1983 Yamaha Maxim Xs400 won't rev?

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chrisd1984

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My bike starts up fine but won't rev when I engage the throttle. I thought it might have been an oil issue but changed the oil today and there was still quite a bit left in the crankcase. Could this be a dirty carb issue? I also have very little gas left in the tank but the bike still starts so don't see why that would be an issue - although the gas was in there late last year so I could always drain that and replace it with newer fuel. Could it be the oil filter?

Any other ideas? The bike was working fine until the other day when I removed the spark plugs to examine the ends. I put those back properly though and the bike starts up anyway. When the throttle is engaged the bike seems like it's going to stop running but keeps going and just doesn't rev. There are no knocking sounds and no blue smoke leaving the exhaust and there was no pieces of metal in the oil when I changed it so I don't think it could be anything within the cylinders.....
 
I would guess carbs unless you filled the tank to the brim with ethanol-free gas and the requisite amount of stabilizer last fall - and have an in-line fuel filter just below the petcock.
If any one or more of those things is missing from the equation, it is almost certain detritus of one kind or another has plugged up something in the carbs.
 
Yeah had a feeling it was that. Have you rectified a non-revving issue before by cleaning these out? I suppose I have to learn it sooner or later though.

Does this video look like a good tutorial?

Thought there was a link somewhere on vx400 too somewhere but can't locate it
 
here's a thread containing vid links

http://www.xs400.com/forum/showthread.php?t=6375

and it sounds like some of your jet passages are clogged inside your carbs, un-clogging them will fix your non-revving issues. Idling works through only some of the jets, so some are probably fine and that's why the bike still runs. Just open it up and probe everything through with fine wire, boil in vinegar, ultrasonic them or what ever floats your boat.
 
I can't watch the youtube vid you posted right now cuz I'm in my work's firewall. But that link looks like a good how-to. Just don't expect your carbs to look the same, cause they don't. But in principle, the idea is the same. Just make sure you take your time and double- and triple- check everything for 100% cleanliness. It takes most XS400 owners a couple tries to get the carbs cleaned well the first time around. Took me 3 tries before problems were solved, just because I forgot to probe some of the passages.

Good luck!
 
So I cleaned out the carbs on the weekend and still have had no luck in rectifying the rev issue. They actually weren't even that dirty: sprayed the cleaner through all of the crevices and holes and made sure there was nothing clogged anywhere in the diaphragm, bowls, etc.. Placed it back on the bike but it still won't rev too much (although it's gotten a bit better). Basically when you try to rev the bike, the bike engine dies. Does anyone else have any ideas?

1. Could there be an air leak somewhere?
2. Could this have something to do with the float height or how the jets are set?
3. Could excess air be getting in through the rubber boots or vacuum?
4. Could an exhaust pipe be clogged?
5. I noticed the electrodes on the spark plugs were a bit black, could this mean something with the cylinders?
6. Could it be a dirty fuel filter or fuel line? (Both look clean to me but the fuel filter is in backwards)


Not really sure what else to do.....already changed air filter, oil and oil filter...
 
1. yes, this could be a problem. Check by spraying something flammable (brake cleaner, starter fluid, carb cleaner, deodorant) around your carbs. If there is a change in revs, there's an air leak.
2. Yes, but not as likely I would say. Nonetheless, check your float height using the instructions in your manual. Jetting would be my last guess, unless the jets have been seriously meddled with with the past (replaced with much bigger ones, for example, but not likely the case).
3. Yes. See answer to question 1.
4. Possible, but very unlikely. Unless someone deliberately stuffed something in there (banana? lol).
5. If your plugs are a bit black, it's OK. Blackness is a sign of your bike running a bit rich (so a bit too much fuel in your air/fuel mixture). Check your idle mixture screw, it should be 2 1/2 turns out I think. Perfect spark plug color is slightly tan. Running too lean will cause white plugs.
6. Yes. But it's easy to check. Pull off your fuel line in the ON or RES position, and let the fuel which is in the filter and fuel line leak out. Should be about two tablespoons worth. Now try starting your bike (electric starter) and your fuel should start flowing out freely. If not, try the PRI setting (without trying to start the bike). If it doesnt work with either, your fuel filter is most likely clogged. If it doesnt work in ON or RES with the bike turning over, but it does work on PRI setting, your petcock is not working.

But before you try any of this, clean your carbs again. Get very fine wire and probe through every small passage. Seriously. Spraying carb cleaner usually doesn't cut it, and it's likely that you missed some passages. Look through your jets to see if they're open, or make sure you see the wire come out the other end. It takes most people a few tries before getting them clean, and you'll be wasting a lot of time and effort trying to find other problems before finding out the carbs were still not clean enough.

good luck!
 
I see, I did check all of the small holes in the carbs to make sure everything was clear and didn't see anything. I sprayed the cleaner as well but also made sure there was no dirt anywhere. Could there be any other reason why the bike would be doing this besides what was already mentioned?
 
Also when checking #6, do I remove the fuel line from the petcock end or from the carbs? It's very hard to pull from the carbs (tried before and couldn't get it) but unscrews easily from the petcock.
 
If you want to see if the fuel filter is not clogged, you should try both. If the petcock works with the fuel line+filter attached to the petcock (not the carbs), both are fine. If the petcock does work without the fuel line+filter attached, but not with the fuel filter on, you know its the fuel filter thats causing problems.

Pretty much the only reason for your bike not revving would be somewhere in the way the fuel/air mixture is delivered. Which comes from the carbs. So the most likely scenario is that you missed one or two small passages in your cleaning, or there was something floating around and it got clogged again straight away. Check all of the passages again, and check if your petcock is working. Also check your float height and check for air leaks.
 
Yeah I checked the petcock and fuel filter last night and both work great. Looks like I'll have to disassemble the carbs again and really make sure I give them a good cleaning. Could it be the way the carbs are synchornized or is that factory set?
 
Poorly synchronized carbs will make the bike run less than ideal, but carb synching is only fine tuning. Have a look on youtube on bench syncing your carbs, that should be enough until you've fixed your revving problem. I recommend doing a valve clearance adjustment and subsequent carb sync after you've fixed the bigger issues (like your non-revving issue).
 
chrisd1984 said:
... I did check all of the small holes in the carbs to make sure everything was clear and didn't see anything
...
Click to expand...
These are xs400 carbs. Just because something is invisible to the naked eye, doesn't mean it isn't there clogging a passage which is also invisible to the naked eye ...
 

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