Carburetors shooting flames!

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Guitarguy1996

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Hey guys! So I've got a 1980 XS400 that I got as a complete project. It needed a new timing chain, piston rings, rocker arm, and head cover. I tore it all the way down and completely rebuilt the motor and it's looking great. Also, the wiring harness was beyond messed up. The previous owner just cut everything not 110% necessary for it to start which left me with reconstructing everything he cut. It's far from done but I've got it to a point where I should be able to start it and hear it run. I tried to fire it up and after a ton of cranking....um.....the dang thing caught fire! Shot a big ol flame out the carb (wasn't running a filter at the time so I could give it some carb clean to help it out) and lit some spilled gas and the masking tape tags I had made for some loose wires 😅. After breaking my fire extinguisher virginity, I realized I put the cam gear in upside-down. Pulled the head cover, flipped that around, re-set the cam timing, and the same thing happened! Big ol flame out the carbs! Both cylinders have 125 psi of compression so I don't think I'm off enough for it to be a valve timing issue if I'm off at all. All valves are adjusted to spec (.10mm intake, .20 exhaust if I remember right) would this be ignition timing? It's got a CDI ignition on it instead of points so finding info on how exactly to time it hasn't been too easy. I'm sure the process itself can't be hard but I wanted to throw it out there before I tore into it again today and set the whole d*** garage on fire 😬 thanks guys!
 
It sounds like a timing issue, either the cam timing or ignition timing. Since you have just re-looked at the cam timing, let me help you with timing up the electronic ignition. First, you need to do a static adjustment and then once the engine does fire, you have to do the final adjustment with a timing light.

Here is a link to some pictures for the static timing: https://www.xs400.com/threads/breaker-help.20605/#post-186626 look at post #8.
The procedure:
1. Rotate the engine over until you are on the "LF" mark in the timing window. The rotor on the camshaft end should be positioned with the punchmark on the upper side of the rotor, if it is not, roll the engine over one more turn and back to the LF mark.
2. Rotate the backing plate of the coil until the rotor and coil line up like the picture in post #8 above.
3. Tighten down the screws for the coil backing plate and the static timing is done.
4. The bike should start with this setup.

For the final timing, start the bike and use a timing light attached to the left plug wire to fine tune the timing. With the bike running at idle, the timing light should show the LF mark in the timing window. Rotate the backing plate of the pickup coil to get it on the mark.
 
Thanks! I've been messing with it quite a bit today. My battery keeps going dead but I'm at least close. Turns out, it helps to have the right coil connections going to the right side of the motor too 😅. I haven't quite gotten it to start yet but it tried right before the battery died on me. I thought the cam was timed pretty spot on but I was rotated one tooth too far back. It looked level to me but I figured I'd try one more adjustment and my compression shot up to 140. I'll definitely strobe it later and see what it looks like off that too. I think now, it's mostly a matter of re-setting my valve clearances and getting the carbs right. Thanks for the help though! I figured it had to be easier than the points on my wife's Honda CL350. God bless Yamaha!
 
Great! Keep us posted on developments. Just getting it this far with a hacked up wiring harness is a win in my book.
 
Yeah no kidding. The guy literally cut everything. Lights, horn, signal, starter solonoid, even the key! Granted, he was just using it as a dirtbike around the farm till it blew a rocker through the head, but still! Unplug it my guy! Don't just go cutting crap! Anyways, back at it now. We'll see!
 
No luck tonight. It backfired pretty good a handful of times but no start. Is it possible the part that holds the gear could've skipped and is now out of time with itself? I can't even get the ignition set to a point that lines up all that great without re-setting the cam back about 2 teeth past where it was initially. Maybe a new cam is in order?
 
Probably your cam timing is off. What reference are you using to set the cam timing?

I set mine using the Yamaha factory manual from this site, and I think it is the gold standard of procedures. Also make sure you only turn the engine one way to get the left cylinder to TDC and take up timing chain slack (if turning it from the left side, roll the crank over counter-clockwise only). If you don't, the chain tensioner will introduce inaccuracies.
 
Yeah I only turn the engine over to the left or to the front of the bike and away from the tensioner. I'm using the LT on the flywheel and the two horizontal lines on the cam with the arrow kinda thing pointing up. Also, the cam lobes on the left side are facing down. It looked more like the lines were level with the head how I had the timing the first time. Maybe tomorrow I'll pull the valve cover again and go backwards 2 teeth on it? Like I said, I can BARELY get the ignition plate to kinda line up with the thing on the end of the cam if the flywheel is at LF so I think I need to go back a few teeth? Idk.
 
Great job! Getting the cam and ignition timing set on an engine are pretty fundamental engine mechanical skills. That first fire always feels good.
 
The weird thing is, this isn't my first rodeo! I've rebuild a good bit of motors (Honda CL350, YZ426F, JR50, Kawasaki Bayou 220 and 300, and I'm sure I'm missing some), the timing marks on the cam gear just don't look very level to me. They look like they're 1 tooth back. I was honestly afraid the intake wasn't gonna close in time and I'd have a valve to dig out of a piston or two. It just doesn't look right but it fired and ran pretty dang well for not fine tuning the carbs or ignition timing yet. It even idles pretty ok. Tad high but not bad! I'd upload a video but it won't let me for some reason. Says the file is too big.
 
Guitarguy1996 said:
Hey guys! So I've got a 1980 XS400 that I got as a complete project. It needed a new timing chain, piston rings, rocker arm, and head cover. I tore it all the way down and completely rebuilt the motor and it's looking great. Also, the wiring harness was beyond messed up. The previous owner just cut everything not 110% necessary for it to start which left me with reconstructing everything he cut. It's far from done but I've got it to a point where I should be able to start it and hear it run. I tried to fire it up and after a ton of cranking....um.....the dang thing caught fire! Shot a big ol flame out the carb (wasn't running a filter at the time so I could give it some carb clean to help it out) and lit some spilled gas and the masking tape tags I had made for some loose wires 😅. After breaking my fire extinguisher virginity, I realized I put the cam gear in upside-down. Pulled the head cover, flipped that around, re-set the cam timing, and the same thing happened! Big ol flame out the carbs! Both cylinders have 125 psi of compression so I don't think I'm off enough for it to be a valve timing issue if I'm off at all. All valves are adjusted to spec (.10mm intake, .20 exhaust if I remember right) would this be ignition timing? It's got a CDI ignition on it instead of points so finding info on how exactly to time it hasn't been too easy. I'm sure the process itself can't be hard but I wanted to throw it out there before I tore into it again today and set the whole d*** garage on fire 😬 thanks guys!
Click to expand...
This is just me, and I'm a rat bike guy. But if it runs AND does that..... Brother that's a feature, send it
 

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