Hello All,

I am a shade tree motorcycle mechanic trying to help a friend get his 1973 DT360 fired after a winter of sitting. I have done a lot of searching on this forum and others (non-Yamaha) to attempt to figure out my issue. I have narrowed it down to a sticking carb slide. I am hoping that the experienced members of this forum can point me in the right direction.

Attached is a picture of the machine, I have videos of it from a few weeks ago when I had it running nicely

Current state:
2 weeks ago I had this machine started but it wouldn't run correctly at full speed (cutting out, lean).
Now it will not fire unless I push the carb slide all the way to the bottom of the bowl and re-tighten the slide cap. Any throttle and the slide sticks, the bike won't fire at all.

Full troubleshooting:
- I received the bike about 1 month ago with a "no start" condition
- I proceeded to add a new 6v battery, as the old one was completely dead. This was about the same time that I read in the original FSM that you don't need a battery to get this bike fired because the ignition circuit system is driven off a mag with coils in it. Oops, every day is a learning opportunity...
- Still no spark, so I pulled the magneto flywheel cover
- I used some sandpaper to clean the points off
- BIKE STARTS AND RUNS!! Woohoo
- Test ride, I discover the bike is choppy and lean at higher speeds
- I pull the plug and gap it to the FSM spec
- A little better on the road but still leaned/sputtering at WOT
- One time I started it and I had to kill the bike with the key because the throttle slide stuck and the bike went max RPMs wide open
- Disaster narrowly avoided, I knew immediately what was happening and killed the engine
- I reran the throttle cable (it was in the incorrect spot, going outside of the headlight instead of inbetween the handlebars near the ignition switch) and used some zip ties to attach it to the frame so it operated smoothly
- This is when I learned about crankcase seals and I started worrying my buddy's bike's seals were shot

2 weeks passes by
- Bike's owner agrees with need to go into the carb, bike sits for 2 weeks while I wait for the flywheel cover gasket
- Gasket arrives, but now the bike won't start.... what in the hell....
- Points must be corroded? I give them another clean and check for spark... SPARK IS GOOD....
- I completely drain the fuel tank and add fresh non-ethanol fuel
- I clean the water trap in the petcock (it will full of debris)
- I replace the inline fuel filter with a clean one that you can see the fuel in
- I fully disassemble the carb, clean it very thoroughly and reassemble
- I compression test the cylinder with 2 different gauges, watch the compression 100PSI+ over the course of 15 mins on both gauges.. compression is good tested with both gauges

STILL NO START!
Then it hit me... the throttle feels wrong. I'm not hearing the "thunk" of the slide within the carb when I release the throttle. I opened up the cap and noticed the slide is all the way at the top of the carb. I push it down to the bottom of the carb, go for a kick and it nearly fires... I got 1 or 2 chugs out of it but I can't give it any gas or the slide sticks again.

I tore into the carb and this is what I found (it looks like the plastic tab in the carb's slide bore has been damaged):
 

Also another point is the small bit of damage on the jet o-ring:
 

Possible Next Steps:
- Tonight after work I am going to loosen the carb manifold bolts to see if that impacts the slide at all. I am thinking they may be too tight.
- The carb's spring is responsible for pushing the slide down... could the spring be worn so badly that it has lost its usefulness?
- ????????

Gentlemen, I am stumped. Can I fix this carb slide? The owner's preference is to keep the bike as original as possible so fixing the old is preferred.

I don't have time to answer much now...  I'm sure many will be along with tips and answers to your questions.  Just wanted to note that you are working on  a 1972 RT2.  Different animal than a DT360 which was a 1974 model. 

Title and registration on these old bikes are often incorrect...  but important to know what you're actually working on.  Tech Library here has VIN tables for positive ID.

Thank you for this reply, the model specifications have thrown me off before... especially when I saw "RT" on the VIN but my friend always referred to the bike as "the 73 DT".

I will confirm the VIN to the tech library. I'm also going to have him bring over a picture of the title. I'd love to know what it says versus the actual year.

I recently had a title come back from PENNDOT for my girlfriend's 1969 Honda CT90 that was wrong on both my last name AND the last VIN character.
 

You're definitely going to need to clean up the plastic nub in the slide bore, looks like it was squished by the slide being forced in out of line. Me, I would gently file to remove the mushroom top until the slide easily glides up/down in the bore. The spring should be plenty sufficient to return the slide as throttle is closed, I've never seen one fatigued. And as you mentioned, the carb can distort if overtightened, and it is possible the flange is already warped. You can use a straightedge or a glass tabletop to see how flat it is. 

Replace that damaged O ring . It's not the cause of your current problem , but once you've
got past the sticking slide problem , it could be causing  you grief .

I'd add inspect the slide very carefully for any sign of scratching. Feel along all the edges for burrs. My experience the tolerances are very tight and even a small burr or scratch on the slide can make it hang up. I removed an engine  with the slide hanging loose on the throttle wire still and must have sandwiched the slide between engine and frame enough to create a small burr on one of the edges. When I put it back together the slide hung up going down in the slide bore. In doing so I made the problem worse by creating scratches in the bore itself. It took a bit of work cleaning up both mating surfaces to get it to slide correctly...almost to the point of trying to find a new slide. Good luck!

Gosh, where to start. Gotta get that slide operating freely. Might be best to look for a good carb with a good slide guide. Check the Oring on the car flange mount isn't too fat which pulls the flange ears on tightening & causes slide sticking. & the small Oring you've found, replace. Re-cleaning points--if you had the flywheel off then pull it off again & check the crank key, needs tightening very tight or eat crank keys. Have you set the open gap so its not too wide. Plug gap too, not crazy wide.

Thank you to all that replied with ideas, I really appreciate it. After tonight's session I feel like I am getting closer... or at very least getting rid of issues.

I resolved the slide operation problem, I can hear the CLUNK again of the slide hitting the carb with spring pressure after releasing the throttle. THAT'S PROGRESS.

Plastic Nub in Slide Bore
I cleaned up the damage to the slide pin and made sure the slide moved freely in that area. While I was completing this work, I noticed that the slide was not hanging on the pin but rather at the top of the bore where the threaded cap lives. When I pushed the slide up, it sticks. Then I squeezed that area and the slide dropped freely to the bottom of the carb body. I could hardly believe this and took a video to send to my friend.

Shaving the Slide
To remedy this situation I coated the slide in dykem blue and sanded it lightly with 80 grit , then 180, then 220 until an even surface finish. It seemed to have done the trick because when I dropped it in, it was able to move freely from top to bottom. I thought I was rounding the plate to home here... so I quickly threw the carb back on the bike and.... couldn't get it to fire. It didn't even burp... instead acting like it had no spark/fuel. So I pulled the plug and checked to make sure I saw bright blue sparks.

Flatness of Manifold
The manifold I am working with does have a bit of light shining through but not as bad as some of the examples I have seen. I am certainly not dismissing this idea but I tend to be overly cautious around the tightness of an old fastener without a torque spec at hand. If it was bent, it was bent long before it stopped starting.

Points
I approached cleaning these with the flywheel on. I do have the proper puller from a previous 81 exciter 250 street bike project but chose to work with a pick and sandpaper in the small window on the flywheel. I will pull it and replace condenser and points.... if I have to. But for now the spark appears strong. Otherwise I did not touch the points gap.

Plug Gap
Adjusted this per the FSM spec but I will have to double check my work. For what it's worth, the bike previously ran on the current gap.

What next...
I am starting to think there is something wrong inside the carb. Mainly with regard to that o-ring that lives on the jet. I am going to buy an o-ring assortment from the hardware store this week and see what difference (if any) this has.








 

I've not had the 73 but I have the 74 and my starting problems have always led to the pilot jet. That passage is so small, it's easy for the smallest micron of gunk to clog it. I've heard people use guitar strings to push through it. I always used the end of a garbage bag wire tie but, regardless, I shine a flashlight into the bowl to make sure I can see a clear pinpoint of light through it. If you can find a way to spray a bit of mixed fuel into the carb end while kicking it from a spray mister (might need a second set of hands to do this) it may confirm it's a fuel delivery issue. Many guys smarter than me on this site but I've always found these bikes are mostly bulletproof and the simplest explanation is usually the one that works. Obviously, you need to confirm the fuel is getting into the bowl first.

O ring is still available at a Yamaha dealer . It needs to be fuel resistant , and size is critical .
I wouldn't recommend the Hardware option .