GT750 J pulls clean to 4500, pops, then goes to redline

[busa1300]

I have a 72 GT750 that I just got running, after setting in various storage areas for untold years. The first 200 or so miles, the bike was slowly cleaning and clearing itself out, and felt as it was pulling smooth through all the gears and full RPM range. I now have about 500 miles on it, and the bike doesnt want to stay idling at a stop for long. And when twisting the throttle pulling through the RPM's at a reasonably quick pace, the bike slows for a fraction around 4500 / pops lightly once / then generally pulls good on to 7000. All the plugs look a little dark, possibly running a little rich just to be safe. Using aftermarket jets, they may not be the exact size as factory, even though they are stamped as the same size. Bowls leak a bit when parked on side stand, so I might have the floats a fraction bit to high also.
Or could this be involving the SRIS system?

[oldjapanesebikes]

Or could this be involving the SRIS system?

Personally I doubt it they would cause the problem you describe but I'm sure others will chime in.

The J model did have a problem with the check valves sticking and so screens were added and then changed/updated part way through the K range. Take a look at Suzuki service bulletin #18 at this link:

http://www.oldjapanesebikes.com/mraxl_G ... etins.html" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

You can carefully check if the SRIS valves are stuck by gently trying to back flush them with a syringe (shouldn't be able to) and then using the syringe to pull the flushing fluid back out if they do pass back into the crankcase. If they don't clear then your options are to try and replace them, or plug them off. The filters are not available from Suzuki, but all the other bits, including the hoses are.

[tz375]

We all tend to roll on the throttle and that makes it hard to differentiate between RPM related events and throttle position events. Does the bike exhibit the same cough at all throttle openings and the same revs or is it really Throttle position related?

You might try snapping the throttle open faster or rolling it on slower and see if the stumble changes with throttle position. An easy way to see that is to warp some white tape around the grip and mark it at 1/4, 1/2, 3/4 and try it at 1/4 and repeat at 1/2 and again at 3/4 and see if anything changes.

If it's totally independent of the throttle and stays at fixed RPM it's probably a resonance effect in either the intake or exhaust where things get out of step. That's exactly the RPM my 76 has a dip in the power curve and it has different ports, exhaust and carbs, so go figure. You may be able to tune it out slightly by raising the needles one clip position - or dropping them.

What happens if you ride at a constant 4500 and what then happens if you change throttle position?

[busa1300]

If I slowly pull through the rev range to the red, it just slightly hesitates then power hits a little harder up to top. If I wind through a little faster through the range....it usually has the slight pop around 4500 then goes to the redline pulling fine. If I twist hard....it will pop several times through 4500 to 5000...then slightly clear up to the redline. But it always seems to be the same general RPM. When I first got it running, it never had this problem...I believe my floats might be slightly high, if that might have any connection. Was planning on pulling carbs again when I got my new intake boots, and draining tank again to check for rust residue, from being cleaned out a few months ago...

Planning on looking tonight at the sris schematics, so i can see how it all works...

Changing fork seals tonight...but found out the bolt holding the one fork together down by the axle, has a stripped hex...and that bolt is harder to drill out than I thought.

new drill bit tomorrow.

[busa1300]

Had to come back and say that I got the issue figured out... My center points gap was too tight. I reset them all at .015, and now the thing pulls like a 40 year old freight train.
Now I need to concentrate on my CBX and my RGV.... And just cruise the summer on the old J model...