Ideas

My '74 850 has been running pretty good this year, no real issues, no modifications. A couple weeks ago, I went out to go for a ride and it didn't want to start. It had been sitting for a week, but it has always started in 2 or 3 kicks in that situation. This time it tried to light but wouldn't catch. After about 10 kicks it finally caught, but wasn't running smoothly and wouldn't idle. I pulled the plugs and they were black and sooty - rich. I changed the plugs and everything was back to normal. I have a single 34mm mikuni. My starting procedure is to turn the gas on, choke on, kick through twice, maybe 3 times, and then ignition on and away it goes. I knew it was on the rich side since I've had it. I've dickered with the air screw multiple times, with no noticeable results. Last night I dropped the needle one notch and set the air screw at 1 1/4 turns out and went for quick ride to the bank and gas station. It seemed to have a very very subtle stumble in 1st year, at the point where I had just enough speed to pick my feet up. Seemed to have a little more snap in the mid range. Other than that, everything was fine until I came to a redlight on the way back. I stopped and it idled fine while I waited for the light to change, the light turned green and as I gave a little throttle to pull away, it stalled flat out. Started right up with one kick and I went the last 1/4 mile to my house. Pulled the plugs and they looked much better; tan instead of black. I played with the air screw some more and then noticed that the left side exhaust was throwing a very small amount of smoke. I shut it down and did a compression test, engine still hot, and got 120 psi on both cylinders. I can't remember what the jetting is on the mik and I didn't have enough time to take it apart last night to see, so I apologize for not having that info available. Any ideas why it stalled? Would it have anything to do with dropping the needle one notch? Initially I thought it was my clutch plates sticking, they need to be cleaned, but it didn't happen at any of the other stop signs/lights during this ride. I hope to pull the carb tonight and get a list of what jets and needle I have in there. Just wanted to see if it could be anything else.

Thanks!
Ben

Could Be Just Spring Carb Cleaning time.

Or your float level. I've never had luck getting Mikunis tuned right, and when you do get them right they dont seem to stay set right. Also check your vent lines for blockage.

I did a quick once over on the carb before starting it the first time this year. Suppose I should go through it a little more thorougly. Probably time for a new inline fuel filter too.
I'll check the float level when I have it apart. I set it once about 2 years ago and it's been good since them. Maybe it needs an adjustment now.

Thanks,
Ben

Tell us what ignition system and how old the battery is for the other side of the factoids to consider shooting in the dark long distance. I've had this happen after diddling stuff under tank and knocked a coil wire loosey goosey. Both me and my buddy have had perfect results a long time maintenance free with our 34 Miki carbs.

Best cold starring procedure I've found on Miki was to just flip the enricher lever on and w/o any throttle or prior kick through step on it and most the time fires right up w/o any extra engine turning w/o lube wear on cam/lifters. Amals get a tickle on 1st cold kick off then whole rest of day with an hour or so between starts, just leave throttle closed and step on it to begin runing at dead steady idle.

May have to go through a hand full of plugs figuring this out as I've had the bad experience of new plugs getting fouled while trying stuff so once the fault actually fixed the plugs kept fooling me afterwards that I was a flat Norton owner failure.

Battery Idea !

Boyer ignition. Battery is 4 years old. Barely took anything to bump it back up on the battery tender this spring. It's a magna power or something like that. Sealed battery. I'll check the voltage tonight. I've never gotten any kickback while starting, so I figured the battery was ok.
Keep the ideas coming.

Ben

I did not see your first text posted in ALL CAPS, that you have already replaced the Boyer trigger wires. So forget any and everything else till you have posted in ALL CAPS that the Boyer trigger wires have been upgraded. if already upgraded then still may be suspect but way way less so than Boyer supplied trigger wires.

Check all carb fixing fasteners with air leaks in mind and exhaust nuts still sealing too.

I have found with Mikunis either single or twin that they are not the same as Amals when it comes to the correct setting for the idle mixture screw. I find that on the Amals the idle mixture screw usually works best when it is set for the highest idle speed or maybe a bit on the rich side. On the Amal the idle mixture screw is really mainly for idling.

With Mikuni VMs I have found that the idle mixture screw effects part throttle operation quite a bit especially when the engine is running slow. This is true with single or dual Mikunis.

What I do is set it first the way you would with Amals for a nice idle when warm. Then I go out for a test ride. If opening the throttle to much with the engine turning slow causes a stumble then try adjusting the idle mixture screw a bit richer (in) When you have it set so that the low speed acceleration is nice, then you will find that the idle is a bit slow and you will then have to speed it up a bit by lifting the slides.

Once you do this though the engine will run better around town and you will be able to lug it more if you want. I have found on the two Nortons I have tuned with single Mikunis that both of them ran best with the needle down one notch from the center. I don't really remember what needles and needle jets were in them though.

It may be that if I had a smaller throttle slide cutaway then things would act differently. I don't know. I think I usually have about a 2.5 cutaway.

The other thing that helps with starting is making sure you have a small enough starting jet in the float bowl. Some of the carbs I use of snowmobiles have a big really rich starting jet that doesn't seem to work. Unfortunately this jet is not replaceable. What I did was drill out the jet and install a small brass jet with a 3mm thread. I got these jets because we discard them at work. They come in pressure gauges and they are drilled with a really small hole (about .010 I think but I haven't accurately measured them) I experimented with different sized starting jets and found that .021" worked well. I think a little bit smaller might be even better, but that is the smallest drill I have. Some of the Mikunis have starting jets .040 or .050 in size. They are way to big.

The starting jet is very small and is near the bottom of the float bowl. If you park your bike for a long while without draining the carb and tank it may plug up, preventing the staring circuit from working at all.

It would be interesting to hear other peoples experiences with these same problems

I think you should try turning the air screw in a bit. That should get rid of the stumble.

My Seeley 850 uisually starts at first bump. A while back I tried to start it after it had been standing for 12 months. It coughed once then stayed dead as a maggott. I had spark at the leads and clear carburettors with plenty of fuel. Seems the plugs had decided to conduct between the electrodes. I changed them and the motor started straight away.
If you lower the needles in your carbs when it is tuned correctly, the motor should cough when you are using the throttle and changing gear for corners, (if the motor is hot). If you don't get response when you are altering the air or petrol screws look for a blockage in the carb bodies.
I use methanol fuel and when I park the bike in my shed, I always remove the jet covers so the float bowls are drained and open to the atmosphere, I put the caps on a shelf until the next time I use the bike . Modern fuels contain ethanol, perhaps you should do likewise ?

Thanks for the replies everyone.
Tricatcent - You have most closely described my thoughts.
I'll clean the carb, get a new fuel filter, check the battery, get new plugs and give the air screw another try.
Hobot - I'll check into the boyer leads and make sure all is well, or fix if not.

Thanks again!

WAIT ! We're just getting warmed up. Corroded fuse holder contacts.

Don't forget to check the blinker fluid and muffler bearings :roll:

bwolfie said:

Don't forget to check the blinker fluid and muffler bearings :roll:

Click to expand...

Mufflers have no bearing on this! Muffle yourself.

WAIT ! We're just getting warmed up. Corroded fuse holder contacts.

Click to expand...

Oh ugh have I been caught out on side of the road on both my Commandos over the years d/t a perfect fine looking glass fuse was bad at its hidden terminals, only diagnosed by, test light or substituting a .22 LR all real Arkasnsasinas carry with their lighter and pocket knives or cig foil, also very common Arkansas item expect for them nasty guys that chew and spit, ugh, or other conductive crap on side of the road or tool kit. When I get aroundtoit I'll convert to blade type even though no Norton part number for that, neither was a under spine brace tube or brake safety spring or modern tires.

I Mean the insides of the fuse when nothing wrong with the fuse holder itself.

To me there is not many hobbys that can test your whittle and stick to it man hood like what it takes to get an almost mythological "fully fettered Commando"

Believe it or not its taken me about 3 yrs with various show stoppers on Trixie to get fully fettered show stopped enough I don't feel like I'm risking my patients hour+ drive to get to me but I'm not there because of some Commando mystery, smoking what I got wondering how it will turn out this time.

[video]http://www.youtube.com/watch?v=1i-Hc5fqdqQ[/video]

I check the blinker fluid and exhaust bearings every time I ride. The last thing I would want is to run low on blinker fluid while on a ride.
Heading out now to fiddle some more. Then I'll take a ride to the autoparts store to get some new plugs and fuel filter.
I'll report back.

Ben