RH cylinder is too rich

Jay H Miller

XS400 Member
Reaction score
Hi All, I'm new to the "forum world". Forgive me is as I learn the process.

So I'm working on a 79 XS 400. The LH cylinder had a real bad plug cross headed. I removed cover and inserted helicoils in both sides. Lapped the valves and put it all back together. I checked the valves with carb cleaner. They are not leaking. I'm confident things went back together with everything in the correct timing. LH TDC, came shaft and sprocket all aligned. I set the gap and the timing. As part of the rebuild I wrapped pipe tape on the exhaust and attached new mufflers. The mufflers have the same baffle. But the body of the muffler from the baffle to the front end is longer than the mufflers that were on the bike.

Started it up. Used my color tune to adjust the idle. sync the carbs with gauges and check the idle mixture again. Not real happy with the color on the LH. Looked too lean. Idled great. When at about 4000 rpm it would miss. I should also mention that the LH side is much hotter than the RH. Missing at higher rpm so I thought maybe the timing needed to be advanced a little. I moved it forward. Still not luck. RH plug is loading up and LH looks too lean. I opened the carbs up and adjusted the main needle position up leaning out the system. (needle deeper into the main) while in the carbs I checked the jets. I wondered it by some chance they were not the same. Both are 132.5. ( I do have a set of 125) The air jets are both 45. No luck. By the way the other things I tried have been; put the old plugs back in. the new ones were different band just in case the new ones were wrong. And verified the gaps again. No luck. Remove the mufflers to see if the new ones caused some issue. No luck. I'm all out of ideas.

What could I have done to have such an in balance between the two cylinders?

Help - Very frustrated at this point.

Ok. So as I lay awake wondering what was "touched" and wondered about the valve gap. Even though i set them. I grabbed a compression tester. The RH is about 45 psi and the LH side is about 80. The LH plug looks real lean and the RH side is too rich. There is nothing i could do to affect that condition. Could there be some issue with the seal of the head and cover. I put in a new gasket at the head and liquid gasket for the cover.
Anybody have any direction?

Last edited:
BOTH cylinders being under 100 psi means the engine is dead if the numbers are real. You need to verify them using another tester as the test equipment as well as the techniques used there can kill your readings as to reality. Yank both plugs and throttle held wide open or the readings are worthless. Use a car battery to take several tests in each cylinder but do NOT EVER start the car up doing it. Bike battery will work but they commonly run down taking several tests.

Helicoils are not really the correct thing to do in a plug hole, they often don't seal and something like a Timezert, will let the plug still dissipate heat correctly, the helicoil often interferes with that.
Not come across a time sert before, looks really much better than helicoil, though they are probably fine for these engines
OK. So I finally got around to verify the compression. I picked picked up a new gauge and don't recall if I has the throttle open the first time. Now I'm getting 140 psi consistently. That's good. Bad think is I'm back to where I started. Why is the LH very lean and RH rich? I plan to check the valve clearances again. Could I have done something wrong with the gasket or torque that would result in the situation I have. Thoughts?
Someone a bit back pointed out a worn needle jet can do it.

I know myself that a small pinhole or the like in a slide diaphragm can make that one slide open slower and then it tends to run rich. What slow slides do.

You can have good compression and still leak oil a bit at a valve seal, that can easily tilt what your readings look like.

Somewhere around here as well I thought I picked up that the engines are not jetted the same L and R and if jetted even that could be some of it.
Thanks for the advice. I took a look at the needle jets and diaphragm. Things look good. Thinking the issue must be related to the work I've done. Went back and adjusted the valve clearances. Too much in fact. Could hear them clattering. But both plugs looked loaded up. Slept on it and rest the valve clearance. Back to where I started. Left side is hot and running lean. Right side it running rich. Looked at the carbs again. Jets match and the main needle setting is at stock which is the 4th groove.

When I replaced the head and gasket, I used some high temp Permatex. Could I have plugged something thereby blocking the flow of oil on the left side which has increased the flow on oil to the right side?

Thoughts anyone?
Never use anything like that on these bikes anywhere. The only place that needs a sealant is the rocker box and for that you only use yama-bond, only in small amounts. Are you going by what the plugs look like or are you using a IR temp gun. Stock 79 carbs use the middle notch on the jet needle (third). My 79 has this and runs fine. Also 132.5 mains and 42.5 pilots. The mix screws are at 2.5 and the bike is fully stock.
I've been trying to verify the model so I can check for the correct jets in the carbs. I find 2L0 105077. I found one website that states this s 1979 XS 400 F. But when I look in the Haynes Manual I have many of the part descriptions do not contain a 400 F. Does anyone have any direction?
2L0-1xxxxx would be a 79-F model. Post pics of the bike.
"F" - that's what i concluded. So the manual states the following for stock jets;
Main = 132.5
Air = 45
Jet needle = 5Z1
Needle clip position = 3rd groove
Needle jet = X-6
Pilot jet = 42.5
I know the pipes are not stock. But not really anything special. Just replacements. See any reason why these jets would not be correct? and the floats are brass and set at 26 MM.

Post pics of the mufflers. The ends also. Nothing will work the same as stock and may need adjustments to the carbs to work as close as they can
Post pics of the mufflers. The ends also. Nothing will work the same as stock and may need adjustments to the carbs to work as close as they can
I'll post some pictures. The ones that were on it were shorter. However the length of the section with the baffles are the same. the additional length is in the front and it is hollow. It ran with the ones that came on it when we bought it.

I have another question. If I were to use a clear tube coming off the bowl plug what should be the the fluid level? I've head level with the gasket surface. I've also head 1 to 2 mm below the gasket surface. Is 26 mm the correct height to set them mechanically? They are brass.