Help Request: Good ol' Non-running Issue! Hopefully NOT the usual suspects...


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Hello fellow XS'ers!
Having been a lurker for a long time before diving in on a 1981 XS400H, I'm grateful for all the information that's been shared on this site. I'm almost at the end of the tunnel (I hope) in terms of getting my project running, but the latest issue has me quite defeated, and any help is appreciated!

I picked up an XS400H for $300-some dollars from a guy who gave up half way through the dealio.

My current issue is that the bike just refuses to start. It's been converted to kickstart-only, but still retains a factory harness. Problem is that I've never heard it start, so I've had to start from the bottom.

In an effort to solve the issue, I've done the following:
  1. Checked compression: 120PSI with the engine cold
  2. Oil & Filter Change
  3. Adjusted valve clearance (0.04" Intake & 0.07" Exhaust)
  4. Set timing on pickup coil (12 o'clock @ LF)
  5. Checked both ignition coils: 4ohms and 9k-ish ohms on both coils
  6. Checked that the coils are wired to the correct side (Grey - Left & Orange - Right)
  7. Replaced coil wires & non-resistor spark cap
  8. New spark plugs (NGK BPR7ES)
  9. Tested stator by checking resistances- tested without issue
  10. New Lithium Battery (12A 12V, fully charged and hooked up to the bike it reads 13.8V)
  11. New solid state rectifier/regulator (3 Alternator Wires, positive & negative, and an alternator sense & ignition switch)
  12. New Mikuni VM carburetors
  13. Replaced fuse box with an ATO style fuse box (20A main and 3x 10A)
  14. Petcock rebuilt and set to ON (I've tried kicking it with the petcock set to PRI too)
  15. New factory-style ignition switch (EMGO replicate switch)

I've done some prodding with a multimeter and ensured that the safety relay is being powered/grounded correctly. The starter solenoid is getting power from the handlebar switch.

The clutch released during the kicking and the bike is in neutral.

I do have "weak" spark on both plugs (by weak, I am just assuming so because I don't see blue when I test them for spark against the cylinder head). I attached a similar image of what they look like.

When I kick, I hear/feel the engine turning and a deep sound that sounds like a very brief combustion. However, the bike has not once started no matter how hard/how many times I've kicked it.

I've been chasing this problem for about a month, and I'm running out of things to check. I appreciate any and all help towards a solution. I figured it's finally time to put my issue on the forums! Have a good day, and thanks for bearing with my wall of text :D

PS: I will update as I continue to tinker. Been raining a few days, so I'm on a much needed break from the bike for now.


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Plugs look a bit fouled. VM carbs are not stock for these and most that go to them use 28mm-30mm. What size do you have? Tuning for these type carbs can and will be a pain as most that try them never seem to get it right. 120 psi compression is low for these bikes and might be part of the issue. 125-155 is the range you want. Coils for the TCI bikes should be 3k for primary and 8.6 secondary. +-10% and 20%
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You can't get a dark plug like that without at least some running. And it's fouled too.

The non-OEM carbs if not custom built for a Y400 (most are snatched off other bikes) could easily foul you instantly if the idle jets are too big. Where so many go and why they cannot make them work.
Quick update after the rain: I threw new NGK BPR7ES plugs in and it's still not kick-starting. Would some electrical gremlin prevent the engine from starting/producing weak spark? I'm going to take a look at wiring for now. Coils tested at 3.6 OHMS on both, not 4 as mentioned (brainfart!).

I have VM 30 carbs on there.
Could be your non-OEM carb setup there, they commonly can do that if not jetted right. Wetting the plugs I mean.

I will say too that most visual impressions of spark are in error, commonly everybody thinks they are weak. Battery needs at least 12.3 volts or wasting your time. Higher is better. If the spark has already shorted down side of porcelain in plug tip you won't get a hot spark at the gap, you already damaged the quality of it.
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Your compression is low enough to not start easy too.

Use a 1 second MAX shot of starting fluid into each carb with throttles held open and then close them and try to start, if you get no pops or anything you have something physically wrong with engine.
Wouldn't cause the bike to not run, immediately, but this line has me concerned:
New solid state rectifier/regulator (3 Alternator Wires, positive & negative, and an alternator sense & ignition switch)
These bike's regulators should have a green for the field coil, brown sense/supply, and ground. Could be that you have the right one and called it the wrong thing or you might not have the right regulator.
Quick thanks to everyone who pitched in. It turns out it was a combination of two things: the fuel/air setting on the carburetors and the coil wiring. I followed the wire colors on my Haynes diagram for my bike model, but for some reason when I swapped it opposite of what the Haynes shows, the bike started right up, no fuss. I'm thinking maybe someone along the way rewired something...? Not sure, but for anyone in the future with similar issues, try swapping the coil connections to the wiring harness. Now to check the regulator as per BBS's suggestion...
Additional question: My bike appears to have electric ignition (not points). Is there a way I can run the bike only on on cylinder safely for adjusting carbs?