Gutless GT250A

[Rimano1964]

Hi everyone, does anyone have an idea what compression readings I should get with a GT250A 1976, also any other suggestions to the symptoms below are welcome.
I knew the bike had stood years when I bought it, work done so far, new points and timing set, new battery, and air filter, carbs stripped and cleaned in a home ultrasonic cleaner, float height set and fuel tap cleaned, new plugs and fuel. The bike starts easily and idle's steadily but doesn't rev well under load, it will reach 70 eventually but slows badly into a head wind.
The plugs on both sides look a light brown colour with no sign of burning crankcase oil.
I've ordered some caustic soda to clean the exhaust inners even though they look quite clean.
Any other suggestions welcome, thanks in advance.

[Craig380]

Hi and welcome.

the compression test should be done with engine hot (like 2 minutes after a ride), and throttle held wide open. Kick hard and quickly 6 or 7 times in a row, then read the gauge. You're looking for at least 120psi in each cylinder, with no more than 10% difference between cylinders. Ideally it should be 125 - 130psi.

A bike will run with 100psi but not very well.

What did you set the timing to? Did you use a dial gauge? The marks on the flywheel are not a good or accurate reference for timing, Suzuki revised the timing figures several times but never changed the flywheel marks, which is why you really need to use a dial gauge to set the timing properly.

Caustic soda alone will not remove the decades of baked-on crap in exhausts. What I used to do with my GT380 was to get an old clutch cable, cut the nipple off one end and fray the strands out so it looked a bit like a small chimney-sweep's brush. Stick the 'brush' end into the pipe (it's best to drop the silencers and go in from the header end), and fix the nipple at the other end in the chuck of your electric drill.

Then hold the cable outer, switch the drill on and move the cable in & out so the frayed ends scrape away at the inside of the pipe. A couple of minutes each pipe, then tip all the crap out. You'd be surprised how much carbon comes out.

Also make sure the baffles are clean. Good luck, the performance is there, you just need to find it

[Rimano1964]

Thanks for the reply regarding compression values, I'm picking up a tester this afternoon.
Regarding timing I set this using the marks provided by Suzuki, I don't have a dial gauge but confirmed the marks are correct by using my digital vernier down the plug hole onto the top of the piston so I'm happy the ignition is set correctly.
Regarding exhausts mine are 1 piece but I'll take on board the cleaning method and the baffles have both previously been cleaned.

[Craig380]

Regarding exhausts mine are 1 piece but I'll take on board the cleaning method and the baffles have both previously been cleaned.

The GT380 outer cylinder pipes are also one piece, and what I found is that the seam where the header is welded to the body of the silencer acts as a magnet for carbon & crud build-up, it becomes like a restricting washer in the pipe, reducing the internal diameter. That's why it's good to go in at the header end.

If you don't have an old cable to hand, those long springy "drain snakes" that are used for clearing blockages in sinks are very effective when fixed into a drill chuck too. Pound shops often have them.

[sportston]

Caustic soda alone will not remove the decades of baked-on crap in exhausts.
Also make sure the baffles are clean. Good luck, the performance is there, you just need to find it

+1 on that. I have had caustic soda method fail on me before on an old GP100. I had to buy a new Gianelli exhaust. Completely transformed the bike!