Suzuki 2 Strokes

72 GT750, slight porting & jemco chambers, just purchased after the bike was completly rebuilt a few years ago & then sat without being ran for a couple of years. Had aprox 300 miles on engine.

My problem I believe is jetting, she idles fine but doesn't have great power. About the time its comming into the needle jets she's crackle badly, I moved the clip up 1 position to lower the needle 1 position (clip is 2nd down from the top) runs a lot smoother. But unable to get into the main jet part of the throttle because it will pop back thru the carbs & I don't want to hurt it. Well today I rode to town & was planning on doing a plug chop opened up as much as possible before the popping occured & that's when she back fires (I guess) blowing the right hand carb out of the boot. I had to call a friend to bring me a phillips head screw driver so I could loosen the clamp & reinstall the carb. Did that but she has a high idle on rh carb (using the choke on that carb slowed it down to normal) rode my couple miles home & shut her off for about 5 minutes & inspected the slide to see if it was hanging up (nope) restatred the bike & now idles pretty high. I'll let it cool down completly before restarting & see what happens.
The old girl gets horrible milage (18mpg) and has a 130 main jet in the mikuni vm34 carbs. 2.5 slide & if I remember right a 6d4h needle.

My guess is that its lean but the bike shop around the corner didn't have any jets I wanted to try, a 160 should be enough of a change in a safe direction to give me an idea of what's happening.

Sounds like the main jet is too large and you are trying to lean it out using the needle. Try 125 main jets - they are readily available from any Mikuni dealer or on line. Alan P used 125 and Suzukidave uses 150 I think, so teh pipe and filter design both have an effect on jetting and what it will need.

Then set the needles back in the center slot and fit #30 or 35 pilot jets.

Get the idle right first and then the main jets and then the needle/jet.

I thought that 5F16 was stock on that motor. Maybe my notes are wrong or the PO changed the needles out. 6DH4 is in the range used by piston ported Yamaha race bikes interestingly enough. What needle jets are fitted - 159 or 188 series and are the main jets hex, small round or large round?

I'd also check ignition system and timing.

New mikuni carbs. Not stock. I'll have to look at my notes I left at work about the needles. Main jet is Small hex I believe, same size as my stock rd350 carbs.
I guess 130 seamed crazy small compared to my RD that uses a 190. Does a piston port engine compared to a engine with reeds tyrically use a small main jet?

New carbs with large hex (same as RD) will be completely different sizes than stock carbs with large round jets that I assumed you had on the bike. I should have paid more attention to the original post You did mention VM34 Mikunis in that thread. I missed it or assumed you meant stock 72 carbs.

I don't have enough data in the database to draw from but 36mm carbs on a full race motor are in the 280 - 330 range, so 34mm carbs with Jemcos and mild port job should probably be anywhere from 180 to 240, but that's a W.A.G.

What needle jets and slow jets are in those carbs? If the bike has never run right after it was built with those carbs, it may have generic settings. SUDCO supply VM34 with that needle and a 260 main jet, so it's time to go up in jet sizes.

Well I've been fooled. I was convinced most of my problems were jetting & finally started swapping out main jets. True upping the mains helped by the popping continues.
A friend and longtime wrench rode the bike today & stated ignition miss is present. Well I have 3 new coils & waiting on my accent electronic ignition to show up any day now.
Also like to note this is going to be a screamer, finally running well enough to show it's potential. Thanks tz & zook for suggestions along the way.