43 Liter slow top speed

My dad and I can only get our

My dad and I can only get our boat to go 30 mph and only revs up to 4000 rpm on our 4.3 liter and will start to cut out on top end its a 1989 cobia 19 ft. ski boat have any tips on how to get to run better

have you done a tune up?

have you done a tune up?

coil and/or wires

coil and/or wires

we have rebuilt the carb. and

we have rebuilt the carb. and changed the plugs and wires and distributor (the thunderbolt 4) but havent messed with the coil but every thing else we have done with the fuel parts like cleaned the tank replaced the fuel pump and water seporator ill try a new coil. is there a way to test the coil we have with a volt meater

What prop are you using?

Ha

What prop are you using?

Have you checked the timing?

cutting out at high rpm's

cutting out at high rpm's can certainly been a coil symptom but isn't the coil part of the dist on a TB IV? cant remeber...

"No, the coil is separate."

"No, the coil is separate."

we have a 18Pitch 4 blade prop

we have a 18Pitch 4 blade prop and we have tried higher pitches and it wont go in to plane and we have checked the timing and tried ajusting the mixture scrues on the carb and dosent seem to help ill try a new coil and see what happens

"James...

I agree about the

"James...

I agree about the coil, but also look at the wires going to the coil and other related ignition parts. Loose or chaffed wires will cause alot of problems at high speed where all the vibrations occure. Just a note about the prop idea. Did the boat previously go faster with this same prop? You would need to go to a smaller pitch of prop to get it to plane faster. The higher pitch will try to get a higher top speed, but puts more strain on the engine at low speed and hard acceleration. Thats why you went to a 4 blade in the first place.

Wrench"

I replaced the coil over the w

I replaced the coil over the weekend and it dident change i also went over all the wireing and dident find any bad connections or wires theres a computer chip type thing were the the two little wires that come off the coil go to. its mounted on the side of the exghust elbow could may be there some thing wrong with that i also redid the timing a bit with the new coil but still nothing happend.

"Do a compresion test, if that

"Do a compresion test, if that checks ok , rebuilb the carb.
sounds like your power valve is gumed up."

"You went the wrong way with y

"You went the wrong way with your prop choice.

The higher the pitch the harder to get on plain and the lower the rpms will be.

I would suuggest going to a smaller picth and the boat should pick up some rpms. Aprox 300 rpms per 1 full pitch change 18p to 16 p

With I/O's it would be a 19 p to a 17 p

A higher pitch typically will give you higher speeds and take longer to get there. A smaller pitch will give you lower speeds and you will get there sooner.

As far as cutting out try running a seperate feul source to the carb. This is a easy way to see if your feul suplly is the issue. Also make very sure your gas tank vent is clear, Very sure!!

Where did you set the initail timing? 6-8 degrees before tdc?

The thingy you describe on the elbow is the INGINTION TIMING CONTROL MODULE. This controls your timing advance which should be full on ~ 34-36 degrees at ~2600-3000 rpms.

Just so you know adjusting IDLE MIXTURE SCREWS only affects the idle and off idle NOT high speeds.

What is your voltage readings at full speed?


Try Disconnecting the grey wire on the negative side of the coil that feeds the Tach and see if that makes any change in high speed cutting out.

Cutting out at high speeds is usually a feul or ingition problem so concentrate on those areas first. Also how hot is the engine getting?"

when you rebuild the carb make

when you rebuild the carb make sure you get the right base gasket; ONLY USE 1/4" THICK GASKET FROM MERC.

Geez Guy...

Whats with the

Geez Guy...

Whats with the thick carb base gasket all of a sudden. I know you just put the thick one on yours. Was the performance gain like bolting on a tunnel ram or what. (wink wink). How's the fishing down there lately? You'll have to send me a couple more of those pics that you have of those bass.

Wrench

WRENCH:

It was like adjusti

WRENCH:

It was like adjusting th mixture on my '72 Charger; you can tell "right now" where the engine stumble points are.

Going fishing in the a.m. for walleye. My brother found a school hanging out in 30' of water in some old tree stumps off of a ledge. I'll let you know how she runs when I return. Send some pics. if I get any.