Does it chime in on both cylinders if you close the relevent air lever? If so,it must be fuel and not ignition.
ET: Engine (Twin) Engine Intermittent Cutting Out of One Cylinder
- Thread starter Rob H
- Start date
I saw one once that would not start because of a tight Valve guide, We are only talking of a few thou, If the valve does not close the seat, We have seen pushrods go tight in the tappet cup or follower, Strange things can happen on these old Bikes.
Worth making sure the tappets are few thou loose, Just to test it, And making sure the pushrods turn free.
Cheers Bill.
Worth making sure the tappets are few thou loose, Just to test it, And making sure the pushrods turn free.
Cheers Bill.
My old Comet, After a winter layup, If it back fires on first start, I lose all compression I take the spark plug out, Bit of WD40 down the hole, No more trouble for the rest of the summer, Not every year, But it has done it often, I have not had it to bits for 15+ years, STRANGE !. Cheers Bill.
I've even had cases where the inspection caps stop the valve closing fully. The ones over the valve spring/retainer...........This was using the gaskets that Neal sells which are thinner than the stock paper type. I don't use them, as I find they melt with the heat and they also split if people over tighten them which is pretty common.
Had an afternoon with the bike so thought i would give a quick update. Had the carbs stripped and cleaned all good, no blockages used a little toothpaste to seat the needles, no leaking of petrol. Also check the valve rocker clearance and made sure had the slightest play. Could not figure out which cylinder was the problem so tried to start the motor with only one plug in at a time. front cylinder fired up straight away ticking over like a well tuned comet! Then went to the rear cylinder which was very reluctant to start and only once run for a short while. It was also noted that I could stand on the kickstart with the plug in the front cylinder but the rear did not seem to have much resistance. I will get a good compression tester and test next week. What psi should I be looking for?
In summary i think I still have a problem with the rear cylinder exhaust valve, I am guessing the repair was not successful and the valve is not seating properly.
Unfortunately I have a feeling I will need to be looking for a new head.... anyone know of a good second hand early type rear thats available?
In summary i think I still have a problem with the rear cylinder exhaust valve, I am guessing the repair was not successful and the valve is not seating properly.
Unfortunately I have a feeling I will need to be looking for a new head.... anyone know of a good second hand early type rear thats available?
Don't rush off and buy a head, If I remember where the rocker was , Was very dry ?, Does it look like there is enough oil getting there now ?, Maybe the piston is in trouble, Did you have the barrel off when you did the valve job ?, Seems strange that you are happy with the running of it most of the time.
Cheers Bill.
Cheers Bill.
Seems to me quite obvious, at least one of three components in that cylinder does not seal well. Get a spark plug thread adapter and pressurize that cylinder with air with both valves closed. Then listen to hissing in the carb mouth, no hissing: Inlet valve OK. Listen at the silencer end: hissing means exhaust valve not tight. Or if both valves OK, listen for hissing from the engine case, at one valve inspection cap or so: Hissing means piston rings not sealing well.
Now you can dig deeper: If one or both valves cause hissing, off with the head and test valve sealing with bearing blue on the faces. That may show a valve that has a runout or else the seat ring is overdue for machining the seat face , depending on the blue marks. If both valves allright, then there is a severe problem with piston rings or cylinder bore, seizure and scoring. Anyway , a decent maintaining of pressure is first action, all else is secondary.
Vic
Now you can dig deeper: If one or both valves cause hissing, off with the head and test valve sealing with bearing blue on the faces. That may show a valve that has a runout or else the seat ring is overdue for machining the seat face , depending on the blue marks. If both valves allright, then there is a severe problem with piston rings or cylinder bore, seizure and scoring. Anyway , a decent maintaining of pressure is first action, all else is secondary.
Vic
The bit Vic doesn’t mention is the sealing between head and liner flange. Presumably the compressed air method will make a cylinder leak here just as obvious.