TOPIC:

Hey all,

For those who have been seeing my posts about my 1981 KZ440, it's starting to come along. At the moment the bike starts and holds an idle just fine, so we're making progress.

The problem I'm having with it now is that when it's cold, there is almost NO power from the engine. It starts up fine, but when revved it sputters and won't make it past about 4k until it warms up a little. Then, once it's warm enough to rev freely across the rev range, if you put it in gear and try to get the bike moving you damn near need to redline the engine with a ton of clutch slippage to actually get the bike rolling. Once moving (I did manage to take the bike on a short ride to see how it would act), it coughs a little bit around 1/4 throttle. Once ridden for about 10 minutes, the bike doesn't stutter nearly as much, but still suffers from a slight loss of power.

Additional info:
-The bike starts on full choke and holds a steady idle, but coughs hard when given any throttle when cold. After warming up for about 5 minutes, it will hold a steady idle off choke and begins to respond better to small throttle blips.
-I've cleaned, soaked, and poked fine wires through every single passage way in both carbs multiple times. The diaphragms don't have any holes, and the factory jets are installed and clear.
-The idle mixture jets are set to factory settings.
-Stock airbox is installed and securely connected to the carbs.
-New coil, new plug wires, new plugs are all installed.
-Timing advancer opens freely, was recently cleaned and lubricated.
-Top end was recently rebuilt by yours truly, with new gaskets (base, head), a cylinder hone, new valve stem seals. Piston rings, cylinders, and valves were all within spec, so they were all cleaned and kept.

Alright, I know that's a lot of info but that's about everything that I could think of that might be relevant. Any ideas?

That sounds normal for an old school carberated bike. The air fuel mixture is set for when the engine is warmed up not cold. My KZ650 runs like shit when cold until it's been ridden a couple of miles. Does it run fine once it's aarmed up? Also check the carbs jetting and make sure it's at oem specs, someone might have gotten in there and messed with the jetting.

Really? I mean, I've had other 80's carb'd bikes that wouldn't take this long to get up to a temp where it would be able to pull away from a stop. I mean, there is NO power from a stop for like 10 minutes after it starts. This coming from someone who has put at least 30k miles on two wheels, so I know how to use my clutch well enough to get moving haha.

Even once it's warmed up (15 minutes idling, another 10 minutes of stop and go riding around town), it still hesitates a hair from 0 - 1/4 throttle.

Jetting is completely factory, carbs are clean, and there are no vacuum leaks (after trying the old starting fluid around the boots trick, replacing intake manifolds, and visual inspection of boot connections). Also has factory airbox.

Does it smoke at all? What colour are you plugs?

In the OP, you did not mention fuel level in the carbs and initial (bench) synchronization. Are they up to spec?

You are running lean. Do both plugs look the same? Where is your fuel level? What fuel are you using?


.

Alrighty, I think I managed to fix the issue with a few things but first lemme tell you guys what the tests were saying.

@Warren: There was no smoke out of the exhaust, and the plugs were on the whiter side (although not horrid). Both plugs were about the same color.

@TexasKZ: I did a rough bench sync while I had the carbs apart, but didn't (and don't) have a vacuum sync tool yet so a precision sync will have to wait for the moment. However, I didn't check the float levels, so I did that this evening. One was dead on spec, and the other was about 4mm low, so I adjusted and got it within spec.

@Nebr_Rex: I believe you're right, I was running a little lean based on the plugs. I just filled the tank within the past week, so fuel was good. Also supply to the carbs was good, as I run a transparent fuel line and there was plenty of flow.

In the end, I corrected the float level in the carbs (one was off), reset my valves (a couple were too tight), and after starting and warming up the bike adjusted the mixture screws. A little richer mixture completely got rid of the hesitation, and after letting the bike cool I tested to see if it would run while cold a little better and it only takes about 5 minutes until it can rev smoothly through the rev range. I'll give it a full test drive tomorrow and report back any remaining coughs in the rev range and the condition of the plugs afterward. Thanks for the help guys!

Okay, I took it out for a test drive today and it ran at about 90%, but its still not all the way there yet.

It starts fine, but it still had just a hair of hesitation right off idle. When I was coming home, it even misfired a few times when I was pulling away from stops. After I got back, I noticed the plugs were still on the whiter side and realized the mixture screw only controls the idle, so it makes sense that I didn't enrichen the mixture at higher throttle positions. Thus, I think I'm still running a little lean.

Before I go pulling and replacing jets, I think I'm going to hunt for vacuum leaks some more. I suspect two screw mounting holes in the airbox that don't have the screws (I needed to leave them out to get the airbox far enough forward to get a good seal on the boot-carb interface), but I'm not sure if that would be enough to give me my lean mixture. Will report back after I get those plugged.

In the mean time, any other suggestions? Thanks!