Should I do an Engine Rebuild? - Intake Valve leak...

Hi guys - like I have been saying in other posts, I have been adjusting my valve clearances on my 700 Maxim X. I did a leak down test with the cams out and I have a significant intake leak on all cylinders...

With the cams out, the valves should be as tight as possible, so I am worried that this is going to be a show-stopper.

Has anyone ever torn down an XJ engine before? How much work was it and how much did it cost?

What are your opinions on whether or not it would be worth it?

Do you think that this leak could be due to the bike sitting for three years? I don't think it would be. My personal opinion is that the intake valves were not shutting due to the incorrect valve clearance that I mentioned in another post - and this caused the seat to get cut from exhaust gases backleaking into the intake manifold. Does this sound like a reasonable theory?

Background - 30,000 miles, I got it to run (not well) for a short period before I tore into the cams. Everything else on the bike is fine. It was sitting for three years before I started working on it.

Thanks in advance for the help.

 

Rich 03, there could be a few things amiss here. Sitting (especially with old oil in it and a humid environment) can be hard on the bike. As with all engines, a cylinder or two will be left open due to the resting position of the cams, leaving a route to open air through the exhaust. It is possible for corrosion to build up in the cylinders and cause a ring of rust crystals to form in the cylinder. These materials can cause the rings to stick and/or tiny pits to be formed in the cylinder walls. As with all long term storage, you need to coat the cylinders with a heavy oil and pop a few of those lovely desicated spark plug doohickies in the cylinder head. Let us assume that your beast has suffered the ravages of aforementioned corrosion. I have successfully fielded a few bikes with this issue and they came on line within a few hundred miles (and an oil change). I would suggest you shoot a cc or two of Marvel Mystery oil into each cylinder and rotate the assembly by hand to coat the cylinders and rings (make that a few more cc's of oil). Let that stuff soak in overnight or over a few days. Then tighten everything back up and fire it up. Let it idle a while (short trip to the grocery store is good), then check your compression again. I would do this at least 3 times before I wrote the motor off to a rebuild. Any motorcycle engine with more than one cylinder is just bloody expensive and I can see why folks avoid it like the plauge. Let the fresh oil and heat cycles free up what they will and then take a look. I'll bet that the numbers improve with a little time. Another point. Have you done a wet test on this bike? If you have significantly higher numbers with the wet test, the rings are to blame, not the valves. I would just try the cheap and easy first if'n I was you. Go to it and let us know how the cookie crumbles. All the best!

 

yes richard03, with the cams out the valves should be closed and seated. If you are having air blowing back thru the intake ports then obviously they are not seating. It could be due to corrosion and they are hanging, it could be weak valve springs ( don't think so ), it could be they were out of adjustment and have been burnt as you were pointing out.

You have the cams out already so why not take it one more step and pull the head off, this way you will find out for sure. You can also check the condition of the cylinder walls.

Costs ?? valve grind should fix seats without need to replace valves ( unless too far gone ), head gasket, and valve stem seals if used. Hopefully you can find a top end gasket set someplace.

I just got my xj750rj fired up tonight. I bought it last year completely apart, and had parts missing. I haven't totaled the amount I've spent, I know it's too much! I put in new bearings, gaskets, seals, wrist pins, and piston rings. The previous owner had valve work done, but it was sitting for so long I took it apart and lapped the valves. When assembled only one of the valves was at correct clearance. I had to purchase three 275 new ( used ) shims from my local yamaha dealer for only $2.00 each. One of my valves was at the zero clearance as you explained. I changed as per the tables and rechecked to find it still out. I had to replace it with a 270. I found that you really don't have much choice as to where you end up with the clearance as the shims are in set increments. I opt to be a bit over instead of a bit under on the valves that don't fall in the ideal range!

 

Thanks for the replies. I am thinking that I will have to take the head off. Bummer. I have never done this kind of work before. I am reasoning that the valve was probably out of adjustment before it was stored, and so the seats are probably cut from combustion gases.

Did you do the valve work yourself or have someone else do it? Is it possible to do it without a machine shop? I am thinking it is. If so, how much did the valve work cost?

 

It depends on what you find when the head comes off.

If the valve seats and valves are in good conditon you may get by with just some valve grinding compound and suction cup thingy to reface the surfaces. You can do this on your own with a small fee for supplies. This is all I did to my head.

I'm assuming you will need a valve spring compressor to get your valves apart, valve grinding compound, and the suction cup thingy, if your going to lap the valves yourself.

If the motors been run some time with the valves out of adjustment you may have to take it to a engine rebuilder to have the valves and seats reground to the correct angles. I would pick up the phone and get some quotes locally. You can ask how much if you take the unassembled head in for them to do, or how much if you take the valves out and you reassemble the valves once they are done. If you do it yourself make sure you label all parts as to where they came from so you can put them back in the same place.

Good luck

 

Well, I talked to some experts at my local shop. Interestingly enough, they had never seen my type of shims before (see other post on Maxim X clearances).

More interesting, the head of the maintenance shop said he VERY rarely sees burnt intake valves, so my problem is most likely a carbon build up due to insufficient clearances. He told me that it should go away once I get the proper clearances. Great news!

I did some internet research, and this is true! Exhaust valves run I think about 1000F hotter than intake, so they are MUCH more likely to get burned.

So, I am going to get new shims and put it back together and see what happens!

Thanks for all of your help...

 

Joy and happiness! I'm so glad you found your ray of sunshine. Wasn't it fun getting here?

 

Yes, it has been an experience.

Another good idea that my local shop had was to grind down the old pads (shims) by hand. I am going to do that today, and install them and run the bike for about an hour or two. Enough to knock the carbon off. Then I am going to order the right size pads with the new measurements I get.

It won't be great for the lifters, but I feel like I have little choice with the amount of money I want to spend. I also feel like if I am careful when I hand grind them, that I won't cause any damage to the lifters.

BTW, I was told that a good way to get the carbon out is to spritz water into the intake with the motor running. INTERESTING!

 

Interesting yes, good way, depends. The amount of water you should introduce into the motor is very, very precise and shouldn't be lightly pursued. The good old B-52 bomber uses water injection for added power at take off (just look at that dirty smoke trail at take off, cleans the motors right out). The expansion rate of water is about 825 to 1 so if you get the quantity wrong, you could do some serious damage to your engine (bent rods, holes in pistons, etc.). Remember, water doesn't compress. I would take the water advice with a grain of salt and start looking into the physics of it all and do a little math before I just sprayed some water in there (I have been tempted too, but I'm too lazy to find out how much myself). If you decide to figure it out, pass out the knowledge you’ve gained! Good luck!

 

Good point! I wouldn't know how to do a metered injection though! I will think about it and get back to you. I might just try it and get back to you. I wasn't too worried about it, because I would spray into the water box, and so anything that got into the cylinder would already be atomized anyway. And since we can ride in 100% humidity, I wouldn't see any difference?

But then again, there must be more to it because driving in the rain would knock carbon off.

I will think and do some research.

 

Without reading all the posts:

Why not just do a top end rebuild?

There are various bike junkyards where you can possibly get a used head cheap. I would start there.

Or take your head to an engine shop and ask them to test it out and quote you.