Nc750 compression figures

[MB5 2]

Hello, I have a 2015 dct with 48000 miles on it, will be doing a oil and filter service soon and checking valve clearances etc. What figures for engine compression tests should I be getting? What would be good readings?

[Rocker66 34,437]

Hi MB5 welcome to the forum . I’m sure somebody better qualified than me will be along to answer your question.

Would yo care to tell us where you are located ?

[MB5 2]

Billericay, Essex, UK

[Slowboy 20,451]

Hello and . 
The manual quotes 257psi, at 450 odd revs (starter motor speed?) anything over 220psi should be ok.

 

If you join as a member (via the donate button to Ted) that might help your need for a manual 😉

 

Hope that helps anyway.

[jeremyr62 2,557]

Holy crap, that's high compression. My old VF500 was around 125 psi on a good day.

[Slowboy 20,451]

1 hour ago, jeremyr62 said:

Holy crap, that's high compression. My old VF500 was around 125 psi on a good day.

I thought that as well. As a working figure I’ve always used anything over 135 to 140 psi is ok. I’ll have another look in the manual, but the figure is as I’ve stated.

[Slowboy 20,451]

Page 1.8 of the manual

“Cylinder compression at 470rpm - 1,775 KPa (18.1 kg/cm2 257psi)” no service limit specified.

 

The same figure is given in chapter 10, page 10-6 compression test.

 

higher than I’d expect but there you go, it’s what the manual specifies.

 

 

Edited January 16 by Slowboy

[Xactly 5,399]

I wonder if it’s because the motor is long-stroke ie doesn’t rev much but the piston travels a long way relative to the bore size. I suppose also that modern bikes with EFI generally run much higher compression ratios than in the past. Certainly 9:1 on an old Brit bike is on the high side. Or perhaps it’s because the reading is taken at 470rpm? Who knows, but it seems high. Whatever, piston ring/ valve wear is rare on these bikes anyway

 

[jeremyr62 2,557]

Diesel engine territory.

Explains a lot