Engine Idle Problem.....

[jackc2]

I have a XS400 that I originally purchased directly from the dealer when I was 18 (50 now ouch). Trying to get it running smoothly so I can ride with and teach my 18 year son to ride as well. Great starter bike. I love it and still do.
It is running and runs strong with only 9K miles on the engine.
Carbs have been completely rebuilt. However, I did not replace the diaphragms as they look to be in good working order and are VERY EXPENSIVE.
The carbs have also been balanced.

So the problem I am having is that the bike after it gets warmed up will not idle.
When you come to a stop it idle will drop to around 1000 RPMs and then it will slow creep up until it is at 4000 RPMs(like the choke is pulled, it is not) at which point I let on the clutch to bring it back down and it creeps back up again. I cannot get it to just stay down at 1000 RPM and of course I do not want my son learning on a bike that has this problem.

Since I have been told that bad value adjustments can cause this problem
I am working to get the values adjusted properly (which I have been struggling with) but with help from another forum here I think my problem is I am introducing slack in the time chain before making the adjustment by backtracking to the mark when I pass it. It easy because the engine wants to flow right past it.

Can anyone offer other suggestions on what might cause this type of problem?

 

[Wolfe_11B]

You'll either need a buddy to help keep the crank from moving, or if you search on here, some people have made tools to hold it in the right spot while doing the adjustment

 

[ToastLoader]

Vacuum leak? Air could be getting sucked in through a loose carb or airbox clamp.

 

[jackc2]

Update, I check for air leaks all along the air intake systems. The method I used was to spray engine starter spray around all connections to see if it changes engine idle and it does not. I have also fully inspected all of the connections and all are in good working order. I also replaced the boots between the engine block and the carbonators with new ones and have tested this area for leaks as well. Feel like I have eliminated this possibility. Thanks for the suggestion....

 

[jackc2]

On the value adjustment issue. Attempted to adjust values again tonight (5th time) and still I do not get a good result. I made 100% sure I am on the exact mark and get their by turning the crank in the correct direction(counter clockwise) and not pass the mark and coming back to is (introducing slack in the time change) but still no love. After making the adjustments I even get loud click noises from the value when I just kick it over with the kick starter. So I must be doing something wrong. Here is what I have been doing according to the services manual (which is not very specific).

My bike is a 1981 - XS400SH (The one with the mag wheels, no spokes)

When getting to TDC I am always turning the crank counter clockwise stopping directly on the mark (without moving back to the mark after passing)

1) I set the time mark on TDC for the left side of the engine (LT Mark, see picture)
2) I adjust intake to .004 inch gap, I adjust the exhaust to .006 inch gap.
3) I then move the time mark to TDC for the right hand side of the engine (RT Mark in manual but on my bike it looks more like RI, see picture
4) I adjust intake to .004 inch gap, I adjust the exhaust to .006 inch gap.

I have include to other pictures that show 3 marks before a LF then a line. This is just in front of the LT line. Manual says nothing about these marks. Does anyone know what they are for?

Start engine, see what I have and all I get is bad clatter in the values (and clicking when I am just turning it over with the kick start).

Does anyone see something wrong with what I am doing?

Jack

 

[xschris]

Make sure your right on the mark. Also you need to mark sure when you are rotating the motor to do the right side that it gets to the correct cycle of the stroke. I think you rotate the fly wheel twice. Turning it from LT to RT on the same turn is not TDC for it.

 

[jackc2]

I will check this tonight. You are right I may not have been at TDC on the right side because I was not going around 2 times. I was right on the marks my pictures are off the marks just so it was easier to see them.

Do you have any idea with the 3 half lines and LF are for?

Thanks for the support.

 

[iamunique127]

You should have your spark plugs out (which you probably do) and the engine won't tend to turn so easily when you leave it at either the LT or RT marks.

Also, the piston in each cylinder should be at the top of the stroke if you are in the proper cycle. You can check with a straw or something in the spark plug hole, or just look with a flashlight.

 

[xschris]

I will check this tonight. You are right I may not have been at TDC on the right side because I was not going around 2 times. I was right on the marks my pictures are off the marks just so it was easier to see them.

Do you have any idea with the 3 half lines and LF are for?

Thanks for the support.

I think they are for timing with a strobe. The LF is the left firing mark for setting the ignition.