T500 balks down anything over 1/4-1/2 throttle open

[keithwwalker]

I took my T500J for a spin today. It starts up easy and I haven't dialed in the idle yet. But good enough for a ride.

Problem is, once the throttle is open more than a 1/4 or 1/2 at speed, the engine balks down. About the most speed I can do on the interstate is about 60. Can't accelerate.
At a standstill,there is no problem with gunning the throttle - but once it is at speed, it can't accelerate.

Any ideas?
Both cylinders are firing.

I am going to pull the carbs and look at them, but they should be OK.

Could this be a symptom of timing being off, and not advancing?

Thanks
KWW

[ConnerVT]

Check:

-- Exhaust not plugged (pull baffles and clean)
-- Slides in carbs not installed backwards (cutout on bottom of slide goes toward air box)
-- Battery is good, and fully charged
-- Point gap and timing is correct (no advance on this bike)
-- Correct Main Jets installed. Pre/post 1973 carbs are different. Pre 1973 are internally vented. Post 1973 have external vents. Early Jet is 150, post is 70 something (at work, can't check now). Original carbs may not be on your bike.

[keithwwalker]

Prior to this problem:
I pulled the baffles, they have been cleaned.
I actually corrected the slides! fwiw, the bike won't idle with them in backwards
I installed powerdynamo system, the timing is statically set (with a depth gage in the spark plug thread hole), but need to put a timing light on.
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What do you mean about carbs being internally vented? The slides, needles and clip settings are correct.
Will check jets tonight.

Also,how do you check to see if the pistons are in the right sides? I assume that if at TDC there is a cutout at the bottom of the intake side, it is installed correctly?

I am following tune up data here:
http://www.ozebook.com/compendium/t500_ ... pspecs.htm" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;
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I pulled down the carbs to double check the jets.

I noticed that the siphon hose wasn't clamped down on the carb nipple.
I reinstalled with a clamp.
I am going to put it all back together and test ride it tomorrow...to be continued!

Check:

-- Exhaust not plugged (pull baffles and clean)
-- Slides in carbs not installed backwards (cutout on bottom of slide goes toward air box)
-- Battery is good, and fully charged
-- Point gap and timing is correct (no advance on this bike)
-- Correct Main Jets installed. Pre/post 1973 carbs are different. Pre 1973 are internally vented. Post 1973 have external vents. Early Jet is 150, post is 70 something (at work, can't check now). Original carbs may not be on your bike.

[ConnerVT]

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What do you mean about carbs being internally vented? The slides, needles and clip settings are correct.
Will check jets tonight.

Suzuki experimented with what they called "Homo-pressure" carbs in the early models of the T500. Instead of the fuel bowl being vented to the outside world (atmosphere), the bowl was vented internally, with a passage through the carb body to the intake side of the carb. This was supposed to improve the "signal" of how the jets responded, and reduce sea level atmospheric differences (riding in Denver, vs. riding by the ocean). It didn't really work as planned, and was abandoned in the 1973 model.

The key difference you can see on the carbs is that the post 1973 bikes have a vent/overflow nipple on the fuel bowl. The early carbs don't. (one bad thing with the early carbs, is if the fuel bowl overflows, it flows right into the engine, instead of the garage floor).

The early carbs have 150 Main Jets. The later carbs have 97.5 (I looked it up. Knew it had a "7" in it )

Obviously, if the 150 jet is put in the later carb, it would be very rich, and be four stroking badly by 1/2 throttle.

I noticed that the siphon hose wasn't clamped down on the carb nipple.
I reinstalled with a clamp.

I have no idea what you mean by "Siphon hose". Please explain.

[keithwwalker]

The siphon hose connects to the fuel petcock diaphram to open the fuel flow, only when the engine is running.
Is part #24 here
http://www.ozebook.com/compendium/T500parts/image12.htm" onclick="window.open(this.href);return false;

[ConnerVT]

OK, I thought that's what you meant. I have never heard it called that, even here on this multi-national forum.

[keithwwalker]

Well who calls it what Suzuki called it on their fiche: "boost tube"
Lolz!

[keithwwalker]

Well I rechecked the jets and they are correct for the 72' T500J. 150 main jets and 30 pilot jets.

I clamped the vacuum line for the petcock, and took it for a ride.

It still balks down at any speed, any gear once the throttle is opened up.

I am 98% certain that it is the main jets coming on when this happens.

So after 1972, the main jets went from 150, down to 97.5.
This seems drastic, was there also a change to the piston???

I look at the online fiche and it says that the pistons were the same except for the first model year:


I am going to pull the heads/cylinders and check the pistons, to see if they are in correctly.

Any advice?
Which way should the piston cutouts be pointing for the left and right pistons?
Perhaps I switched the left and right pistons and didn't notice this - there should be arrows pointing to the exhaust on top of the pistons.

Thanks
KWW

[keithwwalker]

OK, I found photos of my rebuild.
It looks like my pistons are in correctly. I will pop the heads to confirm the arrows pointing forward and they are on the correct sides.
The cutouts are at the back

[Vintageman]

for the 72' T500J. 150 main jets

Any chance you used a hex 150 main instead of a 150 round main?

The later 97.5 has to do possible with air box changed and the fact the carb body between the two years had different bowl venting schemes and I also think the use different Jet Needles so between these three things is why diff.

Can you explain in better detail how its runs (more sound description in words…. Hard to do)) and how it behaves RPM versus throttle position?

Did you pull flywheel off crank and inspect key way integrity. I have seen a couple of these heavy a cranks spin the little key way and if you use timing marks to set timing only you won’t catch this nasty confusing problem.

[GT750Battleship]

Well who calls it what Suzuki called it on their fiche: "boost tube"
Lolz!

Hi,Carbruettor Vacuum hose !!!
Regards,
Roger
GT750A

[keithwwalker]

I think I found the problem, and I never would of guessed it was related to the other issue with the bike, that I thought was minor.

The shifter action was really tight, so I thought I would pull the shifter rod and see what the problem was.
When I pulled the side case to remove the clutch, I noticed that there was hardly any clearance between the shifter rod gear sector and the clutch gear. After I removed the clutch, I also noted some metallic sludge immediately below:


I pulled the rod out, and sure enough the rod is bent. I also didn't notice that there is a washer missing that prevents the rod from shifting laterally to the right. Its part #30 here, and it let the interference happen:


My theory is that there was just enough clearance between the gear shift rod-gear sector and the clutch gear for the bike to run at low rpm. When it was in neutral, I could gun the engine as much as possible (the clutch primary gear isn't moving at that point, so no bogging down), but when it was in gear and on the road, the loading of the engine, would cause the gear to start rubbing against the shifter road gear sector. The gears are not straight cut, but have a pitch on them and will thrust a bit.

If you look at my older photo, posted before, that wear wasn't there when I put the engine back together.

Now I have to get a good shifter rod, to see if this is really the problem.


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A bit of background. I got this bike, not running. I purchased it from a farmer (friend of a friend), and it was seized when I got it. It laid in storage in my parent's heated basement for years, and then I finally got to tearing it down for a complete engine rebuild.

Strip down was a bear, because there was cow crap, corn husks and kernels all jammed into the bottom fins of the crankcase (LOL). The previous owner used to ride it in his fields.
When I did the rebuild, most bits seemed good, except for the crank which I had rebuilt.
I finally got it together, but a move across country delayed finishing. In those years I had forgotten about the shifter being a tight fit. I think the shifter seals were also bad, because I replaced them. I think what happened is that the previous owner went down on the bike and bent the shifter fork. I have literally under 20 miles on the bike, since the rebuild.

Live and learn....

[keithwwalker]

To answer your other question, the main jets are round #150's, not hex.

Thanks for the feedback, I am not out of the woods until I get another shifter rod

[Coyote]

Learn something new every day. I have never seen float needles with retainers before. NEAT! That gives an idea of what might be wrong with my GS1000 that fuels the floor whenever I use 'prime'. Maybe one or more of the float needles are dropping too far and then angle locking in the seat guide hole. That would also explain why it stops after the motor starts, setting up enough vibration to dislodge the needles. I just put the bowls back on for the umpteenth time because I couldn't find anything binding. That almost HAS to be it.

[keithwwalker]

Well I got a new shifter rod, necessary washers & dialed in the idle. Bike is back together.

But I still can't open the throttle beyond 1/2 way - it loses power. This happens in Every gear.

Gonna check float heights - would that prevent the main jets from delivering fuel?

Help me out, guys. I checked almost everything on the bike: jetting, petcock, even changed the 2s oil. What causes power loss at greater than half throttle?