Mercruiser performance Question?

[Stillreelin]

I replace my 4.3 liter V6 mercruiser. New everything from the carb to putting fresh water on the motor. The problem that I am having is it won't go over 4400 rpms and when it is wide open,it will run smooth then will act like it drops 100 rpm then picks right backup. It will do this the hole time it is wide open.At 4200rpm it will run fine no issues. Any suggestions on what may cause this? (It is the same outdrive and prop as before)

 

[300sflyer]

Re: Mercruiser performance Question?

How many hours are on the new engine? Is this a brand new complete engine from Mercury?

 

[Stillreelin]

Re: Mercruiser performance Question?

Went through the 10 hour break in that the remanufactured engine called for. May went over a couple of hours before I took it wide open. Remanufactured engine came with everything except electronics and exhaust and all the pulleys and things.Replaced all the electronics a year ago when I had a bad coil.

 

[John_S]

Re: Mercruiser performance Question?

Do you have Thunderbolt V ignition? If so, might be hitting the rev limiter. That seems too low of rpm to hit the rev limiter though. Was the boat tach ever checked against a shop tach?

How are the plugs looking? One of them different than others?

 

[Stillreelin]

Re: Mercruiser performance Question?

Thunderbolt V is the ignition system that I have. Motor is a 1998 with 4 barrel.Never had the tach checked. Will check plugs this weekend and also check the timing again.

 

[300sflyer]

Re: Mercruiser performance Question?

Has there been a change in the WOT RPM since the rebuild? If so, what was the previous WOT RPM?

My new remanufactured Mag 357 engines, specify a 20 hour break-in period. It seems odd that yours is only 10.

 

[Stillreelin]

Re: Mercruiser performance Question?

The plug looked good and the timing was good.The old WOT was 4800 and new engine won't go over 4400. It is 10 hours of slow break in then for the last 10 hours it said you can touch the WOT for a few minutes the back down not leaving it at the same rpm for a long time. I can tell it is not right because the speed is about 2 knots off. I run the boat around 4200 most of the time away. I just want to get It running in top condition.

 

[Philster]

Re: Mercruiser performance Question?

Fuel... Age? Filter... Age?

 

[John_S]

Re: Mercruiser performance Question?

Thunderbolt V is the ignition system that I have. Motor is a 1998 with 4 barrel.Never had the tach checked. Will check plugs this weekend and also check the timing again.

The 4brl would not have been stock on that year motor. Reman can be different from merc specs. Richen the 2ndarys 2 steps and retest wot. If you gain speed, go two steps more. When you stop gaining back down to the prior. You can then adjust plus or minus one step, but it is probably close enough.

 

[Stillreelin]

Re: Mercruiser performance Question?

Reading from edelbrock the carb is set up for a 350. They give you the right springs and jets and metering rods to use. So I ordered that. Until then I will put on the original Card to see if that is the problem.

 

[John_S]

Re: Mercruiser performance Question?

Reading from edelbrock the carb is set up for a 350. They give you the right springs and jets and metering rods to use. So I ordered that. Until then I will put on the original Card to see if that is the problem.

I assumed that you already had the 4.3 kit in the 1409. Those jets and rods were from the pre-vortec merc specs. You might have some tweaking still to do, once you install it. What was the original carb?

 

[Stillreelin]

Re: Mercruiser performance Question?

The original carb is a weber 4 barrel. It almost is an exact match to the edelbrock.

 

[Stillreelin]

Re: Mercruiser performance Question?

I put the old carb on while I wait for the new jets and metering rods. Run a lot better. It still is down 300 rpm . But i think with a little jet tweaking it should be running in top shape ready for Rockfish trophy season in the fall.

 

[Sinko]

Re: Mercruiser performance Question?

no where as technical as the past posts but just a note that my 1997 4.3 litre has always WOT at 4400 rpm with a 19" prop.

 

[bigdan1]

Re: Mercruiser performance Question?

like previously said , start by changing fuel filter!