Compression test reading

[EddieDay]

Hi all. I had a compression test done at my local bike dealer because I thought the bike was a bit wheezy at 55mph. The reading was 75psi. My bike is a 1958 18S, so compression ratio should be 7.3:1. This should give a compression reading of 107psi (7.3x14.7) so 75psi looks on the low side.
I have trawled through posts on compression for the last half hour but have not yet found one that states whet the reading should be. I have read some interesting stuff on cylinder pressures in a running engine, a post that states the pressure should be 120-145psi (I can't see that with a CR of 7.3) and some tips about putting oil down the plug hole.
I am now thinking I need to take the head off and look for bore damage, broken piston rings or valve damage. Before I do that are there any other thoughts from you experienced guys?
A bit more info:
- The bike has done 30,599 miles, which seems to be genuine according to the MoTs I can see on line back to 2006.
- The bike has covered only 313 miles since its previous MOT in 2011. This is borne out by the colour of the fuel I drained from the tank when I bought it 2 weeks ago. I kind of dark honey colour.
- The bike starts quite well from cold and hot. Typically 3 kicks.
- I would not be worried except that I think the bike should get to 60mph more easily than it does.

[1608]

Did you watch them when the garage did the work. Comp tests are usually a guide on multi cylinder engines for making comparisons btn cylinders. The test should be done on a hot and then cold engine, again for comparison. The throttle should be held fully open. If compression is improved when a small amount of oil is placed directly into the cylinder then this would indicate that there is some wear or damage to the bore /rings. If little or no improvement then the problem is more likely to be valve seating. For peace of mind I would inspect the bore and valves. You may improve things greatly with a hone and new rings and or valve lapping. I have no idea what the cr should be, but my 18s holds my 18st on the kick starter for a short period. good luck and give us an up-date, should be interesting.

[EddieDay]

1608, thanks for your reply. Unfortunately I did not see the compression test done. I have just ordered a cheap compression tester and android camera attachment. I will do my own tests following your suggestions and have a look at the bore and piston through the plug hole.
If anyone knows the pressure I should be seeing I'd appreciate the info.

[SPRIDDLER]

Welcome to the club and these forums.

Your garage may be more familiar with leccy start bikes. Our singles need to be kicked over very smartly about half a dozen times.

A general rough 'n ready yardstick that many AMC single owners look for is around 110/120 psi. minimum.
You mentioned yesterday in your first post that the bike is new to you. If so, and it hasn't been run for a while it may be that the rings are a bit gummed up in which case it might improve after about (rough guess) 50 or so miles of spirited riding.

In an earlier post you mentioned greasing. Be aware that the front wheel bearings don't have a 'proper' seal to prevent the grease getting out and on to the brake linings.
A couple of strokes is all that's needed on the headstock bearings (unless you want to fill the entire front frame section with grease ).

[mdt-son]

Not of any help solving your present problem, but if you decide on a rebore or having a valve job done, I suggest you fit an air filter afterwards. AMC offered air filters as an optional fitment, but most buyers didn't order the bike with an air filter, for some reason. Even today many classic bike owners chose to run their bike sans air filter. The amount of grit, dust (which contains solid particles) and other debris being sucked in is enormeous, wearing heavily on the internals of an engine. Even the lube oil's condition is adversely affected. So, to preserve your engine's health, fitting an air filter is your best investment ever.

- Knut

[EddieDay]

Spriddler, thanks for your welcome. I will follow that guidance when I get the compression tester. I have done about 100 miles now so I don't think the cylinder bore will improve much more if it was gummed up. I hope the little phone cable camera will show the condition of the bore. It only cost £5 plus P+P so not much lost if it doesn't work. Meanwhile I received the needle attachment for the grease gun and have greased most points. I spotted the warning about over greasing the front hub on another post.
mdt-son I was surprised when I bought the bike that it had no air filter. I think I will fit a mesh at the very least. At least it will stop grit etc. going into the carb.

Thanks for the help.
Eddie

[moley]

i would think that 75 is not enough , i would have said got to be over 100. take the head off and test off bike andy

[ajscomboman]

My go to number for a twin or single is anything between 110-125psi is healthy and yes it does reach that number or above on 7.5:1 CR 8.5:1 reach 145psi


Most compression readers start the green healthy area at 100psi and up, below that number there is wear within the motor somewhere.