Logo
FULL SITE MENU

Anyone want to touch this 1

J

jeremy421

Member
"1975 mercruiser 302,rebuilt e

"1975 mercruiser 302,rebuilt engine with new stock crank, Summit racing camshaft, lifters, eddlebrock intake, holly 4 bbl marine carb, Mallory distributor w/ pertronix conversion kit, heads have been worked, valves all good. this runs great in the driveway and has for some time while i have been working out other bugs with trim, throttle cable linkage, impeller, and whatever other odds and ends. Took it to the lake today to test drive it and soon after it was in the water i noticed a rocker tapping (which i am sure i can adjust) but the main issue i am having is popping back through the carb at near full throttle. Has anybody ever switched to an automotive performance cam??? Should the timing still be at 12 BTDC??? This is a 1975 boat the engine block is an 87, and the crank is Pre 1982 but the sling was removed so it would fit in the 87 block, this was done so i could keep the orig harmonic balancer and flywheel. Any help on the cam or other reason for backfire would be appreciated. Maybe this carb is starving for fuel??? I have installed a restricter to keep it at 6psi. manufacturer recomends 4-7 psi so this should be OK. I am hoping for a timing issue. Maybe it was a bad idea to use an automotive cam, but i wanted more power and the smart guys at Summit convinced me this was OK. Help.

Thanx Jeremy"
 
"Jeremy, The marine cams used

"Jeremy, The marine cams used in Mercruisers are similar to truck (i.e. S-10 Pickup) cams. Probably a high performance auto cam (if it is tuned for High RPMS) is a bad idea. You might like to look up the timing settings for the Mercruiser. My GM V-6 is more like 4-6 BTDC. Does Pentronix have a recommendation?"
 
"Pertronix recomends adding .0

"Pertronix recomends adding .05 to the spark plug gap, by the mercruiser book my timing should be at 12 BTDC, summit recomends 10, i think i'm going to try 8 and see how she does, but it appears that i may have a vacuum leak, with a guage on the intake i'm only at like 13 inches of vacuum at 1000 RPM's. Damn i need this done by this weekend>hahaha. Oh Chris... this is a high performance RV cam with a operating range of 1000 to 4500 RPM's so it's more for low end torque, a little added lift but mainly more duration. If anybody has a painfree way of checking for vacuum leak i would sure love to hear about it.

Thanx

Jeremy"
 
Sounds lean with a touch too m

Sounds lean with a touch too much timing.
If using stock exhaust through the drive try the semi-closed exhaust boot from the transom mount to the drive if you haven't.
RPM to high?
 
"Jeremy,

Don't discount

"Jeremy,

Don't discount the chance that your lifter tap and your pop are related. You might want to resolve that issue first. You might find that the problem is one and the same.

Good luck
Rick"
 
Back
Top