1981 kz550 ltd engine bogs down

First post on kzrider, thanks for having me. I'm also new as far as mechanics are concerned, I have a decent idea of how things work but nothing too technical.

Problem:

I bought this 1981 kz550 ltd off of craigslist the other day and rode it home. It has a few issues that I need to deal with and learn as I go, but there is one main one that I can't diagnose on my own. When I test rode it, I pulled into a cul-de-sac, stopped, and slowly turned it around, and the engine bogged down and died. Later as I was driving home (~30 miles) on backroads, it would die down in a few different ways. I would stop at a light, and it would idle fine, and when the light turned and I gave it gas, it would die. I would stop at a light, and it would die. I would be going 20, 30, or 45 mph and it would lose half power, and I would have to give it more gas to stay at speed, or I would be going any speed and all of a sudden full power would kick in and I would have to quickly let off the throttle to not run into the 18 wheeler going slow in front of me. Sometimes when starting up if I revved it up fast it wouldn't die, but if it had already died, it wouldn't necessarily work. Also once it died and I couldn't start it for about 20 minutes, and I felt like the engine was flooded. I turned the throttle all the way and held down the ignition button and eventually it caught.

Another issue is that I see condensation that looks like gas on the inside of carbs where my pod air filters fit on. I rode it home without the filters, which I shouldn't have done, not sure how that would effect it. The vacuum petcock seems to work fine, except maybe be slightly leaky in the 'on' position (very slightly, one drop every 20 minutes, if that). On 'pri' it flows fine. I had left it on 'pri' for a while so maybe it was that extra gas in the carbs condensating out, unsure.

Reading up on the forums and talking to my mechanic friend, it seems like a fuel delivery problem. The guy I got it from said he cleaned the carbs and put new jets/gaskets on all carbs (and he seemed like a trustworthy kid), but maybe he didn't adjust them right. I'm also not sure how the carbs/exhaust system work together, and I should probably just take them apart and clean them anyways. I already downloaded the shop manuals. It seems like the floats could be getting stuck. However, some have mentioned on the forums that it could be an ignition problem as well, or coil connectors.

After I replaced a cracked fuel line and fuel filter today, I took these videos of it idling/revving fine, and then dying out:

running ok:



engine dying:




Any ideas of what I should attack first? Clean the coil connection, buy an Ohmmeter and check the electrics, rebuild the carbs and adjust the jetting, take it to a shop and get the carbs balanced, or other?

Many thanks.

Seems like a carb problem.

looks like this bike has CV carbs if not tk22 carbs, they are very different if you have cv carbs take note that the carbs are very sensitive to adjustments made with the air box and will not run correctly on pods without you being a carb tuning wizard. Without the stock air box the slides will not function correctly and the bike will be running very lean and run like shit.

On another note these bike arnt designed to run on the prime position because the carb needles cannot hold back the pressure from all the fuel in the tank this can cause the carbs to overflow and if they dont have a drain tube it will go into your engine.
Try to always leave the petcock on the ON position unless you run the carbs dry then leave it on PRI for 10-15 seconds till it starts then switch it back to the ON position.

I suggest you smell your oil to see if their is petrol contamination,
It's also be worth taking out you spark plugs to check their colour and confirm that you are running lean.

From my experience always start from square one and follow the directions of your factory service manual and go through the maintenance procedures, eliminating other systems of you bike other wise you will end up chasing your tail..

Very hard to diagnose from a distance, but really with any old bike, new to you, you need to check everything over. A lot of years have passed from when it was new for all sorts of things to have been butchered , get gummed up or wear out. From purely a safety point of view I would advise a thorough once over before riding far and at speed. Get a manual and do all the service and maintenance checks they advise,

This is most likely a fuel, carb issue, but I would advise starting at the petcock to ensure you have a steady flow of clean fuel. I would try running it with the petcock on Prime, the drastic changes in power could well be fuel starvation if the vacuum from the engine is not opening the petcock properly. Check the vacuum line to it. I would then check your float levels with the clear tube test before proceeding further to make sure the levels are the same and none are overflowing. If the float levels are wrong, or the floats bowls overflowing and putting excess fuel into the carbs and engine then you will get nowhere trying to do anything else, until you have that under control. You were told the carbs had been cleaned and reconditioned, but don't assume if was done properly or that they really knew what they were doing. Check for any air leaks on the boots where they connect to the motor,

There is a possibility of an ignition issue too, cutting in and out so you lose power, so if you are getting fuel flow and the carbs are not flooding then I'd check the possibilities here, you may have dirty and bad contacts in several areas from the kill switch to the coil. For what it's worth watching a video and listening to it through computer speakers it sounds like a fuel issue.

When were the Pod's put on they could create a situation where the original carb settings could make the bike run really badly in itself as could ill thought out adjustments by the PO trying to dial them in.

razmo99 wrote: Seems like a carb problem.

On another note these bike arnt designed to run on the prime position because the carb needles cannot hold back the pressure from all the fuel in the tank this can cause the carbs to overflow and if they dont have a drain tube it will go into your engine.
Try to always leave the petcock on the ON position unless you run the carbs dry then leave it on PRI for 10-15 seconds till it starts then switch it back to the ON position.

I'm not sure I agree with this statement, the carbs have to have enough fuel going to them when in the "on" position to keep the carbs fed, I'm not sure that's any different than being in "Prime", if the motor's running. The carb float needles control the fuel flow not the petcock.

Even in your "normal running" video it sounds to me like it is not firing all 4 cylinders. Ed

First of all put the pods back on. They won't run optimum with pods but it's a lot better than no filter at all.

Second video it is not running on all 4. Definite miss. You can hear the cam chain loading and unloading, Then the way it dies it sounds like it's out of fuel. I'd start with the petcock in PRI position and ride it around a bit. It won't hurt to run it in prime just don't leave it in prime with the engine off. If it's better then you should be checking the vacuum line, I think on #2 carb body and make sure it's not cracked, If you don't find any cracks and it is hooked up to the carb and the petcock then I'd test the petcock for proper operation. should not flow fuel in on or res until vacuum is applied to the vacuum line. But should flow fuel in PRI position.

Be sure and let us know what you find.

redhawk4 wrote:

razmo99 wrote: Seems like a carb problem.

On another note these bike arnt designed to run on the prime position because the carb needles cannot hold back the pressure from all the fuel in the tank this can cause the carbs to overflow and if they dont have a drain tube it will go into your engine.
Try to always leave the petcock on the ON position unless you run the carbs dry then leave it on PRI for 10-15 seconds till it starts then switch it back to the ON position.

I'm not sure I agree with this statement, the carbs have to have enough fuel going to them when in the "on" position to keep the carbs fed, I'm not sure that's any different than being in "Prime", if the motor's running. The carb float needles control the fuel flow not the petcock.

If you think of it this way when you have it on prime the fuel valve is open 100% giving maximum potential flow.
When the bike is idling it doesn't need this much fuel, the vacuum diaphragm is actuated based on the speed of the engine. Faster you go the more fuel the carbs get. It ensures your carbs get the amount of fuel they need but not too much.

Dont get me wrong you can run the bike on prime but it can cause problems if your needles leak and, by making sure it is working you make sure you dont have a vacuum leak on number 4 cylinder

Thanks everybody for the responses. I will eventually be testing and rebuilding the carbs as suggested because I really don't think that I can trust the kid's work. Also, I have TK carbs, not CV. I'll also be looking into all the contacts in the ignition-coils-plugs to make sure that they are clean as well, but I've never dealt with anything electric since middle school so that'll be a lot of learning.


First off, I've never dealt with a vacuum operated petcock before, so looking it over, replacing the fuel line, and researching online (and now just reading that last response by jdvorchak), I realized that the vacuum line is disconnected, actually non-existant. The attached picture shows what the line looks like now.

What happens when the vacuum line is disconnected/capped? I guess that means that I can only run it in PRI, but will the rest of the engine stop working correctly as well?

Also, Is the vacuum line the same type of line as my fuel line, or do I need something special?

Again, thanks for the responses, and I'll keep everybody updated as I go through it. Turns out the guy I got it from doesn't know too much.

Assuming that the cap is air tight it will have no effect on engine performance.
You will have to use the PRI setting to get fuel into the carbs, until you connect it backup.
Vacuum line can be purchased from an automotive store, if you have the correct diameter fuel hose it would do the trick in the interm.

Since you have TK22 carbs that are a slide type and from what I have read are easier to tune for pods.

As jdvorchak suggested do put the pods on some air filtration and restriction is better than none.

razmo99 wrote:

redhawk4 wrote:

razmo99 wrote: Seems like a carb problem.

On another note these bike arnt designed to run on the prime position because the carb needles cannot hold back the pressure from all the fuel in the tank this can cause the carbs to overflow and if they dont have a drain tube it will go into your engine.
Try to always leave the petcock on the ON position unless you run the carbs dry then leave it on PRI for 10-15 seconds till it starts then switch it back to the ON position.

I'm not sure I agree with this statement, the carbs have to have enough fuel going to them when in the "on" position to keep the carbs fed, I'm not sure that's any different than being in "Prime", if the motor's running. The carb float needles control the fuel flow not the petcock.

If you think of it this way when you have it on prime the fuel valve is open 100% giving maximum potential flow.
When the bike is idling it doesn't need this much fuel, the vacuum diaphragm is actuated based on the speed of the engine. Faster you go the more fuel the carbs get. It ensures your carbs get the amount of fuel they need but not too much.

Dont get me wrong you can run the bike on prime but it can cause problems if your needles leak and, by making sure it is working you make sure you dont have a vacuum leak on number 4 cylinder

I agree it might make a difference on a bike where the carbs need attention if the fuel flow were restricted by the petcock, I don't know if in reality it happens, but it shouldn't on a bike without carb issues and properly functioning floats and needles. I'm sure the same carbs were used on bikes with a normal petcock and many people do away with vacuum petcocks, replacing them with the manual type and they work fine. The only purpose of the vacuum petcock was so people didn't have to remember to turn the tap on and off and it probably had the benefit of preventing carb flooding as this often is at it's worst when the bike isn't running and is on the side stand.

jostml wrote: Thanks everybody for the responses. I will eventually be testing and rebuilding the carbs as suggested because I really don't think that I can trust the kid's work. Also, I have TK carbs, not CV. I'll also be looking into all the contacts in the ignition-coils-plugs to make sure that they are clean as well, but I've never dealt with anything electric since middle school so that'll be a lot of learning.


First off, I've never dealt with a vacuum operated petcock before, so looking it over, replacing the fuel line, and researching online (and now just reading that last response by jdvorchak), I realized that the vacuum line is disconnected, actually non-existant. The attached picture shows what the line looks like now.

What happens when the vacuum line is disconnected/capped? I guess that means that I can only run it in PRI, but will the rest of the engine stop working correctly as well?

Also, Is the vacuum line the same type of line as my fuel line, or do I need something special?

Again, thanks for the responses, and I'll keep everybody updated as I go through it. Turns out the guy I got it from doesn't know too much.

That would explain one of your problems if you weren't running on "Prime" the bike would only have what ever's leaking through the petcock to run on.

It will still die on me in PRI, just not as often!