GT380 problem

[ecto11]

Hello,
I'm new of the forum, I live in Italy.
Together with my father, I'm a lover of old japanese two strokers, so we are organizing a small collection of them, starting from Suzuki (but we also have a pair ok Kawasaki 500 and a Yamaha 350).

I'm experiencing some issues with my GT380 of 1975 (about 10'000 km) that we recently bought.

The fact is that the left cylinder always starts with difficulty, and sometimes it misses, and its muffler is a bit oily.
We replaced its contacts and small condenser with as a result some better run of left cylinder but problem still present.

Spark plugs are brand new B7ES, gasoline tank is clean (we use our standard 95 unleaded) and it has the tankerite treatment inside.
Carburettors should have been cleaned last year, but I'm going do it again asap.

But the biggest issue is that we cannot rev past 5'000 rpm at full throttle, seems caused by difficulty in main jets to deliver fuel, in fact if I rotate the gas knob gentle I can rev better. Seems to be lean in that situation. These are my opinions but I'm not at all an expert.

Last thing, spark plugs reading shows right cylinder with almost white central ceramic (maybe a bit lean), and central and left completely black.

I don't know what to do.
Anyone has ideas?
Thanks in advance

[sportston]

Snesible process of elimination would suggest; swap the plug cap with the centre cylinder. If the centre then has a misfire and the left works fine you need a new plug cap.
However....I doubt your problem is spark. It sounds like clogged jet in the left carb. I know it is a pain in the bum, but it is probably worth stripping and ultrasonic cleaning the carbs. When you rebuild them, fit an inline fuel filter to prevent future problems and it is worth replacing the rubber pipework.

[GTandcbr]

Hi and first of all welcome to the forum !
Firstly if your right spark plug electrode is covered in white deposits this is a big problem you must deal with straight away. If you run this motor hard you risk burning a hole in the piston crown . There is likely a problem with the air/fuel mixture(too much air) now there are a few things to cause this but here are the usual ones. Airscrew too far out. Cracked or hard carb to cylinder rubber not sealing correctly. Leaking head or base cylinder gasket. You can test the carb rubber by running the engine at idle and spray starter fluid and watch to see if the engine revs rise.
One more thing. It is better to use unleaded petrol at the highest octane in fact as close to 100 as you can find because that is what these engines ran on when they were made. Try to avoid ethanol (E10)
Here is a copy of the carb manual to help
http://www.3cyl.com/mraxl/gt/manuals/su ... /cover.htm" onclick="window.open(this.href);return false;

[ecto11]

Your suggestions are very precious, so thanks a lot!
I'll do some checks and we will see.

[ecto11]

Update:
we noticed that the three little pieces of rubber on the top of each carburetor (where the metal stick of the valve lifter enters in the carburetor) are very damaged, so they allow a lot of unwanted air to enter.
Causing lean carburetion for sure.
I'm waiting to replace them and clean all.

[GTandcbr]

Yes this is a very common fault. It will definatey improve the performance of your engine