Jetting advise for GT550

[tjt1990]

I am building a 73 GT550, engine has stock bore, ports are smoothed but stock width and height, head is shaved .020, pod filters and Jemco 3 to 3 chambers. I got 3 new 32mm flat slide keihnin rip off carbs from a friend, OKO brand I believe. Just trying to find a good starting point on my jets. I know they are alittle too big of carb but want to try them, Thanks for anyones input.

[tjt1990]

The carbs are actually copy's of Mikuni TM-32's I believe

[Vintageman]

that is a tough question, for me, I have no experience with this combination. So my two cents

What does it have for jets now this->
http://www.motorcyclecarbs.com/MIKUNI_T ... 01C776.cfm" onclick="window.open(this.href);return false;.

No air box and pods ( that is a large difference for WOT) and a larger carb 28 vs 32mm so bigger main again by (directly proportional difference 32/28 = 1.15% main).

I find the 73 550 with chambers like large pilot so maybe 45 OK if what it has. Sanger here is when you back off or coast at high RPM with no throttle. Q-2 Needle should be good start but again be careful

Next you shaved head 0.020"and I assume you did not cut into dome taking that off? So maybe you raised compression a full point (e.g. instead of 6.7 :1 for example maybe 7.7 : 1… getting high for 2 stroke IMO),

Those are domed heads not squish type so extra compression and then boost from chambers can, will, may, make you ping due to pre-detonation issue with dome design and raise in combustion temp due to compression increase. Liston for it (ping), I could hear mine ping due to air cooled fins…. That ain’t piston slap your hearing. Also, I only added chambers all else stock. I got ping when chambers kicked in

You may want to retard timing some too until you figure it out.

Richer fuel to air will help cool it a bit too, Use B8SE instead of B7SE I think the 73 calls for.
I am thinking 300 for main maybe more to start. Guys, have burnt up pistons just using pods on stock GT550 carbs and not enough fuel to match the extra / ease of air draw

Here is one guide to start assuming thing are way off

http://forums.everything2stroke.com/thr ... ing-method" onclick="window.open(this.href);return false;

[tz375]

Those OK carbs do not have a very good reputation and are reportedly difficult to jet correctly but I have no personal experience with them. I read somewhere that they are externally a copy of a Keihin PWK flat slide but with internals more like a Mikuni TMX.

That general jetting approach made interesting reading and is as good as any I have read.

[tjt1990]

Thanks for all the input so far. Looks like I have some work ahead. That article helps alot. Any one else please chime in.