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2005 Suzuki (Kawasaki) RMZ 250 Motocross Just decided to take on a project that I couldn't resist - the price was right.

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  #1  
Unread 03-19-2013, 09:24 PM
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Default Suzuki RMZ250 Carb Tuning and Troubleshooting

I acquired this bike understanding that the owner and his friends couldn't make it run. They took the RMZ apart and tried to fix it and just couldn't manage to ever get it running again. I didn't even know what was wrong with it when we bought it but I assumed it would need a valve job and new gaskets. I did the top end work without replacing piston and rings since they looked so good when I had it apart. And while I had it apart it only made sense to do a valve job and adjust them back to Suzuki specs.

I was feeling pretty good about starting the bike up. I decided I would just try my luck by assembling the carb, airbox and then just trying to start it up. No luck and I didn't know where to start troubleshooting. I checked and double checked the valve timing. It was dead on. Then I checked for fuel. I used a factory tool to check the actual float level and it was good. Then I checked to make sure there wasn't any corrosion or blockage - all good and clean. Still no start.



What next? The accelerator pump was surely pumping a stream of fuel when the throttle was turned. Then I held my hand over the air inlet of the carb while my friend kicked it over. There was a mist of fuel mixture spray being produced as my hand was wet from fuel and I could see fuel fog in the carb. So what the heck? Maybe the 48 slow jet is unusually large since 40 is the stock. It's a long shot but I replaced the slow jet (sometimes called pilot jet) back to stock. No go. It would just pop every few kicks maybe. I couldn't help but think it was a timing issue again but I decided to stick with troubleshooting the carb.
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Unread 03-19-2013, 09:31 PM
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Next I wanted to make sure the fuel level was correct. Maybe it's starving out? I borrowed this snazzy factory fuel level check tool. These have a part number from Suzuki/ Kawasaki and works with all sorts of Keihin carbs. I was almost about to make myself one until a good friend of mine said he probably had one in his tool stash. I was lucky enough to just borrow his. The only way there would be a problem with the fuel level is if someone was messing around with the float (bad mechanic) or if there was a hole in the float for some reason. In my case there wasn't any problem at all. I held up the carb and measured the fuel level from the bottom of the carb body and fell within spec of what the factory manual calls for. So no matter what I had a proper fuel level.

If you just remove the bowl and measure the actual float level you would probably be just fine too. But using the gravity fed fuel level tester you know for sure how the fuel level is with the assembled carb in the upright position.

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Unread 03-19-2013, 09:37 PM
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More to come. First you have to secure fuel leaks. Just by taking out the fuel inlet elbow I could tell that fuel would just flow right past the old crusty o-rings. The elbow felt like it was rattling when I was putting new fuel hose on it so by simply taking out the retainer screw and pulling it could see which kind of o-rings I would need. The only o-rings that you will get to fit properly are a factory ring. They have to be right on to allow the fuel inlet to push back into the carb body.



And here we are with new o-rings and a thin coating of silicone grease.



I highly recommend silicone grease for all the rubber seals and o-rings while assembling. The carb bowl seals did leak as did the large diameter drainplug seal down on the bottom. I simply conditioned the o-rings and seals with the grease and I didn't have any more leaks when things went back together.
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Unread 03-21-2013, 10:01 PM
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So to continue with the troubleshooting I put the carb back on the bike and made sure there weren't any fuel leaks. The silicone grease on the dried up bowl gasket and fuel elbow o-rings really did the trick. And when I tried to start the bike I would get nothing for about 5 kicks and then maybe a pop or a backfire. This totally is a sign of a tooth off with the timing chain. BUT I had checked and double checked the timing marks over and over. I even brought in an experienced friend to talk about the possibilities of what could be going on.

#1 I had put my hand over the carb while turning engine over to check for fuel. We have fuel.

I checked the timing a couple times. It's as close as possible. It's not totally exact like I want it but the timing marks that line up on the valve cover head surface are maybe 2mm high on the left when the right is exactly in line. So with the crank at TDC the marks are just right while not to the exact thousandth of an inch in line. Just damn close and so close that moving the cam sprocket or the crank sprocket one tooth would throw it out of whack and not help at all.

This reminds me of a lot of Honda /Acura 4 cylinder engines with their timing belts. When you line the crank shaft timing mark to TDC you want the crank mark just to one side of the timing mark on the block. I forget which side it is but you can line up all the marks perfectly but you have to choose exactly which side of the mark you want the crank pulley line to go on... even with the tensioner installed. One side of the mark will fire the engine and the other side won't. It's a toss up.

On this Suzuki / Kawi 250cc engine I have heard of stories of the cam sprockets actually getting hot and spinning on the camshaft. I couldn't help but think about this causing the timing mark on the exhaust camshaft to be indexed out of alignment that 2mm. This is a rare occurance so I put it out of my head but it did come up in the possibilities when my friend was even questioning the cam sprocket timing mark being off by such a small amount. The only other possibility of throwing off the timing mark is the cam chain stretching. But the engine doesn't have nearly enough time on it to cause this. Stretched timing chains might come up in high hp sportbike engines that hit 50K but not in a dirtbike engine that probably hasn't even had a top end rebuild.

To rule out the timing just follow the service manual procedure. Line up the crank TDC markings. Remove the cam cover and check the timing marks and how they line up with the top of the head surface. Then the manual says to count 28 chain link pins between the exhaust timing mark and the intake timing mark. That's it! As long as you have the timing marks lined up "close" and 28 pins between the cam sprocket timing marks you are golden.

#2 The engine timing marks are lined up as close as possible.

Now to test the spark.

Last edited by Smithers; 03-21-2013 at 10:03 PM.
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Unread 03-21-2013, 10:19 PM
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#3 SPARK

Just for the heck of it we decided to check the spark. And boy did we get some spark. With the sparkplug dangling about an inch away from the exhaust header we could see huge spark connecting between the two. Then when we held the plug right on the header we got a flash of white spark. To most people that means they have spark. To me that just seemed like an odd spark. It was too much spark. You might think of it as a wreckless and uncontrolled spark. The resistor was not working properly. While inspecting the spark plug boot I could see obvious signs of overheating. The boot actually stuck to the spark plug like it had melted so I knew right off the bat before I bought the bike that there was more to this bike than what you could see on the surface. When I came back with a new plug the spark was not as explosive but it was a controlled violet colored spark.

The bike fired right up. I was very happy but not out of the woods just yet. The engine would die as soon as you pushed the choke in and I had to adjust the idle screw very high to get the bike to hold idle. Then when I would back off the engine would die. At this point it wasn't easy to start anymore and it surely wouldn't hold a proper low rpm idle speed. Not even close. Something was wrong with the carb but I just spent all sorts of time going through the carb and checking for proper flow in all the passageways and everything.

What was the last thing on the whole bike that I didn't check? The fuel screw! I knew I would find something wrong. The fuel screw controls a lot of the mixture throughout the whole rpm range and it also has a lot to do with starting. I had backed it out 2 turns to begin with but there was a deeper problem. I removed the screw and it simply dropped out of the hole. There wasn't a spring, washer or o-ring to seal off the outside atmosphere from entering in around the screw into the fuel stream. If you don't put the o-ring and the other washer and spring that holds the o-ring tight against the fuel screw you won't have a chance to get the bike to run correctly. The added air simply leans out the fuel mixture and causes a massive lean condition. This is why the bike would only run with the choke on! The mixture was so lean that the choke counteracted this by flooding the carb! What a way to ride a bike!!

And now you know why the spark plug boot was melted to the damaged spark plug. Overheating was the result. Kinda funny how the chain of event unfolds. When you have a condition in any engine you have to ask yourself what was the root cause of this change? You will always find a reason why you have the obvious problem in the first place.
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Unread 03-21-2013, 10:33 PM
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So someone was monkeying around with the carb jets for some reason and neglected to put the fuel screw spring, washer and o-ring seal back in with it. Why? Because the spring is small but very firm meaning it's a REAL pain in the butt to push back in with the fuel screw because you have to turn the screw at the same time so it will catch on the threads that you are trying to grab to hold it in. It's kinda hard.

Luckily I had a new aftermarket flexible fuel screw that I had been saving for my KTM. The same part will fit in any Keihin FCR carb so I tore the package open and proceeded to install it on the RMZ250. It was very hard to install. The spring was so difficult to push in while turning the flexible screw. It took me a half hour until I was able to get the screw threads to engage the carb body. But I got it.

I kicked the bike with faith and it fired right up and actually idled nearly perfect. It was stumbling a little bit unless I kept on the throttle slightly. Regardless of the idle speed adjuster it was just running a tad rough. With nearly half a turn back out on the fuel screw the mixture was richened up a tad and the bike ran PERFECT. I was shocked. The fuel screw adjustment was really a tiny adjustment when you think about what physically happens when you turn the screw. But it did the trick.

The RMZ250 sounded a lot meaner than I imagined it would. The Pro Circuit pipe really barks loud and hard with the blip of the throttle. With the engine warmed up I wanted to see how well the engine would respond to quick throttle blips. The Honda CRF's I have been messing around with recently have a really weak accelerator pump system which causes the engine to bog when you stab the throttle. If you turn the throttle normal or fast it's not a problem but if you just dump the throttle right off idle you won't be going anywhere too soon. With this RMZ it's a completely different monster. I was really surprised when I turned the throttle fast. It took off! Then I let it idle down all the way and I turned the throttle tube as quick as I possible could rip it open. The sucked just jumped full tilt without any hesitation. I'm very impressed with the carb and it's setup so far that's for sure.

  • I'm just at sea level here in California.
  • Main Jet: #178 (stock indeed)
  • Pilot Jet: #48 (stock is 40!)
  • Everything else is stock except the new fuel screw which has a more aggressive taper
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Unread 03-22-2013, 05:45 AM
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I just love root cause analysis and process management.
Nice job of being persistent and not overlooking all of the possibilities.
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Unread 03-22-2013, 06:04 AM
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Me too! I've been doing this since I was knee high and I just think it's amazing how people get by in this world without basic troubleshooting knowledge when it comes to life in general!

You always have to revert back to the basics and then stem the possibilities from there. I was just using my trusty Italian made BCS rototiller yesterday and it stopped running all of the sudden which is odd because it had fuel in it. BUT the fuel tank bracket was bent! So yes it had fuel getting through the fuel hose AND when I hit the plunger in the fuel bowl fuel did come out. But the fuel level in the bottom of the tank WAS BELOW the fuel level required to properly get fuel up to the float level inside the carburetor! Here is a picture of me checking float level with the factory tool with this Keihin carb again:

If the bottom of the fuel level in the fuel tank isn't as high as the float level in whatever carburetor you are using you won't have enough pressure to get fuel through the carb! Oh this made me laugh. I basically disconnected the fuel hose from the BCS carburetor and held the hose next to the BCS carburetor in the same manner that I held this tool up to the Keihin carb and fuel would not come out of the hose when I held the hose tip in line with the top of the BCS carb float bowl! Mad science strikes again! I put more fuel in the tank and it fired right up.

I'll take a picture of the BCS setup today to post up here to show it. Best rototiller in the world. You like my paper Castrol GTX oil can?

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Unread 03-22-2013, 07:36 AM
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"Fluid seeks it's own level"
While I've never used a device like the one displayed builders have used water levels for thousands of years and the little snaky clear tube water level of today are simple and easy, work well over long(er) distances, and require no batteries (thinking laser levels).
Since your real reference point has to be the level of fuel in the bowl (to bad we don't have window in the bowl) this application is spot on.
All I hear in my head right now Thomas Dolby singing SCIENCE! from She Blinded Me with Science. Strangely in this version the bit starts off with a vintage Bimmer or Ural w sidecar. That was 1982 and I was in my prime...whatever that was.
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