Fresh rebuild from reputable shop, no compression.

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panama86
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Fresh rebuild from reputable shop, no compression.

#1

Post by panama86 » Wed Apr 17, 2019 1:17 am

Hey all,
I had my 51 FL pan rebuilt by a reputable shop here in Australia mid last year, the motor has been on the bench while i finished up other components on the bike. Finally got it ready to go last week and gave it it's first kick, followed 80 more. I checked everything probably 10 times over the weekend to ensure it wasn't something I had set up incorrectly. The motor is mag fired so I had assumed it was just the mag so I dropped another new morris from another bike on with no luck followed by a stock timer setup and still nothing. The bike is on .010 oversize, fresh bore. Basically stock motor apart from the Sifton 412. This shop builds early motors almost exclusively and have built many motors for friends of mine over the years without issue.

Borrowed a compression test kit off of a friend and discovered 25psi in the front and 30psi in the rear. Checked rod adjustment again to be sure, then decided to put a piece of 3/8" fuel line into the plug hole on front compression stroke and just blow through with my mouth. Nothing escaping past the valves but with the inspection plug out I can hear air flowing past the piston.

Called the shop to explain the issue, was told since I supplied the piston kit it was probably incorrect pin height etc etc etc. The kit was a stock 8:5:1 kit. Also was told that it's natural for a little to escape past ring gap and also that the rings likely hadn't seated yet, which I understand but that shouldn't explain 25-30psi. That's not enough to even start the thing.

Shop says they are too busy to look at it for at least a few weeks.

Is there something I could be missing here? I could take the heads off and investigate myself but don't want to give any reason for the shop to void any guarantee.



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Re: Fresh rebuild from reputable shop, no compression.

#2

Post by Excalibur » Wed Apr 17, 2019 3:24 am

That's frustrating!! Sorry to hear things aren't working.

What are the pistons and rings? Brand, type, material etc?

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Re: Fresh rebuild from reputable shop, no compression.

#3

Post by Andygears » Wed Apr 17, 2019 10:32 am

A reputable shop that won't spend 20 minutes on the phone trying to work out your problem? Is not a reputable shop in my book. They start right off blaming you for wrong Pistons etc etc. They built the motor, they're responsible.

Pressure from the timing hole is normal and may not be compression past the rings. I suspect an incorrectly timed gear side but could be other things.

For us to help, please give more details about the rebuild, what Pistons, where the cam came from, was it together before rebuild running right? Perhaps the cam gear is incorrectly located on the cam which would cause this although the timing marks were aligned. Again, more details on what was done in the rebuild.

Andygears

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Re: Fresh rebuild from reputable shop, no compression.

#4

Post by 1950Panhead » Wed Apr 17, 2019 2:50 pm

Remove spark plugs and release push rod covers, turn engine to tdc on front cylinder, if intake pushrod tight rotate engine and verify intake valve opens as piston desends, at next tdc verify both pushrods spin without excessive clearance.
Repeat for rear cylinder.
If valve timing ok and pushrods ok put some oil in both cylinders, rotate engine a few times and recheck compression.
If no change let the shop figure it out, rings can stick to piston and not expand, anyone with experience checks this when building engine.

panama86
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Re: Fresh rebuild from reputable shop, no compression.

#5

Post by panama86 » Thu Apr 18, 2019 3:09 am

Thanks for the replies.

So this motor is, or was, a bit of a dog. Came to me in parts, a lot of mismatched parts some of which had to be replaced. From memory it has new sifton solids, as mentioned has .010 over bore. The pistons kit that came with the motor was new, from memory they were the moly skirted v-twin kit with hastings rings. Had a fresh valve grind done, running black diamond valves. Heads were in good shape. Crank was rebuilt, new rods etc using stock flywheels, bearings replaced. Most of the gear set was in good condition, as mentioned above I decided to run an old sifton 412 cam that i had that was in good condition also. Forgot to mention, the jugs were used repop also, not sure the brand.

I dropped the motor off to the bloke last night, he was actually very helpful and said he would take a look. He mentioned that he suspects maybe it's the cylinder height and that there was some variation in heights between E and F series motors? I don't know enough to verify thats the case. He said he suspects thats what could be causing the low compression. Does seem like something an experienced builder would have noticed though during assembly. He wasn't too fond of the suggestion that it could be the rings or potentially a tooth out somewhere.

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Re: Fresh rebuild from reputable shop, no compression.

#6

Post by RooDog » Thu Apr 18, 2019 3:39 am

These old Harley engines may be simple, but they are not simple. You got to know your stuff and use the right parts to make them work. There is a good 1/8 inch of difference between an 74 and a 61 cylinder height. the bores don't match either. It really takes some finagling to mix these pieces so that is probably not the case, or, your engine builder is an idiot. Now, let the education begin....
....RooDog....

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Re: Fresh rebuild from reputable shop, no compression.

#7

Post by Frankenstein » Thu Apr 18, 2019 3:44 am

OK,
First, the Sifton 412 cam has late intake closure, which will give you less compression and resistance to kicking at cranking speeds. You mention that there was no air leaking from the exhaust or intake ports when you blew into the spark plug hole. Not exactly a leakdown test, but a rough check. So valves are not leaking, perhaps?
Why did the builder? bring up the subject of E vs F engines? There is quite a difference in cylinder height. So, is it truly an "F" series motor from the factory, or an "E"? If you've got a 61" stroke and 74 cylinders, you'll get very low compression, is why I ask.
Shoot a little oil down the bores and see if cranking pressure increases. Then you can narrow down if it's leakage and where.
Keep us posted.

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Re: Fresh rebuild from reputable shop, no compression.

#8

Post by RooDog » Thu Apr 18, 2019 4:01 am

These guys who don't say where they are from/located make it difficult to have a frame of reference to their situation. Yah, I know,we're all Harley guys, but it does mean a lot knowing what sand box we are playing in....
...RooDog.... In Knoxville, Tennessee, US of A....

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Re: Fresh rebuild from reputable shop, no compression.

#9

Post by Andygears » Thu Apr 18, 2019 11:41 pm

If you had 61 inch wheels in your pile of parts, it may have been overlooked in your builders quest to complete a motor.
Isn't there a difference between the crankpin lock location on 61 or 74 flywheels? Or was that between 80's and 74's? If so, that could be viewed thru the timing hole. If not, looking thru the timing hole at a '74 (take a pic) and noting the center of the crankpin in the window and then looking at yours might reveal a difference.

A mystery unfolds
Andygears

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Re: Fresh rebuild from reputable shop, no compression.

#10

Post by Frankenstein » Fri Apr 19, 2019 1:04 am

Crankpin lock screw was at the side on BTSV wheels only. 61" would have a smaller pin nut, don't know if you could determine that from the timing hole or not. Never tried.
Frankenstein, Interlaken, heart of the Finger Lakes, NY, USA (yeah, wine country)

panama86
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Re: Fresh rebuild from reputable shop, no compression.

#11

Post by panama86 » Wed May 01, 2019 7:16 am

Hey all,
Back with some feedback here. I can confirm they were 74" flywheels, I had photos on of the parts that I dug up to check. The builder has come back to me saying he has gone over everything and that nothing is amiss, including barrel height etc.

He basically said the same as Frankenstein that the only thing he thinks could be responsible is the combination of the Sifton 412 with some overlap and stock 8:1 compression. He suggested 2 options, high comp pistons to retain the cam and gain some cranking compression or switch back to the stock cam and see how that helps. I opted for the latter. He sounded like he would be keen to swap for the 412 for the stock cam he has so shouldn't be much out of pocket.

Does this sound like it could be the cause?

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Re: Fresh rebuild from reputable shop, no compression.

#12

Post by nan00k » Wed May 01, 2019 9:01 am

look for my post elsewhere on my 413 stamped cam. YES, your symptoms sound a lot like what I was experiencing, I didn't check numbers as thoroughly as you but having switched back to a stock cam all is good.

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Re: Fresh rebuild from reputable shop, no compression.

#13

Post by RooDog » Wed May 01, 2019 2:32 pm

"Overlap" has nothing to do with cranking compression. It occurs near TDC when both the exhaust and intake are open before the intake stroke begins. What does effect compression pressure is the closing point of the intake. Early closing traps more air in the cylinder during the compression stroke and the later closing of a hot rod cam traps less air and lowers compression pressure. Long duration cams will therefor present lower cranking resistance. Also a miss-installed cam, not properly degreed, will manifest the same symptoms. Perhaps somebody missed the timing mark upon installation, or, though unlikley, the pressed on gear is not registered on the cam properly. Has anybody pulled the cam cover to check the timing marks?
....RooDog....

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Re: Fresh rebuild from reputable shop, no compression.

#14

Post by panama86 » Thu May 02, 2019 1:21 am

nan00k, good to know, I'm going to pick the motor up today with a stock cam in place of the 412 and will report back on the outcome.

Roo, thanks for that explanation. Makes sense. The builder is adamant that the cam is correctly timed but as you say, it could have been an issue with the cam itself. Either way, sounds like the stock cam will solve my issues. I'll update this thread in a couple of days when I've had a chance to try kick her over again.

Thanks for the help.

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Re: Fresh rebuild from reputable shop, no compression.

#15

Post by panama86 » Mon May 06, 2019 1:43 am

Well, I'm back again. When i picked up the motor from the builder last week he mentioned he had checked everything to confirm all was well up top and installed the stock cam. Optimistic, I got the motor back in the bike, double checked everything and gave it a kick. Nothing. Not even a misfire.

Thinking maybe the problem was still with the mag (a recently rebuilt morris) I switched over to another mag which has also been recently rebuilt, an adjustable hunt so I could retard it a little for starting. Timed it with the buzz box..... nothing. Next I swapped the M74B out for a stock super b and tried again, still nothing. Set up a manifold leak test plate, a couple of bubbles that were corrected, tried again, nothing. I've probably kicked the c$#t 100 times over the weekend. Spraying some start ya bastard too also gave me nothing, not even a pop.

Strange thing is, after kicking a bunch with no fire I pull the plugs and they aren't wet and barely smell of fuel. But that being said even spraying some fuel straight into the cylinders and kicking isn't giving me anything either. Checked pushrod adjustment and all is well there.

Both magnetos sit pretty close to the front cylinder, is it possible if there was contact there between the cylinder and the mag body it would cause a weak spark (I can confirm I do have spark but she's pretty dim).

The only other thing I can think of, which with my lack of experience could be nothing at all is that my kicker arm only returns to about the 10 o'clock position on it's own. I can move it by hand back up to 12 though. I thought this was because I incorrectly reamed the starter clutch bushing and once the bike ran a bit it would wear itself in. I did wonder if that could mean i wasn't getting everything I could be from my kick. But the fact it returns to 12 by hand makes me think it's indexed correctly.

Other thing would be a potential exhaust leak, my spigots are pretty clapped out, i find it hard to believe though that a leak there would cause the bike to show no signs of life. I should at least get a pop.

I'm at a loss here with this thing, I don't have enough knowledge to identify what else it could be. So any advice would be much appreciated.

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