Can someone describe the dreaded GT-750 Surging problem?

[buffalodave]

I recently finished my resurrection of a long dormant A model and have done (IMHO) everything that one should do to the motor to revive it. Crank seals/bearings, pistons/rings, Accent ignition and went through everything that I read and saw on these forums. AND I studied BAZ's motor rebuild and carb rebuild dvd's (very helpful by the way) and spent PLENTY of money on it and it came out really nice. It runs GREAT and I have ridden it about 160 miles so far.

I followed the carb dvd to a T and the plugs read like they should. But what I am experiencing is when you are riding at a steady speed it is fine (in any gear) but when you have to roll off the throttle to slow down I feel a bucking or "surging". It goes away when you pull the clutch in and when its in neutral it is not present either. Its only when the motor is under load but not under throttle in gear. I thought maybe the chain could be out of adjustment but like I said when you disengage the clutch its gone...

Any thoughts?

[tz375]

That's it. If really bad it can be felt at small throttle openings say trying to pootle along in traffic or along a country lane on a whiff of throttle.

Did you fit air jets as described in the bulletin. Start with o.9mm(#90) air jets

There's a good description of the problem towards the bottom of http://pinkpossum.com/GT750/carb/BS40carbs.htm" onclick="window.open(this.href);return false;

[buffalodave]

I wish there was some mention of this mod on the dvd...I would have done it. Lovely, I get to pull the carbs again...

My real question...did these bikes do this the day they rolled off the showroom floor? If I had bought one new and it did this I think I would drive it through the showroom window...

if I read the "possum" page correctly...if I add these simple jets in front of the pilot jets on the main body of the carb it will eliminate this?

http://www.ebay.com/itm/Mikuni-BS30-97- ... EI&vxp=mtr" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

[tz375]

In front of the air jets, yes. There are two air jets. 1.0mm pilot air and 0.5mm main air jet. You are fitting a jet ahead of the larger air jet to richen it up under certain running conditions.

[buffalodave]

I already ordered them...and while I am not exactly looking forward to taking it apart again IF it makes it even somewhat better then I will gladly do it...but I am still curious how they sold them like this! Thanks for the info, I really appreciate it!

[Warehouse1001]

Just curious....will be making the same fix.....are those jets you reference on ebay the right ones? Pink Possum said .8 or .9.....those are 1.8 according to the seller.

[tz375]

The air jets come in a whole range of sizes.


There are many sellers and if your supplier says that they only come in 1.8mm, they may have misunderstood the question.


For example:
http://www.hvccycle.net/mikuni-air-jets-bs30-97/" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

http://www.mikunioz.com/shop/bs3097-air-jet/" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

http://shop.napalmmotorsports.com/eshop ... ERIES_.htm" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

Some have a better selection than others.

[oldjapanesebikes]

...but I am still curious how they sold them like this! Thanks for the info, I really appreciate it!

All large displacement 2 strokes are prone to surging - and I think I'm safe in saying that all GT750's do so to some degree. If the carbs are really clean it just usually isn't bad enough to be considered a problem. You could try retarding the timing a couple of degrees and see if that helps.

[buffalodave]

If I bought a brand new bike that was essentially the top of the line machine and it did this I'd be livid. But its also possible that when the bike is fully broken in it may not be as bad or even there...wishful thinking on my part i am sure butI guess I will see what happens...

[Vintageman]

Yup, that's what I call surging

Before you try restricting air flow to pilot circuit via the air jet addition/mod , increase the pilot fuel screw a ~1/2 turn out. That will add a little more pilot circuit fuel mix to the pilot passage hole after the butterfly throttle flap/engine side. When throttle closed, bucks, then this may just be what you need.

I had surging as you described and that fixed it. I did the air jet mod first, but removed it . Better mpg now as well.