Power loss, missing, coughing at high rpms

[Wet Wonder]

I've done a lot of work on the engine and boat side this season, with help from some threads on the forum.

1977 Mercruiser 351 233hp, 2 bbl, elec ign, raw water.

I now have one remaining problem whenever it gets above 3,500rpms.

It should be able to WOT to 4,500 rpms, but when I throttle up and over 3,500, there is power loss, coughs, misses and the motor slows way down. At that point I have to pull back the throttle, wait a second, and then can bring it back up to speed. Same thing happens if I bring it above 3,500rpms again.

I've done the following work this season.

Compression tested:
- strong all cylinders

Cooling:
- replaced impeller
- replaced thermostat

Ignition:
- replaced the distributor, with new springs/weights
- new cap and rotor
- installed Ignitor II elec ign and flame thrower coil
- new plug wires
- new plugs, gapped at .40 for the elec ign

Fuel system:
- tested line for leaks, passed
- doped all fittings and connections
- changed water separater filter
- tested pump for correct vacuum, passed

Fuel tank:
- drained it and refilled

Carburetor:
- Had it rebuilt

Timing:
- Markings were wrong, probably b/c flywheel wasn't set when replaced way back when, so I had to set timing out in the water. Sequence: Made adjustment, ran, anchored, re-adjusted, ran, anchored, re-adjusted, etc., until it seemed right.

So the motor is now running great under 3,500 rpms, but fails above 3,500. So any ideas about what to look at? I was thinking issues could be-

1) timing problem still
2) vacuum problem somewhere
3) internal engine
4) ??? (problem probably here)

Any ideas that I can get started on?

Thanks,
Dave

 

[Don S]

Re: Power loss, missing, coughing at high rpms

1) timing problem still

What, you didn't check it?????

replaced the distributor, with new springs/weights

Are you sure you put in the right springs? Did you check timing advance and compare it to the charts in the OEM manual?

2) vacuum problem somewhere

Didn't you hook up a vacuum gauge and see what it was doing?

Have you gone through this list and checked ALL the items?
http://forums.iboats.com/showthread.php?t=295070

 

[Wet Wonder]

Re: Power loss, missing, coughing at high rpms

When I brought cylinder 1 to TDC, the timing mark on the balancer was well past the indicator. I thought there was something faulty with the location of the mark.

I wired up the firing order based on location of first cylinder and hand set the timing. When I hit it with the timing light, it was showing about 60 degrees (guessing) before the TDC mark. When I adjust the timing to meet the mark, the motor would cut off. When I readjusted the timing well in advance again to about 60 degree, it would idle well.

What did I mess up here? Did I screw up TDC?

 

[snoski]

Re: Power loss, missing, coughing at high rpms

When you brought #1 to TDC are you sure you were on the compression stroke? The intake valve needs to close and then the piston comes to the top to be sure you are at correct TDC. If your mark is not at 0 then something is off. You could make another mark with a dab of white paint and then time from that mark to see what happens.

 

[Wet Wonder]

Re: Power loss, missing, coughing at high rpms

No, not unless you had the timing chain off. Are you certain you are on cyl. # 1 and that you have your firing order correct. See attached photo. Assuming that is all correct let me ask, was this the original engine in the boat? Any chance it had a 302 in it originally?

Let me ask a silly question. As you stand in the boat and face the engine (aft) which side of the engine is the timing indicator on? Left or Right of the harmonic balancer?

It's actually a right hand motor, so the order is 1,8,etc.
The indicator is on the left side of the balancer when looking over the motor from the rear to the front.

I'm at cylinder 1 as far a could tell, doing the finger in the spark plug threaded hole test and feeling the compression build. I'm learing as I'm going, so I could have been mistaken.

This isn't the original motor when I bought it 3 years ago, but it the boat came standard with a 351.

 

[Wet Wonder]

Re: Power loss, missing, coughing at high rpms

When you brought #1 to TDC are you sure you were on the compression stroke? The intake valve needs to close and then the piston comes to the top to be sure you are at correct TDC. If your mark is not at 0 then something is off. You could make another mark with a dab of white paint and then time from that mark to see what happens.

No, I'm not totally sure. I'm learning as I'm going. I presumed the balancer marks were off because I found cylinder 1 well in advance of them.

This is essentially what I did - timed it from a new mark that is a good bit after the original mark on the balancer.

I must have done somthing wrong. Any advice to recheck it?

 

[Wet Wonder]

Re: Power loss, missing, coughing at high rpms

I'm getting a little confused now but I thinkthe indicator is as I said, on the port side - it looks like a one inch spike sticking out perpendicular to the front of the motor and right next to the balancer - and the mark does pass it by about 60 degrees or so when I adjust the timing and hit it with the light.

Someone put white paint on the 8 degree mark and the advance mark to emphasize them, so I know it was used at some point to dial in the timing.

What have I missed? Could a problem here be causing the power loss at WOT, and if so, should I redo it from scratch?

 

[futz]

Re: Power loss, missing, coughing at high rpms

timing chain has stretched or you skipped a tooth.

 

[Wet Wonder]

Re: Power loss, missing, coughing at high rpms

Its really sounding like the engine has been swapped at some time and has the wrong balancer or its marked wrong. (I have seen it happen with a lot of 351s as there are several differnt harmonic balancers for it.)

This has happened here. The intake manifold shows the firing order for standard rotation, rather than right-hand which would be correct, so I probably have some other married parts.

I am going by memory here but I think you can clip on to plug wire # 8 with the timing light and use the existing marks. Sure to be different with a counter rotating engine but the concept is the same. Just find the cyl. that is TDC (compression) when the timing marks line up. (Or make your own marks on the balancer.)

So this brings it back to timing as a possible cause to the power loss at WOT?

I should find the cylinder at the timing mark, and if it's cyl 8 for example, use that to time in the future with that mark? I think that would be easier than finding the right spot to make a new mark, but I don't know.

 

[Wet Wonder]

Re: Power loss, missing, coughing at high rpms

If it is a timing problem causing power loss and coughing at WOT, do you think I would need to advance or retard the timing to correct it?

 

[Wet Wonder]

Re: Power loss, missing, coughing at high rpms

Have you gone through this list and checked ALL the items?
http://forums.iboats.com/showthread.php?t=295070

Yes, Don S., I went through the list to the best of my ability. I even read the fuel system section, bought a gauge and did those tests. But I'm still learning and must have missed something. It's a very long and broad list, so I'm hoping someone has seen this symptom I'm having. There are many threads posting the same type questrion, but it's rare to find the solution posted at the end of the thread, if they found it.