Cam vs static compression

[Ratty78]

Hey y'all

Cold weather is here in NM and my kick only 78 hates me.

I can get her started in as few as 30 kicks when it doesn't get much colder than about 30. Beyond that I don't even try.

So I checked the compression, and only got about 55 and 60 with the engine cold. Warmed it up and ran the test again. No change.

So I put my palm over the carb and I feel 2 (small) puffs of air come out on the compression stroke. So at the very least, air is getting past the intake valves. Readjusted the pushrods a couple times and no change to compression. I even tried running the pushrods loose to ensure no error there and still no change. I also pulled the gear case cover to check the cam timing, and everything lines up as it should.

My cam is an Andrews J grind which has opening/closing events listed as 21/41 and 41/21 respectively, so if I'm interpreting that right, there should be no overlap (also overlap shouldn't be a factor on the compression stroke).

When I freshened up this engine last winter I did the top end .040 with standard compression Pistons (custom chrome) which I believe to be 8:1 although it didn't say on the box. I disassembled the heads, cleaned them really thoroughly, and lapped the valve. The seats ended up measuring at 5/32" where the max by the book is 3/16". So they were out of spec, but the sealing surface came out clean 360* so I decided to run it.

I never checked the compression after the rebuild.

So I think I need to do a proper rebuild on my heads, but don't want to pull it apart if I'm missing something obvious like cam and valvetrain dynamics that I just don't understand.

I think that I shouldn't feel air coming out of the carb on compression, and I should be reading more like 100-120psi of compression.

And insight on this?

[JBinNC]

Yes, with standard replacement pistons (which for a 74 are nominally 8.5:1, and for an 80 are nominally 8.0:1) and a J grind cam, you should have at least 100 psi compression even on a cold motor.

Your cam with the specs you mentioned has 42 degrees of overlap (at .050 tappet lift), not zero, but that is not a factor in a cranking compression test.

Most old shovel heads need extensive rework to function well. Just "lapping the valves" is not going to bring them up to standard serviceable condition. Most need guides and all the old heads with burnished stem valves will need valves as well. I think that is where your problem lies. A decent rebuild of shovel heads can easily cost $800 - 1000, and there are no shortcuts that will give good results.

Jim

[JBinNC]

You may feel an air pulse at the carb when kicking, because one cylinder is always in overlap while the other is on the compression stroke. At cranking speed, air may be forced back through the intake by the cylinder in overlap.

Jim

[farmall]

I think that I should t feel air coming out of the carb on compression, and I should be reading more like 100-120psi of compression.

That's what I'd expect. I like to know what's leaking even when I know it's gonna be a top end job so I'd leakdown test before pulling the heads, and if I pull heads I pull jugs to inspect since what is not inspected is not known. Then I'd test leakdown after reassembly.

Since the top end is coming off anyway I'd pull the carb, exhaust and pushrods then cram a rubber-tipped blowgun into the spark plug hole and listen for leaks at the ports and timing plug hole if ya don't have a leakdown tester on hand.

[Ratty78]

Yes, with standard replacement pistons (which for a 74 are nominally 8.5:1, and for an 80 are nominally 8.0:1) and a J grind cam, you should have at least 100 psi compression even on a cold motor.

Your cam with the specs you mentioned has 42 degrees of overlap (at .050 tappet lift), not zero, but that is not a factor in a cranking compression test.

Most old shovel heads need extensive rework to function well. Just "lapping the valves" is not going to bring them up to standard serviceable condition. Most need guides and all the old heads with burnished stem valves will need valves as well. I think that is where your problem lies. A decent rebuild of shovel heads can easily cost $800 - 1000, and there are no shortcuts that will give good results.

Jim

You may feel an air pulse at the carb when kicking, because one cylinder is always in overlap while the other is on the compression stroke. At cranking speed, air may be forced back through the intake by the cylinder in overlap.

Jim

Thank you Jim.

Would you mind expanding on some of this information for me? Just want a bit more understanding if what is happening and why.

First could you exain the math behind your calculated 42 degrees if overlap?

Second where is the compression going? You mentioned the guides and valve stems. Are you suggesting that there is a bind that won't allow the valves to close all the way? I had considered that possibility although the valves did move smoothly with the springs out. I also considered that lazy springs could cause this. Or perhaps loose guides could allow the valve stem to move elliptically with the bore allowing the valves to seat askew?

Thanks for replying!

[JBinNC]

Overlap is easy: the intake valve is opening 21 degrees BEFORE TDC, and the exhaust valve is closing 21 degrees AFTER TDC, so the overlap is that span of rotation, 42 degrees in this case. (If it's a dual pattern cam, those opening and closing figures are not the same, but the overlap is figured in the same way.)

Jim

[JBinNC]

The main deal with your heads is that a very wide valve seat does not seal well, and as you suspect, a worn guide will allow the valve to hit the seat on one side when closing, so it then has to move to get centered on the seat. That takes time (microseconds) but does effect how the valve seals, and also further wears on both seat and valve.

Jim

[JBinNC]

Shovel valve springs are remarkably long- lived and more than adequate for the cam you have. By '78 H-D was using the H spring set in all shovel motors, and there are thousands of those springs still in service today. I have checked many sets, and rarely see a spring that is not within new spec.

Jim

[Ratty78]

That's what I'd expect. I like to know what's leaking even when I know it's gonna be a top end job so I'd leakdown test before pulling the heads, and if I pull heads I pull jugs to inspect since what is not inspected is not known. Then I'd test leakdown after reassembly.

Since the top end is coming off anyway I'd pull the carb, exhaust and pushrods then cram a rubber-tipped blowgun into the spark plug hole and listen for leaks at the ports and timing plug hole if ya don't have a leakdown tester on hand.

100% agree and will be doing a leak down test before I rip it apart. Just need to go pick up the tool. Need one anyways.

Hoping I won't find much getting past the rings.

[JBinNC]

You have overlooked a common cause of loss of compression in a shovelhead, and that is head gaskets blown to the oil return passage in the gasket surface. The blue Teflon gaskets are very bad for this and should be avoided at any cost. The various "fire ring" gaskets are preferred.

Jim

[JBinNC]

100% agree and will be doing a leak down test before I rip it apart. Just need to go pick up the tool. Need one anyways.

Hoping I won't find much getting past the rings.

A leak down test is always helpful, but with such low compression test numbers, I would just be taking it apart.

Jim

[Ratty78]

I should have mention that I did re-torque the heads after the first compression test. I ran into this early on after I put everything together. Didn't re-torque the heads right away and blew the rear right at the oil passage. Replaced them with another set (of the same blue Teflon gaskets. No big deal to switch over to different gaskets when I pull this apart. I appreciate the advice on that.

My Indy out here is rock solid and been in business since before this bike was made, so I'll prob have them get these heads back in shape while it's still cold out.

It sounds like I need valves and guides, but can maybe save a few bucks on the springs? What's the likelyhood that I will need new seats? They quoted me $130 for a "valve job" without parts. Will the seats have enough meat to get into spec with fresh valves if there is no damage otherwise? (There wasn't 4k miles ago when I lapped them)

[JBinNC]

Shovel valves will run with the seats sunk way beyond factory specs. Indeed, many of the old hot rod cams required that the valves be sunk to avoid valve to valve interference. Therefore, your heads should not need seats.

Your indy's labor quote is way too low for a competent job (in my opinion) so I would look elsewhere for head work. I'm serious about that; pay to get it done correctly the first time rather than to have it done again later. I have reworked hundreds of sets of shovel heads, some of which had "just been done " by someone who did not know what he was doing. Your bike, your call.

Jim

[Ratty78]

You have overlooked a common cause of loss of compression in a shovelhead, and that is head gaskets blown to the oil return passage in the gasket surface. The blue Teflon gaskets are very bad for this and should be avoided at any cost. The various "fire ring" gaskets are preferred.

Jim

This is kind of a stupid question, but should I expect the same compression on a kick only bike as with a electric start?

I've done a lot of compression tests on cars cranking the starter, and it usually takes quite a few cranks before the compression stops climbing. With the bike I open the choke and throttle and kick it through several time, as many as 20, but it stops climbing after about the 4th or 5th. I was wondering if the lower rotational speed possibly contributes to a lower reading? Regardless, it sounds like she's coming back apart.

The bike started first kick pretty consistently all summer (always did prime kicks to lubricate everything first thing in the morning) but the cold is truth

[JBinNC]

Yes, I find that results are lower at kicking speeds. And I find exactly the same: the gauge levels off at the fourth or fifth kick.

The speed matters because you get a little inertia fill of the cylinder even at cranking speed.

Jim