Help me understand this?
 

I have been helping a friend whose boat cuts out around 3000-4000 RPM after a 30-40 minute run. Runs fine up to that point then starts to cut out.

I was reading some old threads on this issue and one of the recommendations was to unhook the tachometer as that may be the problem.

Went out today and ran it until it started to cut out. Then unhooked the tach and ran it pretty good for another 30-45 minutes with no issues. We need to take it out one more time to just to make sure the problem is fixed and are crossing our fingers.

Why would a bad tachometer cause the engine to cut out?

Shorts out ignition as what what triggers it

Tach uses the same wire/signal that fires the coil, if the tach is failing it could increase the resistance in the circuit and fail, I would also suspect the coil, they get hot and fail

We put on a brand new coil and it still cut out. The tack had been bouncing around and acting strange.

Steve is this John's P 29 boat that you are talking about ? If so list what type of ignition set up it has and style of coil or if not John's boat still list info for possible help.

Tach gets its signal form the coil depending on app. The tach also has 12 V and a ground. Believe it or not a bad tach depending on app can even cause a no start condition and or running problems. If this a twin app and the other engine runs good with no problems at all and has a good tach - just swap tachs and see if problems follows. That's what I would do.

Basically what Wobble said gives a script of it.

I had a Sun super 2 tach back in 1982 that I bought at a rummage sale, once you drove for about 20minutes it would start breaking up earlier and earlier, it was drowning out the ignition, no different than if you grounded the tach signal wire to chassis, motor wouldnt even start.

I hear ya ^^^^ I wish I could remember this one to a T but years ago we had a twin Carb HP 500 Mercruiser app - one engine started with ease the engine was either dead - nothing at the key switch or cranked but would not start, I do not remember that part of it but is was either dead or cranked over but would not start. . It turned out to be a very loose terminal on the back of the tach. Took me about 4 hours to find the issue. .I hate that easy stuff and it turns into WTF is going on.

Bingo loose power lead bad ground.