1973 merc 850 problems pulling skier

[scottrbishop]

Hey everyone - I've got a 1973 Merc 850 that's been to the mercury dealer 4 times now for a grand total of $700.00 with no improvement. The motor seems to be lacking power WOT and doesn't have enough power to pull a 140 lb person on 2 large skis. Earlier in the season (May/June about 80 degrees outside) the boat would pull a slalom skier out in about 5-7 seconds, but now the boat isn't powerful enough even to pull someone on 2 skis. The dealer has been adjusting the carbs, but I have a hard time beleiving that just an adjustment could make such a difference in the power I am lacking. The boat planes out fairly well with no skier, but I swear it had more top end at the beginning of the season. The boat is a 1973 Glastron gt-150, so the boat+motor is about 1150 lbs., it's just me driving (150 lbs.), and my brother skiing (140 lbs.), so there shouldn't be a weight issue. I've given the boat a full seafoam treatment in gas tank/cylinders/upper and lower carb intake, carb has been adjusted by dealer 3 times now with new plugs after the seafoam. Tachometer is apparently busted which is making this harder to diagnose. Same 13.25 x 19 quicksilver prop in good condition, fresh gas, new impeller, new fuel line with bulb, stays hard while running, no water in hull. I've tried trimming the boat up and down with little difference in power. I'm going to take the boat out skiing with the mechanic on Thursday, is there anything else I could to make it a happy 4th of July?? They keep telling me it runs fine, I take it out and plow through the water all day without getting up and it worked fine earlier this year, very frustrating! If anyone can diagnose the problem better its this board, thanks for all the help thus far,

-Scott

 

[TD_Maker]

Re: 1973 merc 850 problems pulling skier

Scott,
Are you sure the prop is not slipping? A prop can look perfect but be slipping like crazy under a load.

 

[j442w30]

Re: 1973 merc 850 problems pulling skier

Have you done a compression test recently? Did the mechanic make sure the timing is set correctly as well as the carbs?

 

[scottrbishop]

Re: 1973 merc 850 problems pulling skier

Scott,
Are you sure the prop is not slipping? A prop can look perfect but be slipping like crazy under a load.

How would I check the prop while pulling a load? Or is it just something you have to feel?




Compression and leakdown test were both done by the mechanic and passed. Not sure about the timing, but it was running great a few weeks ago. Is it common to slip out of time?

 

[j442w30]

Re: 1973 merc 850 problems pulling skier

To make sure the prop is not slipping on the hub, make a mark across the hub and the part of the prop that the hub goes into. Try it out and if the marks are still aligned then the prop is not slipping on the hub. As far as the timing goes it usually does not get out of time very easily but the timing and the carbs have to be set together correctly so the engine can accelerate correctly (this process is called a link & sync). How about your filters? Have they been changed recently? Maybe you just got some crud in some gas and its clogging something up.

 

[jebeebe]

Re: 1973 merc 850 problems pulling skier

are you sure there isn't some water getting in the hull in the floatation area? My son had an old fiberglass boat that started slowing down and he found it was full of water between the layers of fiberglass where the foam was. Just a thought.

 

[Chris1956]

Re: 1973 merc 850 problems pulling skier

Scott, Are you getting spark on all four cylinders? Perhaps under load the bottom plug wire is arcing to ground? If the compression is good, chances are the spark is bad.

 

[scottrbishop]

Re: 1973 merc 850 problems pulling skier

Thanks for the advice guys! As far as water goes, it could be taking on water, but I can tilt the boat past 45 degrees and let all the water drain out the bilge, and then let it sit for a couple days to dry out completely and the problem still exists, it also doesn't seem to let much water out, usually just as much as people in the water brought back onto the boat with them. How can I check for spark besides holding the wire off from the plug, the bottom one is REALLY hard to get to. The plugs are new, the wires seem pretty new as well. Anyways, I will try out a few things on the water today and bring the ideas along with me for the mechanic on Thursday if I can't get it going. Thanks again to all for the suggestions, it's much appreciated!

-Scott

 

[emckelvy]

Re: 1973 merc 850 problems pulling skier

Scott, I'm wondering if anyone has pulled the lower unit to check for obstructions in the exhaust passages? It's possible for a mechanical failure to cause a blockages somewhere down the exhaust tower or at the lower unit. Massive carbon buildup can do the same thing.

Unfortunately, there's probably no good way to check for blockage up at the powerhead without pulling the exhaust manifold (if even possible with the powerhead on); pulling the powerhead, or using a scope up the center section.

With an exhaust restriction the engine would run normally but under a heavy load wouldn't generate as much power.

BTW has your top speed changed since having the bogging problem? That's a good indication of something going on.

How's the gear oil? A 'trashed' lower unit might be putting a significant drag on the motor.

Best way to check spark while underway is with an inductive timing lite clipped around the individual spark plug wire. If you do this in the evening, when the light would be more visible, you can get a good idea of spark quality.

How hot is it now where you are? The hotter it gets, the less power your motor's gonna make. It'll also run comparatively richer than in cooler weather. Maybe take it out on a cool evening and see if it does any better. It may be on the "ragged edge" of too rich, and hotter weather makes it fall "flat". I'd take a look at one or both carbs' main jets ('course that lower carb is a Bugger eh??) by pulling the brass outer hex plug to reveal the jet, see what the jet size is. According to the book, it should be .080" up to 4000 Ft in elevation.

Check the carb and distributor linkage to make sure the distributor is advancing fully to its "spark advance" stop. And that past that point, the "throttle stop" is being hit. Look thru the choke shutter with the throttle wide open to verify that the carb butterfly's are fully opened. Do the Link n Synch as suggested if you see anything out-of-whack.

One last thought, if no one has taken a look in the dist cap lately, check it out for "trash in the cap". Can cause a myriad of strange running problems including crossfire, which could definitely harm performance!.

Anyway, there's a few more things to scratch your head about, keep us posted............ed