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TOPIC: RM125B Resurection...

RM125B Resurection... 17 Aug 2016 16:37 #1

  • cpdave
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Not sure how often I'll update this thread, but here's the RM125B I bought on 3 February of 1977 with paper route money:



The first issue I have to deal with is that a mouse moved into the air box and chewed up the air filter retainer bracket (and the one in the eBay air box I bought to replace it; apparently this item is delicious for small rodents):



Dave

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RM125B Resurection... 17 Aug 2016 19:30 #2

  • Scootern29
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Cool thread. I had one of those with Curnutts for shocks, RM80 ignition, R&D stage 5 porting, and a cut pipe. The bike screamed. If that swing arm is a true DG it's probably worth as much as the bike to the right collector. I do notice the super trap for the silencer. That is kind of interesting for a 125.

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RM125B Resurection... 17 Aug 2016 20:49 #3

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The Super Trap was installed because I started running 3 hour scrambles in '79, but I also hole shot a Dist 7 125 B class moto with it that June so it wasn't hurting output much. The motor just has a little clean-up porting and an R&R 4 pettel reed valve in it. It always pulled really hard right from day one, and the little bit I did just made a bit more power everywhere. And yep, the DG swing arm went on the bike in 1978.

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RM125B Resurection... 08 Oct 2016 10:20 #4

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I made a little progress on the RM125B. The custom K&N air filter is installed in the good eBay air box which has been cleaned up with repaired installation nutserts. Ready to go back in the bike's frame today:







When I tried to order a new K&N air filter, I discovered that, wile they do list, make, and sell a filter for my '77 Yamaha DT400, they have dropped the listing for the RM125B,C,N & PE175. Some research, careful measuring, and multiple Email and phone exchanges with K&N engineering got me a custom build on K&N PN: E-9144 (which is a cylindrical filter with a 5.875" OD and a 4.875" ID) changing it's height to 3.07" (in retrospect 3.0375" would have been easier, but their tolerance is +- 0.060" anyway, so go figure on that). As it sits it is a very tight fit in the surviving period top, somewhat loose on the base of the retaining bracket, and had to be tweaked a little at the top & bottom of each of the pleats to shorten it slightly, but it now fits more snugly than the original one did back in 1977.

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Last Edit: by cpdave. Reason: Adding more detail...

RM125B Resurection... 09 Oct 2016 20:18 #5

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Well, it looks like I spoke too soon on the "air box ready to install" statement in my last post. I'm unsure whether the eBay air box is for another model, or just a different part of the same production run, but, while it slips into the frame like it was built for it, only one of the mounting points is in the correct location for the tabs on my frame. :EEEK Sooo... I had to drill the rivets holding the air filter bracket in the new air box out, and install the bracket in the original one. The OEM one is on the right.


In the course of this operation I discovered that the eBay airbox's filter bracket was also installed without a gasket (the origional one from my bike is in the center front of the above photo), which makes me think it was kluged together at some point in it's past. Anyway, below are the parts laid out for assembly:


And with one rivet to go:


And the air box is finally back where it belongs with a K&N air filter fitted:


Yay2 :beerchug

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Last Edit: by cpdave. Reason: because I can't f*****g type despite using a keyboard almost daily for the past 35 years.

RM125B Resurection... 17 Oct 2016 20:41 #6

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I made a little more progress on the bike's engine this past Sunday. I got the top end torn down and the reed valve off:


The valve in the foreground is the R&R Cycles 4 petal valve that I will have to make new reeds for. Another shot in side view:


In the mean time the OEM valve is going back in with fresh Boyesen reeds:


The piston is back in with fresh MC-1 on the big & small end rod bearings and the mag side bearing. I checked ring end gap, it is right at 0.006" on a 0.0059" minimum. Looking at the cylinder, I decided to revisit the port prep, and the exhaust bridge with some precision file work before freshening up the cross hatch washing the cylinder out and reassembling it (maybe next weekend):


Dave

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RM125B Resurection... 15 Mar 2017 16:52 #7

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Well it's been a while, but I've made a little more progress on the RM. When I started to reassemble the top-end I relized that four of the cylinder head studs (the four that didn't get helicoiled in 1978) were pulling out:



So it came apart again while I arranged with a high school friend who's day job is designing, building, and maintaining electro-mechanical biological analysis robots for NIH, to help me with replacing all six studs and helicoiling the four suspect holes. In the meantime i ordered the studs (readily available as Suzuki seems to have used them in more than 80 models), new nuts, and washers. I did the ordering back in Dec, but then broke ribs go-karting on January 8 and came down with shingles, on the same side, the following Thursday. The broken ribs didn't make me miss any work, but the shingles sure did; mostly because I couldn't sleep.

Anyway, a couple weeks ago I finally got together with my buddy (who has a nice bridgeport & lath in his home shop) and we (he) got the studs helicoiled and changed out.



Everything went very smoothly and the Bike Master stud sockets I bought actually worked as advertised both for turning out the old studs and installing the new ones in unblemished condition.



The proof of the accuracy of the project was that the head slid right on over the new studs, just like it came from the factory! :)

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RM125B Resurection... 15 Mar 2017 17:11 #8

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The next item on my list was a carburetor rebuild. I never upgraded the stock 32MM Mikuni as it always seemed to work well and the bike made plenty of power for my needs. So I took it apart, & cleaned it up first with carb & choke cleaner:



Then gave the main body, bowl, & cap a bath in the ultrasonic cleaner I bought off of eBay (I used distilled water with a few ounces of "Oil Eater" degreaser at 60 degrees C):



The slide had effectivly no wear, but was a little loose in the carb body, so I baked on a coat of dry film lubricant to reduce future wear and take up some of the space:



Finally I installed a UFO in the bottom of the slide along with the recommended smaller pilot jet (from a 30 down to a 17.5 in this case):



Then I reassembled the carb' with a new float needle & seat, went out to the shop, lubed the throttle cable, and hung the carb back on the bike ready for the top end to go back together when it warms up a bit:

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RM125B Resurection... 15 Mar 2017 17:25 #9

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Back in 1978 I bought an upgraded reed valve to replace the RM125B's stock one. With their 1977 RM models Suzuki chose to take a hybrid approach, providing a very conservatively timed piston port to promote low RPM torque and adding a 2 petal case reed valve to provide additional flow capacity to support higher RPM power. Over all it worked pretty well, and the after-market started providing hop-up options almost before you could buy the bikes. First Boyesen started providing dual stage fiberglass reeds, then R&R Cycle brought out a 4 petal reed block to provide additional flow area. I tried the Boeysen reeds:



and they made a small. but noticeable impact on bottom end and mid range power, then I switched over to the R&R 4 petal unit, which provided an additional boost throughout the engine's RPM range. In fact I was able to take my 3 season old bike to a race in 1979, where I was bumped from C to B class and got the hole shot on the first moto ("Whoa! NOW what do I DO?!" :O ... I finished the moto in 8th :) ). The down side, though, is that, While Boyesen is still making their RM reeds, R&R has long since passed on. Sooo, I had to make my own replacement reeds:



It was fairly straightforward to trace the old reads onto the new 0.35mm fiberglass, although it was a little hard to see the pencil on the black material:



I left a margin and then used 180 grit paper to round things off and get to my final dimensions.

So this is ready to go back in, I'll be interested to see how it performs and holds up:

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RM125B Resurection... 15 Mar 2017 19:39 #10

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Shingles are fun. I had them for 2 months last year. Build is looking good.

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