Trouble shooting engine start up

BSKZ650 wrote:

....then more this gets kicked around, it sounds more like a carb issue.

Guys, what about putting in another brand new plug, then as soon as bike is started, (engine is running, ok), then give the open carb throat a quick shot of starter fluid. In fact, you probably would not need to remove pod, just spray it. If cylinder starts to fire, well, so much for questions of ignition.

mountain wrote:

... 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.... 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.... #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?

The bottom located pilot screw on engine side of carb functions differently than side located pilot air mixture screw located on the intake side.

Recall the side located pilot air mixture screw located on the intake side of the carb regulates amount of air (from the air passage) entering and joining fuel from the pilot jet at the orifice to be sucked into the combustion chamber.

In carbs with bottom located pilot screw on engine side of carb, air from the pilot air jet arrives in a predetermined fixed amount where it is blended with fuel coming up through the pilot jet. The blend (fuel mixture) arrives at the orifice where the adjusting screw regulates the amount allowed through the orifice. Screwing in reduces amount of mixture. Screwing out allows more mixture.

There is a second orifice (called a bypass outlet) located in bottom of bore underneath throttle slide near a notch at bottom of throttle slide. This orifice is also supplied with fuel mixture (from the same source) and is not adjustable.

With the throttle slide in its fully down position, air intake through the main bore (from the air box or pod filter) passes underneath the throttle slide cutaway picking up mixture from the bypass outlet and goes on through and past the throttle slide notch where it joins mixture from the screw-regulated orifice to be sucked into the combustion chamber.

Long story short -- turning in the bottom located mixture screw leans the pilot mixture and turning it out enrichens the pilot mixture. This is opposite of the side located mixture screw which enrichens by turning in. And regardless of the type pilot system (whether bottom located or side located), a clogged air jet and/or clogged air passage would over-richen the pilot system fuel mixture. With your #4 carb, I would be frustrated enough to just screw it all the way in, or just a tad out, and gradually proceed from there.

The enrichener system (choke) has its own separate jet, pipe, passage and orifice (closed by the plunger).

Believe you are correct about the plug flooding. Here's what I think may be happening. The new, clean-appearing, wet, non-firing plug, is being doused, flooded and rinsed cleaned by the excessively rich mixture. But this can also happen in a perfect carb when leaving the choke on too long, or allowing the engine to warm up with the choke on. When starting cold engine, suggest initial choke with throttle untouched, and upon first running, immediately close choke asap, then work throttle if needed to keep engine running while further warming up. Best to warm up by riding.

Uncertain whether NGK plugs are particularly vulnerable to flooding out. But know from experience after running on three cylinders and removing the relatively new non-firing plug and finding it perfectly clean and pristine looking only to resolve matters by installing a brand-new plug and again hitting on all four. Go figure.

When replacing the #4 plug with a new plug, suggest first using air pressure to blow out the combustion chamber. And then a quick spritz of starter fluid or carb cleaner into the plug hole, again followed by blowing it out with air pressure. All in an effort to get the combustion chamber clean, dry and fuel free before installing the new spark plug.

And won't dare mention the time honored method of "de-flooding" a cylinder by holding a lighted match with long nosed pliers and inserting it into the spark plug hole. But if the idea is irresistible, stand well back and be prepared for a flaming blue whoosh.

Hoping you're in the home stretch.

Post edited by: Patton, at: 2007/04/01 15:00

Post edited by: Duck, at: 2007/03/30 19:30

Yet another possibility comes to mind as to cause of an excessively rich condition in pilot system with bottom located mixture screw which enrichens as it screws out.

If at some earlier time the screw was turned in too tightly, the pointed tip at end of screw might have been damaged, thereby preventing it from precisely metering amount of mixture allowed to pass through the orifice (and consistently be too rich).

Suggest removing pilot screw and inspecting the pointed tip (under good light with magnifying glass). And might want to compare it with one from another carb.

Tip may check out just fine -- hope it does -- but will eliminate another possible source of problem.

I'm out of town, so I can't try any new approaches to remedy supposed flooded #4 carb, but I can't wait until I am able to work on it again. I will post periodically as I try to resolve.

Regarding your statement on cleaning the #4 cumbustion chamber, let me reitterate: To clean cumbustion chamber, I should spray a little carb cleaner in the open plug hole and then blow it out through the plug hole. I am thinking this would be wise to do with the intake valve closed tightly so as not to blow a piece of carbon or any particle back towards the intake port, carb orifices and jets. Is this the right idea?

Although I just replace the NGK plugs with new ones, perhaps I'll pick up a different brand name, say Champions, and see if that doesn't help. Honestly, I can't see that this could make a bit of diference, unless I receved a defective NGK. Plugs are cheap and I'll need some soon for my other project, so why not.

More Tuesday or Wednesday.

This problem ended up being Carberator trouble, ran fine after removing and re-cleaning the carbs. I also installed an inline fuel filter to help prevent any residual particulates from fuel tank.

It happened again! One Cylinder is cold and not firing . . . Argggh. This time with cyl. #2. I pulled the carbs, lowered the float bowl level a touch (tweaking the tangs on the floats) and blew out the oriffices. Put in a new spark plug, and all four cylinders were firing as normal. My only thinking is that the floats were set in such a way to alow fuel to flood past the bowl and into a cylinder or airbox or both. Then a plug would get fouled badly and that cylinder would never fire. That is the trouble with those manual petcocks which can be accidentally left "on". With a full tank of fuel, this could introduce enough pressure to really flood out if the float pin doesn't stop the fuel at it's highset point.
Any way, It was 75degF here in Denver yesterday, so everyone was out riding there bikes. Mine was rippin' fast and clean at 80+ mph down hwy36 towards Boulder, I've been busy and away from the site for a while. It is snowy today, 30deg, typical for Colorado this time of the year. I'll ski Eldora tomorrow morning.
Bike I'm talking about in this post:
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Bike I'm positing to sell on Denver's craig's list:
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I've really missed being on this site, but I moved to a new house and my worksite has been non-existant and will be for a while yet . . . (I'm building it) It's all good.