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Engine backfiring above 3000 rpm

goatneck

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"I have a 1993 four winns sund

"I have a 1993 four winns sundowner with a 5.8 liter (351 ford). It has the spitfire ignition with a 4 barrel.
It runs perfect at an idle and when going thru the harbor at 5mph. Once I get it up on plane and above 3000rpm it starts to ping then back fire thru the carb.
I have replaced the fuel filter/seperater twice. I have also replaced the anti syphon valve as well as rebuilt the carb. I just recently changed the cap and rotor and today I took it out with the same proplem. The old anti syphon valve was clogged and I thought that was it,but no. Need help."
 
The timing was never moved. pr

The timing was never moved. prior to this the boat ran perfect and the timing was never touched. I will double check. Is it possible with the clogged anti syphon valve the fuel pump started to go because of lack of fuel to it? I also have that spitfire ingnition so the only adjustment I can make is base timing.
 
"I wonder if you might have a

"I wonder if you might have a defective knock sensor in there? I pulled this out of the manual

A cylinder head-mounted knock sensor monitors for detonation or preignition, gradually retarding spark advance until the condition is eliminated.
Once activated, the system will maintain the retarded advance until the engine speed changes by more than 50 rpm. Once this happens, the system will gradually increase advance until the normal spark curve is achieved for the engine speed and operating conditions.
The system program provides two separate variable spark advance curves for the ignition module. The normal curve is for 89 AKI octane fuel and it uses the knock sensor to protect the engine and modify the curve in the case of lower octane fuels. There is also a default curve for 86 AKI octane fuel that works to protect the engine when lower octane fuel is detected and or in the event of a defective knock sensor.
The system utilizes a breakerless electronic ignition system which requires no adjustments and almost no maintenance."
 
"If not mistaken that engine h

"If not mistaken that engine has a knock-sensor at the rear of port cyl head, pos #37. Dismantle, clean off properly and reinstall, alternative change.
A naughty way of testing: attach a timing light, rev up engine to about 3000 and knock the head with a small hammer close to the sensor. The timing should drop.

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The diagram looks accurate. I

The diagram looks accurate. I called the local OMC dealer and they said that there is not a knock sensor on that motor.(1993 5.8liter 4 bbl carb motor with the spitfire ignition)
On the diagram Part # 30 34 and 37 are what? Is that a crank trigger and harmonic balancer?

Is a bad or lazy fuel pump out of the question?
 
"If you truly have the EEM &#4

"If you truly have the EEM (electronic engine management) ignition and a carb then this should be your wiring diagram and it should have the knock sensor. The only other 2 options for that year would be a 5.8 with carb and electronic ignition or a 5.8 with EFI.



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Yes Sir you are correct. I jus

Yes Sir you are correct. I just called a different OMC dealer prior to receiving your email and I was mis informed. I will check that sensor tonight. Also do you know where the timing sensor is located? I want to check that as plan B. Thanks for your help. My local dealer has mis lead me twice now.
 
"I haven't seen one myself

"I haven't seen one myself but the book says....

TIMING SENSOR
The timing sensor is mounted on the upper flywheel housing, on the Port side. This is a very simple electronic switching device, receiving power and ground through the EEM module. Two Hall Effect switches inside the sensor sense the timing ring movement and send an electrical impulse back to the module which uses these signals to control ignition svstem operation. Removal and installation of The sensor is a snap, but is also detailed in the
appropriate Engine Mechanical section (usually in the Flywheel Coupling section.

You can also see it in Morten's diagram above item # 25, 37 is the knock sensor and 30 is the timing ring."
 
"If Your engine is running on

"If Your engine is running on all cylinders, most likely not the timing sensor.
However I had a few cases in my time where the knock sensor made too much trouble, and we disconnected it. Drawback: Engine had to be run at min 98 Octane fuel!! What happened was that when running hard in choppy seas the banging on the hull was recorded by the timing sensor and all of a sudden the rpm dropped whenever we hit a wake! With to days fuel quality, You may not run without it."
 
The banging on the hull was re

The banging on the hull was recorded by the timing sensor? Does that noemally happen or only when the Knock sensor is disconnected? I cleaned the knock sensor last night but did not get a chance to take it out. I think I may buy a new one just in case. Anyone know about how much they cost?
 
"It happened when installed no

"It happened when installed normally, but mainly in a special boat brand!
The sensor must be removed and threads cleaned. It is important it has metal-to-metal contact to work properly."
 
I just called the OMC dealer a

I just called the OMC dealer and they said the knock sensor is no longer available. Yikes! Any Ideas fellas?
 
OK I am going to bite the bull

OK I am going to bite the bullet and oeder this part. Because it is discontinued the cheapest I found it was $113. We are sure if this part was bad it would ping then backfire thru the carb?
I still have the fuel pump in the back of my head... If I disconnect this sensor and drive the boat whill that tell me definativly that this is bad?
 
"Look above:
Drive the boat a


"Look above:
Drive the boat at around 3000 and tap the head with a small hammer. The rpm and timing should drop. If not, the sensor is bad . Disconnecting and testing will not give You any answers."
 
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