77 xs400 starting problems


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Hey yall just picked up this 77 xs400 from a motorcycle enthusiast who was downsizing due to marriage. He's been vouched for by my coworker as a solid mechanic and he seems like an upfront dude. Now onto the issue.

When I bought it he told me it wasn't running and the reason for that was that he was running it with no filters and it sucked a turn signal relay into the carb. He looked around for components to try and rebuild the head but instead found a complete head and decided to just buy that. He installed it and said that when he went to start it up it wouldn't. Sat on it for a while, life got busy and he never got around to checking it out so it was posted for sale. I got the motorcycle over to my coworkers garage and put the battery on a trickle charge this afternoon. I was able to get a Clymer manual and have been looking through it. I plan on going over to my coworkers garage tomorrow to see about getting it running.

My question to everyone is, where should I start? My plan was to see if I could crank it first and see what it sounds like and maybe go from there but if any of the more experienced wrench turners here have any suggestions on what to look at first I am all ears. This is my first bike but I remanufacture industrial machinery and do tool and die work for a living so I'm not afraid to get in there and get dirty.
Thanks in advance
I would start with making sure the engine turns and it's not locked up. Reason being is if you only bought a head without the valve cover, it could be clamping the cam too tight. The head and cover are machined together for the camshaft journals, then each are stamped with a four digit number to match them. The numbers are in the center between the carbs. If it does turn easy, then it's the old air, fuel and spark checks for ignition.
Yeah I saw that in a different thread. He gave me the old head and valve cover along with another transmission in a box. I tried turning the motor with the kick starter before I bought it and turns pretty smoothly. I'll double check on the matching numbers today and if all that clears I'll start with what you suggested. Thank you for the advice
I'd just start with the basic checks. Compression, spark, fuel, and timing. Odds are, if you've got compression, valve timing is at least close. Regardless, a valve adjustment is always a good idea and if it's been sitting, it'd probably be a good idea to go through the carbs too. But that's where I'd start.
I think the discussion above is the leading the way you should proceed. Let us know how you make out along the path.

I will add this though. It may just be me, but I find it odd that sucking a relay into a carb could cause head destruction as significant pieces of it would have to pass through the carb and then through the intake valve. I don't think the relays are small enough to make it past the venturi and needle in the carb. If the head got damaged by this, I would also take a hard look at both carbs, one is bound to be damaged inside.
Alright so some updates. Got the battery charged and gave it a crank, it turned and sounded pretty good but didn't turn over. Pulled spark plugs and ground to engine, getting spark. Checked the gap, within tolerances, checked the points, they look good and gap was within tolerance. Went on to check the valve timing and it was off so I went ahead and followed the procedure in the manual and set gap to spec. Went to turn again but this time with some starter fluid and man did it get close to wanting to turn over. Coworker thinks its a fuel issue so he suggested taking the carbs off and giving them a good clean. Captchrome I agree with what you think so this will give me an opportunity to see if they are damaged. Ill get around to it this Tuesday night. In the mean time here are some pictures, ignore my coworkers parts bike lol. Also if anyone has any clues as to what year this thing is lemme know, the number on the motor dates it a 77 and the guy who sold it to me thought it was an 80 so I dunno. It also has less than 10k miles on it which I thought was interesting for a vehicle so old.


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Thanks for the pics - love seeing 'em! It is definitely pre 1980 as it is a points bike and in 1980 on the engines had electronic ignition. The VIN would tell the exact year. If you post back with the model digits of the VIN number (on the head stock it is the first three digits after "JYA" and on the left side engine case it is the first three digits) they can be looked up. The engine and frame might be from different years, who knows?

Also, it looks like you have the pilot box and ignition switch from an XS650 on the dash.
Ah the mystery deepens, it has electronic and kick start. The vin ia 2a2 which indicates a 77 xs400d. Havent really been able to find much on that specific model but I'm pissed because the 77 has the gas tank I want. Im building what I think will be a brat style Cafe racer but am starting to lean a little towards a tracker style. Pulled the seat off and noticed they had cut the frame so I dunno if I am gonna be able to fit the flat seat on it long enough for 2up riding. I could weld a hoop on it but thats for down the road, i need to get it running first
I forgot to ask, when I was doing the valve timing so I took the stator/alternator cover off and around a half quart of oil came out. Is it normal for that much to come out? It was on the side stand when I took it off so I expected some but that was a lot
Carbs clean and ...success! Battery was dead so I had to Kickstart it. Runs pretty alright with the choke wide open but when I close the choke it dies. Im guessing idle needs to be set but damn tachometer isn't working so I'll have to set it by feel. If anyone has any other ideas of what might be going on im open to hearing it
Runs pretty alright with the choke wide open but when I close the choke it dies
Probably the idle mixture screws on each carb are too far in. In general they should be somewhere around 2.5 to 3 turns out to get a good, first start idle with the choke off. If they are screwed in too far, the pilot circuit of the carb won't provide enough fuel to run, so you have to compensate with the enricher (choke).
Yeah that was gonna be the next thing to check. If im understanding this correct, the idle mixture controls each carbs mix individually and then the idle screw in between the 2 sets the overall idle speed?
Awesome, I'll get that squared away tomorrow and then go on the first ride. Can't believe how it sounds with the stock pipe too. Previous owner gave me a shorter one thats less flashy that I was gonna put on but I think that chromed out one is growing on me. Last question, the stock carburetor is a mikuni bts34 right? The Clymer manual had something that appeared different than the one I've got and since this bike is pretty modified I cant be totally sure
Alright capt chrome, did as you suggested with the mixture screws. Fired up and ran without choke, adjusted idle by screwing the idle screw all the way in and backed off like a turn. Bike cranks and idles alright but it's not a nice smooth sound. It skips and hops as though it's gonna miss an ignition point and occasionally it backfires through the carb. Thats something you fix through the mixture screw too right?
Well that is positive progress! Yes, now you need to tune the bike at idle. Basically:
1. Check and set the ignition timing.
2. Set the idle mixture screws by opening one side in ¼ turn increments until the idle rpm does not increase anymore and then turning it back in a ¼ turn. Then repeat with the other carb.
3. Synchronize the carbs.
4. Redo the above steps to make sure nothing has changed.
okay I will knock this out tomorrow afternoon. Question though, when I got the bike I went ahead and checked all the timing related things Including the points, and they checked out with some feeler gauges, clearances were acceptable. Given what you just said, there is a way to advance and retard time then? I just assumed ignition was run off the valve cam shaft. Havent really dove into the point system yet so I am ready to be enlightened lol
oh I wanted to ask about the tachometer as well. The old valve cover that I inherited has the tach cable stilll attached while the one on the bike isnt there at all. I looked at seeing if there was a way to reuse the old one but it seems like the two are mated and it would involve some work to install it on the bike. Any Ideas? I can take some pics if you need to see