Trouble shooting engine start up

mountain wrote:

...wouldn't think that a little too high fuel level in the bowl would massively flood....#4 carb is not overflowing either. Same amount of fuel comes out of it as the others when I drain them.

My understanding is that the correct amount of fuel intake up into the pilot jet (and up into the main jet, bleed tube and needle jet) is at least partially dependent on fuel level in the float bowl as governed by the float.

Fuel level may be too high without having reached overflow level. So absence of overflow does not guarantee the float being set low enough. And yes, many bikes run okay regardless of a leaky float needle seal (as noted by RonKZ650).

Would tend to agree that a too low float bowl fuel level would seem less critical (and probably less noticeable) while riding the bike. This because the leaner mixture continues to fire while it doesn't fuel foul the spark plug. However, a too high fuel level (even it not at overflow level) may allow excessive fuel intake up into the pilot jet (and up into the main jet, bleed tube and needle jet) and sooner or later foul the plug with fuel and soot.

While having always used NGK plugs, have noticed they seem to very readily quit firing when doused with an excessively rich fuel mixture (even before the soot takes over). But maybe all brands do the same.

Purchased many new plugs while suffering a continuous fouling problem in #1 cylinder. Each new plug would start out like gangbusters, then start sputtering and die within 20-100 miles, sooty and black. The definitive and lasting cure was finally obtained by adjusting (i.e., lowering) the #1 carb float bowl service fuel level, and the problem has never since recurred.

From KHI KZ1000 Service Manual: If the level is set too high, the fuel can be drawn up too easily, resulting in too rich a mixture. Says the same about too low resulting in too lean. Believe the same principles apply to your KZ650 carbs.

Ok, I found some plastic hose that would do the level check. Yes it was as easy as pie. The level is right where it is supposed to be: slightly below the float bowl top, 2mm below the line where the float bowl meets the carb body. I'm out of things to try doing. It must be compression. Could compression slip by the rings fast enough to cause the piston not to fire?

Do you have Dyna coils?
If you do, you can switch the 1 & 4 wires at the coils (similar to what you did at the plugs). If the problem moves to #1, then it's a bad coil.

If you've got sealed coils, you can't switch the wires. But it sure sounds like you have compression and spark, so that leaves fuel/air. I think that #4 is getting either no fuel, or too much.

I am at my wits end. Could there be a mouse in the #4 header? I am going to take off the right side exhaust and check to make sure it is not clogged. If it is, couldn't this snuff the firing of my #4 cyl?

Ok, I guess I'm getting desparate. No mice in the header. I'm back to thinking carb problem. I agree, too much or too little fuel.

mountain wrote:

...found some plastic hose that would do the level check. Yes it was as easy as pie. The level is right where it is supposed to be: slightly below the float bowl top, 2mm below the line where the float bowl meets the carb body. I'm out of things to try doing. It must be compression. Could compression slip by the rings fast enough to cause the piston not to fire?

Mountain, I must say, your response and reaction times are some of the fastest I've ever seen. You must be a paramedic.

Compression issue? Honestly, No.

But I'm almost ready to bet my nickel on the answer, if only knew about the spark plug tip, wet or dry, but will muddle forward anyhow, since it almost has to be one or the other.

And guessing the starter plunger is not stuck open. If it is, push it down, intall a new plug, and see what happens.

Okay, here goes (admittedly over-simplified). The carb has a pilot system to provide the fuel/air mixture when the throttle is closed. Recall when the throttle is closed, the carb slides are completely down closing off air from passing through the bore with fattest part of the tapered jet needle filling the needle jet, basically blocking and disallowing any fuel mixture through the main bore of the carb into the engine.

So where does the fuel mix come from to run the engine when the slides are completely down? The pilot system, a separate source, furnishes air and fuel.

So how does the stand alone pilot system work? It must have a separate fuel supply, and a separate air supply, and a way to mix the fuel and air. And a way to control the mixture (richer or leaner)

Pilot Air System 101:
Air supply -- look at the intake side of the carb and see a separate little entrance hole existing alongside the main bore. That's the beginning of a separate air passageway running through the carb body around the throttle slides to a small hole (orifice) in the bottom of the carb bore on the engine side of the throttle slide just before entering the manifold connecting the carb to the engine.

Fuel Supply -- Fuel from the float bowl up through the pilot jet goes to the same orifice to mix with air from the air passage. The amount of fuel is set by size of the pilot jet, and is otherwise not adjustable.

Your carb, I believe, has the pilot mixture screw on the air box side (not underneath on the engine side). This side-located pilot mixture screw is a pilot air screw regulating amount of air entering the orifice from the air passage where it mixes with the fuel from the pilot jet. Screwing in reduces the air supply from the air passage at the orifice, and thereby enrichens the pilot mixture.

So there is air (adjustable via side located screw) joining and mixing with fuel at the orifice. The fuel mixture is sucked from the orifice on through the intake valve and into the combustion chamber where it is ignited by the spark plug.

When the air passage, or the air jet inside the entrance to the air passage, is clogged or blocked, the fuel mixture is way too rich, regardless of the pilot screw setting (can't regulate amount of non-existant air). Some of the air jets are removeable for cleaning. BETTING MY NICKEL if plug is wet that the air jet and/or air passageway is clogged or partially blocked.

How to be certain. With carb on the bench (and with protective eye goggles on), spray carb cleaner into the separate little entrance hole at the intake side of the carb and watch it spurt out of the orifice in the bottom of the carb bore on the engine side of the throttle slide just before entering the manifold connecting the carb to the engine. If spray doesn't freely pass through the air passage entrance hole and squirt from the orifice, the air passage isn't clear. So keep on squirting and cleaning until the air passage is unquestionably open and clear. This assures air is available at the orifice for regulation by the side located screw.

Now if plug is dry. Probably the pilot jet (which allows fuel to the orifice to mix with air from the air passage) is clogged or too small. Most pilot jets are removeable for cleaning.

Would venture to guess the vast majority of carb cleaning exercises by us amatures fail on the first attempt (and sometimes after several subsequent attempts) to get all the holes and passages cleared.
Vaguely recall sixth time being the charm for me.

So have at it, mountain. Hoping a few info tid-bits are picked up from this, and looking forward to your report.

Post edited by: Patton, at: 2007/03/28 21:19

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Post edited by: Patton, at: 2007/03/28 21:21

Patton, you're awesome to give such great attention to my amature attemps. I am gaining ground very fast, though. This set of carbs came on a free 650 I received in December up in Bozeman, MT. I followed my nose and very carefully disassembled the carb COMPLETELY. Everything was soaked in a carb cleaner basket, hand cleaned, shot with air and reassembled. Unfortunately I broke off the choke tube on carb body #3. It's the brass slender tube coming down from the carb body into the bowl bottom. Bummer, but I got a #3 carb body replacement for $35 from Jackson Cycle in MN. I got it all reassembled well, and was proud that I could do it. This is before I found KZrider. I kept all the parts segregated to their original carb bodies. Only bought float bowl gaskets. The carb ran on The bike that I now have totally dissassembled for a custom rebuild. It ran and #4 cyl fired well in January. I got an identicle model 650 in February to have as a daily rider while I rebuild the other. I just swapped the carbs, since I had a clean rebuilt one on the bench. It's been great having two of the same bikes, it makes swapping out things and trouble shooting so much faster and easier. I did the same with the tank, and it also helped me get a good front disc brake situated.

This long tale doesn't help my current problem. The fact that it did run and fire #4 cylinder on a previous bike doesn't make me feel any better. I actually sent my other identicle set in to wiredgeorge today. I spent plenty on carb cleaner and gaskets, and absorbed enough evaporated cleaner to make George's price seem very reasonable. Plus it's all bike tested, etc. It's kinda feebing out as far as do it yourself theary, but so what. When it comes back, if I haven't solved the problem by then, I'll swap in the wiredgeorge set and see if, infact, it is the carbs.

Before then, I will go through these carbs, again, on the bench. I'm not going to buy carb kits, though. I'd rather have George do 'em. We'll see on the first set I get back.

As far as my fast return to the post, I am doing all this work as I post. I am starting to build up some speed skill. It's not about speed, though. I'm just going faster because I'm doing things over several times.

I will not be able to work on the bike till next Tuesday, but I'll be back to it then and post my carb rebuild results here, probabley Wednesday.

Here is the frame of my custom rebuild, powder coated porcelin white:

Thanks for the reply and trust you will enlighten us with the final resolution.

In my case, the pilot jet was small enough to escape attention the first go-around, but the air jet (inside entrance to the air passage is so tiny, a magnifying glass was needed before realizing it even existed and had a screw driver slot :S.

Sounds like you'll be all set in plenty of time for several July-August rides through Yellowstone and Beartooth Pass

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Post edited by: Patton, at: 2007/04/01 14:53

I've been thinking hard about Patton's words of advice. First, my Mikuni VM24ss carbs do not have the air screw on the side, but rather have the pilot, or fuel screw under the carbs at the front of the float bowl facing down. The restricters are are no longer, but the screw stay at where they're set. Set at 1.25 turns out. These were all removed cleaned and air blown. The passages emit air. This was done to all carbs first time through, and again to the #4 carb after this #4 cyl wouldn't fire.

I think I understand the thoery on how fuel dilevery begins when the engine is cold and the throttle is closed. The enriching circuit provides a mixture that will ignite while the engine is cold. As the engine beggins to warm, the choke plunger is depresed and the pilot system takes over, throttle still closed. As one beggins to open the throttle, overlapping of jets and oriffices occurs, the pilot system getting progressively left out as main jet and needle jet take over at higher main throttle valve openings.

The #4 spark plug is wet and may have fired a time or two, but will not consistently fire. At first, when I removed the new plug it looked wet but new, no evidence of any fuel ignition. After several attemps at other trouble shooting, I noticed the plug had a faint bit of ignition evidence appearing on it, still wet and very clean.

My question is, if the cylinder does not ignite originally under rich (enriching circuit, cold engine) circumstances, wouldn't it fire up after a while if the throttle is open, say half way to around 4500 rpm, and begin firing? By then the engine is getting pretty warm and the #4 is by proximity.

Perhaps it is just getting doused by to much fuel by this point, and needs to start cool and rich and heat up, or work up to being able to vaporize the increase in fuel as the throttle venturi opens and bigger jets start taking over fuel delivery.

I'm thinking towards an overly rich delivery causing my #4 cyl not to fire. It's getting flooded. Perhaps I have a clogged pilot air orifice. The pilot/fuel screw seems set right by the Clymer's manual and factory manual. All three of the other carb bodies are set equally and run well enough to fire their cyl. Could a clogged enriching circuit or somethin else possible cause me to flood out?

I've been thinking hard about Patton's words of advice. First, my Mikuni VM24ss carbs do not have the air screw on the side, but rather have the pilot, or fuel screw under the carbs at the front of the float bowl facing down. The restricters are are no longer, but the screw stay at where they're set. Set at 1.25 turns out. These were all removed cleaned and air blown. The passages emit air. This was done to all carbs first time through, and again to the #4 carb after this #4 cyl wouldn't fire.

I think I understand the thoery on how fuel dilevery begins when the engine is cold and the throttle is closed. The enriching circuit provides a mixture that will ignite while the engine is cold. As the engine beggins to warm, the choke plunger is depresed and the pilot system takes over, throttle still closed. As one beggins to open the throttle, overlapping of jets and oriffices occurs, the pilot system getting progressively left out as main jet and needle jet take over at higher main throttle valve openings.

The #4 spark plug is wet and may have fired a time or two, but will not consistently fire. At first, when I removed the new plug it looked wet but new, no evidence of any fuel ignition. After several attemps at other trouble shooting, I noticed the plug had a faint bit of ignition evidence appearing on it, still wet and very clean.

My question is, if the cylinder does not ignite originally under rich (enriching circuit, cold engine) circumstances, wouldn't it fire up after a while if the throttle is open, say half way to around 4500 rpm, and begin firing? By then the engine is getting pretty warm and the #4 is by proximity.

Perhaps it is just getting doused by to much fuel by this point, and needs to start cool and rich and heat up, or work up to being able to vaporize the increase in fuel as the throttle venturi opens and bigger jets start taking over fuel delivery.

I'm thinking towards an overly rich delivery causing my #4 cyl not to fire. It's getting flooded. Perhaps I have a clogged pilot air orifice. The pilot/fuel screw seems set right by the Clymer's manual and factory manual. All three of the other carb bodies are set equally and run well enough to fire their cyl. Could a clogged enriching circuit or somethin else possible cause me to flood out?