951 compression test

[SIMPLEARKANSAN]

I have the motor on the bench. New top end. I don’t have the Reed valves installed, temp sensor in head, exhaust or intake. Can I jump the starter and get an accurate compression?

 

[etemplet]

I've never tried that but I don't see what would prevent a decent reading.

 

[SIMPLEARKANSAN]

I know the starter doesn't spin as fast with a jumper cable connected to a battery. I'll wire the starter directly to the battery with correct length and gauge wire and see what happens. It seems to be spinning much slower.

 

[etemplet]

It depends on the condition and size of the battery cables, You'll still get a decent reading. What are you checking that for anyway? As long as you didn't forget the wrist pin bearings it will be good.

 

[SIMPLEARKANSAN]

I just wanted to check compression. It’s a 2000 GTX millennium edition 951. I’ve seen normal is 125, I’ve seen 135, and some people reporting 150. What is normal for a stock 951 silver motor?

 

[SIMPLEARKANSAN]

I think I found my answer. Stock is 135. I read a lot of top ends start around 125 and approach the 135 with break in.

 

[mikidymac]

You are not going to get full crankcase and cylinder filling without the reeds installed so I would expect compression to be lower than normal. But you aren't going to hurt anything by trying.

Perfect compression on a carbed 951 is 130 psi, on a DI it is 135-140.

 

[SIMPLEARKANSAN]

Mine is carbed and I’m getting between 125-130 without reeds installed so I think I’m good. Thanks guys

 

[etemplet]

I'm curious also as to the compression variances I read about as well.