Hole

[pin]

Hi
A friend of mine has a gt 750 which has put a hole in the centre pot 3 times now I just can’t work out why,
First time he had not had the bike long so just put that down to bad luck,
So rebuilt it with a rebore new pistons ultrasound of the carbs and put an electric ignition on and slowly run it in but after about 400 miles it put a hole in the centre pot again,
So new piston Ultrasounded carbs again and put a new jet kit in checked the timing all looked ok it again run in slowly and has about 200 mile more,
Went out for a ride today doing about 60 mph it put a hole in the centre pot again,
Has anyone had this happen to them and if so what was it
Thanks
Andy

[widget]

Wifes bike did two middle pistons. Went through everything you would normally check after the first one but it did it again. Went through every thing again. double checked the double checked items, nothing obvious. Replaced the lectronic ignition with points (Cheers Baz) Gave the bike to Phil Baldwin to look over but he couldn't find anything obvious. Air filter was a bit Poo so he replaced that. He rebuilt the motor, checked all the carbs, timing etc. changed the plugs. Bike runs a treat now.

The only thing that wasn't stock were the spark plugs. I have run iridium plugs on all my bikes for years. Search on the net reveals there are dodgy copies about, though i buy mine from a local motor factors so would like to think the ones on our bikes are genuine, but looking at the differences on the markings on the plugs they could be dud ones.
NGK were not overly helpful when i spoke to them so didn't bother to send them back for checking. Just got B8ES in them now.

Both times the pistons holed, the bike was being ridden 'enthusiastically'.

It would have been nice to have a eureka moment and find something definitive but nothing that you could put your finger on. Sorry cant be more helpful.

[teazer]

Are you sure that debris from the first failure didn't damage the crank seal? I would do a leakdown test to be sure there are no leaks. It is also possible that a prior owner may have damaged the barrel faces allowing air to leak.

If the seals are all AOK, then put a timing disk on the alternator and check the ignition timing with a timing light on each of the three cylinders to confirm that the timing is correct.

You could also try a plug chop but that requires traveling at somewhat above the speed limit before you chop power and that might upset the local law enforcement revenue raisers so be careful. If you have the stock exhaust where the center pipe is split into teh two small mufflers, check that they are not partially blocked which might cause exhaust and cylinder temperatures to rise.

[pin]

Hi
I will pass on your thoughts but I think he’s thinking of selling it which will be a shame it’s a bike he’s always wanted to have,

[canaletto5]

Something is causing it to overheat. Has he checked the temperatures at the exhaust headers to see if there are differences between the barrels? Holing is generally due to lean running so may be worth seeing if the needle heights can be adjusted to a richer setting. The guys have all talked about air leaks as a possible cause. You don't say what heat range the plugs are.Standard is B8, if they are 7 or below it is potentially asking for trouble. Fit a cooler standard plug (not iridium) If there are B8s fitted, fit a B9. Kev

[pin]

I think now that he has slept on it he going to get it repaired,
It had B8 plugs in and I was there when it put a hole in the piston , it was smoking a fair bit and I remember think my one after the first start up runs clear just a bit of smoke ,
Maybe to much oil caused a weak mixture but he said that checked the oil pump setting and it was ok

[750ynot]

Have a look at this chart, it will help you diagnose your problem.

https://www.thetuningworks.co.uk/assets ... gnosis.pdf

[Kettletimes3]

You say he fitted a new jet kit,Was this OEM or aftermarket.
He may want to look into this middle carb.