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Project Bike-Low Compression

10K views 25 replies 4 participants last post by  sr73087 
#1 ·
So I purchased a non-running 99 R6 as a winter project. I checked the compression last night and got the following: 30, 31, 50, 59. Engine was obviously cold, throttle open. Do any other these engine have decompression valves? My next step I guess would be a leakdown test to try and determine where the compression is being lost, however this is really low. Bike only has 15k on it, is it normal to have issues like this at a moderate mileage? With compression numbers this low, is it likely a number of things worn and leaking?
 
#2 ·
Leakdown test is definitely what needs to be done next, there is no decompression valves on these motors and the numbers you have are extremely low another thing that wouldnt be bad to pick up is a cheap usb camera off amazon or ebay the one we use is like 20 bucks and works great for what we need it for if you have an android phone, im sure they make one of these for an iphone as well

https://www.amazon.com/gp/product/B01DEOAQAM/ref=oh_aui_detailpage_o09_s00?ie=UTF8&psc=1

just need a usb camera app to use it but we can look in the spark plug hole for cylinder wall issues down the intake and exhaust for bent valves etc
 
#10 ·
So I finally got back to this.

1 75% leakage, sounded like mostly intake but it was so loud I couldn't tell if was rings as well, exhaust ok
2 90% leakage, same scenario here
3 90% leakage, same
4 40% leakage, mostly intake but maybe also coming from the crankcase, exhaust not noticeable.

I should have checked the valve lash while I had the cover off but I forgot. With numbers like that, could the valves be that tight and not seal at all on the intake? Could this also be a mechanical timing issue or does cylinder 4 being so much lower reduce that possibility?
 
#11 · (Edited)
So I went back to this since it was bothering me. I checked the valve lash, mechanical timing and leakdown again. Mechanical timing was right on and some of the leakdown #s were different when doing the test without the valve cover on. I misunderstood the degrees to turn the crank the first time. Valve lash numbers are in inches and rough numbers (could be .01-.02 off).

Intake
1 .08 0.05
2 both >.05
3 both >.05
4 .08 .08

Exhaust
1 .1 .1
2 .05 .1
3 .1 .1
4 .08 .06

Leakdown
1 75%
2 75%
3 55%
4 35%

So can these alone being out by this much cause compression and leakdown numbers this poor?
 
#12 ·
There's no way those are in inches. Your valve lash readings don't make sense. If they are in mm, then you have some that are very small, and yes, they will hold the valves slightly open. Where are you detecting the leakage during the leak down test? Valves, or crank case?
 
#15 ·
Adjust the valve clearances, then check leak compression, again. I'd say, you have at least a couple valves being held open by tight clearances.

Oh, and you might want to get some precise thin feeler gauges.
 
#16 ·
Best/easiest way to check and adjust them is with the radiator out and carbs off with 45 degree gauges? Even with the 45 degree gauges I was having some difficulty getting a good measurement, especially on the ends. Its obviously doable but I want to make sure there isn't a better way. In the middle I was going under the center and then sliding it in from the side if that makes sense.

I am alittle concerned why they are so far out of spec. Is it at all normal for them to "naturally" be that far out? Or is it likely that someone adjusted them inproperly? Below is the correct specs for a 99 correct?

0.11" or .2mm intake
0.21" or .3mm exhaust

Other thing I am worried about is if the valves were bent, that would affect leakdown and compression but have nothing to do with valve lash right?
 
#17 ·
First of all the correct spec is:
Intake .11 mm (.004") to .20 mm (.008")
Exhaust .21 mm (.008") to .3 mm (.01")

Secondly, if you had bent valve, the valve wouldn't close all the way, and your clearance would be huge. You would be able to actually SEE the gap (as in, you might have a clearance of a few milimeters).
 
#19 ·
Ok so likely not bent valves.

Adjustment wise, some of them are out but not by a ton. So based on this they "should be" sealing much better than they are right? I would imagine not perfect but not the leakdown and compression I am getting. Could the seats be toast or burnt valves? Other than that what else would cause a leak on the intake?

I think I am going to bore scope it like others mentioned before and redo the leakdown again and try to better determine where its going.

Ive read a cracked cylinder#1 is common on these correct?
 
#20 ·
You are making things much more difficult that it needs to be. Don't try to keep jumping ahead of yourself. If your clearances aren't set, then a leak down doesn't tell you any useful information. Go in order, otherwise, you are spinning your wheels.

If you insist on leak down tests: Once you have the cams out to replace shims, do it then. That way the valves will be fully closed, and the valve lash won't affect the readings at all.
 
#22 ·
Since you have so many that are out of spec, you'll probably just want to buy the hotcams kit. Personally, I just have a bunch left over from various bikes, and what I don't have, I work out a swap out deal with a local shop.

Yes, set them on the loose side of the spec.

The tensioner on the 1st and 2nd gen bikes isn't as prone to failure as the 3rd gens. The 99-05 bikes tensioner is strictly spring actuated. The 3rd gens have an oil assist that tends to be inconsistent, and the spring isn't as stiff because they were relying on the oil pressure to do the job.......didn't work out so well.
 
#24 ·
One thing I overlooked and could be a very bad sign. The bike did not come with a full cooling system. I kind of overlooked it, since a bunch of stuff was taken apart that is functional. Oil looks ok, but I worried about the possibly of a blown headgasket or more than likely a crack somewhere. I think I have to fill the system and retest before I go further right? Is there a way other than a teardown to differentiate between a cracked block or something and a blown headgasket?
 
#25 ·
Ok so I adjusted the valves and then retested the leakdown. The numbers are very high still, nothing noticeable coming from the intake, exhaust or the cooling system. A ton of leakage in cylinders 1 and 2 coming through the crankcase. So to double check everything further, I filled the cooling system and it held without pressure for about a week. Then I pressure tested it and it dropped from about 15 psi to 14 in about 20 minutes and then holds for several hours at 14 psi. The cooling system had a redish residue in it, not sure what it was. Drained the oil and checked that out and it was full of fuel but did not look to have coolant in it.

I'm thinking it may be rings. Is there anyway to really know this for sure?
 
#26 ·
Pulled the head. Valves seem to hold liquid even with air pressure applied to the intake and exhaust runners. Pistons have no holes or marks from a collision. Bore looks decent, still some crosshatching to be seen and no lip on the upper cylinder wall. Should I just lap the valves while have this apart? Also, is there a way to know it is definitely the rings at this point? It seems like it almost definitely is this issue based on what I am seeing so far and the leakdown results.
 
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