Funky motor jetting:

[jgas]

I just had a 274cc Eric Gorr motor with porting for all low end torque put in my 250 XC. It now feels like a GG300 from idle-midrange, then quickly signs off and quits pulling. Occasionally it won't stop pulling hard, but will rev out like any normal 2t. It sounds like it's loading up bad on top end. It pulls hard and smooth from idle into the midrange, then quickly starts sputtering and sounds like it's not running cleanly enough to rev out all the way. I don't need a drastic/long/hard top end pull, but would like a bike that will pull cleanly into the upper rav ranges and not sputter and feel like it's loading up, refusing to rev out all the way.

Again, to clarify, it sometimes pulls hard and long like any 250 or 300, sounds clean and smooth with a normal top end pull. Other times it revs and runs perfectly until the upper midrange, then sounds like the choke was pulled out.

My guess is that it needs a smaller mainjet, sounds like it's getting too much fuel. I am kinda afraid to lean it out too far and toast a piston since I just had it rebuilt. All above is my uninformed guessing. If I lean it out on the main jet, how far can I go lean without melting a piston? Would changing the needle help? If I go far leaner on the main, can I take plug readings to determine if I'm creating a dangerous situation or not?

I know I am probably not explaining the characteristics right, but I gotta try and make the problem understandable.

 

[Jakobi]

Considerations.

What jetting are you currently running? Have you made any changes to it since the work was done? Was it jetted correctly prior to the work?

The increased capacity (read bigger pump) will now create a larger vaccuum and thus pull more fuel through the carb. I have found the 250 needs to be about a step richer or more everywhere than the 300.

The upper rpm are the upper rpm. Jetting deals with throttle position. I assume that you mean when you give it a handful to get into the upper rpm its breaking up.

It could either be rich down low and loading the plug up so that when you go to fist it its having to clear its lungs. This would make sense as to why it sometimes does it and other times not. It could also be the main jet is simply too large and is breaking up under light load and pulling clean when you have it on a hill or in deeper sand etc.

If you think its the main, I suggest dropping one size at a time and seeing if you see improvements. Alternatively you could try leaning the needle 1 clip and evaluating how it runs through the mid in comparison. Key is to only make one change at a time and see how it effects the bike.

 

[GMP]

Once you say EG motor all bets are off, could be anything. Too many EG horror stories for me to even consider doing something like that.

Think about this, does the drastic change in top end performance coinside with proportional changes in temperture, altitude, or humidity? If not, then you have a problem with the motor. He has to screw with the PV flap to get it to work with the overbore(and not hit the piston). My guess is its hanging up before fully opening.

 

[Jakobi]

Once you say EG motor all bets are off, could be anything. Too many EG horror stories for me to even consider doing something like that.

Think about this, does the drastic change in top end performance coinside with proportional changes in temperture, altitude, or humidity? If not, then you have a problem with the motor. He has to screw with the PV flap to get it to work with the overbore(and not hit the piston). My guess is its hanging up before fully opening.

That was my initial thoughts when I read EG. I have done some reading in the past and it seemed that for every positive outcome there was an equally opposite one.

 

[Rick]

Seemed like a 300 jug would get better results with OEM parts......less $$$$$.

 

[Jakobi]

Seemed like a 300 jug would get better results with OEM parts......less $$$$$.

There was a thread running on this topic and most suggested just that.

 

[GMP]

Seemed like a 300 jug would get better results with OEM parts......less $$$$$.

Thats exactly what I suggested. Funny, right after that one came up from LAWMAN's part out and I grabbed it.

 

[Jakobi]

Mhmm.. Just curious, how much does EG charge for his services?

 

[Rick]

Save your self a ton of heartache and find a 300 partout and start over! At least you will riding the bike and not trying to figure how to make it work.