< 1/4 throttle running odd? (while maintaining speed)

[zk0ot]

So. While riding the bike (gt380) I noticed while maintaining any speed the bike isn't making a smooth exhaust sound. It becomes poppy like its 4 stroking ? Its fine as long as I'm accelerating. Idles fine. Iridium plugs.

[CBWELLS]

I'm no expert & I'm not familiar with the term "4-stroking". Never heard that phrase before, but what you describe sounds perfectly normal for a two-stroke bike. My experience it that they like to have a load! When you're in that "dead-zone" between engine loaded & engine holding back, they tend to pop & hunt for "something to do"... either pull the bike along or slow it down. I usually just tolerate it, or shift to another gear. My original (then new) 550 did it way back in '74 & my present 550 behaves the same way. The 380 is probably affected even more by its comparatively higher mass/engine displacement, reflected by the 6-speed transmission. If it idles well & accelerates well, there's probably no problem, but ya might check the air filter to be sure its not filthy. Good Luck to ya.

[tz375]

Sounds to me like typical two stroke behavior. I have no personal experience with a GT380 so I cannot say for sure if your is typical or not, but two strokes don't like light throttle running as a rule.

You are correct that it feels like 4 stroking and in your case may be worse that it should be if the filters or mufflers are clogged. On light throttle it will tend to surge and sputter and feel uncomfortable. On the 750 the factory recommended cure was a smaller slow speed air jet to make it richer only under certain conditions.

I would suggest cleaning out the carbs and exhaust baffles and fitting a new filter and then try adjusting the idle air screws say 1/4 turn each way and see if that helps or hurts it.

[GT750Battleship]

Hi,what's the idle like ? a high idle rpm on the 750's is quite often reflected in the bikes hunting/surging problems on a trailing throttle ? Spent a lot of time getting mine to idle happily at 1,000rpm,this got rid of the "hunting/surging" problem,without resorting to "anti-surging" jets.
Cheers,
GT750Battleship.

[Vintageman]

My 74 gt380 runs perfect at any throttle position. And, at any throttle position you can sweep the full rpm range smoothly (understand this point? important a great street riding experience IMO)
Since your bike runs fine once you give more a little more throttle (if what you mean by accelerate), what my experience has shown me is there is something not up to par with carburetor lower speed circuits. If 1/4 throttle probably not pilot jet although it plays a role. Most like Needle jet is worn or corrode, Even a little make a huge effect. Remove it and look at it carefully with bright light and magnifying instrument. Is the out let still true round and it shinning smooth inside where the Jet needle pass through. If not fuel will not atomize well with air. Of course this assumes all else is well, float level, right jets, carbs are reasonably synced etc.
You did say Popping, but don't think you mean it goes Bang! out of mufflers where the ignition system cuts out and then back in causing the buildup of fuel to ignite. Also assume timing is set OK. ALos pipes an dfilter cleen as those who responded before me

[zk0ot]

The bike idles perfect. And revs up awesome in nuetral. Also drops down fine as well.
The bike is running chambers and uni filter.
I cleaned the carbs. and rode to work this morning. Noticed the bike started doing this as it warmed up to full temp. Its not a bang.... Its an in between of letting off throttle and with throttle.....more of a poppy sloppy sound
One of the carbs has a gnarled emulsion tube. That is creating more turbulence in a carb.... Maybe a rebuild kit is in order?

[Suzukidave]

You should be able to get the emulsion tube's separately ?

[zk0ot]

ivans has them with their rebuild kits...

[Vintageman]

Emulsion tube do you mean Needle Jet?

Not sure what year your bike is Early or Late GT380, but OEM does not offer Needle Jet so don't think you will be able to find it new from Mikuni (Sudco or Jets R US for example).

Ivan offers the Needle Jet?

I thought he provided a custom tapered Jet Needle, not the Needle Jet. Hmmm

[zk0ot]

yeah sorry needle jets. its a 72. (early)

his are slightly modified along with his needles. the hoods of his are half the size he said.

[T350guy]

His H2 kits work with 72-73 as the 74/75 h2's had oil pumping to the bowl thus changing the mixture so I just used the jets.
I also have them in my 72 H1B and had to go up from a 30 pilot to a 35..works great. Planning to get a kit for my 73 S2A
His Suzuki kits would certainly be worth it

[Vintageman]

That is impressive (offereing needle jets too that is. Particularly if he had them made from scratch).


T350Guy.

Did know that about later H2's putting injection oil into carb bowl for a sort of premix. Anyone else do this. So you have H1's H2's and S2. Dang! I wish I had at least one Kawa triple…. Even if an S1. Or what about even an A7. So hard to find now. This summer there was a 1975 H1 in my area (Cavendish VT…. Anyone here buy it?) on CL for sale. it had been repainted a non OEM dark gray, no decals, chambers, pods (not sure if he had stock air box or pipes for it), had 10K miles stated. Was down to $2800.

Oh well I diverged because jealousy.

Back on track. I wonder what the needle jet series is for the GT380 to see if it is available. If not and you need them. Ivan’s kit sound perfect. And, supposedly makes for a better ride

I await his results on the T500. That bike for me has been very difficult to get every throttle position spot on an d I think mainly it is the Needle not quite right taper for all positions.


Also would be nice if he does kits GT550, GT750 and RD400.

He should do them just for an excuse to build up his bike collection.


Wonder if he is hiring test helpers.

[T350guy]

My 74 H2b had some tuning issues with the carb kit ( needles ) due to the pump feeding the float bowls as well as the crank
I just couldn't get it to run correctly because kits were made for the early H2s. I ended up using the jets which work just fine.
He does make them from scratch using NOS jets then spends a LOT of time on the needles.
On the Kawi's he leans them out on top and richens a bit on the bottom..Like I said my H1B works well with the kit.

[zk0ot]

ordered mine few days ago... talked on the phone for a bit... nice guy