My XS 400 only goes 25 mph.

J

Jamin Smitchger

XS400 Member
Messages
22
Reaction score
0
Points
1
First of all,

Hello everybody, My name is Jamin Smitchger, and I like to tinker with old broken down pieces of machinery.

I recently got an old 1981 xs400 motorcycle which had been sitting out a year in the weather and wasn't running. I tore the carbs apart and cleaned them, tried this and that and finally got it running yesterday. It will not start without starter fluid, but eventually it gets going. I was taking it out today and I noticed that the bike would accelerate up until third gear, eventually getting up to 25-30 mph, and then it would not go faster. I kept on shifting up, and although the rpms decreased, the bike kept going the same speed or slightly slower. The throttle was wide open at 25 mph in 2nd, 3rd, 4th, and 5th gear with no added effect on speed. The air filters were not on the bike at the time, but I do not see how that would make much of a difference.

Any idea what the problem is? A friend of mine said I need to rebuild the carbs.

Sincerely,
Jamin:)
 
You don't necessarily have to rebuild the carbs, that is, replace parts, but your carbs are still dirty. I guarantee it. Strip them all the way down and boil them in 1/4 vinegar water for a couple of hours, turning them into different positions every 20 minutes or so.

Also, how are the plugs? new or cleaned and gapped properly? How's the fuel flow to the carbs? How are the air filters?
 
I agree with hough...

The carbs are still dirty I am sure. Did you remove all the jets that come out, including the needle jet which runs up through the middle of the carb (remove main jet and diaphram and push out through the top, taking care not to damage the thread, and might need some persuasion if it has not been removed for awhile)? you may need a piece of fine wire or torch tip cleaner to clean some of the orifices. Clean all the orifices and blow them out with compressed air.

Check to make sure the diaphrams in the top of the carbs are ok, no holes.
 
When I was cleaning the carbs, I sprayed brake parts cleaner through all the jets until it came out the opposite side of the hole. That cleaned out a lot of brown junk and got the cycle running. I did not actually remove the jets.

I do believe it is a fuel problem because the bike has plenty of power in first and second gear but the third gear is weak and the fourth gear is weaker. The air filters were not on the bike when I was running it so clogged air filters are not a factor. I only checked one of the diaphrams in the top of the carbs, but that one was fine.

The spark plugs were the ones in the original bike. They seem to work fine from what I can tell.
 
If it won't start without starter fluid, chances are the carbs are still dirty. Possibly, you have very little compression, but my bet is still on the carbs. The fact that you can get it to rev in lower gears where there is much less load on the engine does not mean the carbs are clean. It means with less load, less fuel is needed....but get a compression check. It's not the transmission, it's the engine.
 
You need to pull your emulsion tubes (the tube that the needle rides in and out of and where the main jet screws into the bottom) this tube has small holes along the sides that allow air to mix with the fuel coming up from the main jet. I would bet (almost guarantee) that these are clogged and you're not getting a good midrange/upper fuel supply.

Also, you need to make sure ALL your passages are PERFECTLY clean. It usually takes multiple cleaning attempts to get all passages, expecially the idle/pilot circuit clean. If you need starter fluid, they are still dirty.
 
Thank you everybody,

I will try cleaning out my carbs with a fine tooth comb. During my initial cleaning I did not know that the jets unscrewed so I will give that a try. Hopefully, I won't break anything.

I will let you know how it goes.
 
Did you say the air filters were not on the bike when you were running it?

If the stock jets are still in the carbs, then that might be your problem. With no air filter there will be way too much air in the mix.

The stock air filters were somewhat restrictive and necessary for the bike to run right.

This is a problem for these bikes. The stock air filters are not readily available nor are they cheap. If you still have original stock air filters, chances are the foam in them has disintigrated and they are letting too much air in as well.

A lot of us have gone to the pod air filters. With those you will need to change the jets anyway. If you search my posts I have some pics.

It sounds like you have a salvageable bike. Without those filters and proper jets you will be running dangeroulsy lean.
 
Mcrowell75 might be right. Before you start to strip-off the carburators again, mount the airfilter, the bike might run way too lean without. Instead of buying new filters, its ok to glue on the old ones some filterpaper from kitchen hoods. It has amost the same resitance like the original paper, which has to be removed before obviously.
Cheers from Munich Hannes
 
Last edited:
mcrowell75 said:
Did you say the air filters were not on the bike when you were running it?

If the stock jets are still in the carbs, then that might be your problem. With no air filter there will be way too much air in the mix.

The stock air filters were somewhat restrictive and necessary for the bike to run right.

This is a problem for these bikes. The stock air filters are not readily available nor are they cheap. If you still have original stock air filters, chances are the foam in them has disintigrated and they are letting too much air in as well.

A lot of us have gone to the pod air filters. With those you will need to change the jets anyway. If you search my posts I have some pics.

It sounds like you have a salvageable bike. Without those filters and proper jets you will be running dangeroulsy lean.
Click to expand...


Good advice. These thing have a tendency to run a little lean even with the filters in place.
 
I was running the bike one day and I noticed that one cylinder was not firing.

So I STRIPPED the carburetors and replaced the needle valve and etc. Both cylinders are now running just fine. It feels great to have fixed that problem.

However, now that the second cylinder is running, there is black smoke everywhere, which is coming from the top of the exhaust pipe on that side, Oil is also leaking from that location. Oil is also leaking out of the tail end of the exhaust pipe and smoke is coming from that exhaust pipe.

So any ideas, I am not really sure how to diagnose the problem. in my opinion, it could be....

1, Blown gasket that connects the engine to the exhaust pipe.
2. Blown head gasket
3. Worn piston rings or a combination of all of the above or both.

Any opinions would be appreciated.
 
oil is either valve/umbrella seal or blow-by, perhaps piston ring. bike has been sitting for a while, or no? ring may be stuck. try trans fluid in the spark plug hole for a few days, then spin the bike without plugs, then reinstall and fire away.

that or take the guess work out and do a compression check;)
 
Yeah the bike has been sitting for a while. I am a little new to working on motorcycles. What will the compression test rule out?:)
 
compression test will tell you that either A.) there is a loss in compression, either by bent/burnt valve, cracked/broken/stuck/burnt rings, or a cracked/broken/burnt piston. could even be caused by bad mechanical (cam) timing. or B.) all are good and you have adequate compression for proper combustion.

a good reading checks off alot of bad stuff:)
 
It is interesting to note though that the bike has way more power than it did when only one cylinder was firing. It is way more fun to ride.

By the way, do you think riding the bike around as is has the potential to damage the engine?
 
potential damage, yes. if compression check goes good, i bet it could use a valve reshim and a good thorough cleaning of the carbs. life should be good at that point. pinpoint where the oil is coming from, because that is going to mess with your mixture, at the very least. hopefully is something minor and not something big.

but, it sounds like the bike runs okay, its just off a tad.
 

Similar threads

Pmcbee
My xs400 adventure begins!!!
Replies
1
Views
221
CaptChrome
CaptChrome
E
78 XS 400 2E won't start!
Replies
6
Views
481
Ed Dunham
E
AmbientMoto
Glad to be here and the XS that was meant to be...
3 4 5
Replies
81
Views
10K
AmbientMoto
AmbientMoto
Aedan Graves
Clutch Issues
Replies
0
Views
698
Aedan Graves
Aedan Graves
Travis Lowe
Finally finished my '81 bobber
Replies
4
Views
747
got20z
got20z
Back
Top