Clutch drag and suspect parts, pavement and two strokes, and compression.

[Cannon00]

I've got a couple questions about engines, hoping some board members can answer them. My friend has an old 92' YZ250, and it's in absolutely mint condition, aside from the fact that the clutch drags, it lurches forward badly when clicked into gear and tries to stall unless you give it throttle while putting it in gear. The clutch is adjusted fine, and the basket and hub were replaced only a month ago because they were badly grooved. After the basket and hub replacement the clutch still drags badly(although not as bad as before). What should we be looking for now? Which parts could cause dragging? The springs are mixed with every second spring being heavy duty(higher resistance) and every first spring being normal(standard resitance). Could that do it? I know warped friction plates can cause dragging, but could the steel plates be a problem also in some way? How about the pressure plate and pushrod? What should I look for on the various parts?

2nd question: I've heard pavement can cause seizures when you roll off the throttle on a two stroke, there's this stretch of abandoned pavement near my house that seems like a good place to practice better balance in wheelies, I was wondering if I avoided throttle roll-offs and pulled the clutch in when not using the throttle, would it be an ok idea to practice wheelies on the pavement? I know the lack of cush in the driveline is a problem, but since I have to ride on a few abandoned roads to get to the trails nearby I figured I would live with the transmission risk, I'm just trying to avoid a seizure.

3rd question: What's the compression you should expect from a modern 250 MXer? How about a 125? I've read vastly different numbers from different sources for 125s, as high as 190psi and as low as 120psi being normal. What are some real compression guidelines to go by on stock engines?
Thanks in advance for any answers

 

[ononewheel]

Yeah I also want to know the same about riding on road. Will it blow it up or what? There is a few roads near me that go up to the crosser tracks and woods, can I ride there???

 

[KiwiBird]

1. Different springs could cause the pressure plate to warp. Have you tried a different oil? My Gas Gas clutch really loves Shell Rotella 5-40W synthetic diesel oil available from Walmart.

2. Yes, you can seize it when rolling off the throttle and coasting down. Pulling the clutch and blipping the throttle will help but I doubt that it would be a problem when doing wheelies.

Disclaimer - doing wheelies or any other form of public displays of speed is frowned upon by the law and the pavement is very unforgiving when you crash/loop out. But I know how it is ;) - remember, the back brake is your friend!

3. I'll leave to someone else

 

[Cannon00]

Thanks for your answers. I've already looped out on pavement before, I'm not doing wheelies in front of people on the side of the road or anything, just practicing them on this abandoned stretch for better balance when I wheelie in the dirt, so I'm not worried about the road rash or the cops. Can a pushrod wear out?
Hope someone can answer about the compression.

 

[KiwiBird]

Did you replace the clutch plates (steel/aluminum and fiber)?

It's important to have the same height of plates or the clutch pushrod will act on a different part of the cam and may not travel far enough to give a clean disengage.

Doubtful the pushrod is worn out, but it could be bent. Some systems have a ball bearing between the rod and the throwout bearing.

 

[COLEMANAPP]

I had similar problems with clutch drag on a bike that sat for some time. The fiber plates were stuck to the alum plates (steel for some). The bike was near new but sat 10 years. I tried cleaning the fiber material off the alum plates but just the smallest amount left still had the clutch dragging. I finally replaced the alum plates and all was well. Just another area to check.

 

[ericlachance]

riding on pavement is not the greatest idea, the hubs on an mx bike are not mounted with rubber dampers like street bikes. the tranny will be absorbing any hit the rear wheel takes.... not great for the trans parts.

 

[Cannon00]

When you say the hubs lack rubber dampeners, where are these rubber dampeners located on streetbikes, is there a rubber dampener between the sprocket and the hub, or the hub and the spokes, or in some other place?

 

[WWR]

Both my street 2-strokes have no problems seizing after a WOT run. Proper jetting and engine coolant setup prevents this from happening. (1994 Honda NSR250, 2000 Honda RS250)

And neither of them have cush-drives in the rear wheels. This is mainly a 4-stroke thing, as the 4-stroke heavy flywheel effect is the culprit here.

I have no idea what is going on with your clutch. The only time I have seen this is with a grooved basket and/or hub.

Your compression will depend on your compression ratio, port timing, and ring condition. The best way to monitor it is to check it with new rings, and see how much it drops per hours on the machine. Check it like every 10-15 hours of riding.

 

[Cannon00]

I was kinda just looking for some guidelines to go by for the compression thing, I've heard such drastically different numbers for stock 250s that I don't know what to go by anymore. Guess I'll just get a good reading after the next topend like you said. The clutch problem was not caused by a grooved basket or hub (though they WERE grooved), since we replaced them both and it still dragged, but now they are grooved again after a week of riding. Thanks for all the input.

 

[JasonJ]

Dont know why you would use different PSI springs in the clutch, In my mind, that would cause warping of the plates. People go to great lenghts to make sure the clutch springs are of the same compression rates. Also did you replace the clutch disk? friction and center plates? Synthetic oil is awesome in the gearbox.

Cush bushings are rubber bushings. Most street bikes sprockets are mounted on a hub that has like 5 or 6 splines about 2 inches or so long that fit in to wells in the rear rim. Rubber cush bushings are infront and behind each spline fin to dampen both accelerations and deccelerations forces and prevent a resonating affect. Using the clutch to ease the transition is needed on a lighter 2 stroke with no cush, Frequency resonation (bounce, or slap) is a force multiplyer that can and will shred parts or even toss our bike out of control, this is the same affect you can see causing SS race bikes to "fish" under race conditons and if the rider dose not react the bike finaly goes sideways and or tosses the rider.

Cyl compression is not a cut and dry fact. It is a result of the bore and stoke and the size of the combustion chamber AKA the compression ratio. So, things like a shaved down cyl head, thicker or thiner head gasket, cyl boaring, piston type, will affect the calculated PSI with the size of the combustion chamber at TDC bing the largest variable. Unless you have a # from a book, youll have to do the math on it. That being said, a leak down test is a much better test of your motors integrity. Low PSI (like 50?) of air is pumped into the cyl and a guage is watched for about a minute to detect loss of PSI. Many 2 strokes have blow by since many have only 1 ring, at the speeds that the motor is running it is not relevant though. Soo, like was said in the other post, check the PSI on YOUR bike when the rings are fresh and use it as a basis.