Can I do a Compression Check with the Risers Off and no Hose Connected?

[KM7]

Can I do a Compression Check with the Risers Off?

5.8 Ltr Ford. I removed the risers (Elbows) because I will replace them. (See other post "Overheating Possibly Caused by Restricted Thermostat Housing Port")

I'd like to do a compression check before I spend a lot of money. The boat has been running fine, but I figured I might as well. Here are the issues: Since the risers are off, should I NOT have the hose connected? I'm not worried about heat because the engine will not be running since all the spark plugs are out, but the impeller will be spinning dry. Is that a problem? It only takes maybe 4 revolutions to get full compression. There is another issue: the engine will be cold, so I will not get the full compression. If I did this, it would only be to check for very big differences in the compression between cylinders. What are your thoughts?

Thanks

 

[alldodge]

but the impeller will be spinning dry. Is that a problem?

It can be, I would disconnect the pump or remove the impeller to be safe

Cold is better than nothing and should provide some insight

 

[Scott Danforth]

with the risers off, you should NOT have a hose connected.

easy to pull the impeller on a cobra. or just pull the cover, squirt in some soapy water and put the cover back.

 

[KM7]

Update: I did the compression check: All spark plugs Removed, the lead to the coil disconnected (and protected with electrical tape). No water connected. Throttle wide open, and carb held open with a screwdriver. I left the fuel pump connected.

Around 6 cranks of the engine to get a steady reading:

1 - 150 psi , 2 - 150 psi , 3 - 150 psi , 4 - 150 psi
5 - 149 psi, 6 - 141 psi , 7 - 140 psi , 8 - 145 psi

Max difference: 6% OK

 

[KM7]

I'm planning on changing the impeller anyway, so if spinning it dry for a few turns hurt it, no big deal