CR125 running like hell even with mods

[jowoo919]

I'm calling on those of you who know more than me for guidance!

Its a 2003 cr125,
134cc big bore kit from eric gorr
Ported for mid to top and pump gas
Boyeson Rad Valve
FMF Fatty (Small dent in mid section)
FMF Shorty
13/53 sprockets
Running stock needle, 40 pilot, 400 main right now

Now the bike screamed when I got it (stock reeds, stock engine, just the fatty and shorty) would pop up easy in second gear without popping the clutch

At some point the performance just disappeared and I didn't readily notice cause i'd been off the bike for a while doing cosmetic mods after i'd taken a fall and got a concussion, I put in vforce reeds and a 51 rear sprocket.

I've changed everything around trying to fix these issues, had vforce, went to stock reeds, got boyeson reeds. Froze the fatty pipe to expand the dents a bit. Spent hours and hours changing needles and jets at the track. Finally shipped everything off to eric gorr thinking a big bore would fix whatever the issue was and it didn't.

The bike has a power band but I have to abuse the SHI* out of the clutch to get there regularly and then its weak and short when I do get there. I don't know what it could be i'm to the point of wanting the sell the bike. I get on my little brothers yz85s and with a little twist they just scream like hell and I have to hold on to keep the front down, i'd be raping the track if my bike ran like that, and it used to.

What could be causing these issues? Plug gap? Power valves not functioning properly? Something with the bottom end of the engine? The small dent in the expansion chamber? My searching has said no to all of these things.

Guys! What gives?!

 

[_JOE_]

I'd start with a leak down test and then verify power valve operation. If that checks out go back through the carb starting with a thorough cleaning and inspection.

 

[helio lucas]

i´d inspect the electrical system of the bike as well. conctions, stator and timing.

 

[jowoo919]

Hey guys thanks for the replies! I'll have to take the bike somewhere for a leak down test I know the tools to be expensive...

Power valves should be working unless there is a problem in the crankcase because they are still freshly clean and like I said i've been constantly messing with the carb, fresh vent hoses, everything SHOULD be fine.

Also I think the electrical should be fine but i'll definitely look into that as well the best I can. Timing is automatic as I understand it on the newer honda twostrokes? I looked into that before and didn't see how it could be changed.

Really I guess it could be a crankcase leak? But I can't imagine that it'd make it this bad even when i've been adjusting the carburetor jetting...I'm gonna try and check those things out though.

 

[pl690]

First install a new spark plug..BR9EG @ .024 gap. Plug gap doesn't affect performance so much as long as it's in the .022 to .024 range. Once a plug is failed it can't be cleaned. The BR9EG works best for my 125 and believe me I've tried them all. Then I would check compression just to rule it out. It's a lot easier than a leak down test. You should have about 170 on a warm engine new top end with a 134 kit. Any thing less than 150 something isn't right. If the right crank seal is leaking it will appear as running rich because trans oil is bleeding into the crankcase. If the left side is bad it will appear to be running lean. These bikes are notorious for bad crank seals even from the factory. I jetted the **** out of my 05 until I had the lower end rebuilt with both crank seals replaced and that was the biggest fix I could have ever made. It cured all my jetting issues because the right side was leaking trans fluid and fouling my plug. I would also talk to the previous owner and find out what fuel and mix ratio he or she was running. I run 93 octane 32:1 in a stock setup but I yanked that POS mikuni and installed a Keihin 36mm Quad vent. Now the bike rips. Not sure tho if that carb is big enough for a 134 kit. I had a 134 kit and hated it. Next rebuild I would go back to stock. Unless You put in a new high performance crank with a new crank arm chances are you might snap the crank arm in half (speaking from experience...lol) From what I,ve been told by the shop that rebuilt my lower end, the stock crank isn't designed for all that extra power. Also, I think the 03 has the electronic power walve so the cable has to be checked periodically and adjusted. There is a cover on the left side of the cylinder to check the slack. If all else fails...buy a YZ!

 

[jowoo919]

So far i've determined my left side crank seal was leaking, I just got a replacement from motosport, (I ordered a prox crank seal kit but only got one seal?! I was going to replace the wet seal on the clutch side but only got the dry side seal which is all I seem to need). I'm going to install that and hope things get better!

I'd really like to avoid a full bottom end rebuild. I got the 134 kit from eric gorr and all i've read points towards the bike being able to handle the kit just fine.

I don't have the money for a new carb right now either :\ the power valve is mechanically operated by an arm that extends into the crank case. I've read bits and pieces that you are supposed to stuff the pocket in the crankcase where the powervalve control arm goes with moly grease? I'm not sure if there could be an air leak there. The only other place I think there could be a leak is on the exhaust manifold and i'm gonna see about some stud sealant for that.

I've put so much into this bike I don't want to sell it! I know I can get it screaming again some how. But believe me if I had the money i'd buy a new yz or ktm 125/150.

I'm gonna check my plug gap as well. My compression is way low for what people say...I checked it when I put the big bore kit on and its at about 90 I think. Way lower than 150!

Thanks for your post pl690!

 

[jowoo919]

Guys i'm drained, its hard to have time to worry and worry about this thing. But i'd love to get back out riding.

I checked compression with the engine cold last night, throttle wide open and kicked a good 10 times, came in at 100psi and thats with the new crank seal on the ignition side, havn't had time to ride but with some research, everything I read points to 150psi or at the least 120psi.

Suggestions? :\

 

[_JOE_]

How's the power valve working?

 

[FruDaddy]

No suggestion here, but I disagree with the plug suggestion above. The EG plug is fine for a 125, but for big bores you should switch to the BR9ECMVX OR ECMIX. The ground electrode on the EG plug is likely to break off due to the way it is welded on with the big bore. The ECM ground is welded in a bit better. You should definitely consult Eric Gorr for a plug recommendation before using the EG since he did the enlargement.

 

[jowoo919]

Power valves are working. I went to the track and the bike was running okay, I think it's still a good ways from being where it should be though! But the crankseal definitely helped alot.

I'm set with the br9eg, those are good plugs and have the biggest electrode...

As of now i'm gonna just throw my bike to my local pro-action guy who just rebuilt my little bros suspension, I spoke with him and he does engine work as well. Have him take a look and see why i'm not screaming down the track...have him do a revalve on the suspension too I reckon.

Bye bye $$$$, and hopefully hello to what feels like a new motorcycle!