I need a specialist.........seriously!!

[razorboy]

Ten ways to Sunday....
That's how many attempts I have made to get my 1988 CR250 to pull through the midrange. It is friggin' hopeless.
Seriously, I have done everything that I can think of but nothing changes that bloody midrange problem?

I have bottom end and topend but "where's the beef"?

Here is a brief list:
1) New piston, rings, topend gasket kit (Athena Graphite Coated)
2) Uncoked power valves and got those working
3) Replaced the ignition system with a newer one(Including CDI module)
4) Replaced the reeds
5) Bought a jet kit with everything from 160-200 main jets and went from 170 through to 190 with no change to the midrange (stock is 185)
6) Check for crankcase pressurization by inserting tube into bottle of water (no bubbles) and no coolant in oil.
7) Double-checked HPP to make sure they were opening. They were!
8) Replaced cylinder head with one that was completely in spec.
9) Tried a hotter plug
10) Pulled the carb off so many times, I lost count.
11) Air filter is clean as a whistle
12) Plug is new (well, one of about a dozen new ones that I have used to test.
13) Compression is excellent (starts on 1 or 2 kicks) and it even idles pretty darn good.
14) Has a Pro-Circuit pipe.
15) Mixing at 32:1 at sea level using Honda Synthetic (Prolube I think it is called?).
16) Checked timing and it is dead on
17) Prayed to the Honda god and I think I felt something hit me on the forehead (damn seagulls)

It feels like I am chasing my tail. Messing with the carb in an attempt to fix something else that is happening.

I am convinced that this is NOT a carb problem. There is absolutely no way that I could make so many changes that had zero effect on the problem.

What about that HPP system? When are those valves supposed to open? I have read that it takes a pretty high RPM to get the centrifugal action happening. Mine open when I get on the throttle but the bike isn't screaming or anything.
Can I remove the valves temporarily to see if that may be the issue? Or at least get them to stay open or closed?

Bike revs smooth as silk on the stand but doesn't do squat in the midrange when moving. It just farts and sputters on and on.

The bike WILL foul the plug after you ride it for about an hour or so ----yet will bong, bong, bong on return to idle after and good blast like it is running lean and suffering detonation?

Its a friggin' enigma!!

I even took it to a reputable shop and they couldn't figure it out either?

Surely gawd, someone out there has experienced something similar to this?

This bike is almost a parts bike now as I am nearly at the end of my rope with it. I hate to give up on something because I can't figure it out but I can tune my sons KX65 to within' an inch of its life yet can't even go for a ride with him because of this piece of s@#t!!

This is the VERY last time this bike will get mentioned on the board. I have wasted enough time on it already.

Cheers
Bernie
:bang:

 

[76GMC1500]

How does it run on top? My 87 is a little soft in the midrange, but then again, it has the biggest flywheel weight known to man on it. With a little bounce on the front wheel and a twist of the throttle, it certainly has no trouble getting the front wheel up in gears 1-4.

 

[just_a_rider]

You could always add flywheel weights, that will give you some torq. Changing your gearing will help, droping a tooth on the front sprocket will lower your gearing as well as adding to the back sprocket. Going a tooth smaller on the front is like adding 3 teeth to the back and will be cheaper than a back sprocket but you can fine tune the gearing by changing the back sprocket a tooth at a time. I assume your looking for mid range through corners? Gearing will do it.

 

[razorboy]

Hi Guys,

No, this is a general running problem, not a "I don't have enough poop" problem.
The bike runs like crap through the midrange. It sputters and bogs like it is leaned to the max yet it fouls the plug after an hour of riding. Will running VERY rich also make the bike run like a dog? I have dropped the needle through every position and even run the airscrew as far as 4 to 5 turns out. No fixing that midrange problem.

Once I get through the midrange, it goes like a scalded cat! However, after I bring her down, she starts to make those bong, bong, sounds like its detonating.

I can also get it to wheelie through 1-3 gear as long as I don't pull the revs into the midrange. Let's say from idle to 1/4 throttle it is great and from 3/4 to full it is great but a hunk of junk from 1/4 - 3/4 throttle.

Cheers
B2

 

[76GMC1500]

Sounds like your main jet and low speeds stuffs are right and you need to play with the taper of the needle jet.

 

[just_a_rider]

Fouling plugs after an hour sounds like a jetting problem beings you said the compression was good, and yes runing rich will afect the running. You need to play around with some more jetting, start at 170 and go down since the stock jetting is always rich. My 96 KX250 sports a 45 pilot and a 158 main with the needle in the second slot from top and the air screw 1 and an 1/2 turns out and I still need to go down a little. The float settings can also effect the rich or lean issue.

 

[kx125412]

Yeah when I first got my bike running it was wayyy to rich and would just bog until it got on the main jet then just screamed. I leaned out the needle and it just about fixed it completely. I would say get a whole leaner sized needle and then go from there. And its not that often where the float settings need to be played with, only if your going for that perfect jetting.

 

[razorboy]

Hey Guys,

Does it make any sense that if I was running rich in the midrange and had the air screw way out that I would have noticed some difference?
This is what is perplexing is that no matter how aggressive I get with the air screw or jetting (although 170 being the lowest that I went) nothing seems to change in the mid?

Cheers
B2

 

[snb73]

Have you changed the pilot jet?

 

[2strokerfun]

My air screw never seems to make any difference except on pilot jet idle. I've had it vibrate out during a ride and didn't notice until next time I started the bike.

 

[razorboy]

Have you changed the pilot jet?

I did change the pilot but put the original one back in after not noticing a difference. I put a leaner one in.

Tell you what.
I am going to mess around with it some more today and see if I can get anything to change in the midrange again. I am also going to triple check the HPP system.

Cheers
B2

 

[76GMC1500]

 

[zewcrew]

It's my understanding that the air screw should not really play any role in the midrange, so I would set that back to around stock and work on the jet needle. Is the needle worn? Flat spots or anything like that?

 

[kx125412]

For the airscrew and pilot all its good for about 1/4 throttle. To set it let the bike idle nice and turn the airscrew to 1 1/2 turns out from all the way in. Go in and out until the bike idles the fastest then re-adjust your idle to wherever you want it to idle. Over two turns out get a smaller pilot, more than a turn in, get a bigger pilot. Its most likely your needle and clip which comes into play at mid throttle. The pilot jets for 1/4 throttle, needle and clip for half and main jet for full throttle. Just keep playing with that needle and leaning it out until it cleans up.

 

[whenfoxforks-ruled]

Needle and slide sounds like your target. But, I got to ask is that cdi you changed for that model year specifically?

 

[rockers109]

Sounds like your carbs slide is shot. I had that problem with a 88 kx125 I had. I had to get a new carb because the carb body was also to worn out. That bike ripped after I got an after market carb and rejetted it.

 

[actual mechanic]

Just a silly question but are you sure the piston is in the right way?? Arrow forwards?? I had a late CR125 with very similar probs, thats all it turned out to be,intake port on piston not letting in enough fuel/air mixture... i agree that it doesn't sound like carb as your changes have made no difference, have you physically checked crankseals? Sucking in gearbox oil? Worn slide is a possibility though..

 

[Redrodent]

You can chase your tail all over the place if it's the wrong needle. Moving the clip up and down won't change the profile. Try this. Turn off the gas, do some roll ons in the middle range. If it really clears up and picks up power as you run low on gas, well, you need someone smarter than me to suggest a different needle.

 

[Jamz]

I had a similar problem on my 00 YZ250 at sealevel. I would lower the needle all the way down and it would help a little but was still a mess. So I ordered a thicker needle now it runs great in the middle clip. The biggest thing is deciding what needle. They come in different tapers and thicknesses. You want a leaner mid so you definitely want to go thicker (lets less fuel through). But then to decide where exactly your rough is kicking in and decide the taper from there. I know for my YZ the different tapers = 1/2 clip movement. But as said before the Main is from 3/4 up, airscrew is for idle to maybe 1/8-1/4, pilot is idle to 1/4 and the needle is your bear 1/4 to 3/4. But the airscrew nor the main will affect mids or at least that is what I have learned reading every how to jet post here when I was trying to get mine right :-D You have done alot to that bike and a needle is not that pricey so it wouldn't hurt to order one or two and give it a try. If it is sputtering in the mids then it is rich and will have no power....if it bogs in the mids it is lean and will have no. Good luck hope you get it running right soon.

 

[BSWIFT]

I'm thinking your CDI is not right. The ignition should advance at the same time the power valve opens. If they are not in sync, you will have a bog. It will appear as a jetting issue but is harder to detect.

 

[Morvo]

I would say it's more to do with the electrics aswell, possibly the CDI or maybe one of the coils are breaking down perhaps?

 

[dezryder]

Old thread.

The bike may have been sent to the dumpster already! If not, you might consider trying a different pipe if someone has a compatable one you could borrow. If you still have the OEM one put it on for an experimental test ride.

Expansion chambers can really change the characteristics of the powerband depending on what they are formulated for.

Just a thought since it sounds like you've tried/checked everything else a billion times. (Very frustrating for sure.) Good luck!

 

[76GMC1500]

Now that I read through some of the posts again, it sounds like the governor is kicking the power valve open early. Either that or exhaust pressure is opening them because the power valves are no longer tied in to the governor.

 

[snb73]

Now that I read through some of the posts again, it sounds like the governor is kicking the power valve open early. Either that or exhaust pressure is opening them because the power valves are no longer tied in to the governor.

I agree. Make sure your power valve is preloaded and/or operating correctly.

 

[dezryder]

Okay guys, what if the OP bought the bike used and the previous owner got a pipe for cheap that wasn't the right one, but it fit? (different year/application) whatever. I'm shootin' in the dark here because the OP seems to have covered a ton of the basics already.

I'm no expert, but been wrenching on my own bikes, done some dealership work, and for a while now. I mentioned the pipe angle because it hadn't been discussed in this thread at all to my (skim reading) knowledge.

My two cents.

PS?~ If you removed the power valve all together...would the bike still run? I already know the answer...do you?