Points, Plugs, Coil, Carby - What Is The Problem?

Muddy · Post by **Muddy** » Sat Apr 23, 2016 4:18 am

G'day Guys

It has been a while since I posted here, but I have been lurking in the background. I've got a problem that has just about beaten me and I'd really love some help if you can assist please.

I have a pretty good condition '64 C72 that I've rebuilt and has been running just fine up until recently. Since the rebuild some time ago, I've been riding around the local streets with the bike running very well and pulling great for a little 250. The old girl has pretty open “Overlander” mufflers and a conical pod air filter - things to make the engine run lean. Admittedly the bike did sit around for a while - maybe 6 months, without being started while I worked on another project.

When I went to start the old girl a little while ago - she wouldn't go. There seemed to be number of issues:

- no fuel getting thru the rebuilt petcock - had to put a drill bit thru the swollen gaskets

- no spark on one cylinder - replaced points, condenser, plugs, coil, leads, plug caps (5k ohm) – good spark on both cylinders now

- carby looked “dirty” possibly from old fuel – tried to clean it out with carb cleaner and compressed air – no luck, so had the carb ultrasonic cleaned

The carb is a Mikuni VM22H – the one that is supposedly difficult to get parts for!

Anyway, got the new points, condenser, plugs, coil, leads & 5k ohm plug caps in yesterday. I went to start it and it fired up pretty quickly but ran really rough with lots of black smoke out the exhaust - sounds and looks like a 2 stroke. The motor would not rev well and stops when the throttle is let close after being open. The choke seemed to make no difference to running unless it was fully closed, where the engine would stop.

There seems to be no real difference in running between idle and throttle well open - all runs rough and lots of black smoke - spark is advancing properly, but engine is running really rough.

Even after running for a short time the plugs were very black and sooty. I changed the plugs, but had no real change – they became black and sooty quickly. It all looks to be running very rich to me.

I appreciate that the general consensus is to use non-resistive plug caps and I have a set of these coming and have just used the 5k ohm resistance caps until they arrive. However I would not have thought this was the source of the problems I’m seeing?

I’ve been focusing on the carby because everything else is new and appears to be OK. Timing is spot on, battery is good, tappets are set fine, spark is good. So what is wrong?

I’ve set the pilot screw out 1 – 1.125 turns out as per the “tuning guide”, but have found a little info that suggests that the Mikuni carb pilot screw should be 1,5 – 1.75 turns out – I’ll try this tomorrow.

I take it that screwing the pilot screw out adds more air and makes the idle leaner – is this correct?

Any comments, assistance or help most welcome.

Regards

Muddy

DianneB · Post by **DianneB** » Sat Apr 23, 2016 6:34 am

Black, sooty plugs is DEFINITELY a running-rich issue, as are some of the other symptoms you describe, so that points to the carbs.

Did you do a carb overhaul at the time of cleaning? New gaskets, jets, clean all the little passages, new float valve, new intake manifold gaskets?

Muddy · Post by **Muddy** » Sat Apr 23, 2016 3:58 pm

Hello DianneB

No new parts for the carby - seems like gaskets, o-rings jets etc are almost impossible to get for the Mikuni carb. I've even tried going direct to Mikuni - the newer jets while looking the same, appear to have a different pitch thread.

Float height is set to 26 mm which I believe is correct for this carb and has not changed since stripping for the clean.

I'll try opening the idle screw further today and see what happens.

Thanks.

Regards

Muddy

Muddy · Post by **Muddy** » Sun Apr 24, 2016 1:28 am

OK, so here is what I found.

I managed to uncover a Mikuni VM carburettor tuning manual for newer VM carbs, but it helped a lot.

Seems I had the air screw not turned out near far enough resulting in a very rich mixture. I was working on around 1 - 1.25 turns as per the Keihin carbs, but that was nowhere near enough. Ended up with the air screw turned out 2.5 turns and it is running like a "Dream". To quote another poster here, "It is running before the start button has come back out".

Boy am I happy and relieved.

Muddy · Post by **Muddy** » Sun May 28, 2017 5:39 am

Just to close out this epic...

It has been over a year and I've continued to have problems with continual dry black sooty plugs. Even after a couple of minutes in the shed, the plugs were black and heavily coated with carbon. I've had the carb in and out a myriad of times, adjusting the float level lower and lower, but without success. Compounding this, there is not much info around on the Mikuni VM carb.

Because it looks to be running rich, I've dropped the float level in the bowl several times and also lowered the needle into the jet. I've had the float sitting in only a couple of millimetres of fuel in the bowl, but still the plugs carbon up. I’ve taken off the after-market, pod-style air filter, hoping that it may be undersized or restrictive, causing the problem.

I was misled by false hope at one stage, with the float very low in the bowl and a quick belt around the street for 15 minutes looking very promising. Pulled the plugs and all looked very good - nice and tan. However, next weekend when I started and ran it again, back came the sooty plugs.

I checked the floats for leaks, but they seemed good. I checked the needle (float valve) and seat and again, they looked fine. I checked the needle and seat by blowing into the fuel line, sure enough the needle (float valve) seated very well and shut off the flow.

I checked with loudmouse who was a great help and walked me thru ensuring the valve and ignition timing were correct. In loudmouse’s words, the carb is often blamed when really the problem is with timing. But still, black sooty plugs.

In the end, I bit the bullet and ordered another Mikuni carb from eBay in the USA, convinced the carb was the problem. The carb was stripped and cleaned when it arrived. I reassembled the carb, checked the float height to 26 mm, believing this to be correct.

Well, what do you know, the old girl seems to run fine. I’ve been around the local streets for a couple of 10 mile runs with no problems. The plugs look nice and tan.

So what have I missed with the original carb? The needle looks fine; the needle jet looks fine; float looks fine, float valve and seat look good; air filter is removed – why is it running rich?

I do plan to try and compare the 2 carbs and check jet sizes to try and establish what is wrong.

Any suggestions to what I may have missed would be greatly appreciated.

border0_3 · Post by **border0_3** » Sun May 28, 2017 5:40 pm

Nice looking bike Muddy.

Sometimes carbies are just plain worn out.

All the best. Richard

Muddy · Post by **Muddy** » Mon May 29, 2017 5:12 am

Thanks Richard. Yes I suppose the carb could be worn out, but the bike doesn't indicate a lot of miles. The speedo only had 3000+ miles on it - admittedly the speedo cable was broken. Valves, pistons and bore all look very good. Clutch plates appear to have next to no wear and it still has the original tyres with lots of tread.

All this makes me think the carb should be better than it is.