Triumph T120 R splutter

[step2534]

 So Im going up a slight gradient doing 65MPH on my recently re built 1971 T120R. Its pulling like a train.
 Suddenly loss of power and it feels like left cylinder not firing. Quick thinking Petrol starvation so turn on left tap. Bike splutters then picks up and to me runs normal.
 My lad is following and told me that what looked like white smoke came out of bike near my left leg(I reckon that was airbox area) ( Was it smoke or vapourised petrol)
 From then on its blowing a little white smoke from exhaust when under accelaration, OK on over run.
 Still starts first kick and ticks over fine with just a slight hint of white smoke from left silencer. I would say the left plug is slightly more discoloured than the right one. Carbs re conned 300 miles ago as was head, new Morgo barrel ,pistons , tappet followers virtually everything from cams up has been renewed or re conned.Its electronic ignition. I dont want to really start stripping down . Any thoughts, Carbs ?? muck in carbs ??, fuel starvation. Leave it and let it develop? Thanks in anticipation,   

[raptorkev]

was it your first run after rebuild?
have you done a compresion test?

[Revband]

It does sound as though you had a partial seizure on the left piston, I would be inclined to lift the barrel and check all is ok.

Fresh rebuild, 65 up hill, perhaps just pushing it a bit too far?.

[triman65]

Did you rebuild the engine yourself??. If so did you try the gap between the cylinder wall and piston, as this should be between 4 and 5 thou clearance. Some rebore firms can make the gap to small and result is seized pistons. This happened to me and I had to take it back again for correct clearance. Like you all the problems seem to be with the L/hand cylinder.  I would recommend at least 1100 miles before taking the speed above 50MPH. Most of my problems sorted them selves out after plenty of use.

all the best
Mick 

[rogerwilko]

I used to get white smoke and sparks out of my exhaust on a GT Ducati on one exhaust when it started to starve of petrol, until i switched to reserve.I wouldn't panic yet.

[step2534]

Thanks for your thoughts. The bike has a Morgo 750 conversion which is new. The gap was 5 thou. It was running great Id done about 250 miles on it before this happened. Im changing the petrol pipes for them ethanol proof ones and going to check carb first cos it did feel more like fuel starvation. What Ive been doing is slowly building revs up then shutting throttle back to about 2000 rpm I havent let it labour or over revved it I think it might have been doing about 4000 rpm when this happened,it still starts first kick and ticks over OK.  Theres always something but that owd bikes eh??

[step2534]

Revband, You were right Partial Seizure on left cylinder, Bu--er it, more expense. You always wish for the best but hey ho thats life,we ll just start again.

[Revband]

Hi Step
Obviously I cant see how bad the seizure was, but unless the piston has been badly damaged, Just clean out the bore with a flap wheel in an electric drill, smooth the piston with fine wet or dry paper clean both well and fit new rings.

[rogerwilko]

A partial seizure wont cause the cylinder to cut out, just slow you down maybe?? The plug will still be firing.

[step2534]

 Just a quick update and something that might be off interest if it happens to anyone else.
 Both pistons had melted on the inside just above the gudgeon pins. The barrels were virtuall run in. No body had seen anything like this there was a deposit of aluminium on the bores at the left and right hand side where the pushrod tubes are.

 Now I accept that the centre area runs hotter cos there are no fins there but to get this hot is quite extreme. Took it back to Morgo and they re placed everything, that excellent service (but did say Id run it in too hard)

Now this is the interesting bit Talking to a knowlageble engineer this evening he said he had come across this before, BLOWHOLES in casting between cylinders

 Makes sense ,gets hot ,air expands , cylinder tightens up on piston this causes extreme local heat, rings melt and so does side of piston. Apparently you can test for blowholes with Ultra violet, it shows a black spot in cylinder wall.
 Guess what mine are off for testing before they get put on this time. .

[Revband]

Hi Step
Just a thought, the T120 is very sensitive to spark plug type and even one grade to hot can cause overheating, it might be a good idea to fit cooler plugs and possibly set it a little on the rich side to run it in.