850 w/Mikunis running problem

First of all, just wanted to thank everybody for this forum and for their help. I've got my first Norton running and would not have gotten everything together properly without this forum. Thanks again!

After diiagnosing a loose spark lead, I got my 1974 Commando 850 to start right up and settle at idle. New plugs in the bike, set the valves and cleaned the carbsan which are dual Mikunis (appear to be VM 34s).

The bike started, idled and accelerated well, but after 1/2 mile, I heard a few pops from the left pipe, then a loss of power and it died. Once dead, it would not restart no matter what. Fuel flowed from the petcocks, and I'm sure their was gas in the bowls but it simply would not fire. Fortunately, the garage was downhill!

I may be imagining this, but it almost seemed easier to kick after it died. I would kick and kick, but not so much as a pop would emerge from the pipes. The bike has great compression cold and started right up, but will not take me very far.

Has anybody encountered anything like this before? This is the secobd time it has happened, and I would love to be able to ride my bike further than a half mile! Thanks again.

Nomad — you don't say if you checked for a spark immediately after it died. This would be the first thing to do, if you have not already done so. Mostly, ignition faults are the main reasons for no go. If it starts from cold but only runs for 1/2 mile, it could be a faulty coil. What ignition system do you have? You could also check for loose connections in the wiring, and especially loose earths. If you have a Boyer, the pick-up wires inside the points cover can fracture inside the insulation, so check the connectivity with an ohm meter. Once you are satisfied that the ignition system checks out, then it's down to fuel delivery to the carbs. Did you drain the float bowls to check if there was petrol in them after it sat down on you? Is your petrol cap properly vented? Sorry if this sounds obvious, but all of us have been caught out on occasions by simple things.

Let us know what you discover.

I pulled off the plug leads this evening and both basically fell out of the coils. I little bending and cleaning followed, and they fit snugly in the coils. Two kicks later (with the starter plungers depressed) and the bike roared to life before settling at a comfortable idle of about 900.

One mile later and the left side cut out. The bike died and I tapped the carb, pinched the fuel line, implored the gods of British motorcycles, and the bike came back to life. It did this four or five more times, popping and backfiring before it eventually sputtered. By the time I got home, the bike was limping on one cylinder and I was limping on one leg.

Each time it died, I was able to revive it by depressing the left - but not the right - started plunger. It has got to be the carb, but is there too much or too little fuel? I think too little, but am not sure why. Could the float valve be sticking?

Too exhausted to check the plugs tonight. Will do so tomorrow and check back in. Any suggestions are greatly appreciated, but I think I defintely need to tune these carbs.

Nomad, do you have a fuel pipe from each petrol tap? Do you have a double banjo on each carb with a short section of pipe connecting the two carbs? If you don't, and I guess this is a long shot, it might be causing an air lock. How about the petrol tap itself? You could take the pipe off and see how well the fuel flows from the tap, and for how long. If it stops flowing after a while, lift the tank cap. If that doesn't make a difference, and assuming you have enough fuel, then it is the tap itself. In which case, remove it and blow through it. If that checks out, is the fuel pipe itself kinked? Do you have a fuel filter in each fuel line? If you do, remove it and see how it behaves.

If all of the above check out, then take off the float bowl and check the float needle and seat. I have come across a sticking float needle only once before, but not on an Amal. The float chamber is vented, so check that the vent is not blocked (i.e. if you invert the carb, fuel should flow out).

Keep us posted.

I have had the same issue with the dual Miks on my cafe racer. Pull the carbs and thoroughly clean them. Pay special attention to the posts that the floats slide on. Evaporating fuel will make them very sticky and they tend to hang up and not open fully or close fully. This can cause the float bowl to empty and starve the engine after a short run, or if it sticks open, will cause it to puke fuel out the bowl vent. I'm not sure why Miks seem to be especially prone to this, but mine are.
The long term solution is to ride it more! Otherwise, drain the bowls if it will be sitting for any length of time.

I would go for some sort of fuel blockage - perhaps a plugged vent. The tickler doubles as the fuel bowl vent. if the rolled pin holding the cap in place is plugged up, fuel will not flow freely into the bowl, causing a starvation. If the bike is running lean, it also runs hot, which can lead to vapor lock in the fuel lines.

Check that fuel flows freely from both taps. If you take the line off each tap, fuel should come out in a nice steady stream, about 1/4" diameter. If it doesn't, check the filter screens for blockage. The common replacement tap uses a rubber gasket between the petcock and housing, which often swells or doesn't align with the outlet, resulting in very poor flow.

There is also a nylon filter screen inside the banjo fittings in the fuel bowls. check that they are clear, too.

The fuel lines that came with my bike were in an 'H' formation, with inline Tees forming a crossover in front of the carbs. The new fuel lines I installed have double banjo fittings on the fuel bowls, putting the crossover between the carbs, about three inches further away from the head. I think its a better arrangement, as it keeps the fuel as far away from the heat of the engine as possible. I noticed the fuel would boil out of the crossover in the former arrangement if the engine was hot, making hot restarts problematic.

My mistake! Of course, your carbs are Mikunis! I should have read your post more closely.

Dave

If you have a Boyer or similar so that your coils are series and fire together you can do a quick ignition check by swapping plug leads. you might need to borrow a car lead or two to test that.

If it has been sitting I'd also suspect in order gas cap vent, (yes, sometime only one side will cut out with limted flow), crap in the tank, the petcock or filter, vents, and plugged carb passages. I have also heard the sticky float story elsewhere but have not seen it myself.

Here in CA the crap that gas turns into seem to just ignores conventional carb cleaner these day, both the spray on and my soak can. I've had some plugged passages that were a nightmare.

Regardless I've had great luck cleaning carbs and petcocks by boiling them in water, I just cleaned three petcocks and a couple of screens that way. Get a burner and pot for outdoors as this won't go over well in the kitchen, it smells like an old gas tank when boiling.

George has told me about haveing some very strange problems with a spider in a Mikuni vent hose so I always look there.

Good luck!

Cookie said:

Regardless I've had great luck cleaning carbs and petcocks by boiling them in water, I just cleaned three petcocks and a couple of screens that way. Get a burner and pot for outdoors as this won't go over well in the kitchen, it smells like an old gas tank when boiling.

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I've heard about the same trick but using 50/50 mixture of water and simple green. I have yet to try it but I just scored a big Coleman dual burner camping stove to test it outdoors. Now I just have find a ratty old pot to use. I can't imaging the girl friend would be very pumped on me using the stainless steel Lagostina stock pot as a carb cleaning tool.

I have currently retired my Coleman propane stove and now have a single burner electric from a thrift store. For a buck my wife got me a teflon pan with a handle and a lid that does not quite fit.
The electric burner is much more convenient to put just outside the garage door to merrily boil the pot.
I couldn't believe I was able to save a petcock that had sat in fuel until it evaporated. The screen was clogged solid up to two inches. It take boiling them in a few changes of water for a good result, I've not tried adding a cleaner but I will if the results are good.

I just bought (on e-bay) a small ultrasonic cleaner, it also heats up the solution. I have tried it on a couple of items with water and soap ( I use Hertel available locally which by itself cleans grease very well) and the parts came out very clean and free of any grease. Aluminum parts come out shiny too. Can't wait to try it on a gunked up carb.


Jean

I just bought (on e-bay) a small ultrasonic cleaner, it also heats up the solution. I have tried it on a couple of items with water and soap ( I use Hertel available locally which by itself cleans grease very well) and the parts came out very clean and free of any grease. Aluminum parts come out shiny too. Can't wait to try it on a gunked up carb.

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Perfect, Jean! I bought one like that to clean my scuba regulators when I service them. I have since used it for a couple sets of Amals. A little dish soap (or Simple Green) takes an amazing amount of crap out of the carbs.

I love the idea of those ultrasonic cleaners but I've yet to try one. Heard nothing but good about hem.

Not sure what size VMs I have on the bike, though I think they are 34s. Is there a stamp on the carb anywhere, or do I have to measure the engine-side inlet with a ruler?

I think I have a spare pair of 34 mm Mikunis home, I'll look for a stamp after work.

Last night I pulled the fuel lines off the carbs and noticed that they were barely dribbling gas even with both taps wide open.

The bike sat in a garage for almost a year with the taps off, but I suppose it is possible that whatever gas was left in the taps has turned into goo and now restricts flow through the nozzles. The tank itself is very clean, so I do not suspect any debris. Hopefully the problem will be obvious when I get the taps off.

Will keep you posted, and will post pics when I've identified the problem.

I just cleaned up a couple of forty year old taps that had sat with a quarter tank in them to evaporate. After a few times of boiling even the screens cleaned up. I did finally take a drill to clean out the passages through the tap, that stuff sticks again like superglue if you don't get it out quickly.
You are familiar with the old boil them trick aren't you?

I went back one page and saw we already had this discussion. Senility allows you to enjoy new stuff every day.

Senility or no, the advice is greatly appreciated. I drained the tank and removed the taps tonight. Tomorrow I will go after them with carb cleaner and, if need be, a coffee can full of boiling water.

I noticed that one of the tap filters fell off in the tank. Any way to repair or replace them?

The Norton is so close to being ready. I can't wait to finally take it out!

Pics to follow.

Mikuni 36

i have Mikuni 36mm diameter on my cafe racer,PW3 camshaft and opened chanels in cylinder head and i cannot take lot of power out that engine.
Does it one carburettor enough that engine or should be two?.

Does it one carburettor enough that engine or should be two?

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MrNorton if you are looking for power forget the single carbs.
Try for instance the Keihin FCR 35mm flatslide set (available by www.coloradonortonworks.com and www.hollandnortonworks.com) you'll be amazed!