Misfiring Issues

83 KZ750 LTD H4 - Cut to the chase, my bike has a misfire that isn't constant through the RPM range. In the morning, the bike is a little rough to start up - too much choke and it wants to just shut off, but not enough choke and it's going to shut off. Not sure if that part is common or not, this is honestly my first vehicle with a carb. Most mornings I need to have the choke half way, get it "started" (stumbling and misfiring), then roll on the throttle a little to bring the RPMs up. After about 10 or 15 seconds, the bike will idle high which I think is normal with the choke on. I lower the choke as the engine starts to warm up over the course of the next 4 or 5 minutes.

- The bike will rev up just fine - no stumbling at the first flick of the throttle.
- Once I put a load on the bike and take off from a stop, the bike will sputter and misfire until it reaches about 3000 rpm. The misfire doesn't disappear so much as it fades away right about 2800 rpm, then maybe a misfire or two until 3000 (like when I am riding around my neighborhood and the RPMs are low, 2800 - 3000 is when it starts to clear up).
- 3000 - 4000 RPM, the bike runs great. No problems, no matter how much throttle is on.
- Then 4000 - 5000, the misfire kicks in again under normal cruising throttle. I mean, as soon as the tack hits 4000, there is nothing from one ( or two?) of the cylinders. It is really weird that is so cut and dry like that. Then about 4800 - 5200 it starts to clear up, and then the engine is golden. If I have 40% or higher throttle in that RPM range, the misfire doesn't happen as often, and if I were to hard accelerate in say first and second, there would be no misfires in this range. Under extremely low throttle, like just trying to maintain speed or a slight deceleration, there doesn't appear to be a misfire, although I can't hear it that well.
- 5000 RPM - Redline, the bike runs great. I mean, I think it could be running great and it’s not, but on the highway it has no issues accelerating at all in fifth gear, the bike will run well above 85 mph. Under a hard launch and wide open throttle, the bike has no issues in high RPMs and pulls strong.
- Playing with the choke does not fix any of this. This will happen when the choke is on or off.
- The bike runs better in the morning when the air is cooler, but yesterday got up to almost 80 and it was definitely worse. Engine temp doesn’t seem to matter too much, meaning it will act the same whether I’m riding the bike just after it was started or if it has been running for a half an hour.
- The bike is completely stock - stock exhaust, stock airbox.

I just got the bike a short while ago, and it wasn't running when I first got it. I changed the plugs and wires, changed the oil and oil filter, drained the gas and put some new gas in. When I changed the plug wires, I bought the 7mm wire and NGK caps from Z1E and reused the stock twist on caps at the coils. I honestly hate this design, to me it is not a robust solution, and at first I thought it was a poor connection between the wires and the coil, so I pulled everything back off and made sure of a good connection (as sure as I could be).

Could my carbs need to be cleaned? On one hand, I have no idea the condition of the carbs, but I feel like if the carbs were gummed up, the bike would run for crap at high RPMs. Also, if it was a carb issue, I feel like the choke would help relieve some of these issues. I thought it could possibly be a coil pack that took a crap, but if that was the case, I feel the bike wouldn’t idle right, and it also wouldn’t matter what RPM range or throttle position I was in.

I am at a loss here. I don’t like just throwing parts at the bike just to try and fix it, I want to understand what the issue is and address it. I’m sure the carbs could be cleaned and the coils could be replaced, but I wanted to wait until the winter time so the bike wasn’t down for a bit this summer (I'm sure it will take me a while), but I would rather have the bike running right. Any help or insight would be much appreciated!

Adjust the valves first. If it's been sitting clean the pilots in the carbs. Coil could be starting to fail but you won't know unless these other things are done.
Steve

Too many unknows in play here. I agree with doing some maintenance, like checking the valves, but also pull the carbs and go through them too. Hold all the jets up to a light and make sure the orifices are open. Pull the pilot screws and spray out the passage, then reset them at 2.5 turns from lightly seated. When everything is done be sure to vacuum sync the carbs. Replace the air filter if it's dirty. Check voltage at the coils.

Regarding the plug wires, did you screw the wires on at the coils. There is a threaded spike on the coil and you need to work the wires onto the spike before the screw caps are installed.

HI guys - thanks for the replies! Forgot to add a bit of information...

Voltage at coil packs was 0.1V less than battery voltage. Resistance across the primary coils was 2.9 ohms.

The second time I took the wires off the coils, I did screw the physical wire onto the spike, gave the wire a gentle tug to make sure it was on tight, then screwed the cap down.

Not trying to argue, just for my understanding - any reason you suggest to adjust the valves first?

First off they are the most neglect thing on these bikes. They take time, effort and even money to do so the problem must lye elsewhere. :S If too tight they will cause symptoms you're describing. Also the first thing to rule out. You've already dealt with the coils so carbs would be next. Check the spark advance that it is not sticking and moving freely. One thing at a time and you will get there.
Steve

If you need a manual go here www.750turbo.com/manuals/ and the GPz 750 Base Manual down towards the bottom of the page will cover your model. It has a good section on the pilot circuit in your carbs

If you dont have one already a fuel filter is a good thing to have on these old bikes

Not adjusting the valves is one of the primary reasons these old bikes hit the scrap heap; with mileage the clearance reduces and at some point the valves don't close anymore and burn. For some unknown reason a lot of people think adjusting the valves is optional, even though the factory maintenance schedule clearly states the contrary.

Adjusting the valves is always one of the first things I do when rehabbing an old bike, and more times than not there are some valves without any clearance.

BTW, I bought a spare engine off a member here while rebuilding my 750E. My idea was to break it down and hand select the best parts between it and my original engine during the rebuild process. At some point while this was ongoing another member here needed a cylinder head so I sold the parts engine head to this guy. Shortly after receiving it he reported that there were chunks of valve seats missing. Stupid me, I didn't pull the valves and look before shipping off the head. I wound up refunding this guy his money, including the considerable shipping cost. Reason for mentioning this is because (with hindsight) I'm certain that the reason the head failed is because the valves were never checked and the seats burned.

Wow, that was a poke in the eye. :blink:
Steve

Thank you again everyone for the quick response.

Sounds like I really should just suck it up and do everything now. Inspect the shim clearance, while I am waiting on new shims to ship (I can almost guarantee at least one will be out) I can pop the carbs off and give those a run down. Also, I would like to check out the spark advance like Steve mentioned - the cover for that is damaged, I wonder if inside there got damaged as well... Halfway contemplating just changing out the coils as well, because new coils never hurt anyone and they are probably due anyway.

That's okay, it will be raining the end of this week anyway, so maybe I can start to tackle this stuff now and over the long weekend and the bike won't be down for more than a couple weeks...

I appreciate the advice - I will admit, I did not think adjusting the valves was super critical. Owning cars with hydraulic lifters my whole life, I obviously never had to even think about it. I honestly thought adjusting the valves was only something you did when you rebuilt the engine and had new cams, new valves, ground seats, etc.

Definitely check undrr the points (eldctronic ) and advacer cover particularly if damaged. Theres only 2 bolts holding it on. I woul ddo that before even looking at carbs

Just in case anyone is searching through threads in the future, I thought I would just cap this one off - I took the carbs off the bike and tore them down for cleaning. The carbs were pretty clean like someone had gone through them previously. Turns out one of the carbs was actually missing a plug for the pilot jet. I replaced that (with one from another set of carbs I got off of a parts bike), cleaned the carbs really well, then put them back on the bike. The bike runs fantastic now. Thanks again to everyone for their help!!

7-Fiddy LTD wrote: Sounds like I really should just suck it up and do everything now. Inspect the shim clearance, while I am waiting on new shims to ship (I can almost guarantee at least one will be out) .....

Hmmm.... How would you know what shims to buy it you have never checked the cam clearances?
Do you have a factory service manual? Even the small owners manual probably has a section on cam lobe clearances and how to estimate what shims you need to compensate for normal wear.