Merc/GM 4.3 tops out at 4000 rpm

[huzer1]

I have a reman 4.3, installed last year. It was ordered as a long block and I put the motor in myself. Since I've put it in, I haven't been able to get over 4,000 RPM under load, which the old motor was able to run 4400 all day long. It also seems to have a difficult time getting on plane with an 18 pitch 4 blade prop. Sitting in the driveway on muffs I'm able to get it to rev out to about 4600 RPM. When I put the carbs back on the motor they seemed very clean in the inside. There was no build up at all, and all I have ever run in the boat if e-free fuel.

Timing wise, I'm sitting at about 12 degrees base timing and automatic advance is sitting around 30 degrees at 4000 rpm. I'm at 6,000' in altitude.

When I got the new motor it was stamped as a GM block which also have me suspecting that the GM cam might be in there.

Here's a link to a video showing the problem.

Thanks for any help.

 

[Scott Danforth]

Is the prop same?

Did you service the fuel system?

Reving in neutral doesn't mean anything. The motor would rev to destruction running on 2 cylinders.

Your base timing seems high. I think spec is 8 BTDC

 

[Rick Stephens]

That sounds terrible. New motor - do a compression check for baseline. This also checks the valves are good.

 

[achris]

When I put the carbs back on the motor they seemed very clean in the inside.

How many carburettors on this engine?

 

[jimmbo]

Did you order it as a Marine Engine, or just any old Long Block?

 

[dubs283]

When I got the new motor it was stamped as a GM block which also have me suspecting that the GM cam might be in there.

Possible the wrong cam was installed, all 4.3 GM blocks are cast as GM, GM manufactured them. Reman means nothing (at this point) because any engine builder can put any level of quality parts in a GM 4.3 block, including a different cam

Did you check/adjust valve lash on the new long block assy?

As stated base/idle timing for mercruiser 4.3 engines is 8-10 degrees btdc depending on vintage/ignition style. I don't think this is your issue though as you find 30 degrees of total advance which should produce acceptable WOT rpm if propped correctly

 

[huzer1]

Is the prop same?

Did you service the fuel system?

Reving in neutral doesn't mean anything. The motor would rev to destruction running on 2 cylinders.

Your base timing seems high. I think spec is 8 BTDC

If by the same, you mean factory, then no. The factory prop I believe was a three blade but I replaced it when we bought the boat for a little more low end power to pull wake boards out of the water faster.

I replaced the fuel filter with the motor, but I hadn't thought of that. I will replace it again for this weekends test.

That sounds terrible. New motor - do a compression check for baseline. This also checks the valves are good.

I'll give this a shot after this weekend if fuel filter doesn't pan out.

How many carburettors on this engine?

just one, not sure why I always refer to it/them in a plural sense.

Possible the wrong cam was installed, all 4.3 GM blocks are cast as GM, GM manufactured them. Reman means nothing (at this point) because any engine builder can put any level of quality parts in a GM 4.3 block, including a different cam

Did you check/adjust valve lash on the new long block assy?

As stated base/idle timing for mercruiser 4.3 engines is 8-10 degrees btdc depending on vintage/ignition style. I don't think this is your issue though as you find 30 degrees of total advance which should produce acceptable WOT rpm if propped correctly

I did not check the lash as the company I ordered it from said it was a ready to go block. Add oil, carb, alternator, water pump and exhaust manifold.

 

[Scott06]

Is the prop same?

If by the same, you mean factory, then no. The factory prop I believe was a three blade but I replaced it when we bought the boat for a little more low end power to pull wake boards out of the water faster.

By the same he means are you running the same prop on the new engine vs the old engine?

Have you always been at 6000 ft altitude? Did you rejet the carb for that altitude?

Compression is good place to start, and what do the plugs look like when you pull them

And did you get a marine reman or automotive ?Difference can be in camshaft which would certainly affect holeshot

 

[Lou C]

I’m hearing a surging there, fuel delivery issue?
Anything else change besides the new long block?

 

[itsathepete]

I’m hearing a surging there, fuel delivery issue?
Anything else change besides the new long block?

I hear/ see it too on the tach. Almost like a rev limiter. Possibly poor connections on ignition wiring causing coil to cut out?

 

[Lou C]

Are you sure that the carb throttle valve is really wide open when the remote control throttle is wide open?
2 bbl or 4 bbl?
6,000 ft elevation is big. Air is less dense so the carb has to be jetted leaner. How did do the plugs look? I’m at sea level though so I don’t have experience with this but theoretically that’s an issue for sure.
What size boat is this?
My 20’ has the 4.3 4bbl and came with a 19 pitch prop. Only pulled 4200. Then went to 17, got 4600. Ended with a 15, now get 5,000. My boats heavy @ 4200 lbs so this works best to minimize strain on the little 4.3.

 

[nola mike]

I hear/ see it too on the tach. Almost like a rev limiter. Possibly poor connections on ignition wiring causing coil to cut out?

Don't think so, it sounds gradual. Missing on 1 cylinder you probably wouldn't even hear. But you can throw a timing light on there and see if you're getting a constant spark on each cylinder.

 

[Scott Danforth]

at 6000 foot elevation, you are down almost 20% on motor power (3% per 1000' of elevation)

if you were always at 6000 feet. that is one thing.

 

[huzer1]

Back from the weekend. Replaced the fuel filter, didn't make a difference. I tried to play with timing some more, 12 degrees seems to be the spot. 8 degrees and I was capping out around 3600. More then 12 was the same result.

I pulled the plugs and checked gap, they were at .040 and the book calls for .045 so I did open them a bit. Compression was as follows
1: 120 psi
2: 105 psi
3: 115 psi
4: 110 psi
5: 90 psi
6: 95 psi

I was told by a dealer that this carb doesn't need re-jetted for altitude, but looking in the service manual they seem to offer an array of jets.... Boat originally came from lake Michigan.

 

[Lou C]

Plugs look ok but comp test results are low. Not sure how much lower you’d expect them to be at 6,000 ft elevation than at sea level, that data is probably available though.
Just as a data point my old ‘88 4.3 V6 comp tests are 165-175.

 

[Scott06]

Plugs look a tad rich

found this compression test compensator https://sovietsteeds.com/forums/viewtopic.php?t=48747

says at 6 k your readings will be 16% lower , but that only brings the low cylinders up 110 psi at sea level… which is low. With a fresh engine i would expect them tighter and higher 130 should be a new engine at that altitude.

One question didn’t get answered were you running the same prop with old engine

 

[huzer1]

Plugs look a tad rich

found this compression test compensator https://sovietsteeds.com/forums/viewtopic.php?t=48747

says at 6 k your readings will be 16% lower , but that only brings the low cylinders up 110 psi at sea level… which is low. With a fresh engine i would expect them tighter and higher 130 should be a new engine at that altitude.

One question didn’t get answered were you running the same prop with old engine

Same specs but new prop. Old one was beat up.

It was also a harbor freight compression tester so accuracy may not be there at all.

 

[Scott Danforth]

Retest. Compression that low is time to rebuild.

 

[ScottinAZ]

Ill also say the compression is crap. Not so much the ultimate value, but the spread between cylinders is excessive. You typically want to see no more than 10% between high and low cylinders. yours are much more than that. Time to start looking at a long block

 

[Rick Stephens]

Just as a data point my old ‘88 4.3 V6 comp tests are 165-175.

Lou, your gauge reads much high. Unless you put high compression pistons in that motor, factory new and broke in won't top 150.

Back from the weekend. Replaced the fuel filter, didn't make a difference. I tried to play with timing some more, 12 degrees seems to be the spot. 8 degrees and I was capping out around 3600. More then 12 was the same result.

I pulled the plugs and checked gap, they were at .040 and the book calls for .045 so I did open them a bit. Compression was as follows
1: 120 psi
2: 105 psi
3: 115 psi
4: 110 psi
5: 90 psi
6: 95 psi

I was told by a dealer that this carb doesn't need re-jetted for altitude, but looking in the service manual they seem to offer an array of jets.... Boat originally came from lake Michigan.

As with Lou reading high, you could likely be reading low. Compression gauges are notoriously inaccurate. Usually what is important is that the cylinders are close to each other, with ~5-10%. You might consider finding a parts store nearby that you can borrow rent another compression tester to verify numbers. Offhand, if those are correct, the motor is needing rebuild.