Topic: compression test 77CB550F

[scuba steve]

my compression is
cyl 1 - 145
cyl 2 - 125
cyl 3 - 105
cyl 4 - 142

I checked it while the engine was hot with a standard autozone compression tester kit.  used the kickstarter, choke and throttle open. The bike has 41k miles and is running but not totally smooth and leaks oil around the cylinder head. Is this a big spread for compression numbers?

[hesselfuzz]

Unfortunately they should be within 10% of one another.

[scuba steve]

decided to go through top end. engine is still in frame, I have the cam cover off and am wondering if i need to mark something before I remove the camchain?

the head gasket definitely needs to be changed but the gasket below the cylinder block looks good. If I want to change the piston rings do I need to pull the engine?

[scuba steve]

just realized this thread should be on the sohc/4 bikes board, can someone help me move it there?

[TwoTired]

Probably too late now.  But, you should set the valve clearances before doing the compression check.

Do check the cam lobes for wear.  Those can effect compression, too.

[scuba steve]

thanks two tired, yeah I should have set valves clearances before the compression test, it would have given closer to actual comp numbers. Either way the head gasket is leaking bad so a rebuild is in order.

I am still on the fence with pulling the engine. Can the rings be changed with it in the frame? when you pull the cyl block and fit new rings on the pistons can you slide the block back onto the pistons?

[TwoTired]

Can the rings be changed with it in the frame?

Yes

when you pull the cyl block and fit new rings on the pistons can you slide the block back onto the pistons?

Yes,
The bottom of the cylinder sleeves have a taper to aid assembly, it gets thinner when boring.  But, you still have to compress the rings into the piston grooves during fitment in either case.
Are you going to measure the cylinder bores for wear, Pistons? Ring lands?
Are you boring the cylinders?

I seriously doubt the head gasket was leaking. The oil seal orings are far more likely.

Cheers,

[scuba steve]

I decided to put the rocker cover back and set the valve clearances. After setting them I checked compression again and got:

137
133
135
135

So I guess before this the valve clearances were not within spec and giving me false compression readings.

After seeing this i put it back together to see how it runs. Its running better but not totally smooth, roughness more noticeable at idle.

There is some smoke coming out the exhaust, kinda white but maybe light gray in color.

Considering the compression and valve clearances are good:

1. Can it still burn oil? Maybe leaking through the valve stem seals?

2. Can something be not right with the valves or timing internally OR does it need more checking/tuning on the carbs and/or timing externally?

The leak around the head gasket starts to smoke pretty bad once it gets up to temp but right now this is just a nuisance, I really just want to see it running smooth.

[scuba steve]

Does anyone know where I can find Float bowl gaskets for a 77 CB550 F model?

They are five sided and shaped like a house. I tried some circular ones for a 76 but these wont work.

[TwoTired]

They are all round when new.  30 years of being held to a different shape tends to reform them.
I use a thin coat of Gaskacinch, applied with a toothpick, to hold them in the groove during assembly.

[scuba steve]

so the point gaps were way off. got em set right and its running better but I still haven't synced the carbs, thats def next.

check out this video of how she is currently running:

CB550 -1

[lucky]

thanks two tired, yeah I should have set valves clearances before the compression test, it would have given closer to actual comp numbers. Either way the head gasket is leaking bad so a rebuild is in order.

I am still on the fence with pulling the engine. Can the rings be changed with it in the frame? when you pull the cyl block and fit new rings on the pistons can you slide the block back onto the pistons?

Before checking the compression the cam chain needs the slack taken out of it FIRST. Then valve adjustment ,THEN compression test with fully charged battery and throttles wide open, and all spark plugs removed except the cylinder being tested.

[scuba steve]

thanks for your response Lucky. I had started taking apart the top end and realized I should check comp with valves adjusted correctly first. So I put the rocker cover back on and and set valves. At this point the carbs, air box, and battery were all removed when I checked compression again. so holding the throttle open was pointless. The battery was out, but how does this effect anything on compression readings?

You say all spark plugs should be out except cylinder being tested, you meant ALL of them im sure bc the one being tested has compresion tester hose installed.

I checked my points again last night and noticed they were not totally dead on but close. I set the gap again but this time I removed the .014 feeler gauge from its set and let it sit inbetween the points while i tightened the adjusting screw. Its running even smoother now than it was in the video above. The points sure are finicky.

My problem now is chattering noises coming from the bottom end or transmission at idle lower than 1600. It will idle ok at 1200 but this slight chattering has me a little concerned. Is this normal? it smooths out instantly when I rev it.

[TwoTired]

My problem now is chattering noises coming from the bottom end or transmission at idle lower than 1600. It will idle ok at 1200 but this slight chattering has me a little concerned. Is this normal? it smooths out instantly when I rev it.

Normal.  Mostly.  It is effected by carb sync or other things than can cause uneven cylinder power pulses among the four.
The primary chain has no tensioner.  Small changes in crank rotation speed move the chain slack top to bottom to top, etc.
What you are hearing from the bottom end is most likely transmission gear clack, as all the big teeth in the trans are always engaged.  The gear mesh is not perfect and the changing speed causes the tooth face contact to alternate front to back an the driven speed changes.  Disengaging the clutch usually changes the timbre of the sound as the inertial mass is changed by the clutch components.

Cheers,

[scuba steve]

Two tired when you say "Small changes in crank rotation speed move the chain slack top to bottom to top, etc."
- the small changes in crank rotation speed are caused by uneven cylinder pulses? caused by carbs not synced right or some other tuning issue?

so if a 70s fours were 100% properly tuned the chattering would not be noticeable? or did these bikes have some chatter even when new?

[lucky]

thanks for your response Lucky. I had started taking apart the top end and realized I should check comp with valves adjusted correctly first. So I put the rocker cover back on and and set valves. At this point the carbs, air box, and battery were all removed when I checked compression again. so holding the throttle open was pointless. The battery was out, but how does this effect anything on compression readings?

You say all spark plugs should be out except cylinder being tested, you meant ALL of them im sure bc the one being tested has compresion tester hose installed.

I checked my points again last night and noticed they were not totally dead on but close. I set the gap again but this time I removed the .014 feeler gauge from its set and let it sit inbetween the points while i tightened the adjusting screw. Its running even smoother now than it was in the video above. The points sure are finicky.

My problem now is chattering noises coming from the bottom end or transmission at idle lower than 1600. It will idle ok at 1200 but this slight chattering has me a little concerned. Is this normal? it smooths out instantly when I rev it.

"You say all spark plugs should be out except cylinder being tested, you meant ALL of them im sure bc the one being tested has compresion tester hose installed."

Your words.

The reason you want all the plugs OUT, except one will have the compression tester in it , is so the other cylinders do not drag down the rpm and give you a low compression reading. Battery must be fully charged.

[TwoTired]

Two tired when you say "Small changes in crank rotation speed move the chain slack top to bottom to top, etc."
- the small changes in crank rotation speed are caused by uneven cylinder pulses?

Yes

caused by carbs not synced right or some other tuning issue?

Yes
Imagine an idle where #2 gave 100%, #3 82%, #1 - 94%, and #4 fired at 97%.  You think the crank would maintain an even speed during the firing order?  It could with a big enough flywheel, I suppose.  Bu,t the CB550 only has the alternator rotor and the crankshaft mass to contribute to that flywheel function.

so if a 70s fours were 100% properly tuned the chattering would not be noticeable? or did these bikes have some chatter even when new?

Brand new, there was very little gear tooth wear and the primary chain was still tight both top and bottom.
I don't recall much, if any, chatter on my 1500 mile 74 CB550 in 1975.  My 77 CB550F  trans chatters like a mofo at 1000 RPM.  And has since I got it with 8K miles.  But, it's pretty quiet right after a tune up.  Still, I've bumped the idle up on that one to about 1250.  Hard to complain about it, as it is probably my fastest CB550.  I'm not taking it apart to find out why, though.

Cheers,

[scuba steve]

hey two tired thanks for your insight on that low idle chatter. More motivation for me to get this thing properly tuned.

I tried using a borrowed sears timing gun to get my timing set but it wouldnt strobe. when i pull the trigger the engine bogs a little and the strobe light doesnt come on. the red/black alligators go to the battery and the pickup goes on #4 plug wire, correct?
I read in the forum you should use a separate battery to power the gun, does the light draw to much power?

so I tried to set timing using the static method. I rigged up two test leads to a 12 volt bulb, one to ground, one to points terminal. when rotating the crank, The bulb stays on when the points are open, off when the points are closed.
If im connected to the 1-4 point, and rotating the crank, the bulb goes off as the points close long before the 1-4F mark comes up. the bulb stays off for a bit of rotation then comes on as the points have just opened. I stop rotating at exactly this point and the mark is around the 1-4 advance double hash mark - at this point i should be lined up with the 1-4F mark. so this tells me the timing on 1-4 needs adjusting. So i rotate the crank and line up the 1-4F mark then loosen the 3 baseplate retaining screws. The bulb now stays on and turning the baseplate to any point within its allowed movement doesnt turn the bulb off. not sure what to do next. am i trying to have the marks line up when the bulb goes off or on? i.e. when the points first close (bulb goes off) OR when the bulb comes on again (points start to open)?

I checked all the plug gaps when i had the them out but I will recheck them. also I think i should check out and lube the advance mechanism, but that should not effect basic static timing, right?

[scuba steve]

update - I bought a new timing gun. The old one was def broken. I can pretty much replicate with the new timing gun what im seeing with test bulb static method.

on the 1-4 point the timing gun shows them set far to the right of the 1-4F mark and Im out of adjustment room on the baseplate. what does this mean??

on the 2-3 point I was able to line the marks up perfectly within the allowed plate movement.

the advance mechanism seems to be working properly, when i bump the throttle to 2500 I can see it advancing up 20 degrees or so then stopping- let go of throttle and it swings back

[TwoTired]

I wrote a FAQ entry about the timing plate movement and shimming to stabilize the timing and fix the "out of adjustment" issue.

[scuba steve]

thanks two tired, I read your timing FAQ and it makes total sense, I did notice the baseplate jumping around a little. So I guess the basic idea is to get it steady with a shim so you can rotate it smoothly while its running? Where exactly do you shim? Does anyone have a pic of a good starting place for the shim? or is it pretty much trial and error?

The timing on 1-4 is still out but I wanted to sync the carbs anyway since my uncle gave his homeade sync rig:

CB550 -carb sync
 

[TwoTired]

The shims on mine were mostly at the lower right post (as you look at it).
But, one bike needed the shim on the lower left post to enable both points set to come into proper adjust range.

The shim goes between plate and post to prevent radial distance motion relative to the points cam.

Carb sync is really the last thing to do during the tune up check list, btw.

Cheers,

[scuba steve]

Two Tired, I thought you were talking about shimming inbetween the screws and the plate inside the slot. But now im thinking  by "posts" you mean inbetween the plate and that tab im pointing at in the pic. Is that right?

My plate sits in those tabs pretty snug, do I just wedge a thin one in there?

One thing I wasnt doing before that got me closer was starting with the baseplate in a position that sets me up all the way opposite from the direction I need to go.

Its like this: set 1-4 gaps, check 1-4 timing, now i see i need to rotate cw so i rotate plate all the way ccw. reset 1-4 gap, check 1-4 timing and rotate plate all the way cw and im a lot closer than before but still not there.
I repeated same procedure for 2-3 and got closer but still not all the way on.

So i need to figure out this shimming business

[TwoTired]

Two Tired, I thought you were talking about shimming inbetween the screws and the plate inside the slot. But now im thinking  by "posts" you mean inbetween the plate and that tab im pointing at in the pic. Is that right?

Yes

My plate sits in those tabs pretty snug, do I just wedge a thin one in there?

The springs on the points push the loosened plate around as the point cam rotates.  The springs make it "seem" to be tight, but the plate can sometimes be seen to move while the engine is running and the plate hold down screws loose.  Defeat the point springs by propping open both points, loosen the plate screws and see if it is still tight within the posts.

Once the plate is shimmed in the correct and truly tight position, the gaps are set, then only rotation of the 1-4 plate is needed to achieve proper timing and the gap will be maintained during plate rotation.  If the 2-3 points run out of adjustment range, then you need the shim at another post and you have to start over, doing 1-4 gap first, timing, then 2-3 gap and timing.

Cheers,

[scuba steve]

Still no luck. Looking for more suggestions.

I removed the baseplate and advance mechanism to check everything out. The small pin on the back of the adv mech that slips into crank was not bent. I cleaned the points cam until it was shiny and greased it. advance mech is working perfectly, sprayed it with a little PB blaster. put it all back and removed the points so I could shim. removing the points did not release any tension because the plate fits inside the three mounting posts perfectly. I was able to get a my .0015" shim into the top post, but it was very tight.

None of this had any effect on getting me closer. Still can only get it to within 15 deg or so. What am I doing wrong?