Thruxton won't start. - Triumph Forum: Triumph Rat Motorcycle Forums
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post #1 of 19 (permalink) Old 09-18-2019, 03:57 PM Thread Starter
Minitwins
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Thruxton won't start.

2004 Thruxton, 69000 miles. I was riding yesterday running fine, and it just quit. I thought I might have heard/felt a pop, but that could have just been the sudden loss of engine running and the compression. The engine would turn over with the starter, but would not fire.

I got it home, tried an igniter that I know is good, still nothing. Checked the pick-up coil per the manual, it is ok. Checked the coil, per the manual it might be bad, so I swooped it out with one from my 2008 Bonneville. Still nothing, the starter will turn over the engine, but not fire. Just for the heck of it I put the coil from the 2004 in the 2008 and the 2008 runs.

Gas flows from the tank fine. There is gas in the float bowls as when I open the drains I get plenty coming out.

Turning the engine over, I feel compression from the exhaust by putting my hand over it.

With air filters off, I feel a lot of back pressure from the right carb with my hand over it, none from the left carb, this is with the engine turning over with the starter. Also too, no pressure out of the left spark plug hole, some from the right one, turning the engine over with the starter, this by just holding a finger over the plug holes.

Besides not good, any thoughts? I have my suspicions, but I am looking for other opinions before I go from here. Thanks for any help.
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post #2 of 19 (permalink) Old 09-19-2019, 02:44 AM
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Also too, no pressure out of the left spark plug hole, some from the right one, turning the engine over with the starter, this by just holding a finger over the plug holes.

Do that test again but with the throttle wide open. It should blow your finger right off the plug hole.


You want unimpeded air to enter the combustion chamber for compression tests, the only thing is that the test is usually performed with the engine at working temperature so your overall figure might be a little lower than normal.

Compression should be 150 - 180 psi, anything less than 120psi needs looking at. It should really be measured with a suitable compression tester gauge though.


If you still get no pressure it indicates either broken piston rings, a holed piston, valve clearances too tight holding the exhaust valve off its seat, or a stuck valve.
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post #3 of 19 (permalink) Old 09-19-2019, 07:26 AM Thread Starter
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It seems as though it simply is not drawing in air/fuel into the engine. From what I am seeing it appears as though the valves are not working/working properly, i.e. exhaust valves closed on one cylinder and open on the other, same for the intake. I have never heard of the cam chains breaking on these, but anything is possible, I suppose. At any rate I will give your suggestion a try. I have to wait for another free day. Thank you for your reply.
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post #4 of 19 (permalink) Old 09-19-2019, 02:01 PM
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It seems as though it simply is not drawing in air/fuel into the engine. From what I am seeing it appears as though the valves are not working/working properly, i.e. exhaust valves closed on one cylinder and open on the other, same for the intake. I have never heard of the cam chains breaking on these, but anything is possible, I suppose. At any rate I will give your suggestion a try. I have to wait for another free day. Thank you for your reply.
If the camchain breaks it would seize the engine solid.
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post #5 of 19 (permalink) Old 09-21-2019, 01:05 PM Thread Starter
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dead

So, pretty well dead. With the valve cover off, and spark plugs out, turning the rear wheel. No pressure from left cylinder, both pistons rise and fall, however, absolutely no movement of the cam chain.
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post #6 of 19 (permalink) Old 09-21-2019, 02:53 PM
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So, pretty well dead. With the valve cover off, and spark plugs out, turning the rear wheel. No pressure from left cylinder, both pistons rise and fall, however, absolutely no movement of the cam chain.

That is intriguing. It's certainly the first time I've heard of a camchain breaking and I've been here for about 10 years...there's also the possibility of the crankshaft cam sprocket stripping its teeth, also unheard of, so far.
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post #7 of 19 (permalink) Old 09-21-2019, 04:02 PM
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That is intriguing. It's certainly the first time I've heard of a camchain breaking and I've been here for about 10 years...there's also the possibility of the crankshaft cam sprocket stripping its teeth, also unheard of, so far.

Don't forget Mr F, that the cam chain only goes half way up to an idler shaft. The cams are gear driven from there, so the problem may be with the idler gear further down. I have no idea how its held to the shaft but if the drive pin or whatever has sheared, the cam chain would work as normal but there would be no movement of the camshafts.

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post #8 of 19 (permalink) Old 09-21-2019, 05:08 PM Thread Starter
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To be more specific, I can lift the cam chain a very small amount off the sprocket in the head. Looking down into the tunnel it looks as though the cam chain tensioner is still functioning as I can see that it is pushing against the chain, i.e. a bow in the chain. When I lift up on the cam chain as I turn the back wheel, I can feel it making contact with the sprocket on the crank. Also to, all the intake followers (lifters) are at their lowest points, none making contact with the cam lobes.

Sorry if the first post wasn't more specific, I'm kinda bummed. I have had this bike since it was new, and we have some history. Yes, I know, it is just a machine.
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post #9 of 19 (permalink) Old 09-21-2019, 07:08 PM
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You will need to shine a bright light down into the camchain tunnel and try to spot where it goes around the crankshaft.
You are looking for "sprocket" teeth on the crankshaft - they should actually look more like gear teeth.
Could be very hard to see without splitting the cases or at least getting the cams etc out the way.

RichB in AZ
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post #10 of 19 (permalink) Old 09-21-2019, 07:36 PM Thread Starter
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Yep, already tried that, can't see well enough.



Anybody know any good engine rebuilders, I am in Ohio.
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