Suzuki 2 Strokes

You made a lot of changes all at once, which make its awfully tricking to jet those aftermarket carbs properly.
Making the exhaust & air filter mods with stock carbs is a lot easier because you are starting from proper jetting with the stock exhaust & air box+filter.

I have a ’72 GT380 with a ’74 motor. Has two changes: Higgspeed 3-into-3 chambers & K&N pod filters. Used new fine-wire plugs. The stock carbs got a good cleaning + the typical pilot&main jet increases. And the bike runs sweet!

Some forum members have successfully used aftermarket carbs. I don’t remember seeing a writeup on this forum detailing what it took to get them jetted correctly. Would be interesting & useful for some of those forum members to make a new topic an explain what it took.


Craighjr, your plugs may have been the culprit because they were crudded up via the process of getting you bike set up.
Bigger pod filters usually work better than smaller ones on these bikes. Which Uni pod filters were you using?

The bike has always ran so terribly, every part replaced was in a effort to get it running better. Which nothing ever did. I didn’t change everything at once. Crazy to see but I will work on the jetting seeing how now I know where a major hurdle in the bike being streetable was.

Glad you're making progress. Keep us posted.

Hi all,

Apologies for stepping in on another members post to post a similar problem that I am experiencing.

My GT380 running on chambers and K&N pod filters has developed the "fart, stuter & surge" syndrome as described by Craig380 since installing GT550 carbs although, the bike runs a lot better and smoother on this option.

I will try out tapeing up of the filters as described to get some feedback on whether they are running rich or lean.

Based on the outcome my query is: Would the pilots need altering to a smaller size if running too rich or is there an alternative way of increasing the airflow that I could try out first even though it may not be a permanent fix?
Should they be running too lean I assume larger pilots are the only fix - correct?

Apreciate any feedback.

Thanks
Carlos

adjusting pilot air screws might make a slight difference. But may not be sufficient if your jets are way out of range for your motor

I got some genuine Mikuni jets and dropped the main one size to 90. Now the bike runs like crap all the time with or without filters. Is there a chance that the dual layer foam pods just mess with the air flow so much they can’t be used and the 95 main jets are the best I’m going to get?

I am late, but a think this is a common problem/challenge. I absolutely agree with Zunspec4s reply. I have a GT380 and will explain what I think it is or is not

I want you guys to be happy and enjoy the little GT380 at least for a while for if it proves not to be for you or you sell for it is not your top 10 and you only have room to store 10 bikes.

When you get that bike to run properly it is awesome. Arguably, one of the best sounding two strokes with those Jemco' s. It's only 371 cc so not a torque monster . but can run so smooth, slow, fast or anything in between. The Jemco's don't take away too much of the broad power band per my butt dyno (which counts a lot street riding). They will give you a little more top end... you will occasionally find that easy to enjoy.

If this is the first time you try to restore a certain model bike, my advice is to rebuild it to stock specs first. I am very sure the bike runs very well if you repeat what Suzuki intended. If it does not ( that whole why did it get parked in the first place mystery that baffled everyone back when) you indeed have a real problem with some function and should become obvious... guys here have seen some weird stuff.

If you can't do that (stock) due to parts availability OK, but I don't think that should be an issue for GT380 if you are a little patient at most.

I have a 74 GT380 with those 3 into 3 Jemco's and stock pipes that I ran first. Jemco's was not a problem in general and I didn't need to do anything to adjust for Jemco's to run well... thousands of miles has proven that to me so far. Disclaimer: I am not saying you don't need to adjust jetting to ensure it is not on lean side for example. Always be wise when changing anything. I just don't thinks it's the pipes especially if new.

I would think its Air/Fuel path: Carbs, jetting, and lack of factory air intake boot, air box and filters. If you search any post about two strokes tuning for new carbs and/or just pods it takes much patients. Sometimes they change jet values so much, it is hard to believe they got everywhere correct. Street riding is very hard to tune due to wide usage needs and so easy to feel problems when not perfect. Even Just a bit off is enough to make the experience poor vs great. It ls just like a switch for 2 strokes: Love it - Hate it

So start stock, fresh brass bits, ensure it runs right, then if you want to run bigger carbs and pods you know for sure where the problem is (and can switch back too). This is usually the issue and it just won't run right everywhere in RPM range vs Throttle Position. vs Load You may never have a piece of mind street riding when modding the intake side, always thinking you need to tweak it bit more to be better (at least my mental state which alone makes riding not enjoyable as it can be).

If your stock carbs software brass metal bits were bad, there is a person (Ivan's performance) who claims to have jets kit for later GT380 model carbs. He spent much time tuning. I have not tried. Also. he solved the problem of OEM needle jets obsolete... I think jet needle are also obsolete?

Stock Points are easy to set up for timing perfectly, unlikely its the electronic unit, just saying. For example it is so easy to to change a bad condenser or points in case one is a fluke (always by OEM)... Sometimes, I wish I had extra known good solid units to debug modern (Ok technology been around for long time now) engines (Some cases I do now). Some of the aftermarket items are low volumes have know quality issues.


Good luck

So true, most of my issues are the stock parts were so trashed the replacement was done as an upgrade but nothing ever made the bike run right and I finally ended up so far from stock. I am local to Ivan’s and he doesn’t want to touch the bike if not stock. I have to try the stock air boot I have with the velocity stacks rotten out of it to see if it’s an improvement over the pod filters. The bike is rideable with 95 main jets and no air filters. I may just leave it there for the low mileage it will acquire it may be fine.

Have you investigated air leaks, I mean down to the last seal/gasket.

It made a huge difference, installing the all to often neglected ondulated metal shim on the exhaust, on my CR, for instance...

I even leak-proofed the silencer rivets, to tell you the level of paranoia I got from this bike, not acting the way it's supposed to.

Now if it does not start fourth kick, I have to take the plug off and reinstall, to release compression (and avoid the dreaded kickback); never a case before.

A two stroke gotta be tight.

Hope it helps

Cases are resealed with a rebuilt crank. Top end is fresh with new exhaust gaskets. I got velocity stacks with screens since the bike runs with no filters but they are doing something to the air flow that it will not run with them on.

Just a few questions so we can give the best advice;
What size pilot jets do you have?
What needles are you running with?
Are your carbs definitely clean as a whistle, even the bits you cannot see?
Do you have a fuel filter fitted?
Have you double checked all 3 float levels are spot on?
Have you balanced the carbs so that they reach maximum throttle at exactly the same time?
Almost certainly it will be harder to get it running right without a standard airbox fitted. An airbox smooths intake pulses out nicely, with pod filters or trumpets you don't get that.
If you go without airbox you might need larger main jets judging by the sound of your symptoms (it gets worse with 90s instead of 95s).
Unless it is simply impossible, try to get hold of an airbox and fit that. You will make your life easier and improve the usability of your bike.

Did you say that the performance was just as bad before you started to upgrade parts?

Carbs are brand new but may have become clogged after sitting for a while, but it sounds like it's something more fundamental like a crappy connection in the connectors or switches, but you fitted a PowerDynamo ignition which would replace points, and much of the wiring. Did that come with new coils or did you use the old coils? I read that you used new plug caps, but didn't mention the coils and I can't remember if the PD system comes with new coils.

I wonder if the kill switch has bad contacts and is allowing the relay to trip in an out.

On the air filters and velocity stacks, many of them partially cover some of the air passages and that upsets airflow and doesn't allow the carbs to work correctly.

Does it run fine without any air filter/stack or is it just better but still not right?

Thread from the dead....turns out the last oil pump I put on it was assembled 180 out (because the pump that came on it was 180 out). There was too much oil at idle and tapering off to nothing as the revs climbed. I figured this out after building a test bench out of the ruined case halves and running the pump with a drill. I am getting some detonation occasionally while riding, but I have put more miles on the bike the last week than the 4 years it had been built before that.

My questions now are;
Do these bikes ping a bit in 90 degree weather?
Do I avoid steady throttle cruising? The bike seems to be happier in lower gears revving and coasting
Where do people see the power band with chambers and filter pods on these 380’s?

It shouldn't detonate / ping at all. Back off the ignition timing a little, as mentioned earlier in this thread, to about 2.2mm BTDC or even 2mm.

Power should come in at 5,000 with chambers and pods. On a steady throttle, my own 380 was happiest either above 5,000 or below 4,500. In the 4,500 to 5,000 range the chambers & carb jetting I had caused odd resonances and the bike would 'hunt' a little on constant throttle. However it accelerated through that range fine. I could never be bothered pissing around with the jetting to try and fit it, as it was only a minor hassle.

Great information in this thread as I am starting on my first GT380 (since 1972 anyhow) largely from scratch build.