1973 TX650 RPM / Idle increases a lot when warm

[1973 TX650 Chopper]

Okay guys, I'm a newb here so I'm hoping for a little advice on a problem I'm having. I bought a 1973 TX650 that was 95% completed by another owner. I knew it needed a little work but I'm having an issue I can't figure out.

This TX650 is a custom chopper/bobber with a lot of aftermarket parts. It has aftermarket open exhaust and pod air filters. When I purchased the bike it started easily but ran a little rough, though it would go down the road and was driveable. It would start right up and it would drive, but it would run rich and poorly (no acceleration) for the first 10 blocks or so. Then it would stop running so poorly and the acceleration would return to normal and I drove it like that a few times. It seemed to run okay once the initial no acceleration problem disappeared. But then after the bike ran better and warmed up, it wanted to increase the RPMs on it's own and surge rather high. I've had to shut it down a few times to avoid it racing to dangerous RPMS. A few times I activated the choke temporarily and that seemed to keep it from RPM increasing. Also, I could feel it wanting to rev up while it was in gear, but if I left it in gear while I slowed down the RPMs would drop back down low enough it would usually not surge, until it reached a high enough RPM and then it would want to surge again. I originally thought the carbs were junk since they were the original ones. I ordered new Mikuni VM Roundslide 34mm Carbs by TC Bros and I installed them along with a new throttle cable and new intake manifolds. Once I replaced those parts the bike started right up and I was tinkering with the idle screws and making minor adjustments to the carbs. I took the bike down the block and back and it seemed to run well, but then the exact same thing happened with the RPMs increasing dramatically and I had to shut it down to avoid damage. Now, I'm not sure what to check. The carbs are brand new and tuned for my set up out of the box by TC Bros.

Can anyone point me in the right direction? I have the owner's service manual and the Clymer's manual for the bike and I do have a fair amount of mechanical aptitude, but I'm not certain how to troubleshoot this occurrence. I wanted to test ride some more, but I live in the upper midwest and it's really cold here now. The TX650 has an updated charging system I believe, but it still has the original points type set up. Any help would be appreciated. Thanks for your time.

 

[MaxPete]

Hi Chopper: I had a similar problem with my '76 Standard at one point during the wiring troubleshooting extravaganza. I had the tank on and off MANY times and somehow along the way, I put the throttle cable in a position that caused it to open the throttles whenever I went around a corner (it was....stimulating, when that happened the time I was right behind a city bus).

Try this:
- start the bike and let it idle;
- now turn the handlebars all the way right & left from lock to lock (without touching the throttle at all).

Does the idle speed change when you move the bars? It definitely should NOT.

On my bike, turning the bars to the right would cause the engine to race. All I had to do was pull just a little more slack out from under the tank - toward the front of the bike - and all returned to normal. In fact, I guess you could even do this test with the bike not even running.

On mine, I could actually see the throttle lever move as I turned the bars. BTW - I'm sure you know this, but the throttle lever is on the carb - left hand side - it is a little lever with the cable attached to it.

Pete

 

[5twins]

You probably need to service your ignition advance unit and the advance rod it connects to that passes through the head. These parts need to be kept lubricated, which pretty much no one ever did, lol. The advance unit is supposed to turn the advance rod about 1/8 or 1/4 turn as the RPMs increase, and that advances your ignition timing. It sounds like when cold, it could be sticking in the retarded position, hence your poor acceleration and performance. Once hot, it could be moving to the advanced position but then sticking there, hence the high RPMs.

 

[weekendrider]

BTW - I'm sure you know this, but the throttle lever is on the carb - left hand side - it is a little lever with the cable attached to it.

His isn't and yours wouldn't be either if you had his carbs.

 

[MaxPete]

His isn't and yours wouldn't be either if you had his carbs.

Sorry, I don't understand...?

 

[5twins]

He put VM carbs on it. They don't have cable arms, the cables go into the tops of the slide towers and connect directly to the slides.

 

[MaxPete]

You probably need to service your ignition advance unit and the advance rod it connects to that passes through the head. These parts need to be kept lubricated, which pretty much no one ever did, lol. The advance unit is supposed to turn the advance rod about 1/8 or 1/4 turn as the RPMs increase, and that advances your ignition timing. It sounds like when cold, it could be sticking in the retarded position, hence your poor acceleration and performance. Once hot, it could be moving to the advanced position but then sticking there, hence the high RPMs.

Yup - that very well could be Choppers problem. In fact, I'm going to have a look at my advance unit <insert embarrassed facial expression here>. I am ashamed to admit that I have never been inside that thing except to lube the weight pivots.

Nonetheless, the throttle cable thing was a real issue for me and adjusting the position of the cable under the tank cured it immediately.

Pete

 

[weekendrider]

Yes Max that can be a real problem for everyone. It is not limited to the XS650 nor is that problem only on CV carbs. That problem is usually detected without the bike warming up and I've never had one rev more than 1-2K rpm because of cable routing. But I glad you found it before you ran up a bus's muff.

 

[5twins]

Yes, I've encountered well lubed and greased advance units that were mounted on totally dried out advance rods, lol. Doing the advance unit is only part of the job. Yamaha never mentioned servicing these parts in any of the manuals but they should have. I guess maybe they didn't figure the bikes would be around this long, lol. It's not something you need to do often but it does need doing, I'd say maybe every 5 or 6 years.

 

[weekendrider]

OP watch this. It is 5twins was describing.

 

[MaxPete]

OP watch this. It is 5twins was describing.

WOW - that is a pretty graphic illustration of the problem. Thanks for sharing that little video.

Pete

 

[1973 TX650 Chopper]

Thanks, MaxPete, but I did replace the carbs and the cable and ended up not resolving the issue at all, so I'm positive the cable is fine. Thanks though.

Thanks, 5Twins. I was going to start checking the timing and points and hopefully stumble across the problem if I didn't get any good tips. The mechanical timing advance unit and rod will be the first thing I check when the temperature here gets back above a high of 5 degrees. Hopefully that is the issue and I can resolve it. I may not have needed the new carbs at all, but the old ones were original and I think the new Mikunis will perform a lot better once I can this resolved.

I love this bike and I enjoy working on it when I have free time. I'm sure I'll be looking for more advice as I go and I hope to help others that have questions I can answer with the experience of working on this amazing bike.

 

[MaxPete]

Okey doke - well, stick at it Chopper.

Pete

 

[1973 TX650 Chopper]

I did solve this issue. The advance mechanism was not working properly at all so I replaced the entire old (points) ignition system and upgraded to the Complete Pamco High Output Standard Ignition Kit SKU: 14-0912. I set the timing and now everything is working properly. I am having a little trouble getting the upgraded carbs from TC Bros dialed in, but I'll keep at it. Thanks everyone for the help.

 

[MaxPete]

Very good - you will nail all of this down and have a great ride there soon.