I ran a compression test on my engine today because I bought my 06 blaster used, I did it cold and only got about 92 psi and I know it's supposed to be in the 120 130 range. But when I'm riding it has plenty of power. A friend of mine told me to do it while the engine is hot and to put a tablespoon of oil in the cylinder. Can someone tell me the correct way to do a compression test, thanks
Compression
- Thread starter Monsterman10
- Start date
No need for the oil and heat.
With the throttle wide open and kicking until the pressure stabilises, if it is less than 100 you should look at a refurbish.
By the way it is now time to do a leak test.
With the throttle wide open and kicking until the pressure stabilises, if it is less than 100 you should look at a refurbish.
By the way it is now time to do a leak test.
doing it cold is fine, it may gain a few psi when warm ?
were you holding the throttle wide open while kicking until the needle stopped moving ?
is the o-ring on the tester good and sealing completely ?
how many kicks does it take to start it usually ?
hard starting is usually the first sign of low compression
were you holding the throttle wide open while kicking until the needle stopped moving ?
is the o-ring on the tester good and sealing completely ?
how many kicks does it take to start it usually ?
hard starting is usually the first sign of low compression
I didn't open the throttle and the tester is fine and 9 times out of 10 it starts first kick hot or cold
Retry it with throttle wide open like said. Never use oil to get a True reading. The oil will help seal the rings and give a false reading. Adding oil in a cylinder is a 4 stroke way of doing a cylinder leak down test and determine if the rings or valves are leaking.
I re did the test with the throttle open and I got 95 psi. im going to have my cylinder bored to 68 rather than the stock 66 and new piston and rings
Boring to a larger size will NOT make a noticable difference in power. And dropping some oil in the cylinder is a good way to see if the rings are the issue but not in this case really. Test and if its low, add a bit of oil and if the reading goes up, the rings may be at fault. The oil goes around the rings and helps to create a seal which will temp. boost the compression.
if the cylinder and piston are in good shape you may just need new rings which is significantly cheaper than a new Top end. If it were me, I would try the oil and see what happens to determine if its the rings.
I would also make sure the compression gauge you are using is accurate.
I would also make sure the compression gauge you are using is accurate.
Pretty pointless exercise adding oil to a 2 stroke, as a 2 stroke adds its own oil from beneath the rings which is not scraped off by an oil ring.
I have run my KTM 125 bike down to the 80psi region and it ran remarkably well and still had a lot of power and speed. What you lose with compression is quick starting and torque off idle. A 2 stroke often has surprising power at peak RPM even with very low compression.
The most extreme case in point for me was a time I dunked my 125 many miles off road. The bike went down while I was on the throttle and the engine was completely submerged. Had to flip the bike upside down and crank it over to drain the water from the crankcase. It had NO compression, because it had broken a ring I later found. In spite of this, the bike started on a push and ran well enough to get me home. It would not idle, even with the idle screw fully in, and would not start on a kick, but would push to a start.
Steve
The most extreme case in point for me was a time I dunked my 125 many miles off road. The bike went down while I was on the throttle and the engine was completely submerged. Had to flip the bike upside down and crank it over to drain the water from the crankcase. It had NO compression, because it had broken a ring I later found. In spite of this, the bike started on a push and ran well enough to get me home. It would not idle, even with the idle screw fully in, and would not start on a kick, but would push to a start.
Steve
I took off the head and took a look at the cylinder and it looks perfect. I fulled out the reed pack and they aren't closed all the way, maybe that's the problem. on the piston it is marked 66.42 wich im assuming would be size so could id be bigger than the 66 stock
No, reeds have NOTHING to do with compression. Take the reed block out and you will read the same compression. A scratched up cylinder wall will destroy compression but a smooth one is no guarantee of no problems. Likely you can probably get away with a set of rings, but always good to measure the bore and check the piston for cracks or skirt damage. Blasters can break skirts, so if you plan to rev it, plan on a new piston.
Steve
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