Marine: Boating, Sailing, PWC, GPS and Navigation - 1986 Mercruiser 4.3 L misses when hot

I have a 4.3L with thuderbolt ignition and it uses the 4 barrel Rochester carb.(205 HP)

The boat runs real fine for about an hour and a half, I live in very hot Louisiana. Then she starts to backfire through the carb, especially when accelerating. If one operates the trim system it immediately backfires. So I know it is electrical. Batteries (two seperate) are fine so is alternator. Checked all wiring cannot find a bad spot anywhere.

Starting to suspect coil or thunderbolt ignition. Once before had a problem where she would not run well above 4000 rpm. Needed to replace dist cap, rotor and HT leads. May be same again although present problem only when engine hot and being running a while. This has happened the last two outings. Any ideas?

I would recommend putting a timing light on the #1 cyl lead
and see if you have intermittent spark.
bickfiring in the carb is generally a sign that you are running lean,
but it could be severly out of time. check that the
timing is at about 6* btdc.
the next thing to try is disconnecting the tach lead at the coil
Simetimes the tach can cause this type of failure if
it is going bad.
let us know what you found.

Hi. The timing is fine at 6 degrees. Has not slipped in years. The trouble only occurs when the engine is hot. I slow down for a no-wake zone for about 5 minutes and then it starts. After this first happened a few weeks back, I serviced the carb and checked float level which was fine.

The commonality is to slow down for this no wake zone and then she starts farting after speeding up again and operating the trim tab (current draw?) exaserbates the problem.

The negetive battery lead to engine on one of the two battery banks is a bit weathered but I don't believe this is the problem as it happens no matter what battery bank i use.

Can a coil start to give problems once it gets hot? Original coil 18 years old.
Cab Thunderbolt ignition module do same when it is hot?

thanks

could be the coil. or it could be temperature in the engione compartment
too hot, or vapor lock, check the fuel lines and change the coil.
it sounds like you are on the right trac.
i dont think the sensor would be that intermittent.
but if it was when it failed it would lock the timing at 6 deg.
ie, no advance. so you could check that as a last resort.
good work,

:)

The problem was both the coil and the distributor cap. In the latter the carbon contact at the top of the distributor that contacts the rotor had worn down so bad that a bad contact resulted when the engine compartment got hot.

Thanks for the suggestions.