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Post by Tobyjugs on Oct 21, 2017 15:27:23 GMT 1
Hello chaps, I have been home for a few days and was feeling a bit bored. I have a set of 250 carbs with two brass screws in place of the brass balls everyone like to remove. I have never had any problems with these carbs other than that irritating 6000 rpm stutter, they were even used on my hybrid which was making 65 hp at that moment. I changed them for some other 31K carbs and it made no difference in performance.
Any way back to my question. I removed the brass screws and found the area behind the screws clean but deeper in where the small brass bush is with the hole i could not see any thing and they looked completely blocked off. Is this normal?
I'm not up to date with the mods for these carbs.
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Post by jon on Oct 22, 2017 8:01:50 GMT 1
The air correction jet is small, typically,around 0.5mm.
It's difficult to see through this.
With the balls out, squirt something through the jet such as WD40 or carb cleaner and look for the exit to check they are clear.
Jon
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Post by Tobyjugs on Oct 22, 2017 8:28:46 GMT 1
No they are blocked, but are they supposed to be blocked? Was there a mod from Yamaha to block them?
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Post by oldelsieboy on Oct 22, 2017 8:48:09 GMT 1
No they are blocked, but are they supposed to be blocked? Was there a mod from Yamaha to block them? The mod was to the early 4L0-00 carbs, they were manufactured without a brass ball to block the circuit, a blob of araldite was the suggested remedy. OEB |
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Post by jon on Oct 22, 2017 8:51:33 GMT 1
No they are blocked, but are they supposed to be blocked? Was there a mod from Yamaha to block them? The mod was to the early 4L0-00 carbs, they were manufactured without a brass ball to block the circuit, a blob of araldite was the suggested remedy. OEB They were only araldited after drilling a 3mm hole to feed the jet from outside the airbox. Jon |
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Post by Tobyjugs on Oct 22, 2017 10:00:02 GMT 1
Ok if i understand this correct there should still be a passage way or air correction jet. Yamaha blocked the entrance to this hole off with araldite. My carbs were blocked off with bronze screws.
If that hole is blocked they shouldn't really work, or is this only effective at very low loads or rpm?
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Post by jon on Oct 22, 2017 10:20:13 GMT 1
Tobyjugs, the air correction jet must be fed with air to work.
Originally on early carbs the hole in the 6 o'clock position was left open allowing air to get to it via the airbox.
The mod was to block off the hole in the 6 o'clock position with araldite, then drill a 3mm hole at an angle through the side of the carb to intersect with the air circuit.
I'll post a picture of the mod if I can find it.
Jon
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Post by Tobyjugs on Oct 22, 2017 11:36:05 GMT 1
Thanks Jon
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Post by jon on Oct 29, 2017 10:33:30 GMT 1
Tobyjugs, Just found it.  Jon |
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Post by Tobyjugs on Oct 29, 2017 14:00:27 GMT 1
Tobyjugs, Just found it. Jon Thanks for the info jon. I just measured the holes in the carbs. The hole that must be modified to 3mm is 2.5mm on my carbs. The hole behind the brass screws which go to needle jet was blocked, i have let them soak in some degreaser and now this hole has a diameter of approximately 0.6mm. Does any one know what size this should be? What i cant get my head around is if you have a hole of 0.6mm feeding air into the needle jet area why do you need a 3mm hole to feed this hole as it will be restricted by the 0.6mm hole? Is there fuel and air being mixed before the 0.6mm hole which is being fed into the needle valve area? |
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Post by jon on Oct 29, 2017 14:37:09 GMT 1
If you've got a 2.5mm hole, then I'd leave well alone.
Of course originally there was no hole.
As you say it should easily feed the air jet.
Jon
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