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View Full Version : 86 pcm 351.. what kind of compression numbers


milehigh970
08-31-2013, 09:34 PM
Ok guys. Decided today i was going to figure out my secondaries that dont open. I replaced my carb with a new 4160 at the begining of the season, no luck. I changed out the vacuum diaphram when i installed a quick change cap for the spring. I have tried every spring and its a no go. I have hooked it up to a vacuum gauge and played with the needles but can only get about 12in of vacuum at idle. Today i was playing and noticed i can screw a needle all the way in and it will still run... vacuum leak!! I sprayed some carb cleaner and noticed a small hicup in one spot. Pulled the carb and replaced both gaskets, now i cant duplicate with spray. Still only have 12in of vacuum. I pulled the plugs and did a compression check.. all cylinders around 95.. seems a lil low. Maybe I am just fighting a tired motor? What do you guys get for compression numbers?

cbryan70
08-31-2013, 09:47 PM
Did you do the test with the motor hot?

thatsmrmastercraft
08-31-2013, 10:16 PM
Ok guys. Decided today i was going to figure out my secondaries that dont open. I replaced my carb with a new 4160 at the begining of the season, no luck. I changed out the vacuum diaphram when i installed a quick change cap for the spring. I have tried every spring and its a no go. I have hooked it up to a vacuum gauge and played with the needles but can only get about 12in of vacuum at idle. Today i was playing and noticed i can screw a needle all the way in and it will still run... vacuum leak!! I sprayed some carb cleaner and noticed a small hicup in one spot. Pulled the carb and replaced both gaskets, now i cant duplicate with spray. Still only have 12in of vacuum. I pulled the plugs and did a compression check.. all cylinders around 95.. seems a lil low. Maybe I am just fighting a tired motor? What do you guys get for compression numbers?

Could also be a blown power valve. If you are dumping that much fuel you could have washed your cylinders and have a low compression reading. Pull the primary bowl and remove the power valve. Hold the hex end in your hand and try to turn the small end. If it turns you have a ruptured diaphragm.

milehigh970
09-01-2013, 06:21 PM
Motor was warmed up.. ignition disabled and throttle open. I havent checked the power valve.

Greenwood87
09-01-2013, 09:20 PM
My 351 Windsor in my 89 with a fresh head gasket and completely rebuilt heads has 120 on all cylinders. How many hours are on your boat?

Xjet2011
09-02-2013, 07:11 PM
I believe the vacuum that is connected to the Ignition Vacuum advance increases with airflow. Unlike intake manifold Vacuum that is greatest at idle. Vacuum Advance vacuum is taken from the venturi area of the Carb. After Reading your original post I see you are working on secondary carb issue not ignition advance. Disregard

MikeyOrange88
09-03-2013, 10:44 AM
I pulled the plugs and did a compression check.. all cylinders around 95.. seems a lil low. Maybe I am just fighting a tired motor? What do you guys get for compression numbers?

Last year when I had to replace a cracked head I got approx. 130-132 on all cylinders, except the bad one which was 125. '88 motor with approx. 605 hours on it at the time.

milehigh970
09-10-2013, 04:33 PM
Xjet- marine distributors do not have a vacuum advance. If your boat has a vacuum advance distributor I would look into changing it.

Im not sure really how many hours on it. My hour meter stopped at 415, never have got around to changing it. If i had to guess i would say 600-700 hours. Looks like my numbers are pretty low... i been wanting to do some gt40 heads anyway so i guess ill do a lil refresh this winter ;)

Xjet2011
09-10-2013, 09:02 PM
Thanks. I was just remembering mechanics from older vehicles. Your right I do not have Vacuum advance distributer. Thanks Xjet