Topic: compression question for the experts

[Redline it]

is it common or even logical that a failed head gasket could drop compression for all cylinders? or could the ex valves all get burnt by a super lean condition?  cb400f running ok not using oil. not smoking. steadily declining in low end by the ever famous slight miss coming off of idle. plus the eternal honda head seeping oil. so i decide to put on another rack of carbs that i thought was ready for a spare. rebuilt a year ago. very slightly gummed i clean that out. blow it out. one of the main towers was corrosion hit but the jet o-ring would still fit in the slight eaten away tower.

the bike ran like a monster in mid-range power, like a 600 power.  like hugh power, low and high end wouldn't run worth beans. i rode a few miles thinking my old carb might be way off. but then it started to get real weird and would hardly run. so i took the spare carbs back off and was gonna put the old ones back on. then i though might as well do a compression check, carbs off, i get 50psi on all 4 (they weren't all to impressive when the motor went in at 110) so out of curiosity i squirt some oil in nos 1 and 4 and it came up both 4psi.  there's slight different color oil on the front of the head, like black, instead of the clean seep it's always had. thanks if you got time or not.

[MikeSimon]

Maybe your head is not torqued down properly.

[Tracksnblades1]

No expert. Have you verified the basics, proper valve lash, timing, mechanical advance mechanism, sparkplugs, wires, boots, etc...?  If you think your exhaust valve are all burnt, Can you verify that...? Have you access to a leakdown tester...?

[BomberMann650]

Improper Cam timing and valve clearance would have a negative effect on compression.  Even with a leak free chamber.

Discovered this building an ascot 500's motor.  Factory crank marks proved to be retarded by two cam sprocket teeth.  The retarded timing pulled 90psi - properly degreed cam pulled 180psi.

[Keith]

I doubt it would even run with 50 lbs. compression. I would adjust valves first.

[TwoTired]

Re-examine your compression tester.  Does it add volume to the cylinder?  Where is the check valve/ release valve?  If it isn't right next to the spark plug hole, it adds cylinder volume and alters the test numbers.
There is a post in the engine FAQ about this.  And, it is especially critical for cylinders with very small displacement.

Better to have a leak down tester if you don't have compression tester intended for small engines.

The leak down test will allow you to find exactly where the compression is being lost.

Cheers,

[PeWe]

Full throttle while doing the comp test?

MLS head gasket?
These have rivets that can be placed between the mating surfaces and cause gap with huge leak if not removed or hammered flat + drilling small dents for them on both surfaces. I have seen this on a CB750 K6 application.

[Redline it]

thanks a lot for these possibilities, i'll go through them and get back the info one by one checkin them off. but i could pointing to me not taking care of some things right. being younger at least i thought i knew what i was doing, now that i'm gettin old i can see that there is a pattern, long story, the short version is i'm seeing a stack of motors, this is #3,all with similar problems and wear.
i'll start checkin off the list now,
twotired, the compression tester has the release valve away from the cylinder so it does add volume, i'll look for a check valve tomorrow, i don't think it leaks, and it's not small engine tester, i didn't think it mattered, i'll check out the faq. i believe the last and only leak down i ever did had #2 not doing the best results and leaks into the ccase, the test weren't done correctly, i'll try to get it done tomorrow, i'll start there and go down the list, to check them off.  thanks again , oh and PeWe, think it's factory stock head gasket, and carbs are off. it's always had a seep of clean colored oil aroung the oil port, there's a lot more oil now than usual, some of it is black around the front exhaust.

 

[PeWe]

Do not forget to tell us what the problem was. I bet there are others that have same problems or will get it, myself included

[Deltarider]

Redline, in the past 35 years I must have done about three or four attempts to do a compression test on a CB Four. Let me confess rightaway: I never managed to do it right. Then I read this Danish book. In it the author Damkier somewhere mentions that Honda engines were the first that had a much more precise piston/pistonwall clearance: 0,03mm instead of the conventional 0,07mm before. That gave me great comfort and I never bothered to do a test ever after. BTW, a mechanic tought me his basic trick. 1. Remove sparkplug 2. Press thumb or finger firmly over plughole 3. (Let helper) operate starter (full open throttle, choke open ofcourse). 4. Evaluate: when thumb is blown of the hole: compression is sufficient. When thumbs stays put over hole: compression is bad. Enjoy your riding!
Bike on the cover is a K7.

[Redline it]

Do not forget to tell us what the problem was. I bet there are others that have same problems or will get it, myself included

I certainly will, it could be longer than i hope for, but if any longer than that, i'll have to find a psych dr for some thorazine. i was having a blast  and now it's broke, my only other running bike is a trash found 33cc chinese scooter (beer wagon, it'll never keep up with the demands of the 400 hauling christmas trees home (also left out on the curb) couple weeks after christmas. already almost burnt the clutch on the scooter draggin a milk crate loaded with wood home. my next ride is a wheelbarrow, there's noway i'm taking that to the drs office.

[Redline it]

I’m no expert, but you asked numerous normal things, so heres a few normal replies...
-Yes, a blown head gasket can effect compression numbers, but to effect all 4 equally, the gasket would need to fail in multiple places. Not likely.
-Yes, super lean conditions eat valves. And if bad enough, valve seats.
-The performance issues you describe, sound carb related, but given the compression issues, I’d begin to think about valve adjustments being suspect.



I wholly agree a leakdown test is a great indicator of problems, and pointer to the source. You’ve got a few things rumbling around, so start with ignition timing, then valves. Then a leakdown test. See where that gets you. Then install some carbs you know are good and clean, and pull some plug chops. You’ll find the source, guaranteed. 

thanks i'll try to follow that order, valves ok with .002 both in and out, i used to run .002 and .003 for exhaust, i only had 750 clymers manual for the first 25 yrs, you know the one that says to dump the used oil into the trash.  i think that's where i got the .003? i got a leak down piece of aluminum with the restrictor but with a 2 or 3 ft steel braided aircraft hose. but then toetired mentioned the compression check is critical for the small bore checks with the check and release valves having to be near the plug hole, check valve?. i thought that was the schrader valve on the ga. the dump button is up there too. the leak down thing i got i used 1 time before, but being alone i couldn't hold back 100psi so i reduced it to 25psi and divided the leak pressure down to whatever it was, that with the .002 that is too small of a-hey oh i almost forgot,speaking of carbs that i know work for sure...do you got any? or know anyone that does. i'll buy new kehins not aftermarket for 2 times as much as i paid the bike. i don't want to the 300 rebuild either, that'd be another trip to phych dr. carburetors, a rebuilt running 400f, or even i'd ship the motor off for someone who's got good experience like shaws guy, somewhere close to the western most states ca, az, nev, i don't know about or, i might be able to beat that quality control. i got 2 more motors they're the same year, same amount of wear on worn standard bores. heads look ok, cam bearing wear i think i can live with, i got a 466 kit waiting to be put in, but it's a tossup cutting that much material or trying to find first over or .015 over pistons, rings etc, maybe a hone can do the job i don't know. not me but some one with some experience and mounts for the right alignment. i'd try it, but it wouldn't work after that. i barely made it out of a family of farmers, cotton pickin farmers from Mo and Tex. i didn't get far though, i farmed dirt, high production. thank you.

[Redline it]

Redline, in the past 35 years I must have done about three or four attempts to do a compression test on a CB Four. Let me confess rightaway: I never managed to do it right. Then I read this Danish book. In it the author Damkier somewhere mentions that Honda engines were the first that had a much more precise piston/pistonwall clearance: 0,03mm instead of the conventional 0,07mm before. That gave me great comfort and I never bothered to do a test ever after. BTW, a mechanic tought me his basic trick. 1. Remove sparkplug 2. Press thumb or finger firmly over plughole 3. (Let helper) operate starter (full open throttle, choke open ofcourse). 4. Evaluate: when thumb is blown of the hole: compression is sufficient. When thumbs stays put over hole: compression is bad. Enjoy your riding!
Bike on the cover is a K7.

sounds much more like i could handle that. the small tolerances and detailed test, and testing devices are holding me up and plus the results i'm finding the older i get are in fact way off. i never sync'd by any vac sync'r only by sound. and it worked for 35 yrs then one day just recently i buy that one from oregon or washington for a hundred, i sync them, they sounded and ran awesome for a week, then on their own they went back to running just like they've been running for the whole time i've been on it, since 82. still got the stock grips, stock plug wires and caps and all the orig electrical except the start button,i rewired that to a little 3 wire toggle switch that sets just between the key and the bars. like the carbs bench testing following the float level 21mm rules, it took me a month and 20 full bowl disassembles and retry readjust, or 19, and i was so pissed one random bowl either would be a runaway overflow that wouldn't quit, an the next try wouldn't get fuel in that bowl. it got so bad that on the18th try i only made it look like i did an adjustment on it, faking the observant carbuetor out, and you know what happened it did the exact same opposite extreme as if i really did change the level. then the next try, i took all the jets out, floats and the needle jet fuel inlets out put em in a pile and shuffled them, put it together and i #$%* you not, it worked, for 3 yrs. i did figure out a way to side step the manual timing static order with a slightly worn points plate/case mount, that goes all over the place with the lights. works #$%*en, only hard to explain but it's simple, safe and fool proof (manual proof.) you make one guess with #1/4 dwell, within a running sound that's ok, once you got that you won't need to know the gap or if it changes after setting the timing then the 2nd sets only to get the first step off again, by pass it and finish it simply by outhinking it. set it off the same degree it always falls on but only the opposite way by the same amount, then bring it back with the plate, if you run out of adjustment, then set it "off" a little less in the same or opposite direction, until the plate adjustment brings the F mark and dwell right into the sweet spot, after that don't move the plate and then 2/3 are done deal, i've got it down to less than five minutes. not the endless tries, no shims to drop and can't find and can't fit or just add to the #$%* only the kids can see. anyway thanks for the tip. i gotta sackout.

[Deltarider]

It never ceases to amaze me how many sync and resync their carbs here. Could it be that they forget to put a bit of loctite on the adjusting bolts?

[PeWe]

I’m convinced that uneven idle, low rpm is seen as carb issue when ignition is not equally set.
Both points can be set within the spec but still cause crappy idle.

Set them both eqally and notice the difference!
Easier with dwell meter when numbers eill be seen and guide how to adjust.

It must be possible to set both with feeler gauge. Then adjsut one of them when engine idles until engine runs smoother. Its a huge difference. Test closing a little, if worse, other dir to best idle.
Check ign with lamp, idle as well as full advance.

I usually need to adjust a few times since point gap and ign setting affect each other. If you do not believe, try it

This has been dicsussed several times. Find MRiecks thread about dwell.

[Deltarider]

I use tools like dwellmeters, strobes and others, but the final say have my ears and what my fingertips feel at the end of the exhausts. Long time ago an experienced mechanic demonstrated me, that although his 4 vac gauges showed identical underpressure, a final tweak by him made it run just that little bit better.

[Redline it]

I use tools like dwellmeters, strobes and others, but the final say have my ears and what my fingertips feel at the end of the exhausts. Long time ago an experienced mechanic demonstrated me, that although his 4 vac gauges showed identical underpressure, a final tweak by him made it run just that little bit better.

and so the leakdown test that i plan to do  right after the compression finger test,  is also going to be-forget the numbers, for now-add a little air on tdc and listen  in the exhaust and case, or intake for one being a little more of a hiss than the rest, right? thank you for that.

[Redline it]

I’m convinced that uneven idle, low rpm is seen as carb issue when ignition is not equally set.
Both points can be set within the spec but still cause crappy idle.

Set them both eqally and notice the difference!
Easier with dwell meter when numbers eill be seen and guide how to adjust.

It must be possible to set both with feeler gauge. Then adjsut one of them when engine idles until engine runs smoother. Its a huge difference. Test closing a little, if worse, other dir to best idle.
Check ign with lamp, idle as well as full advance.

I usually need to adjust a few times since point gap and ign setting affect each other. If you do not believe, try it

This has been dicsussed several times. Find MRiecks thread about dwell.

i try to set the dwell for both right on equal. the last time i set em i had just learned to set them at the longest dwell on the high side specs for hotter spark, i was getting an oscillation of the light more on #2/3, i had another bolt that was straight, and it took care of 1/4, and 2/3 at the time. couple days ago #2/3 is doing the vibrating/oscillating timing at F. the advance are ok on both. i appreciate the help you guys here put in here, as you're probably busy doing things, and i don't want to waste your time at all, and what you just said about the timing, made me realize one thing that's been  happening quite often in the last year. i'll shut the bike off via the thumb kill switch, and the battery will run down to 0volts. sometimes i don't catch it till morning, the battery isn't hot anymore, but i'll check the water and hit a 12 volt schumacher at 50 amps jump start for 30 seconds or about there and if it's not 11.5 i'll leave it on 10 amps for another few seconds, and finish it off with a yuasa trickle charger. i gotta do the jump start or it won't charge, i'll check the water after the charging with loose caps if it's low i'll add distilled water and put it back on trickle charge or run it. it's happened at least 8 times now, the battery is a walmart correct size but it's 3 yrs old, i got a yuasa non activated battery i was gonna charge on to use as standby for when i forget to shut the lights off, i won't have to cancel riding, but what i'm getting at is the points, they could be taking a beating from leaving the key and lights on, right? they're good points, the kind. i gota a bunch of new spares but the junk kind, trash. i'm looking for some hook ups for points, carbs, plug wires and caps, the coils, can they , as i've seen lately be given a new spark plug wire? either drill it out or pull it out and put in a new wire? if that's causing problems down the road, cause i've never changed them, then coils will be added to the list, not the aftermarket trash either, in todays market, people will me right in the eye, knowing their new aftermarket parts are trash, and say without a bit of hesitation that after selling thousands of them, they've never had a complaint. i don't want to name names or companies but those deal on jets, everyone i've known that appears to be honest will say, don't put em in, they're 50/50 chance of being ok, too much work to have to take apart again new ones too. but i'm gonna do a fast compression feel and add some air to the cylinder to see if there's a big leak, if there is i'll check the valve timing, if they're ok or like as they were few years ago. i'll check the points, condenser closely and then get it on again, after i put the carbs back on. back to the list a spare running good 400f, looks aren't that important is something i'd like to add to the spares. or know anyone who rebuilds em, honest and good mechanics, i tried to email ichiban moto, he must be busy. but the schwan guy does he still put em together. maybe i can pm his account here? i'll try it. my points, i gotta look at em. 

[Don R]

 If it ran like a scalded dog and suddenly didn't, the new carbs are probably suffering the same malady as the old ones. Is your tank rusty? Fuel old? These jets are tiny and easily plugged.
 Check the timing with a strobe timing light on both sets and all four wires. Plug fouled? Get an infra red temp gauge and compare the pipe temps toward the bottom of the bend. Are the plug caps original? Do the plug wires arc to the frame in the dark? Just a few random ideas.

[PeWe]

One more thing I have noticed with 2 different cranks and adv units. There is a rather big play when the nut is not tightened. Move advancer unit max to the left (counterclockwise) and keep it there while tighten the small center nut. T and F will then match the case mark.

I have verified this several times with piston stop and degree wheel.
If the advancer will max at the other direction ignition will be too advanced when setting it relative case mark. Too advanced ignition on low rpms can behave as carb issues running too lean. Pinging on higher rpms can blow the pistons. I'm sure that's why I blew 2 RC 836 pistons which cause I finally is shure about 32 years later. Higher CR, quicker it will go bad if wrong.

I have used an alternative case mark around 4 degrees later on case (to the right of stock mark) to match max clockwise position of the advancer. This is difficult to get right.  I have learned that ignition need to be more retarded than I have thought to get it to run well. My unit has adjusted springs to avoid too advanced too soon.
I'll set advancer to max counterclockwise position next time.

[PeWe]

One more detail with photo.
I did some lightly peening of the crankcase bosses all around the plate to snug it up.
Before it moved sideways when adjusting, loosen the 3 screws holding it making the adjustment to a never ending story if getting it right.
I'm sure this made people to go electronic and ban points.

When moving sideways point gap will change. Adjust and ignition change, loosen bolts and change again. I got ignition in a bad location when I got the oscillating readings with ign lamp. Now much better. My avatar K6.
Here on my K2 build not yet started.

I used this type of hammer, the sharper side, hammer lightly straight on the bosses.
Not sideways as shown in the link that has good info about the ignition. https://www.motorcycleproject.com/text/sohcign.html
I did not want to crack the aluminium, only make it to swell. I hammered with point plate in, screws not tightened so it can move equally sitting centered. I tapped lightly once on each going to next, next going around until all 3 bosses had got 3-4 taps.

Plate will not move sideways now when loosen the screws adjusting ignition.
Hammer as the photo, sharp side worked really well. Hammer really light, you'll see when it make marks making alu to swell.

[Redline it]

i'm a little slow at it but what i did was was the thumb comp chk and it was good, all four felt the same. then instead of just compressed air i screwed in the leak down tester and on each tdc i added air until it was around 40 psi just to see the differences. it was the first time (2nd ever to date) that the crankshaft was lined up w/T enough that it didn't break over tdc and push the piston down, so the 40 was good enough to see that it was comp across the 4 w/all 4 right around 2psi lower, #2 or 1 was maybe 3psi,that's the difference.

and to my common sense the valve timing has got to be close enough to call it "on" (with the T mark lining up with CS and the pistons holding by themselves adding 40lbs to each hole) without having to take the valve cover off to chk the cam gear marks, correct me if it's not good enough. the only way V timing could get of is if the chain is stretched beyond specs or if it jumped a tooth which would be pretty obvious?

the tank is non rusty at all. stock clean as new. one thing i did was a few years ago was i used a cut bucket handle to use as the tank filter, and i wrapped a garlic bag around it after a scientific test that it was fuel compatible passed. i looked in and it looks ok. i didn't pull it off though.
 
the point timing is spot on except the 2/3 t-light wiggles slightly, i've never used special nut to turn over, never trusted that. the advance isn't double spring'd. one thing i've done different lately is just use #3 and 4 wires to strobe light check, that could be an issue throwing everything off, and consistent with my luck lately.

the next step, after studying pewe's points/advance cures (forever checking the adv rpm change they all have been nearly identical of the beginning of adv movement to full adv, and they've never been smooth, once it starts it's goes to the stops, pegs in between the ll.,) i still haven't even checked to point contacts for major pitting on account of the key left on so many times, ill try to do a static test (carbs are still off)   i'm either going to check the t chain tension bolt. or take the head off and put a gasket and orings.
 
gas quality is fresh 1 fill a month. the gasoline were getting these days are fking us big time, it could be constantly plugging the jets, we either got to file a class action lawsuit against the opec or corn farmers because of the destruction of our tools when they're saying it's safe to use. and if there's product of conditioner like stabil to off set the clock starting of erosion then they could put that in when mixing the gas/corn. every one of my chain saws within 2 months is straight screwed in need of a carb overhaul, and if i can  buy a quart of straight gas on the shelf of lowes then why can't i buy it at the pumps? sure the station owners are getting rich(er) by not carrying the straight gas. but to imply all is good, when it's not is bs. this last set of carbs i took off there was greenish powder residue  in every bowl that resembled a copper corrosion. i couldnt put my finger on that one at all, that was the set that ran like a striped ape. the wheat is in the boot stage just before heading, some already half out with healthy full heads, not a sign of disease nor insect attacks and 20 degree nights just happens to show up with a plan to stay. dangit.

[dave500]

were lucky so far here in Australia every gas station has normal fuel and the "E10"option,thats 10% ethanol,i think theres also a higher percent option at some,this is for cars designed for it,still a lotta #$%* though!we can leave fuel in chainsaws for years,just top it off and they fire up three/four or five pulls on the handle,having said that though im just repairing my brother inlaws crappy jonsered with crumbling fuel pump and lines,he said he used it bout 8 months ago?maybe he put E10 in it?

[Redline it]

were lucky so far here in Australia every gas station has normal fuel and the "E10"option,thats 10% ethanol,i think theres also a higher percent option at some,this is for cars designed for it,still a lotta #$%* though!we can leave fuel in chainsaws for years,just top it off and they fire up three/four or five pulls on the handle,having said that though im just repairing my brother inlaws crappy jonsered with crumbling fuel pump and lines,he said he used it bout 8 months ago?maybe he put E10 in it?

you guys ARE lucky, looks like they got their foot in the door with "optional #$%*" their selling. keep an eye on em.  i had an outboard gas tank in the garage had gas in it from the 50s, not a trace of corrosion, the metal was clean. i don't think they'll let me in Australia, or i'd go. and over here the way it is mixed or injected takes place at the trucking level somewhere at the pumps or long after the refinery, or even when he's loading his truck, it's just cutting it is what it sounds like. so the retailer or station owner has some options on every tanker, to make more money on his gasoline sales? they only need 15% real gas to work barely. we're relying on their honor to be honest. we got physicians over here, that have reached that level of occupational degree that are prescribing opium based pain medication to drug addicts or peddlers, making millions of course doing so, and when busted they loose it all and get 30 yrs in prison. there's multiple gas stations almost on every corner owned by Arabic accented guys. now that's a weird twist, lots of quality opium comes from the middle east, not many would think twice doubting the trust of who came up with the burning bush stories (that currently are not working out so well.) it the dr i'm kind of surprised about. 

[Redline it]

today reading a 350f post about the guy selling his bike but it doesn't start stay running or something, might have some messages in it for my situation. i have been trashing a 2 or 3 yr old batter by accidently leaving the key on, it's happened maybe close to 10 times. up till the day i parked it a few weeks ago the battery appeared ok by never dragging while starting. no unexplained dead battery. the process of recharging from a left on key was a simple momentary high amp boost then a trickle charge would top it off. and it'd work fine until the next left on key night.

i trickle charged it a week and half ago. yesterday it was almost dead at 12.2 volt, so i put it back on the yuasa trickle charger. after that did a volt check while simply turning the ign key on and while cranking. with key on the voltage is dropping pretty fast and while cranking from what the charger says fully charged it drops to near 12 but only comes up to 12.2 when stopped. that could have some influence on crappy running. might as well take the head off, try to slow the oil leak some.