Krumbsnatcher

This past weekend i ran into motor troubles after the first season. The problem was known to me, so I thought i was taking it easy. The problem was running her lean.

The motor is a blown 540 with all the goodies in her to handle the power.

My only problem was a big one fuel pressure at higher rpm's. My question is this she ran lean with a big backfire out the exhaust, she kept running but at idle she popped every once in a while after a good run.

I immediately thought i tuliped a valve. I shut her off and called sea tow back to the marina. Once back at my house, i started looking for the culprit found her on Cylinder #8. The spark plug center was melted away (lean), however the ground strap was completely in tact.

I performed a leak down test on all the others and they were strong as to be expected for a new motor. Now for my biggest fear, was I going to find a burnt hole in the piston? or was it going to be valve?

Well i flexed my wrists and started to dismantle the blower, manifolds and removal of the head. I inspected the cyclinders and she looked new with no hole in piston and no marks either on the valve or top of the piston.

Could a lean out condition (supercharger induced) cause a ring landing problem in one cylinder identified by the leak down test or would you guys bet on the head?

I know the engine was hard to start and just layed down prior to calling for sea tow which is probably why i do not have an issue with valve to piston collision.

I will have the head looked and tested sometime this week. I just want to know what you guys think and your experiences with Lean conditions.

Thanks OSO Brothers.

blue thunder

What was the result of the leak test in #8? Any pics of the underside of the suspect head?

BT

Lmarth

What were the leakdown numbers,especially for #8? What did other plugs look like? You missed an opportunity to do a leakdown test together with a stethoscope when you had the blower manifold and exhaust manifold off PRIOR to removing the head. With the stethoscope, you could have listened to the intake and exhaust ports for #8 as well as in the crankcase and compared those subjective findings for the other cylinders. My experience is for a decent head, you'll hear more leakage past the exhausts than past the intakes which makes sense given the conditions the exhausts run under. You should hear a real 'whoosh' if you drop the stethoscope pickup tube down into the crankcase for even the best cylinders. It would be noticeably worse in #8 if you burned out the top ring or distorted the ring land(s). If the head checks out ok and you had a bad leakdown in #8, then you've got to pull that piston to find out what's going on. Just one persons opinion. Keep us posted.

28Eliminator

Bummer, Krumbsnatcher
I would definitely think the exhaust valve is bad, but if it isn't, then the top ring land may have pinched the ring. I would fix the head and bolt it back on without attaching anything else and do a leak down. Then you would know if the ring set is bad or not.

Krumbsnatcher

I will post pics of the head tomorrow, the numbers were at 60lbs of pressure on all the cylinders about 52psi, #8 was 22psi.

She looked like she was flubbering so a real reading on the plugs could not be obtained due to the fact of her running rich then, the result was all black and sooty.

Thanks guys.

blue thunder

With that much leakage you should be able to see light throught the seat area. Shine a bright light around the closed valve and look inside the intake runner to see if you see light. Do this to both valves in that hole.

BT

Krumbsnatcher

BlueThunder

Thanks I will do that test tonight and let you know the results.

JJONES

Ive Dropped Valve Seats And Tuliped Valves From Running Lean At Top End!what About Leak Down Test?check For Cracks In Head Between Valves!

Strip Poker 388

iTrader: (6)

Look at the intake valve relief on top of the piston next the the cylinder wall , I have lifted them there from a lean condition . That could be your 22 .

I always set the leak down gage air pressure to 100psi so the leak down readings will be easier to reed ,like if it read 22psi would be 22 %.


With it back firing thru the exhaust could of been the spark plug firing at the wrong time making it pop in the exhaust..

Also the pop in the exhaust could be the exhaust valve sticking in the guild.



The piston read 48 % it did not want to idle very well,probably had positive pressure in the bottom end at idle

Lmarth

Rob's making good points. Just personal preference, but I also prefer 100#'s for leakdown tests. You'd think that with only 22# in #8 that there would be obvious signs of what happened. Guess we'll just have to wait a bit