Single most #1 cause of poor top end performance?

[delvel]

Ok you've all been a great help on this odessy( corroided wire harnesses, bad sw. box/rectifyer,stuck impellers,clogged pee hole). I've fixed all these.
Yesterday I took the boat and motor to a local lake and power tested it on the launch ramp( I don't trust this motor to take it out on the lake yet!).
Ok it starts and idles(purrs) just fine. Shifts in/out of gear fine, pumps water fine.
BUT.......At full throttle it does not seem to have the sound of a healthy all out great running machine. I don't have a tach for it, but I can tell she's just not winding out!

I'm about to quit and sell this whole thing "As Is"
Two "marine repair?<more like paying a traffic fine!>shops haven't helped in the past so I'm not putting more hard$$ into this thing.

What Is the "SINGLE" most cause for this? I'll try one more session of tinkering w/it if I get a consensus>

these old Mercs(72' 500,50Hp) are just a big hole in the H20 to trhow $$ into!

[Bflaherty]

My old Merc1000 (100 HP, inline 6) had a similar problem and it turned out to be the dwell settings inside the distributor (the early six cylinders had two coils so there were two sets of points [one set fired 1,3&5 the other fired 2,4&6]) once we got the internals of the distributor sorted out that motor ran like a top all the time and I picked up 10+ MPH!!!

Alternatively, you could have more serious internal issues like worn out reed valves or possibly an engine that someone has severely overheated and the cylinder walls may be so badly scored that high end compression bleeds off to where it wont run right (this was the case on my brothers '90s Johnson)...

Good luck!!!

[TheBronze]

I'm no engine wizard like Tom, but I have done the visual watch of the thunderbolt ignition and it is amazing. Spark advance or retard based on the movement of that big armature. At idle it is at rest and isn't doing anything. As soon as you move the throttle, that arm moves to advance the spark. I'd look there: dwell, etc.

[Tim_Mattson]

One thing on some Mercs is the WOT setting on carb is adj while running and at full speed or rpms.You twist until it roars and away you go.
Timing at wOT is just as important,you`ll burn it up if set too high.
Borrow a timing light and see what the marks on flywheel light up at idle and then when throttled.
Mark the line you want to hit or see with paper white-out or scratch it with a pick.
Point the light down carbs to make sure all are drooling.One dry hole will fry a powerhead.One cyl losing spark could do the same.....
Hooked to trailer won`t simulte free-floating so may take some courage to detach and see what it will do.....sorta like a bird flying while you hold one leg.
Goodluck,
TimM

[delvel]

Progress? not really?

Thanks Tim et all:
So we took boat out on lake yester day.
No improvement:
at WOT it pushed boat @ abot 4 kts.(16' Starcraft alum runabout).

Used timing light(all cyls firing ).
Tach( if it was reading correctly, <portable car tune up tach>) approx 1000 rpm, wouldn't go more.
Can't figure how to adjust carbs(can't access them since front cover plate that hoses SW Box, & rectifier blocks any resonable acces w/ tools I have) seem to be wide open from position of throttle shaft.
NO power surges
Slight miss occassionaly
No fouled or dammage plugs <after about a 20-30 min puttsing about>

At least we didn't get stuck out on lake!

P.S seperate issue:
now it won't go into neutral or reverse: only runs in forward?

[Bflaherty]

P.S seperate issue:
now it won't go into neutral or reverse: only runs in forward?

That sounds like your problem may be entirely related to a bad fast idle linkage setup... If the fast idle linkage is not set properly it will make the system think it is in fast idle all the time which prevents the throttle from actually advancing, and prevents you from shifting in or out of gear (since high idle speeds can destroy the gearbox). If you open up your cowling can look down at the base of the powerhead you should be able to see the shift linkage and watch it move, there are notches in the throttle arms which allow the shift linkage to pass through only when throttle position is safe for shifting. If these levers get misaligned you will either lose shifting or throttle movement, or sometimes both...

I wish I still had my outboard as I would take some pictures to make this more clear... Perhaps someone else out there can snap a picture of the linkage on a similar motor??

[delvel]

I did put a new impelled in,perhaps I Did something wrong in process.

[Bflaherty]

I did put a new impelled in,perhaps I Did something wrong in process.

It is very possible you got the alignment piece in backwards down at the lower unit. On my merc there was a white UHMW collar that had a step in it that had to be oriented one direction or shifting and throttle movement was impaired. If you have had the lower unit off, I would start there and just double check it... It's super simple mistake that I have made a few times...

[Tim_Mattson]

I believe you had to put it in FWD to lift it up so driveshaft could come out.....Possible you had upper linkage in fwd while the lower unit was in reverse.
Get Motor up so it can accept the lower unit.Insert it when in nuetral while making sure shift linkage is in the middle of the track where it slides.Might even be a nuetral index there on the black guide.
At idle you should see butterflies closed in carbs ,then horizontal or fully open for the top end.Unhook throttle side so you can manually move while looking inside carbs.If it hits the WOT throttle stop and isn`t fully open,then you need to adj something.
TM