12mm thread. I would not use the rubber tipped testers, they are cheap and will not seal 100% which will give you inaccurate readings. Its hard to hold against something that will be pushing back 150 psi.
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Modest '78 CB750 Build
- Thread starter Pabeye
- Start date
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- Tags
- build build thread cafe sohc
Pabeye
Older than Dirt
12mm it is then..........thanks, have a great day!
CB750 supply has a tester kit for $58 that looks like it would work decent http://www.cb750supply.com/products/9/tools , just dont use the rubber tipped part they send with it use the threaded adapters. Plus they support this forum and are good to deal with, have used them for parts before.
Pabeye
Older than Dirt
I have already bought several things from them and I'm very pleased with their service. So, what are good compression numbers for a 78 CB 750 with 15K miles?
I would be really happy with 150 but 120 will still run good any lower and it is getting too low to be considered a healthy motor in my opinion. I dont know that I have seen a number actually published by honda, there may be but I havent seen it. All Honda says is if they are all within 10% you are good but then again all within 10% of 75 psi wont work lol.
amc49
CB750 Guru
100 psi is pretty much junk motor status like implied, it's a number I worked out after many years of working on cars and bikes, the engine cannot run hot enough to keep plugs running clean there. Compression = heat. That number does not really apply though to some very small 2 strokes, they will run that low and fine, being they make twice as much heat as a 4 stroke.
Pabeye, none of that was aimed at you, it just gets frustrating to hear all the malarkey that's out there. In between lie all the people who will never get their stuff lined out due to archaic ideas tied bulletproof to refusing to give a minute to solid thinking, it's so much easier to just wreck things it seems. They (not you) spend endless hours chasing ghosts when the engines are dead and never will run right. So many think you work on the carbs and it's fixed. I used to buy and sell based on that and you can count on it being cast pretty much in concrete. I've made lots of money over it.
My bad.
Pabeye, none of that was aimed at you, it just gets frustrating to hear all the malarkey that's out there. In between lie all the people who will never get their stuff lined out due to archaic ideas tied bulletproof to refusing to give a minute to solid thinking, it's so much easier to just wreck things it seems. They (not you) spend endless hours chasing ghosts when the engines are dead and never will run right. So many think you work on the carbs and it's fixed. I used to buy and sell based on that and you can count on it being cast pretty much in concrete. I've made lots of money over it.
My bad.
Pabeye
Older than Dirt
AMC..........thanks for the clarification. I am a retired teacher, so I'm quite familiar with folks who do not hear or follow good advice. Like you once in a while used phrases that were perhaps not quite appropriate for the situation. I appreciate your explanation. 
Back to the bike.....I did not want to wait for the arrival of my nifty new compression tester to come in the mail so I bought a cheapo at Harbor Freight. Had to use the rubber tipped extensions and could only test the outer cylinders. #1 ( the cylinder with the dark plug) came in at 110 psi and #4 at 120 or so. I also checked my Chiltons manual and they call for 140 to 170. So far it would seem that the old four banger is not in such good shape. I've no intentions so far of pulling it apart like I did to my last project bile which was a lot simpler 2 cylinder XS650. ( That one had a 1/2" home in one piston) I'll check again when the new tester comes arrives. Also will check the other essentials in the mean time. HUMMMMM MAYBE some Marvel Mystery Oil will fix it?
Back to the bike.....I did not want to wait for the arrival of my nifty new compression tester to come in the mail so I bought a cheapo at Harbor Freight. Had to use the rubber tipped extensions and could only test the outer cylinders. #1 ( the cylinder with the dark plug) came in at 110 psi and #4 at 120 or so. I also checked my Chiltons manual and they call for 140 to 170. So far it would seem that the old four banger is not in such good shape. I've no intentions so far of pulling it apart like I did to my last project bile which was a lot simpler 2 cylinder XS650. ( That one had a 1/2" home in one piston) I'll check again when the new tester comes arrives. Also will check the other essentials in the mean time. HUMMMMM MAYBE some Marvel Mystery Oil will fix it?
Did you check with the throttle wide open? make sure the battery is fresh and cranks over fast. Have you verified valve clearance yet? 170psi is high, there wont be many of these bikes that will ever show that high.
Pabeye
Older than Dirt
Battery fresh ( on trickle charger) No tank in place, but yes Throttle open. Have not checked the valves yet.........more to follow. Thanks dirt for keeping an eye on my thread.
One other note....I did squeeze about 1 TBS of oil into # 1 and #4 and the PSI jumped up maybe 20 psi. The really weird thing is, the bike runs strong......and I say this from a bit of experience....I've owned 14 o5 15 bikes over my 45 years of riding.....everything from a CBRXX which I can say from personal experience will do 175 mph to a 125cc Yammie dirt bike. Go figure, the numbers so far on this old girl don't seem to match her "seat of the pants" performance.........
These will run fine on 120psi, but would more then likely run better once freshened up. If you went with the next size overbore and had the head freshened up you would be surprised by the extra power you would get. But you need to wait for a better compression gauge before condemning the motor as it sits. 1 TBS is a lot of oil for the smallish bore, you may have inadvertently givin yourself a false increase by that amount of oil, that much could effectivly bump compression. You want just enough to wet and seal the rings not reduce chamber volume.
Pabeye
Older than Dirt
YUa know I never considered that. When I Googled "how to do a compression test" I got a Popular Mechanics article...( I trust those guys) and they said 1 Tbs......never thought that they were talking about a typical AUTOMOBILE engine and an MC would be less.....
amc49
CB750 Guru
'...you need to wait for a better compression gauge...'
Absolutely. Compression readings always have to be taken with a grain of salt, they are very often unreliable. You NEVER use a gauge that simply presses up to the hole if you want a real number. I use a car battery too to make sure the spin is the same at first cylinder and last, I take 3-4 readings off each cylinder. All plugs out, leaving one in can tilt your numbers. Do NOT use a battery in a car with the car RUNNING!; the higher alt amp output can fry bike electrical parts. Battery by itself though will do fine with no issues. Let each cylinder spin up until the gauge quits increasing and even the amount of speed indicates an issue, lower cylinders will spin up to their max slower. The gauge quality will make you or break you too, there are far more crap compression gauges out there than good ones. You are modifying the compression ratio with any added volume from the gauge, some have so much area inside you can lose 10 psi over that alone. The gauges often if rental ones will often have the wrong valvestems in them off tires too, a proper valve stem for a compression gauge takes like 1/10 the pressure to blow open it a tire stem does but the parts stores don't care about that on rental tools.
Absolutely. Compression readings always have to be taken with a grain of salt, they are very often unreliable. You NEVER use a gauge that simply presses up to the hole if you want a real number. I use a car battery too to make sure the spin is the same at first cylinder and last, I take 3-4 readings off each cylinder. All plugs out, leaving one in can tilt your numbers. Do NOT use a battery in a car with the car RUNNING!; the higher alt amp output can fry bike electrical parts. Battery by itself though will do fine with no issues. Let each cylinder spin up until the gauge quits increasing and even the amount of speed indicates an issue, lower cylinders will spin up to their max slower. The gauge quality will make you or break you too, there are far more crap compression gauges out there than good ones. You are modifying the compression ratio with any added volume from the gauge, some have so much area inside you can lose 10 psi over that alone. The gauges often if rental ones will often have the wrong valvestems in them off tires too, a proper valve stem for a compression gauge takes like 1/10 the pressure to blow open it a tire stem does but the parts stores don't care about that on rental tools.
Pabeye
Older than Dirt
Thanks as always for the advice AMC......Just purchased a Sears Craftsman gauge. It will arrive in a few days. It seemed to have more flexibility than the one sold by CB750 supply. Will keep you & the group updated with any progress......26 degrees here this AM....... way too cold to ride for this old man....almost too cold to work in the unheated garage too!
Pabeye
Older than Dirt
Just picked up the sears compression tester. It looks to be of very nice quality and has lots of attachments to fit different scenarios.
Will give it a try ASAP........QUETSION.........do I have to do the test with the engine at operating temp.....sounds difficult especially for those inside cylinders.....
Will give it a try ASAP........QUETSION.........do I have to do the test with the engine at operating temp.....sounds difficult especially for those inside cylinders.....
your best results will be from a warm engine but a cold compression test will still show you if something obvious is wrong. i like to do both cold and warm engine tests
amc49
CB750 Guru
X2, cold won't kill you but warm is better, it also adds the small amount of cylinder wall oil that raises the number which won't be on walls that have been sitting a while. The rings are straighter because the piston has swelled out more to correct size. The rings as well have swelled out to be rounder too and seal a bit better.
Pabeye
Older than Dirt
AND, the results are in........tested each cylinder 2X this is the average of the two tests. Engine was cold, throttle open.
# 1 130psi #2 132psi #3 125psi #4 137psi...........#3 seems a bit low but I guess within limits..........thanks one and all for all your help up to this point. I did have a panic attack at one point. The 12mm adapter I was using stayed screwed in the plug hole when I unscrewed the hose. UGHH! luckily it was the #4 cylinder. Had it been one of the inside cylinders I don't know how I would have gotten out!
# 1 130psi #2 132psi #3 125psi #4 137psi...........#3 seems a bit low but I guess within limits..........thanks one and all for all your help up to this point. I did have a panic attack at one point. The 12mm adapter I was using stayed screwed in the plug hole when I unscrewed the hose. UGHH! luckily it was the #4 cylinder. Had it been one of the inside cylinders I don't know how I would have gotten out!
I should run ok with that compression. Just for giggles I would check it again when you can warm it up. Have you verified valve clearance? I would guess that it is valve leakage causing the lower compression, the bottom ends last a good long time normally but the valves, seats and guides are what wear the fastest. A fresh valve job would probably kick things up a bit.
I have gotten the adapters stuck before, had to use a needle nose to get them out.
I have gotten the adapters stuck before, had to use a needle nose to get them out.
Pabeye
Older than Dirt
DD I will give it another shot with the engine warm. But, at least for now, during the winter months, I feel more comfortable spending more time cleaning , polishing, touching up paint and so on because I'm at least somewhat confident that the motor is OK.
I also used needle nose pliers to unscrew the adapter..........I don't think I'd have been able to do that with #'s 2 &3..........
I also used needle nose pliers to unscrew the adapter..........I don't think I'd have been able to do that with #'s 2 &3..........
Pabeye
Older than Dirt
Well, well........it was warm today (55 degrees which is warm for December in NY) I did not put the tank on but hooked up my DIY test bottle to warm up the engine. I worked as fast as I could to do the compression test while the motor was still warm......and the results are...........
150 - 155 PSI in all 4 cylinder. I was quite pleased with that. i Tried to check the valve clearance first but did not have a offset feeler gauge of the right size. In trying however with a straight gauge it seems almost impossible to check that clearance! Do you guys have a "trick"
In addition I tried running the engine again both with the stock air box and the cheapo Emgo pods and there was not too much difference, although the air box may have been a bit better. I shot a video so that perhaps some one may be able to further the diagnosis. I'm still feeling like something is up with that #1 carb. That is the cylinder in which the plug always gets black an sooty and the other don't. That being said you guys know way more than me about such things. Here is the link:
http://www.youtube.com/watch?v=z9ZAEQnJxP0
How difficult is it to do a carb sync? Can I use the DIY method I've seen or do I need to buy those gauges? Input always appreciated!
150 - 155 PSI in all 4 cylinder. I was quite pleased with that. i Tried to check the valve clearance first but did not have a offset feeler gauge of the right size. In trying however with a straight gauge it seems almost impossible to check that clearance! Do you guys have a "trick"
In addition I tried running the engine again both with the stock air box and the cheapo Emgo pods and there was not too much difference, although the air box may have been a bit better. I shot a video so that perhaps some one may be able to further the diagnosis. I'm still feeling like something is up with that #1 carb. That is the cylinder in which the plug always gets black an sooty and the other don't. That being said you guys know way more than me about such things. Here is the link:
http://www.youtube.com/watch?v=z9ZAEQnJxP0
How difficult is it to do a carb sync? Can I use the DIY method I've seen or do I need to buy those gauges? Input always appreciated!
