Ilovemountains

Ok, I bought a 1977 Yamaha XS400D as a winter project a while back, its got 8,000 1 owner miles on the clock and sat since 91. I just got a new to me set of carbs off of a 1981 which used the same engine. I cleaned them and got the bike to fire but it is back firing like crazy, I don’t have the air box installed but I still don’t know why its popping through the carbs and back firing out the mufflers. Could it be the timing? I have never messed with breaker points on a bike before and haven’t touched them in my VW for a few years. Any help would be great on this one. The bike has aftermarket pipes on it and freaking GROWLS, it’s an awesome bike for a 400, actually quite rare, they were only offered in this model for about a year before they went to the little mini cruiser look. This bike has front and rear disks, and a 6 speed. It won’t hang with my 955 Triumph but for my girlfriend, which you all may remember had a bad motorcycle accident last summer when she put her 600 Katana down, it will do just fine! Here is a link to some pics. I think I got the carbs cleaned out good; they could be out of sync but I’m not 100%...

http://pg.photos.yahoo.com/ph/saabbo.../ph//my_photos

jx94148

I'm no expert but i'm gonna guess that its running a very rich mixture. When you take carburators off of X bike and put them on Y bike with N exhaust you will have air/fuel mixture problems.

All I can say is that you will need to re-jet the carbs. I'm not sure if you are running lean or rich, but the air/fuel mix is causing the problem.

In my experience, timing is somewhat non-adjustable on that vintage of Japaneese bike.

krasher

I agree, if you are running good fuel through it and continuously miss, then you have a combustion issue most likely related to the jets.
Good luck, keep us posted.

VolcomTacoma

I have an old Honda ATC that was backfiring. I disassembled the carb, cleaned it thoroughly with compressed air, then used a rebuild kit from Dennis Kirk to put it back together. Didn't backfire anymore after that. Even the smallest piece of dirt can clog one of the jets. Another thing to make sure of is that your gas is fresh.

krasher

For kicks, you can try to run straight seafoam through it. My Shadow sat for a few additional months after winter and sputtered and killed. In additional to the Stabil already in it, I ran about a half can of seafoam through it and she fired like she had been running all summer.

Ilovemountains

I really don’t think its a jetting issue, it could be but I ran a 600cc sport bike with stock jetting a K&N filter along with a full Yoshimura exhaust system that ran fine. This bike is back firing through the carbs which leads me to believe the timing is out. It may need new jets but I doubt it, the head pipes are stock and it will have a stock air box set up so I can’t imagine that installing megaphone style aftermarket mufflers on a 400 would require a jet kit! Correct me if I’m wrong but if you back fire through the exhaust usually it’s a carburetor related issue, but if it back fires through the carburetor then it’s most likely timing. I think that the timing is out and it’s firing while the intake valve is still open throwing the power stroke back through the carbs, but I’m not sure! This bike uses breaker points and condensers and I have not way of knowing what to set the points to since my book hasn’t come in yet! I am going to order 2 carb rebuild kits along with 2 new carb intake boots since mine are a cracked up mess. Ill keep you all informed though, what did you think of the pictures? It’s going to be a cool little bike. I am going to re-foam the seat and cut the drivers section down some, I also found a NOS set of K&N clubman drop bars. I am also going to install a café style headlight fairing. I've been taking pictures of the bike as I go and will keep them posted. I just ordered new tires, the stock Bridgestone’s scare me ! Link to pictures= http://pg.photos.yahoo.com/ph/saabbo.../ph//my_photos

Ilovemountains

The pictures are in order from day 1 up to what it looks like now, well I haven’t taken any of the carbs yet…. Since day one I’ve scrubbed the entire bike down, rebuild the fuse box, changed the oil and filter, new spark plugs and the new to me carbs. I also cleaned the tank and petcock and made sure it the diaphragm in it was OK. The carbs I got are rather clean, I pulled all of the jets and cleaned them with very fine wire, I pulled the vacuum diaphragms and needle valve out and cleaned them as well. They operate very smoothly. I also put a gallon of fresh 89 in the tank! I think I may take it to the local vintage bike shop and have the timing set and make sure the carbs are synced. I don’t want to take the chance of burning a valve. There is a guy that runs a sweet shop call Motorcycle Classics of Maryland. http://www.mcofmd.com/ He was a master mechanic for Yamaha for many years and is just a good person to work with! A lot of shops including the Yamaha dealers in my area won’t even look at a bike if it’s older than a 1992, which pisses me off but what can you do?!

Doc279

I've got several of the old Yamaha XS twin cylinder 650's
one of them I've owned since high school - 30 years ago. They are all
cold natured sumbitches !!!! I'll usually kick start it with the choke on
and keep the idle kind of high - enough to run.

I turn the idle up to 1200 - 1500 and let it run for 5 - 10 minutes before attempting to ride. Once warm it will start and run all day.
Cold will make them cough - spit and sputter until warm. Nature of the beast !

Steveh29

I talked to a buddy of mine who works on bikes quite a bit. He had this to say:

Timing could be an issue as well as maybe a vacuum leak of leak in the carb boots. I would run the engine only with the air filter on because the carbs are jetted to have that restriction. Without it the bike can run lean and that can definately cause popping.

Sounds like you've gotten lots of good advice on this thread too.

Ilovemountains

My carb mount boots are a cracked up mess. I am not going to run the engine again till I get new intakes, get the carb kits installed and get the air box fully installed. I don’t want to take any chances. I did just win an auction for a new Avon Super Venom front tire on EBay for $35 That’s freaking SWEET. I had the SVs on my old FJ600 and they were great tires. I need to find a rear but since I got such a steal on the front I can justify spending the $$ for the matching rear. Anyway I am still waiting on my book.. I want to check and make sure the points are set right. I am also thinking about installing all of the new parts and just dropping the bike off at the shop to have it tuned and have the tires mounted and balanced. I’ve got under $200 total in the bike now, so I think its worth dropping $150 in for a tune up. You can’t find a bitchin old school bike with 8,000 miles that is this clean for $350! Anyway if any one knows how to set the points on an XS let me know Ill check them when I get home from work this afternoon! Thanks again, Matt