Is there a fan on the back of your LED head light bulb? If so, that's probably your compressor sound. I would check to be sure the battery is fully charged, then look at reversing whatever wiring you did.

 

Not sure if the LED headlight has a fan but it's not plugged in and not on the bike anymore I switch back to the incandescent bulb. Also since it's an older bike I haven't been using the battery on it it doesn't require the battery to run. I've undone the wiring changes that I made tonight if I have time I should be able to do some more testing to see if it's running better now or if it still has the same issues.

 

Just an update, I was going through the carb again to make sure it didn't have any debris in it and realized that the last time I put it on I left the o-ring off between the carb and intake manifold. Going to put that back in later today and see how things are running. Hopefully that was the issue.

 

That was your surging almost for sure, your intake vacuum would be bad and it would run lean.
Might want to check your alternator coil outputs once in a while to make sure they all work still.

do you know what is inside a full wave bridge rectifier? <- 4 diodes, you are using diodes to power diodes. LED is just a diode that happens to emit light as well as heat, they run on about 1.5 volts of reverse bias voltage.

 

Let us know

 

I have a Suzuki TC120 that I've been restoring and I had everything up and running and tuned in pretty well and the bike sat for a week or so unused while I was getting some parts in. I decided to try and add a negative ground 12V DC circuit to the bike to try and power an LED headlight and LED turn signals but using a bridge rectifier and a 2200uf capacitor to stabalize the voltage. It worked fine at first, but then the headlight stopped working (headlight itself is still good, but the bridge rectifier/capacitor no longer power it. I only mention the electrical work in case its involved with the issues I'm having now.

I took the rectifier/capacitor off and left the light unplugged just to start up the bike and ride it during the day, but then I began having problems with the bike running. It would turn over but wasn't idling correctly, I'd have to keep revving the throttle and the bike wouldn't always rev properly, it would tend to hunt and surge, rev up a bit, then almost die, rev up again, then almost die until eventually it would just sputter out. I've noticed a few times that right after the bike dies I hear what sounds very similar to a loud blast of air from an air compressor. I'm not 100% sure of exactly where the sound is coming from though, although it seems to be the exhaust.

Any ideas what could be causing the issue and where that air blast noise is coming from?

I'm thinking worst case scenario I may have blown the capacitor on my stator (I have a spare, I just don't like to mess with stator wiring unless I have to), which could be causing issues with the ignition spark.

Also, does anyone know of any good kits/instructions for converting a 12-16v AC circuit to a negative ground 12v DC circuit?

It hurts to read this, why are you trying to make 12 volts DC on a bike that was designed to output 6 volts when you only need 5 volts to power LED devices ? LED can even run on AC, it will just flicker visibly because LED's don't have latency, they are solid state. They sell AC LED's if you like, they just team 2 regular LED's together in opposite direction to each other. Your stock alternator is barely up to what you are wanting to do too because I don't think you have a 3 phase alternator, from memory you have 2 coils, one with a centre tap and I think only one is critical to ignition spark if the battery is in there to carry your lights. You could try raising you idle a whole bunch that might take away the surging and if it does the problem is definitely the capacitor not charging fast enough.

 

In hindsite, I have no idea why I decided I HAD to do the headlight/turn signal wiring in that fashion. Moving forward I'll just try and find LED equipment that runs fine off an AC current. At this point I'm fairly certain the issues I'm having aren't really related to the electrical changes I made and have since undone. Unfortunately I broke one of my own rules and changed several things without testing between each step which makes diagnosis harder but at this point, I think everything is pointing towards an air leak/gasket issue. Right now the spark is good and when I try to start it it will give a few good pops and tries to start but doesn't start running/idling. The smoke coming from the tailpipe is pretty white, although I don't hear the decompression/backfire noises like I had been. So far I've noticed that one of the gaskets on the intake manifold is cracked, the gasket between the top end and crack case is broken in 2 places, and the head gasket seems to have some fowling on it and I don't think its making a good seal. After trying to start the bike last night I took off the top part of the top end and cleaned all the metal surfaces and the copper gasket to make sure it could get a good seal. Tried starting the bike again without luck, checked the head gasket again and noticed that it was oily on the top and bottom parts of the gasket, like some of the oil was able to get into the space between the gasket and metal surfaces of the top end.

I've ordered replacement gaskets that should be in later this week and hopefully installing those will fix the issues I'm having. Here's a short list of the changes I made before having these issues:

1. Used bridge rectifier/capacitor to add negative ground 12v curcuit for headlight (have since undone those changes)
2. Took off carb and cleaned it multiple times, noticed the O-ring between carb and manifold was missing and that the gasket between block and intake manifold was cracked(will replace when new parts come in
3. SInce I was taking things apart I replaced the old piston with a new one I had ordered, noticed the gasket between the top end and crankcase was broken in two places. Compression with new piston seemed fine (around 125 PSI) but I've since put the old one back on since it had a slightly larger diameter than the new one and also since it was working with that piston in the first place.
4. I drained the crankcase oil and refilled it with 20w-50 ( I couldn't find 20w-40 anywhere)

 

From what I remember the TC120 has a copper top head gasket that crushes after the first use. The bottom gasket was paper and super easy to damage.
It's posi-force oil injected 2-stroke so the engine only needs to service the transmission and clutch, you don't even need multi-grade oil for that you could use hydraulic/transmission oil same as they use in a small farm tractor transmission.

... I wanted a TC120 real bad when I was a kid but I had the TC90 instead.
... and they ate spark plugs real fast.

 

Ahh ok. I had a hard time getting information on exactly what type of fluid I'd need to put in the crank case since I can't find a manual for the TC120 online anywhere. Would ATF Transmission oil work fine?

 

Bike won't work good if your bottom end seals are bad, air pressure will leak out there and the engine will lack for everything.

 

Oil level screw will be the funny looking hex/star head one with the fibre washer right in front of the kick start, bike needs to be level to use that.

 

Service the clutch engagement assembly down inside the engine too, it's steel on nylon, Sil-glyde lubricant works good for that. If the clutch engagement still feels mushy after that replace the cable. It will be behind the rectangular cover on the left side engine case. It's the multi-start screw thread part in nylon that needs cleaning and lubricating fairly regular to keep the clutch nice.

 

Ok, I got it filled with hydraulic transmission fluid and my gaskets came in today so I have brand new gaskets for the top end, cylinder, and carb. I got everything back together and tried kicking it over and so far it still hasn't been able to start, although its humid as hell right now so I only gave it a dozen tries or so, it may just need some fine tuning. Fuel flow is good, have a good spark, and cylinder compression seems good. The onyl issue I noticed the other day that I hadn't mentioned before is that when i took off the exhaust from the top end there was a fair amount of inky black oil in the exhaust so I let that drain out before putting everything back together today.

 

Compression test should tell you if the bottom end seals are any good, the oil/water/carbon in the exhaust is normal you are going to want to clean that all out occasionally. Other then that the spark has to happen close to the right time.

 

Hmm, I think I'll look into the timing aspect of things. I did notice that the stator plate has some room to move to the left or right by a quarter inch or so where the mounting points are. I'll try adjusting the position of the plate to see if that has any impact

 

I seem to recall using a meter to set the ignition timing as providing the best results, and measure the point gap along with it. The points and condenser need replacing occasionally if you can still source those.

 

Well I decided to get a kit for new points and a new condensor, if anything it will be nice to have around even if I end up not needing them. I've found that rotating the stator plate can help (the engine will start to ilde for a few seconds then die as opposed to not kicking over enought to Idle) but I haven't found the sweet spot for it yet. Not sure if the stator/crankcase have markings on them to show what the default stator location should be for normal timing.

 

Isn't there a mark on the flywheel for TDC and another mark several degrees preceding that one?
I have a service manual for the TC90 and it has 2 pages dedicated just to the ignition timing. Standard ignition advance on that bike is 20 degrees advance and they give a piston distance from TDC as 1.96mm
The manual outlines the process for setting timing with either the meter or a timing light. Highly recommend you procure the suzuki service manual for your bike.

 

Ok, got my new contact points/condensor installed today and it started right up, so at least I'm back up and running again. I guess my main questions now is how can I power an LED headlight using the original electrical system without modification? The problem I had run into earlier was that all the LED lights that I have seem to require a negative ground lead to function. If that's the case can i run them directly off AC? Could that potentially cause issues with the LED's burning out?

 

If you are using LED lights anyway, why don't you just stuff some batteries in the headlight shell and run it dead loss, nothing says you need to have a way to recharge batteries that power your lights. Or you could just install a rechargeable battery and plug it into a tender. Might as well get use to that, it's what they want us doing with our cars soon.
Most important electrics are for the ignition.

AC won't make a diode burn out any faster but if the power supply has a cycle that is slow enough you will see flicker because LED is a solid state device it has no illumination latency like an incandescent lamp filament. Over-voltage will take out an LED and that is why they work better on a well regulated power supply. Diodes are one of the most frequent components to fail in my experience. Made a lot of money replacing toasted diodes in one thing or another.

 

That air compressor sound and poor running still concerns me your bottom end seals might be leaking. Or did you replace them.

 

I got the gaskets on the top end replaced and I'm not hearing that air compressor noise anymore. I'm thinking that was related to the timing being off. Once I'm done with work I'm going to take it out for a ride to see if its running better now. It seems to be idling better than it had been

 

If the bottom end seals are good it will idle with a nice crisp pop, pop, pop and produce good power.
If the bottom end seals are poor it will idle uneven, sound more like ying, ying, ying, ying and not return to idle nicely.
... and it will be harder to start.

Sounds like you are well on the way to getting it sorted

 

Took it for a test ride yesterday and the first one went well, although it would bog down a little bit in second gear sometimes. Came back out for another ride later on and had a hard time getting it running, bike would bog down while giving it throttle. I could ride it a bit but it would almost die while giving it throttle, then lurch forward, start running, almost die,etc,etc. When I get home today I'm going to take the carb off and give it a good cleaning, my guess is that the gas had been sitting for a while and some junk settled in the bottom of the tank, got sucked into the carb and is partially blocking fuel flow. I had also changed the tip on the main jet at one point, so that may be causing an issue too, but at this point I think any issues in seeing are fuel flow related.

 

Well I played with it a bit more last night and it seems a bit better, but still off. Here's a link to a video I took this morning of it running, it still seems to hunt and surge a bit and the throttle is inconsistent.

 

Dang, I’m coughing just looking at the vid. Is that with any choke on or have you tried it with some choke?

 

Ok don't do that any more, it's painful to watch.
Ignition sounds way advanced so that means the points and condenser don't sound at all right. Audio recordings indoors of motorcycles is sketchy at best. My totally wild guess from thousands of miles away is your point gap is too close

Work on getting it to idle first, it almost sounds like it wants to. That is all about the ignition timing, the air screw (low speed jet) and the carburetor slide.
Your air box is off? I can hear the induction on the carburetor way more then you should, you won't be able to tune the carburetor with a wide open intake, note the rubber snorkel connected to the carburetor is shaped like a velocity stack for good reason and there is function in an airbox design.

... and your bottom end seals are still in question until you do at least a simple compression test. I bet you can rent a compression tester if you don't want to buy one. Sticking your finger in the plug hole and turning the motor over does not count as a compression test.

 

That was with no choke. I've tried with and without choke but the result is about the same. I have an actual compression testing gauge I can use once I get home just to verify everything looks good. Last I checked I was getting a pretty consistent 125psi. I don't have a leak down tester, although I'm not sure if I would need that.

It may still be timing related, I was going to try rotating the stator plate a few mm at a time to see if it gets better/worse.

Right now I have a 'performance' air filter attached to it that's was lying around, but not the stock air box. The stock one was really dirty and the inside was full of rust flakes, but I can get it cleaned out when I get home and try it with the stock air box.

If the ignition is too far advanced can I adjust that by rotating the stator clockwise or counterclockwise? Do you happen to know of any good videos/resources showing how to adjust the point gap?

Also, just FYI,I was able to find a service manual and get it ordered, bit it probably won't be here for a few weeks.

 

I have the TC90 service manual and your bike is very close, they say set point gap to 0.35 first
Then use timing light and meter to verify the timing mark on the crankcase lines up.

This OEM manual is very good, but sadly they don't give any details on compression testing, mostly critical inspection of the individual parts

Important part on the air intake is the rubber cone part, the rest you can get fancy with, the intake does not require restriction but the service manual will assume no changes to the bike so you might need to adjust things differently to accommodate changes. Your bike is very simple piston port type, not a lot of things to go wrong with no valves at all, but not the most efficient system either.

 

Ok, finally some good news. Thanks to your help I was able to get the point gap set correctly. I also noticed that when I was tightening the spark plug boot that it read causing the actual wire to twist instead of screwing the boot in more, so I trimmed the twisted part of the cable and reattached the boot. Finally I noticed some black fuel/oil mixture bubbling past the spark plug so I used a 14mm chaser to clean out the threads on the spark plug hole, cleaned up the spark plug threads and got the plug back in and there's no longer any leaks there. It's running better now than ever. Only real issues now are that after riding it for a few hours and having it sit I took it back out for a ride and it seemed more sluggish, like rpms were running a bit higher but not getting the same speeds so I'm going to adjust the carb/throttle/idle mixture to try and tune things in. I've also noticed that that one in a while when shifting to second gear it doesn't actually shift. I may need to tighten the clutch cable a bit more (or loosen it?). I'm also going to re-read your comment about lubing the clutch to see exactly what I should lube up.

Also if anyone knows of any decent led headlights that will run on ac power let me know. Right now I'm using a tractor utility light which is nice for letting other drivers see me, but not that great for illuminating the road.