Bogging Down/Choking out issue

Hey KZRider,

I recently purchased a 1982 KZ750-E that has been in great running shape up until recently. The motor will start up fine and hold idle while it warms up and is being ridden for the first few miles. Once warm and the choke is moving down towards fully open, it will seem to bog down and die at stop lights. While in the process of dying at a stoplight I have tried to give it throttle which produces a bogging noise and doesnt increase the revs. When I try to bring the idle back up by pulling up on the choke, the revs go screaming out of control on the tach and jump to 5-6k. To counteract the high revs, I push the choke back down and it bogs down. It has done this twice to me over the last 30 miles. I have let the bike sit for about a week not having an opportunity to ride it. It was low on fuel so I gave it about a gallon from a jerry can I had and tried to turn it over today. Again it idled fine for a moment and then bogged down and died. After it died the first time today, it will no longer turn over and almost seems like it is not getting fuel anymore.

Has anyone had this issue or have any insight as to what is going on? I filled it with regular under advice from a friend I ride with then realized the prior owner of the bike had used mid-grade or premium so I am wondering if the bike is suffering from a symptom of the change in fuel grade.

Thanks in advance!

Once warm and the choke is moving down towards fully open, it will seem to bog down and die at stop lights.

Moving the choke leveler down towards fully close = no fuel/air enrichment = normal riding, (engine warm up). Moving the choke leveler up is moving fully open = fuel/air enrichment with higher idle for cold start.

What you describe can be caused by a very lean fuel mixture. This can be the result of clogged pilot circuits, cracked and leaking carb holders, missing air filter and such.
Your bike should run just fine on regular grade gasoline. Thhere is no advantage to using higher octane fuel than the manufacturer recommends.

By the way, the carbs on the 750 LTD have enrichners, not chokes. A choke closes off (chokes) the airflow, causing the mixture to be extra rich for cold starting. An enrichener allows more fuel to flow, thus creating a richer mixture. Two methods to achieve the same goal.

Thanks TexasKZ for the clarification

When was the last time the valves were adjusted? Do the basics first then move on.
Steve

Definitely start with basic maintenance .

Most folks who buy a vintage bike want to hop on and ride but I second the basic maintenance suggestion. Check the valve clearances and compression, change the oil/filter as well as brake fluid and examine brake hoses carefully. Installing new brake pads is also a prudent step. A new set of fuel hoses with inline filter is critical. Cables need to be lubed.

Once you do t his stuff,, and prolly some things i didn't think of, it is time for getting the bike to run right. You have CV34 Keihin carburetors. These carburetors are pretty good but have some things that need checked or serviced. #1, new primary and secondary main jets. #2. good plugs above the pilot jets and NEW pilot jets. #. internal passageways clear. 4. Set float levels to proper level. 5. The fuel tee and two fuel rails have orings which really should be replaced.

These carburetors are a bit fussy to get sorted but once sorted, work great. Operation on closed choke only indicates pilot (slow jet) circuit dirty and in need of clean/service. The rev up when warm with full choke on is normal.

You would do well to stay off the bike until it is serviced fully. Oh yeah, check the date code on the tires. It is on the sidewall in an oval and is four numbers. First two are the week such as 16 is the the 16th week but the biggie is the second two numbers which are the year. 02 means 2002, etc. If the ties are older than a year or two, no bueno, remove/replace.

Typically a change in fuel grade won't normally cause a good running bike to suddenly turn into crap. Of course if requirements call for premium and you fill with regular you probably would get some crankiness when you got on it in the form of detonation. You might have picked up some bad gas. Fouled plugs? But then I wonder why the PO used something other than regular. Like the others have said a valve check is first in order. You can't adjust the carbs without the calves in tolerance.

I would also find out what the bike requires in the form of fuel. Your fiend could be wrong. I have a bike that lives on regular and one that requires at least 90 octane.

Thank you for the reply! I have plans to dive into servicing the bike this weekend finally and will be address the valve clearance, oil, hoses etc. The carb stuff I need to do a little more reading on before I am comfortable with opening them up since I don't have much experience working on carbs. I have much to learn about working on old bikes!

One of the bigger questions I have on this bike with regards to service is the manual cam chain tensioner. The prior owner had installed an aftermarket CCT on the bike and the bolt looks like it has way too much thread left which makes me scared that an adjustment could result in a broken CCT. AS I have ridden the bike since purchase, I am starting to hear more and more "chain slap" from the motor which in my mind equates to the cam chain loosening over time. I have been searching for a good write up on adjustments of the CCT but haven't had any luck. Do you have any advice towards adjusting the CCT?

Thanks in advance!

I am not a fan of the aftermarket cam chain tensioners, especially in the hands of an inexperienced owner. It relies too much on feel that a new owner may not have. Suggest you buy a Kaw manual cam chain tensioner. It consists of a piston that is spring loaded and a set screw/nut. You install, let the set screw loose and the piston will zip out and contact the chain (the rear cam chain guide) and put pressure on it. Once it is out, you turn the engine over by hand using the nut on the ignition pickup up side of the crank (big nut; there is a small one as well) . Turn the engine over a time or two (this is best done with plugs removed as you can't really do it with full engine compression). Once the chain is tensioned by this process, screw the lock screw in and snug up the locknut. This becomes a maintenance item that needs done now and again but not often. There is also an "automatic" cam chain adjuster for your bike but they are a big complicated and a pain to set, if removed or put back in.

As far as advice if you want to muck around with the aftermarket cam chain adjuster, you loosen the lock nut and turn the adjuster nut clockwise to take up slack. How much? As noted, it is a feel thing and that makes how much hard to quantify. If the carbs are something you are not comfortable with, see my post in the vendor section... might help you out.

wiredgeorge wrote: I am not a fan of the aftermarket cam chain tensioners, especially in the hands of an inexperienced owner. It relies too much on feel that a new owner may not have. Suggest you buy a Kaw manual cam chain tensioner. It consists of a piston that is spring loaded and a set screw/nut. You install, let the set screw loose and the piston will zip out and contact the chain (the rear cam chain guide) and put pressure on it. Once it is out, you turn the engine over by hand using the nut on the ignition pickup up side of the crank (big nut; there is a small one as well) . Turn the engine over a time or two (this is best done with plugs removed as you can't really do it with full engine compression). Once the chain is tensioned by this process, screw the lock screw in and snug up the locknut. This becomes a maintenance item that needs done now and again but not often. There is also an "automatic" cam chain adjuster for your bike but they are a big complicated and a pain to set, if removed or put back in.

As far as advice if you want to muck around with the aftermarket cam chain adjuster, you loosen the lock nut and turn the adjuster nut clockwise to take up slack. How much? As noted, it is a feel thing and that makes how much hard to quantify. If the carbs are something you are not comfortable with, see my post in the vendor section... might help you out.

Thank you for the advice! Very helpful.