1987 3.0L OMC - Hesitation Issues...any ideas?

[bsjcg1]

I bought a 1987 OMC 3.0L 7 years ago from the original owner. The original owner had the engine rebuilt prior to selling (provided documentation). Over the years, the engine has had an intermittent hesitation I have never been able to trouble shoot.

Understanding that I bought a 1987, I have replaced or rebuilt the carb, fuel pump, alternator, ignition wires and ignition kit, (condenser, points, cap, plugs, etc). I pulled the ball check and run with a straight fitting...still hesitates. I have used a mechanic to set the dwell, timing, fuel/air mixture...but continue to have a hesitation when under load. The hesitation is intermittent. For example, I can start out cold and run with no hesitation, come off plane, repower and start to hesitate, typically at 3500 rpm and above. Other days it will hesitate cold trying to plane....and once warm, the hesitation will slowly go away. Some days the hesitation will not allow me to plane. Other days there is no hesitation at all....runs perfect. However, the engine will always start and typically run smoothly 3000 rpm and below.

The filter to the carb is clear. I have checked the fuel pick up in the tank and it is in good condition. The fuel tank was completely drained and refilled with marine fuel only.....no more ethanol, (seemed to help some). I have removed and checked all the fuel lines....all are fine. I have replaced the fuel/water filter every season. The system has the orignal stainless steel fuel line from the pump to the carb.

Hesitation under load continues to point to the engine starving for fuel....but I am out of ideas on how to resolve. I am wondering if the cause is vapor lock or air getting into the fuel line....and if so, how to resolve.

Any ideas are greatly appreciated!

 

[mpdive]

Re: 1987 3.0L OMC - Hesitation Issues...any ideas?

Does it catch up to itself right away after the hesitation? I have the same engine and there is always that lag in accelaration due to it being carbureted. To me that's normal, and hence the reason for fuel injection. I can't think of a car when I was a kid that didn't have a momentary lag. Is this something that just started or has it been like that since you have owned the boat. There is always a momentary pressure differential between the inside and outside of the intake when the throttle is opened. Not trying to discount your problem, but just trying to make sure it's above and beyond what the setup is capable of.

 

[bsjcg1]

Re: 1987 3.0L OMC - Hesitation Issues...any ideas?

Definitely not a lag...but a loss of power like it is starving for fuel typically after initial acceleration. The hesitation is not only at first throttle.....but can happen while on full plane for 20 minutes at 4000 rpm

This problem has been occurring in varying degrees since I purchased the boat....some days it does not do it....other days that is all it will do.

Good to know of the acceleration lag...thanks

 

[Big_Eddy]

Re: 1987 3.0L OMC - Hesitation Issues...any ideas?

Could be a few things.

Check your spark advance. Is it advancing properly and consistently? Hesitation could be caused by timing that is not advancing properly. If you have a timing gun and can check the timing while it is hesitating, that's an easy check.

Could be fuel starvation. Is the fuel tank vented properly? Have mud daubers filled in all the vent holes so that you are pulling a vaccuum in the tank?

Check your plug wires, distributor cap and rotor. A cracked cap or a bad wire could cause the same.

Those are the easy things I would check first.

 

[mpdive]

Re: 1987 3.0L OMC - Hesitation Issues...any ideas?

I have to agree with Big Eddy on the fuel starvation. If this were my boat(I alway's say that when giving advice) I would get an auxillary tank and hook it directly to the fuel pump. Make sure your pump bowl filter is new and your carb filter. Then I would take it out for a ride and see what happens. The reasoning behind this is, you may have an anti-siphon valve going bad coupled with a routing issue of the fuel lines, especially if a water seperator was installed. Why is this a problem? Manufacturers have tried to make the quietest boats they can and by doing so have isolated the doghouses with insulating materials and tight seals. Heat buildup can and does generate vaporlock conditions especially in that environment. Any unnecessary 90's and bends in fuel lines just adds to the problem. It appears that you have been thorough in trying to correct this and by isolating half the fuel system you'll be one step closer to solving it. Anti-siphon valves can be a real pain. Again, that's where I would start.

 

[Bridar]

Re: 1987 3.0L OMC - Hesitation Issues...any ideas?

Do you have a canister style fuel filter? If so, is it a 10 micron filter or closer to a 30 micron filter? The 10 micron filter is better at catching moisture from the fuel, but for some setups it can be too restictive under WOT. If you try a 30 micron filter and the problem still persists, at least you've ruled out one more potential cause.