Help with Carbie installation.

[montynut]

I have been trying to get a 38mm round slide Mikuni to run correctly on my '80 Montesa Vf250. The present status is.

1) Bike runs perfectly with the OEM 38mm Bing. The carbie is worn (both body and slide) which makes low speed running slightly erratic and hard to start particularly cold. The bikes runs perfectly mid range and top end although is getting progressively harder to start and tune at low speed.

2) Engine is in good condition. Standard bore, new piston and ring, exhaust is clean (internally), new clean 'twin air' filter element, New NGK plug (not iridium) of correct heat range, standard Motoplat ingition timing checked and re-checked, new plug cap.

3) 25:1 silkolene with Caltex 98.

4) with the Mikuni installed and bike running there are no air leaks on the engine side. I did find a large air leak and thought I had found the problem but NO  . Have checked for air leaks with bike idling and spraying all joints with aerostart. Bike does not change speed and there is no effect on engine operation at all. If I spray enough I start to get high but the bike idles on perfectly

5) Mikuni set up is 38mm VM, 3.5 slide cut away, 6FJ6 needle, 159#Q6 neede jet, 35 pilot, 280 main. The carbie is new and I have checked all passages, jets and circuits clear. It does have long overflow hoses (250mm) instead of the standard short ones. The settings are similar to Husky, Bully and Maico values I have seen. The carbie and setup was supplied by a very highly regarded Montesa specialist.

The problem.

The bike starts easily and warms up with the choke for 30 sec then choke off. The bike will rev freely in neutral with plenty of smoke and the engine sounds crisp and responsive once the warm.

When I try to ride the bike the moment I try to move off in gear, once the throttle is opened the slightest amount under load the engine just dies away instantly and stops. I have tried adjusting the idle mixture both richer (in) or leaner (out) by progressive steps of 1/4 turn at a time until fully in and 4 turns out these adjustments seem to have little effect on idle mixture. There is a slight increase in fast idle speed at 1 turn out from idle screw fully in.

If I scream the engine and slip the clutch until moving (basically a race start) the bike takes off and as long as it is in the the top half of the rev range goes great.

I have increased the pilot jet from 35 to 45 with very little change in the behavior described above.

A number of other Montesa 250 riders have fitted these carbies with this setup and have had no problems. I remove the Mikuni and reinstall the bing and bingo the bike runs perfectly again.

Any suggestions  HELP please :'(

[Lozza]

What does the plug say?
Forget what anyone elses bike is jetted to just concentrate on yours.
If the idle mixture screw is making no difference, the pilot jet is to big. Go to a 30 (RGV has a 27.5 as standard) and see if the air screw has an influence on idle speed. Next will be RAISING the needle circlip toward the blunt end of the needle. If you find this improves the response, but the needle is at it's upper most groove you need a leaner needle.

[montynut]

The plug reading for high speed operation seems good, just slightly rich (dry but very dark brown/grey). Is there any way of doing a plug chop at 1/4 throttle? At the moment I can not get the bike to run at a constant throttle much below half anyway.

One other thing is that I assumed it was lean at the bottom end because if I turn the choke on when trying to take off it initially improves before flooding out it moves about 10ft but is super rich.  forgot to mention that. I didn't put much importance on that as the change from 35 to 45 made little if any difference.

Is there any other way of determining an over rich bottom end? Can I put a new plug in and run the bike at low revs in neutral (it will not ride at low revs remember) to get a plug reading. I did not think this would reveal much but keen to make headway.

[jimg1au]

i went down to a 27 pilot on my cr250 it ran better then and also downt to 260 main.you have to muck around with the jets and do plug chops to get it right.i set my carbies up rich and then lean down doing 1 change at a time and do plug chops.you also need to do this at the track.
cheers
jim

[All Things 414]

The pilot jet must be pretty close as it does respond once you turn the screw out one turn (a slight increase). Can't imagine any of these things (pilot jet, needle, air-screw) is making it do what it's doing to the point of where it'll just die under load. I'd be having a close look at everything again. Then throw away the Mikuni.....

[Lozza]

You can do a plug chop at any rpm you want, if you ride off and it dies then you will have a plug reading. A Mikuni is infinitely variable and very tunable.

[brent j]

I may be way off line here but a few thoughts from what I have found.

When I put a Mikuni on my XT500 I found a recommendation to use a 159 series needle jet on a four stroke. Initially I had a 247 or 248 in the carb I used and it ran badly down low.
Recently I needed a Q0 so borrowed a jet off a mate from his YZ250G (248 series) and it ran badly again even though the size was what I wanted. A 159 series Q0 is great, now that I have one.
Perhaps a different series needle jet???
Are the other guys running 159 needle jets?

A 3.5 cutaway will run leaner than a smaller one. When I ran large cutaways (4ish) I had to go up in pilot sizes to about 65.
I understand that too large a cutaway will affect fuel atomisation.

I know 4 strokes behave differently but maybe something here will help.

Brent

[montynut]

Thanks for all the help. It helped to steer me in the right direction. Turns out that the Carbie came with the wrong slide from the original supplier, it was then fitted with the correct jetting. The slide was a 3.5 when it should have been the standard 2.5 cut away. This reinforces those old rules.

1) Never assume anything
2) Even if the last 10 carbies from a supplier are correct don't assume anything.
3) Murphy's laws 1 through 99. If something can be incorrect it will be wrong the only time you don't triple check. Don't assume anything.
4) Tuning a carbie is not fun!

In hind sight (like most people I have 20:20 hind sight) it is obvious that if the mixture was OK at idle and OK above about 1/3-1/2 throttle the problem was in the slide cut away or needle. The fact that the bike stopped the moment the trottle was opened with load on it that points to the slide cut away more than the needle as the needle would not have come into play. Simple really  :'(