Will XJ750 Carbs work on a XJ650?
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From: noreply on 19 Aug 2007 12:24 It is good to be conversing with someone knowlegible. And I appreciate you looking and researching. Your description of the pilot screw is correct. I was lucky that mine were not under a brass plug, but rather were covered with red silicone, I think from the factory. Apparently some years had the brass plug, while other years they used red silicone. I have had the pilot idle mixture screws out and cleaned all of them, and made sure the outlet port is clear. After looking closer at the factory service manual that I have, and reading online about cleaning the carbs, I was confusing the "pilot screw set" with what I thought was other parts under the pilot jet, in the bottom of the carbs. I am pretty sure the idle mixture port is clear, because I have blown compressed air and carb cleaner through it. Also, the butterflies were closed when I remounted the carbs on the bike, and I have only made minor adjustments to sync the carbs. I have also tried reducing the idle screw to close the butterfiles. I can't get it to die by backing down the idle setting, it just goes down to about 1000 RPM's and then sits there, idling fine, even though I continue to turn down the idle. Because of this, I think it is fine getting fuel through the idle circuit. I don't think the problem is compression because when I borrowed by friends carbs, it started fine. I think the starter bypass circuit is ok. If the bike is warmed up and running, starting to open the bypass circuit even a little causes the motor start to sputter and eventually die (too rich I assume). The starter jets in the bowls are clear because I can easily shoot carb cleaner through them. This is really baffling me. I have got so many hours of labor in this bike now, the thought of giving up now makes me cringe. I've been looking at used carb bodies on e-bay, but with that you are just getting someone else's problems.
From: Albrecht via MotorcycleKB.com on 19 Aug 2007 14:35 noreply(a)noreply.com wrote: >I have also tried reducing the idle screw to >close the butterfiles. I can't get it to die by backing down the idle >setting, it just goes down to about 1000 RPM's and then sits there, idling >fine, even though I continue to turn down the idle. Because of this, I >think it is fine getting fuel through the idle circuit. If the engine continues to idle with the throttles closed all the way and the idle mixture screws turned all the way closed, perhaps the throttle shaft seals leak air. You could spray some water at the throttle shaft seals to see if they leak. If the RPM changes, you know it's a leaky seal. The bypass plungers have been known to leak air. You can find out of they leak air by putting your finger over the inlet hole, which is probably in the inlet bell of the carb, on the same side as the plunger. >I think the starter bypass circuit is ok. If the bike is warmed up and >running, starting to open the bypass circuit even a little causes the motor >start to sputter and eventually die (too rich I assume). Maybe the pilot mixture screws are too far open? The pilot jets are #40's, which is quite large for an engine that size. I would expect the correct setting to be about 1/4 of a turn open from lightly seated. Another possibility is high fuel level in the float bowls, which makes it too easy for the engine to suck up idle mixture. >This is really baffling me. I have got so many hours of labor in this bike >now, the thought of giving up now makes me cringe. I've been looking at >used carb bodies on e-bay, but with that you are just getting someone else's >problems. It seems to me like you should be able to fix the carbs you already have. http://houseofmotorcycles.bikebandit.com/partsbandit/ShowSchematic.aspx?deptId=653498&machineId=8214 #21 Jet, Pilot supplies fuel to the idle mixture circuit. The jet may or may not be crossdrilled to add air to the fuel/air mixture at this point. #22 Jet, Main Air supplies emulsion air to the cross-drilled holes in #18, nozzle, main, AKA "needle jet". The tapered needle moves up and down in the main nozzle to control fuel/air mixture in the butterfly's midrange. #23 Jet, Pilot Air supplies air to the idle mixture circuit If you remove the diaphragm cap and the diaphragm and slide and squirt aerosol carb cleaner through the pilot air jet, it has to come out through the pilot jet, the idle mixture port and the three transition ports. If it doesn't flow freely out of any of those four places, a passage is plugged up. The idle mixture circuit acts to emulsify fuel and air to make fine droplets that the engine can burn when the throttle butterflies are closed, but the idle mixture circuit continues to flow and ever-decreasing amount of fuel/air mixture until the butterflies are wide open. -- Message posted via MotorcycleKB.com http://www.motorcyclekb.com/Uwe/Forums.aspx/yamaha/200708/1
From: noreply on 20 Aug 2007 18:47
Yes, that's it. Any suggestions? I have cleaned and cleaned. I think it must be a vacuum issue, because all the passages are clear. kylentz(a)mchsi.com |