Topic: What should my compression #'s on a 76 550K ??

[Otto]

I did a compression test on my 550 this morning and I was puzzeled at the readings. What should they be?

Here's what I got for #'s:

# 1  - 80
# 2   - 73
# 3   - 80
#4   -  67

I was expecting lots higher but, I have no reference?

Thanks-

Bob 

[heffay]

considering they are all that low... i'd go borrow another compression tester.

[bwaller]

Open throttles full to test.

[TwoTired]

Numbers should be within 10% of each other.

Throttle wide open, choke NOT applied during test.

Does your tester have a big hose to add chamber volume on it like this?

[heffay]

2tyred... i don't really understand the need for extra hose.

if the regular hose is an extension of the combustion chamber then why should you have to extend the extension of the combustion chamber... its just a guage

edit:  if it were possible, i'd rather mount that gauge straight to the spark plug hole!

edit again:  you know the theory, to air up a tire efficiently you need a compressor w/ a tank as big or bigger than your tire's capacity?...  well, it seems that if you were trying to strictly measure compression of a cylinder you would strictly limit it to the size of the cylinder.

[bryanj]

I think what TT meant was if you have a long hose you will get a low reading because of the volume of air in the hose and gauge so you need one with as short a hose as possible and those readings are WAY low

[Otto]

Yes, the compression gauge is like the one pictured... Brand new Craftsmen.

ok, I went back out. choke off, throttle wide open, he's my results.

#1 - 145
#2 - 125
#3 - 130
#4 - 110

Now what ?

bc

I hate to sound like a rookie, but, I  haven't ever heard of having the throttle wide open to check compression? What's that all about?

I have not adj. the valves yet either. Did adjust points and timing is advacnce ~2 2 degrees? Does that matter fot this test?

I was just getting reqady to pull the motor and start the restore. Do I need to do the motor too?

[TwoTired]

Yes, the compression gauge is like the one pictured... Brand new Craftsmen.

The internal volume of the hose adds to the chamber volume and changes the compression ratio.

I hate to sound like a rookie, but, I  haven't ever heard of having the throttle wide open to check compression? What's that all about?

Compression is made by squashing a certain volume of air.  If you limit the amount of air being compressed by closing the carb throttle slides, less air is available to be compressed.

I have not adj. the valves yet either. Does that matter fot this test?

Of course it matters.  The valve opening duration affects how long air can enter the chamber.  The valve adjuster clearance affects when the valves open and close in a given cycle.
If the intake valves don't open the same lift and duration for each cylinder, it affects how much air there is to be compressed for each cylinder.

What do you think the compression would be in a cylinder if one intake valve didn't open at all?

You can also expect better compression numbers when the engine is at operating temperature, too, as that's when all the metal bits have expanded to their "as-run" size.

Cheers,

[Otto]

Thanks.

I will do it again tomorrow engine warm..will adjust the valves too. Thanks for the explanation.



Bob

[Otto]

Ok,

This morning I adjusted the valves, (motor cold).

Ran motor for 4 minutes or so to warm it up.

Performed compression test, choke off, full throttle.

#1 - 160
#1 - 140

[TwoTired]

#1 has two readings?

What is the engine history?  Has it been sitting a lot.  How many miles put on it in the last month or six months?
Did you try a few drops of oil in each cylinder to see if the compression numbers change?  (To make a determination of ring leakage or valve leakage.)

[Otto]

Not sure what happened to my post, only hale of it posted... Here's the rest.


Yes , it has had very little use in the last 3 months. maybe 1oo miles.


Ok,
 
This morning I adjusted the valves, (motor cold).
 
Ran motor for 4 minutes or so to warm it up.
 
Performed compression test, choke off, full throttle.
 
#1 - 160
#2 - 140
#3 - 150
#4 - 110
 
so, I added a bit of oil to #4 , 4-5 squirts from an oil can..and it went up to 210! Becasue, 1,2, and 3 were within range of each other , I did not put oil in them and test again.
 
Looks like I need rings.
 
I saw a 550 overbore to 605 for sale maybe I should buy it huh? Check it out... http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=260312223746&fromMakeTrack=true&ssPageName=VIP:Watchlink:middle:us
 
What do you recommend? .030 over, .060 or leave it alone?
 
 
What would you do?
 
bc

[TE 450 Pilot]

I just bought a kit like that off ebay for $405.00.  If you can get it cheap i would go for the big bore kit. Keep in mind i'm new so i may not know anything !

[TwoTired]

I said a few drops of oil!  More is NOT better and it distorts the pressure readings.

The 550 combustion chamber is only about 15cc.  If you make that smaller by filling it with uncompressible oil you change the compression ratio!
550/4 = 137.5 cc per cylinder.  @ 9:1 comp ratio, that makes the chamber 15.2cc when the piston is at TDC.
How much is 4-5 squirts?   A teaspoon is about 5cc, making the chamber 10 cc instead of 15.2cc and, thus, changing your compression ratio to 13 or 14 to 1!  Of course your compression numbers will increase!

Inactivity can make the rings sticky in the pistons and they lose best seal against the cylinder walls.  Running it with a good oil often, over time, gets the rings to seat properly again.

However, it sounds like you have convinced yourself to do a top end job.  With a metric bike, it is not a .030 or .060 over affair.  The pistons come in metric oversizes, IF you can find them.

How was the bike running?  Does it run so bad that it warrants a top end job (which you can do with the engine in the bike on a 550)?  Is there some compelling reason to throw more money at the bike?

What I would do is run the bike for a while, 500miles before a compression retest using the same equipment and technique as the recent test and compare numbers. (assuming the bike is running well.)  Unless it is burning oil and leaving a smoke trail?  Or, misses on the weak cylinder at idle?.  Or, if I absolutely MUST have a new engine, then go for oversize pistons, and an extra $400 on the top overhaul.

How many miles do you plan on driving the bike?

Cheers,

[OldSchool_IsCool]

I would let her sit over night to let all that oil drain out of #4!  She is pushing huge numbers because she is trying to hold half the US Strategic Oil Reserve that you pumped in there which doesn't leave much room to compress the captured air.

Tomorrow double/triple check your tappet adjustments, fire her up to burn off any oil left in the cylinder then try it again but this time, only about a teaspoon of oil.  You want just enough to coat the walls thus decreasing the blow by space of possibly weak rings.  

The only TWO reasons I would bore that engine is:
1) I want to
2) I have to in order to salvage her due to severely scored cylinder walls which you really can't tell until you crack her open.

If it's just weak rings, those are fairly easy and inexpensive to replace.

[Otto]

Um,

Sounds like I pumped in too much oil. Drove it for about 10 miles after. Run very good. I small stumble when decelerating, just a small one.

4-5 squirts is what I did with a little oil can, I estimate a little more than a teaspoon.

No need to throw $ at it. Just wanted to beef it up over the winter. I am also doing the head to Stage 1 specs. Well Hondaman is gong to do it sometime this winter , that is. I just saw that  overbore kit and thought it might be cool to beef her up. Thinking of doing a 650 cam too.

I am pulling the bike apart this winter for complete rebuild... Just the way I like to do things, I guess. I will getting running 100%, then rip it apart, and redo all bearings ,  fork seal, brakes, tires, paint, polish the motor to chome like finish,etc. When I saw the compression #'s were so far apart, I freaked and figured as long as the top end was being done , why not do the bottom end. If things aint right, they aint right. I don't even like being off the tolerable 10%. I'm picky.

I do the same thing with my boats, they aren't done till everything is ripped out, done over and repainted. That way, I know what I have when I am done and it's truly mine. I enjoy it - and Maine winters are looong and coold. Gives me something to do.. The last bought I did, a 1973 Mako Cuddy Console, now 28 with a 250DI on a 30"setback bracket, was in Classic Yacht magazine last spring. Before that I did an old 69 Mini Cooper 998 and that was in Mini World Magazine too. I am not bragging, I just enjoy the details and the "journey"of the build. Sometimes when I am done with a project , I sell it cause I am bored with it.   

Thanks for all your advice,

bc
 

How much oil should I ad next time I try it? hehehe

[TwoTired]

A teaspoon of oil is too much for these little engines.  Fine for a 350 cu. in. V8. But, you only need enough to wet the rings for the circumference of the piston.
I don't know your oil can.  But, I'd guess a single squirt would be plenty to tell if the compression rises because of improved ring seal.  BTW, your are trying to differentiate between ring leakage and valve leakage.  Oil won't seal valve leaks.

If you are striving for perfection, then you will likely want to do a top end rebuild.  I think it is rather silly to go into the lower end (except maybe for the primary chain).  These bottom ends last over 100K miles in stock form.  There aren't many home mechanics that can build a bottom end as well as Honda did.

But, if you looking to tinker, do as you will.  There are certainly more expensive hobbies.  
Just don't expect to get your money back in resale.

Cheers,

[Otto]

So maybe I could get away with new rings and be happy.

Will retest with a suirt of oil in the cyliners and let you guys know how it goes.

Resale, the 550's seem to be the most unappreicted bike out there for resale.  This one is a fun bike - I appreciate it.

750's are nice, I have had a few...  but at this point anyway , the 550 just fits me.   I just like it!



stay tuned!

bc

[MRieck]

 A leak down test is much better at diagnosing what is going on in regard to engine sealing. I've said this many times and Jay at APE has said the same thing.

[Otto]

OK,

I'll give it a shot. How do I do a "leak down test" ?

bc

[MRieck]

OK,

I'll give it a shot. How do I do a "leak down test" ?

bc

You need the tool. basically it pumps 90 to 100psi into the cylinder (with the piston @ TDC) and you can hear where you are losing charge i.e. intake valve, exhuast valve , rings etc

[OldSchool_IsCool]

So if it hisses out the carb, your intake valve/seal is suspect, and exhaust hiss means exhaust valve/seal leak and a hiss out the breather or dip stick means rings.  Is that about right?