4.3 bogs at 2500rpm

[jeffjod]

I have a 93 regal with a 4.3L, idles fine and runs good up to 2500rpm then bogs and wont go over that. The dealer is telling me I need a C.D.I. unit, just wondering if anyone has had a similar experience. The dealer tells me it is $1800.00 and non-refundable if it does not fix the problem, he will not guarantee that this will repair the boat. Thanks for any help.

[Hazard]

Originally Posted by jeffjod

I have a 93 regal with a 4.3L, idles fine and runs good up to 2500rpm then bogs and wont go over that. The dealer is telling me I need a C.D.I. unit, just wondering if anyone has had a similar experience. The dealer tells me it is $1800.00 and non-refundable if it does not fix the problem, he will not guarantee that this will repair the boat. Thanks for any help.

___________________________________________________

Hi Jeffjod.

In my opinion only, the principal advantage of a CDI system is the ability to present a superior spark to the air/fuel mixture inside the combustion chamber, thus maximizing burn efficiency. The easiest way to get a bigger spark is to increase the spark plug gap size. However, increasing the gap distance also increases the voltage necessary to ionize the air/fuel mixture. And the resistance of the air/fuel mixture increases as the mixture is pressurized in the cylinder, requiring even higher voltage to spark across a plug. A CDI system provides the higher voltage required by the increased spark plug gap size, thus providing very intense spark.

A CDI ignition system can create spark potential as high as 37,000 volts. Most engines only need about 20,000 volts for reliable ignition. The stock system begins to 'droop' as the rpm goes up. At higher rev's, the spark voltage becomes more and more marginal, averaging about 18,000 volts. With a CDI system, the step up process is very fast compared to a conventional 12-volt induction. This assures a more consistent spark delivery across the plug gap, even at very high crank speeds (rpm).

Note: The consistently higher voltage will cause neoprene-type insulation to break down rather rapidly. You need sparkplug wires with better insulation, such as silicone or non-metallic.
If the dealer tells you cough up $1800.00 and no garantee, I would be very careful.

[Frosty]

jeffjod --does the engine free rev above 2500 (out of gear)? if it does, its not the cdi.

Have you just bought the boat? Is it the right prop?


When did it start doing this?

[jeffjod]

Thank you for the replies guys, much appreciated.
This motor consistently bogs at 2500 in neutral or in gear. Last year it was an intermittent problem, the person I bought it from said he had got a batch of bad gas a couple of years prior to selling it to me, but had that problem fixed and was working good. The marine shop said they put the timing light on it and could see the mark fine until it hit that 2500 RPM and then the mark was jumping all over the place. I am wondering if the CDI unit also works as a rev limiter as well as premium spark. Another thought I had was putting a distributor in this engine and tossing the CDI.
Thanks again.
JJ

[marshmat]

For one thing, a capacitive-discharge ignition box on a 4.3 is NOT worth fully half of what the motor goes for on the rebuild market. A second dealer's opinion wouldn't hurt. Everyone and his uncle services the various forms of the 4.3, it's probably the single most popular boat engine out there.
Is your unit rigged with the Merc Alpha drive? The same engine can be seen in many different configurations.
A big part of the reason for the move to CDI in both outboard and inboard engines was that it cured the slow, inevitable deterioration in performance caused as the old contactor points wore down. Of course, that slow deterioration has thus been replaced by a new ailment, the sudden and complete failure of the whole darn thing. It seems odd that a CDI would go out only at a particular speed though; I was under the impression that when they die, they go flat-out dead in about a microsecond.

[jeffjod]

I took my timing light and checked it out for myself, at idle timing is right on spec, 8 deg and advances fine as rpm's increase until about 24-2500 rpm and then it starts to retard and the more throttle you give it the more it retards. To me it seems like an improper input into the cdi, or ignitor box. Does anyone know the purpose of the shift interruption switch, this is one of the inputs into the ignitor, i'm wondering if its not giving it the wrong input and limiting the rpm's.
Thanks
JJ

[Frosty]

A proper dealer would solve this problem by 'substitution' (Plug in a new one and try it).

Unfortunately the man in the street doest have new ones to substitute.

I would do what marshmat says and find another dealer-- and another after that.

[marshmat]

Does anyone know the purpose of the shift interruption switch, this is one of the inputs into the ignitor, i'm wondering if its not giving it the wrong input and limiting the rpm's.

This switch cuts off power to the ignition for a few milliseconds while the clutch dog/cone is in the process of engaging a gear. If a power stroke were to correspond with the point where the clutch is only just beginning to make contact, the sudden pulse of torque would take some metal off the edge of the clutch. Do that a few hundred times and the gearcase is shot. It's just an on/off switch, it either works or doesn't- almost certainly not your problem.
Get another dealer's opinion. Complex CDI problems are beyond the backyard mechanic.