Mikuni Carb Settings

[themoudie]

Aye Blaine,

Try cutting a circle of lead sheet flashing about 3mm (1/8") thick, so that it fits inside the top of the slide, then remove the centre for the throttle cable and needle. Not only does it pre-load the spring a wee bit, but also helps stabilise the throttle slide, rather than fluttering about due to the intake pulses. Works on AMAL MKI and MKII concentric throttle slides.

Good health, Bill

[Blaine.Hale]

Aye Blaine,

Try cutting a circle of lead sheet flashing about 3mm (1/8") thick, so that it fits inside the top of the slide, then remove the centre for the throttle cable and needle. Not only does it pre-load the spring a wee bit, but also helps stabilise the throttle slide, rather than fluttering about due to the intake pulses. Works on AMAL MKI and MKII concentric throttle slides.

Good health, Bill

That's a great idea. Though, I'm not exactly sure where to get a lead sheet around here. Any online retailers in the US have way more than I need and it's pretty pricey.

[themoudie]

Aye Blaine,

Try a plumber or roofing contractor. Over here in the UK these are the tradesmen most likely to have some lead.

I do not know if lead shot is still used in shotgun cartridges in the USA (over here it is steel shot), or there are lead fishing weights, eBay looks a good source and for a reasonable price. Melt them down in the bottom of a steel tin can with a blowtorch until you have sufficient depth of lead. Leave to cool and then extract from the can. If the lead is a bit thick, then you can beat it with a wood/nylon mallet to spread it out a bit, before cutting out the disc with a pair of tin snips.

Good health, Bill

[veloduke]

I have tried a VM28 on my Road 250.
As Duccout experienced, 1/4 throttle was a problem area. This was fixed by using a 1.5 cutaway slide.
I ended up with
25 pilot
1.5 slide
169 02 needle jet
5F21 needle
150 main jet

The bike rides very well with combination.

I have another issue that the revs don't die immediately the throttle is closed, but I might start another thread for that.
It's not the carb, I've tried a new Amal and that behaves the same.

[Jordan]

Thank you.
Does it have an air cleaner?

By the way, does VM28 mean the bore is 28mm, or is it a nominal description of the body type?

Sluggish return to idle could be a cable or twistgrip problem?

[Duccout]

Thanks Max.

Those settings are very close to mine, except that I have a 2.0 slide, which Motocarb are adamant is correct, although if it were an Amal it would be the first thing that I would change, as it exhibits all the characteristics of a weakness at the slide cutaway. The problem that I have now is whether to persevere with the Mikuni and keep throwing money at it, or forget it and buy an Amal, or even go back to the original Amal and refurbish that.

Colin

[veloduke]

Hi Jordan
I have tried with and without the air cleaner, and the throttle snaps shut when released. I also had it running with the petrol tank off (so no trapped cables). VM28 refers to the bore size rather than the carb body size.

Hi Colin
Yes that is the dilemma. I now have a brand new Amal and a brand new Mikuni, same issue with both.

I'll start another thread for this.

[Duccout]

The Mikuni settings are strange; the Triumph Daytona settings for a VM 28 are: main 140, pilot 35, slide 2.5, needle 5F21 and needle jet 169-04. My 250 came jetted with: main 140, pilot 25, slide 2, needle 5F21 and needle jet 169-02. Max's settings are: main 150, pilot 25, slide 1.5, needle 5F21 and needle jet 169-02.

The discrepancies in main jet are normal, and the needles and needle jets are almost the same, but what about the slides? All engines are 250cc cylinders, so why such a large variation? Should I bite the bullet and buy a 1.5 slide, or try a 35 pilot jet?


Colin

[Duccout]

As a further to all this, last week I stripped the head off again to make sure that either of the valves were not sticking, and predictably, everything is in good order, so that was a waste of time, but had to be done to eliminate a possible cause.

No matter what changes I make, the pilot air adjuster has no effect on the speed of tick over, although it does make a difference in how the engine runs, ie, if I turn the screw out to weaken the mixture, it is noticeably weaker low-down when riding. When the engine is ticking over, every now and then it will cough and either miss, or cut out, any ideas anyone? Possibly this is connected to the lack of effect of adjusting the pilot air screw. I need to solve this; is this hiccup the cause of carburration problems or a symptom of them? There is only one more thing to try and that is Jon's suggestion of a faulty ignition switch, and I plan to by-pass the switch to eliminate this, and then I am out of ideas.

Colin

[Duccout]

A very hot test ride today, with progress. I raised the needle one notch and bypassed the ignition switch. Bike started first kick and ran, albeit very richly, with the cold-start lever on, which it had not before, then after turning the lever off, it ran perfectly, with no spitting back at all, which is the first time it had done this. So, either raising the needle worked, or bypassing the ignition switch worked.

Next step is to lower the needle back down and see how that goes.

Colin