1981 GS450 Tracker Project

CupidsDeathDay

Been Around the Block
Hey guys. I will be posting pictures of my project soon, but i have a few questions about an issue i am having. Hoping someone may have some input.

I was working on cleaning and prepping the carbs, cylinders, tank, electronics, etc. to get my bike road worthy again after a 2 year hiatus. But i seem to have come across an unusual issue with my scoot on my first drive attempt.

After getting everything cleaned, and tank was cleared of the surface rust after sitting for a while. Cleaned the petcock, carbs, and gaslines. All of this seems to be clean and seals are all good that i can tell. I have good compression from the cylinders and good spark on the plugs on both sides. But there seems to be an issue with one cylinder firing really lean. The idle is super high when i start the bike, and really low after warming up. I cleaned up the spark plugs and verified again that spark was good. Both carbs are getting gas and i know the floats are working. But when i hit the throttle it accelerates very slowly. Once i get moving its as if one side is not firing as i get no acceleration. But then *BOOM* it accelerates hard for a split second, and bogs right back down to the slow acceleration again. I cannot find any airleaks that i can see. I tried spraying the boots connecting the carbs to the engine, but it does not rev when sprayed from either side.

So i accelerated, the best i could, downhill and dropped the throttle closed and came to a halt to check the plugs. One is white(the problem cylinder), and the other is almost a perfect Tan/brown.

Could this still be an air leak on that carb that i am not seeing, or could this be a timing issue?

Fully open to suggestions as this has just become a frustrating pain in my ass. I checked and verified i have all matching jets(in case i put the wrong size in one). The diaphragms for the slide has no holes and the needles are all straight. This bike ran great before i stopped riding...

P.S. I am running a 2-1 exhaust, and dual 2-stage UNI's for the filters on stock Mikuni CV carbs. For my jets, as a starting point, I am currently running 35's on the pilot, and 145's on the main. Could it just be that i need a Higher pilot jet?
 
I think i may have figured out the issue. I don't like what i found, but i found it.

Looks like i have a head-gasket leak on the left cylinder side of the engine. I plan to retest the spot i found with the starter-spray test. If it fails the test, I will pull it apart this weekend to verify. Gaskets aren't old, but this has been sitting dry for a while, so i guess i am not surprised. Updates to come. Hopefully will be uploading pics soon too. If anyone else has any suggestions while i am tearing into the top end, let me know.

I plan to go ahead and have the sleeves honed, and probably clean the intake/exhaust ports of any carbon and such. Also will be making sure timing and valve clearance is all good, while i am in there.
 
So after doing some testing i found that there is a small break in my head gasket on the Left side Cylinder. So it looks like a top-end "Rebuild" is in order. New Gaskets, shims, possibly valves, and honing of the cylinder/sleeves. So this is going to be a fun fall/winter for me...
 
The picture of it all together was the what it looked like when I picked it up from my parents storage a couple months ago. Finally got it apart, and also decided I am going to big-bore it. Gonna upgrade to 73.5mm pistons. I am getting it bored out in a couple of weeks, and going to work to clean everything up while I wait for parts!
 

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But I also have a quick question about this. Especially since this will be my first overbore job!

From my reading and understanding of doing a bore increase, the rule of thumb is stated as a minimum 0.003-0.0035"(0.0762-0.0889mm) over the size of the piston, for any bore sizes between 2.500"-3.625"(63.5-92.075mm). The 81 GS450 manual says it's 71mm piston/bore size, but doesn't give me exact for either/or.

So if I get 73.5mm Suzuki replacement aftermarket pistons, that would put my total recommended bore size at a minimum of 73.5762mm. I figured I would round it to 73.6mm on the bore size, as it's almost the midway point between the recommended minimum of 0.003"(0.0762mm) and max of 0.005"(0.127mm), while leaving room for "error". Would this work?

I just want to be sure I am calculating this correctly.

Any input would be appreciated!
 
I would get the pistons and send them and the barrels to a machine shop and let them do the math for you. If your head gasket was leaking, it would probably be worthwhile to have them surface the head and mating surface of the barrels as well. Myself and a lot of guys on here use MotoTech USA in Ohio. Call and talk to Chris, he will help you out.

Also, reading back on your carb issues, I have a GS450 that I chucked the stock airbox and am running UNI pods. I will need to double check on the jet sizes, but I had to shim the crap out of the needle in order to get the mid range to be rideable. The CV carbs do not care for pods, but it can be made to work. I am currently running 4 shims underneath the plastic needle spacer to raise the needle and that has all but eliminated the surge/flat spot.

Here’s a pic of mine, cosmetically restored, haven’t touched the motor yet outside of swapping the oil pan. They are great running bikes. Also fully electronic ignition so timing should not be your issue.

Image1727186386.340710.jpg
 
Take the cylinders to the machine shop for evaluation and measuring before you buy pistons.
 
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