1981 XS400 Special... help!

[Mberg9000]

Hello everybody!

This is my first post and I'm hoping that I will find some good advice. To start, I'd like to thank everybody in advance who gives me any information. I am by no means very knowledgeable when it comes to bikes... I'm pretty green. But I do have SOME mechanical knowledge, as well as a well equiped garage at my disposal.

Ok, so here's the problem: I bought a 1981 Yamaha XS400 Special last summer. It worked without any problems for 4 months last summer and for the majority of this summer as well, except recently there has definitely been somthing wrong with it. When idling, it will occasionally "die" for about half a second. The engine will lose most power, the RPM needle will dip down getting further and further each time, and the neutral light will flicker a bit as a result. Sometimes it takes several minutes before this starts happening, and sometimes it happens about every five seconds. Regardless, it eventually does it more and more frequently until it dies completely. Also, when starting the bike cold the engine "putters" more than it has in the past and acts a bit jumpy. A friend thinks it may be backfiring through the carbuerator. When driving the bike in higher RPMs than an idle there are no signs that anything is wrong.

In my effort to find out what's wrong, I think I've ruled out a few potential areas. I've charged the battery and have even run the bike with the battery charger attached and running; the problem persists. I have checked the air filters; they seem relatively clean to me, and I've blown them out just to make sure. I've also tried some spray-in carb cleaner into the air intake on the carbeurators, but there was no noticeable difference at all. The problem persists regardless of where the petcock is.

Has anybody had a similar problem? Does anybody have any ideas what the problem might be? Is it a timing issue? Is it not recieveing enough fuel? A wiring issue? Would it help if I created a short video of the symptom?

Any help is really, truly appreciated. Thanks!

[drewpy]

I'd go back to basics..

check contacts if you have them

check ignition timing

valves gapped

ensure the battery is good, not just charged, but has enough amps to drive the ignition

carbs may have a partial blockage, so may need stripping and check the tank for rust flakes which will starve the fuel to the carbs

check air can replace fuel in the tank ( fuel cap vent)

[Mberg9000]

Thanks for the reply drewpy. I'm hoping that it really IS just some small basic problem. But my lack of experience with bikes especially makes it difficult for me to know where to check for all of these things.

Ignition timing, for example. How would I go about checking that? What exactly do you mean by "valves gapped"?

[drewpy]

Thanks for the reply drewpy. I'm hoping that it really IS just some small basic problem. But my lack of experience with bikes especially makes it difficult for me to know where to check for all of these things.

Ignition timing, for example. How would I go about checking that? What exactly do you mean by "valves gapped"?

first you need to buy a manual, haynes or clymer and read up and understand before you attempt anything. If you don't understand something in the manual you can ask on here!

[Mberg9000]

I picked up a Haynes manual from my local library today and checked a few things out.

I opened the cover for the timing points (one of the screw heads was very stripped, somebody has obviously been in there before...). Inside was not the adjustable mechanism that the Haynes manual detailed, but instead a round black plastic unit with fixed points built in. I also opened up the circular observation panel on the left side of the engine. The LF marker appears to line up properly with the firing point/cam shaft, but the marker for right cylinder lines up when the points are almost past each other. I don't know if this is a bad thing or not, but I didn't think rotating the entire black plastic unit was a good idea.

I also checked the valves by opening up the four "observation ports" underneath the gastank on the top of the engine. Unfortunately I didn't have the proper feeler gauges to reach down and check the gaps (I only have straight feeler gauges, something with a 90 degree bend would be needed for that). Just by moving the valves around though, they didn't seem to have an unusual amount of play (or a lack of ANY play for that matter).

I checked the fuel filter; no problem there. I also checked the sparkplugs, which also appear to be fine.

I guess the next thing on the list would be to dismantle the carb? Hopefully it's easier than it sounds...

[OllieB]

I picked up a Haynes manual from my local library today and checked a few things out.

I opened the cover for the timing points (one of the screw heads was very stripped, somebody has obviously been in there before...). Inside was not the adjustable mechanism that the Haynes manual detailed, but instead a round black plastic unit with fixed points built in.

From 1980 on, these bikes had solid state ignition, not points, hence the black module.

You are going about it the right way. For those w/o any knowledge of engines, it is indeed a steep learning curve but you can do it. Lots of folks here in the U.S. experienced the same a couple years ago when our gas prices went up to $4/U.S. gallon. Thousands bought the cheap Chinese scoots from Internet vendors ("dealers" in name only) and found themselves with a machine they had to assemble, prep and trouble shoot to get it running properly. With the help from forums such as this, folks like yourself slowly became knowledgeable about small carbureted engines and how to maintain them. It also helps if you can find a local friend with some knowledge. Keep at it and you will have a feeling of accomplishment in the end. Lots of very knowledgeable folks here will assist you.

[drewpy]

you will need to re-time the "points" using a strobe. its possible that someone has messed about in there before causing trouble.

I use straight feeler guages and bend the ends to fit. I have always been taught that the job comes first, the tools last. so if it means ruining a tool to get he job done, so be it.

valves need to be checked on compression stroke and with the correct timing lined up. get those sorted first before you attempt the carbs.

[Mberg9000]

I took some photos of my timing module just to be clear about what I'm doing:

The Haynes manual said that it was first necessary to match the left cylinder timing up with the LF mark on the rotor by loosening the two screws (top right and bottom left in the pictures, kinda hard to see) and rotating the whole timing unit. They then said you need to set the right cylinder timing with the RF mark by rotating an "inner" section of the timing unit accordingly.

In my case, there is NO RF mark on the rotor. (There IS an RT mark... I don't know if it's relevant, but I posted the photo anyways.) I assume that since the left and right points can't be adjusted seperately, just the LF mark is to be set at the appropriate point? Is there any method of setting this without using a strobe? If not and I buy (or find) a strobe for this purpose, what is the method exactly?

By the way, thanks again for all of the replies. It's really great to have such practical and knowledgeable help, especially when I'm learning all of this for the first time.

[drewpy]

RT and LT is for your valve timing

LF is the firing mark. I'm asuming that the whole module would move so setting the LF correctly, would automatically set the right side as well.

why don't you borrow or hire a strobe, you'll only need it for a half hour ( so long as the engine is running) as once set up, the electronic ignition should stay in tune!

[Mberg9000]

Good news!

I finally found some time today to set up a strobe and check the timing. It was more retarded than it should have been, firing about halfway in between the LF and LT mark. I advanced it as neccessary, and it runs fine now, even better than it did before!

Thanks for the help, drewpy especially. You're the man!

[drewpy]

your welcome

[Joeleol]

Where can I get a timing kit like that for my XS250 (its the same as the 400 I believe) I really hate points.

[drewpy]

Where can I get a timing kit like that for my XS250 (its the same as the 400 I believe) I really hate points.

should be a "how to" in the xs400 sticky

[SPUDGUN]

I have the same solid state module in my 1980 xs250 and a bought a manual that only had info on the points type so this has been a great post. Thanks, BTW the manual description I bought included my model so Ive written to the seller for a refund.