Mac behaving badly

[Bill Peck]

Help! This has me baffled.

In the last practice session on Friday at Oreville I thought I heard a slight "tink" coming off the hairpin, then the head temp jumped up over 400 and the the next time off the hairpin the engine bogged a little bit. OK, air leak right?

So Saturday morning I pressure tested the engine (91B-1) and it was tight as a drum, so the leak must be in the carburetor. I replaced the base gasket and trimmed a half inch off the fuel line to get a fresh end and went back out. Now it's running at 370 head temp (my normal max) AND 4 cycling about half way between the kink and the restart box. Then when I pulled into the pit lane it just shut off.

I tried to restart it in the pits and it wouldn't fire. I pulled the plug and it was badly oil fouled. I replaced the plug and it started and ran with the rear wheels off the ground, but as soon as I stepped on the brake it stalled. Changed carburetors and it did the same thing 2 or 3 more times. OK, change engines (that's why I missed qualifying). So now at home, I pressure tested the engine again; tight as a drum.

Oh, by the way, I was also loosing clutch oil at an abnormally high rate. It's the Margay gearbox wet setup where the oil bath is in contact with engine PTO seal. Could it be that the PTO seal is sealing under pressure but leaking under vacuum, thus sucking clutch oil (and air) into the engine? In my demented mind that would account for all the symptoms including the "tink", which could have been the seal spring blowing through the engine without hitting anything important. Or am I grasping at straws?

Anyway, I just wanted to see if anybody could shed some light on this for me before I tear down an otherwise perfectly good engine. Thanks.

Bill.

[doug tenney]

I suppose that you could test your PTO seal theory with a hand-squeeze vacuum pump.
Are you certain that the oil bath housing has adequate venting?
Is there any difference in the amount of compression the engine has now versus before heading to Oreville? (Maybe your head gasket pushed out the side?)
Did you loose any springs holding your pipe to your header? Your air leak could be at the header flange or downstream of that.
Check the clamps for your pipe, to make sure they haven't slid over to partially block the outlet.
A couple of years ago my son bought me a "borescope" camera attachment for my Android phone. He said it was about $30, I recall. If you have something like that, you could take a look without having to tear it down.

[David Luciani]

if you are gonna be anal about checking seals the vacumn test is recomended.
you definitely could have a reverse leak rare but not unheard of.
i'd replace the seal just because it's cheap insurance.
d

[Bill Peck]

Doug - the housing is well vented, the compression is still excellent (that's why I don't want to tear it down if I can help it), and the header and pipe are as they're supposed to be. I thought about trying to rig up some sort of vacuum test apparatus, but as Dave pointed out, that would be anal!

Dave - thanks for your pragmatic solution. Do you think I can pop that seal out without pulling the crank out? But I tend to be prone to tunnel vision. Could be missing something?

[Kurt Bogerman]

Bill,

Earlier this summer, I replaced the PTO seal, in situ, on the MC90 I ran at Oreville and at TBO. You can pry it out, but you have to be very careful to not gouge the crankshaft nor the seal bore. You'll need to rig up a long tube to drive the new seal, as it'll have to slide over the whole PTO and most sockets aren't that deep. (EDIT: Actually, the need for the long driver was an issue on the flywheel side. I ended up using the aluminum body of a cheap LED flashlight) Again, you need to be careful, but it definitely can be done.

As far as checking vacuum is concerned, I use a MightyVac MV8500. It can apply both pressure and vacuum. In my vast experience (insert eye roll here) of building two engines and testing a third, I've found it to be as accurate as the test needs to be. I've been using it for setting pop-off pressures, too.

I hope this helps,

Kurt

[David Luciani]

bill,
somewhere in old old posts you can find pix of the vacunm/pressure tester i built.
it was a neat thing and actually practical for a bench tool.
i put it away long ago as too anal.

replacing seals without disassembling the engine is doable but my best remark is it's kinda lazy if you're at home.
often just doing it correctly the first time will save you grief whjen the shortcut fails.
just sayin!

d

[ted johnson]

Truth is, if you take it apart to replace the seal, you just might find any other issues at the same time. TJ

[richard Engel]

Hi , You may have a Stuck Ring that you can not see If you came down the strait and did not give the carb a quick burp than you could have burnt up the gas and oil on cylinder Richie, Richard Engel

[Bill Peck]

So it turns out I was right (that doesn't happen too often)! The PTO seal was missing its spring. When I took the stuffer off about 2 oz. of clutch oil ran out all over the bench. Took everything apart except removing the piston. Cleaned all the oil & carbon out, put it all back together and fired it up in the driveway. Sounds fine on the stand. We'll see what happens when I get it out on the track at Cuddy. Thanks to everyone for helping me think it through. See you there.

Bill.

[David Luciani]