Fix or Not to Fix that is the quesiton

So I've done some dianostic work over the Canada day holiday and I need to know just where I stand.
-Basically my #1 cylinder is only getting 100 psi compression with 152 psi when oil is added - the rest all get 148 to 151 psi.
-When doing a colortune all cylinders show a nice bunsen blue when tuned to 2.5 tunes out with the rich on hard throttle and slight leaning out at higher RPMs. But my #3 cylinder won't respond to the adjustments to the idle mixture, no matter how rich I try to make it (leaning still works).
-I've sprayed carb cleaner all over the carbs while running to check for vacuum leaks but I cannot find any.
-The oil pan has a slow leak though the oil levels remain fine.

The bike seems to run beautifully though I am still wary of taking it for a long trip as I'm not too sure how it will hold up and it really hates to idle and will die after about 15 seconds if allowed to.

So basically I'm wondering how bad is it to run a bike with poor compression in one cylinder and should I be worried about the fact that the idle mixture adjustments to the #3 cylinder have little effect?

 

i had the same problem with my bike. i would try to colortune my #1 cylinder and either way i turned it it would not make any difference. The promblem was my carb to manfold boot was not sealed on the manifold air tight. So i took the boot off put a nice bead of black silicone around it, let it dry for a day and it worked like a charm. I dont know if that is your problem but it think it might be a place to start. Check the seal on that one boot on the particular plug you cannot get to change.

 

Usually when a compression test with oil on the piston leads to a higher reading the pistonrings have gone bad, or the cilinder wall is scrathed I'm afraid...


grz Thijs

 

thats why i suggessted a place to start. when you are trying to get a bike running you and i both know we dont want to think engine rebuild!

 

How bad would it be to run the rest of the season with the poor compression? I think I remember some vertical markings on the cylinder wall from the last time I had it apart (4 weeks ago to replace the head which had been killed when the oil pump sprocket popped off) so I think the walls need to be re-honed.

I a little reluctant to do the new piston rings because I think I would have to re-hone to an oversized piston ring and that would be a fair bit of work and money. Though I am not sure if I would have to rehone and get new rings for all 4 pistons at the same time.

I'm just wondering if you guys think I really should just bite the bullet and do it, or if it is ok to just let these things slide for now.

I am going to want to take a few trips on the bike in the next couple weeks so I really want to find out if you guys think the bike can handle a couple good 5 or 6 hour rides with the current issues.

 

its a gamble.

 

What sort of damage am I looking at?

 

If it runs on 4 cilinders you should be able to ride it, but the crank won't like it. How far are you planning to ride? Maybe a secondhand cilinderblock with pistons could to the trick for you.

If you lose that much compression (about 1/3th!) I don't think a hone is going to cut it. (Unless a pistonring is broken and causes the leakage) If not, then I think it should be bored and honed with 4 oversize pistons and rings.

Therefore maybe a secondhand cilinderblock with pistons could be a more cost-effective solution. Eventhough I would hone them also and use new rings.


grz Thijs

 

I have another set of cylinders from the scrap engine that I pulled the head from though they are not in great condition and have a fair bit of surface rust on them, but I guess I should look into rehoning them as I could still ride the bike while they were in the shop then it would be a pretty quick job (one day) to pop off the head and put the rehoned cylinder in with new piston rings. But the cost of going this route would be around $300 ($200 for new piston rings $100 for machining I guess) so I'm not too eager to jump on it. I put new gaskets in when I did the head 4 weeks ago but I guess I would still need another new set.

I guess the weird thing about the whole thing is that when I pulled the head off I had about 110 psi in the #1 cylinder and like 120-135 psi in the rest so I figured they were close enough to each other (though bad in general) that I could just keep riding the bike as is. Though now the the other cylinders got magically better (the bike hadn't been run much previously so I guess they just needed to work in a bit) and the #1 has got a little worse I am starting to get a bit worried.

To save money is it possible to go with a 4th oversized rebore and piston ring set on the #1 cylinder only?

 

I forgot to mention that I currently ride about 15-25 minutes on the bike daily going to friends homes or the gym after work sort of stuff. As far as I can tell the bike runs fine, good acceleration, no shakes or rattles that shouldn't be there and it has a nice even fireing sound to the engine when running. Though I find that it still won't idle for more then 15 seconds on its own even though I have cleaned the carbs and checked for vacuum leaks (I guess this could be due to the poor compression).

I would like to take it for a couple trips up to Cape Breton later this summer which would be about a 1200 km trip over two days sort of deal.

 

Hey fore, when I bought my XJ650 I rode it for about a six hundred miles with broken rings on #2 and #3 cylinders (didn't know it). I discovered the problem when I did a compression check and only got 80psi on those two, which explained why my top end barely pinned the needle. Pretty tough engines.

 

Man that's pretty impressive, did that do any damage to the engine or were you able to just pop a new set of rings in to get your compression back?

 

Boring only one piston is not done. That will definately result in an unbalanced engine. (piston weight issues) I don't say it won't run, but it will definately throw the crank off balance in a certain time.

Those 15-25 minute rides shouldn't be a problem at all I guess, but the 1200 km (or do you mean miles) are quite a lot. I don't know how fast the compression dropped on cil 1? (as in, how many miles) That could give you an indication of what the life could be of the cilinder.

A sticking ring could also be an issue, but that's something you really should have noticed when you've replaced the gaskets :wink:. The ring could also be worn. There is no way of being sure without taking the engine apart.

Remains only one question though, how did you measure the compression? Warm engine, cold engine? All cilinders the same amount of "pumps"? Throttle open/closed?

Usually to determine differences between the cilinders it's best to warm up the engine, take all the plugs out, open the throttle fully and then crank the engine until you've felt/heard that it has made 4 or 5 compression strokes. That'll give you a better comparision than cranking until the meter doesn't climb anymore.


grz Thijs

 

Thanks for the info Thijs. When I had the engine apart I didn't notice any stuck rings (and I would have if they were sticking do the hole process of getting the cylinder walls back over the pistons. I foolishly didn't check the wear on the rings so I guess that might be a good job for this weekend.

I suspect that there is also some vertical wear on the cylinder walls that maybe helping to reduce.

When I did the compression check I warmed the engine on the quick ride then I took the spark plugs out and turned things over making sure to allow the dial to reach maxium pressure (a half dozen turns usually). However I didn't open the throttle but hopefully since the plugs were out this would have a limited effect.

Anyway I'll do another compression test this weekend as you suggest.

Is there any way to measure for wear on the cylinder walls and should I check for anything else besides the piston ring wear while I'm in there?

 

In order to do an accurate test you need to have the throttle blades all the way open on the carbs. I would advise a re-test.

MN

 

Fore4runner, to answer your question, no, I didn't fix the engine. It wasn't worth it so I replaced it with a 750 that was taken care of. Now I can pin that speedometer needle very quickly.

 

Yep, you can have the bores in the cilinder block measured at an engine or machining shop. They have special tools to do so, and it shouldn't cost you that much money. They can measure if the cilinder is oval or not.


grz Thijs

 

Ok a few more question, if I have to go to say a 4th oversize do I also have to replace the pistons?

And is there any possible downside to leaving the #3 cylinder in its current state where I can't adjust it to be rich even though I have a decent color in the colortune?

 

I wouldn't go to a 4th Oversize unless that poses the LAST Resort.

You have to buy a set of Oversize Pistons.
Depending on the Bores ... you'll need different sets of Rings.
They Bore the Holes taking the Minimum material needed to repair the Cylinder Wall and STAMP the I.D. on the Block.
You might need Varying Sized Pistons, too.
Complicated.
Can get expensive.

It would be MY Personal Opinion to:
Find a USED Block that might only need to be De-Glazed or would need the 1st Oversize Rings.

Try not to get yourself into a situation where every move you make is going to require some further adaptation.
When you get beyond the Second Oversize ... those factors kick-in.
Different Pistons.
Different Rings.
Availability of "Odd-ball" Nature parts.

All the expenditures for going beyond the 2nd or 3rd Oversize are eliminated by acquiring a decent set of Jugs and keeping things simple.

 

Yes, FULL throttle is the only way to get an accurate reading. Start with #1.