T90 Problems

[brooksy]

Hi,

My father in law has just bought a T90 (68) which doesn't run right
The engine runs on tick over but pops, bangs and misses when revved up.
I have fitted a Boyer as the old electrics looked suspect but to no avail.

I have checked the carb and all jets are correct and no obvious air leaks

I have also just reset the valve timing but the inlet valves are noisy when you turn the engine over - they seem to shut violently with a lot of   noise.                                                                                          
Has anyone any suggestions please before I strip the engine and start from basics.

Thanks

Brooksy

[Rex]

I have also just reset the valve timing

What does this mean? You've checked it and it was OK, or you've changed it in some way? Don't know what this bit means either-

 they seem to shut violently with a lot of   noise.          

Before stripping anything I'd recommend using a manual to ensure everything is set as it should be, valves, ignition and carb. Something's amiss, but I don't think you'll find it without going through the basics first.                                                                                

[brooksy]

Sorry, I knew what I was trying to say.

I reset the valve timing to the marks as I have heard this the only real way of doing it. The manual says they only line up every 94 revolutions so I just removed the intermediate pinion and lined it all up again.

The noise is when you wind the the crank forward and the inlet valve opens and then shuts with a bang as if the the cam profile is very sharp. My t140 doesn't make a noise like this and opens and closes smoothly. This makes a metallic sound as if something is hitting the valve.

Thanks for your help
Brooksy

[L.A.B.]

Just a thought, could the tappets have partially seized in the tappet blocks?

As I believe it is possible for the tappets to seize if the engine has not been run for some time, the tappets/blocks become dry, so are more-or-less running dry when the engine is eventually started once again, so can rapidly overheat and start to seize before any oil begins to drain down to them from the valve gear?

As I said, just a thought. 

[brooksy]

Hi,

Thanks the bike has been standing. The Father in Law has had the rocker box off this afternoon and can't see anything wrong, but we haven't thought of this. I am wondering whether a valve is sticking open and then snapping shut. I guess the only thing to do is to take the top off.

The original problem was banging an popping when acclerating -what do you think?

Thanks
Brooksy

[Rex]

I wouldn't think there's any problems with valves sticking; it would tend to stay either open or closed rather than slide to closed after a set period.

I recommend you refit and check the points and ATD etc  and check the timing, capacitor (or for people born in the 1920s "condensor") , then check the various jets and drillings in the carb and then for for air leaks.

You sure you've got a good fully-charged battery too?

[brooksy]

Thanks for all the suggestions - I'll try them out and let you know of the developments

Cheers.

[brooksy]

Well, having spent a hot day in the Father-in-Law's garage progress has been made. I removed the rocker boxes and watched the pushrods move. The inlet and exhaust rods on the same pot were rising and falling together. The last job I did before was to set up the engine timing on the marks which resulted in the metallic banging noise. During the week the Father-in-Law removed the head and found the inlet valves damaged - they have been hitting the exhaust valves. I established that the timing pinion on the inlet camshaft was put on in the wrong keyway. There are three keyways on the pinion. Before when it was banging and popping the settings must have been a guess. All that needs doing now is to put it back together and fire her up.
The real lesson to be learnt here is to thoroughly check the bike over before you buy it. I don't think my Father-in-Law is ready to hear this at the moment after the grief he has had - espescially from the Mother-in-Law.

Thanks to everyone who offered advice.
Brooksy

[Rex]

[The real lesson to be learnt here is to thoroughly check the bike over before you buy it.
Maybe the real lesson is not to trust anyone else's work, and check everything first?

Nice bike, the T90, very under-rated. Best of luck with it.