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Discussion Starter #1
:eek: I Have been trying to get this YZF750R to run. It has been to a number of shops all through last year to no avail.

Anyway, last year I was able to get the bike running fine, however the bike would have a problem starting in the mornings. I replaced emulsion tubes, choke plungers, stock jets, adjusted the floats, and gave the carbs a good cleaning.

Since the bike had troubles starting in the morning, it was sent to a shop to get things sorted out. They disassembled the carbs and could not figure it out. They installed a primer to get it going, but since they messed with the carbs they could not get it to run. This shop has since then closed its doors.

I have put the carbs back together and got the bike running, poorly I might add. It had boggled in the 3000 rpm range then clear up. I raised the bowls slightly and am having a hard time to get it started. As it was turning over it shot a HUGE Flame out of the exhaust and the loudest backfire I have ever heard. I have lost my hearing in one ear (I hope it returns) and the bike will not start. :(

Sorry for the long post but I need some help. Any ideas???
 

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rebel litre
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you might wanna check your valves(if you did'nt already) i had the same problem with my zx600a a few yrars back you may need new valves. just my 2 cents
 

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Discussion Starter #4
I have not checked the valves, the bike only has 10000 km, and it ran decent before (not saying that they aren't off). I was going to check compression, but my tester does not have an adapter small enough. I have been fooling around with all the carb adjstments but nothing.
I just pulled the plugs (which have less than 100kms on them) only to find they are fouled to the point to which they barely fire. I am going to put a fresh set of cr9e's in it to start it up and try to fiddle with them some more.
 

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Did you buy this bike non running? Did it start running like crap on you? It does sound like carb problems to me, possibly running rich. There is many problems that it might not start easy in the morning. Oil too thick for example. If you don't have a repair manual get one. It will help you to adjust the carbs, and\or figure out what else the problem might be.
 

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second chimp in space
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My old gs 550 sometimes would take lots of coercing to start. Turns out every valve clearance was too tight. Fixed that and it started great.

Also, if they're way too loose and difficult to get right, you may have head problems. When I brainfarted and held a valve open while turning the crank (hey, I thought it would be easier to get the shims out that way...) and cracked the valve guides, those two valves would be way too loose. Those two cylinders wouldn't fire at all and I'd occasionally get a backfire.

I think something would have to be really wrong with the carbs for the bike to not even start. With the float level close to what it should be, stock jets/needles and idle screw close to stock (2 turns out for my kawi and my old suzuki) it should at least start. It may not run too well, but it should run well enough to fine tune. Only time it didn't start for me is when I had water in the system or when the carbs were really dirty. It looks like yours are squeeky clean, though.

3 things for an engine to run: fuel, compression, spark

Don't forget the ignition. Do you get a good spark? I used to have weathered spark plug boots that needed to be replaced. One cylinder wouldn't fire for half a minute and another wouldn't fire for a couple miles. Try starting the bike in the dark. If you see arcing, there's your problem. You can also try holding the boots and see if you can feel the current (it has to be really bad to do that, but it feels good!). You can also try taking the boots off and just snipping about 1 cm off the wires to get to fresher wire and put the boots back on. That joint gets corroded.
 

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Discussion Starter #7
Well I replaced the plugs and the bike fired up first crank. However the bike runs like crap while under load untill 3000 rpm them pulls hard after that. As it warms up it gets considerably worse. Any suggestions??
 

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Its my understanding that all sportbikes run like crap under 3k. No power, slack in the drivetrain etc.

HOw is the idle????
 

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I dont want to bastardize your post, BUT in your best interest, you may just want to get your ear checked. About your bike, i have no idea.
 

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Have you checked for vacuum leaks? Maybe a hose got kinked or a gasket got pinched in all of the troubleshooting...

You'll hopefully feel better when it's running fine... be persistent... don't give up and let it win... If al else fails, use a hammer... ;)

Seriously, try checking for a vacuum leak... I don't know if your carbs have accelerator pumps like older Chevy Rochester Quadrajets, but those would occasionally go bad and cause problems like that...
 

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second chimp in space
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3V402_YZF600R said:
Well I replaced the plugs and the bike fired up first crank. However the bike runs like crap while under load untill 3000 rpm them pulls hard after that. As it warms up it gets considerably worse. Any suggestions??
If it gets worse as it warms up then it's too rich.

Where is the throttle when it runs like crap? If its just cracked (like in sane city riding) then it could be that your idle is wrong. Since your carbs were taken apart this is a good possiblitity, but the good news is that idle is easy to get right:

Warm the bike up and use the collective idle adjuster screw to set the idle high enough that it doesn't die. The scew I'm talking about is the one that sets a lower limit on how much the butterflies have to stay open. I forget the proper name. It should be accessible from the outside, like on the side of the rack, or between the middle carbs, or wherever.

Locate the idle mixture adjustment screws. These may be plugged by the factory, but I bet your plugs are already removed. They're flat head screws, probably located on the engine side of the carb on either the top or bottom. If they're on top, they control air: in = less air (richer), out = more air (leaner). If they're on bottom they control fuel: in = less fuel (leaner), out = more fuel (richer).

Stock position is usually close to 2 turns out.
With the bike warmed up and idling, turn the screw on one carb until you find the highest engine speed. Lower the idle with the collective, and go again. Repeat on all cylinders. You can probably start out by turning the screws 1 turn in the leaner direction. Turn the screws slowly so the engine has time to find its new speed. I found it easiest to determine what the engine was doing by staring at the tach and listening. That way you have two senses going at it. If you're close to the proper mixture, then you should see a difference with 1/8 of a turn. If your max engine speed is at the borders of the adjustment, you need a different pilot jet.
 
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