Poor revving

[Larry Perkins]

I am out of ideas.  Please read all before any help with diagnosis.

250 HS ran fine when put away.  Sat sometime and previous owner got it out to race and ran poorly.  I bought it, cleaned carb, petcocks, and tank thoroughly.  Float level is correct, floats are good, gas valve(seat needle) is good.  Has Mikuni and reed valve.  Reeds are not broken.  Has good strong spark.

When you start it it acts like it is loaded up or like choke is on.  It won't freely and cleanly rev.  If you turn the gas off as it runs down it revs cleanly like it is no longer loaded up but as soon as you turn the gas on it acts loaded up again.  Nothing has been changed in carb since it ran right and it has good compression.

I am supposed to race this at an AHRMA National Saturday and I am at my wits end.  Was sonically cleaned and disassembled at a local shop and they saw nothing wrong with carb.

HELP!

Larry P

[skiracer]

Sounds like the needle is hanging up allowing the gas to just flow into the motor.  Ahhh if only Mikuni had see through float bowls..... Don't you have another carb laying around that you can use for the weekend?

1976 MC 5 Original Owner
1982 Suzuki PE 175
1976 Penton 175 XC
1985 20' Hi Point trailer

[454MRW]

I am assuming you have a good filter on the gas line. Take the float off and blow compressed air into the needle seat with the gas line off. You could possibly have a line deteriorating or a small piece of debris, just enough to slightly hold the needle off the seat. If turning the gas off makes it run better, it almost has to be a matter of excessive fuel. Mike

Michael R. Winter
I enjoy rebuilding and appreciating Pentons!
1974 250 HS Pentons-1978-1979 MX-6 175 & 250 KTM's
1976-78 125-400 RM's & 79 PE250 Suzuki & 2012 DR650
2007 CR125R Honda
1977 MC250 Maico

[Larry Perkins]

It is new fuel line, and new fuel filter.  Everything on carb was blown, cleaned, and checked.  They said float level was right.  I just checked it myself and it seems a bit high so I have lowered it.  Will see if that helps.  I have not worked much with Mikuni.  Bings I know.  I thought if float was high or seat not sealing I would see gas out overflow.  But none of that.  No other carb to use.

Larry P

[Mickey Sergeant]

I had that problem in the past on the mikuni on my 100. was running out of fuel on long uphills, bent tabs to much to get more fuel in the bowl and the float needle would almost fall out. it was binding and not allowing fuel to shut off. Also if that is the case then you should see fuel running out of the overflow tube

Mickey
74 Birshire
74 sixday
73 jackpiner

[Larry Perkins]

No fuel coming out overflow but lowered float level made a difference.  Much better.  A bit boggy in the first quarter throttle.  Should I lower float level a little more or turn airscrew out some.  It is at 1.25 out.

Larry P

[firstturn]

OK, so you know you were getting too much gas and you lowered the floats and it helped.  You can play with the air screw and see if it helps(easier to do that floats).  If it doesn't help lower the floats a small amount.  I would also consider a new needle and jet as I do not know how much time is on the carburetor?  You also need to take a plug reading so that you do not go too lean.


Ron Carbaugh

[KJDonovan]

Larry,

There should be a fiber washer on the float valve seat before it is threaded into the carb body.  If that was inadvertently left out when cleaning it would cause the condition you are describing because the float would need to rise higher to fully shut off the gas.

Thanks,

Kevin

[slvrbrdfxr]

Could it be drawing fuel through the choke circuit past a bad choke plunger ?? My other thought would be to double check your exhaust pipe to be sure some critter hasn't made their home in there causing restricted flow.
Dave McC

[Daniel P. McEntee]

Any kind of screen type spark arrester or silencer that is carboned up will give you fits. It can happen from one ride to the next it seems, or very slowly over a period of time so that you hardly notice it until the bike sits for a while, then you go to ride it again. Been there and done that also. Might be a combination of carb/exhaust issues due to sitting a long time.
  Good luck atthe National,
   Dan McEntee

[Larry Perkins]

Thanks but it is a tiny silencer on a handbuilt MX pipe.  I did check it though and it is clear.

Larry P

[Rain Man]

Maybe swap the. Carburetor if you have a spare. At least this will tell you if it is the carburetor or not..
 Too bad there's no way to measure the actual spark current on these single cylinder engines.  I'm always curious how strong the spark really is... 6 volts seems to get lost occasionally on some of the older ignitions.

Raymond
 Down East Pentons

[Richard Colahan]

A good idea to pull, examine, and clean the "choke" plunger. As we all know, it's not a choke as such, but a seperate circuit allowing extra fuel to be sucked up into the venturi when the lever (or pull knob...) lifts the plunger.
The rubber seal at the bottom of the plunger can harden (or rot)and no longer seal when the plunger is lowered.

Has happened to me twice...on Mikuni as well as Bing...

If seal looks rotted or rock hard...try to replace it with one swiped from another carb...or maybe even gently "resurfacing" it so you have some degree of confidence that it is sealing.

quote:Originally posted by slvrbrdfxr

Could it be drawing fuel through the choke circuit past a bad choke plunger ?? My other thought would be to double check your exhaust pipe to be sure some critter hasn't made their home in there causing restricted flow.
Dave McC

Richard Colahan
1969 V1225
Upper Black Eddy PA

[mike scholl]

Larry,

This might sound crazy but by any chance is your Mikuni slide in backwards? I rebuilt a motor for a friend and he was having the same issue! I was going crazy trying to figure out was wrong so I finally hopped in truck and went to him... after a couple minutes of tinkering it hit me... the groove was on the opposite side so when he would put the slide in he didn't notice it was backwards... put another slide in with the correct slot... Done! Runs like a champ!

Mike Scholl
Team J & J Cycles
Motocross & Enduro Specialist

[skiracer]

Hey Larry,  Did you get it fixed? Were you able to race it? Do tell....

1976 MC 5 Original Owner
1982 Suzuki PE 175
1976 Penton 175 XC
1985 20' Hi Point trailer