my #3 cyl is reading just under 120psi on a compression tester causing a misfire rough starting and horrible performance, i put it on a cheap HF leak down tester and got nothing... i turned it up so it was pumping bout 100psi to the cyl and still nothing... no pressure loss no noises no bubbles in the coolant nothing through the exhaust or intake or through the oil fill

im not too sure where that leaves me? should i go get a better tester that allows more than 15psi before its in the set position? but even then its weird when i crank it up and ignore the leak reading i can not hear any air escaping?

ive seen some people suggest looking at the valve train when they've had the same experience but what should i be looking for? if a valve was stuck open or bent wouldn't i hear air escaping from the intake or exhaust? i had no metal in the oil or any reason to suggest major damage and i was completely expecting to notice air noise in the case when i did the leak down test i would think it was ring or cyl issues but so far i can not confirm that

i was a little low on coolant but none in the oil. i guess i could top it off to be positive theres no bubbles in it when i pressure the cyl, would the thermostat being closed hinder seeing bubbles or is the small hole in it enough for enough air to pass to be seen?

ill start by filling the coolant as high as possible just incase and go from there im open to any and all suggestions and appreciate any and all help.

thanks

 

coolant was topped off and compression line was connected directly to the compressor since i wasnt getting any noise or leak % anyways the compressor was set to 100psi. i got a few bubbles then i squeezed the coolant hose to get the air out as i topped off the radiator and slowly released the hose to try to get any existing air out that might have been there and since then i have no more bubbles coming out of the radiator.. and still have no noise in the crank case, intake or exhaust and im back to where i was but i feel pretty confident that its not a cyl to water jacket head gasket leak so thats something i guess

i dont know if this is a possibility but could i have a head gasket leak between cylinders that doesnt leak to the water jacket? only thing being i would think that if that were the case than more than just cyl 3 would have compression issues? but i guess i could confirm that by removing 2 and 4 plugs and seeing if i hear any air escaping from the plug holes?

 

Most compression testers store the air in the gauge. They are designed to show the maximum pressure developed by the cylinder. The gauge doesn't go down until you release it via a button/valve. How does the compression of the rest of the cylinders compare?

 

the other cylinders were about 145psi, ill run 1/2/4 tomorrow again since its been awhile but from what i remember thats about what they were at. cyl 3 read ~118 and has always fouled the plug quicker than the other cylinders.

yeah i ran just a standard compression test first using both of my compression gauges and got the same results between the two gauges so i know their accurate, also used anti seize on the threads and oring so it is fully seated and not leaking.

after i ran the standard compression test i ran a leak down test, which sets the psi being pushed into the cylinder via a compressor/regulator which is monitored by the first gauge and then has a second gauge which monitors the rate which the pressure is lost. because its connected to the compressor and it pushes air into the cylinder if theres a air leak somewhere i should be able to hear it; if the intake valves leak i should hear air leaking through he intake, exhaust valves are heard in the exhaust, ring leaks can be heard in the crank case etc. problem im having is i know i have low compression in cyl 3 but when i run the leak down test i cant hear where its losing its pressure?

if i cant find where im losing compression with the engine in the bike ill just have to pull it out again and start with the valve train then the head and go from there but ideally i could find whats going on before starting to take things apart but if its just a head issue thats way better than a lower end problem and much cheaper to repair, im not in a place right now to spend the money on a complete rebuild especially on a old 600

 

Here's my thoughts. You get what you paid for...

The difference between 145 and 120 is not a whole lot.
Yes, it's outside the recommended range, but not that severe.
It's still holding most of it's pressure, so it's a small leak.
It probably isn't causing your running problems.
I've seen engines with much worse number, that ran fairly well.

Going by your 'flouled plug' comment, I'm guessing that it's probably worn rings in that cylinder.
#3 carb has problably been flooding that cylinder for a while with too much gas.
That will foul plugs, cause idle rough and not perform well when warm.
It will also wash the oil off the cylinder walls and cause accelerated ring wear and eventually low compression.

The leakdown won't tell you anything because it's a small leak.
It's leaking past the rings, into the crankcase, then up through the breather tube, into the airbox and then out the intake (past the filter).
You get my point, long path so you are never going to hear it.

If it was me, I'd fix the running issue first, rebuild the carbs.
Once the cylinder isn't being flooded with fuel, you may find the compression number comes back up a little.

FYI, my old honda civic has numbers 150,75,85,130. Slightly rough idle stoplights, ran fine on the highway.
Drove it for 2 years like that.

 

sounds like a good plan of attack, ill clean the carbs i already have some smaller jets so ill size down #3 see how that goes, i do have a k&n filter and muzzy exhaust so i have previously rejetted and it ran perfect for years but i went from sea level in fl to a mile up in new mexico which might be part of the issue but somehow my current tune still gave a 15.4 afr on the wideband but ill start back at stock jets for #3 and see the results

it was weird though for months after being here it still ran fine and started no problem until one day when it just started to change, maybe as you said it was running rich and that washed the cylinder oils causing the issue to get worse, i should have been more careful with it after moving here would suck for another f2 to be ruined due to poor ownership after years of caring for it in FL

i was worried it was the rings it seems like the most logical answer but i can completely get what your saying the leak could just be so small that ill never hear the air escaping from the rings but as long as i can get it going again i can live with that for a while. i had attributed the fouling to incomplete/no combustion of the fuel due to low compression, i never thought about too much fuel thank you for that input and thought

 

well no luck with rejetting, i brought #3 to 132 mains instead of the 140s it had, i also put in a new plug and couldnt get it to start. i tested spark on all 4 and they all have spark, timing hasnt changed unless the belt jumped but i dont have a reason to suspect that, i smelled fuel and saw that the plugs were wet and i used starting fluid at one point so im comfortable with the idea that the plugs had fuel so its not a fuel issue i dont think (not saying that the jetting/tune couldnt be off it very well could be) but as far as what an engine needs to run i can positively say it has fuel and spark and im very confident the timing is fine

im rerunning a compression test across all 4 using 2 different gauges, granted im leaving he carbs on while doing it but im opening the slides all the way and holding the throttle wide open so theres very little air restriction. the numbers were quite different from what i remember/last saw them at. im running he tests twice so ill post up when i have the 1/2 cyl numbers, my battery got too low so its on the tender right now

 

just finished getting the compressions for all 4 cylinders, ran the test with two gauges one than the other with slides pulled up and throttle wide open choke was off, but i pulled one plug at a time to test one cyl at a time instead of pulling all the plugs then testing. happy to do it again if its recommended to pull all plugs before running a compression test

compressions are as follows:
cyl 1: 120 then130
cyl 2: 135 then 140
cyl 3: 120 then 125
cyl 4: 120 then 125

cyl 2 is the highest but nothing crazy but the rest seem to be pretty even, and whats weird i ran the test without thinking about it the first time before the tests above and i didnt move the slides or open the throttle and i got

cyl 1: 135
cyl 2: 100
cyl 3: 118
cyl 4: 100

not sure if hat helps at all crazy how cyl 2 and 4 dropped so much especially 2 but again that could have been from not opening the slides or throttle. but assuming the first tests were good then i would think that i should be able to at least start the bike i have compression thats somewhat equal spark and fuel. given compression test results and looking at the manual, the manual suggests looking at the valve train guess i could always check timing valve clearance etc

any thoughts?

 

Yeah, couple thoughts. For the compression test, all plugs should be pulled. You want the engine to spin and breathe freely.

For the carbs, I think your jetting was probably ok.
From your description, the problems appeared suddenly, that's not a jetting problem.
Things you should look for are torn or cracked boots, orings on the idle needles, vaccum diaphram for the slides.
Check the rubber bits, they are 27+ years old at this point.
Also check for any contamination or plugged passages.
More than a few things can cause over fueling on a cylinder.
K&N over oiling is a big one, but that effects all cylinders.

Also to test rings, test the compression first.
put a squirt of motor oil in the spark plug hole.
Then run the test again.
If the rings are good, there shouldn't be much change.
If there go up, then it points to rings or cylinder wash.

 

just toook the carbs off and the plugs out, carb boots i know are good they still are soft and only a couple of years old at this point, ill check the rest of the rubber pieces when i look at the carbs and ill probably go back to 140s on cyl 3 to keep he balance the same as it was. it ran perfect with this tune for a while and had great idle and throttle response across all rpm i was very happy with it

i havent cleaned or oiled the k&n filter in wayyyy to long so i know its not over oiled if anything its dirty haha

heres the compression numbers with the plugs out and carbs off breathing as good as it can, again i ran the test on each cyl with both gauges (just for my records the purple one first then the sears one):
cyl 1: 120 then 125
cyl 2: 120 then 130
cyl 3: 120 then 125
cyl 4: 125 then 130

they arent the highest compression numbers in the world but im also a mile high (5300 ft+) and their all pretty damn close to each other. im starting to think that compression isnt the issue i would think the motor should be able to run with those numbers. which is weird i was able to confirm the engine has spark, compression is pretty even, i saw/smelled fuel on the plugs and even used some starting fluid which makes me think it isnt a fuel issue.

im going to take a light and turn it over a couple of times and watch the valves to make sure their moving right and opening, might even pull the cover off and measure the valve clearance and check the timing it has been a few years since i last adjusted the valves and checked timing

at least through all this i can confirm it seems that #3 isnt extra low on compression so something else must be the reason for it fouling the plug, ill test the coils maybe the spark it has is just weak

EDIT:
just visually inspected the intake valves, they are are moving nicely, they all look to be opening roughly the same amount even though thats not a precise measurement theres no weird noise or any valve that isnt opening or stuck open, im going to test the coils before i move forward with valve inspection granted i just visually inspected the intake and not the exhaust

primary coil resistance is jumping around a bit for both coils which is kind of weird but hovering for the most part around 3.1 to 3.4 does randomly dip down into the 2.9 range but hovers mostly from 3.1-3.4 mostly staying above 3.1 which the manual lists as the upper limit so there could be a little weak spark. ill test again with my other multi meter i have a southwire and craftsman both have always seemed accurate to each other so i dont think its a cheap tool issue

Edit 2:
carbs are coming apart and are being cleaned as we speak: i forgot the number of turns out on the mixture screws but from what i recall they were about the top of the manual specs but it has 140 mains and 42 pilots for future reference. im thinking im not going to pull out the idle mixture screws (can and maybe i will) but i had recently pulled replaced orings and replaced one broken one last time i tuned it so im pretty sure their still good but ill double check if i still have issues after cleaning, the sync was just so good i just dont want to mess with that haha

and just incase any one has some extra tips or advice for carb cleaning, im pulling all jets and floats out using brake cleaner (currrently out of carb cleaner) to wash the jets using a needle to manually clean all holes then using compressed air to dry and give one last blow through

cyl1: had a slightly clogged pilot jet, main looked fine and slide vacuum diaphram looked fine still in one piece not hard or brittle
cyl2: everything looked good no signs of clogs bad rubber nothing
cyl3: everything looked good again
cyl4: slightly clogged pilot again but thats it no rubber damage and everything else looked great float needle worked smooth like the rest of them and all passageways were clear

just put them back on put fresh supreme gas in a auxiliary tank gave it some starting fluid and still no start

given the carbs are clean not leaking and have the same balance and tune they did when it ran great, and that it was given starting fluid this has to be a ignition problem unless 120 is too low of compression but i dont think thats the case, it should at least be able to start.

it has fuel, it has compression, so i know the exhaust valves arent stuck open, i know that the intake valves look good. once in the past i somehow had the timing pick up come loose and that made it loose spark and i know that it has an adjustable timing pick up gear i guess its possible that somehow that gear has shifted or loosened and thats messing with the timing/spark? thats the only grey area as far as timing goes but i see spark on all 4 making me think that hasent happened again. i really hate throwing parts at a problem but i know the primary coils for both ignition coils are both high ill check the secondary coils and the wire resistance before i pull the plug and get new coils but it seems weird that both would go back at once unless one was already on the edge which made it a little tough to start and then the other went bad making it not start all together

sorry for the long post just giving all the info ive gotten as i get it