Mikuni Carb Settings

[Duccout]

Right listen up, I think that I have the definitive jet setting for a 250 Ducati with a 28mm
Mikuni (well maybe!).

My bike ran well with the Mikuni, but was troubled by weakness just off idle, which Motocarb said definitely was not the slide, but it was the obvious culprit. I tried to buy a 1.5 slide to replace the 2.0 which was fitted, but none are available, so I checked and found that each half increment equates to 0.8mm, so I put the 2.0 slide in the lathe and turned 0.5mm from the base, giving in effect, a smaller cutaway. Although I haven't road tested it, I am confident that the weakness is cured, the bike is thumping away and not spitting back, and the pilot air adjuster is working. So, if anyone is looking at fitting a 28mm Mikuni, here are my settings:

Main jet 140
Pilot jet 30
Needle jet 169 0-2
Needle 5F21
Slide 1.5
Main air jet 0.5

Of course, individual adjustments will always be necessary, but this is a good base setting.

Colin

[Bevel bob]

Cutting the slide down puts the needle lower in the jet by the same amount ,so you may need to raise the needle a notch.

[Duccout]

Good point, thanks Bob.

Colin

[Duccout]

It is time to revisit Mikuni woes.

I contacted Allens Performance here in the UK who are the other Mikuni suppliers here, and they use a different needle/ jet combination to Motocarb, who supplied my carb. For a 28mm Mikuni, Motocarb use a 5F21 needle and a 169-02 needle jet, but Allens recommend a 5DP7 needle and a 182-02 needle jet, so I have ordered the different combo from Allens.

Will this saga never end? I am mindful of the old saying 'when in a hole stop digging'.


Colin

[Duccout]

Well, I tried the new needle jet/needle combo........ Just running the engine in the garage, it seems OK, but the revs will not drop at tick-over, it is racing at about 2,500 RPM and will not come down. It did not do this with the previous needle/jet combo, so I have just about had enough of Mikunis, they are a bottomless money pit.

I thought that when I bought the Mikuni it would have been jetted ready to go; but no, there is just too many combinations of settings offered by different suppliers; I am sure that this is not the case with Amals.

Colin

[themoudie]

I feel your pain Colin! Whilst the set ups may be simpler with the AMAL, opinions/ recipes are as numerous as an ants nest and getting the right combination as easy as herding cats! Whilst AMAL have a combination for the Spanish 250s and 350s, for which theirs was the "standard" instrument, unless you are running the standard carb and everything else is standard ticketyboo, it is still very much a "suck it and see" process.

The 450 still coughs as it comes off the pilot onto the slide and yet the plug is now a lovely tan colour around the top of the insulator and on the earth electrode.

Number drills maybe the way to go, as AMAL pilot hole size is not given as an ID, but as a flow rate of "cc of fuel per minute". With the increased amount of alchohol in the fuel, I suspect something between the 25 and 27.5 pilot jet is where I need to go, if I'm to clear the "cough".

Keep on keeping on!

Good health, Bill

[Duccout]

Thanks Bill, I hope that your repairs are coming along OK.

I don't understand why changing the needle jet and needle can stop the engine revs from falling; surely the revs are controlled by air flowing through the pilot system? The slide is fully closed and there is play in the cable, and I have confirmed this by putting my finger in the bell mouth and holding the slide down and it makes no difference. I will ring Allens on Monday, but I am baffled, for the revs to rise there must be an increase of air through the carb, but the slide is closed.


Colin

[Duccout]

Right, next installment of the never-ending Mikuni saga.

I sorted out the high tick over and went out for a ride to try the new jet/needle combo. Bike started fine and ran well, I pulled away and the engine died, just like it was out of fuel, spat back through the bell mouth and the engine stopped. When it wouldn't start I found that the carb had blown off the manifold.

Pushed it back to the garage and in a fit of pique, tore off the bloody Mikuni, while cursing the Japanese and anyone else who I felt was responsible for this mechanical abomination. So, out of the drawer came the original Spanish Amal, I cleaned everything thoroughly and blew them off with the air line, rebuilt the carb and fitted it, reasoning that at least all the jetting was correct and I wouldn't have to spend all my time and money on R&D. When it was ready I went for a long ride........

Unfortunately, the problems with the Amal that prompted me to buy a new carb in the first place were still there, namely a flat spot between idle and midrange, causing a lag and a jerk when opening the throttle, the engine was also vibrating too and not happy. Grrrrr. Came home and fitted the Mikuni again! This time I raised the needle one notch, remembering how sensitive these carbs are to needle position.

This morning took the bike out and it ran perfectly, no problems at all. So, is that it? Who knows? The spark plug was still slightly rich, but not as much as with the original 5F21/169-02 needle/jet combo, but I am running a 1.5 slide and Allens recommend a 2.5 slide, so, do I just leave well alone and see how it goes, or do I waste more money and buy a 2.5 slide to try and risk a return to the previous spitting and cutting out at low revs?

Colin

[themoudie]

Aye Colin,

Personally, I would leave well alone, have a quiet word of thanks too the fairies, keep your sheckles in your pocket and get some miles under the wheels.

I suspect that a lot of the settings prescribed by the agents for these instruments are based on "suck it and see" trials, rather than specific dyno runs under controlled conditions. "Suck it and see" trials are what we all rely on, as hourly atmospheric changes, machine wear and a changed fuel type from when many of these prescribed settings were calculated are all part of the process of getting an engine to run.

A final thought! The wee grub screw holding your AAU onto the timing gear shaft hasn't slackened and allowed your timing to wander?

Good health, Bill

[Duccout]

Thanks Bill, I will check the backplate screw. When Mikunis are dialed in they do give beautiful, responsive carburation, but the problem is that there is no common knowledge of settings, and the main agents seem to have their own ideas about them. The various settings can be made to work by adjustment, and there is more than one way to skin a cat. The common problem is that of stalling when pulling away when the engine is cold, and most Mikuni equipped bikes seem to suffer from it, and Rick Parkington in Classic Bike even mentioned that he 'has never had much luck with Mikunis'.

Cheers,

Colin