KZ440 with KZ400 B2 Engine with hanging idle and backfire problem

Hi everyone,

Despite a lot of research on this forum and others, I have a problem with my 1983 KZ440/400 that I can't solve. It must be simple but, being quite new to mechanic, I can't find the solution. Let me try and give you the most info I can.

Here is the deal. Last year, I bought a KZ440 LTD for a few hundred euros (I am French). It wasn't running when I bought it and after a few weeks of discovery/cleaning (carburetors mainly and electronics)I got it started. After a few more weeks, I realised that the engine was that of a Kz400 B2, with a kick (so I have no idea what's the real mileage on this bike). The meter says 9 000-ish... Another important info, it was missing the airbox cap, the part with the filter in it. A friend with a 3D printer managed to create one for me based on the dimensions of the original one (found on various forums) and I have now something that is the closest to stock airbox. I did valve clearance, checked the compression (150-ish psi on both cylinders), synced the carbs and adjusted the fuel levels with clear-tube methods. I set the pilot screw to 2 3/4 out as stated in the FSM. It was running well but maybe too rich (it lacked power and, despite reaching 100km/h, as soon as I went WOT in 5th gear, the engine would start losing power and I would have to release a bit of throttle to have it run smooth again). To adress that, a mechanic suggested that I give it a bit more air. I modified the openings on the air cap, making them wider, and it gained a bit of power but still this high-end problem at full throttle.

Recently, I realised that I wasn't running stock spark plugs. The PO had B9ES and when I had replaced those, I bought the same. Thinking that this might be the key to my high-end problem (remember I am new to mechanics), I bought the stock ones (B7ES NGK) and tried starting the bike. No luck. Worse, it now shows signs of running lean with a hanging idle and backfire on my left cylinder when I give a bit of throttle at idle. When starting it cold, it needs 30 seconds of choke to warm up and idles fine after that but as soon as I open my throttle, it stays in high rpms and takes some time to go back to normal idle.

I checked for leaks on the boots (there were none but I still replaced them with new ones) and on the vacuum lines, no success. Today, I tried replacing the spark plugs, putting back the old B9ES ones, but the problem is still the same. So, while this annoying situation started when changing spark plugs (this is litteraly the only thing I changed before the whole idling situation went crap), it doesn't seem to be related. Which means I am completely lost. One more precision, it has electronic ignition and not mechanical points and the timing advancer is well lubricated so I don't think that the problem comes from there (I checked the ohm resistance on ignition coils and the pick-up coil and it looks good). Also after testing the compression again today with the engine well warmed up, I noticed that my left muffler was less hot that the right one. And, last thing, I have a small oil leak on the top of the engine, where the tachometer is connected to the engine.

 Does anyone has a clue as to what might be happening here ? That would really help me. I attach a picture of my two carbs with new boots and ignition.

Thank you for your time.

A B9ES a pretty cold plug.
The in-cylinder temperature would have to be high to need a plug like that.
The cylinders would be running really lean.

Can we see pictures of all the spark plugs?

 

Here are the four plugs. The ones in the outdoor pictures are the B7ES used only at idle, I didn't ride with those on. And the B9ES that I rode all summer with. In each picture, the left spark is the one for the left cylinder.
 

The two B7ES don't look too bad.

The two B9ES look like they are too cold as they are carbon fouled.
You also may be running richer closer to WOT.
If you get too rich for the spark plug, then you will start to misfire because the spark plug will be covered in carbon.
When you lower your rpm the cylinder may heat up enough to clean the carbon off of the spark plug.
The spark plug temp needs to be between 500˚C-800˚C to stay clean of carbon but not overheat and cause pre-ignition.

The PO may have changed plugs to cover up an issue with the ignition system or carburetors.

Others will chime in.
 

Thank you for the input F64 !
So, based on what you are saying, the B9ES being cold + running too rich is an explanation for my lack of power when at WOT. And the change of spark plug was a good step towards a better response of the engine, right ? So why did it create a lean condition suddenly, especially since everything seems adjusted to normal specs ?
 

Your carburetor may be too lean in certain circuits.
You will need to do plug chops on new plugs to see which carburetor circuit(s) are lean and which are rich.

The B9ES plugs are too cold for your current air/fuel ratio.
The B7ES are closer to the correct temp for your current air/fuel ratio.

You may be able to cover half of your airbox opening with tape to see if it runs better using the B7ES plugs.
This will cause a rich richer condition than you currently have.

 

Will do ! Could you explain how I should do to check the different circuits with plug chops ? To my understanding the plug chop is going WOT on a straight road (in any gear?) and then pull in the clutch lever and turn the ignition off and coast until stop, then immediately check the spark plugs. So, should I also do a plug chop with throttle half open and throttle barely open ? Would that be the way to check pilot jet, needle and main jet ?

In your opinion, what is my hanging idle and popping issue due to ? A vacuum leak ?

Thank you again for having taken the time to answer me !

Alright, so I fixed the hanging idle/popping issue... one of my pilot jet was closed by some dirt... morality, always double-check.
I still have a loss of power as soon as I go WOT and I am not sure how to fix that.
Is it something to be adjusted by tweaking the main jet size (I have stock ones on my carbs) or is it a problem of coils (my coils and pick-up coil look alright to me but I am no expert) that might no produce a strong enough spark ?
 

www.cycleworld.com/story/bikes/kevin-cam...to-tune-carburetors/

You need to use new spark plugs for the plug chop.
You will be looking for a ring down inside at the bottom of  the spark plug.

rehermorrison.com/tech-talk-89-the-eyes-inside-your-engine/

Sorry for the late reply, didn't have much time with work and family.
I haven't had the time to do the plug chop but, I have some questions

I realized that, since I have a KZ400 B2 engine, some Keihin carbs that look like they are CV36 (how do you identify carbs btw?) and an airbox for Kz440 with a custom made air filter,
and given that the kz400 B2 is supposed to run with Keihin VB32 carbs and has a different airbox than the kz440, can I find the proper adjustment with the gear I have ? Or should I find new carbs and a new airbox (both are very expensive and hard to find)?

I am really inexperienced with all this mixture tuning (I read the cycleworld link you sent me F64, and it is really informative) and I would appreciate any help on that.

Thanks for your input !

The 36s should work.
One thing that concerns me is the intake boot being the same diameter at the cylinder head as the cylinder head opening.
I'm wondering if there is a step there because of the 440 boot and the 400 cylinder head.

Try this page.
www.factorypro.com/tech/tech_tuning_proc...wer_rpm_engines.html
 

So I am definitely looking at rejetting then ? Never done that before but based on the links you sent me, this is what I think of doing:

Given that I have a lack of power when I open the throttle 3/4 to  WOT, I assume I should change the main jet.
There are two main jets in the Keinhin carbs, the primary (I have a #70) and the secondary (#85).
Should I just go for one or two sizes down on each ? Or just change the primary ?

After that, I cannot adjust the needle clip as there is no clip on the needle in my carbs. Just a screw to hold it in place.
So straight to float height and, finally, just tweak the pilot screw (I have those at 2 3/4 turns out, as per the manual).

Does it sound like the proper thing to do ?

Another thing. Instead of rejetting, could I just assume that my airbox is not letting enough air in and open it more ?
Or is there just too much fuel going in anyways ?

Why is the diameter on the intake boot a concern ?

Thank you again F64 !