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I bought a Yamaha XJR 1300, 4 cylinders air-cooled with carburetors.

It has a problem known from the sale (hence a very decent price), when the engine is hot, the rpm tend to stay higher than the idle setting after a throttle blip. I felt confident I could fix this easily but I am still struggling. I already tried some basic things, replacing the admission pipes (they were cracked), adjusting the TB sync (it was way off), checking for any obvious malfunctionning of the starter. Basic maintenance was done (oil+filter change, air filter, spark plugs).

The problem was greatly reduced by the TB sync (like 80%) but is still present, on a standard ride from home to work I have to lower the idle screw some 2 turns to have an acceptable idle when hot.

It is not only a idle setting problem but really a great difference between idle and RPM after a blip, and I feel a strong lack of engine braking when cutting gas at low to mid rpm.

Next thing I will try will be to disassemble the carbs ramp to set everything to stock settings if needed (idle mixture, needles).

One more thing, it sounds like it has a tappet clearance problem as it does a very clear ticking sound on the left side of the engine head. Can it be linked to an admission missbehaving?

I will take any advice on what to check/set to fix this, thanks in advance.

Update : I think I got a strong lead, I removed the carbs ramp and the two right carbs have a power valve spring too weak, they are very slow to go back in place after I push them up. The rightmost one even stay half-way and makes a metalic tinkle when I push it. So I'm going to buy 4 new springs and hope this fixes the problem.

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    Is your throttle cable sticking/dragging slightly? The lack of engine braking suggests the throttle may be slightly more open when you experience this... – Digital Lightcraft Mar 8 '18 at 10:06
  • Can you explain "admission missbehaving"? – Pᴀᴜʟsᴛᴇʀ2 Mar 8 '18 at 12:05
  • sorry, english is not my native language. I wonder if a bad tappet clearance can cause my problem, engine rpm staying too high and not lowering to idle. @DigitalLightcraft the cable setup looked OK but I will dissassemble and reassemble it to be sure ! – TNCreator Mar 8 '18 at 13:12
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    @TNCreator - I think tappet clearance is very unlikely to be the cause. – Digital Lightcraft Mar 8 '18 at 14:07
  • @TNCreator, I'd suspect an air leak as this only happens when hot. Sounds like you are running lean (more air in your air/fuel mix) caused by metal expanding and gaps forming. I support the propane (or other) method of testing. – Dacid Salin Sep 28 '18 at 4:32
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It sounds like the carburetors are sticking when hot or you have an air leak between the carbs and the head. I'd recommend using an unlit propane torch, or something like WD40 or spray carburetor cleaner. Spray it around the carburetors and the intake ports. If the idle changes it's an air leak. You could try tapping on the carbs with a screwdriver if it's a sticking slide or tapping on the linkage if the linkage is sticking.

  • thanks, I have to try the spray method, I think brake cleaner should work as I use it a lot as starter spray. – TNCreator Apr 23 '18 at 8:48
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You definitely have to do some checking to come up with an accurate solution.

The first thing I’d do is check the cables. As noted, it does sound like something is causing the throttle to stay stuck too high. In theory this could also be due to a choke issue. Just warm it up or do whatever you need to to cause the symptom, then (with it in neutral and ideally the clutch depressed) blip the throttle and watch the cables where they connect to the carb. You should see them return fairly sharply with a snap. Otherwise you have either dirty/dragging/damaged cables or weak/damaged springs or some serious gunk. If this is bad, try to determine if the issue is the cables or the carb pulley. You can disconnect the cables and manually action the carb pulley to see if it snaps back hard, if it does, then your issue is probably cable-side, otherwise there may be bad springs or heavy gunk carb-side. If you get a lot of resistance when trying to action the carb, then I’d recommend tearing it out for a good cleaning.

If that’s all fine, then check the carb. Make sure everything move and returns smoothly and without much resistance or sticking.

As a note caution, disassembly and reassembly of carbs can be a bit tricky. There are lots of little parts and fine tuning actions that go into it. If you’re very confident and comfortable then go for it! If you’re worried or hesitant, then I’d recommend getting professional assistance.

  • thanks, I checked all cables, but everything is fine, the pulley returns firmly to its stop without any drag. I untighened the starter cable too, to be sure it wasnt still a bit pulled when moved all the way back. I think I will have to pull the carbs out. I have a bit of experience with this so i'm confident, only did for single and twin carbs though, it will be twice the fun ! – TNCreator Apr 23 '18 at 8:53
  • well that last part about everything returning smoothly into place seems to be the problem, as the power valves do not. There is a big difference in speed getting back in place between the 4 of them. I ordered four new springs. – TNCreator Apr 26 '18 at 11:49
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Had the same problem, it was the alcohol gas ... had to drain it from the tank, pull the carbs, and clean then replace and turn down the idle and the engine drag back returned... the alcohol was dissolving the aluminum of the carbs ..looked like snot inside lol

  • I don't know what alcohol gas is, is it ethanol ? I never had any problem with 10% (sp95 E10 here in france) but I will try "pure" sp98 next, thanks – TNCreator Apr 23 '18 at 8:50
  • yes it is..the 10%.... after cleaning use only the pure and watch the results – Edward Evans May 23 '18 at 15:23
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The actual problem was the idle mixture. The screws were not evenly set, and the overall setup was wrong, almost fully closed, while being recommended around -3.5 turns. The problem is now solved, I only need to do a new throttle body sync since the idle screws were uneven when I did the first sync. Thanks for the suggestions.

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