No Compression After Top End Rebuild

[bcware]

1978 xs400 2e

So it appears my top end rebuild has not been successful. As it stands right now I have about 90 psi on the left side and 60 psi on the right. Prior to the overhaul everything was fine.

The first start went well; it ran on the second kick after the rebuild (near freezing temperatures as well), but only for about 15 seconds before it died. Everything sounded normal and the idle was steady at about 1,500 rpms.

I thought it was a carb problem at first because I saw some flooding, but I took a look through the plug holes and what looked like droplets of water were scattered over the piston heads. I really didn't want to believe it was oil because it looked so thin (like water droplets) and 20w50 at near freezing temperatures is very thick. Unfortunately I took a sample with a swab and it won't burn easily or at all. Obviously if it was gas it would flare up vigorously. I cleaned the piston heads with long swabs, but the droplets come back after turning the engine over.

I replaced all the gaskets, seals, and torqued everything to spec in the proper pattern very slowly. Literally, I spent 20 minutes each tightening the head and cam cover. Everything was generously lubed with motor oil and I lapped the valves.

One mistake I may have made was to not clean all of the lapping compound out of the intake/exhaust port area below the valve sealing/seating area. Honestly, I think I forgot to look through the ports and not just through the seat. I can see how this could ruin the valve seal and cause a loss of compression, but is it likely that the valve seals are toast as well? Both would have to be damaged in order to explain the oil and the loss of compression.

What seems most likely? The carbs were flooding a lot; could I have washed out the cylinder walls?

If I pull the motor again and open it up will I need a new head gasket and cylinder block gasket? I have no idea of these can be re-used.

 

[Shaun Chandler]

Hey Bc

Is there any chance you got the timing abit out of sinc when you had the top end apart?

You didn't do pistons and rings right ?

Shaun

 

[bcware]

I took the cylinder block off, but I did not remove the rings. The barrels looked just fine.

So, I added oil in the plug holes and I have compression; 150 or so on each side. This means the valves are fine, right?

I cannot, however, stop the carbs from flooding. I backed way off on the jets and float height and the intake tubes are all wet and so are the carb boots. It is possible I keep washing out the barrels/rings with fuel. Right now I am 2 up on the mains, stock pilots, needles on the 4th position from the bottom (1 down from the top), 30mm float height.

Would kicking the engine over many times cause this even with correct carb settings? Fuel is not pouring out or anything, but everything appears too wet.

Perhaps I will drain the oil and double-check the timing. Could I have set the timing on the wrong stroke?

 

[bcware]

Well, the oil appears to have a scary amount of ultra-fine metal flake in it. It looks like a pearl paint job.

The timing is set on the correct stroke on both sides (both valves closed, light off on RF), but it is slightly off. This is likely due to the damn lash in the cam chain.

 

[Shaun Chandler]

Not certain but you mentioned you had a lot of gas go into the cylinders, perhaps you just diluted the oil and loosened up old fillings in the motor. Scary though.

When you had the jugs off did you make certain the ring gaps were staggered ?

Shaun

 

[bcware]

That I did not do. I really did not touch the rings or move them at all. Is there a high chance of them lining up when I put the block back on? The silly manual did not mention anything about making sure the rings are staggered, but maybe its in the ring replacement section and not the general assembly section.

 

[Shaun Chandler]

If you didn't touch them they are probably fine, I'm not sure the manual mentions it I've just learned its good practice. Well atleast thats what I was taught.

What about the ring gap? Did you measure it ?

Shaun

 

[drewpy]

the staggered rings are in the manual, but I think it was the official yam one!

if a lot of gas is evident, i would say the bores have been washed of oil and you really need to stop that problem! you'll wear out the bore in no time

when I first connected the tracker it flooded, but a tap with the screw driver on the carb float chamber and it soon settled down!

always time the valves as the engine rotates to the timing mark, ie don't back it off if you go past it. that is the way the Yam manual says you have to do it.That will eliminate any lash.

I'm just a bit concerned that you didn't clean all that valve paste off. Its really really bad for the engine and I suspect its ground down the bores as it passes through.

I sincerly hope not coz that's a bummer!

the only thing to do now is take a deep breath and take the head off again ( new head gasket) and don't skip any steps, check the bore for marks and wear, check rings and hone it for a 45' cross hatch! take out valves and clean down that head.

one more thing, before you go mad, did you time the valves using the RF marks?

 

[bcware]

I used the RF/LF marks, yes.

When I time the engine with the plugs out it is very hard to get the crank to stay at the RF/LF line; it always wants to fall quickly past it. Should I keep the plugs in when I turn it over? I don't know if this would help.

It probably was the paste. It explains everything. It's unfortunate because I was so careful about assembly, but not cleaning that crap off.

If I hone the bores do I need new rings? Should I just get new ones anyway? And, do I need larger ones or would I just stick with the same size?

 

[drewpy]

I used the RF/LF marks, yes.

Erm, you use LT/RT marks for valve timing! T = timing F = firing

When I time the engine with the plugs out it is very hard to get the crank to stay at the RF/LF line; it always wants to fall quickly past it. Should I keep the plugs in when I turn it over? I don't know if this would help.

when using the LF/RF for ignition timing, it always does that and I have to support it using ratchet wrench and a hammer handle on the ground

It probably was the paste. It explains everything. It's unfortunate because I was so careful about assembly, but not cleaning that crap off.

If I hone the bores do I need new rings? Should I just get new ones anyway? And, do I need larger ones or would I just stick with the same size?

try timing it on the LT/RT marks first and see what happens

 

[bcware]

Is that right? The manual says to time it on lf/rf and that lt/rt is top dead center. If that's how you do it though I suppose I can try it.

I think I am confused though! At first I didn't notice it, but now I see you are talking about valve timing and ignition timing. How do you time the valves? I have never read anything about valve timing. I thought the ATU just gets plopped on and fits on a dowel on the cam.


Edit:

Wow, I missed it completely. I found it in the book; I can't remember if I did time the valves properly. I should check!

 

[bcware]

Bollocks! It appears that the cam timing may be one tooth off!

 

[Shaun Chandler]

Hope it works for you now Bc

Shaun

 

[xs400neophite]

Wow bc crazy!! Hope it all works out! Just think of all the valuable knowledge and experience you are gaining! School of HardKnox.

 

[bcware]

Ok, the manual says the lines on the cam sprocket face need to be parallel with the main bearing, but they are slightly off. Even the photo in the manual shows the lines slightly cocked. Moving one tooth over puts it way off. I think I had it right originally.

 

[bcware]

Photo time! The engine is at "LT" dead on, rotating counter-clockwise only and the cam chain tensioner is off. As previously mentioned the ignition timing was only very slightly off (retarded) due to me backing the engine up clock-wise when I did it.

At this point I am considering pulling the heads and block to get a look at the bores and rings.

Edit:

I'm pulling it; I just found oil in the exhaust ports; yummy! I bet it was leaking into the intake port as well. Valve seals must be toast. It had to be excess lapping compound.

Other than being kicked over a bunch it has only run for 10 to 15 seconds. Maybe I will get lucky and the damage will be minor

 

[bcware]

Update:

The top end is back off; I pulled it off in the frame.

Observations:

milky pearl-white substance in oil (wtf?) upon removal of top end.

top rings on pistons were stuck; came free with finger nails, but it took some effort.

rings were staggered properly, but they look a little dirty (carbon?)

the barrels have a decent cross-hatch pattern already. there are a few lines that would be parallel with the rings and look like scratches, but I cannot feel them; they feel perfectly smooth.

Questions:

How do I check ring end gap properly? Do I insert the ring in the top of the barrel or the bottom? I get much different readings when I do this. I suppose there is more wear at the top. For example the top ring from the right piston measures 0.019 inches at the top (beyond service limit by a hair) and 0.013 inches when placed in the bottom (within acceptable range). Is there a trick to inserting them squarely, or is this just done by eye?

Edit:

I don't know if I mentioned it already, but I did wet and dry compression tests on both cylinders before I tore the top end down. My dry readings were always 60 psi to 90 psi, but wet tests gave me 150 psi. Now, I have been reading a little and it seems like everyone recommends doing these tests with a warm engine. How might near-freezing temperatures and a cold engine effect the tests? I want to make sure I fix this problem if the rings really are bad.

 

[Shaun Chandler]

Hey Bc here is a link to a wiseco guide for ring gap. If its different top and bottom that means the bore is worn. You want it to be around the same top to bottom

http://www.wiseco.com/PDFs/Manuals/RingEndGap.pdf

Is the measurment you took at the bottom within the travel of the piston?

You are this far in I would say get a set of first over rings and have your cylinders honed or bored professionaly. I think your looking at mabey 150 to have it done. If your pistons are just abit worn and you don't see any major damage you can have the same shop knurl your pistons aswell.

Shaun

 

[bcware]

I didn't push them in far, only a quarter inch or less since I don't know where I am supposed to put them and I can't visually square them if they are away from the outer edges. I really didn't want to remove the pistons because I thought the wrist pin clips had to be replaced if they are taken out or something like that.

I tried searching for more info on ring end gap in motorcycles, but everyone seems to have a different opinion; some measure from the top, some the bottom, some both, etc.

From what I read so far no one was alarmed about some taper in the barrels.

Doesn't the piston expand when hot? Would this close the gap further at temperature?

 

[Shaun Chandler]

I was taught to check top and bottom.

As for the hot cold issue. Measuring your ring gap will be done cold so I wouldn't worry that the values would change when hot, thats been accounted for.

As for taper, as long as neither is above the spec size I'm sure you could live with some taper. but if top or bottom is above spec your bore would need to be rehoned or bored.

Shaun