Harley 45 flathead, one cylinder running lean

[Buster]

Stock recently rebuilt Harley 45 flathead. Both cylinders were running fine but front recently started running lean. I have NO intake manifold or head gasket leaks, I have repeatedly tested with pressure. Compression in front cylinder was 40lbs after rebuild and before valves were seated, but has now dropped to 32 lbs. The rear correctly running cylinder had 45lbs after rebuilding and has increased to 58 lbs after running and rings seating. I have leaking exhaust valve in the front lean running cylinder, could a leaking exhaust valve cause a lean condition, If not, any other ideas about the cause of lean condition in front cylinder? Thanks!!

[tgarcia]

Yes that is a problem, if the exhaust valve doesn't have a proper seat it will cause the intake stroke to not have the full vacuum to pull a fuel air charge in past the intake valve, basically it is a vacuum leak at the exhaust valve causing the loss of compression and lean condition.

[Architect]

Those compression numbers seem low for a 6:1 WL motor, should be more like 85-90 psi.

[T. Cotten]

Its not a true "leak-down" test, Buster,...

But putting a gentle air source upon the sparkplug and/or timing hole can give clues by listening at the exhaust, carb, etc. as you turn it through, especially if its a leaky exhaust valve.

A "fresh rebuild" suggests that a valve wasn't leaky to start with, but got that way.

Properly dressed, valves and seats 'beat in' no more than the toolmark allows, and as per Hastings over the phone, "modern rings don't 'seat'" either. If they do, they are worn out.

....Cotten

[Buster]

I pulled the top end and found the front exhaust valve sunken too deep into seat seat. I have ordered an oversized valve. Speaking of compression for a 45 flathead. Is the noted 85-90 lbs correct? I thought it would be lower than that. Maybe in the 60-65lb range. I have 6.0 aluminum heads which I thought were around 5 to 1 compression ratio. Thanks for the help.

[Architect]

#6 aluminum heads are to provide 6:1 compression. WLA and I believe Servi's had #5 heads, 5:1 compression.

[T. Cotten]

I pulled the top end and found the front exhaust valve sunken too deep into seat seat. I have ordered an oversized valve. Speaking of compression for a 45 flathead. Is the noted 85-90 lbs correct? I thought it would be lower than that. Maybe in the 60-65lb range. I have 6.0 aluminum heads which I thought were around 5 to 1 compression ratio. Thanks for the help.

Sunken seats flow poorly of course, Buster,

And get hotter than they should, but something had to change to suddenly run lean.

Did it burn? Does it leak? Is the stem still straight?

If one atmosphere is about equal to 14.7 psi, then 5:1 should give just shy of 74 psi, and 6:1 should give a little over 88. Sunken valves lower compression slightly.

....Cotten

[dukekleman]

Buster,
Without seeing a copy of the rebuild service (Parts and Labor) advice can only based on assumptions. It's unfortunate that you had to reopen the engine again.
My assumption based off the information that you gave on the front exhaust valve is this...I believe that the valve was probably reground and the seat recut.

Valves that are reground to far on the margin will tulip dropping them into the seat further. This causes the valve lash to diminish, the valve not to cool properly and eventually stay open due to lash loss.
Exhaust valves are very prone to this...

A new valve, proper valve seat work and maintained valve lash clearance should remedy the problem.
I would revisit and look closely at all the other valves and seats.

As Tgarcia's post early on, proper valve operation is crucial to proper fuel delivery.

Hope this helps,
Duke

[Buster]

No copy of rebuild service to see, I rebuilt the engine. Since last posting, I now have all oversize valves and freshly cut seats which were lapped. Valve seal is good. After I installed cylinders and heads, I cranked motor over by hand with finger over plug holes and found very good compression on rear cylinder and on front cylinder compression would barely beginning then immediately disappear. I removed cylinder heads and cranked over by hand. Watching the valve action, I noted there is very slight valve overlap on front cylinder. Rear that has good compression has no valve overlap. Motor has been running since rebuilt with no change in cam positions, however Beginning to think a cam out of time. Pulled cam cover to see if cam timing marks are aligned. All timing marks are lined up. Pulled cams out and compared cam mark and cam lobe locations to another set of cams I have. Found no difference between the two sets of cams. Put cams back in and still have cam overlap at front cylinder only. Seems like intake valve opens one tooth too soon. Only thing I know to do is retard front intake cam one tooth and put together and check valve openings. This front cylinder does have a badly sunken intake seat with oversize valve. Could this significantly affect valve timing to cause this overlap. Any ideas appreciated. I don't believe a 45 flathead should have any valve overlap, especially on only one cylinder. I believe this valve overlap on the front cylinder was the cause of my front lean running cylinder.

[Rubone]

No copy of rebuild service to see, I rebuilt the engine. Since last posting, I now have all oversize valves and freshly cut seats which were lapped. Valve seal is good. After I installed cylinders and heads, I cranked motor over by hand with finger over plug holes and found very good compression on rear cylinder and on front cylinder compression would barely beginning then immediately disappear. I removed cylinder heads and cranked over by hand. Watching the valve action, I noted there is very slight valve overlap on front cylinder. Rear that has good compression has no valve overlap. Motor has been running since rebuilt with no change in cam positions, however Beginning to think a cam out of time. Pulled cam cover to see if cam timing marks are aligned. All timing marks are lined up. Pulled cams out and compared cam mark and cam lobe locations to another set of cams I have. Found no difference between the two sets of cams. Put cams back in and still have cam overlap at front cylinder only. Seems like intake valve opens one tooth too soon. Only thing I know to do is retard front intake cam one tooth and put together and check valve openings. Any ideas appreciated. I don't believe a 45 flathead should have any valve overlap, especially on only one cylinder.

Check your camshafts. On later two piece cams it is possible for the gear to spin on the shaft allowing the timing to be way off despite marks lining up. I had that issue on a police Servi-Car that had seized an alternator jamming up the timing train. When setting the valve clearance I noticed the valves on one cylinder just about opening at the same time. What cams are in your bike?

[Buster]

I have a set of the early model pre 1958 OEM cams.

[dukekleman]

Buster,
I agree with Rubone,
That your cams may have moved out of time.
By chance do you have a degree wheel? You could find out where the cam events are at real quick using that.

Do the lobes themselves appear the same, one lobe isn't worn oddly by chance is it?

Duke

[Buster]

I've never used a degree wheel and motor is in bike. Planning on using info in manual showing when valves open in relationship to TDC. Going to make fixture to bolt to cylinder deck to hold piston at TDC and check all cams. Is this reasonable? Leaving in day or two (2,500-mile drive) for Borrego Springs Road Run so it will be after I get back. I'll get back with results or more questions when I return home.

[dukekleman]

Buster,
I believe your plans of measurement will lead to some helpful insight.
Have a safe trip!
Duke

[T. Cotten]

I've never used a degree wheel and motor is in bike. Planning on using info in manual showing when valves open in relationship to TDC. Going to make fixture to bolt to cylinder deck to hold piston at TDC and check all cams. Is this reasonable? Leaving in day or two (2,500-mile drive) for Borrego Springs Road Run so it will be after I get back. I'll get back with results or more questions when I return home.

Hope this helps, Buster!

PROTRCT3.jpg

Resize if you need to, print and cut it out so you can tape it centered upon your motorsprocket nut at TDC; a piece o' wire wrapped around a primary cover screw makes a pointer, so you can set it to 'zero', and then turn the motor through degree by degree.

If you have a second cam, I'd just eye-ball the two to see if something's amiss.

....Cotten