BP-35 engine/carb problem

[Francis]

UPDATE
Passed crankcase vacuum and pressure test.
Passed spark test. Champion CJ-8 gapped 0.025” per manual.
Failed compression test - 30 psi (I disassembled an old spark plug and soldered a brass elbow to it so the gauge could be attached directly to the elbow).

Viewing the cylinder walls and piston skirt through the intake and exhaust ports:
Cylinder walls have minor scoring. Cylinder has a wear line.
Intake piston skirt is clean and has no vertical scoring.
Exhaust piston skirt is light brown and mottled.
Piston top and exhaust outlet have carbon deposits.

Pull cylinder. Measure piston, rings and cylinder vs tolerances and wear limits - after weather gets bad again. Regroup after that.

I find it hard to believe that the engine was running at all speeds with 30psi compression vs a manual minimum of 85psi - but I have little experience with two cycle engines. What do you folks think?

Thank You. Have a good weekend.

[KE4AVB]

I probably not as experienced as many members here but do know most modern 2 cycles usually will not even hit under 100 psi; although, I have seen one that would hit at 90 psi. It didn't run very well at all. So if yours is hitting and running at 30 psi it might be the compression gauge setup you are using. Different gauges setups can affect the compression readings. For most small CC engine the check valve needs to at the end of hose closest the cylinder due to the low volume of compression gases. Gauges with this valve at the gauge itself can read off because you must compression the gases in the hose too before the valve opens thus resulting a much lower compression reading.

Personally I am using an Actron compression gauge setup with the check valve at the cylinder end of hose seems and it seems to be very accurate. I just tried an Azuno digital gauge which had the check valve at the gauge end of the hose and the readings are very low when compared to my Actron test gauge. Actual readings was Actron 110 psi vs the Azuno 17.5 psi.

[bgsengine]

UPDATE

Failed compression test - 30 psi (I disassembled an old spark plug and soldered a brass elbow to it so the gauge could be attached directly to the elbow).

Theres your problem,.. WHYYYY? what do you need an elbow for? the proper type of 2-stroke compression gauge should not need any such modification - should just screw right in to spark plug hole.. by adding your modification you are adding a LOT of cc's of compressible air that do not get registered to the compression gauge (which is why to avoid automotive gauges in the first place) - That compressible air before the schrader valve and after the combustion chamber is going to make any compression readings very suspect.. you DO realize that compression is a function of the difference between the largest and smallest volume of space the cylinder and piston create, right? adding drilled out spark plug and brass elbow, you are increasing the combustion chamber space and thus, reducing the compression ratio (and the compression) of the machine.

[Francis]

The 30psi coupled with the fact that the compression felt good when pulling the cord, said something was wrong. My 30 year old Sears offset automotive compression gauge held tightly to the spark plug hole gave me 90 psi. I thought of stopping my compression comments with the last statement but there’s something to add - the automotive gauge has the Schrader valve inside the rubber tip - you press the valve stem with your finger tIp to release the pressure.

Part of the elbow setup used a commercial adapter that had a bad “0” ring seat. What I have now has the pressure relief valve 3-1/2” Inches from the top of the spark plug hole - including the elbow. I get 85psi with this setup. The reason for the elbow soldered to the base of an old spark plug was that I was by myself and didn’t want to disassemble the unit further to get additional clearance. My neighbors are hunting, but one stoped by today and pulled the cord while I held the automotive compression gauge in the spark plug hole.

Thanks for all the good advice. The weather is nice and I have things to do outside before winter. I’ll get back to this later on this month.