I have a 1987 Polaris trail boss 250. It has a brand new piston and bore but it only at 75psi compression????? And it will sometimes have suction to the carb so it will start then it will die and it wont have and suction to the carb and wont start..??? Im stuck here.
2 Stroke Atv problem
- Thread starter JeyK
- Start date
very little chance of properly measuring the defacto compression on a two stroke engine
if you cannot keep the crank anywhere near idle speed for more than a couple dozen revolutions.
IDK what Setup the polaris is using tbh..
but in case it's a reed valve engine and you are suspecting an issue or air leak;
My first guess would be to check said reed valve..
those vanes can sometimes break and sometimes they just get clogged... sooo worth to check.
'sid
if you cannot keep the crank anywhere near idle speed for more than a couple dozen revolutions.
IDK what Setup the polaris is using tbh..
but in case it's a reed valve engine and you are suspecting an issue or air leak;
My first guess would be to check said reed valve..
those vanes can sometimes break and sometimes they just get clogged... sooo worth to check.
'sid
Also I forgot to mention without the engine being broke in yet it won’t read the correct compression psi also.
My brand new top end is the same way. It reads 75ish but is supposed to have double that. Engine builder said break it in first then check it.
My brand new top end is the same way. It reads 75ish but is supposed to have double that. Engine builder said break it in first then check it.
a piston port is even worse to measure the actual runtime compression of the cylinder..
since by design the crankcase leaks against the carb even more than a reed valve would.
Anyways.. you cannot ever measure the compression of a non running 2stroke engine
as easily as you can measure a 4 stroke..
so no matter if broken in or not, the static compression will always be way lower than expected.
and way lower than the compression generated by a running engine.
The only way to get a somewhat accurate 'static' reading would be to remove the carb and exhaust, and plug the ports tightly,
then carefully put the engine into compression (crank case port just closed), thread in the compression tester and bring it to TDC
that should show about at least 80ish percent of the compression depending on combustion volume
the remaining few are the ccs of the exhaust port in the cylinder wall..
And that only works if there's no decompression valve installed (or it's disabled)
The perfect way of measuring the true compression is to remove the decompression valve and use that port on a running engine,
if you don't have a decompression valve and you do not want to add one,
you'd need a second head , drill in a port for the tester and then test the running engine.
Since again, the true compression of a two stroke also depends on the rpm and the exhaust (it's back pressure at that rpm)
A piston port engine should be in the realms of 6 to 9 atmospheres
a reed valve in the realms of 9 to 10 max IIRC (much above that and you risk self ignition)
Since I'm not too good at your ametrican freedomunits I can only only eyeball that to
88-132 psi for a piston port and ~130-150 for reed valves..
With that the 75psi read from a static test are a not too shabby value considering
it's a static test on a two stroke and 88 is enough for a solid running engine
frankly.. with the exhaust in place, the carb wide open and a static test..
turning the engine over with the starter motor (and roughly 1000 rpm)
I'd say you read just over half the actual compression
which would put the reading of 75 in a true value of 130-140 psi
So nothing to worry about IMHO as of yet.
'sid
since by design the crankcase leaks against the carb even more than a reed valve would.
Anyways.. you cannot ever measure the compression of a non running 2stroke engine
as easily as you can measure a 4 stroke..
so no matter if broken in or not, the static compression will always be way lower than expected.
and way lower than the compression generated by a running engine.
The only way to get a somewhat accurate 'static' reading would be to remove the carb and exhaust, and plug the ports tightly,
then carefully put the engine into compression (crank case port just closed), thread in the compression tester and bring it to TDC
that should show about at least 80ish percent of the compression depending on combustion volume
the remaining few are the ccs of the exhaust port in the cylinder wall..
And that only works if there's no decompression valve installed (or it's disabled)
The perfect way of measuring the true compression is to remove the decompression valve and use that port on a running engine,
if you don't have a decompression valve and you do not want to add one,
you'd need a second head , drill in a port for the tester and then test the running engine.
Since again, the true compression of a two stroke also depends on the rpm and the exhaust (it's back pressure at that rpm)
A piston port engine should be in the realms of 6 to 9 atmospheres
a reed valve in the realms of 9 to 10 max IIRC (much above that and you risk self ignition)
Since I'm not too good at your ametrican freedomunits I can only only eyeball that to
88-132 psi for a piston port and ~130-150 for reed valves..
With that the 75psi read from a static test are a not too shabby value considering
it's a static test on a two stroke and 88 is enough for a solid running engine
frankly.. with the exhaust in place, the carb wide open and a static test..
turning the engine over with the starter motor (and roughly 1000 rpm)
I'd say you read just over half the actual compression
which would put the reading of 75 in a true value of 130-140 psi
So nothing to worry about IMHO as of yet.
'sid
OKay thanks. I figured out I had a blown crank seal so I replaced and everything was good. expect now I have either a really weak spark or no spark?!?I didn't touch the wiring and I used to have amazing spark. The only thing I did was hook up a battery to see if it did anything and it didnt.