Problem with rpm

[Germanivette2010]

First happy holiday to everyone i hope this new year will be better for everyone.
I have a question i have a maxum 3000 scr with twin 4.3 l i just replaced a new engine but the max rpm im getting is 3600 rpm i can never go to 4000 rpm like the other engine we play with the timing we just put it the same as the other engine and went up from 3200 rpm to 3600 or 3700 rpm full trottle but thats it some mecanic told me to replace the ignition module but what do you sujest cause that ignition module cost $600 any advice thanks in advance

[mmwjr]

A little more info would be nice; carb or MPI?

Issues could be timing not set correctly.
Throttle not opening all the way.
Plug wires not in correct firing order.
One cylinder not working.

The only ignition issue would be not achieving full advance. Was this just a long block replacement or is everything new including ignition and fuel system?

[speedysprocket]

My rebuilt 7.4s do 4900 and 4500 rpm WOT. I can’t see any differences in the engines to account for this, so I’m bringing the props back to the shop to get checked this winter...

[LaborOfLove]

I have similar problem. Runs great up to about 3600 rpm. Tested everything with ignition and props. Recently someone suggested that the flapper valves in the exhaust may have failed, resulting in a banana stuck in the tailpipe type situation. Apparently this is a cheap and easy fix, but since it’s winter here I won’t be down to check it out until the spring. Let me know how you make out as we’ve got the same symptoms.

[Germanivette2010]

Is a carburator engine but i bought the long block new carburator new manifold . The only thing is old are the intake manifold . Yesterday i did a sea trial only run 3800 rpm and the startboard engine wich is the old one when up to 4000 rpm

[mmwjr]

I would get a phototach from an auto store to verify the tach reading. Have you verified the props are the same pitch and diameter? Also drives are same gear ratios?

[jrsick]

Sounds like you are over-propped. Like Mike said, what’s your prop pitch and drive ratio?

[shrew]

Can you swap Tach. gauges and see if the same occurs in reverse? If so the issue is in the gauge. If not, it is further upstream.

[Germanivette2010]

I swap it and was the same rpm

[Germanivette2010]

The props are the same that had with the old engine and with the old engine was getting to 4000 rpm

[jparsons121]

Time to verify the basics.
Verify compression on all cylinders
Verify the new carb is set to same specs as the old one.
Verify base timing and full advance, under load at near full throttle. (Note timing procedure in the manual for Thunderbolt ignition)
Verify that valve lash is set properly.

I had similar issue when replacing one 5.0 in my 3000 and each of these contributed to having low RPM compared to previous engine.

Proper timing was the key to seeing the increase in RPM that I was chasing.

[shrew]

Originally Posted by Germanivette2010

The props are the same that had with the old engine and with the old engine was getting to 4000 rpm

Max RPM is 4400 - 4600. If you were ever only getting to 4000, there is already a problem. The most common causes are weight and the pitch of the prop.

What size prop and (more importantly) what is the pitch?

Is the boat currently loaded?

[Brianmaxum3000]

I have a 1998 3000 scr with 4.3’s. Ive replaced both engines in the last two years and changed both over to 4 barrel. My first question would be at wide open throttle does it seem like the one engine is really making that many more RPM’s? And i have never seen 4500 rpms as someone stated. 4100 is about max. Did you replace both carbs or just one. I find it hard to believe its a module but you could always swap it from the other engine and see what happens. And you can get a complete distributor with cap and wires for $350. So the module is not 600.

[Germanivette2010]

Yes is load it i will let you know the pitch when i go to the marina tomorrow

[mmwjr]

$350 for a complete distributor would be an aftermarket and not factory Mercruiser. The Mercruiser parts are $$$.

[Brianmaxum3000]

Originally Posted by mmwjr

$350 for a complete distributor would be an aftermarket and not factory Mercruiser. The Mercruiser parts are $$$.

That is for a factory GM/Deloco Marine distributor. While i guess its not technical a mercruiser one but GM/Delco probably builds a lot more distributors for there 4.3 motors than mercruiser so I’d say it’s good enough.LOL

Well i looked at again and this is what it says...


Genuine 4.3L GM Delco Voyager EST marine electronic distributor kit. Made for ALL 4.3L marine engines made from 1985-current. This is an excellent replacement kit for "points" distributors and faulty MerCruiser Thunderbolt ignition systems. This kit is U.S Coast Guard Approved and has the official marine SAE J1171 stamp on the bottom of the distributor. This is the EXACT kit that Volvo Penta, OMC, Marine Power, etc use on their 4.3L marine engines. 100% compatible and operational with ALL Mercruiser 4.3L marine engines.

This kit includes:
4.3L Voyager electronic distributor
Coil with bracket
Spark plug wires
Wire retainers
Timing shunt
Pig-tail plug
Coil-to-distributor wire
Easy to follow, 7-step installation instructions

[mmwjr]

Originally Posted by Brianmaxum3000

That is for a factory GM/Deloco Marine distributor. While i guess its not technical a mercruiser one but GM/Delco probably builds a lot more distributors for there 4.3 motors than mercruiser so I’d say it’s good enough.LOL

Well i looked at again and this is what it says...


Genuine 4.3L GM Delco Voyager EST marine electronic distributor kit. Made for ALL 4.3L marine engines made from 1985-current. This is an excellent replacement kit for "points" distributors and faulty MerCruiser Thunderbolt ignition systems. This kit is U.S Coast Guard Approved and has the official marine SAE J1171 stamp on the bottom of the distributor. This is the EXACT kit that Volvo Penta, OMC, Marine Power, etc use on their 4.3L marine engines. 100% compatible and operational with ALL Mercruiser 4.3L marine engines.

This kit includes:
4.3L Voyager electronic distributor
Coil with bracket
Spark plug wires
Wire retainers
Timing shunt
Pig-tail plug
Coil-to-distributor wire
Easy to follow, 7-step installation instructions

Yes but the OP has the Mercruiser ignition which all that comes with a Mercruiser marinized engine which is the number one provider of I/O power in North America.

[shrew]

I think that GM Marine is marinized. Whether it is the OEM for Mercruiser, I could not say.

[mmwjr]

Originally Posted by shrew

I think that GM Marine is marinized. Whether it is the OEM for Mercruiser, I could not say.

Yes both marinized engines but with different ignition systems.

[Brianmaxum3000]

Originally Posted by mmwjr

Yes both marinized engines but with different ignition systems.

Maybe they are different but I bought 2 of these distributor kits and they both plugged in and worked perfectly on my 1998 maxum with 4.3’s so I’m sure they would work for him as well and are cheaper than $600. Ang just curious do you think the mercruiser 4.3 is different than a regular Chevy 4.3?