Dead Spot at 3100 RPMs

My boat has a 1993 Ford 351 engine (Carburetor). It runs great except when I get into the 3100 to 3200 rpm range and then I hit a dead spot (runs extremely poor) once I get above 3200 rpms the engine runs well again.

Is this a carburetor problem or an ignition issue? Any suggestions would be greatly appreciated?

skilew

Sounds like your secondary diaphram and or spring may be bad.Your secondaries may be trying to open too soon.

I had a similar problem with my 93', I had the Pro-Boss motor, mine was related to the Pro Tech coil pack, it became worse the hotter the boat got. Replaced Pro-Tech with new electronic distributor.

79nautique wrote: Well then if you don't think it's the carb your foolish because that's about the only thing it can be at that RPM and if the person who rebuilt it isn't very good then odds are it's the carb and it isn't adjusted correctly and needs fine tuned a little. It sure sounds like a secondaries issue. At 3100-3200 rpm the advance should all ready be maxed, it could be a fuel pump issue but that wouldn't be limited to such a specific rpm. The carb transitioning would be the most likely issue. Now if it was a computer controled engine that would be a little different, but it's a carb. When the carb was re-built de they replace all of the diaphrams? I don't believe all needed come in the kit and you have to purchase them separately.

Sounds like good advice to me.

You can also likely rule out the pro-tech coil packs causing the problem because I dont believe he has a Correct Craft.

Here is some additional info.

This is the Pro-Boss motor, I have removed the Pro Tech coil pack and replaced Pro-Tech with new electronic distributor two years ago.

The carb was rebuilt this season by National Carburetors. They have a very good reputation on rebuilding carburetors for marine applications. It wasn't cheap $200.00 but everything was replaced, bench tested and a live engine test. This is still not to say that they could have missed something on the rebuild. They live engine tested it but not under a load.

There is one other thing that the engine will do that may or may not be part of this surging problem at 3100 rpms. If I run the boat at WOT and then bring it back to idle it wants to idle at 1200 rpms vs the normal 650 rpms. The only way I can get the idle back to 650 rpms is to cut the engine off and then restart and then it will idle at 650 rpms.

Thanks again for all the help.

Now that I hear the whole story I have to vote with 79. Sounds like an intake problem of some kind, carb, sticking pcv valve or a bad vacuum leak resulting from loose or non-fitting parts somewhere. Check your hoses, pcv valve, and make sure you carb is bolted down tight. I would also torque the intake manifold bolts.
If all is tight and you don't have a vacuum leak, it must be the carb. Just because the carb is fresh does not eliminate it - in fact just the opposite. Maybe you can disconnect the secondaries and see if you still have the problem.

If your holley has a metering block for the secondaries then focus on that. With the RPM your seeing somethings wrong, sometimes the carb cleaner test doesn't work also. Double check for flatness on the spacer plate you should be able to remove the carb and not hurt the gasket since you just replaced it, unless you glued it down. Also most PCV valves ports are smaller than the hose for the baseplate so put a hose clamp on it if there isn't one already. What is you timing advance like, around 8 at idle then you want to see it maxed by 2500-2800 rpm in the low 30's if it still increasing then you need stiffer spring on the mech advance. Seems really odd having an issue backing off the gas? how slowly are you moving the throttle when this happens? if your featering the throttle slowly then it just maybe you causing it and feeling the curcuit transition in the carb. Another thought that power valve do you have? is it a 6.5, 2.5 or something else?

Jim I'm like you and need to look but I believe your right, might even be a loose screw on the diaphram sucking some air. Diffenently check the linkage to everthing, something must be in a bind. Seems like the only think that could cause the slow down issue.

might make a couple of reference marks with a sharpie to use as a starting point or have some one else go and have the motor box open and watch as they drive, just stay clear of the pulleys and do it early when the water is smooth.

I have heard that the carb bases can wear allowing the butterfly shaft to wobble too much. It seems I have seen a rebuild kit or procedure for those worn bases. I can see where a carb rebuilder could let that slip through the crack and not catch it.

I went over this with my 92 earlier this year. About 3k or so, it would start to stall out and sputter and then possibly backfire through the carb. Did this out of the hole also, but through the rest of the range it was fine. I came here, and got some help... tried it all.. had my carb rebuilt, still not right, but better. I ended up tracing my steps back to when I pulled it out in the spring. In the spring, this when I changed my plugs, (after I burn off the fogging oil, then put new ones in) I got new plugs from SkiDim, what they told me. I didnt pay any attention to the ones that were in there, I pulled those, put the new ones in and hit the water and had the issue. So tracing my steps back, I changed to the correct plug and everything was fine. No problem throughout the RPM range and even ran better now.   
I think the ProTech is very picky about everything from voltage it is getting, (if your stereo is sucking the life out of the 12v system, it may screw with your ProTech) to the plugs you use. In my case, it was the plugs...

So you may have a bad plug in the V8. Pull them out and check them all... better yet, just replace them. It's easier and makes your CC happy again

If the boat ran fine after installing the distributor the springs should be OK unless one of them came off or broke. Just put your eyes on them and see if they lok OK. I am not familiar with your distributor, but if it has a centrifigal advance you should be able to move the advqance plate by hand and watch the springs return it when you turn loose. You will have to take you distributor abort to do this but you can leave it in the engine.

To eliminate the distributor as the culprit you may want to pull it and take it to a distributor shop to have tested.

The problem could still be carb related though.

Have you tried running the boat with the secondaries disconnected? It should run well without the secondaries connected - it will just have less power.

There is a small c-clamp that holds the vacuum dash pot arm onto the secondary linkage. Just remove that c-clamp and pull the vacuum dash pot arm off the secondary linkage and let it hang free. Your secondaries will remain closed under all conditions. This will allow you to run on the front two barrels and see what happens. The carb should perform the same as a small 2 barrel. There should be no dead spot.

Like 79 said earlier, the secondaries could be coming in too soon, too late, or too suddenly. The rate at which the secondaries open is determined by the spring located inside the vacuum dash pot. You have to remove 4 screws to get to that spring. The color of the spring tells you the rating of the spring. Usually you don't have to worry about the spring but you said your carb was rebuilt so there is not telling what they did - no matter how good of a shop they are. There is a small port that allows a vacuum to be pulled on the dash pot. If that port is clogged you could also have problems with the opening of the secondaries. You had a problem with the carb not returning to idle. It could be problem in the vacuum dash pot hanging the secondaries open.

(I have been calling the vacuum actuator a dash pot because I do not know the proper name.)