help with compression 76 kz750b

Hi,
I've looked everywhere here and cannot find anything about acceptable compression. When I first got my bike a couple weeks ago Ipulled the spark plugs & poured some marvel mystery oil down it just in case there were some sticky rings and etc.So I guess I just did a wet compression test. I should have done the dry 1st. Another contributing factor at least I think is if I dont apply all my weight and have good fast kick Im not getting all the stroke . Anyways when I did the check I got 45 psi on one cylinder and 85 psi on the other. I've yet to get her fired over and have no wiring so it has to be done by kick. SHould I get someone with a little more butt behind them to kick it? Im about 5'6" 130 Lbs.The bike has been sitting for I dont know how long but its not frozen up .

petunia76 wrote:

Hi,
I've looked everywhere here and cannot find anything about acceptable compression. When I first got my bike a couple weeks ago Ipulled the spark plugs & poured some marvel mystery oil down it just in case there were some sticky rings and etc.So I guess I just did a wet compression test. I should have done the dry 1st. Another contributing factor at least I think is if I dont apply all my weight and have good fast kick Im not getting all the stroke . Anyways when I did the check I got 45 psi on one cylinder and 85 psi on the other. I've yet to get her fired over and have no wiring so it has to be done by kick. SHould I get someone with a little more butt behind them to kick it? Im about 5'6" 130 Lbs.The bike has been sitting for I dont know how long but its not frozen up .

On the 750 twins:

Nominal compression is about 150 psi

Service limit: 115 psi with less than about 15psi from min to max.

These are HOT readings, no cold specs are given in the manual.

NOTE: the engine will not run with 45/85 compression.

how did you do the compression test did you hold the throttle wide open and kick it until the gauge quit climbing or as just once and take the reading. just trying to clear things up.also try taking both plugs out and then trying it it will make it easier to kick over it is a big twin after all.

I just kicked once and took reading then switched gauge to cyl 2 and did the other side.
I don't even have a throttle yet.I want to make sure my motor is ok 1st .

you have to kick it until the gauge stop moving usally 5 or 6 good hard kicks

petunia76 wrote:

I just kicked once and took reading then switched gauge to cyl 2 and did the other side.
I don't even have a throttle yet.I want to make sure my motor is ok 1st .

Compression test requires plenty of air being sucked into the engine.

Usually accomplished by holding throttle twist grip in wide open position while cranking over the engine with electric starter or kick starter.

May also be accomplished with carbs removed from engine, with nothing such as rags stuffed inside or blocking the carb holders that would prevent plenty of air being sucked into the engine while cranking it over with the electric starter or kick starter.

Good Luck!

If the compression still reads too low after a properly performed compression test, the low compression may be resulting simply from too tight valve clearances.

So the valve clearances should be measured and assured to be within specs.

Even if valve clearances are within specs, one or more valves might be leaking instead of fully sealing when it's supposed to be closed in its seat. Such valve leakage will also cause loss of compression. But often it's only the clearance that needs adjusting.

A leak-down test helps determine exactly where any compression loss is going, whether past the pistons/rings, or through exactly which valve(s), or past a leaking head gasket.

Good Luck!