1980 KZ750G (Ltd, twin, whatever) weird lean condition, maybe? I'm flummoxed

Hey guys, I appreciate the expertise on offer here.

Here's my problem. I replaced a stolen EX500 with a 1980 KZ750 twin with a sidecar. classoc story: the dude i bought it from was a 90 year old lifetime rider and electical engineer whose doctor made him stop riding. So i figure this is finally a safe buy. 

Imagine my shock when riding the bike home as the bike starts to sputter and die above 1/8th throttle. It seemed like it was starving for gas, but i wasnt sure and the sun was setting, so I limp it home a quarter of a mile at a time. I can occasionally get it into 2nd gear at 1/8th throttle and it'll move, but if I open it up at all it bogs out and dies.

I first consider float bowl hight, but I think it might actually be running lean because the old man put pod filters on and can't remember the size of jets he installed. Also, he gave me what he thought was the shop manual for it, but it was in fact the Clymer's for the kz750 four. Red flag, I know.

So I find a complete airbox on ebay, order new boots from Great Britain, and check the needles while I wait. The clips were set at the 2 position, whereas the g1 supplement to the OEM service manual specs 4. So I change the clips, and it kind of improves but the condition remains the same. At this point, I'm starting to notice occasional backfires and surging at 3/4 throttle. I can't get it to run long enough above pilot to do a proper plug chop, so I figure I'll install the new airbox and that will at least give me some good info one way or another.

Well, everything arrived and I did the ridiculous job of half disassembling the bike to put the airbox in. Got everytbing buttoned up just in time for a sunset test ride and... no change at all. Except the enriching circuit seems to have more effect and I'm idling high. Like 5-6k. Still bogging and dying, still backfiring.

So I'm stumped. I checked the plugs and they're NGK BR6ES, which I gather should be running similar enough to the factory B6ES plugs. The boots are new and supple, the diaphragms are clean and in good shape, fuel is running fine. The bike pulls great when it surges and has good HP at 1/8th throttle in first and second.

What's my next move? Replace the jets and needle with known sizes? If so, anyone know what sizes and from where I should buy?

Thanks for the help. Enjoy a few pics of the bike.

Welcome to KZRider 
Your next post MUST  be in the new members intro area  as laid out in our  site rules.
Please read the post titled New Members Please Read
Regarding your problem  , You don't mention a complete rebuild of the carbs  to clean out the  small passages 
Also check for air leaks  in the  rubber  boots

Thanks, sorry for missing the site rules. Intro post made.

The previous owner did a full carb rebuild. I was hesitant to pull the carbs off and redo it if the pod filters were the cause. I've been working 70 hour weeks, so time is in short supply. I've never worked on CV carbs so I kind of wanted to leave well enough alone. More fool me.

I checked this morning and it looks like the left pipe is bluing and the right is not. I guess it's time to order factory jets and pull the carbs. 

I'll do the spray check for leaks when i get home tonight. They look really good, though.

Do you (or anyone else) know a good source for factory jets and needles? 

Before we get too far into this, did the bike run well at all when you were taking it home?

In other words, did it run well and then start acting strangely, or did it never run properly for you? You said "starts to sputter" so I think it ran okay at first but that's not 100% clear.

Welcome from the UK

I wouldn't have thought putting pod filters on was a thing an old man would typically do. They can work if they're set up right, but don't old guys usually stick to stock? 
Also, the fact he gave you the wrong manual is a bit confusing, unless he had a collection and was just getting forgetful.
Do you think it's a possibility the bike may have belonged to his son or grandson, and he was getting shot of it because it had been sitting there for ages?

In any case, in that situation what I would do is assume the pod filters were set up correctly, i.e. put the needle clips back to where you found them. Just service the carbs, get them completely clear, and see if the bike runs properly. If it does, leave it at that. That would be my approach - if he says it worked then see if that's true, because it might be true. If you want to return in to stock that's always an option but parts for rare bikes like that one are getting expensive and it may take you some time. Blocked passages in the carbs and blocked jets, and general maintenance, might be the culprit for the bike not getting enough fuel-air mixture.

Before we get too far into this, did the bike run well at all when you were taking it home?

 

In other words, did it run well and then start acting strangely, or did it never run properly for you? You said "starts to sputter" so I think it ran okay at first but that's not 100% clear.

I test rode it before buying and foolishly never got it out of first gear, low throttle. Didn't think to run through the gears because the sidecar was so new to me.
I think it has gotten worse, though.

Michi wrote:

I wouldn't have thought putting pod filters on was a thing an old man would typically do. They can work if they're set up right, but don't old guys usually stick to stock? 
Also, the fact he gave you the wrong manual is a bit confusing, unless he had a collection and was just getting forgetful.
Do you think it's a possibility the bike may have belonged to his son or grandson, and he was getting shot of it because it had been sitting there for ages?

In any case, in that situation what I would do is assume the pod filters were set up correctly, i.e. put the needle clips back to where you found them. Just service the carbs, get them completely clear, and see if the bike runs properly. If it does, leave it at that. That would be my approach - if he says it worked then see if that's true, because it might be true. If you want to return in to stock that's always an option but parts for rare bikes like that one are getting expensive and it may take you some time. Blocked passages in the carbs and blocked jets, and general maintenance, might be the culprit for the bike not getting enough fuel-air mixture.

I thought so too, but it's a two owner bike and I think he put the sidecar on for stability in order to keep running. I have the original bill of sale from the first owner.

I suspect someone took the box out and didn't realize how much of a chore it is to reinstall it, so went for pods.

I think the wrong manual was confusion because of the bike's name being all over the place from Kawasaki. The bike say "750 ltd" so he went and bought the clymers for that model, and just figured it was close enough, not realizing there was a 750 ltd 4 and twin.

I actually already sourced an airbox, cleaner, and boots and installed everything. Thats part of whats throwing me, because that amount of change in airflow should have some impact, you know? So if it's not airflow then it has to be "above" the lamilar flow in the order of operation,  right? Leaving spark and fuel (or even compression, but it seems solid).

So I suppose pull the carbs and clean them. Could I get away with pulling the jets and not pull the carbs, do you think?

Wookie58 wrote:

Welcome from the UK

Thanks mate!

Oh, and I noticed he'd put NGK BR6ES plugs in as opposed to B6ES. I imagine the resistor plugs don't run as hot as the stock B6ES, could it make enough of a difference to cause this behavior?

I ordered some non-resistor plugs but they won't be delivered til the weekend.

Resistor plugs are not a problem unless you also have resistor boots or wires. If just one of them is resistor, you will be ok.
Dirt or rust in the fuel tank could cause the problems you describe. Pulling off one of the float bowls will tell you for sure.
The factory service manual will be an invaluable tool.  www.kzrider.com/modules/ServiceManuals/K...l%20KZ750%20Twin.pdf

The carbs cannot be properly cleaned in situ. The pilot circuit, which dominates the mixture at small throttle openings, consists of some very tiny passageways that are very easily clogged with dried fuel, fine dirt or rust. The only way to be sure they are clear is to completely disassemble the carbs and carefully clean everything. Here are no shortcuts to carb cleaning.

I agree that the only way to clean the carbs is to take them all apart.  However, since the carbs in question are BS38s both the pilot jet and and main jet are located in the float bowl you can find what sizes of jets you have currently by removing the float bowls.  

Not sure if this link will be of help or not.  Here is a video on rebuilding a set of BS38s.  Keep in mind that the set in the video came off of a Yamaha 650 and may not be exactally like the ones from a KZ twin but they are very much alike.



As an aside I think, not certain of this but think the main jet is a N100.606 and the pilot jet is a BS30/96

For some more info on the carbs that came stock on the 750 twins check out this link   www.wgcarbs.com/index.php/using-joomla/e...-categories/93-KZ750