Mikuni Carb Settings

[Jordan]

Any chance you have a tooth out on the timing gears?
It could explain why you run out of adjustment slot.

[Duccout]

You have the same problem that I have with my Mikuni, weakness off idle. Mine runs OK but is too rich and I can't see an answer, as the engine is telling me it wants more fuel, not less. How does it run with the Amal? Are you experiencing mixture problems? The refusal of the revs to drop down must be due to high vacuum in the port holding the slide open, but why?

Colin

[veloduke]

Update on my problem with revs not dropping when I blipped the throttle (250 MT Road):-

The new Amal carb was jetted as per original. I couldn't find a significant air leak. So I changed the .105 needle jet to a .106 - problem solved!

The engine now shuts down normally when closing the throttle. I saw the same issue when using a Mikuni, so I am going to try that again but with a larger needle jet - surely the same issue as the Amal?


Colin
I will post the Mikuni settings should a larger needle jet solve the problem.

Blaine maybe you need a richer needle jet? I know the needle jet is not supposed to affect idle and transiton to slide, but it certainly did on my bike.

[Duccout]

BTW, I wonder if various Mikuni problems are due to the downdraught angle of the carb? I have mine on a straight manifold, so the problems with transition may be due to the angle of the carb.

Colin

[George]

BTW, I wonder if various Mikuni problems are due to the downdraught angle of the carb? I have mine on a straight manifold, so the problems with transition may be due to the angle of the carb.

Colin

Colin if you have carb at downdraft angle and carb is designed to be level would that not effect float level.

[Duccout]

I have just spent a wasted hour resetting the Electrex ignition timing to the full advanced position, but after going through the motions it was set exactly on the FA mark anyway. Oh well.

I'm contemplating going back to the original Amal carb, which needs a new needle and jet, which is why I bought a new Mikuni in the first place; certainly the Amal is miles easier to carry out adjustments on a Strada/Forza, due to the frame design, but having said that, when I removed the spark plug to check the timing, the colour was perfect, so maybe I am very close to getting it right, with just the weakness when cold to sort out.......

Colin

[Duccout]

Hi George,

How is the new bike coming along? Yes, Japanese bikes that used Mikunis always seemed to have them on manifolds that held them level, but all the manifolds that I have seen for fitting them to classic bikes are straight, which angles the carbs down. Mototrans always bolted their Amals straight to the cylinder head, at an angle, but Ducati always used curved manifolds. I don't know...... It will be a bit embarrassing if we are all running around in circles trying to fix problems caused by the angle of the carb!

Colin

[George]

Hi Colin thanks for asking. have posted update

[Blaine.Hale]

You have the same problem that I have with my Mikuni, weakness off idle. Mine runs OK but is too rich and I can't see an answer, as the engine is telling me it wants more fuel, not less. How does it run with the Amal? Are you experiencing mixture problems? The refusal of the revs to drop down must be due to high vacuum in the port holding the slide open, but why?

Colin

I've never had an Amal to try out. I have the stock Del but I'm missing the correct slide needle to test it out.

I feel like the carb angle wouldn't have that much effect but maybe the distance from the cylinder might? Has anyone actually successfully gotten the vm26 running perfectly on a 250 here? By that, I mean not being rich. Mine runs really really well and I've put about 100 miles on it since I got it, but it's just way too rich.
Here's a pic of the setup from Sudco with the manifold they offer.

[Duccout]

Mikuni say that max angle is 20 degrees; I've just checked mine and I think that it is 10 degrees - possibly the Mototrans Strada frame holds the engine a little more upright than the Italian frame.