The last two times I had the GT380 out, I noticed something quirky wiith the throttle response. Say I am in fourth gear at about 50 MPH (and about 5000 RPM) and I crack the throttles. The engine begins to rev and the bike starts to accelerate and then the revs drop and the bike acceerates more modestly, until it approaches 6000 rpm. The bike does not doe this in the first three gears, but will do it in higher gears at speed.
What could this be?
Sounds like a jetting issue. The change of the seasons is probably throwing your A/F ratio off. We call it "four-stroking" in the two-stroke world and it is commonly associated with running too rich under full engine load, though I can't make that conclusion via message board.
I just watched a video of a four-stroking two stroke and my bike is nothing like that. I think that it is simply loading up. I.E. The at that RPM and load the engine is gettig too much fuel. I have noticed that under load, the engine accelerated better with 3/4 throttle than with W.O.T. Above 5000 rpm. I have no such problems. Between 5k and 7k she pulls hard. It is supposed to make max HP at 7500, but it feels like it starts to run out of steam a few ticks past 7000 (maybe 7200 or so).
Also, it is only really noticeable when I know the throttle under load. I am think it could be normal for that engine.
I wish I knew the best rpm range for the engine. Lugging an aircooled engine is bad news, but I don't want to beat the tar out of it.
Are the needles in the carbs adjustable? If you think its a little rich, try raising the clip on the needles to the next position. That will slightly lean the mid range.
Welcome to the world of two stroke tunning soon you'll be an expert or a wall of shame parts collector... Bet you have 5% alky where you are right? seems to realy change mid range jetting. One of my older brothers is racing the rd350 again and has same ishue with pump gas but if he uses mid octane race fuel it runs great. It's not street plated so has little time to jet it right for pump gas.
When I was having trouble before, I started using Seafoam on the advice of a mechanic. It seemed to help, but I may have overdone the Seafoam. I wonder if that could be part of the problem. I think we have more than 5% alcohol here in NEPA.
In reply to HappyAndy:
The are Mikuni VMs so the needles are adjustable. However, the three carbs have a common top setup so everything has to come apart. Maybe later this fall.
When a 2 stroke develops problems like this, it's almost never related to jetting. Jetting does not change over time, develop odd habits, or make any difference because of the inclusion of E5 or E10.
In reply to Zomby Woof:
Then what is it?
To be clear, the engine is not "four-stroking." It idles and pulls normally. It has a bit of a surge when I snap the throttle suddenly when the engine is under a moderate to heavy load.
How would I know? You just said it might be normal for this engine. That would lead me to believe that you don't have a problem 
Two strokes are very simple. Look for air leaks, change the plugs, check the filters, and clean the carbs out. Jetting does not change on it's own.
Problem solved:
I remembered that when I first bought the bike, the choke lever would move due to vibrations. Sure enough, when the choke (really a fuel enrichment system) was open, it was closing just a hair. I made an adjustment and the bike runs wondefully.
Moparman wrote: Problem solved: I remembered that when I first bought the bike, the choke lever would move due to vibrations. Sure enough, when the choke (really a fuel enrichment system) was open, it was closing just a hair. I made an adjustment and the bike runs wondefully.
Nice work. I'll have to keep this one in mind anytime a problem like this comes up for me.
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