Gt750 Rattle from engine

[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.

Wee?

I will blame auto correct. Obviously it's a pisser.

[teazer]

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

Look carefully at the head and piston crown for signs of detonation. Check the end of the spark plug too for shiny specs of aluminium.

All should be black with unleaded fuel. Leaded gas leaves a light gray color and oil adds color but with unleaded. its hard to see. But all 3 should be similar. What that shows is that the left ran hottest, and right was coolest. The reasons could be timing too advanced on the left or running lean or maybe all 3 are lean/advanced but the right has an oil feed leak. Remove the carbs and before you clean then, remove the float bowls and spray wd40 through the air and fuel jets. Do one circuit and then repeat on the other two carbs to compare flow. Repeat on the next circuit (jet) and if one or more are partially blocked it should be fairly obvious. Don't forget the starter jet in the float bowl. http://pinkpossum.com/GT750/carb/BS40carbs.htm

If you have a dial gauge, you can bolt it to the top of the barrels and check the ignition timing. All 3 should be the same. The workshop manual allows for the fact that the outer plugs are at an angle so the distance is different. With a dial gauge on top of the piston, all 3 should be identical.

[mizzytheman]

If using a dial gauge to set up the ignition timing, the timing on the L+R cylinders should be 3.64 BTDC and 3.42 for the centre cylinder. The reason for the difference is because when the dial gauge is inclined it has to travel further to obtain the same measurement as the centre cylinder. Effectively all cylinders fire at 3.42 BTDC but due to the inclination of the L+R cylinders it appears that these cylinders fire 0.22 later than the centre cylinder.

Or just use a strobe light which is close enough on this type of engine which is in a low state of tune.

[PaulD738]

If he’s got the cylinder head off Martin he can just use the dti as normal

[mizzytheman]

If he’s got the cylinder head off Martin he can just use the dti as normal

Ah yes, just read Teazer’s post again and can see what he’s saying. I was thinking with head on

[Alan H]

I was thinking

ROFL.gif (22.98 KiB) Viewed 451 times

[burgs]

Hi
no problem with the Dial indicators I have a few, but with the head off and being able to fit the Dial indicators to read square to the piston/bore therefore I will set up as per no. 2 cylinder.
I have Barry's video on setting the carby's up and have a a good set of carby's on hand so will do, the original ones are worn out badly around choke rod and levers, also the spare set has got the nipples/ports for vacuum testing, they came off a later L.

Engine gasket sets turned up today, wife works this weekend so all looking good .

I have a couple of customers who have a thing about thinking, every time they say it, I say that's going to add to the bill, big time

Burgs

[burgs]

Well I need to make a steel plate up to mount the magnetic base to, not enough cylinder liner to hang onto , looks like a quick foreign order for tomorrow at work .
Got a couple of rough readings off one and three, one is close to 2.51mm and three is 3.94mm, I checked so I have a reference to check back at, points as were, timing as is.
Had a good look at the the plugs and they reflect the pistons and combustion chambers, especially # one, I expect to find that # two is running retarded as well.

I will check again, take note and recheck the points gap, to make sure they are within spec, and then time from there, head is ready to refit, so should be able to test run over the week end.

Burgs

[Alan H]

When you're doing the timing with a dial gauge, it might be interesting to note where the timing marks are as the points open to see how accurate they are.
I always use the marks on all my bikes and they run ok, but could they run better?...........

[teazer]

Good point (pardon the pun). That way, if the propeller is close enough, it can be used for future checks. But better to dump points and fit an electronic ignition that stays where it is set. Accent is probably the simplest and most straightforward and very well made.