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Old 08-09-2018, 09:52 PM
slyrydr slyrydr is offline
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Default 4.3 efi mercruser engine

so i buy a slightly used (140 hrs) boat, very clean...has 2 4.3efi merc engines, drive it around the lake near where i live, everything checks out....change out all the filters, gas, oil, outdrives, etc....take it to Lake Powell, hot there, day temps around 100 F, everything goes well first day....second day, i notice one engine tachometer fluctuating from 3800 to as much as 6000 rpm...not the engine itself, just the tach, no problem i figure, just a connection somewhere...short time later i start getting some serious pops (backfires) out of the throttle body, pull power back, backfires go away, power back up, runs for about 5 minutes, then same thing....runs good at idle and up to 2400 to 3400 rpm, then problem repeats itself depending on how long its been running...runs good in the morning when its cool, but as day gets warmer, problem repeats....anybody with 4.3 or similar problem on another engine have experience that might give me a place to start ?....i'm thinking electrical, but not plugs, wires, or rotary cap because for first hour engine runs great, gets full rpm, but after cruising for hour or so, problem begins as described...any ideas ?
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Old 08-10-2018, 05:30 AM
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The heat described in scenarios leads me first to detonation.
What grade fuel are you running?
Try some octane booster.

Just an amateurs opinion.
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Old 08-10-2018, 10:29 AM
slyrydr slyrydr is offline
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premium fuel from name brand station...thanks for input though
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Old 08-10-2018, 11:45 AM
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Does this have an HEI distributor?
Could be the coil or moisture in the cap.
The coil sends the signal to the tach unless u don't have a distributor, then tach signal would come from a crank sensor.

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Old 08-10-2018, 06:31 PM
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has distributor, backfiring reminds me of a lean condition, but it's definitely related to temperature of running it for 45+ minutes
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Old 08-10-2018, 06:35 PM
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Could be fuel pump getting hot and not producing enough pressure?
Hook a pressure gauge on the fuel rail and check it while driving.

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Old 08-10-2018, 10:15 PM
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Quote:
Originally Posted by summ8rmk View Post
Does this have an HEI distributor?
Could be the coil or moisture in the cap.
The coil sends the signal to the tach unless u don't have a distributor, then tach signal would come from a crank sensor.

has distributor, the coil comment has merit i think, will be looking at that angle....the fuel pressure also...thx
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Old 08-11-2018, 06:28 AM
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The backfire is being caused by the engine being out of time. Either the distributor timing is way off, you have a worn distributor or possibly it has jumped time on the timing chain. In any case, backfires on a boat are a good way to blow the whole thing up.
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Old 08-11-2018, 12:14 PM
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yup....that's what i'm trying to prevent
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Old 08-11-2018, 12:41 PM
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Default

I would agree on the timing. I have heard of guys trying to advance timing on those motors....maybe someone has done that and gotten carried away? Maybe it's on the edge and presents itself more when hot? Total guess.

On another note....I have that same motor in my boat and the tachometer does the same thing, but never had a running issue. Some times if it's acting up I can tap it and it fixes itself. So the 2 issues may be completely unrelated.
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Old 08-11-2018, 12:45 PM
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Old 08-11-2018, 04:29 PM
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Old 08-12-2018, 09:22 PM
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Old 08-22-2018, 05:32 PM
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Old 08-22-2018, 08:44 PM
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Old 08-23-2018, 02:55 PM
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