Mercruiser 7.4 Bravo three stutters at 3500 rpm

[ludovic815]

Hello,
My mercruiser 7.4 (carb) stutters at 3500 - 3700 rpm. I am unable to rev it at more than 3700 rpm. Same behavior in neutral and in gear.
I made a short video showing the issue: https://youtu.be/c0YzhaPgSN8

Engine serial number: 0F811188

I changed the gasoline filter, it had no effect.
How should I continue to fix this issue?

 

[tpenfield]

Try disconnecting the tachometer from the engine and see if the same thing happens.

 

[ludovic815]

If I disconnect the tachometer, how do I know how fast I am revving?
Any hint about which cable exactly I should disconnect?

 

[alldodge]

Appears to me that your tach is off and your hitting the rev limiter. From the sound your way up there in rpms, get a hand held tach and check

 

[tpenfield]

If I disconnect the tachometer, how do I know how fast I am revving?
Any hint about which cable exactly I should disconnect?

You won't exactly know, but what you should be looking for is if the engine 'shutters/stumbles' at the similar speed/throttle position. When tachometers go 'bad', they can take the ignition system down with them, causing the effect that you are experiencing.

There should be an extra gray wire at the coil that is the tachometer lead. I think there are 2 gray wires, so a 50/50 chance of finding it the first time.

 

[rock doc]

I bought a boat that was under propped and the tach was slightly off. Took me a while to get it sorted. Based on your vid it looks like you are hitting the red line - did you change or damage the prop recently? I recorded my tach readings at different speeds and used a slip calculator to figure out that my tach was off. https://www.mercuryracing.com/prop-slip-calculator/

Mine was only underpropped a little bit so it was hard to figure out I was actually hitting the redline (as opposed to an ignition issue) and the tach was was off by approx 300rpm too low. In your case I think it will be obvious if that is the problem.

 

[rock doc]

I also noticed you have some wonky reading on your other gauges - looks like your instrument cluster is acting up.

 

[scoflaw]

Like 0 oil pressure ? That is what rev limiters sound like

 

[ludovic815]

0 oil pressure is another issue and it is not new (oil pressure is actually OK) : https://forums.iboats.com/forum/engi...-pressure-loss

I did not change the prop. Last year I was able to rev at 4600 rpm (without stuttering at wot, rev limiter not reached) and my top speed was much higher. This is why I do not think that the tacho is off, but I will check anyway

 

[rock doc]

Looks like the depth sounder is acting up too. If the tachometer is messed up it can be causing ignition problem like tpfield said . unplug it at see if you get your top speed back.

 

[ludovic815]

I just disconnected the tachometer and used a hand held laser tachometer. Tried a few times. Each time when this happens, the laser tachometer shows between 3400 and 3450 rpm. This means that I am not hitting the rev limiter and that I have the same behavior when the tachometer is disconnected.

 

[ludovic815]

I also opened the distributor cap. Tried to clean everything inside. No change.

 

[alldodge]

Measure the voltage at the coil + side, and keep watching as the rpms increase

The distributor needs some cleanup and check for corrosion on the contacts of the pickup

 

[ludovic815]

For the coil, if I understand, I should check on the +12v connector and on the ground? I should have +12-14v, right?

 

[alldodge]

Just on the + side only, put other meter probe on a ground.
When motor is turned on you should get battery voltage 12.5 to 12.7V
Once motor starts and is reved once you should see 14 to 14.5V

What your looking for is as rpms increase see if voltage stays steady above 14V or starts getting lower

 

[ludovic815]

There is a big round connector on the harness. I cleaned the contacts with a contact cleaner. Now all my problems are gone. The oil pressure gauge works like a charm and I can rew up to 4400 rpm in neutral and up to 4100 rpm at WOT when cruising. I also changed the spark plugs and cleaned the distributor cap.

Thank you!