First post! My Cafe Project

[cthor]

Hey everyone! I'm a Graduate student and needed something to do to get my mind away from school! I've been building cars for years now and as I was browsing through Craigslist the other day, I came upon this 1978 Yamaha XS400 for $400. It was not running. I've never messed with carburetor based motors before and wanted to learn. I picked her up because she turn over and all. Took her home, immediately checked for spark and she had some.

I changed the plugs and took the carbs out yest. Researched a but about carbs and cleans them up along with rejetting them. Carbs were pretty filthy so I kind of figured this was the problem. Installed every and behold, started on first crank!

Now for the diagnostics that everyone hates.

She idles fine but when she gets hot, the idle starts to die down and eventually kills the bike. I raised the idle screw and the idle was good for a bit but symptoms returned, so today I set the timing statically, and she idles great now. Between 1200-1500. I took her around for a ride around town and she's a keeper! She runs great. Even did a highway run and got her to max out at 70mph, about 6500-7000 rpms in 6th gear. This normal?

I knew I shouldnt have but I checked the compression. SMH...The left cylinder has a compression of 100 and the right 120. The manual says should be 140-170. Normally this would be a huge deal but the bike runs great, what do you guys think? After that, I noticed that there was more backfiring coming from the left exhaust and the exhaust flow was not as smooth as the right. Kind of a blep blep blep.

Anyways, besides the compression, everything was sound. I hope to have this turned into a Cafe Racer by the spring and will post up pics as I go. This is my winter project. But before I do any cutting and fabbing, I want to make sure the engine is OK. Let me know if that compression is normal for a bike 34 years old. I'm kind of worried about the left cylinder bc of that and the sound.

Here some pics of the bike.

 

[drewpy]

welcome cthor

6 -7 k revs @ 70 is right, but they rev up to 9.5k!

might just want using to bed the bike down, oh and the carbs will need cleaning!

don't ask how to, do a search and you'll see multiple threads on the subject

 

[bcware]

Keep plugging away.

Pull the carbs and clean them again. Seriously! They need to be super clean in order to run properly.

Repeat a dry compression test on both sides and record the results.

Next, put a teaspoon or so of motor oil in each spark plug hole. Turn the motor over a couple of times (try and keep the bike vertical if you can) and do another compression test.

If the "wet" compression test raises compression your piston rings are suspect.

If there is no change your valves are suspect.

Wait 24 hours for the engine to get stone cold and adjust the valves.

Adjust the timing.

Synch the carbs.

Set your float height to 26 mm if it isn't already.

 

[cthor]

Will be doing the carbs again. I did find a small leak on an intake manifold. Prob going to replace it soon. I have to find a manometer to try and sync my carbs.

 

[bcware]

Make a manometer. Long wooden board, fish tank tubing, 2 barbed adapters, heavy weight gear oil, zip ties.

I can explain more if you need it!

 

[jmd_forest]

If you don't have a lot a practice taking compression readings it easy to get poor readings. I'd try again after running a week or so and when the bike is warm. Be sure to pull BOTH plugs before beginning and crank the throttle wide open while testing, as well as having a fully charged battery.

 

[cthor]

If you don't have a lot a practice taking compression readings it easy to get poor readings. I'd try again after running a week or so and when the bike is warm. Be sure to pull BOTH plugs before beginning and crank the throttle wide open while testing, as well as having a fully charged battery.

that might have been my first mistake. I pulled only the plug that i was testing it on while just disconnecting the wire on the other one...

 

[cthor]

Make a manometer. Long wooden board, fish tank tubing, 2 barbed adapters, heavy weight gear oil, zip ties.

I can explain more if you need it!

please elaborate if you dont mind.
thanks!

 

[xs400neophite]

Hello and congrats on the bike. Welcome to the forum. Know that pefection is unattainable with these old bikes but they are fun as heck to ride! Enjoy!

 

[bcware]

I used barbed adapters to bridge 3 pieces of tubing. The fastest way to get the oil in the tube without bubbles was by sucking it up like a straw. I did this with the short central "U" section and made each side longer by using straight barbed adapters. You could skip the adapters, but you'd have to suck it up a much longer length of tube.