Mikuni on Montesa

[keychange]

I picked up a flange mount Mikuni (measures 24mm) on eBay and wanted to try it as a replacement for the Amal in my 1976 Montesa. It appears to be an old VM model as fitted in a 1971 DT1

Slide looks new and the carbie looks in good general condition.

I can get it started but it is way too rich with fuel blowing back out the air inlet (filter not connected) and I mean spraying out and no way it will idle ... well it did once then it went silly again. It is clean as a new pin... I have played with float but at no stage has there been fuel overflowing. I have replaced the setter o-ring with a generic (good fit) could this be the issue?

When fitted is sits leaning forward on approx 10degree angle. I just can't figure out what's happening but it must be something obvious that I am missing.

 

[whenfoxforks-ruled]

Are you sure the top end is good? The piston is not backwards? Pipe and silencer? Timing is spot on? All is good, put the filter on and try it again. Vintage Bob

 

[keychange]

Yes - the bike runs ok with the Amal it could simply run better.

 

[Ol'89r]

Compare the jets between the two carbs. The Mikuni should have close to the same jetting as the Amal. Also check the slide cut-away. There should be a number on top of the slide. This number should be at least a 3. If the cut-away is too small, it will make the carb run rich through the whole range.

 

[Rich Rohrich]

It's worth taking a close look at the starter/choke circuit as well to make sure it's sealing and not dumping excess fuel.

 

[keychange]

Rich - I think you have it... but to be honest I can have a look but I am not sure what I am looking for :(

I am tempted to just block the choke inlet for now just to see what happens - can you see any problems with that or suggest alternatives

 

[whenfoxforks-ruled]

I was waiting on the somebody to verify that the 10 degree estimate was not a big deal. No worn parts, and similar jetting sized parts is the ticket? I have never been a fan of the Amal, even the 1 I had on my Montesa. A dirtbike has no place to be tickled.

 

[2strokerfun]

Are the jet size numbers in a Mikuni similar to what Amal uses?? Not so in a Keihin/Mikuni swap. Make sure you are comparing apples to apples when comparing jet sizes......

 

[Rich Rohrich]

Rich - I think you have it... but to be honest I can have a look but I am not sure what I am looking for :(

I am tempted to just block the choke inlet for now just to see what happens - can you see any problems with that or suggest alternatives

The choke is nothing but a plunger that opens or closes a fuel passage. It's common for corrosion to build up and prevent the plunger from closing the circuit completely. When you have it apart look at the assembly and the passages and the operation will be fairly obvious. Here's a diagram just in case.


2stroker brings up a good point on the jet sizing. Not all jets are the same.

Mikuni hex head jets are numbered by bulk flow, while Mikuni round head jets and Keihn jets are numbered by inner diameter in mm.

The flow difference between a Mikuni 175 hex jet and a 180 is about 6cc ~3%) . Keihn jets are numbered by jet id, and the difference between a 178 and a 180 is about .0008" (I'll let those so inclined do the math to determine the flow difference) .

To take it a little further and confuse the issue a bit more keep in mind that air/fuel ratio is based on weight of fuel and weight of air, but jets essentially meter by volume. So we really need to know the weight of the fuel flowing through a jet to understand all this. In simplest terms fuel weight is a function of the area of the jet multiplied by the value of the square root of the fuel head pressure multiplied by the density of the fuel.

It looks like this:
weight of fuel = jet area * ( SQR Root (head pressure * fuel density)

Ok, that's probably more than anyone wanted to know, but I'm sitting on a useless conference call and it's

B O R I N G :bang:

 

[whenfoxforks-ruled]

What size is the engine? Rich, Thank you, more copy and pasted!

 

[keychange]

Sorry I should have mentioned it is a Cota 247

This mikuni carb has the round head (8mm) main jet originally a #90 - I live in a remote spot and have ordered some new jets but in the meantime I drilled the 90 out to the same ID as the #150 Amal (from memory about 1.2mm) - and for a short time ( 5minutes) it ran well then started blurting again.

I haven't had a chance to pull the choke apart - hopefully tomorrow I will get a chance and see what lurks within

 

[whenfoxforks-ruled]

Unless you are a machinist, pot luck on drilling jets. Hit or miss big time. Even for 76 a 247cc engine on a 24mm carb? Or is it the model number and the engine is smaller?http://www.cybermotorcycle.com/euro/brands/montesa.htm ,http://www.cybermotorcycle.com/gallery/montesa/Montesa_Cota_247_c1976.htm http://www.dirtrider.net/forums3/showthread.php?t=5104 . Ahha! Now I see. A good dirt bike query, usually ends up back here, go figure.

 

[keychange]

Yes I understand the hit & miss aspect

I made a rubber seal and blocked the choke - no change!

I have noticed that with fuel off once the level starts to fall then it runs much better... fuel level??? I have double checked and as per VMManual for VM26 it is set spot on

 

[2strokerfun]

Do you know what size the Amal was?? A 24mm carb is pretty small for a 247cc engine as pointed out earlier. You might just need to buy some brass and start experimenting. My experience is that if you downsize a carb, the jets may also have to be downsized because of the decrease in pressure due to more air going through a smaller venturi. In other words, when I run a 34mm mikuni on my bike, I run a 240-250 main jet and when I run a 36mm mikuni on my bike, I generally run a 270 main jet. I run the same pilot jet on both carbs. But there are so many other variables when you switch carbs besides jet size. Obviously, the needle matters. As does the needle jet, which you can change on most VMs. And, as pointed out, the slide matters too. It can get damned expensive and very frustrating to properly set up a carb from scratch. If you can find someone that had performed this switch before, the information they can give you will be invaluable.
And in the end, if you are downsizing the carb, you might gain some low end but normally give up some top end power. Might be easier to give some attention to the Amal.

 

[whenfoxforks-ruled]

Replace the float valve and seat if applicable. When jetting seems confusing, there is a problem elsewhere.

 

[keychange]

Yes the Amal is a 627 (27mm) I had other issues a while back and I did borrow a Mikuni 24 off a mate and it ran quite well (didn't fix the other issues - they were unrelated). That unit ran a 150 main and 30 idle and so I have ordered a range of mains up to 150 and the same idle jet. They should be here before the weekend.

What surprises me with this carbie is how difficult it was to get it to run at all and then this confounded flood of fuel. I just wouldn't expect the jets to do that - particularly when they are smaller than the recommended. Someone has suggested a worn needle valve ...

Part of the my problem is identifying exactly which carb I have - is marked with the number 286 then 03 on inlet side. The inlet throat measures 24 mm approx - and it has the independent floats and side mounted mainjet - the nearest I can find that looks the same is the DT1e (c. 1971) - it was sold to me as off a DT175 but I can't find any DT175 specs that show this unit.

The mikuni that I tried before is numbered 248 ???

 

[whenfoxforks-ruled]

1 word, Sudco. http://www.sudco.com/ . I forgot they carry so much!

 

[2strokerfun]

And people at Sudco are among the most helpful people in the world when you call (along with Wiseco [if you get an old timer on the phone] and Sprocket Specialists).

 

[WoodsRider]

Sounds like your biggest problem is you really don't know what size Mikuni you have. It's been a couple years since I looked up anything for a Mikuni, but the number you listed above sounds like it may be a 28mm. I could be wrong though.

I have a Bultaco Sherpina 250 with an Amal 627 and the Mikuni conversion is a 28mm. The Amal is a pretty simple carb and parts are still available out of the U.K. The main problem is the slide, not being chrome plated, tends to wear out. The bike starts running horribly and no amount of brass swapping can fix it. There is a company that will repair worn Amals and chrome plate the slide. Can't remember their name, but it costs about 1/4 the price of the Mikuni conversion kit (assuming a new carb).

I have ridden a Sherpa T with the Mikuni conversion. That bike was a 325, so I really can't compare throttle response, but the bike did get better fuel mileage when compared to mine.

 

[keychange]

My new jets arrived so I installed the 150 main and #30 pilot (I lowered the float about 2mm) and it was much worse..fuel spraying back out the air inlet.. so much so there was a constant drip from the rear fender - so I pulled the &^*#@ mikuni off threw it away and reinstalled the Amal.

The Amal has a dead top end and an annoying occasional hesitation opening from low revs - but at least does work. I have spent 6 months on the Amal so maybe a reconditioned slide is the answer but I am hesitant to spend any more money on thing.

Thanks everyone for your suggestions.

 

[whenfoxforks-ruled]

Well that is not good. By lowering the float, it was letting more fuel in?

 

[WoodsRider]

You can get a new chrome-plated slide from Amal.