Checking compression on a non running 750

TeddyG

TeddyG

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Location
Portland Oregon
While waiting for parts to arrive so that I can re-start this 750 I recently bought, I tried a compression test manually w/o the engine running. I'm only seeing @ 50lbs each cylinder, so I'm either doing it wrong or there's engine trouble. One spark plug is screwed in tight and the compression gauge is seated into the other cylinder. I'm noticing that it's fairly easy to advance the motor in gear, using a torque wrench directly off the crank in a clockwise direction. The carb and choke cables are disconnected and waiting for new cables. I know compression tests are usually done on a running engine, so hopefully I'm overlooking something here, or I'll have a nice garage ornament. Thanks for any insights/ advice.
 
No, not run on a running engine that I know of, Teddy! I have one that I hold against the plug hole, both plugs removed, FULL THROTTLE or no carbs, run the starter - you should get a good reading. 50psi won't run an engine at idle - my blown left cylinder was reading 50psi - I think it may have kicked in a little at maybe 3500+rpm.
It should be quite difficult rotating the engine from the crank nut when it's coming over compression. You could have valve leak or piston leak-down.
 
Anything over 120-130psi should be enough for the engine to run ok, a good engine will show up towards 180psi. As Quentin said, both spark plugs removed and the tester seated in one plug hole and spinning over as fast as the starter can do it with fully raised (or removed) carburettor slides. It will gradually build up pressure over each compression stroke until it is maxed out. This is the most common form of compression testing and is good enough for what you are trying to find out. A leak down test involves more complicated equipment.
 
Also a balanced compression test, Compression tester variance and unverified absolute accuracy, cylinder to adjacent cylinder, ie twin is not as obvious as a triple, 150-150-120 dry test, repeat with oil in each cylinder 210 210 200 last reading rings/bore look dodgy even though its close to the working others. Made my own leakdown tester, that takes a bit of skill to fabricate and interpret the readings. Normal compression and HC will be different, of course.

Dont omit to protect the ignition system when doing a compression test. j
 
If the engine hasn't run in a while, you'll have low compression due to gummed rings.
Run it for an hour or so before worrying about compression.
Those 750s should run with very low comp anyway.
Paul
 
From my limited knowledge protecting the ignition system involves keeping a plug attached and out of the engine while doing these compression tests, it dissipates the energy away from the black box. Spent most of my life never knowing about this. Take care where this sparking plug sits, don't go igniting any fuel or you will have more to worry about. Happy to be corrected on this as all I learn comes from keeping my ears open. The above was completely unknown to me till very recently, hope it's not BULLSHIT.
 
Luck of the draw, Vince. Some leave HT leads flapping about... no damage, others not so fortunate... and yes ignited fuel... rare though possible..j
 
Points SF ... no prob turning them over without plugs in or ground to motor - have done same with Ignitech without prob, but fried a Bosch sys in a triple in the 70s while working at the bike shop - first I'd ever heard of such an issue, probably because nearly all other bikes in the day like Kwaka 9s and Honda Fours still had points, but even when ND introduced elec ignition I never blew anything up - seemed to be a Laverda triple thing.
 
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get some Redex, the real stuff not a clone, and give both cylinders a good dose of the stuff, helps if you can position the bike/motor so that the cylinders are vertical and the tops of the pistons horizontal, put plugs back in loosely, turn motor by hand, remove plugs and add some more. plugs back in with fingers and then wait two weeks, on returning, crank the motor a few times (plugs out) with the threads covered with cloth, to try and get some of the oil out, then attempt to start it, you will oil the plugs a couple of times, and when you d get started there will be clouds of smoke. Thuis metod has saved me from a total engine strip on two motors, one was A Vauxhall Carlton, the other a Jag V12, both were low on comp and burning oil, both motors recovered the comp, stopped burning oil and lived on for many thousands of miles, in the case of the Jag 70,000, before I crashed it badly.
CLEM
 
Thanks guys- I obviously am not a mechanical genius but perhaps I,m seeing only 50 lbs while manually advancing the motor clockwise? I,ve read that the only way to get an accurate test is by starting the motor several times, and advancing it manually isn,t fast enough to build compression- is that true? A friend w more experience has offered to stop by w a leak down test so that should indicate the state of the engine. Since the bike arrived w the wrong 7 plugs, I hope the pistons aren’t holed. When the bike arrived I briefly started it and it wouldn’t hold a steady idle, probably due to dirty/broken air filter, dirty fuel taps, and a loose choke cable not engaged into the carb mount. I don,t want to start the engine again until new oil is added ( the old oil was black sludge) and new oil filter gaskets arrive. I was hoping I could get a compression reading in the meantime just to see the general state of the motor.
Based on some things neglected, bodged or done incorrectly to this bike make me question the extent of the “engine rebuild“ that the seller claimed. On the bright side, the valves were in speck and carb slides fairly smooth w no major wear. The bike is cosmetically good w no missing parts and supposedly has 15k miles.
 
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TeddyG said:
The carb and choke cables are disconnected
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The throttle slides need to be wide open (or removed) as mentioned above to obtain a meaningful reading when cranking the starter. Removing both plugs will give the starter an easier time and faster cranking. Cranking by hand is almost pointless. Measuring on a running engine is a new one on me.
A leakdown test will provide further useful info. You'll be able to hear where air is leaking (ports = valves; crankcase breather / oil filler = rings/pistons
 
Rob said:
The throttle slides need to be wide open (or removed) as mentioned above to obtain a meaningful reading when cranking the starter. Removing both plugs will give the starter an easier time and faster cranking. Cranking by hand is almost pointless. Measuring on a running engine is a new one on me.
A leakdown test will provide further useful info. You'll be able to hear where air is leaking (ports = valves; crankcase breather / oil filler = rings/pistons
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Measuring on a running engine will probably lead to fire as petrol vapours spewed out of the plug hole are ignited by the ignition.
As they should be in the combustion chamber..
Paul
 
CLEMTOG said:
get some Redex, the real stuff not a clone, and give both cylinders a good dose of the stuff, helps if you can position the bike/motor so that the cylinders are vertical and the tops of the pistons horizontal, put plugs back in loosely, turn motor by hand, remove plugs and add some more. plugs back in with fingers and then wait two weeks, on returning, crank the motor a few times (plugs out) with the threads covered with cloth, to try and get some of the oil out, then attempt to start it, you will oil the plugs a couple of times, and when you d get started there will be clouds of smoke. Thuis metod has saved me from a total engine strip on two motors, one was A Vauxhall Carlton, the other a Jag V12, both were low on comp and burning oil, both motors recovered the comp, stopped burning oil and lived on for many thousands of miles, in the case of the Jag 70,000, before I crashed it badly.
CLEM
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REDEX! Bloody hell Clem- that’s the source of your mystery fluff! 😂
 
piranha-bro2 said:
Points SF ... no prob turning them over without plugs in or ground to motor - have done same with Ignitech without prob, but fried a Bosch sys in a triple in the 70s while working at the bike shop - first I'd ever heard of such an issue, probably because nearly all other bikes in the day like Kwaka 9s and Hinda Fours still had points, but even when ND introduced elec ignition I never blew anything up - seemed to be a Laverda triple thing.
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For the "no problem here" folk....You run the risk of stressing the ignition coil insulation on the windings, may not have a problem today, the high voltage reflected wave will have an impact, when there is no designed plug gap to limit high standing wave voltages, a standing wave has the potential to take say 35Kv voltage and double it to 70KV. Will take out electronic ignitions, if not a coil, it will reduce reliability as some stage.

While the points may survive, the ignition coil or coils are in danger, for any type of motor, when the HT lead is not terminated.

Makes little difference to me, what is done at other workshops, each person decides on the practice and its effect, pass on technical knowledge or practical experience for other to make use of or no. Technical fundamentals are sound and well know. HTH j
 
The Sock said:
And that man speaks with an authority and experience.
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I remember an article in Bike magazine where the journalist was checking for a spark on a round case 750 SS with the plugs out, laisd on the head and kicking it over with the tank still on.
Fumes accumulated under the tank and woosh!
No more 750 SS.
Paul
 
iis_iis said:
For the "no problem here" folk....You run the risk of stressing the ignition coil insulation on the windings, may not have a problem today, the high voltage reflected wave will have an impact, when there is no designed plug gap to limit high standing wave voltages, a standing wave has the potential to take say 35Kv voltage and double it to 70KV. Will take out electronic ignitions, if not a coil, it will reduce reliability as some stage.

While the points may survive, the ignition coil or coils are in danger, for any type of motor, when the HT lead is not terminated.

Makes little difference to me, what is done at other workshops, each person decides on the practice and its effect, pass on technical knowledge or practical experience for other to make use of or no. Technical fundamentals are sound and well know. HTH j
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Yes, coils are like humans, they need to discharge.
Paul
 
Having said I've run spark checks without a plug earthed in the non-check side, I do actually fit a plug whenever I can ... and be very careful of fuel vapour spoiling the party - nice way to kiss your green frame goodbye!!
 
TeddyG said:
Based on some things neglected, bodged or done incorrectly to this bike make me question the quality of the supposed “engine rebuild“ that the seller claimed.
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WARNING SIGNS RIGHT HERE!! Understanding that you bought this bike as is and have no prior confirmation of exactly what was done, i would be removing the motor, pulling the top cover and checking that the cam timing is correct - piss easy job that could save you major time and grief. The number of bikes with 'won't idle' issues that have been caused by one tooth out on the cam timing outnumber the ones that have had a simple idle mixture circuit problem. Far more logical than BOTH carbs having exactly the same idle mixture circuit issues.

While the top cover is off, I'd pull the head and barrels and give the whole thing a decent going over. Don't trust the 'was recently rebuilt' line unless you personally know the mechanic who did it. All it will cost you is a few gaskets.
 
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