Tachometer bouncing

[SSTKO81]

1981 SeaSprite Mercruiser 140 with Alpha drive, converted from points to EST two years ago.
Recently when I get RPMs over 1800 the tach will begin bouncing as it works its way up to 4K. Simultaneously there is a rattling noise almost like valve train heard from the engine. (Possibly unrelated?) I back off to 15-16k and all goes back to normal. Thinking this could be an issue with the EST I changed out plugs, wires, cap and rotor, but the issue persists. Not sure where to start in this particular case. I checked base timing and I’m sitting at 8 BTDC.

 

[kenny nunez]

Be sure the bracket and bolts that secures the starter brush end is tight against the engine block.

 

[Scott Danforth]

Use a shop tach to verify.

You may have timing issues causing detonation causing the rattle you hear

 

[SSTKO81]

Because it’s only happening as RPMs increase I wondered about some kind of electric interference. -this setup worked fine last year I’m just not sure what changed.

 

[tank1949]

1981 SeaSprite Mercruiser 140 with Alpha drive, converted from points to EST two years ago.
Recently when I get RPMs over 1800 the tach will begin bouncing as it works its way up to 4K. Simultaneously there is a rattling noise almost like valve train heard from the engine. (Possibly unrelated?) I back off to 15-16k and all goes back to normal. Thinking this could be an issue with the EST I changed out plugs, wires, cap and rotor, but the issue persists. Not sure where to start in this particular case. I checked base timing and I’m sitting at 8 BTDC.

Have friend exercise electrical connections and see if tack bounces, while you are at helm. Not real sure about EST, but I'd make sure coil wire is connected and tight and good ground. Wires get corrosion.

 

[SSTKO81]

Have friend exercise electrical connections and see if tack bounces, while you are at helm. Not real sure about EST, but I'd make sure coil wire is connected and tight and good ground. Wires get corrosion.

A video tells the story. The needle jumps all over at RPMs over 1800, but it pulses even at idle with a noticeable “miss” you can hear.

 

[SSTKO81]

A video tells the story. The needle jumps all over at RPMs over 1800, but it pulses even at idle with a noticeable “miss” you can hear.

 

[Scott Danforth]

Fix the miss and the tach will calm down

 

[SSTKO81]

Fix the miss and the tach will calm down

Tried some higher octane fuel as it seems to have what sounds like pre-detonation. Also some possible air leaks around the intake. I’ll have to get a gasket for that. Biggest issue is I noticed bubbling around the center head bolt . -tried to make sure it was torqued to spec and when I put the wrench to it, it just spun. When I pulled it out to inspect it had the threads from the block in the bolt. This has quickly become a big problem.

 

[Scott06]

Tried some higher octane fuel as it seems to have what sounds like pre-detonation. Also some possible air leaks around the intake. I’ll have to get a gasket for that. Biggest issue is I noticed bubbling around the center head bolt . -tried to make sure it was torqued to spec and when I put the wrench to it, it just spun. When I pulled it out to inspect it had the threads from the block in the bolt. This has quickly become a big problem.

You will need to remove it and helicoil or put a timesert or similar in there

 

[SSTKO81]

You will need to remove it and helicoil or put a timesert or similar in there

Figured as much. Guess I know what I’m doing this weekend

 

[airshot]

Check the flatness of the head, they are prone to warping. Also check the top of the block for flatness as well. Soak the broken thread with penetrating oil for a couple days, try using left hand drill bits, when they bite in, it often unscrews the broken off threads. Now would be a good time to resurface the head, grind the valves and check springs and seats.

 

[SSTKO81]

Check the flatness of the head, they are prone to warping. Also check the top of the block for flatness as well. Soak the broken thread with penetrating oil for a couple days, try using left hand drill bits, when they bite in, it often unscrews the broken off threads. Now would be a good time to resurface the head, grind the valves and check springs and seats.

This head only has about 10-15 hrs on it since it came out of the shop a couple years ago. -In theory should still be good. The bolts came out fully in tact, just took the threads from the block with them. I’ll mic and check the block while it’s off but helicoiling should fixt me up. I am going to check compression prior to doin this just to get a base line.

 

[Scott Danforth]

If the threads were pulled from the block, the bolt was over torqued

 

[SSTKO81]

If the threads were pulled from the block, the bolt was over torqued

Guess I should have paid more attention to the Seloc manual

 

[SSTKO81]

Fix the miss and the tach will calm down

Head bolt repair taken care of. Tach is still bouncing over 1800 RPM. I can turn the needles on the carb all the way down and she stays running! My guess is carb rebuild may be needed. Looking down the throat it’s wet at the top of the venturies. I don’t recall if this is normal or not

 

[tank1949]

Head bolt repair taken care of. Tach is still bouncing over 1800 RPM. I can turn the needles on the carb all the way down and she stays running! My guess is carb rebuild may be needed. Looking down the throat it’s wet at the top of the venturies. I don’t recall if this is normal or not

Go buy an inexpensive tack, attach at motor, and test to be sure that your tack is bad or good. The wires may have a break somewhere from old tack to helm. A new tack installed will help diagnose where problem is. You will need the tack again. Trust me!!!

 

[Scott06]

Head bolt repair taken care of. Tach is still bouncing over 1800 RPM. I can turn the needles on the carb all the way down and she stays running! My guess is carb rebuild may be needed. Looking down the throat it’s wet at the top of the venturies. I don’t recall if this is normal or not

The float bowl is overflowing due to too much fuel pressure, faulty needle and seat and or float

You can sometimes get the tachs to start working again by cycling the cylinder selector switch back and forth but likely need a new tach would leave it disconnected as a bad tach can short out the ignition system

 

[SSTKO81]

Go buy an inexpensive tack, attach at motor, and test to be sure that your tack is bad or good. The wires may have a break somewhere from old tack to helm. A new tack installed will help diagnose where problem is. You will need the tack again. Trust me!!!

I’ve got a tach feature on my digital voltmeter. I’ll hook it up and see what happens

 

[SSTKO81]

Update and finale’ here! Found the heated air choke hose to be broken. Carb was sucking in extra air for who knows how long. Took care of that and the tach issue vanished. Still need to make some minor adjustments to the carb but all issues are now resolved. Thanks everyone for the help