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Post by infrared on Aug 25, 2015 17:55:14 GMT -5
I have a theory that I am going to test. Theory: Diaphragm type carburetor has very temperamental idle due to the inclusion of a diaphragm. Yamaha jog carburetor has same approx. dimensions but no diaphragm. I ordered one off of eBay today. I will let everyone know the outcome. I bet my scooter will finally idle at one speed. I have had two scooters that won't idle right. That tells you it's a design issue. I was never able to get either scooter to idle at one speed.
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Post by scooterpimp on Aug 25, 2015 20:25:42 GMT -5
Probably a vacume, air leak , possible not setup correctly.?
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Post by paulmohr on Aug 25, 2015 20:38:07 GMT -5
Mine is the opposite, it runs no matter what you do to it. Turn the idle mixture screw all the way, doesn't die. Pull the air box off, still runs. Plug the air box up, still runs. Well unless you plug it all the way up and starve it for air, then it dies. And with mine, mixture adjustment, metering valve adjustment, or jet changes the performance is pretty much the same. 9-10 second 0-30 times and 45 ish top end.
I think I am going to get a different carb tomorrow. I think someone monkeyed with this one before I got it.
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Post by infrared on Aug 25, 2015 21:48:52 GMT -5
Eliminated vacuum. New hoses, manifold. Remember, I had the same problem on both bikes. Think of it further, the level of complaining about idle points to this being a design issue. Not all engineering is perfect. And take notice of the fact that Yamaha is not including a diaphragm on their 50cc scooter, the Jog. Why is that? Could it the right design?. I think so. The jog carburetor that I ordered will be in on Thursday. I bet you the up and down idle will go away.
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Post by paulmohr on Aug 25, 2015 22:03:20 GMT -5
Honda just said screw it and went to fuel injection lol.
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Post by paulmohr on Aug 26, 2015 13:28:40 GMT -5
I am confused, what kind of scooter are we talking about? In your other thread about it not idling you were referring to a taotao 4 stroke. Is this what the "other" scooters were as well, 4 strokes? The Jog carb is for a 2 stroke, are you going to try to put a 2 stroke carb on a 4 stroke engine? If that is the case and you think the cvk carb is hard to adjust, you haven't seen anything yet. A 2 stroke carb works different than a 4 stroke and from what I can tell from a quick internet search jetting it to work on a 4 stroke will be a nightmare.
And also, the diaphragm should not have any effect on the idle. When the carb is in the idle circuit the butterfly is closed and it draws fuel from a small port on the engine side of the butterfly. The main jet and the metering rod should not have any effect in this circuit unless something is seriously wrong or out of whack. Like if your idle screw is turned so far that it is cracking the butterfly open enough to bypass the idle circuit. Then you would be at idle in the off idle or power circuit, which would make it run odd. It should also idle pretty high that way. If it isn't idling high with the butterfly starting to open then something else is wrong.
Are these bikes completely stock? I am assuming they are since you had another thread discussing running stock vrs modified and you were in favor of stock. Because any changes can effect how the carb will tune. A high lift cam would effect idle, as would any air box or filter changes.
It kind of sounds like a leak in the airbox or ducting, or maybe a pilot jet issue. I would be curious as to what the manifold vacuum is while its doing this. It could also be the electric auto enricher, but I would think it would even out once warmed up.
As for it being a crappy design, the cvk type carb is used on a lot of small engines, even harley uses them. I think the sportsters have the same type of carb, just larger. And I would be willing to bet most of the 250cc street bikes use the same style, like the 250 ninja and honda rebel. I could be wrong though, I didn't look it up, just guessing.
Why you have two scooters with the same problem I don't know, it is odd, but not completely out of the realm of possibility.
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Post by katastroff on Aug 26, 2015 16:38:05 GMT -5
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Post by infrared on Aug 26, 2015 19:51:29 GMT -5
I am talking about a TaoTao atm 50. The other bike was a Shanghai Meitian Motor Company 50. Same exact problem. I just find it amazing that people just don't want to give an inch on the design issue. I will follow through and let everyone know. I bet you after i install the carb and it works, people will find some reason other than obvious to say that it wasn't a design issue. I work at an engineering firm. Our plan review folks find errors everyday in the designs that we are asked to review.
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Deleted
Deleted Member
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Post by Deleted on Aug 26, 2015 19:59:11 GMT -5
The only thing I see in common with his two bikes that are not able to idle is probably the fuel he is using. Water in the fuel can, and does, raise hell with the idle circuit. If using gas with alcohol in it as most of us do, it gets even worse.
If it is not the fuel, I have no idea what is wrong. My bike idles great on my new carb and also did fine with my old carb.
Bill
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Post by katastroff on Aug 26, 2015 20:01:48 GMT -5
Good luck tuning it and keep us posted.  |
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Post by paulmohr on Aug 27, 2015 11:19:40 GMT -5
Ok, if we are going to look at this from a logical standpoint we can't just say these carbs are crap and its a poor design. And just blame it on the diaphragm. In what way is the design flawed, and how is the diaphragm causing a poor idle problem? Like I said the diaphragm is not even in the idle circuit so how would it be causing the problem? The only way this really causes a poor idle is if the the metering rod is too high and/or the main jet is much larger than required. Or if the idle set screw is turned in too far causing the butterfly to be partially open during idle. And this should cause a very high idle. If it doesn't there is a problem somewhere other than the carb.
This is actually a fairly popular design of carb used on many small engines. There are even some larger engine designs that are similar.
I had two scooters in a row that left me stranded with less than 100 miles on them. By that reasoning we could just say scooters are a design flaw in general.
And yes if you do manage to get a 2 stroke carb to work there is still an argument to be made. If you have to mess with the carb to get it to work and idle correctly the question would be did you spend the same amount of time and effort on the CVK carb to get it to work. Now if you just bolt that carb on and it works with a few simple adjustments then yes you may be correct.
We also have to consider that maybe in changing the carb you fixed another issue that was not related to the design of the carb, but a problem with the installation or something in the related parts.
On the original carb did you try different pilot jets, metering rod adjustments or main jets to see if that was the problem?
I know it looks like I am going out of my way to argue with you, but I promise I am not. I am just pointing out some logical points and continuing the discussion.
I really do hope your idea works for you.
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Post by Fox on Aug 27, 2015 11:39:38 GMT -5
Erratic idle is usually always a vacuum leak. I have wrenched on literally hundreds of these scooters and I can tell you from experience that the idles are rock steady if the engine is assembled and tuned properly.
The fact that infrared says both his scooters do it kinda backs up my theory that it's owner error and not a design flaw. There's something fundamentally wrong somewhere in his procedures, not the design of the carburetor.
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Post by infrared on Aug 27, 2015 19:50:12 GMT -5
It's not a vacuum leak because the idle varies consistently. Watching the tach it spends an equal amount of time too low, too high, and in the middle. If it was vacuum it would only be too high. I have nothing against diaphragms. I just think they didn't get it right with this carburetor.
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Post by scooterpimp on Aug 28, 2015 6:08:20 GMT -5
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Post by katastroff on Aug 28, 2015 6:37:24 GMT -5
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