View Full Version : Engine does not run smooth at low rpms in or out of gear

09-24-2007, 11:35 PM
"I have a 7.4GL volvo penta, 4-bbl holly carb. W/DP-E179 drive unit in a 24' pursuit denali 1996 that I just bought. A volvo certifed mechanic has replaced cap, rotor,wires,props outer bearing in outdrive,water pump,adjusted timing,rebuilt carb,pulled and checked disterbtor checked compression,checked fuel pump,after all of this the boat ran the same. It runs good at high speed 3000rpm or more. but has vibration at lower speeds the fuel pump does make a whining noise at low speed ,mechanic said that was ok.3-plugs fowl out after about 5hrs running time this could be leaking seals on valves but why would the engine not run smooth when the plugs are just put in new and It runs great at high speeds for a few hours.my next step is to have the heads rebuilt and exhaust manifolds checked out. does anyone have other ideas my check is running short. thanks tom"

09-25-2007, 01:08 AM
he should have done a leak down test

09-25-2007, 05:58 PM
I am useing a AC-MR43t plug I was told to try a hoter plug like a AC-MR45t?

captain William Myers 1
09-30-2007, 06:58 AM
Do not change heat range on the plug as you could hot spot the piston and burn a hole through it. The problem needs to be solved in another way. It runs good at high air flow speeds through the carb I would look for a leak around the base of the carb. It would be at its worse when the engine is at low rpm and low air flow speed through the carb, and be better at the high flow speed."

10-01-2007, 06:39 PM
"You might have what is becoming an epidemic problem in older, carbed boats. To wit:

After wasting a bunch of time and $$$ over the last few years, trying to cure a flooding carb, I now find that new fuel pumps are so poorly made that they put out WAY too much fuel pressure: like 7 1/2 psi. My old Carter AFBs flood above 5 psi, so I've added an Holley fuel pressure regulator. I can now put 10 psi on hte thing without a leak! Something to consider.


PS: Jegs sells them for about 30 bucks."

11-08-2007, 09:51 PM
"I replaced the pick up module ,that didn't do anything for the problem until I put champion plugs in,instead of ac delco plugs. The engine runs like new. I still need to put more hours on it to be sure but I ran out of summer here in Ohio. Tom"