1982 xs400 seca dohc problems starting and stuff

[bcware]

Hanging rpms are a classic sign of a lean condition, but this is not always true. Carb tuning can get complicated, but to narrow it down further are the problems worse when the engine is hot and at full temperature? Is it any better when the engine is cold and you first start it up? If the engine is up to full temperature you can also try turning the idle down. Adjusting the idle has reduced or eliminated these kinds of symptoms for me in the past, but it may not be the best solution.

If you need to blip the throttle to lower rpms and it seems to be "bogging down" you may be running too rich.

 

[Jataylor]

The condition is definitely worse when the engine is warm. It only lasts for a minute or so, but I can rev the engine up to 3-4,000 without it hanging. This only happens when first starting up from cold. I turned out the idle mixture screws about 1/3 of a turn and it doesn't want to die on takeoff anymore. They are now turned out just under 3 turns. If I am riding it and I leave the choke on after it is warmed up, the rpms will surge up to 4,000 on their own. If I flip the choke off, it will do things I talked about it the previous post. This would seem to support the notion that it is running rich. I suppose I can check this by looking at the spark plugs, is there anything special I need to do before checking them to get an accurate idea of what's going on?

 

[bcware]

It's normal for the rpms to surge when choke is on. Leaving it on will foul your plugs due to a very rich condition.

The manual recommends a very lean idle mixtuer (1 and 1/2); 3 turns or so out is what people around here recommend.

The manual float height is also often wrong and this can effect the idle too.

The simplest way to read plugs is to note the color and surface condition. White and/or blistered means lean. Tan/brown is normal, very dark or black and/or carbon-fouled indicates a rich condition.

There are more complicated ways to read plugs, but this is a good start.

 

[Jataylor]

I've got the DOHC, and the manual for that one says the idle mixture screw should be out 2 and 1/2 turns. This is what I previously had it set at. Does this mean I should turn them out even more? I will check the plugs after riding it around for a bit, and I'll stick some pics up on here hopefully not too late tonight. I attempted to set the float bowl heights, but I couldn't seem to get an accurate reading. I stuck some tubing in the float bowl drain hole, let out the screw, and it came out to a certain level. However, when i moved the tubing up or down, the gas level in the tube didn't seem to move to the previous level that it was at, so I wasn't sure where the level was. I set it as accurately as I could, but it could definitely be somewhat off. Is there any way I can set this more accurately? Maybe I need the special tool for this job? If the float bowl heights were off, making it richer, would it cause the condition I'm describing? What else could cause something like this?

 

[Jataylor]

Alright, so I got the motorcycle started up, warmed up and rode it around for a bit. Here are some links to pics of the spark plugs:

Left one:
http://dl.dropbox.com/u/23735436/Photo Nov 07, 8 00 08 PM.jpg
http://dl.dropbox.com/u/23735436/Photo Nov 07, 8 00 27 PM.jpg
http://dl.dropbox.com/u/23735436/Photo Nov 07, 8 01 34 PM.jpg

And the right one:
http://dl.dropbox.com/u/23735436/Photo Nov 07, 8 03 46 PM.jpg
http://dl.dropbox.com/u/23735436/Photo Nov 07, 8 03 57 PM.jpg
http://dl.dropbox.com/u/23735436/Photo Nov 07, 8 04 17 PM.jpg

From what was described, it looks like it was running rich, but I really don't know.

 

[Jataylor]

Oh and if they look funny, I was holding them in the socket i used to get them out. They were hot!

 

[bcware]

This article goes in to a fair bit of detail on reading plugs: http://www.4secondsflat.com/Spark_plug_reading.html

Here is a photo you'll find everywhere on the internet: http://www.aa1car.com/library/reading_spark_plugs.jpg

Yours don't look too bad. Bad would be white and blistered or black and sooty. If you're no where near these extremes you're probably just going to lose some performance at worst.

Yes, your idle mixture should probably be more turns out; about 3. If you adjust the screws while the bike is running sometimes you can hear the change in idle clearly.

Are you measuring the floats themselves or the fuel level while the carbs are on the bike? Measuring a fuel level is probably the best way, but you should do both just to keep track of where the floats are as a reference.

If the float height is wrong (or the main jets are too big) excess fuel could be released by the main jets when the throttle is open. The extra fuel starts to pool in the carb bodies, air filters, carb boots, and intake ports. The extra fuel could cause the engine to surge or hang until it is burned off. Blipping the throttle open allows a quick burst of air to enter without allowing enough time for the needles and diaphragms to rise. If the needles and diaphragms don't rise more extra fuel cannot be released and instead the extra air combines with the extra fuel and is burned off more quickly returning idle to normal. This is sort of how all that works from my understanding!

 

[Jataylor]

Alright! I'm starting to get excited about solving this problem! I've got some more info that might be useful. Last night I was tinkering with the idle set screw (not the mixture screw). I found that if I turned the idle way down the hanging rpm issue completely disappeared. I could rev it up and around 4,000 and the rpms fell like a dream. With the idle set so low, however, the bike wouldn't idle itself, I had to hold the throttle open a tiny bit to keep it from dying. I tried turning the idle screw slowly back in until the bike would idle on it's own, and then revving it, but the hanging rpms came back.
Then I started messing with the idle mixture screws. If I turned them leaner, I had to raise the idle to compensate, but it helped the hanging rpms. I got the hanging back to 2,500 from the 4,000 that it was at before, but I couldn't eliminate it without the bike's idle dropping so low that it would die.
What I didn't try was turning the idle mixture screws out, making it richer, and turning the idle speed down. I just didn't think about it, but now I wonder if that would have worked...
In the end, I turned the screws back to 2 and 1/2 and set the idle speed to normal, just so I knew exactly where I was at when I come back to work on it again. I'll turn them out to three when I get a chance. I feel like I'm so close! I'm not really sure what I'm going to do next, I'll try messing with the idle and idle mixture screws a bit more, and then try the floats?
When I measured the floats the last time, I had some tubing lying around that fit the hole, so I put it in the drain hole and let out the screw. I didn't check the actual level of the floats, just the fuel. How do I go about doing that? I've got no idea.
Is there anything else that I should be looking at that might be the culprit? And I know my posts have been a bit wordy, I've been including lots of detail about everything.

 

[bcware]

More detail can only help and if you explain how you do things people can sometimes point out procedural errors.

I have a custom exhaust and big main jets on mine: I turn the idle speed up for starting purposes, but once the engine is warm it will surge and hang unless I turn it down. It sounds like yours behaves in a similar fashion, but you still need some adjustment if it wants to die.

To measure the float height you need to pull the carbs and open the float bowls up.

You can also check how synched your carbs are. Stick your hand in front of (not on) the muffler outlets when the bike is running. Can you feel a difference in pressure from one side to the other? If you can feel a noticeable difference you're well out of synch, but the lack of a perceptible difference does not mean you are synched. You need to make or buy a manometer to get a real reading and balance them. Doing this will improve all-around performance, including the idle.

 

[jscgdunn]

I ma having a very similar problem.

On the intake manifold (carb holder?) there are rubber o-rings. Can these be sourced and replaced?

Thanks Jeff

 

[Jataylor]

Hey Jeff, I found some new O-rings at an automotive store in my town, they were a bit hard to find, but not too bad. If you've got a dealership nearby I'm sure you could pay up the nose and get them there. That said, the O-rings didn't do the trick on mine. They were still leaking air so I got some gasket material and made some gaskets to stop the air leak. You might try gaskets without O-rings and see if that works.

I set the floats as per the manual, but it didn't really make a difference. I've still got the hanging rpms and I sprayed all around possible spots for air leaks with both starting fluid and WD-40, to no avail.

The weird thing is I can unscrew the idle screw a bit, so far that the bike won't idle on it's own, but this completely eliminates the hanging rpms. I can get it to do one or the other, but I can't get it to both idle and return nicely to idle at the same time.

The bike is also really hard to start, I don't know if this is connected, but it is definitely a problem I need to deal with.

 

[XSLeo]

With your bike standing straight up, a clear hose on the drains, hold the hose up by the carbs, open the drains, the gas will flow into the clear hose up to the same level as the gas in the float bowl. The level in the hose should come up to about 1 mm below the gasket surface.
If the level comes out too high or to loww you need the remove the bowls and adjust the floats by bending the tang that pushes on the float valve, this is explained in your repair manual. Measure both sides of the float, both sides should maesure the same. Adjust by bending the frame that holds the floats.
Leo

 

[XSLeo]

If totally backing off the screw won't idle and is hard to start it could be the idle and enrichener circuits in the carbs are still dirty.
This was written with the XS650 in mind but the XS400 carbs are very simular. www.comamckayltd.com/carbguide.pdf.
Reading through the carb section of the XS400 TECH at the top of the page might help to.
Leo

 

[AMeatHead]

also sync your carbs with a vacuum gauge if you don't have one go here to learn how to build one.

page one has a manometer and page two has a bottle types.
http://forum.yamahabikersclub.com/showthread.php?tid=1547&page=2

 

[XSLeo]

I looked at that link. The first pic in that thread won't work. When you start the bike it will suck whatever fluid in the tubing right into the engine. you don't need two lines as shown. You want just one that goes from one carb over to the tool, down one side up the other and back to the other carb.
Like this.

 

[Lionheart1980]

I looked at that link. The first pic in that thread won't work. When you start the bike it will suck whatever fluid in the tubing right into the engine. you don't need two lines as shown. You want just one that goes from one carb over to the tool, down one side up the other and back to the other carb.
Like this.

That's a interesting manometer picture. Hmm