71 Triumph Tiger project bike

elizilla

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My sweetie acquired a project bike last fall. It is a 1971 Triumph Tiger, mostly original, and it has 7000 miles on the odometer. It is 650cc, a twin, with one carb for both cylinders. He worked on it quite a bit this winter and finally got it running by replacing the carb with a brand new reproduction Amal unit. With the new carb, the correct new spark plugs, and fresh gas, it started on the first kick and ran smoothly.

Note that whoever had it before must have had the heads off at some point - they broke one of the cooling fins and a motor mount was missing. We do not know the history or what the previous owner might have been doing in there. But with the new carbs it seemed to run pretty well.

So last weekend we trailered the Triumph and the Ural to Tellico Plains. The first day, we were tired from the drive and the weather wasn't great so we just rode 50 miles or so on the Tellico Plain side of the mountains. The rain finally stopped around 4pm and after dinner my sweetie and one of our riding buddies went out for a short run. They made it to mile 12 on the TN side of the Skyway, and then the Triumph died. It had to come home to Michigan in a pickup truck.

Back home in the garage, inspection reveals that the right piston is holed. The left looks perfect.

Anyone know what might have caused this? Remember this bike has just one carb and the left piston looks fine. If it was a carb issue you'd think both cylinders would have it.
 

tawilke46

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That is puzzling Katherine. One carb, and I assume one split intake manifold. Maybe a spark plug running too hot on that side?....
 

Warren

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I agree with Sam that it was most likely that the timing was out of adjustment.
 
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My first thoughts are like Sam's, air leak on that cylinder or pre-ignition.
This bike has just one camshaft and cam drive doesn't it?

Brad
 
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just my free thinking thoughts....

What is a "holed" piston? I'm thinking overheated and blew a hole in it. Was it due to overheating and piston blew into crankcase? ....or was it mechanical damage from hitting valves?

My experience is with single carb, single set of points, single coil, single mechanical ignition timing advancement, 2 cylinder engine. If the Tiger has 2 set of points and /or 2 coils, it may be ignition timing on the one cylinder. It would be hard to have ignition timing troubles in one cylinder with single set of points, single coil, setup and not have ignition timing problems in the other cylinder. As mentioned, correct plugs also.

Possibly valve timing also.

Single carb sorta rules out too lean of fuel/air unless the one piston failed because "one of them had to be first" syndrome.....the 2nd piston may have not been far behind it.

Recheck valve and ignition timing components, carb, rebuild and try again.

Sorry about the bad luck.

...or it could have been ethanol, it is the cause of all engine failures. Ok, I admit I said that to stir things up.
 
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Sam, I used to have a '70 Bonneville. Only differences - smaller gas tank, 2 carbs, different manifold. The bike has two sets of points, fixed mounted to a plate. Plate is movable to adjust the timing of both cylinders at once. Dwell is independent.

Brad, bike is a pushrod twin. Unlike the BSA 650's (I had one of them also), it uses 2 cams, one for intake and one for exhaust. The cams are gear driven from the right side of the crank. Exhaust cam drives the ignition points.

Katherine, you two need to check everything, including degreeing the cams.

I could have been more help 40 years ago.
 
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elizilla

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Thanks for all the suggestions. They are helpful!

We probably won't be tearing into it this weekend. Steve's calendar is full pretty much all weekend, I'm not going in there alone, and Greg, our stray motorcycle mechanic, is fully occupied with a new-to-him CX500 project bike. So any more diagnostics will have to wait for another weekend.
 
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If there was a very lean condition, reading the spark plugs may show clues to that. The plug from the bad cylinder may also be damaged too much to read though.
Anyway I would give them a look see if you haven't already.

Brad
 
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