Misfire after sitting a few months-not fixed...

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it is not easy to spot by looking but with the covers off you should feel a difference in pressure to open a valve with a lever.

jamesp said:
Ok, I will look for that. Will it be easy to spot? Thanks.
 
it maybe unlikely but an easy check. also with it parked with a valve at full lift a possibility be it slim. it would not be the first one I have seen with a broken inner spring either.

Biscuit said:
Ran fine last season, so extremely unlikely a valve spring broke from just sitting.

at this point it is something to think about and is not that hard or time consuming to check

Biscuit said:
You've been jumping down enough rabbit holes chasing this problem, leave this one alone.
 
I pulled the valve covers anyway since I want to check the sdjustments. They are so far down that I could not see the inner springs with a decent flahlight. I've got the carbs back together, will get them back on by the weekend.
 
Rohan said:
Biscuit said:
The parts hanging from the carb bottom into the bowl area is the main jet (brass) which is screwed into the jet holder, (steel) and screwed into carb body from the bottom. A needle jet (brass) is screwed into the jet holder at it's threaded end before it enters the carb. Put a wrench on that steel jet holder and it will all become clear.

Aha, Good comment.
If the 'bits hanging down' haven't been apart yet and checked for cleanliness, then they need to be.
The needle and needle jet are what fuels the bike at medium revs,
and if they are gummed up or not clean, then this may be where your problems are.
The needle jet will wear as will the needle. So that can cause the bike to run rich.
 
jamesp said:
Thank you, Rohan, for replying. I will try the cable pulling.

Hate to give up, but thinking of turning it over to someone more experienced than me. So far,not in exact order, I have gone through the carbs (again), checked the timing several times, installed new Tri Spark with their coils and HT Wires, checked and cleaned grounds,checked/adjusted valves, checked compression, checked pet cocks, pulled on cables, changed plugs several times. Still misfiring. Thanks for all your help.
 
So it misfires on each cylinder individually if you try and rev it up by tweaking the throttle cables ??

One thing we haven't mentioned is try strobing the ignition timing, and checking that the timing actually advances with revs ?
If this hasn't been glossed over, or I missed this bit.....

Someone also suggested (broken ?) valve springs.
Although this happening to both cylinders seems a little unlikely...
 
I tried pulling on cables with tank on using needle nose pliers to grip each one. Each cylinder seemed to stutter at first, then rev ok. Not sure how high I revved it.
 
I've not read this thread thoroughly, but I had a similar 'ish' problem on my own bike many years ago. After a catastrophic RTA, which left my bike and the car that pulled out in front of me, as burnt out wrecks, I salvaged what I could and eventually rebuilt it. After all being fine for several months, it started getting an occasional misfire and this got worse and worse. It bugged me for several weeks, but I could not track down the problem. Eventually it died completely (on the way back from Norvil as it happens) and I had to be rescued by the RAC (roadside recovery for those who don't know). The rescue guy was a motorcycle specialist and he was determined to track down the fault (our bikes a very simple after all!!!). He persisted for a couple of hours, but eventually had to admit defeat and we loaded her into the back of his van.

After getting back home and kicking the dog, I tried kicking the bike and, of course, it started first kick. The bike was in the garage, it was getting dark outside and I had not put the garage light on. In the gloom, I thought I saw a spark from around the oil tank area and sure enough, there was the problem. Due to some inexpert wire routing when I fitted the new harness, a power lead was rubbing on the frame. Over weeks and months, the coating on both frame and cable had worn down to the metal.

Different factors, such as engine revs, gear selection, road surface and angles of lean, determined whether or not the cable was in a position to short out or not. This point of shorting was very difficult to see without knowing where to look and, of course, I went through a whole manual of possible reasons for the misfire before having the good fortune to start the bike in near darkness.

Sorry if this possibility has already been discounted, but the information may help someone else. Otherwise, I can only offer my sympathy as I know how totally pissed off you must be feeling right now.

Bob
 
Yes, it is a mk.2a with black box. In reference to the possibility of a short, yes it is possible. However, the misfire is consistant, and not random. No matter, sitting on stand, or cruising the neighborhood, it is predictable, consistant.

Is it possible to check valve spring pressure with engine mounted in frame? Thanks.
 
Someone has probably mentioned this, but how good is the earth from the battery to the engine, indeed all of the earth connections?

Sad story mate, I'll bet it turns out to be something stupid.

Dave
 
The air cleaner element is about two years old with not that many miles. I fashioned a seperate earth wire from under the tank to the engine block where it bolts to the connector to the transmission. I did it a few years ago when I replaced the original head steady. Other earth wires are also connected here. I took a tap and cleaned all the threads. I'm sure it is in the wiring or valve springs.
 
Alrighty perfect example of Commando being a thinking man's hobby, - so glad to hear ya narrowed it down to just wiring or valves. I solved a long interval of intermittent miss fires and mysterious decreased power intervals dead in middle of my pasture by 1st looking in tail light just for the shear hell of it & was just easier to check than repeating the prior failures. Of all the things I've lost I miss my mind the most.

Used voodoo neurology once to solve electric short blowing lights in chevy van I'd put a ziilion screws in frames and panelling over urethane spray covering all wiring - with a impeding trip - used muscle testing with gal friend to ask if front or back, indicated back, then R or L to get L then up or down, said down, then a brace with three screws t
get upper one, so pulled it out replaced fuse again and volia! Technically a version of radiothesia, ie: dowsing, by forked sticked, bent wire, pendulum swinging or plain ole ESP with inanimate objects. Better than coin toss on which cycle to buy or leave alone.
https://www.youtube.com/watch?v=-MM0Z_jCrGo
 
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