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Motorcycle Forum / General / Yamaha / August 2007Will XJ750 Carbs work on a XJ650?
Will carbs from a 1982 XJ650 work on a 1982 XJ650? Is the carb body the same? I figure I need to check all the jet sizes and so forth... Thanks, Kyle >Will carbs from a 1982 XJ650 work on a 1982 XJ650? Yes. ;-) But your subject line asks a different question. >Is the carb body the >same? They seem to be very similar. Is the XJ750 a bored out XJ650? In that case the carburetor spacing would probably be the same. The typical spacing of carbs on an I-4 is that the pairs of carbs on left and right are close together and there is a larger space between pairs. You can shoe horn carbs into place even if the spigots are larger and the spacing is off by 2mm or so because the rubber boots will yield to force and lubrication. http://www.powersportspro.com/pages/parts/home.aspx XJ750: Carburetor Ass'Y 1 (15r-14901-00) 1 -- 3 Carburetor Ass'Y 2 (15r-14902-00) 1 -- 4 Carburetor Ass'Y 3 (15r-14903-00) 1 -- 5 Carburetor Ass'Y 4 (15r-14904-00 XJ650: 2 Carburetor Ass'Y 1 (5n8-14901-00) 1 -- 3 Carburetor Ass'Y 2 (5n8-14902-00) 1 -- 4 Carburetor Ass'Y 3 (5n8-14903-00) 1 -- 5 Carburetor Ass'Y 4 (5n8-14904-00) I dunno if "15r" and "5n8" are just some Powersportspro model designator or what? > I figure I need to check all the jet sizes and so forth... http://www.powersportspro.com/pages/parts/home.aspx >>Will carbs from a 1982 XJ650 work on a 1982 XJ650? > >Yes. ;-) MAYBE-if you rejet correctly and the volumetric flow is adequate. it will not be bolt and go. >But your subject line asks a different question. > >>Is the carb body the >>same? > >They seem to be very similar. Is the XJ750 a bored out XJ650? no. >In that case the carburetor spacing would probably be the same. The typical >spacing of carbs on an I-4 is that the pairs of carbs on left and right are [quoted text clipped - 26 lines] > >http://www.powersportspro.com/pages/parts/home.aspx Thanks. Sorry about the typo - I meant to ask if XJ750 carbs would work on a XJ650, and if the carb body's are the same. I have a XJ650 that I can't get to start well. I've cleaned the carbs numerous times. I know it is the carbs, because I have put a set on from a friends XJ650 and it ran great. I am looking at a used set of XJ750 carbs on e-bay and was wondering if I could use those. Thanks again. Kyle >I have a XJ650 that I can't get to start well. I've cleaned the carbs >numerous times. I know it is the carbs, because I have put a set on from a >friends XJ650 and it ran great. I am looking at a used set of XJ750 carbs >on e-bay and was wondering if I could use those. Well, it sounds like you might have some gum and varnish in the idle ports and passages. I won't go deep into telling you how to clean those passages out, because I've typed it out a few hundred times and posted it in rec.motorcycles.tech. What you need to know is that the starting enrichener won't work unless the throttle butterflies are almost completely closed. And, if you have dirty passages inside your carbs, you have probably turned the idle speed knob to get the engine to idle. So, you've unknowingly sabotaged your carburetors. >> I have a XJ650 that I can't get to start well. I've cleaned the carbs >> numerous times. I know it is the carbs, because I have put a set on from a [quoted text clipped - 13 lines] > > So, you've unknowingly sabotaged your carburetors. Don't forget to clean the port in the bowl for the enricher as well, it clogs easily if there is an crap floating around.... Are you up for a challenge??? I just can't seem to get my current carbs to work. I have put close to a couple hundred hours of labor into them. I think some evil spirit has cursed them. Any help or further advice would be greatly appreciated. Here is what I have already tried: ==Thoroughly dismantled and cleaned the carbs three times (details on how below). ==I borrowed a different set of carbs from a friend who has an `81 650. I cleaned them for him (part of the deal), and put them on my bike, and then it started and ran great, perfect. Because of this, I am pretty confident it is a carb problem. As near as I could tell, his carbs were identical to mine (jets, etc.) and had the same model number on the bottom of the carb bowls (both were Hitachi, model 5N8). ==When I clean my carbs, I take them completely apart, except for the pilot screw set (as described on the XJ CD set, see below). I do not dismantel the rack, I keep the four carb bodies bolted together. Each time I soaked the carb bodys in part cleaner for 12 hours. Blew compressed air through all passages, replaced pilot and main jets with new jets (first time only). I've paid special attention to the bypass valve and passages. The inside of the carbs are spotless. I even boiled the carb body and bowls at the recommendation of a friend. They are very clean on the inside. ==When I clean the carbs, I can't get the “pilot screw set” out. From what I have read in the XJ CD's, this is three parts below the pilot jet. How do I get these out? Do I need to take them out? I've tried toothpicks, a jeweler's screwdriver, and compressed air, but can't get them out. Any suggestions??? Do these parts screw out, or is it just held in by the o-ring? The “pilot screw set” consists of a washer, spring, and o-ring, correct? If these were dirty, would they cause the problem as I described? ==Checked for air leaks by spraying starter fluid around the intake manifolds and boots (did not increase idle). Because it did not increase the idle, and my friends carbs worked, I don't think it is a vacuum leak. ==Replaced the air filter. ==Synchronized the carbs with a vacuum guage, on the bike. ==Set the fuel mixture on the carbs with a "colortune" spark plug. ==Once started, the bike idles and runs beatifully, very smooth. Just won't start when cold without a small amount of starter fluid sprayed into the airbox. ==I have the original owners manual, service manual, and XJ CD set, and have researched them thoroughly. ==I've checked the float level. ==I've checked the tank for rust, removed the rust, etched and coated it. ==Installed a fuel filter. ==Replaced the fuel line. ==Checked the fuel petcock. ==Had it at the local bike shop, they gave it back to me unfixed after 5 weeks, not fixed. ==The local bike shop checked the compression and said it was 120, 120, 130, 140. Is this high enough? I have been assuming since it ran OK with my friend's carbs, it is high enough. ==Posted in various newsgroups before and got lots of feedback about fuel additives. Tried two. Ran through a tankful. Also tried getting up to about 60 MPH, pulling in the clutch, dropping it down 2-3 gears, opening the choke wide open, and popped the clutch (to create vacuum and suck any debris out of the bypass valve openings. …I don't think fuel additives are the solution here. >I think some evil spirit has cursed them. Too much drama. >==When I clean the carbs, I can't get the “pilot screw set” out. From what >I have read in the XJ CD's, this is three parts below the pilot jet. From what I can tell by looking at the online parts diagram (which doesn't show the idle mixture screws at all) they are on top of the carbs, under a metal plug that has to be drilled out. There's an aluminum sprue sticking up, just forward of the diaphragm cap. Is there a round metal plug in the sprue? To remove the brass plug, you have to drill a pilot hole, taking care not to hit the top of the idle mixture screw. Then screw a small sheetmetal screw into the plug, and pull the screw and plug out of the hole with a pair of pliers How do >I get these out? Do I need to take them out? I've tried toothpicks, a >jeweler's screwdriver, and compressed air, but can't get them out. Any >suggestions??? Do these parts screw out, or is it just held in by the >o-ring? The “pilot screw set” consists of a washer, spring, and o-ring, >correct? If these were dirty, would they cause the problem as I described? If you can see the end of the pilot screw, it has a slot for a common screwdriver. The pilot screw adjusts the flow of fuel/air mixture through the single idle outlet port that is all by itself. The other three outlet ports are by the throttle butterfly, they are the transition ports that allow extra fuel air mixture to flow when vacuum decreases as the throttle is opened. But you need to get the pilot mixture screw out of the hole so you can clean out the single ide outlet port. The engine will never idle correctly if this port (and the passage feeding it) is plugged. >==Once started, the bike idles and runs beatifully, very smooth. Just won't >start when cold without a small amount of starter fluid sprayed into the >airbox. The single idle mixture port is probably plugged and your engine is idling off of the three transition ports because you have opened the throttle butterflies a little bit by turning the idle speed knob up. Doing that defeats the starting enrichener. It needs all the vacuum available so it can suck gasoline out of the float bowl. >==The local bike shop checked the compression and said it was 120, 120, 130, >140. Is this high enough? I have been assuming since it ran OK with my >friend's carbs, it is high enough. It's low, but the engine will run if it has 90 pounds of compression. It just won't have any power. I would do a valve job and re-ring an engine that only had 120 to 130 pounds of cranking compression, but the bike would have to look nice before I would bother with it. >== Also tried getting up to >about 60 MPH, pulling in the clutch, dropping it down 2-3 gears, opening the >choke wide open, and popped the clutch (to create vacuum and suck any debris >out of the bypass valve openings. Now that was a bad idea. The idle mixture ports are tiny, and if they are plugged up, engine vacuum will not suck the gum and varnish out, you need a strong solvent under pressure or ultrasonic cleaning. 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. >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=6 53498&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. |
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