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Very low compression - non running


CB750 Enthusiast
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The generic question: should I

A) drag this bike to the road and hope someone hauls it off
B) look into an engine rebuild at this point and if so go ahead and put 900 top end on
C) check and fix the valve shims and get it running to test again

The backstory:

1980 cb750c that turned over with a jump when purchased almost two years ago. (Wish we hadn't stared it after seeing the fuel system condition.) It has set since then. I've pulled the carbs to clean and rebuild now that I've seen them. I checked compression today. It's cold and dry from sitting. All four read between 62 psi and 70 psi. Added oil and all four went up 10 psi.

I don't want to keep throwing money at something if I'm not addressing the right problems. I can tell that the valve cover gaskets are dry and cracked/chipped along the outer edges.

Any advice/slaps across the face/ etc welcome. Just trying to learn. Thanks!
Were the carbs installed during compression check? If so, did you check compression at WOT? The throttle slides will cause enough restriction to give an artificially low reading unless you twist the throttle wide open when cranking.
Thanks! No, the carbs were removed, all spark plugs removed, jumper cables from my truck to the battery terminals (no battery on bike.) the oil and filter are what came in it.

I'm really just leaning towards C. I'm in the process of repairing the tank and have the carbs on the bench with the float bowls off. Think I'll rebuild the carbs, shim the valves where necessary, change plugs, oil, filter and see if it will just run at all after that. But to do it right I still need to buy:

Carb insulator boots
Carb rebuild kit
Gas cap
Bucket tool
Valve cover gasket

That's a good bit for me to invest in something that may not work when I'm done. Kinda would like to have gotten some "light at the end of the tunnel" feedback with the comp test and didn't. But, it being abandoned so long I'm not even half sure a compression test is valid. That's why I was asking opinions.
Pretty odd for all 4 to be that low and yet even, it screams suspect your compression gauge. They are always highly suspect anyway. If one that is not top quality or one that simply pushes against the hole instead of threading in, well, garbage. You'd be amazed how many gauges that will cover. I weed out about 90% of them as junk myself.

There is a valve stem to release pressure manually and one up inside bottom of plug fixture to hold it trapped when gauge is used, many people replace that second stem with one from a tire and a silly stupid error, the one that traps has a spring about 1/5 the tension that a tire uses, it has to be super light to catch every bit of compression pulse or you get super low readings even with an expensive gauge. Press the stem in, you can easily tell the difference.

ALWAYS run multiple readings (3-4 each cylinder) when running compression checks and a hot battery (car better than bike) as well, motor needs to spin as fast as it can or worthless numbers. Open throttle like said too. Don't spend a cent if you can't get engine over 100 psi, it is a failed lost cause as the plugs will never develop enough heat to stay running clean. I personally never touch one under 120, too many problems that never sort out.

Run the valves, common on these for lack of maintenance there to utterly destroy compression, there for a while I was buying/selling these on a regular basis due to people letting that go away, then they commonly wreck the carbs trying to 'fix it'. I have never understood the American males' need to show testosterone by rebuilding carbs they won't understand in a million years. I made plenty of cash doing that, as much as another $1000 for $25-$50 in parts. Use .005" minimum when sorting out compression issues, they burn valves absolutely for sure at .002" and try to at .003". Forget the damn service manual on that, it's WRONG! Problem being the .002" is not real and most likely .000 due to the way cams in caps on these are looser than the valve clearances, the valve springs then push cams around in the looseness to fake your readings bigtime, why .005" is not as loose as it sounds. You can pretty much subtract .002" off every reading you get reliably.

Don't believe me? Check one several times dead on a mark and roll engine around a couple times and then go right back to that mark and check again. If you can truly use feeler gauges you will find that the number will not repeat. Hardly ever. Almost impossible to check on the lobe backside base circle like you can with most engines. All you can do is provide a little too much to guarantee not holding valves open. The engine will love you for it.
Wow! Thanks for the response! Yes, I suspect it's crap as it did come from HF. It's going back soon. I am mostly through the carbs and I've ordered new insulators/boots. I'm going to get it running and borrow a good compression tester from a local mechanic friend to check again at that time. So on the short list is valve check/adjust, adjust cam chain, remount clean carbs, new fluids and filters, run to warm up and recheck with a good tester and deep cycle yellow top out of my truck. Then go from there.

On the test, I read somewhere people bypass some of the bike electronics and put the hot lead somewhere (maybe starter?). Any idea where and if this is beneficial at all as opposed to the battery terminals? Also, anything else I should do check prior this?
WHY? You risk melting threads at starter terminal and dumb. And not nearly so much control as simply hitting the starter button with key on. NOTHING to need to bypass as long as you are NOT RUNNING CAR to have a 100+ amp alternator up and running, the amps can burn 20 amp bike parts out instantly but battery by itself will NOT. I simply run jumper cables on bike battery terminals to car battery and leave it mounted in car. Done it so many times I can't count. Obviously jumping in reverse polarity another stupid move, LOOK at what you are jumping there.

Having car running is positively where stupid things happen, the amps racing through there will rip a new one on lesser rated bike parts like regulator. Car off is simply a 12 volt battery that makes no more power than needed to flow through whatever wires you got there.
Thanks for clearing up that what I'm doing is fine. The battery I'm using isn't hooked up in the truck. I've got a dual battery system and haven't connected this one yet. It's just sitting there. I'll update this thread again when I've done the test next time.
Haven't done another test yet, but wanted to update.

Put cbr600 coils, new 7mm plug wires, new ngk boots, changed oil and filter, adjusted valve clearances (all between .004"-.005" well, one comes up a few turns at .006"), new carb intake insulators and mounted rebuilt carbs. Hooked up battery with plug caps off and spun it a few times. Hooked up caps and it cranked right up! :cheers:

Tach isn't working, but going off sound it's idling a touch high. Blip throttle and RPMs fall quickly. After it warms up (a couple of minutes at most) the RPMs jump to "really high" (my technical terminology). Best guess would be 4-5k. Throttle wasn't sticking, so I killed it to check the timing advance. But before doing that I looked at the idle control and saw it was turned in. I backed it off enough that the plate opener arm isn't touching anymore. Haven't had time to recheck. I feel idle was set too high and when warm just ran where I had it set. So far no oil leaks. Fuel leak seems to be appearing between the 1-2 and 3-4 carb bodies though. I may have to pull them and break them apart, which I didn't do last time. No knocking or weird engine noises. A little white smoke out of the 1 and 4 exhaust that dissipates within 10" so most likely water. I hope to get my hands on a comp tester soon and redo the test warm. Another update when I can.

Edit : forgot to add, in case someone comes looking for answers to similar running conditions - there was no air box or filters on carbs. Just carbs. Jetted to 115/75 and ps out 3 full turns to start with. And I'm sure the exhaust potentially has holes/rust/etc. Test was only to see if the motor was viable since it should not have cranked with my psi numbers.
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Oh, it may well crank and run just not well. Will wet the plugs out quick to foul if compression is truly low. Below 100 psi will give trouble forever and no power that low. Very odd though for all 4 to be that low. Usually 1 or 2 only. Hard to find a good sealing compression gauge on that small a plug.

Give any seals time to expand with exposure to fuel, lots of time all the seals between carbs need. 24 hrs. later they may quit leaking.
Well that's terrible news about compression and great news about the carbs. We shall see. I've just done a bit more fiddling with it and here is the vid.


Several things to note. All four exhaust were hot after this last run. But cylinder one is the only one blowing white smoke. I can't tell you the last time cyl 1 actually ran. That carb was the worst and the coil was completely corroded away. I know they say white is moisture, but could it be something else? Carbon? Etc. Also, if you hear a little ticking it's coming from exhaust 4. There is no collar/seal/crush washer there. It's just stuck in from PO.

Here's coil for cyl 1 and cyl 1 wire how I recieved it. (And on the new coils it is stock electrical wiring, no relay mod yet)

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Plug looks lightly wet fouled to me, use like acetone or alcohol to dry it off and when you put it back in instead of snapping plug wire on simply put it on right to snapping but don't snap it. Any slight gap then makes more spark to possibly not foul as easy. That one cylinder may need to just run awhile to clear out crap in it. Smoke will either go away or not.
Well here's an idea. Does anyone know how much fuel this bike should use at idle? I can't find anything on it. I did find where someone was sync'ing the carbs with a manometer and said the fuel in the float bowls should be enough to sync, but they hooked up an aux tank in case. I drained my aux tank of about 6 oz of fuel in under 3 minutes at idle and my oil smells like gas. The oil that came in it also smelled like gas, which I drained and changed along with the filter cold. If that's too much consumption then I believe carb for cyl 1 may be dumping fuel (stuck open) down into the sump. Would explain that ones affinity for smoking like a chimney possibly (although it's usually black smoke.)

I ordered a new compression tester that will be here Friday. Hopefully better than the last one. My plan is to get it warm, pull the plugs and do the comp test. Then drain the oil and change while warm. Pull the carbs and test for leaks off the bike while assembled, then pull the bowls and test each float manually to see if they are shutting off. Also have a new set of gaskets coming from seang and will be breaking these apart and going through them again. Silly to think I'd be in the 1% that got them right the first time. Lol
Another update. Carbs are definitely leaking around fuel rail o rings. New ones on the way. I've run it and done a warm compression test. My numbers:

Cyl1 - 110 psi
Cyl2 - 130 psi
Cyl3 - 130 psi
Cyl4 - 130 psi

I'm not rounding. No joke. Each was dead on a hash mark. So I realize I'm on the low end of all of them and unacceptable on the variance and lowness of cyl 1.

I've drained the oil again and pulling the carbs now. Since this was so neglected I plan to get the carbs right and just run it a few hundred miles when it's road safe. Take a new Comp test and change the oil again at that time. If still low at that time do a leak down test and see if it's valve seats or rings. Then make a tough decision.

Thanks for the patience and all the help
JD, you're making great progress... Don't give up and never take option A...
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Double check valve clearance and make sure the low cylinder has at least .005" on the valves. Then get bike running and drive the crap out of it. Sometimes if the exhaust valves have a bit of carbon buildup on them to leak a bit the hard dogging of engine then lets it get beat off if the valve is a bit loose to allow it. More valve clearance = a harder impact.