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SmalltownSarah's 1976 RD400C

Discussion in 'Projects' started by holypiston, May 1, 2012.

  1. dood.


    SmalltownSarah just bought a pretty decent 1976 RD400C that is setup pretty well as it sits. I like how the seat makes the bike feel lighter and look smaller. It has older, "period" Spec II pipes and rearsets, italian Tarozzi clip-ons, a very nice BMW rear brake master cylinder (HVC modifies them to work on the RD400) and some cool brushed aluminum turn signals. The problem w/ the bike, was that it would idle somewhat, but would not rev. Sarah said that there were pieces of what appeared to be "wire brush" type material inside the left side cover. I know Sarah through my GF from another forum and they know that we are both RD nuts, so Sarah came down and dropped the bike off.

    We pulled the cover and i could see the silvery material w/ black stuff all over. Come to find out that behind the stator, the brushes had completely broken off and shredded the copper track on the ignition rotor (also shorted it and ripped the top loose, found a replacement off ebay for 30.00 bucks total w/ shipping, not bad. (had the guy check it w/ a multi-meter to make sure it was good!).


    replacement rotor


    and potato chipped the armature,


    along w/ completely melting the field coil connector. It's a good thing that i bought the OEM connector style crimp tool & a bunch of spare period type connectors.


    Sarah's bike had those awful K&N Style pod filters, so I suggested the Y-boot setup (HVC online) has the tuned length for better running and uses one large filter. Much better setup over individual hard end pod filters.


    We are still waiting on new banshee intake manifolds & crossover tube to help tune out the flat spot most stockers have around the 5k~5.5k rpm range. Makes the bike rev more smoothly. NOTE: banshee manifolds need to be modified to fit RD, we cut the rubber dogears off the backside, but are careful to leave the "formed-in" o-ring seal intact for better seal, obviously. I just send those to <---cranks, top end, bore jobs, carb plate mods...has the jigs already for converting RDs to o-ring heads...squish mods, dome, c.c. all that stuff. NIce guy too. If you know Scott Clough racing, he used to work w/ Scott long ago and they are friends. Scott is the goto guy for a lot of porting work for many different two strokes. RB used to port, but leans more to the machining side of things...does a lot of head work for off-road dirt bikes, atvs and all things stroker. And RB Designs is local, lives right off Skyline Blvd. in Portland....his brother owns Cycle Metrics. See a bunch of bikes out there every time i go buy...right on a nice curve too.

    I was in a rush to see if that fixed the rev problem and poor running, so didn't do any other fine tuning and started it up to see. I found that 1 turns out on the airscrews was difficult to start. I turned the airscrew to 1/2 turn and that did the trick and it started right up!

    Here's a video (not ready to run by all means...still some fine tuning, but sounds much better than it did)...nice and crispy...problem w/ turning the airscrews in is the bike is a bit strangled....hence the need to go to a larger pilot, but we'll see after i time it first!!

    So the pilot circuit is telling me the pilot is too small. I'm "guessing" that it is close to stock pilot. Optimal setting for a stock unmodified carb is good between 1.0~1.5 turns. Turning the airscrews out, allows more air to flow, turning them inward allows less. if the pilot is too big, the airscrews need to be turned out, pilot too small, obviously many outward turns leans it out and you keep turning in until it is satisfactory. So I know i have to at the very least, first get it timed, carb synch, injector adj...possibly jetting. Will update w/ a video when i get it running more better.
    Last edited: May 1, 2012

  2. that's a sexy beast!!!!
  3. Wow, that brings back some memories. My first street bike was a '76 RD that Dad got for me in the summer of '77. That was a cool bike to own at 15 years of age. :mrgreen:
  4. Thanks guys. Easy to work on and a lot of fun when they run right :).
  5. looks like the problem we had on our 1976 RD400 we bought not more than 2 yrs ago.


    injector is leaking (HVC makes an oil injector rebuild kit).

    spark plug wires are automotive. Need NGK resistor caps and solid core wires.

    Gonna pull the motor apart and have RB Designs take measurements on the crank & top end. Want to make sure it's in decent shape before i button it back up. I don't believe the crankcase has been apart recently, as I see "old" dirt (like it's been sitting) on the top of the crankcase. I can just imagine an air leak at the seals. Pistons show signs of dark blowby (OEM), bores look ok, but you need special tools and skill for that left to a pro to verify.
    Last edited: May 4, 2012
  6. :scratchea I don't know why any one would want to ride a two stroke?
  7. I'll trade you my daytona for your CBR900RR. It's kinda ratty though, but i think you like it like that.

  8. mjn

    mjn Forum Admin Staff Member

    That is just too damn cool man..

    I wish my cousin wouldn't have sold this one;
  9. That's a beauty and the 350s can rev higher more safely because of the shorter stroke. I like them because they are simple. Hats off to guys working 4 strokes...LOTS of parts to keep track of on those bikes!! Not my cup of tea though. Simple bike for a simple mind.

    Did you know you can pop an RD350LC (canadian) top end on that...get an electric auxiliary water pump and weld attachments to the downtubes for a radiator? An engineer did this in N.C. and had to regear. Probably a few mods to make an exhaust fit, but i bet it would be well worth the effort if you like crazy projects (Already been done and works like a banshee, literally).
  10. looks great, one of my alltime fav bikes the RD also RZ's
  11. james1300

    james1300 Track School Dazed

    Great Post! Awsome bike!:mfclap:
  12. a few updates:

    well i found a couple things.

    The injector leaks...needs rebuild. Wiped that area clean then rode it a short while (prior to teardown), saw it pooling in the side cover.

    One of the heads has some prior detonation damage. They should benefit from a nice re-cut to stock compression and a decent squish..surface it level for a good seal. So will be sending those along w/the other stuff to get spec'd out.

    I had to use a gasket scraper to get the reeds off. Has some old boyesen reeds in there that need to be replaced...look kinda wrinkly. So a new set of those. if she had the original reed stoppers, i'd probably be recommending the stock steels (pull better down low and where i suspect she'll be riding...i've heard good things about 85 YZ125 reeds where you break off one petal and bolt up). Boots look like old original OEM rubbers...they are really weak compared to the banshee intake manifolds. the reed cage surface even had the old "locating" roll-pin for the old style intake gaskets. That had to be removed, as the new gaskets don't come w/ the hole in them anymore.

    Looks like maybe Permatex Hi-tack was used on the intake. Hard to get off...can't just use a piece of wood and a hammer and "tap" off...i had to wedge it w/ a thin gasket scraper. I only use Hi-tack as a last resort for, say, a side cover gasket that likes to migrate w/ vibration. Can be a hassle to get off, but the gasket ain't going nowhere!!

    The thing i am worried about is a seal can be okay, but if the crank is better safe than sorry. I looked inside and it looks okay, but it needs to be spec'd out to be sure.

    Intake parts: got everything but the special banshee style "carb clamps"...will be ordering those soon! I use the RD400 reed cage gaskets and not the banshee style. better sealing. looks pretty cool w/ the "n" shaped banshee crossover tube hooked up. I spent an easy hour carefully using a box cutter blade and dremel w/ sand drum and another small rount metal head cutter bit for the hard to reach ends. I usually have my machinist do this, but you can do it if you are very careful (avoid cutting the square outer seal). Came out pretty good...the square dogears are just too large for the stock reed cages, so cutting them out is easiest way to fit. YOu can also match the manifolds to the reed cages (dremel reed cage) for smoother running, but i don't want to cut into a friends stuff...will work pretty well as it is.


    I also read about this german made "syncrometer" that old VW's use in weber carbs. I can't wait to try this "snail-meter" tool out for carb synch. Especially for the RZ...should be easy o nteh RZ since there is one cable for two carbs. A little bit more time consuming on the RD's w/ separate slide cables. Fits right in the huge bell mouth of the carb.
    Last edited: May 5, 2012
  13. Was just about to split the cases and now i have circlips from hell that won't allow me to access the crank.

    Why someone put "C" style circlips in a piston w/ no removal pocket (so you can stick needle nose to grap it, is beyond me.

  14. use a needle or something small to get it to just lift it enough that u can get a small flatblade screwdriver in there and get it out,
  15. That's exactly what we tried for 2+ hours before we decided to bring it over to our Machinist. He had more precise tools & got it for us.

    So what we found so far in this engine is:

    1) Piston blow by, and it's on the last bore. We have (increasingly less) spares for RD400s...but luckily just what she needs! Our last set of cylinders which we were told should clean up @ 65.5mm (6th overbore).

    2) One of the head's combustion chamber was riddled with piston pieces..someone obviously holed one on that side at some point and didn't bother to fix things. I just got back from RB Designs and the domes look almost brand new...sweet. He slightly altered the squish band for a bit more performance, nothing dramatic.

    3) Bottom End: Where she missed out on total catastrophe was the crankshaft is in decent shape, needing only end bearings and to be tightened up and re-trued, as the rod end play was at the limit but fixable. However, the rest of the bottom end didn't fare so well; RD transmissions are nearly bulletproof, but when a P.O. slams too many 2nd gear wheelies...well, it was goodbye 2nd & 6th, on the countershaft. The dogs were severely worn and the #1 shift fork was bent so bad it didn't look like the proper part!

    So I dug into my last 1A1 bottom end and split the cases yesterday, and although the Crank in this spare was hopelessly rust-stuck, I was hoping somehow the transmission would be okay. To my delight, the gears and shift forks looked nearly new, and I brought the countershaft and the set of rear shift forks over to the Machinist.

    The pic makes it look like there's some rust forming, but it's a weird optical illusion, 'cause there was ZERO rust and the picture also doesn't do it justice as both sets of parts look better in person:


    Last edited: May 22, 2012
  16. RB Designs work on the heads. Love these pictures.

    NOTE: Kevin, since you posted under my log on here, I thought I'd correct your picture text. I also thought initially the damage to the combustion chamber was detonation, but when I looked closer (and felt the protrusions) it turned out to be pieces of exploded/melted down piston fused to it. That's why he was able to salvage them so well...had it been detonation craters as deep as the pieces protruded, well, I'm not so sure they would've been fixable.

    Last edited: May 23, 2012
  17. Ah nice to get a little break. Judy's been helping out alot and mainly the "parts runner" back and forth to Ron's shop. If you guys have RD's, this is the man that has the tooling specifically for the RD series motors to convert your aircooled heads to o-ring heads,...i've blast tested (seized/holed pistons/lean jetting) and the o-rings held up (hi-temp silicone o-rngs). Best seal for the heads you can get. Ron converted my Daytona heads to o-ring heads and I've run o-rings the past 2 yrs. ZERO failures. a 10 pack of o-rings is about 5 bucks plus 5 dollars shipping from McMaster-Carr (can't beat that!). You do that, you might as well have the squish/angle/c.c. done. Bike sounds tits w/ both cylinders cackling at equal volumes...might even run cooler.

    I'm gonna have a carb divider installed in my carbs also...for better low end...

    Check out his website if you have any interest in dirt bikes or RD or similar two strokes!

    My bike w/ the head mods:
    Last edited: May 22, 2012
  18. Are you going with the O-rings on Sarahs bike? I don't see the cuts
  19. Not on Sarah's bike. Trying to keep the money spent to a minimum, just enough to make it roadworthy. The 400F model (1979) requires porting also for the o-ring mods to work, but not on the 76-78 RD400s or RD350s AFAIK. Ron did say the old wiseco pistons were OLDER THAN SIN. I had a replacement set of RD400 cylinders, so those will be bored for banshee prox pistons (middle intake tab filed off). I keep the revs under 8.5k and get quite a few miles. 14.5k on my last set..could have ridden longer, but the clearances were getting to .0045"!! Still ran pretty well.
    Last edited: May 22, 2012
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