Gt750 Rattle from engine

[burgs]

Hi
My GT750 L engine has around 12000 miles on it since a complete rebuild, for a while now I have been hearing a rattle/knock that I put down to the fuel tank, but since replacing the Bromleck? 3 into one exhaust with a 3 into 2 system of my design, the knock has become much louder than I thought due to bike running a lot, a real lot quieter.

At idle there are no undue sounds, revving it up with no load doesn't produce any undue noise, but under load noisy, my guess at this stage little ends, big ends, or pistons but not convinced it is anyone of them.

Anyone experienced similar?
It would be a couple of years ago that I first noticed the noise but it was always there, but now I can hear it yuk maybe put the noise exhaust back on

I have another L that needs new bottom end (seals) so looks like I will need to finish it off!

The new exhaust system made a hell of a difference to the bad hunting I have been experiencing so was a second good result.

download/file.php?mode=view&id=27116
download/file.php?mode=view&id=27115
Any advice greatly accepted.
Regards
Burgs

[burgs]

Hi
I guess my next move is to send my spare crank for rebuild, from there I will remove and rebuild my second GT750 L to standard specification as about the only thing that is not stock is the rear brake light switch.
After this is done I will strip my original GT750 L to see what is making the noise, I will remove the head then cylinder block and move on from there.

Bets Regards
Burgs

[mizzytheman]

Hi Burgs,

Is the noise coming from a particular part of the engine? Is it louder on one side of the engine to the other?

[teazer]

Deep knocking sounds are often big ends or a loose clutch nut.

I would try a couple of things. Pull the clutch cover off and check it's all tight in there.

If that's OK. remove the head and slowly rotate the motor over tdc and then push down on top of thr piston to feel for free play. If the piston can be "clicked" down, a big end is shot. Repeat on the other two cylinders.

A lighter tapping sound could be rings hitting a port top from insufficient chamfer around the ports.

But first, try the old screwdriver to the ear trick (poor man's stethoscope) and see if you can determine where the noise emanates from.

[burgs]

Hi
Good advice.
Haven't used the screw driver trick for a while now, time to bring it out.
Never thought of the clutch, appears to me to be something in the conrod area though.
When riding the the noise appears to be directly below fuel tank, that is why I thought it was the fuel tank rattling against the frame but could not find any evidence of same.
Will try screw driver and then lift the head.

Regards
Burgs

[burgs]

Hi
Tried screw driver test and there is noticeably more noise in cylinder one, slightly less in two and three sounds about normal, two or three I would think would be noisiest in a good engine considering the primary drive is between both so my thoughts are number one is the culprit.
I have boxed up the spare crank and bottom end rebuild kit and am sending it off to C&C Motorcycle Engineering in Thomastown Vic Australia tomorrow, he said he will start on it as soon as he receives it, so a good chance of having it back before the rallies start up in the New Year when things are a bit cooler, already 40C here today and it is spring?

Best Regards
Burgs

[burgs]

Well head is off and still none the wiser?
Cannot feel any play in the piston/bearings/big ends, bores look fine, number 3 piston and head looks like it has a bit of excess oil, not much but definitely wet compared to 1 and 2.
I will keep playing might check the ignition timing now the head is off and see where it is set at, although that might be a bit like clutching at straws so to speak.

Certainly nothing to say yet that it is about to fall to pieces?

The original crank was shot when I first got the bike going but it didn't make any undue sounds, just would not stop when backing off for corners, very scary, the mains, seals and big ends all had bad wear in them, yet it sounded ok, so this has me beat.
Burgs

[teazer]

If there is no perceptible play in any of the rods, and the noise is from the left side, I would remove the left side cover and wee if the starter clutch or gear are loose.

[Alan H]

If there is no perceptible play in any of the rods, and the noise is from the left side, I would remove the left side cover and wee if the starter clutch or gear are loose.

Wee?

[burgs]

Hi
Looking at the cylinder head #1 is light tan #2 is a bit darker, and #3 is black something is happening there?

Noise is only there while under heavy acceleration!

Burgs