low compression on VF700F

Well, finally got around to doing compression test on my '85 vf700f. Cylinder 1: 140, Cylinder 2: 145, Cylinder 3: 75!; Cylinder 4: 150. My heart dropped when I reached Cylinder 3. I checked and rechecked the valve clearance and it is w/in factory specs. I put some oil in that spark plug hole and the number shot up to 170, so is it definite my rings are worn and will need replacement? Should I be happy its that and NOT valves? I am really not looking forward to taking the heads off again but I suppose it could be worse...Any suggestions for this process would be great.

Thanks,
Lucas

EDIT: just thinking, the engine only has 5700 miles on it, could rings really be worn on that cylinder?? Maybe I'll do a leakdown test to make sure...Thoughts?

 

I did my leakdown test w/ #3 at TDC on compression stroke. Pumped 80 psi into spark plug hole and got hardly any decrease in pressure. I could feel air escaping out of the intake valve, nothing from exhaust. Could it not be the rings after all? If the timing is off would that give me a low compression? If I have to take the heads off again would it be worth it to do a valve job? Didn't do this before because I thought it might be over my head and not worth it for the amount of mileage...

Thanks,
Lucas

 

Starting to feel like I'm talking to myself here...Anyway, took rear head off, removed valves, springs, etc., checked measurements, cleaned everything up real good and re-lapped all the valve faces and seats. Re-installed all the valves, put some starter fluid in each combustion chamber and definitely have some leakage in Cylinder 3 exhaust valves. I checked other valves and they seem OK, though I did shoot a bit of compressed air up through the intake/exhaust ports and got a tiny bit of bubbles on the other valves.

With this information, should I try taking Cylinder 3 valves out again and re-lapping and re-installing or am I looking at new valves? Is it possible there is some gunk on the valve seat preventing a proper seal?

Given the compression on Cylinders 2 & 4, should I leave the front head ALONE or would it be worth it to clean up the valves on that as well? Once I resolve my leaky exhaust valve problem on Cylinder 3 I was planning on re-installing the rear head and re-doing my compression test to confirm everything is OK. I can proceed with the front head from there if that is NECESSARY.

Oddly, this has been my favorite part of the whole rebuild process.....

 

redo the # 3 cyl again,loo at the valve seats and valve seats for pitting.you may have to cut the seats or replace the valves.

 

re-lapping the valves on #3 seems to have done the trick. I'm going to re-do #1 for good measure.

Other than the time, is there any down side to cleaning up the valves on #s 2 & 4 as well? I know this is sort of a "if it isn't broken don't fix it" scenario but the OCD side of me is pushin to have clean valves on all four cylinders...

 

If it gives you piece of mind go ahead,I am going to do a vfr750 motor swap for my bike,have the same decision to make,but i know i will feel better if i do it even when the compression is at 200.

 

Well, got rear head back on and re-did my compression test. Practically NO CHANGE from the first time. #s 1, 2, & 4 all within the 145-150 range. #3 still showing ~70. I guess I was hoping and praying it wasn't the rings and that the valves just needed a good cleaning. Figured it was more time/cost effective to test out this theory then go right into splitting the cases again. Though during my leak down test I got no air escaping from the crankcase or breather hose. This all baffles me and if anyone can offer a reason why I have low compression on that one cylinder that is other than rings I would gladly welcome it. Assuming it is rings, what am I looking at cost wise? While I am not ready to give up on this motor, I would consider 2nd gen motor as a replacement if that will even fit in an '84 frame.

Thanks.

 

Confirmed stuck rings on piston #3. I am really hating myself for not checking all this the first time I had the cases split...totally naive move on my part but lesson learned for next time! On the plus side I feel like I know this motor inside out and it has been awesome to see how it works from the inside! Since I have all the pistons out would anyone recommend new rings for everything or just re-use if it is all within spec?

 

Nice write up!!! I would go for new rings/I mean WTF right? You broke the glaze on the cylinders right? I had a situation where I had to replace one piston and rings, I re-ringed the old piston. It was a twin though. The bike did not have many miles on it.

 

Yea I'm thinking I might as well replace rings on all of 'em. I found these on Ebay, seem to be factory: NOS Piston Rings Honda STD VF700 VF750 13011-MN0-305 | eBay

Is this too good to be true? Cheapest ring sets I could find elsewhere were ~$30 PER set and this Ebay listing is the same part number..

I thought I remember reading somewhere breaking the glaze is not necessary/recommended when replacing rings? Can someone confirm?

 

Way to stick with this! If you have 'em, a few photos of the build would be great to see.

 

It is my experience, FWIW, that deglazing is necessary for proper ring seating. I would carefully measure the pistons paying particular attention to the ring grooves and lands to make sure that they are in spec and not damaged.

 

If everything is within spec what grit stone should I use for honing?

For this piston with the stuck rings should I go ahead and replace the whole piston?

 

Check out this site, good info on cylinder deglazing. Untitled Document I prefer the 280 grit. Whether or not to reuse the old piston depends on it's condition. If the piston shows too much wear and/or the ring grooves or lands are damaged, a new piston is in order. My next thought is....what is the condition of the cylinder walls? Are there scratches? Is there a significant ridge at the top of the cylinders? It would be a good idea to mic the cylinders to see if they are within spec.
If I had any suspicions about the condition of the piston(s), I would replace all 4.

Here's info on crosshatching. http://www.hastingsmfg.com/ServiceTips/cylinder_bore_refinishing.htm

 

Thanks for the info. The cylinder walls all appear to be in excellent condition, no scratches but a very light ridge at the top. I am going to get a bore gauge to take more exact measurements. Pistons are also in great condition. The one with the stuck ring seems to be OK but I am still in the process of removing the ring via solvent soaking. My guess is if it is this difficult to get the ring out I should probably replace it. Might pull the #3 piston out of my spare case and see how that one looks.