Spark, fuel, compression, but no start!! By mossy173 (Read 17,307 times)

[mossy173]

Nov 12, 2012 19:57:13 GMT -6 Alleyoop said:

Also to check that you can take the Air Filter off and put your hand over the carb and crank it IF you get air hitting your hand the intake is open on the EXHAUST STROKE.
Alleyoop

THe mystery continues. I reversed the cam sprocke 180. I did the straw trick and watched very carefully what was happening while I slowly turned the engine--piston comes up, piston goes down, exhaust valve opens, piston goes up, exhaust valve closes and fuel valve opens, piston goes down, repeat. your standard four-cycle motor doing its thing, except this one is still not igniting.

I went through and triple-checked the basics. The spark plug gets wet, so there's fuel. A finger over the spark plug hole shows that there's a fair amount of compression. There's spark. The timing checks out. I've tried adding fuel through the vacuum port, as well as starter fluid. Nothing. I think it might have possibly turned over a bit faster at one point, but that might just be wishful thinking.

[mossy173]

You know, I'm thinking of just reinstalling the old cylinder and trying to cut my losses. Maybe 50cc to 100cc is a bit much, anyway....

[Alleyoop]

DAMN Mossy!! If you have compression, fuel and spark it should fire. And because the PLUG is wet I would throughly check the plug and the number of the plug. It is obvious it is NOT sparking when the plug is in the motor if it does not even cough at least once with all that fuel.
Did you put on a different head on that thing. Like I mentioned before the head could be to thick and the plug DOES NOT STICK out a little into the combustion chamber and that will cause it to GROUND out on the head. Or you are using the wrong plug and it is shorter than what it takes.
What is the number on the plug and or put up a picture of it.

At this point Get a Video going while your attempting to start it. I presume you have the panels off so the engine bay can be seen.
Alleyoop

[mossy173]

Here's your video:

I spoke to the fellow who sold me the big bore kit, and he's using the autolite equivalent of the same plug I've got in there, the ngk C7HSA that the engine manual specifies, gapped to 0.025".

[Alleyoop]

Well I must say it sounds really good EXCEPT for NOT a spit of a fire. Have you tried holding the throttle open some because if gas is coming out of the tail pipe that sucker is sucking in a lot of fuel and it may be it is just plain flooding it.
Just for crap grins take the VACUUM hose going to the PETCOCK and plug it up so that no gas is being pumped into the carb and see if it at least gives a cough.

I notice that your carb has a pump and when you do pump it, it will pump fuel into the carb, SO just hold the throttle open and do not pump it YOU ARE ALREADY getting a lot of gas down the motor.

Alleyoop

[mossy173]

oh, I've tried it every which way. Gas on the vacuum port, plugged vacuum port, throttle open a little, open a lot, everything.

[Alleyoop]

Well I have to go back to SPARK again, If the timing is good sounds like it has good compression and you are definitely getting plenty of fuel I really don't think the Spark plug is Fireing while in the motor. Do you get a good NICE BLUE SPARK or a yellowish spark when you tested for spark.

I have to ask though what is the Valve COVER vent hose connected to, looks like a clear plastic piece then some other contraption going into a hose. Can you take a PICTURE of the engine bay so that all the hose connections can be seen. Also that plastic piece appears to be full of oil. Now it could be that the crankcase is not venting and is creating a lot of pressure. Ususally we take the Valve cover vent hose and put a fuel filter on the end of it and tie it up high on the frame. That HOSE has to vent if it doesn't it creates a lot of crankcase pressure and can blow out seals.

I would take the Vacuum hose going to the PETCOCK and block it off so it does not pump anymore fuel into the carb. Also I would Turn the Fuel Ratio Screw CLOCKWISE until it stops and then turn it out 1 1/2 turns and see if LESS FUEL gets sucked out. Right now if it is coming out the tailpipe it is getting to much fuel. Don;t forget the ENRICHER is also feeding it extra fuel.
Or even take the Air Filter Tube off and see if more air helps it.
Alleyoop

[mossy173]

The spark was strong and blue when I tested the plug out of the motor. I went out there with a multimeter and tested for continuity between the positive pole of the plug and the head, just in case it was shorting, and that checked out. I don't know, it seems like it should be sparking in there, no?

I'm not in front of the scooter at the moment, but I'm pretty sure the valve cover breather hose has a way to vent. It connects off to the left in that video to the air filter.

It seems like it has no good reason not to run, doesn't it?? Could it be that the fuel mixture was thrown off when I replaced the main jet?]]

*edit* oh, didn't see that last bit. Alright, I'll play with the mixture. Looks like I'll have to remove some sort of cover from over the mixture screw--is there a safe way to do that?

[Alleyoop]

Yes, your correct that puppy should fire, but from the looks of it right now IF fuel is coming out the tail pipe with the cranking that you did on the VIDEO that sucker is flooding. You are now telling me that the FUEL RATIO SCREW has a plug on your CARB!!(DAMN MAN) you need some MOJO.
Well before you try and Drill the Plug out TRY disconnecting the air tube from the carb that will make it suck in a lot of air. What we are just shooting for is a COUGH ONE LOUSEY fireing then we know we are on the right track.
Alleyoop

[Alleyoop]

I also have to tell you that depending on the carb that is on there There MAY not be a screw with a slot to adjust it behind the plug. Some put in a screw with no slots so you cannot adjust it. Most guys will just order the same carb with the Fuel Ratio screw available and use the old carb to play around with and get to know the ends and out and for spare parts.

On those carbs I would get a small hose to fit inside and force it on the screw without any slots and try to unscrew it out that way and if sucessfull you then can cut a slot in the screw and then are able to use a screwdriver to adjust it. Or you can try a drill bit in reverse and see if you can unscrew it out that way.
Alleyoop

[mossy173]

Well, I've already drilled out the plug and tried the screw at 2, 1.5 and 1 rotations out from all the way in. Nothing there, even with lots of cranking from a freshly charged battery, probably half again as fast as it was in the video. I'll give the lots-of-air tack a shot tomorrow.

I double-checked the spark plug yet again, and I can definitely say there isn't a problem. Held against the cover it gives good blue (almost white) sparks, and held a couple inches from the cover, it actually arcs all the way to it. That's a good spark.

[Alleyoop]

Ok, at least you have a Fuel Ratio Screw now. Yea try more air, also check your AIR FILTER it may be soaked with gas.
It could also be that the BOWL is not cutting off the fuel after filling the bowl and it keeps on coming in and flooding it. That is why I say to cut off the FUEL from filling the bowl by disconnecting the Vacuum hose from the PETCOCK. Also when you do this make sure the PETCOCK is actually not allowing fuel to flow with no Vacuum.
Alleyoop

[mossy173]

Here's a question in the meantime--I had the carb off while I was putting the new cylinder on, and had a look at it. It seems to have a butterfly valve at the narrow opening, and in the middle something like a slide valve. i've had some Japanese motorcycle carbs apart, and they use a slide valve coupled to the needle exclusively. What role does the slide valve kind of thing in the qmb139 carb play? Also, it doesn't seem to move in response to the throttle, is it normal for that to feel limp?

[Alleyoop]

That is operated by air pushing the Rubber Diaphgram up and making the cylinder rise carrying the needle along with it allowing more fuel to flow out of the main jet. Yes at the intake side it has a Butterfly which is controlled by your throttle. That is what the throttle opens and closes everything else is automatic on these CVK carbs. The throttle all it is doing is allowing more or less air to be sucked in by opening or closeing the butterfly.
Alleyoop

[Alleyoop]

Here is a good animation of how CVK carb works:
Alleyoop