Strange problem

Status
Not open for further replies.
R

Roxie

Joined
Jul 25, 2017
Messages
14
Country flag
Hey guys. This is a weird one. Mk3 that Ive had since new. Boyer ign that has been flawless for 15 yrs.Single mikuni. About a month ago it started running lousy. I figured it was crap in the tank. Cleaned and flushed tank.Cleaned carb.When it continued to act up i replaced carb. It starts 1st or 2nd kick and runs perfectly untill it gets hot then the rpms dont come down when throttle released as in shifting. Put new cable on and cleaned and lubed throttle. The slide and needle are dropping as they should. The problem seems to get worse the hotter it gets which would indicate an air leak. I cant find one. Valves are within specs. Head bolts tight. Would this be a symptom of a cracked head? The only other mod is a pcv valve in the breather line. Any suggestions would be greatly appreciated. Thanx. Don
 
Lever on carb. Remember this is brand new carb. Also. Plugs are brownish. I would think if it was big giant air leak plugs would be white?
 
The slide and needle are dropping as they should. The problem seems to get worse the hotter it gets which would indicate an air leak.
Click to expand...


you are at sea level with stock supplied jetting just as 6DH3 needle, in middle notch?

reputable Mikuni vender equipped it with correct stock number slide?
rpms not coming down when throttle backed off usually means too lean idle circuit, which you are on when backing off

is air mixture screw about 1 and one half turns out from seat should be?

try turning it in to one turn out and see if the rpms come down when backing off throttle
 
Are you absolutely certain the slide is returning properly? I ran VM mikuni for a while it always used to hang up I put an amal spring inside the mikuni spring in the end no more problems after that apart from the usual lackluster performance a single vm gives you, cheers
 
Don,

The Single Mikuni VM34 carb kit is a fine modification however the standard throttle spring is prone to throttle slide sticking so I always recommend to put also the original Amal throttle spring inside the wider Mikuni spring to prevent throttle slide sticking and for better throttle response.

Good luck,

Constant Trossèl
http://www.hollandnortonworks.eu
 
Yes. Slide is slamming home the way its supposed to. Carb supplied by rocky point and i checked all internals. This is a new carb and i had same symptoms with old carb.it is definately heat related as the longer i run it the worse it gets. Mikuni has been absolutely trouble free for decades.
 
Are the coils getting hot to the touch when the bike starts running rough?
 
Roxie said:
Yes. Slide is slamming home the way its supposed to. Carb supplied by rocky point and i checked all internals. This is a new carb and i had same symptoms with old carb.
Click to expand...

Checking your carb stationary doesn't tell you anything about this particular sticking problem. Baz and Holland Norton Works are right because the sticking throttle slide is caused by vacuum suck and this can be cured by putting the extra Amal spring inside your Mikuni spring. Just try it :!:
 
true some Mikuni supplied with weaker springs, don't know why as all come from Mikuni same slides right?

just bought and installed two new Mikuni 34's from CNW within the past six months, no slide issues at all

can't imagine Rocky Point somehow getting weaker springs than other venders but...

have you got an old Amal spring lying around from years ago to put inside the Mikuni spring to try out this theory?

question, you say it is a new carb, how new? did you have this non dropping rpm problem immediately when you installed the new carb from Rocky Point and has this developed after a while?

just to humor me will you screw in the air screw until seated and count what turn fraction is was at?

then put it at one turn only out, richer setting, and see if the idle comes down

the condition you describe, a non rpm dropping upon throttle shut down is often leanness in the idle circuit
 
I appreciate all the input from u guys. Everything except the xtra spring has been tried or checked. Yes, this problem showed up about a month ago. Carb that was on bike was about 10 yrs old and dirty & worn. I thought that was the problem so i replaced it. New carb worked fine until it got good and hot then started doing same thing. You are absolutely correct in saying stationary tests dont count but i dont know how i can check slide while underway? Air screw 1 1/2 turns out. Turning in does not help .starting to think its not a carby problem at all.
 
Have you strobed the Boyer ? Problems with the ignition system can give the same symptoms as problems with the carbs. So it is important to sort the ignition system first, that way you remove one variable. The Mikunis spring problem can be very bad. Years ago we fitted Mikuni carbs to an XS650 Yamaha speedway sidecar and could not stop the slides from hanging up. That is the reason I have Mk2 Amals and not Mikunis. Mikunis are primarily a two-stroke carburetter - they don't have as much vacuum.
 
Adjusting the pilot (mixture, idle) screw is not the proper method to resolve idle issues. The pilot screw setting is based on the size of the pilot jet. The proper method for adjusting this is with the engine at operating temperature and turning the screw in until the engine starts to stumble then slowly turn out until highest idle is achieved. Then set the idle speed with the mechanical adjustment screw on the throttle plate.

A floating idle could be caused by a number of things, but adjusting the mixture screw in an attempt to correct it is not the proper thing to do.
 
Yes that is the correct way to set idle and i have always done so. I am traveling and dont have many tools or parts with me. At present we are in NH. I will try to scrounge up a spring and timing light.So no one thinks this could be a cracked cyl head?
 
Also as i said earlier I have never had a problem with mikuni springs. I put the 1st mik on in 1982
 
Cracked head ? MK 111 models used Rh 4 or Rh 10 type heads. Rh 10 being the best as thicker around the intake guides , the Rh 4 being more prone to crack there from inexperienced brutal guide replacement. Burning oil is a clue... not your symptoms. It's stamped over the Right hand exhaust rocker cover. But you likely have a different issue. Maybe check valve clearances with a little wiggle when cold as you are on the road travelling with or without the feeler gauges ?
 
Roxie said:
Yes. Slide is slamming home the way its supposed to. Carb supplied by rocky point and i checked all internals. This is a new carb and i had same symptoms with old carb.it is definately heat related as the longer i run it the worse it gets. Mikuni has been absolutely trouble free for decades.
Click to expand...

I just re-read this and you are saying the symptoms were the same with the old carb...thinking Toronto may be right you should check your valve adjustment.
 
Valves were a little loose but not excessively so. I would think valves too tight would be more symptomatic.It is RH 4 head.complete valve job about 2500 miles ago. No weepage between cyl head and barrels. Plugs dark brown.
 
Well then all I can think of is a fractured slide , or a leak introducing air to the mix. Does the slide snap back with the tank on , or off ? I've had cables hang up when the tank is put back on.
 
Status
Not open for further replies.

Similar threads

E
Tri-Spark unusual problem
Replies
6
Views
515
Tornado
Tornado
bpjthornton
A four cyclinder starting problem
Replies
7
Views
440
Onder
Onder
robs ss
Front Iso - strange?
2
Replies
36
Views
2,787
PeterJoe
PeterJoe
Bigbossmonty
commando year problem?
2 3
Replies
46
Views
3,028
mdt-son
M
S
Horn Electrical Problem
Replies
18
Views
1,319
Tornado
Tornado
Back
Top