I'm looking for a little advise here on where to go next to cure my sick scrammy.

I aquired a 1995 400 Scrambler that was pretty rough. I stripped it down over the weekend and cleaned it up quite a bit.

Here are a few of the issues I came across.

The pipe was missing a spring where it exits the motor. That joint was cocked pretty bad to the point 20% or so of the exhaust was coming out right at the engine. I fixed this with a new spring, although I probably need to get some new doughnuts since I'm sure the old ones are shot.

There was quite a bit of oily residue at the front of the jug that looked to be coming from between the head and the jug. I pulled the head and the bolts did not seem to be that tight at all. The piston looked quite good. In fact almost too good for the age of the machine. It had zero carbon build up on the dome and was very shiny and new looking. Shouldn't a piston be showing some signs of use after even 50 or 100 hours. I'm guessing maybe it had been rebuilt and the head had been undertorqued? Either way I cleaned it up and bolted the head back on.

I pulled the carb and cleaned it. It wasn't too bad looking but I replaced the needle and seat anyway. I set the air screw at 3/4 turn. The airbox looked pretty good.

I drained the gas tank and cleaned it. There was a fair amount of water in it, but not much in the way of debris.

After that I fired it up, no problems at all. It starts and idles fine. However when you hit the gas it studders and misses a fair amount. If you hold the throttle wide open (or close to it) the engine comes to life and stops missing, but as soon as you let off and go to hit the throttle again, it goes back to missing until the engine spools back up.

So, help a newbie out! Where should I look next to try to get my scrammy healthy again?

 

have you checked the compression? to low of compression could cause these types of gurgles, as could badly set rings, or incorrect jetting.

 

Sounds like there may be something in your carb. Check the pilot jet. Could be a little bit of debris in there. Is it hard to start? If so then it could be low compression. If not then start looking to your carb. It sounds like it is running out of fuel (clogged jets possibly).

 

I didn't check the compression, but it starts very easily. Not even the slightest prob starting. Also I had the carb completely tore down when I replaced the needle and seat. It was pretty clean.

I was thinking it was starving for gas myself, but if so they why does it run better when you give it more gas?

 

When you give it more gas it's on a diff. fuel circuit. For slow speeds and idle the pilot jet is what takes care of things. When you go over 1/4 throttle then the needle jet takes over and over that is the main jet which operates wide open throttle. You can have a pilot problem which is slow speed stuttering which could be lean(not enough fuel) or too much fuel (rich). You could also probably have your choke engaging a little causing too much fuel to be let in and bog things out. Check the choke cable (on dash) and see if it's hard to pull out. If so you could be engaging the choke at all times causing all kinds of jetting headaches. Make sure this is right first then go from there. If it's not then take off the carb and look at the pilot jet. It could have some trash in it. You'll have to take off the bowl to access the pilot jet.

 

choke has very easy engagement and was also pretty clean when I had the carb apart. Is the pilot jet directly under the drain on the carb bowl?

 

Let me ask another question. When I had the tank off, the fuel filter seemed fairly clean, but the flow through it was not exactly seamless. I mean I could blow through the line and fuel flowed through it fine (i drained the tank through the line), but it took a little force to push air through the filter. It certainly had more resistance then blowing through a piece of fuel line. Could that be starving the engine at low rpms?

 

Anytime you take the carb off and take off the choke cable it's easy to not get the cable routed just right and cause the choke to open just slightly. If it feels free then that's probably not it. The pilot is accesed by taking off the bowl.

On the fuel flow thought. Take off the fuel line going to the carb. If you get a good stream going to the carb then more than likely it's fine. Here's a site that you can see the exploded view of your carb and the parts. It shows the location of the pilot jet. Just go to "VM Round Slide tuning handguide". It's in adobe format so you'll need adobe acrobat to read it. It's free so you can download it for free if you don't have it.


Mikuni

 

hmmm...looking at the diagram, I didn't mess with the pilot jet so there is a chance its still dirty. By the way, thats some good reading, thanks for the link!

 

You bet anytime.