's a lean main, do a plug chop.
How do i do a plug chop?
You will need somewhere long and straight.
Warm bike with old plug, stop, and fit new plug.
line yourself up for a long straight run, start bike and PIN IT !!!!!!
run all the way through the gears wide open and keep it pinned in top for maybe 30 seconds, then, and here's the trick, all together pull in the clutch, back the throttle off and hit the kill switch and coast to a stop with the motor dead.
Pull the plug out and check the colour.

[holeshot buddy]

so are you saying that when its
cold its ok

[crabman]

yes when it is cold / first two or three laps it is ok. I am starting to think beyond the jetting to an air leak through crank seals or via motor. Anyway I am really appreciative of all the help. Thanks keep the suggestions rolling.

[holeshot buddy]

if it had air leak it would run away
sounds like ignition coil swap it for another and try

[evo550]

Yep, I'm with rusty, sounds electrical to me.
A leaky crank seal or engine gasket sucking in air would have it screaming it's head off as soon as it fired, and MAYBE slow down as the bike warmed, depending on the severity of the leak.
Faulty electrics run well when cool and as they warm start to fail............just burn it

[maicoman18]

Can also check fuel tank breather pipe isn't partially blocked. Fuel line isn't blocked.

Ed

[Hoony]

it could possibly be a faulty stator coil breaking down when hot. if the insulation enamel between windings is poor/brittle it can short out between turns effectively giving a lower output.

hope you sort it out.

[crabman]

Hi Gents
Just a quick update, it would appear that my problem lies in the stator coil, however before I replace I will that I get access to the right timing specs for the PVL ignition. I have printed all of the historic posts and I will also speak to one of the former Maico GP mechanics in Europe.
Anyway I will not give up based on what  I saw in terms of the 490  Maico's  in action in the USA. I still cannot express in words how impressed I was with the race Maico's. I did however buy one fully speced Maico 490 motor. I am excited very excited. I cannot wait to get my motor.

[Tahitian_Red]

Hey Crabman,

I'm sorry I missed meeting you on your recent trip to Arizona!  The guy who stayed behind and rode Dave Coupe's Maico, at the AHRMA race, was really fast.
 

I asked the PVL folks about your 490 timing settings and I hope to have an answer soon.

[Tahitian_Red]

1.8mm BTDC