Sort of a long story...
Last weekend my trusty '75 Yamaha RD125 experienced an engine issue that left me stranded on a 100-mile group ride. Prior to this I had done 1200 miles of trouble-free riding without a single reliability issue since a major recommissioning service a few years back. I took the 125 because it has been so reliable. Still not sure exactly what happened or what caused it.
I had recently serviced the carbs, installed new B8HS plugs (about 800 mi ago), installed new UNI NU-2229 filters (about 1100 mi ago), and adjusted timing (L=1.83mm, R=1.82mm). Carbs are original Teikei 18mm with stock jetting; #94 mains, #44 pilots. Floats set at 20mm, airscrews at 1.5 turns. Points are new, coils are original. The bike has a little over 2K miles from new.
The bike always started on 2nd or 3rd kick when cold (1st kick when warm), ran on the rich side, and performed flawlessly near my house. At home (sea level) the temp rarely gets over 80F and is usually in the 70s. The ride was only about 300ft higher elevation than home so not a factor, and the temp was in the low 90s.
During the ride, I got about 50-miles in and the bike started to lose power and bog down. It progressively got worse and would no longer pull 5th gear and the slightest hill would cause the speed to drop down to 25-35mph. I was thinking that maybe I had fouled a plug or had a dead plug. I reluctantly pulled over and flagged down the sweep vehicle so I could be hauled to the end point since I was way behind the group.
Did some brief troubleshooting after I got back and found that the right side was not firing. It would not restart. Pulled the right plug which was wet and oily but it still had spark when grounded on the head. Wiped it down and reinstalled and the right side started to fire a little but then dropped out again. Left side looked fine and the plug was still showing a little rich. Kept trying to kick it over and it would sometimes try to run but was very difficult to keep it running and it would not idle. When I had it running briefly and revved it up a little it was blowing fuel/oil out the pipe and it was dripping on the ground. The right side was completely loaded up with fuel. We bounced the bike with the front slightly lifted to try getting seme of the fuel out and did get a little out of the pipe. I installed brand new plugs and tried to restart but the right side would not fire at all. The right plug has spark when removed but the right cylinder does not seem to fire when the plug is reinstalled. Decided to call it a day and figure it out later when I had time and it was back in my garage.
Still not sure exactly what caused the problem. I'll dig into it when I get a chance. I plan to peek into the cylinder with my borescope, remove and disassemble the right carb, check the points, and pull the right pipe off to drain it and clean it out.
I'm open to any theories anybody might have regarding the root cause.
crank seal would be my guess. leakdown time.
Need to get some bits to fab up a leakdown rig for the little 125 since my standard one doesn't work. The RD125 does not have rubber carb boots and the exhaust ports are only ~27-28mm.
So...in the meantime I did a little preliminary investigation and data collection.
As a refesher, the issue is the RIGHT side.
Plugs (R on the right, <1K miles):
plugs-post-laguna_1Kmi.jpg
Compression test (about 1K mi ago it was ~120 psi both sides):
Left ~117 psi
RD125_L-psi_2024-07-20.jpg
Right ~114 psi
RD125_R-psi_2024-07-20.jpg
Checked the top of the pistons and bores with my borescope:
Left (intake, exhaust, crown)
20240720131350_L-int.jpg
20240720131402_L-exh.jpg
20240720131411_L-crown.jpg
Right (intake, exhaust, crown)
20240720131602_R-int.jpg
20240720131621_R-exh.jpg
20240720131632_R-crown.jpg
Piston skirt (top, middle, bottom):
Piston skirt_R_top.jpg
Piston skirt_R_mid.jpg
Piston skirt_R_bottom.jpg
And this is what drained out of the right muffler even after we tried to tilt & bounce the bike to get some of it out. And after sitting for a week.
20240720_142333_pipe-R-spooge.jpg
about 270cc (9oz)!
20240720_142914_pipe-R-spooge.jpg
wow! Right crank seal would almost have to pop out of position to pass that much oil?
Quote from: RDnuTZ on July 20, 2024, 07:43:15 PMwow! Right crank seal would almost have to pop out of position to pass that much oil?
It's mostly fuel, however I can't tell if it has any red hue from trans fluid because there's so much carbon in it. You can smell the bike from 20ft away the gas smell is so strong.
The guy that was following me on the ride showed me his helmet visor afterwards and it had a fine mist of oil droplets on it. I double-checked the trans oil level to see if I had lost any and surprisingly it looks as if it's just a hair overfilled.
Tomorrow I'll be pulling the carbs to go through them and look for any anomalies, since I need to pull them anyway for the leakdown test.
good luck!
We have a winner! Two guesses I received after the issue happened have turned out to be correct.
Congrats to so-cal-sdr200 & Beavis who both said it might be that the main jet became unscrewed and fell into the bowl. :thumbs:
Today, after I pulled the carbs and carefully removed the float bowl on the right side I found this:
20240721_123529_carb_R.jpg
Not sure how that came unscrewed, but with the Teikei design when the needle jet unthreads the entire NJ/MJ assembly drops out the bottom of the carb body ...and in this case, into the bowl while the engine was running. Doh!
That succeeded in acting like an unmetered fuel pump and just pulled fuel directly from the bowl and into the crankcase and out the pipe.
I still plan to do a leakdown on it after I can source the parts for fabbing up my modified test rig.
Then I'll pull the top end for inspection and take some measurements so I can decide on the best course of action.
Yup. I called it! :celebrate: IIn a phone conversation last night]
Sometimes I hate when I'am right, but, not this time. :dawg:
The same thing happened, main jet dropping into bowl, to my XL350R.
The XL has two carbs. Primary, and secondary for full throttle runs.
Primary main dropped. Bike would idle, then ~1/3- 2/3 throttle it
would sputter and smoke. High RPM and full throttle it would run
pretty well. Confusing to diagnose and I chased my tail for a while.
I hope the leak down test shown no leaks and you can clean up the
spooge. Get down to some more summer riding.
Cheers, Russ :vroom:
Same thing happened on my DS7, chased my tail for a while!
I'm just hoping the fuel rinse that the engine internals got while it was still running didn't do any damage to the bearings or cylinders. That flood of gas washed the oil lubrication layer off. I'll know more when I have the top end off.
DOH! ive had that happen once a long time ago. sure beats doing a crank seal. good thing fuel is stinky.
Looks like USPS isn't going to make this project any easier. As they seem to do quite often with my shipments, they appear to have misdirected a package of mine. Some gaskets I needed were supposed to arrive yesterday according to tracking. They didn't show up and now tracking shows: "Delivery Date Unknown - Moving Through Network"
As of this morning my package from the Sacramento area, which is only 400 mi north of me had found its way to Louisiana. :rolleyes:
Jul 24 2:20 am
Arrived at USPS Facility
SAINT ROSE, LA 70087
Damn main jets!