XR400 Head Porting

[Troy_Bell]

I have a 2002 XR400. It has the typical "Gordon's Mods" but instead of a modified stock exhaust, it has a White Brothers E-series with 6 discs.

Lately I have been noticing that it hasn't been running the same as it used to. It seemed a bit sluggish and I would occasionally hear a faint knocking sound. It was also a PITA to start after sitting for awhile. I also noticed that over a period of time, the oil level would become low. It wasn't leaking onto the ground so that must mean that it is getting burned up somewhere. Since there was a lot of black soot at the exhaust tip, I figure there was a valve seal and/or guide problem.

I took the valve cover off to check the timing chain and guides and they were fine. The chain hasn't seemed to strech any, and the tensioner still had plenty of travel left.

Next, I took the head off and I could clearly see the the right exhaust valve was allowing oil to pass through. I also noticed that the area on top of the piston directly below this valve had seen a lot of oil. It appears that I at least need new valve stem seals.

Since I didn't do a compression test, I poured gas into the intake and exhaust ports to see how well the valves were sealing. The results showed that this same exhaust valve was also not sealing well. In fact, it was quite bad. This could possibly explain some of the performance issues.

So anyway, I took the head apart and cleaned everything up really well. Surprisingly, there wasn't much carbon build-up, so cleaning was pretty easy. The valves and guides were within spec, and it appears that lapping the exhaust valve with some fine compound will probably fix the sealing problem.

Something kept telling me that I should check the cylinder and piston. I didn't want to go this far, but I figured if everthing checked out OK, I might be able to get by with a hone and a new set of rings. I pulled the cylinder out and removed the piston. Overall, both were in pretty good shape. I did notice that oil had been making it's way past the rings in the area where oil had been sitting on top of the piston.

I proceeded to clean up the piston and cylinder. When I placed the piston inside the cylinder, it seemed very sloppy, so I measured everything and realized I couldn't get by with a new set of rings. The cylinder was way out-or-round. In fact, the piston would not slide inside if turned 90°. Currenlty, it is at the machine shop getting a bore job and piston. Fortunately it only required .010" over to clean it up. Now it will be a little bit closer to a true 400cc.

Now for the reason of this posting. (Sorry for all of the babbling)

Since the head is off and in pieces, I figured now is the time to do some head work if I want to. It is kind of hard to leave well enough alone when presented with this opportunity.

I have read many posting to find out what other people are doing, so I followed suit.

I matched the intake boot to the head intake port. It didn't need much, but now the small step is gone. I also knocked down the small ridge down along the inside of each port that was left from the casting process. I also sharpened the bridge between each intake path. I left the inside as-cast finish the way it was and didn't remove any other material.

Next, I turned my attention to the exhaust ports and head pipe. Years ago, I had ground down the welds, but I noticed that the exhaust ports are significantly smaller than the head pipe ID. I took a quick and dirty measurement of the head pipe ID and the exhaust ports before I left this morning and there appears to be roughly 0.200" difference. That's a little more than 3/16".

I come from the school of thought that these pathways should always match each other as closely as possible so you maintain good flow characteristics. This however is a BIG difference. I would have expected this to be far less.

I know that Honda could have cast these heads with a larger diameter, so this makes me wonder why there is such a difference in size. Could it be that Honda made it smaller for a reason? Maybe this smaller size produces a specific back pressure? Would increasing this diameter mess up this "carefully calculated back pressure" and cause a loss of low end power?

These are the questions that come to mind and make it hard for me to start grinding into the head again.

So I guess my question is, should I match port the exhaust or leave it alone?

[XR250rdr]

Port velocity is very important. I wouldn't enlarge the exhaust ports much, if any. Stepping from a smaller diameter to a larger one like that does not cause nearly as much flow losses as a sharp step down.

You really should be replacing all the valves. Usually the intake valves are the first to go and if you've had an exhaust valve go south its time to replace them all with good quality stainless valves. The stock intake valves are coated and cannot be reground.

Are you going to bump up the compression while you have it apart?

[Thorndike]

That a good question and I don't have that answer. I would think the exhaust would flow better if they matched. I don't know how you would get the flange bigger though.

When I did the head on my 435cc I used the Kibble & White valves with new guides and seals. I also had a three angle valve job done. This seems to be the best way to go and seems to lasts a long time.

I had the head ported at the same time. Matching the intake, sharping the bridge, roughing up the intake ports and polishing the exhaust ports. The valve guilds were shorten and narrowed also.

It's hard to say how much performance I got doing just that. The biggest difference came from raising the compression to 10.5 to 1 and going to 435cc with a pumper carb.

[Troy_Bell]

I do agree that now would be the time to replace the valves and guides, but surprisingly there is no play in any of the guides, soI'm feeling OK with using them as-is. It is obvious though, that the valve guide seal was bad and let oil to get past and allow it to deposit on the valve seat.

It is already costing me a chunk of coin and to rebuild the head would easily double that cost.

I'm staying completely stock and not bumping up the compression.

[XR250rdr]

I wouldn't worry as much about the guides as the valves themselves. I would seriously consider replacing the valves. Intakes for sure, exhaust valves may be able to be reground but don't quote me on that. There are a few other guys who can chime in.

There was another thing I recalled a few minutes ago. The port sizes on XR heads on some bikes were not identical for each pair of ports. Meaning each intake and exhaust port were shaped differently from the other port to widen the powerband. I don't know if the 400 is like that, I know for sure Honda sized the intake ports on the dual carb bikes differently. I've heard stories of people porting those heads only to have a bike that made less power afterwards.

[Baja trail Rider]

You just got great advice to replace all valves.You dont want to see, when the Ex valve stem snaps in two.Also on head porting xr small clean up no more.

[TajMan]

this interests me, I want to port or have my XR400 head ported later.

I do some porting work myself, but its different on turbos and turbocharged car heads etc etc

you just must know what you're doing for that specific engine..