Shake rattle and Roll

[teazer]

This GT750 Suzuki is getting to be a PIA. After fixing all the leaks from new components, I have it running really nicely except below 2000 rpms. At idle it shakes much like an old British twin except that it's a side to side shake and the front shakes not the back.

It has new pistons in fresh bores, new crank and seals, the same 145psi compression across all three cylinders. The plugs and wires and caps are all new. Carbs have been rebuilt and then stripped twice to see if I missed something. Ignition is a new Accent electronic from Germany. Carbs are synched and are spot on.

All I can think that I have not checked is the exhaust. The motor is rubber mounted and so (in theory) is the exhaust. I am starting to wonder if it's mounted too rigidly at the rear end and that acts like a pivot around which the motor vibrates and it magnifies it at the front end. Time to pull the rear exhaust mounts off and fire it up with a flexible temporary mount and see if that smooths it out.

 

[clem]

Look at this on eBay http://www.ebay.com/itm/222295685534
I used these on the rear of my ds7.

Sent from my SAMSUNG-SM-G920A using DO THE TON mobile app

 

[teazer]

Thanks Clem,

First I had to see what difference it made to free up the pipe and it made a huge difference. Clearly that was part of the problem. The frame has a rubber bush but the Jemco pipe bracket was touching the frame. I use those on our CB160s and have a couple on hand somewhere.

Next up, I stripped the carbs again and on closer examination I see that one of teh new main jets is not correctly machined, but a bigger issue was a slight ding to the surface of the float bowl right beside the pilot jet and I suspect that was leaking air and upsetting things. I have a spare and I "surface ground" all three bowls, so we'll see if that improves idle.

 

[xb33bsa]

the 3 throw 120 crank has a rocking couple so you are better to let it float

 

[teazer]

Exactly. When I built it, the exhaust bracket didn't line up with the frame mount, so I made a small connecting piece but didn't notice that the original bracket was now just touching the inside of the frame, so that instead of floating, it was pivoting. I removed that bracket and side to side shake was much better but still worse than any of the GT750s I built and ran before.

I have now rebuilt the carbs again and have fixed that tiny internal air leak and replaced all three main jets just to be sure. Next I'll slip the back on and see how they look on vacuum gauges. Normally those shake too much with each pulse, so I'll have to fit suitable restrictors (main jets) to damp things down. If that doesn't bring the shake under control, I'll have to resort to a programmable ignition and see if one or other cylinders need more or less advance than the others. I doubt the ignition is the problem but the step changes in indicated revs on the electronic tacho could be pointing to something.

Race bikes are much easier, no one cares if they don't idle smoothly, but oon this one it has to be right.