GT550 jetting with chambers

[Iron Spade Cycles]

Did you do a plug check at midrange? Ride it up to speed and throttle position where its running funny and kill it but don't let the engine start up again or drop in rpm under power. Pull over and pull each spark plug and check the condition of the plug, it will tell you which way to change the jetting. Also something else I've learned is to jet each cylinder as if it was a separate engine. The middle cylinder might need to be slightly richer to help with too much heat. Learn to read your plugs and adjust accordingly.

Good luck. Later Travis

[tz375]

Bill,
I agree that one could expect the mid throttle setting to be lean. You described it as being like a 4 stroke and if you meant lumpy and firing every other revolution, that's rich. If you meant flat and no power, it could be the pipe design or lean.

Plug chops at anything other than flat out are misleading to the point of possible failure under load because the plugs are not hot enough to burn off deposits at light load conditions. Exhaust Gas temps and Air:Fuel ratio analysis are useful at all revs/throttle positions though.

The trick here is to separate out throttle position from revs. On a slide carb throttle is most important and revs are less important until we get very close. For example we know that at 1/2 throttle needle/jet are the main controller, at wide open it's 99% the main jet.

A great way to start is to mark the throttle at 1/8, 1/4, 1/2 etc and see how it runs at each setting and then see how it changes from say 1/4 to 1/2 and from 1/2 to 3/4 - regardless of the revs the engine is pulling.

That will give you a much better indication of which jets need to be swapped out.

Do that first.

If we have a difference in fueling across the rev range - AT CONSTANT THROTTLE SETTING - then we tweak the air jets or height of the needle jet very few of us get to that stage of fine tuning. To do that (after you get the jets in the ball park), you ride along at say 1/4 throttle and let the bike accelerate and note what happens as revs rise. The best way to do it though is at 100% throttle on a suitable private road/track. It may be smooth all the way up or there may be a flat spot (pipes) or it may go rich or lean (all at 100% throttle). That will give you information on which to choose jet height, air jet size etc and will tell you what the pipes are doing and where they are completely out of phase.

[Iron Spade Cycles]

I completely agree with TZ375, and the only reason I suggested the plug chops method is most people don't have/use EGT gauges. I do have them in place on my RD350 but not my GT550 as I've not yet seen a triple gauge system. Plus looking at that gauge while maintaining a specific throttle position, making a mental note of the EGT for each cylinder and avoiding traffic, dogs and street signs can be tricky for even the most skilled riders.

Good luck with the most fun process we all face after a performance modification.

[tz375]

Westach make a triple EGT gauge and it's available from SLP, Accu-Products, Aircraft Spruce and others.

In the real world you are right, it's hard enough to sneak a quick look on the main straight just before grabbing a handful of brake for turn one and dangerous and difficult to use an EGT on the street. I have a triple EGT gauge here, but I'd really like a data logging set that I can read at a safer time with EGT and lambda, TPS, revs etc, but that is not available in my price range

[bill in okc]

Thanks all. I have my carbs apart again. The center needle jet has a lot of scale deposits inside near the bottom. Also I had used the needles (not the other jets though) from a keyster kit and they are narrower than the stock needles starting about halfway up. I held them side by side and slid them into a caliper. They started out about the same, then I opened up the caliper a bit more to about .097" and the Keyster Y49 could slide all the way through while the stock 5DH21 was too thick to go more than about halfway. Not too scientific but this might cause some richness I think. I will soak these needle jets in carb dip for a day or two and go back to the stock needles, they do not look too bad.

[bill in okc]

A minor setback as one of my original needles rolled off the arm of my chair and was swallowed whole in the cushions... I'll have to perform chair surgery to recover this precious bit possibly tonight. If it had been the Keyster part I would just leave it.

edit: I've found an available Mikuni needle, p/n 5L1 that is supposed to be an upgrade to the 5DH21 - a better taper or something? SUDCO crosses to it as a racing/competition part. I am thinking about trying them out instead of the chair thing...

[tz375]

Bill, I don't have the specs for a 5DH21, but a 5L1 was stock on RD400C/D models. It has the common 5 series 2.518 root diameter but a very thin (rich) point, so it will be richer at the top end in all probability.

[bill in okc]

The 5DH21 was found and the original needles were put back in service with positive results. My 3 Keyster Y49 needles measure ~.097" or 2.464mm at their widest point. It runs much better with the original needles, actually surprises me how hard it pulls. I still have a a little 'off' where I think it transitions from pilot to needle but half throttle and up feel good now. Plugs look a bit rich still. I am going to replace the coils before working with the carbs any more. Bill

[Allan k]

I looks like your engine block is made for the nikasil type cylinders with those big ports...anywats

I am running a gt550, with jemco's, mild porting in the intake and crankcase ( no cylinder port modifications).

I am desperately looking for advice on jetting...

I have run a hole season last year with a 3in1 with 105 mains in all three carbs, otherwise stock and it ran great and startet fine

This year with Jemco's, and mild porting. Any suggestions??

[Allan k]

I did a new test run today, but with 110 main jets instead of 105, and raised the needles 1 step from stock position, it was running much better, and idle was better too.

The engine pulls hard after 6000rpm, so much that the clutch is slipping!! The clutch plates and springs was replaced with new ones last year and has only done around 5000km. I have only used 'wet clutch' motorcycle oils...

Should i get some stiffer springs for the clutch?