Hi Starr,

I'm unsure of the acetylene, caustic soda subjects except knowing they're dangerous. The pilot jet on a Mikuni is marked on the side. You'll see a little square logo thingy and a number. The main jets are marked on the top of the hex.

Most two stroke carbs follow the same design where there are multiple circuits (air passages and fuel passages) for different throttle settings. The summary looks something like;

Idle to 1/8th throttle, Pilot circuit

1/8th to 1/4 throttle, slide cutaway (That angled bit on the slide facing the airbox)

1/4 to 3/4 throttle, Needle jet and needle

3/4 to WFO, Main jet

There is overlap but this is pretty much the map. The air for the pilot circuit enters in through those holes you see in the bell mouth of the carb. They also provide bleed air for the other circuits in the carb. The theory is really complex on how all these holes work. Think Doctor Who explanation of the TARDIS. All Fuely Airly wibbly wobbly. Not really necessary to know however. All you really need is the map above and to set the float height correct. There's a lot of good info on the sticky Mikuni thread at the top of the forum and the web is full of info on this type of carb as it has been standard on many two strokes over the years. Oh and when you clean the carb use compressed air to blow out the passages.

the front pipe is easy to burn out....set the oxy cutting torch just inside the exit of the header pipe until the carbon starts to glow red, you don't need massive amounts of heat, just enough to get it to glow red..then remove the oxy torch and gently blow compressed air in...with safety glasses on you can observe inside the pipe and watch the carbon burn itself out being regulated by the amount of compressed air you're feeding in (don't burn your face!)...pay attention not to allow the metal of the header pipe to glow red (not disastrous but its preferable not to heat it that much). you may have to heat from both ends of the pipe until no more carbon comes out

the centre pipe will have a fine steel gauze inside much like a metal scouring pad....it melts very easily so less heat is best until you get the hang of it.....primarily, heat the inside of the pipe like you did with header pipe, the carbon will glow and you'll blow it out with compressed air....keep a large rag handy soaked/dripping in a bucket of water and continually douse the aluminium outer surface with this to keep the outer temp under control...if you have a friend to do this that's good....expect a lot of smoke from the centre pipe....take your time, burn it slowly...remember, less is best, keep burning it, swapping ends until your satisfied with the amount of ozone depleting smoke you've created....when your done, the inside will look near new....the outer aluminium surface will probably have a somewhat etched appearance to it....

you may be able to use a high volume LPG powered heating torch with same effect, but I have not tried...

I did the exhaust today but I used a leister heat gun for welding pvc flexible sheets for roofing and lining, it gets up to 700 degrees when turned right up. It worked a treat and totally cleared both front pipe and middle box although there was a point when flames were shooting out of the ends of the pipes! I don't think they have ever been cleaned out and I can now see the honeycomb type material in the middle box and no carbon left inside, I think I will repack the silencer again and see how it runs now, I just hope this has done the trick.

Andy.

It worked a treat and totally cleared both front pipe and middle box although there was a point when flames were shooting out of the ends of the pipes!

Heh heh love it! With a choice of fire or chemicals I'll always go for fire too.

Last night I deceided to take the top end apart to check how coked up it was, it wasn't to bad and I scraped the oily carbon deposit off of the head and the top of the piston with a wooden scraper. There were no scores in the bore or on the piston and only slight burn marks below the rings on the piston so I ordered a new set of rings just to be safe. The small end bearing and crank were also in good condition thankfully, once the rings arrive I will get it back together and fingers crossed it will run ok now.

Andy.

I put the bike back together last night and started it thinking my problem may be solved!!! No its still the same, could it be that the main jet and or the needle be worn in the area of 1/2 to full throttle? as the bike runs fine in every other area.

Andy.

Edited February 3, 201412 yr by starr870

It's possible. Just as an experiment try dropping the needle one or two grooves. See if it runs worse around the bad throttle setting but runs better where it's bad now. You might also try a new plug cap if it's breaking up at high RPM. As always reseat connectors, clean grounds and if you have the stock kill switch bin the damn thing. They are evil.

Dan,

thanks for the reply, I will see if I can pick up a new plug cap tomorrow and also try dropping the needle to see what happens.

I'm not to sure if I have the original kill switch mine has a square button and works fine.

Andy.

Edited February 4, 201412 yr by starr870

Well I've changed the HT lead, plug and cap and dropped the needle to its top groove and its still not right, the revs take along time to drop when you rev it up and its still smokes as you rev it hard.

What am I missing here it is really starting to do my head in now!

Andy

Ok no panic. If turning left don't work turn right. Try going the other way on the needle. Tuning is always about fine tweaks and sometimes regardless of experience it all comes down to trial and error. Good tuners are patient.

Edited February 8, 201412 yr by dan williams

I'm riding in a trial next sunday I may just leave the needle dropped to the lowest groove and see how it runs.

Do you think I risk damaging anything? will it be running lean?

Andy.

I wouldn't just set out to ride a whole event with new jetting without trying it out with at least warming up the engine to full temp and testing for a bit.

Then again I have done that.

Well I rode at bransgore today with the needle clip in the top groove, the airscrew 2 1/2 turns out and super unleaded mixed at 75:1 at first the bike felt better but was smoking a bit and someone at the event told me to try opening the air screw another 1/8 of a turn. All this seemed to do was make it smoke more and not rev so cleanly, I did check the plug straight after the first two laps and it was black and wet, any ideas?

Andy.

Maybe it's time to forget what we know and start over. I'm not trying to be thick as I know you noted some of this already but troubleshooting sometimes requires a ground up approach.

What is your current jetting?

Does the bike sound like it is pinging under load?

Are you using the stock heat range of plug?

How does the bike run at steady throttle at different throttle settings?

Has the ignition timing been changed from stock?

What are the condition of your reeds?

Good solid grounds?

Kill switch not sticking?

Dan,

Main jet is 150, pilot is 27.5

it doesn't ping under load

I have tried a 5es and a 6es plug and it's been the same with both.

The bike runs fine at different throttle positions until you try to open it past about 3/4 throttle.

As far as I know the timing is stock although I have never had the flywheel off.

The reeds are fine with no damage to the petals and they seal fine against the block.

I cleaned all the electrical connections 2 weeks ago.

The kill switch is working fine.

I spoke to a trusted trials shop today and described the symptoms that the bike is showing and have been told that is probably the crank seals that are leaking and pulling gear oil into the combustion chamber causing the excessive smoking and black oily plug.

Does this sound likely?

Andy.