I have been trying to tweak my fueling to correct two scenarios.

First engine stumbling on 1/8 - 1/4 throttle which I corrected by moving the needle clip up one position.

Second a slow lurching when in motion and letting the throttle return to the idle position. I corrected that by turning the air-screw in, enrichening the idle.

When combining both changes the issues were better than at stock settings but not as effective as when only one change was in effect.


I had a second carb that I bought and cleaned but had never tested. I installed it using the same throttle valve and needle but reverted back to Yamaha recommended settings, 2 turns of the air-screw from fully closed, and the needle clip at position two. And the fueling was perfect. I ran both configurations within 45 minutes of each other so heat and humidity were essentially the same.

Each carb was using the stock jet sizes but the each had their own set,  230 main, 25 pilot.  The throttle valve has the stock cutaway of 2.0, the needle is a 4J13. The needle jets are both 0-6.

The original carb has some internal pitting. Could that cause fueling problems?  Or any ideas what else could cause these things? The jets had been given a very thorough cleaning though.  If I have the time I will keep adding parts from the old carb to the newer one, one at a time to see where things go bad.

I should be simply happy that the fueling is where I always wanted it to be but after a year of trying to get there, I am itching to find out what was wrong with the orginal carb.

thanks for reading.

 
 

Needle jets and needles both wear.

Yes my thoughts too-as Mark says. Also, the small Oring sealing the bowl onto the needle jet [jet setter]. Unlikely though as would affect it badly on full throttle. Are we talking CT3, looking at earlier posts.

I was not aware that the needle jet was a wear item. I will test with that first. thanks

Yes. in trying to provide as much detail as I could, I wound up forgetting the bike model. dam details always get me.

But the pitting isn't a concern?

The pitting could definitely be an issue.

Yesterday, I received a request to photograph the rear of a AT2 - CT2 carburetor with a picture very similar to what you have posted… I don’t want to give the eBay requestor’s name. But I replied that I had 13 pictures in my listing. Was that you. 

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Yes it was indeed.

Okay, I took today off work I’ve been going 7 days a week for about 10 or 12 days now, that is why I said Thursday. I think MarkT is more equipped to help you regarding your motorcycle. My telephone number is listed as Yama-Land and you can certainly call, but I don’t circumvent eBay to save money. I buy their shipping labels, and have drastically increased my numbers on auctions.

Don’t go too lean, I keep all my needles in the third clip from the top and almost never go up on the suggested pilot jet. I buy that Ace Hardware “online VP 94” small engine fuel for $8.00 a quart and they email me 45 minutes after payment online to pick it up at my chosen store. Don’t go there and buy it for $16.00 a quart. Our gas in California is very bad on bikes. It’s worth $8.00 to see how your bike performs on VP 94 …..

I will get that photo for you that you asked for…

a

 

In an extreme case of needle jet wear you can eyeball it up to the light & see the elongated hole.
Not sure how bad the old Yamy's get but i've had an extreme case on my YZ125K "83 & in a case like that dropping the needle [raising clip] only masks the problem & mucks it up around that point & beyond by leaning it off.
In an even more extreme case at work we sold lots of LTF250-300 Suzy quads to dairy farmers & i'd say we'd replace the needle jet plus needle at the rate of one a week on average.
Could see the egg shape in the jet at the top. [metering is only at the top few mm or so--so look from the top & roll it around while looking.
Best bet is replace with a new 'pair'--jet & needle combo if in doubt.
An over rich idle setting can change things a little too when shutting down from speed & bringing throttle up again.
Had a problem that took a while to track down [lean not rich] where my DT1 had grown stale fuel residue the total length of the needle jet so encompassing the needle full length. Bike had been parked for a year or so but was likely happening from the previous hibernation & just carried on. Our fuel is terrible stuff!!.

Carbs are a wear item, and while a 50+ year old carb, will work better with a new needle and needle jet fitted, finding these parts might be very difficult, as OE carbs often use manufacturer specific parts, which are not the same as those used in generic after market carbs.

In most cases simply fitting a new carb is a much better idea, and need not be all that costly if a Chinese Keihin clone is used. The clones branded "Nibbi" work well, and aren't that far from Japanese in terms of quality.