Mag. 52 4 stroke won't run where to look
 

Thanks for looking.

The engine is a Magnum .52 4 stroke that has been running well for the last 12 years. This is one of the first to hit the US market.

3 years ago I installed a new ring to bring the end gap to spec and compression back up. Currently the engine will not run.

Problem statement:

Fuel will not draw from the tank to the carb preventing the engine from starting.

Observation:

Fuel will gravity drain from the fuel supply line and is not restricting flow.
The muffler pressure tap is open.
The engine if exhaust plugged will pull fuel to the carb. W/o plugging the exhaust opening it will not draw fuel.
Compression measured by the old flip test appears low but the new ring only has about 2 hrs run time.
Fuel Cool power 15%, Prop 11x6, Plug OS F

Actions taken.

Carb pulled and disassembled cleaned and all o-rings replaced
Back plate pulled and verified tight seal.
Verified valves gap and timing.
Plug new OS F
Fuel tank cleaned new lines and verified will gravity drain.

Found in the crankcase a white (teflon?) pad. Might be from piston as there is one teflon pad that prevents the pin from sliding out as in OS design.

Tomorrow I will fully disassemble.

Currently nothing other than the teflon pad is out of place. No air leaks, no scoring of the cylinder, bearings still good, no piston damage.

I am open for suggestions. I could replace the cylinder liner, piston/ring but they currently are not available.

Any help would be appreciated. Thanks vettdvr.

RE: Mag. 52 4 stroke won't run where to look
 

If it pulls fuel when you cover the exhaust, that would suggest there is a leak at the exhaust valve. If the engine has a lot of run time the valve stem could be carboned up and not letting the valve fully close or there could be something under it. If you replaced the ring you should have good compression after running it once, the ring seal will of course improve with more run time but not by much, at least that's my experience. The teflon pad probably got knocked off when you put the piston back into the sleeve and you didn't notice it but should be fixed or you'll score the cylinder with time.

RE: Mag. 52 4 stroke won't run where to look
 

I will check the valve today. But at first look it appears to be sealing but perhaps not. I run full synthetic oil to prevent carbon build up but this engine has lots of time.

RE: Mag. 52 4 stroke won't run where to look
 

You were right on! [sm=thumbs_up.gif]

I pulled the head this morning removed the valve and there was heavy carbon on the valve stem that was hitting the valve guide.

I carefully polished the valve stem back to shiny metal. While at it I cleaned the valve seat and carbon in the valve pocket.

I also took forceps and carefully rotated the clean valve against the seat with light thumb pressure to reseat the valve.

I haven't run it yet, but spinning the prop will now draw fuel and yesterday it would not. So with it drawing fuel I expect it to run.

One question. My auto valve C compressor is just a bit too big to remove the valve retainier. Could not help but wonder what you use.

And of course my thumb nail slipped and I fired off the valve retainer cap across the shop. Fortunately my bifocals allowed me to find it.

Thanks again. I expect the cub to be airborne soon. vettdvr

RE: Mag. 52 4 stroke won't run where to look
 

Glad it worked out for you, hopefully will run like it use to now. What Vett do you drive? Have a '67 that is driven now and then, had it since '72. Fun cars to have and drive.

RE: Mag. 52 4 stroke won't run where to look
 

Sold red 64 327/4speed conv. (87000 miles) riverside red to fund son's college

Sold 87 teal (76,000 miles) to fund NEW 99 C5

C5 now 87000 miles thinking about C6.. UMMMM also thinking about buying 1 to 1 airplane instead and keeping 99.

Looking at Cessna 172 for cross country and might build (REALLY BIG MODEL) RV7A cruise at 200 mph max ceiling 21,000 range 1200 miles

Lots to think about in retirement,, tough work but someone has to do it:D

Do a search on Vans RV7 and look at the one with nose wheel.

RE: Mag. 52 4 stroke won't run where to look
 

Update. After doing the valve work it would draw fuel. I started the engine and it ran a few seconds then quit. It would no longer draw fuel.

There is very little compression and I suspect the cylinder liner is worn out. I have ordered new piston/ring/liner for replacement. They won't be available

for about 3 weeks. I am considering replacing the engine. The cost of replacement parts is about 45% cost of new engine.

This engine only has about 10 yrs of heavy run time on it.