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skilew
09-25-2006, 09:30 AM
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

92 190 PS
09-25-2006, 10:07 AM
A buddy had a similar issue on his 93 and chased if for a couple weeks...I think there is even a post on here for his...In his case it was the check valve out of the gas tank that was clogged...There is a 90 degree elbow on the top of the tank (where the fuel line comes out of the top of the tank) that and for some reason this was an issue for a couple folks on this site....He cleaned and flushed that and boat has run well ever since. Simple and worth checking before you go through anything else...

east tx skier
09-25-2006, 10:11 AM
Interesting about the check valve. My boat has a flat spot at 2,175 rpms. If I run it there for a few seconds, the rpms will drop, then return repeatedly. You don't feel it under normal accelleration. I'd wondered if it was a vacuum leak.

92 190 PS
09-25-2006, 10:11 AM
Here are the links to his issue...Good luck

Starving for fuel thread....
http://www.tmcowners.com/teamtalk/showthread.php?t=10801

Fuel pump thread...
http://www.tmcowners.com/teamtalk/showthread.php?t=9783

skilew
09-26-2006, 02:08 PM
I removed the gas tank check valve and it appeared to be clean and work properly. I recleaned and installed but it did not fix my problem. Should I go ahead and replace this check valve anyway?

skilew

east tx skier
09-26-2006, 02:17 PM
How are your gaskets (don't know that this is a problem, but wondering since I suspect I may be due for a switch and it might be causing my flat spot).

LakePirate
09-26-2006, 02:27 PM
Does the 93 still have points?

east tx skier
09-26-2006, 02:31 PM
Electronic ignition was standard in 93.

skilew
09-26-2006, 07:42 PM
Does not have points and gaskets are in good shape.

Jim@BAWS
09-26-2006, 08:22 PM
That 3100-3200 PRM range is where you secondaries are susposed
to open kikcking in the 4 barrels. Run the boat with the motor box
open with the flame arrestor off and see if they are kciking in

Jim@BAWS

skilew
09-27-2006, 02:25 PM
The boat runs good above the 3200 rpm range and at WOT so the secondaries are kicking in.

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.

This may or may not have anything to do with my initial problem. Just thought I would muddy the waters some more.


Thanks again,
skilew

east tx skier
09-27-2006, 02:35 PM
The boat runs good above the 3200 rpm range and at WOT so the secondaries are kicking in.

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.

This may or may not have anything to do with my initial problem. Just thought I would muddy the waters some more.


Thanks again,
skilew

Do you have perfect pass? My boat did this with the idle because the throttle cable was binding. Make sure the cable is as level as possible.

skilew
09-27-2006, 02:40 PM
I do not have perfect pass.

RickDV
09-27-2006, 02:53 PM
Don't mean to threadjack here, but I had a similar problem on a previous boat. It was a stern drive that would hesitate only at a certain speed and only when the trim/tilt was at a certain point.

Turns out that the wires running to the trim limit switch on the lower unit had insulation wearing off the wires. At that speed and trim water sprayed onto the cables and caused brief, intermittant shorts in the electrical system. I replaced the wires and all was well.

My point is to consider odd-ball causes like this. It took some out-of-the-box trouble shooting to figure my problem out.

Good luck!

Hunterb
09-27-2006, 04:06 PM
Check the secondary float level as well. Too low could cause a hesitation as you describe.

Bruce

rcnjson
09-27-2006, 04:35 PM
Arm chair expert here to chime in my $.02 worth. I think it could be carb/linkage related or timing related. Not having experienced this dead spot I can't say for sure. Carb or linkage, if the linkage is binding some where, could be binding or sticking from rust in the cable, or a cable that is stretched, there could be a problem. There are differences between mechanical and vacuum secondaries, we'll focus on vacuum for the time being. The secondaries cannot open unless the throttle is advanced far enough. Also, the secondaries cannot close until the throttle is returned back to the idle position. If the secondaries were still open when your throttle lever is back at neutral the engine will idle higher. More air and fuel are entering the engine and it will idle higher. But you say yeah but the problem goes away when I shut it down and start it again. I would say that this is consistant with my thoughts. Vacuum is required to keep the secondaries open. When you shut the motor off, the vacuum is lost and the secondaries will close and still be closed when you start it again. You could check this quickly. Run it with the dog house off, flame arrester off and look at the secondaries when it is ideling normal. Then run it up and bring it back down. If it is ideling high, are the secondaries still open? That is one possibility. Timing is the other. Same deal here something could be sticking or binding. The weights in the distributer determine the timing advance for the motor. If they are sticking, the advance may not happen at the RPM it is supposed to when you accelerate, and it may hang up and keep a higer RPM after you returned to idle postion. Either type of iginition, electronic or points, uses weights for advance curves. If it is me, I'm checking the carb first, that is a 5 minute confirmation run out on the water. If that checks out I'm focusing on ignition. That is me, the arm chair expert, keep in mind I haven't been in your boat so I could be way off.
k

skilew
10-03-2006, 04:12 PM
Ran boat this weekend and checked the linkage on the secondaries. Did not note any binding but also could not get the boat to duplicate the high idle problem.

But problem still exists with dead spot between 3000 rpm and 3200 rpm.

How do I check the distributer weights. Remove cap then what?