Compression Question

Forum rules
Please do not start new topics here, but here: New Panhead and Flathead topics
Post Reply
fatfatboy
Posts: 100
Joined: Fri Jan 01, 2010 4:04 pm
Bikes: '03 Fatboy
'14 Slim
'60 FLH
86 XT 350
Location: St. Louis
Been thanked: 2 times

Compression Question

#1

Post by fatfatboy »

Greetings gents. I've been busy with life and have finally got to spend some time with Pandora.
I got got her plumbed and juiced up and ready to start,, or so I thought. Got a few puffs and a ball of fire but never a run.
I did a compression test, with a teaspoon of oil in the jugs, and the rear cylinder pumps to 30psi but at the front the needle goes up but then falls back down. Can't gt enough compression to read.

Through research in the K-base (thanks to all for the K-base) I found a helpful thread that told me how to test by adding air via an air compressor and I believe my valves are the issue.
With air pumping into the front cylinder through the plug hole and turning her over with the kicker air was constantly rushing out the carb or the exhaust and never stopped escaping.
At the rear cylinder, with air pumping in, the air would stop coming out as long as I was pushing down on the kicker. So I'm not 100% sure there is an issue there but I'm only getting 30psi.

Everything I've read states to do a compression test on a warm motor but thats not possible because I can't get her to run therefore the rings are not set.
My question is should I be getting more than 30psi without the rings being set?

Thanks for all your help.



Kirk



Bigincher
Posts: 2789
Joined: Tue Dec 18, 2007 2:10 pm
Bikes: 1941/59 EL
1952 FL
1977 FLH
1994 Fatboy
Has thanked: 22 times
Been thanked: 109 times

Re: Compression Question

#2

Post by Bigincher »

Sounds to me like the valves aren't fully closing. Check the pushrods adjustment.

fatfatboy
Posts: 100
Joined: Fri Jan 01, 2010 4:04 pm
Bikes: '03 Fatboy
'14 Slim
'60 FLH
86 XT 350
Location: St. Louis
Been thanked: 2 times

Re: Compression Question

#3

Post by fatfatboy »

Thanks Bigincher.
I checked the push rods and if memory serves me correctly the rods should spin free when the piston is at TDC.??? Which they are.
This how I adjusted my XLCH and never had any issues.

I'm beginning to think my valve job is botched. Just hoping for some insight before pulling the heads off.

Kirk.

58flh
Posts: 3337
Joined: Sat Jun 26, 2010 1:54 pm
Bikes: 1958flh 1969sporty,had a knuckle but sold back to original owner
Location: NEW JERSEY
Has thanked: 195 times
Been thanked: 267 times

Re: Compression Question

#4

Post by 58flh »

KIRK---As stated it sounds like valve issues.Try this-- & you will be 100% sure-Pull the Pushrods-- put air in thru the sparkplug hole/-It wont take much if theres a leak.You will hear it out the carb.-EX.-& Timer hole.You just may need a good LAP-JOB,-Cant say for-sure until you pull the topend off!.---The Rings might be good!-The smallest hole from an unseated valve will give you those numbers.--Do you have ALOT of miles??-Pans with HYDs. & Stock cams should have the heads serviced at 30K//-Solid-Tappets every 20K or 2yrs.& don't forget to put NEW ROLLERS on!--RESPECTFULLY----RICHIE

PanPal
Posts: 1328
Joined: Wed Apr 21, 2004 8:57 pm
Bikes: 50 EL custom
59 FLH (Sold)
58 FL (Sold)
76 FLH Original Paint
08 CVO Road King
Location: Maryland
Has thanked: 108 times
Been thanked: 78 times

Re: Compression Question

#5

Post by PanPal »

Alsoo check your clutch adjustment. Pop the timer cover and make sure it is always turning forward through a compression kick.

fatfatboy
Posts: 100
Joined: Fri Jan 01, 2010 4:04 pm
Bikes: '03 Fatboy
'14 Slim
'60 FLH
86 XT 350
Location: St. Louis
Been thanked: 2 times

Re: Compression Question

#6

Post by fatfatboy »

Thanks for the tip Richie.
I pulled the push rods and did the air test. Air came out the timing hole only. Nothing out the exhaust are carb.
I did a compression test again without the push rods and got 35 psi on the rear and 25 on the front. A little higher than what I had with the rods in.
The motor has no miles on it since it's rebuild. It's been about 3 years since it was put back together and has never been started. Could this be part of my issue?

Kirk
Last edited by fatfatboy on Wed Mar 11, 2015 3:31 am, edited 1 time in total.

fatfatboy
Posts: 100
Joined: Fri Jan 01, 2010 4:04 pm
Bikes: '03 Fatboy
'14 Slim
'60 FLH
86 XT 350
Location: St. Louis
Been thanked: 2 times

Re: Compression Question

#7

Post by fatfatboy »

PanPal,
You may have something there. I pulled the timer cover and the cam did not spin the whole time I was kicking her over.
I'll check it tomorrow night.

Thanks.

Kirk

RUBONE
Moderator
Posts: 6273
Joined: Sat Nov 15, 2008 4:09 am
Bikes: Multiple H-D, Ducati, BMW, Triumph, BSA,...
Has thanked: 139 times
Been thanked: 1331 times

Re: Compression Question

#8

Post by RUBONE »

You CANNOT do a compression test without the pushrods. The engine needs to draw air in in order to compress it. Without the pushrods you are merely wasting your time and the readings mean nothing. And the throttle needs to be WIDE OPEN for the compression test. What kind of camshaft was used in the rebuild? Some have enough overlap that they make the cranking compression numbers very low. If the rings are stuck in the pistons from sitting for that amount of time you also may need to free them up. Squirt some Marvel Mystery Oil in the spark plug holes and rotate the engine a bit, then let it sit so the gummed up oil dissolves.

1950EL
Posts: 53
Joined: Sun Jun 21, 2009 1:51 am
Bikes: 1950 EL, 1983 Honda CR480, 2012 FLHTK
Location: United States
Has thanked: 3 times

Re: Compression Question

#9

Post by 1950EL »

It sounds like Richie's suggestion to pull the pushrods served it's purpose of eliminating any question of loss by mis-adjustment and I'm sure he knows that we can not do a regular compression test that way. The mmo is a good suggestion- I run it regularly in the gas. The slipping clutch sounds like the culprit I didn't think of. Maybe between the two.....
Mike

58flh
Posts: 3337
Joined: Sat Jun 26, 2010 1:54 pm
Bikes: 1958flh 1969sporty,had a knuckle but sold back to original owner
Location: NEW JERSEY
Has thanked: 195 times
Been thanked: 267 times

Re: Compression Question

#10

Post by 58flh »

RUBONE---Putting air thru the plug-hole can tell you a lot of whats wrong.Its another tool that quick & Easy to do----- to check over the DRY & WET comp.-test numbers.Also its not just for valve & piston checking for blow-by.I have used this ALOT of times/Sometimes just to AFFIRM that other tests were good//& have used this method to simply find what cyld-head to pull & therefore I can adjust the price for the work that needs to be done.I use 12lbs. in each plug hole.I found this is the best poundage for diagnosing.Also its fast & EASY!.I understand that 10-men & 10 motors with the same issues Will be FIXED!---But there will be 10-different ways that it got finished.Most important is a GOOD-JOB & FINISH.Afterall its your name on that fix!-(MUST BE GOOD!).Im mot saying that this is the OLNY way to do the job!/But simply another effective tool to add to the arsenal.----Respectfully----RICHIE

Bigincher
Posts: 2789
Joined: Tue Dec 18, 2007 2:10 pm
Bikes: 1941/59 EL
1952 FL
1977 FLH
1994 Fatboy
Has thanked: 22 times
Been thanked: 109 times

Re: Compression Question

#11

Post by Bigincher »

I always wondered how much damage can be caused by rolling the motor over without pushrods.
The piston is going up and down, nowhere for air to come in or go out unless it makes it's own path.
Somehow that can't be good.
In this case, since the motor has never run, the rings aren't seated, so the air probably just went past them.

RUBONE
Moderator
Posts: 6273
Joined: Sat Nov 15, 2008 4:09 am
Bikes: Multiple H-D, Ducati, BMW, Triumph, BSA,...
Has thanked: 139 times
Been thanked: 1331 times

Re: Compression Question

#12

Post by RUBONE »

Richie and 1950EL,
Read my post..
It was directed at the original poster who said he was planning to do a "compression test" with the pushrods removed. Now if either of you want to explain how that works I'm all ears.
Richie,
I use a Snap-On leak down tester to determine condition of rings and valves. Been doing so for the last 35 years.

Huck
Posts: 636
Joined: Tue Mar 15, 2011 5:35 pm
Bikes: 58 hd
91 hd
09 custom
Location: Wake Forest NC
Has thanked: 15 times
Been thanked: 34 times

Re: Compression Question

#13

Post by Huck »

Seems odd the compression test comes out basically the same with or without push rods.
The valves need to close & open, I’d verify that first.
Seems odd to me someone would butch a valve job that badly.
You understand they’re two top dead centers, correct?<< I asked because you didn't say so.

panhandler
Posts: 135
Joined: Sun Nov 05, 2006 5:30 pm
Bikes: 1963 FL mod
Location: Alberta and Mexico
Has thanked: 4 times
Been thanked: 2 times

Re: Compression Question

#14

Post by panhandler »

I had the same thing happen when my bike was in the shop getting the mainshaft seals replaced. He couldn't start the bike and compression was low, in the 30s.....funny cause it started fine before it went in......and he's been working on pans for a long time....he finally called back a day later somewhat redfaced and found the prob when he put the clutch plates and fibres back....crap between them was hanging up the clutch....I'd start there as it seems the simplest to check....just my 2 cents.

regards,
Keith

james
Posts: 827
Joined: Tue Nov 18, 2008 7:33 pm
Bikes: 2003 FLHRCI
1952 FL
Location: Wrightsville Pennsylvania
Has thanked: 49 times
Been thanked: 53 times

Re: Compression Question

#15

Post by james »

Many good points here. In regards to pumping air into the cylinders with the push rods removed. You say no air is escaping to the carb or exhaust only the timing hole or crankcase. Like Robbie said try the MMO in the cylinder. The air coming out the timing hole is due to a poor ring seat to the cylinder wall for whatever reason, stuck ring, broken ring, worn rings, tolerance's off, there's air pushing past the rings..
No air out the carb or exhaust means the valves are sealing if the pushrods are adjusted properly. Sorry to sound like I'm repeating what has been said already, you can't check compression with the pushrods removed and the slipping clutch is one more thing to check into.
Jim

Post Reply

Return to “Motor”