Hi everyone I need some advice I've done a compression test in my vfr750 fj 1988 and my first results were 30 psi per cylinder I reground the valves in and retested now getting 60 psi pre cylinder I've checked the valve timing and they are right the bike starts and runs well but I'm at a lose as why I'm 100psi down on each cylinder Thanks Regards David
Stuck rings maybe? This thing probably sat for a long time? My dad had a trick using diesel fuel to get the rings freed up. The other things to consider; do you have the throttles open when testing? Are the CV slides impeding airflow and this maximum compression?
Had a bad gauge cause me some trouble once, always double check now if it is off equally on all cylinders
From my VFR Manual: View attachment 25897 Click on image to enlarge You might want to squirt a little oil in cylinder to seal rings
always can do a leak down test and nail your problem. if it comes up with oil in combustion chamber then you have ring issues
A little hard to run tests at normal operating temperature! I could not tell you how much difference that would make, both my manual and articles on line say to run when engine is at normal operating temps. If your cold tests were close on all four cylinders, that good news.
this, this.. and this a compression test will just tell you there might be a problem. A leak down test will not only tell you if there's a problem, but where it is.
Thermal expansion will raise it a bit, cold test is fine. Will just be about 10% lower on all 4 holes
Combustion engines will run as low as 10psi, but you wont get any power out of it. Before I rebuilt my old escorts motor it was in very sad shape with the cylinders at 10ish psi. It ran but wouldnt do anything. Try a different set of gauges and also try warming up the engine a bit. Doing a leakdown test will tell you which cylinder is the cause but it wont tell you why. You said you reground in the valves. Did you put a proper 3 angle grind to the seats or did you just polish the valves in to the seats? If you polished them in then you most likely need to pull the head again and have the seats ground properly. While your at it check the valve faces for the tattle tale line and see if its near the middle of the face or not. Before you pull the heads, try adding a couple ounces of clean diesel fuel in the cylinders and let it sit for 10 mins then crank it out the spark plug holes. If you have major ring issues then youll see the thinner oil floating on the engine oil in the little window on the side. Else after cleaning the ring seats a bit add a teaspoon of clean engine oil to each cylinder and re do the compression test. If it comes up more than 15psi then you have dirty rings and seats. Means pulling the bores and cleaning everything.
zen i beg to differ on the leakdown test, air out the exhust means exhust vavle damage, air out the intake means intake valve damage. air out the cranckcase means compression ring damage. so it does give you exact cause on low compression
We can argue this in another thread, but leak down testing will not tell you about cracks that can and do happen but show falsely that its one of the three areas. What a leak down test will tell you is how much has to come apart. Are you doing a head job or taking the bores off? Its kind of like a compression test only give you an indication of cylinder health and engine health.
Pottsy, do you have a bore scope? If you do, put it in a spark plug hole and watch the valves as you rotate the engine by hand. It occurred to me that the valves may not be closing either when they should or not fully. There are two timing points that the engine will run at, only one will make it run right. Also you never said why you are even looking into this. What symptoms did you get that caused you to even open up the top end?
Thanks guys for all your info I can get hole of a scope my bike runs ok and pulls ok it just felt down on power and seems to be using a lot of fuel so I did the first compression cold compression test results were 30 psi on every cylinder so removed the heads and re lapped the valves in then did second cold compression test got 60psi on every cylinder
You dont have to lap the valves (lapping is not recommended anymore) lapping showed you that there is a valve issue. Have a proper valve grind done. This is a full 3 angle grind of both the seat and valve face. Usually costs about 200$ up here. Try the oil trick to see about rings before pulling the heads as they may be stuck or not sealing.
I remember saying that at post 1 Dad used to pour diesel down the carb of a running gasoline engine until it flooded out. That allowed the walls and rings to get completely coated. Then let it sit a few days and try again. Probably a lot easier on some V8 with a single carb than to do on a four carb system.
I still use diesel fuel to free up old engines for model A cars and the like. Works good too as long as your careful to watch the sight glass for new oil then there is no issue for doing to a bike. But then again I was referring to the use of engine oil to seal test the rings. Add a bit of oil, not a lot just about a table spoon or two and redo the compression test. If it jumps up then you know youe doing a ring job and touching up the bore with a bore cutter and a good hone. But after determining rings you can try to wash them with diesel to see about deposits fouling them from seating. A cheap fix if thats what it is and gives you time to order parts and keeps you riding.
