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Old 24 June 2013, 14:53   #1
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problem with Yamaha carburettors

Hi I am working on getting my old yamaha 40HP. 3 CYL up and running for this year but it seems like its one thing after another having sorted most things out up to now with all the help from everyone on this site I am now faced with trying to get it running well and setting up the carbs as its just not running right and I think setting them up is beyond my capabilitys, is there anyone close to me that can help or has anyone got any advice on what I could do. I know go on say it 'SCRAP IT' sorry to late for that! I know I could maybe put this into a local repair shop, but I have a limited budget and seem to have spent quite a bit already.
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Old 24 June 2013, 16:18   #2
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The only needed thing to be adtjusted on multi carbs are fue/air screws, That info should be stated on service manual, check how many turn backwards is needed for those carbs, probably all 3 must have same adjustment, or probably don't ? Once done adjust overall min idle rpm carb screw which sets same all 3 carbs.

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Old 24 June 2013, 16:40   #3
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This evening while trying to adjust the mixture settings on the carbs, I found that they were all seated at various lengths, one was very loose and the spring offering almost no resistance and the other 2 were very tight, almost to the point where I could not turn them. So I took them all out individually, cleaned the threads and put them all back in again. But on seating them back in fully, I realised that I could only seat them in as far as the spring was allowing, which seemed to vary from one to another, depending on the springs. So do you think this meant they were all fully seated when they were turned as far as they will go? So on turning them out 1 1/2 turns, this should have been right, but still seemed to run unevenly and cutting out when revs where dropped.
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Old 24 June 2013, 17:01   #4
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All screws must be fully seated in and then backed out some turns which in your current engine don't know about. Let me check if possible to find that info, much faster, call any Yam Samaritan dealer to provide you that info over the phone...

In order not to screw the interior carb threads mixture screws must be seated slightly tigh on spring and then backed out...

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Old 24 June 2013, 19:40   #5
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Problem with carb screws that may have been over tightened Is that often the screw seat will put a ring or mark a groove around the tapered part of the needle / screw which then makes fine adjusting an all or nothing .
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Old 26 June 2013, 15:07   #6
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Yamaha running rough

I eventually got up the courage to take the carbs off and set about cleaning them, they were very dirty, which made me think this might have been the problem all along, alas I was wrong, this is not all of the problem but has helped, starting and idling a lot better. But while running with the revs up a bit, it gives a jump every now and then, sometimes followed by a squeak. If the revs are up it carry's on going, but if the revs are low it cuts out. On trying to set up the mixture screws, I had a tachometre on each individual ht lead, 2 were very similar in rpm but one was almost double and no matter what I did I could not alter this with adjustment. Does anyone have any ideas what would be causing this, thinking maybe it's time for a compression test??
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Old 26 June 2013, 16:18   #7
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The jump and squeak is probably just a sneeze or backfire. Most 2 strokes do this when free revving ie not under load and it is because the timing is not correct for engine speed or load. I would get the carbs spotlessly clean and if possible use an ultrasonic bath too to make sure you have got everything out and put them back on. You'll need to balance them too. To do this you need to back off the idle speed screw completely to shut the butterfly completely. This will only be on one carb not them all. You then need to slacken the adjusters on the other two carbs and ensure all the butterfly's are closed completely and then re tighten the adjusters. Bear in mind some are left hand threads but it should be marked. Once you have done that screw in the idle screw till it touches the linkage then go another full turn and use that as a starting point. With the tachometer I'd not be too concerned with that at the moment until you know the carbs are 100%. Hope this helps.
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Old 26 June 2013, 16:29   #8
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Thanks for getting back to me, I'm always grateful of any advice. Bought all 3 carbs into the house last night and ran every part of them through the ultrasonic cleaner, finished the last carb this morning and I was pretty happy with the results as before cleaning them, there was quite a lot of crystallised fuel hanging around, gunking everything up. So I cleaned out fuel lines, pump and filter, so am now pretty sure everythings ok....or as sure as I can be. Had a go at starting and running it tonight, is now running better and starting much better when warm and the idling is quite settled, but there is still this annoying jump and squeak then it cuts out. Will give a try to what you have suggested.
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