Wildguzzi.com
General Category => General Discussion => Topic started by: sauldgold on March 01, 2019, 09:07:35 PM
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Hi everyone. I have a '78 G5. I just did a compression test and found 120 psi on the left cylinder and 100 psi on the right. For what it's worth, PO let the bike sit for extended periods of time.
I am going to disassemble over the weekend and go from there, but this is my first time down this road, so I have some questions:
1. Is it possible that this is just a matter of replacing piston rings and nothing more, or is that wishful thinking?
2. If necessary, is it possible and / or advisable to over bore 1 size and are such pistons even available? (I haven't seen them for sale anywhere)
3. Since I'm going to be spending money on either machine work or cylinder/piston replacement kits, is it best to keep everything stock, or are there any advisable motor upgrades? Or is that just a recipe to piss away money for little gain and make even more headaches for myself in the process ?
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Before you start potentially unnecissary dissassembly, How did you perform your compression test? What is the mileage? Are the valves adjusted properly on the right stroke. Any other info on the bike?
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Before you tear into it, I would suggest doing a leak down test , it will give you a much better idea where the leakage is occurring and you can then proceed.
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The mileage is 62,000.
I used a psi gauge from autozone. I ran the engine and warmed it up, removed the spark plugs, screwed in the gauge, opened the throttle all the way and cranked the starter 5-6 times until the gauge wouldn't pressurize further.
I haven't checked the valves. I don't have an air compressor for a leak down.
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Pull the heads and barrels, hone the bore if it looks OK and isn't barrel shaped, (Probably won't be, Guzzi big block piston speeds aren't fast unless you rev the snot out of them.) measure the ring gap, (It will be bigger than spec.) and fling a new set of rings at it, (Make sure they are rings for a cast iron bore, not Nicasil.) then take the heads to a head shop and ask their opinion. If it needs valves it will need the guides K-lining too. Get the heads serviced and put it back together.
If the barrels are severely worn just get a set of Gilardonis.
Re face the rockers and shout it some new spindles at the same time.
It'll need a camchain and a proper tensioner too at that mileage.
Pete
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Ditto on finding someone to do a leakdown test. Might just be a valve not seating well..
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Also, I checked my manuals but can't find an answer, what is ideal PSI for each cylinder ?
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I checked my G5 compression when it was bedded in
from new and got 184psi on one side and 187psi on the
other.
Malc
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I checked my G5 compression when it was bedded in
from new and got 184psi on one side and 187psi on the
other.
Malc
I would guess this is a perfect world scenario to gauge your perfection at.
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Oops , not taking into consideration a leak down check .
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In my "old car" days, the process was, do a compression test, if one was low squirt a bit of oil in. If the reading stayed much the same it was valves, if it shot up it was rings.
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Ditto on finding someone to do a leakdown test. Might just be a valve not seating well..
This. Several times, I've seen low compression that can be cured by "staking" a valve, an old aircraft mechanic's trick. Airplanes generally sit more than they are used, and may have corrosion or carbon built up on valve seats. The method is to have the leak down tester on the cylinder pressurized, then hit the rocker arm on the valve in question with a copper hammer a few times. Many times, you'll see carbon and or corrosion blowing out the exhaust stack (on straight stack engines,) and the valve sealing up with each swat. :smiley:
I have brought 80/20 cylinders into the 80/70s range doing this.