Starting / running issues GT 550 ???

[Skyguyz]

This was posted on the FB forum, so.....


So.....

I did a restoration on this 1976 GT 550 and the bike has some initial starting running issues. Now I have restored quite a few of these vintage 2 stroke Japanese street bikes from the 70’s, (Kawasaki triples, GT 750's RD’s, etc) So I am not a stranger to these bikes, (Especially the Miuni carbs) but this one has an issue with I believe is fuel related. It’s hard to start and does not stay running at lower RPMs.
Now, it has had all the seals replaced on the crank, inner & outer. Standard points ignition timed at 3.42 mm BTDC (vertical plug) and left and right cylinder 3.64 mm BTDC (inclined plugs). and points gap at .35-38 mm. Resistance check of wires to the points is good very little voltage drop. I used 3 different sets of plugs: Plugs used are NGK B7ES, B8ES, B9ES, (good spark as measured with adjustable spark tester and plugs out of cylinders grounded, very good big blue spark. So, I am confident it's not the ignition, plugs are good.
Carbs....
I have done a lot of these early Mikunis without much difficulties. Yes, one would suspect the pilot jets and associated pilot jet pathways. These have been addressed twice and 100% confident pilot system and mains are all clear, (Ultrasound bath, non-clorinated brake cleaner spray, compressed air. The spary come out of all the correct holes and the emulsifier tubes and pathwyas are exceptionally clean and working great. Floats set at 27.6 mm +-.5 mm Pilot and main jets are standard size. 1.5 turns on the pilot air screw also. The 2 enrichners, (read chokes) have been adjusted and work smooth and the brass plunger goes to the bottom and seals well in the "Run" position. These carbs have been cleaned to new-like condition.
The exhaust are brand new Higgspeeds, clear and perfect. I even tried starting it with the airbox on & off, same results.
It seems to start only with the enrichner, (choke) on which leads me to believe it either has an air leak or is lacking fuel.

I checked and double checked all the carb fittings, (new boots, fore & aft) and they are tight with no signs of air leaks. Now if the outer or inner crank seals are bad, then this could be the issue, but the crank was done at a well known machine shop that specializes in the GT series Suzukis cranks here in the states, (Bill Bune enterprises.) The crank was installed correctly with pins in place and seals not touching the webs. The crank came back to me looking almost brand new. I don’t want to do a leakdown test just yet as I will have to remove the intake & exhaust. *Yes, I know I should have done one after the engine was complete.
Compression on all 3 are within 3 psi of each other and at 119-122. *As measured with ultra reliable motion pro compression tester. New pistons with the ring gaps set properly. Honed jugs, lie new condition.
After the bike runs for about 10-15 seconds, it seem to starve itself of fuel??? At higher revs it revs freely and shows no sign of ignition faultering The performance at a little higher RPM is spot on. Low RPM = garbage. But in any case it will die shortly after running regardless of enrichner position or throttle position, again, fuel??? But the carbs are spot on, (hopefully???)

Maybe I should just send my carbs over the pond to Baz???

It is a very hard to start bike as it sits right now.

What did I miss?

*Video of poor running:
https://i.imgur.com/zQbRLJM.mp4


Thanks for reading this long post.

[asco58]

Standard points ignition timed at 3.42 mm BTDC (vertical plug) and left and right cylinder 3.64 mm BTDC (inclined plugs). and points gap at .35-38 mm. Resistance check of wires to the points is good very little voltage drop. I used 3 different sets of plugs: Plugs used are NGK B7ES, B8ES, B9ES, (good spark as measured with adjustable spark tester and plugs out of cylinders grounded, very good big blue spark.

The timing you've quoted is for the GT750. The GT550 is supposed to be timed at 3.37mm BTDC on all cylinders for USA models. A standard USA GT550 M onwards uses B8ES spark plugs. The 550 and 750 have their points on opposite sides. The 550 engine spins clockwise if you are looking at it from the right hand side. With a dial gauge, turn the crank clockwise to find TDC. Then turn anticlockwise up to 3.37mm BTDC. Double check this as well. I'm not questioning your ability to time ignitions but I mention this because I once turned the crank the wrong way and set the timing to 3.37mm after TDC and the bike ran like you describe, didn't idle but high rpms were ok. I think you could try timing the bike again to the correct spec and see if it runs after that. If not then it's one more thing ruled out.

[Alan H]

I've always used the timing marks on all my 550s (since 1972!) and they have all run well.
550s will always need choke to start from cold, and for more than 15 seconds too. I usually use 'pri' to fill the carbs - with the bike level, not on the side stand (I prefer some clear fuel line so I can see it flow) then back to 'on' or 'res' depending on how much is in the tank. Start her up and less choke as the engine warms. I usually find that the bike won't pull up my sloping drive without another 'dab' of choke. After that they all run fine.
Plugs. 7s are too hot unless you only do town riding and 9s are too cold unless you race. 8s are correct. Hopefully you have genuine ones and not knock offs.I always use non resistor plug caps.
Make sure all the carb slides are set so they all open together and evenly, then set the tickover with a warm engine.

Must admit, all the carb sets I've had from Baz have only needed tickover adjusting, as he uses a flow meter to set them up.