5.7L 1984, Rough idle at start up, then ok after a minute or so

[flashback]

A compression test on the cylinders when it's cold may give you an answer.

 

[tfret]

A compression test on the cylinders when it's cold may give you an answer.

Ahh that’s a great idea. Thanks

 

[tfret]

A compression test on the cylinders when it's cold may give you an answer.

Yes I adjusted the valve lash when I replaced the heads a few years back. I have seen the technique you’re talking about and yes that does sound messy. But I may not have a choice. Thanks

 

[Drcoffee]

Collapsed lifter sounds plausible. How does the battery negative (black) cable look? If it gets corroded that can reduce voltage from the battery until the alternator can overcome the voltage drop. It can also stop the battery from getting fully charged, and reduce power to spark plug on start up. Those electrical gremlins can be hard to find.

have you checked the battery? It could have a dead cell. The battery runs the engine until the alternator spools up.

 

[tfret]

Collapsed lifter sounds plausible. How does the battery negative (black) cable look? If it gets corroded that can reduce voltage from the battery until the alternator can overcome the voltage drop. It can also stop the battery from getting fully charged, and reduce power to spark plug on start up. Those electrical gremlins can be hard to find.

have you checked the battery? It could have a dead cell. The battery runs the engine until the alternator spools up.

Battery terminals are clean and tight. Also the battery is only a few years old and I always get the best one available. 14 volts when idling. Yes electrical bugs can be a buger. I’m an old EE with a lot of troubleshooting experience. I chased that about as far as it goes thinking it was an ignition thing initially. But Electrical bugs don’t typically go away after the engine gets hot. As soon as I get the opportunity I’ll do a cold compression test. At least I’ll know if the valves are holding tight. Then I’ll probably have to run it without the valve covers to see how they move.

 

[Scott Danforth]

Most likely leaking Q-jet welsh plugs and misadjusted choke

Would explain the black soot

 

[nola mike]

Doesn't explain the tach though. I still think you're misfiring. Did you ever do a cylinder drop test? Also, I'd throw a timing light on each plug wire while it's misbehaving and make sure you're getting a consistent spark to each cylinder

 

[tank1949]

Most likely leaking Q-jet welsh plugs and misadjusted choke

Would explain the black soot

Quads are bad about leaking or dripping. Been there. I also had a plug leak one time and just pitched carb. But, that was back in days when they were a dime a dozen. If all else tests fails, boater may have to install an Elderbrock (sic).

 

[Scott Danforth]

Doesn't explain the tach though. I still think you're misfiring. Did you ever do a cylinder drop test? Also, I'd throw a timing light on each plug wire while it's misbehaving and make sure you're getting a consistent spark to each cylinder

bad plug wires and condensation would account for misfire, however not the tach.

only way tach can go crazy, is if the motor is going crazy or the signal from the coil is junk.

@tfret being a 1984, have you serviced the points?

 

[nola mike]

only way tach can go crazy, is if the motor is going crazy or the signal from the coil is junk.

@tfret being a 1984, have you serviced the points?

Motor going crazy?

 

[Scott Danforth]

Motor going crazy?

RPM hunting rapidly

 

[nola mike]

RPM hunting rapidly

Well, that's the tach acting as it should. And obvious. Doesn't sound like what's going on here.

 

[flashback]

I experienced a crazy tach and a tip I picked up in a thread recommended I turn the selector switch to clean it up. So far so good.

 

[nola mike]

I experienced a crazy tach and a tip I picked up in a thread recommended I turn the selector switch to clean it up. So far so good.

Yes, but in that case the engine runs normally. Wonky running + wonky tach = missing spark until proven otherwise imo

 

[Drcoffee]

Might be worth swapping in a known good ignition coil. It might be shorting internally. And the suggestion of using the timing light on each plug wire Is a good idea. Could also be a bad plug wire. Sounds like a electrical issue

 

[Lou C]

Quads are bad about leaking or dripping. Been there. I also had a plug leak one time and just pitched carb. But, that was back in days when they were a dime a dozen. If all else tests fails, boater may have to install an Elderbrock (sic).

Nick’s pronunciation lol!

 

[tfret]

Ok finally spent a full day on it before winterizing. Ruled out a lifter problem as there is no tapping at all. Ruled out stuck valve as compression test shows good on all cylinders when cold. Used a spray bottle to mist fuel into carb and nothing changed so it’s not a fuel delivery carb problem. The tack gets its signal from the coil. I think it’s an intermittent ignition problem. Maybe the coil. However it could be a flakey ignition sensor in the distributor. I’ve had it fail twice. Both times it just stopped dead and the ignition would not spark. Could be this time it mostly works but is going bad. I think it’s one of those two components. I forgot to mention it has Thunderbolt IV electronic ignition. I’ll change the sensor first and go from there.

 

[Scott Danforth]

I would test the sensor prior to throwing money at it.

 

[tank1949]

bad plug wires and condensation would account for misfire, however not the tach.

only way tach can go crazy, is if the motor is going crazy or the signal from the coil is junk.

@tfret being a 1984, have you serviced the points?

Or a loose nut on coil or gauge... Been there too!

 

[achris]

.... I’ll change the sensor first and go from there.

Is it the old 2 wire sensor or the new 3 wire? (The old 2 wire have unsealed circuit board and the solder connections do get corroded, and could be getting resistive when cold, bit of heat and they conduct better. That would explain the tacho too.

Chris.....