Suzuki 2 Strokes

Hi I've restored a 1977 gt380 but have a really annoying issue with it .
It starts on 3 no problem but after about 5 mins it dies on the left cylinder.
I have done the following to try and resolve it.
Rebore pistons and rings
New plugs
New condensers
Checked the timing
Had the carbs ultrasonically cleaned
New coil
Carb is full of fuel and sparking ok on that cylinder, can anyone spare some wisdom n. This it's driving me to distraction

Hello and welcome.
First of all, we like to know a bit about you - like which bit of the UK are you in and what's your bike history, especially your repair knowledge/capability.
Only then will we suggest that you've possibly got either a slightly stuck choke, slightly high carb fuel level(s) or slight air leak between carb and cylinder.
Last is easiest to find - simply spray carb cleaner around the suspect area with the engine running and if tbe engine picks up speed, there's the air leak. Favourite issue here is old perished rubbers between carb and cylinder.

Hi and thanks for the reply.
I am in Warrington Cheshire , I'm now 56 and have had bikes since I was 14.
I suppose having messed around with bikes for years I have an idea but no expert.
Will try your suggestions but I think my concern was crank seals, although I have replaced the outer seals

If the outers were u/s, it very likely the rest are dud, unfortunately. Fi gers crossed on tbat one. I've soaked cranks in situ with diesel on 3 x GT550s and managed to save seals and bearings, but if you've been running it, bets are off. I've a mate in Warrington that recently sold his Kettle that may be able to help. PM me your details and I'll pass them on.

Will it run on 3 cylinders at all after the left pot has died? After the engine is warmed up, will the left pot chime in above a certain rpm, or does it remain dead?

If the left outer crank seal is failing or has failed, it usually makes the bike hard to start from cold, but it shouldn't stop it running altogether when the engine has some warmth in it.

Have you replaced the plug caps as well? For what it's worth, I don't trust the resistor caps so I fitted non-resistor ones.

Have you a compression tester to check the compression in the cylinders?

No once it dies on the left cylinder it won't kick in again until it's cooled down.
I have replaced the outer seals when the engine was stripped for blasting.
I haven't replaced the caps but there is a good spark still when that cylinder is off.
Tried spraying carb cleaner in earlier and it seemed to come back in but the carbs have been cleaned ?
Compression is good

Where did you spray the carb cleaner, just around the inlet rubbers? Or down the carb throat? Either way, if the left pot chimes in when you do that, it sounds like a fuelling problem.

Is the float bowl vent tube clear and not kinked / trapped? That's the tube from the bottom of the floatbowl that pushes onto the brass spigot. It isn't just an overflow: if that tube is blocked, then there's no atmospheric pressure getting into the float chamber, and that will starve the cylinder of fuel after a minute or two. Best way to check is just to pull the plastic tube off completely, and squirt some carb cleaner up the brass spigot to try and clear it (not with the engine running).

Definately sounds like a stuck choke piston to me. Starts when cold then floods the cylinder killing the spark. Starts again when cold. The bottom of the choke piston has a rubber seal if that is damaged choke won't switch off

I had a set of GT750 carbs last year that looked OK but one cylinder was cooler than teh others so off came the carbs again and parts all looked OK. Then I used a Vacuum tester and found that the cool cylinder choke did not actually seal well, so I replaced it.

The rubber tip does get hard as it ages - unlike guys - so now I always check that teh choke plunger does in fact seal off teh fuel passage. Get a MityVac or similar tool and test all 3 crabs.

tz375 wrote:I had a set of GT750 carbs last year that looked OK but one cylinder was cooler than teh others so off came the carbs again and parts all looked OK. Then I used a Vacuum tester and found that the cool cylinder choke did not actually seal well, so I replaced it.

The rubber tip does get hard as it ages - unlike guys - so now I always check that teh choke plunger does in fact seal off teh fuel passage. Get a MityVac or similar tool and test all 3 crabs.

Interesting. How exactly do you use the vacuum tool to check the carb chokes?

Good advice all stuff I hadn't thought of .
There aren't any pipes on the overflows will that matter ?

The choke hole is 'f' close the choke piston and apply the vacuum to 'f' it should not allow air through.

Multimts12 wrote:Good advice all stuff I hadn't thought of .
There aren't any pipes on the overflows will that matter ?

It doesn't matter for getting the bike running. The tubes are only there to dump any overflowing fuel on the road, rather than onto the tp of a hot engine, where it could be a fire hazard

Just make sure the passage from the external brass nipple into the float chamber is clear of obstruction.

Hi, I’m the guy in Warrington AlanH was referring to
If you pm me your details I’ll get in touch and see if I can help you.