I'm struggling to find the cause of a misfire in my 1915 T. Replaced the timer (which was worn) - to no avail. Now I want to try cylinder leak down tests, but how do I keep the cylinder at the top of the compression stroke without the engine rotating/ piston dropping to the bottom of the cylinder. Releasing the parking brake just permits the car to move.
Appreciate your thoughts on both items.
Chock the wheels and put the car in high gear?
A good way to do this is to remove the manifolds and the valve cover. It's not critical to be at the top of the compression stroke but rather to just be on the compression stroke. I just did this on my car a few weeks ago and found that one of the intake valves was not seating. Removing the valve cover let's you see that both valves are in the shut position as you rotate the crank. Next, set the brake and chock a front wheel. Put a little air to the cylinder and listen at both valve ports (manifold) and in the oil fill hole by the generator. Mine showed bad blow by on all of the cylinders ( into the oil pan) and one intake valve passed a lot of air. It's a very good way to find the cause of low compression. Needless to say, that engine is apart at this time.
Sorry. I put that wrong. I meant to chock a front wheel and throw the clutch lever forward into high gear... as Mark said. If you put too much air to it, she will climb over the chock, so you want a push button air valve, like on a blowgun.
Leak down tests are done with the piston on TDC. Since at that point the crank throw is straight up and the rod is pushing straight down it will not move. You have to be within a few degrees of TDC.
Totally unnecessary in my opinion. Take a dry/wet compression test and you'll get all the information you need about the condition of your rings & valves. Also a classic case of not enough info in the original posting. Give out some more info on exactly what the engine is doing and we can possibly send you in the right direction and save you some pointless work. A leak down test wouldn't be a logical next step for a rough running/miss firing motor.
A leak down test is a slight specific program, two pressure gages are used with an isolation (shut off) valve between them, both pressure gages are pumped to cylinder pressure immediately the valve is closed to the outboard gage to maintain the pressure, the inboard gage will "bleed" off you read the time of the bleed off. A good cylinder would be very slow a bad cylinder would be very fast. For each cylinder a stopwatch should be used to record the time. Between each cylinder the outboard gage is bled off by depressing the snifter valve