How low can it go?

Bought a 1974 R90/6 this summer. Just a little clean up, some electrical issues, cleaned up the carbs, new battery and it's running strong and starts easily. Did a compression test and was surprised to find both sides in the 80psi range. A I doing something wrong, or can a reasonably well performing bike have compression that low?

Rode it for about 15 minutes. Disconnected and removed both plugs. Placed caps where they wouldn't ground. Twisted the throttle wide open and cranked it over. Both sides about the same, 80ish psi. I read 120 or so means a bad engine. What's up???

I think you have an artificially low reading. I assume the bike has CV carbs. The throttle plate may be open but the carbs need vacuum for the diaphragms to retract the slides. You don't get much vacuum at cranking speeds.

If you don't allow enough air in, there isn't enough to compress and you will get a low reading.

I would redo the tests with the carbs removed and pulled back away from the heads. I think you will be pleasantly surprised by the results.

No 120# would not be a bad engine! Have you adjusted the valves? Not sure but you may have to remove the carbs. Are you certain your test gauge is accurate? 80# is really low! How many rotations did you give it to achieve that figure?

Not sure about the gauge. It's a loaner from Autozone. I cranked it 2-3 seconds until it looked like it wasn't going any higher. I'll try it with the carbs off. How do you keep the engine warmed up while pulling the carbs off? Or is a warm engine necessary?

rcockrum said:

How do you keep the engine warmed up while pulling the carbs off? Or is a warm engine necessary?

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I generally check mine at end of season. Ride the bike till its fully warmed up and then once I get home do the test. I secure the carbs with cord (don't be hanging them by the cables), and loosen the clamps enough to pull the carbs off the inlet spigots to the cylinder. I want to keep the plugs grounded, I leave the one installed on the side I'm not testing and secure the other to the cylinder. You had said you put the caps where they would not ground?? I think thats not a problem for a contact ignition (original on your bike), bad practice for electronic ignition.

I first tested mine like you did, cold and carbs on. The advice from the forum had me do it warm with carbs off and the results were ~ 15 psi higher. But I went from 105 to 120, your first result of 80 does seem low.

best test is a leak down as I understand it to give an overall health of the top end. It is surprising how much compression you can get with leaky rings and valves, on my 525 still had high compression but everything was simply worn out with greater and 60% blow through where 5to10 is max. A leak down test needs a 100psi source and a tester ~$150. I have yet to check the R100GS yet but is what I use on my enduros which are easy to lock the crank at TDC, the R100 I have yet taken the time to figure the best way to lock it down... any suggestions? Now that I have a tester it is what I plan to use to monitor the engine health once or twice a year.

grahamm said:

I have yet taken the time to figure the best way to lock it down... any suggestions?

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This was an eye opener for me the first time I used my leakdown tester...bought at Harbor Freight for about $50...nice unit. Pressurized the cylinder...thunk...pushed the pistion right to BDC. What I ended up doing was rigging my 10mm wrench onto the alternator bolt and tape it to a frame member somewhere. It's not much force, so just needs a little restraint.

I'm not sure I'd use a 10 mm wrench on the alternator bolt as a crank locker. 100 psi puts a lot of force on the piston and, if the crank is much past or before TDC, could put a lot of torque on that little bolt.

I'd get the rear wheel off the ground, put the trans in high and roll the piston up to very nearly TDC, get a helper to apply the brake and turn the air on.

rcockrum said:

Not sure about the gauge. It's a loaner from Autozone. I cranked it 2-3 seconds until it looked like it wasn't going any higher. I'll try it with the carbs off. How do you keep the engine warmed up while pulling the carbs off? Or is a warm engine necessary?

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Just about any gauge's designed for working on car engines don't work well on the smaller cylinders of a motorcycle engine. Put some oil down the cylinders and do it wet and see if there is a difference.

If the bike has sat for awhile then the compression is going to be low. Ride it for several hundred miles and then check again.

If it pulls good, plugs are a good tan colour, starts easily ( doesn't crank a long time before starting) then I wouldn't worry about it.