Yamaha RD400 test harness question

CALfeRacer

CALfeRacer

Fat man on a little bike
Hello everyone, been a little while. I'm helping my friend get the RD400 which I bought to spark and show some signs of life, but the wiring is pretty hacked from a previous owner. Long story short, he's going to buy it off me if the bike sparks and has compression. We passed the compression test, so now time for spark.
I put on a new set of points/condensers, and tested the resistances on the coils, both of which look good. Problem is there is no continuity from the coil power to the power at the fuse box, there's no key and wiring is cut so I'm thinking there was a sketch attempt to put a new key in and it failed. I want to just hook the coils up to the points and a battery with a super minimal "just for spark" harness to confirm it will spark regardless of the timing being close (not being sketch with him, he said its cool when I ran it by him).
I found this picture online:
1e534f5b.gif

I was wondering if I could get away with just running a new wire straight from the resistor to a good battery, the signal wires from the points, and a ground connecting the battery to the frame. Again, just need to see if it'll spark with a few good kicks and make sure I won't fry anything by sending power to the coils without the rectifier pictured above.
 
You are talking about wiring a total-loss system. Very simple. Battery positive will go to the coils positive (the resistor would be in this part of the circuit if one is being used). The coils ground goes to the points, there should be a wire already there hopefully. The battery ground will go to the frame/engine as usual. Put a switch on the battery to coil hot line if you like, or just disconnect the battery ground to arm/disarm the whole thing.
No coffee yet, and 20 years since I wired one this way but I think I got it right. My first RD350 race bike ran total loss.
 
I don't recall the RD having or needing resistors for the ignition coils. Those were usually reserved for the much larger bikes like the XS1100.
 
Shoeman has the plan. Orange and grey are the "ground" from the coils that go to the points, though the coils themselves don't have colored wires so you will have to go with what's left of the harness wires. Just unplug the not orange or grey coil wires and connect the battery plus, or if a resister is present to the input side and disconnect the harness. I think red w/white stripe is the wire color for the ignition hot coming from the key switch. Can't remember if that color is maintained past the resister. The system should be fine for testing (and brief running) without the resistor if needed. Battery (+) to the coils and (-) to the chassis should have it running if the points/condensers are wired up correctly. Don't worry about the rest of the electrical system as it won't interact if you test the ignition this way.
 
Cool, thanks guys. The resistor is there, I tested it and its reading the correct resistance (1.6 Ohm). The red/white wire is what is going from the resistor to the coils. I'll give it a shot as soon as I can.
 
Quick update, I ended up messing with the stock harness a bit more to see where I was going wrong, realized the toggle switch that I thought was hooked up to the ignition circuit actually wasn't, and the wires to the ignition were just loose in a ball of electrical tape up front. So, I cut the ends off and twisted them together, hooked everything up, and we have spark! Probably should've double checked that before I started thinking about making a jumper harness :o Thanks for the help though!
 
I know it isn't an electrical question, but I didn't want to make another post so I figured I'll just post a reply here and hope for a response.
I did a compression test this weekend and got 60psi on both cylinders. I've opened the engine up before (few months ago?) and the top end looked good, I was also told it had been bored to 420cc with new Pistons at some point before I bought it. I tried putting some oil in the cylinders (remember seeing that can help tell if rings are bad or something?) and doing it again but still got 60 on each.
I'm not sure if I'm doing something wrong or if the gauge is off, but the fact that I'm getting the same on each makes me skeptical that they're both that far off.. Test was done with carbs and pipes off, about 5 minutes after torquing the head bolts to 180 in-lb (~15 ft-lb).
Any help is appreciated!


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Make sure that you have no leaks where you plugged the intake and exhaust ports. How many times did you kick it over?

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What do you mean plugged the ports?
I kicked it over a good few times, the needle behaved like it did last time I did a comp. test (on a KZ650)


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60psi won't be enough to run. If you stick a finger over the plug hole and kick it over, does it feel like there's compression?

More to the point, what make of compression tester have you got? If it's HF, throw it away. They are notoriously inaccurate.
 
I bought a couple of those and so did a mate and all three started out ok,. One now reads 60psi on everything, the second reads 30psi after any number of kicks and the third one didn't make it out of the box.

You can probably borrow one form your local autozone store and get a real reading.
 
Sorry, I thought you were doing a leak down test on the engine. At 60psi it wouldn't be anything to kick that over by hand! If you do a leakdown test check this one out.
http://vintagesmoke.com/product/2-stroke-leak-down-test-kit/

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