1973 mercury 850 not getting over 4200RPM @WOT

[StriperLover]

i have 1973 mercury 850 that is running fairly well but there are a few quirks with the motor i would like to solve if at all possible first off the motor is on a 1986 sylvin 16 ft aluminum boat customized into a center console. so the boat is very light. the problems i am having is this, at WOT i am only getting 4200 rpm and 36 mph(GPS) with a 19P prop so i figured it was the wrong prop.I went down to the prop shop talked to the guy there and I told him my situation, told him i thought i needed a 13P - 15P to get the motor into its normal operating rpm range of 4800-5500rpm as suggested by mercury and he reccamended that i try a 17P prop so i did as instructed now with the new prop it is weird i am only getting 4100 rpm and only 32mph so i think there is an unknown problem with the power head. so i went through everything i could think of.( i rebuilt the carbs when i first got the motor this winter ), so i started with the tuning procedures in the SELOC manual (timing and carb sync. ) and got no change as to the original settings weren't that far off from where the book said they should be. i also redid a compression check and got 5 psi more than the original 115psi i got from when i first got the motor and it not being run in over 2 years. as to what else i could check i am stumped so any help would much be appreciated.

thanks in advance Billy

 

[StriperLover]

Re: 1973 mercury 850 not getting over 4200RPM @WOT

also i wanted to stay i tryed adjusting the carbs @ wot with a friend driving and again didnt get any improvement

 

[diaric]

Re: 1973 mercury 850 not getting over 4200RPM @WOT

did u run this boat before with the original prop? and how did it run then?

 

[Clams Canino]

Re: 1973 mercury 850 not getting over 4200RPM @WOT

ALWAYS start with a compression test.... If it passes... the rest is gonna be relatively easy to fix.

-W

 

[StriperLover]

Re: 1973 mercury 850 not getting over 4200RPM @WOT

did you read my post? i already answered both questions in the first post. but here goes again.

compression: 1st got the motor this winter 115-120psi even on all 4 cyl.
2nd compression check 3 days ago 120-125psi even on all 4 cyl

now the 2 props i used:

1st one a 19 pitch 3 blade aluminum got 36MPH on my gps and 4200RPM at WOT
2nd one i tried a 17 pitch 3 blade aluminum 32MPH and 4100RPM.

 

[diaric]

Re: 1973 mercury 850 not getting over 4200RPM @WOT

that wasn't my question. have you run the boat previously (like last year) and got different performance. was it always like this or have things changed. ie. if it ran good at one time with the original prop then the pitch is probably not the cause of low rpm's

 

[StriperLover]

Re: 1973 mercury 850 not getting over 4200RPM @WOT

no i just got this motor this winter and have went right through it after i did a compression check and it was satisfactory. i have rebuilt both carbs and the fuel pump i have also replaced all of the broken and decayed wires i have replaced the water pump. and i have also taken the distributer apart and cleaned it and i have also changed the switch box due to a no spark issue in the very beginning after all this it runs well but i have the problem of the RPM under wide open throttle under load conditions not getting over 4200 and i have changed the prop to a smaller pitch and i have lost RPM and speed. this is my problem.

 

[ricksrster]

Re: 1973 mercury 850 not getting over 4200RPM @WOT

Try adjusting your tilt bar up or down a notch.

 

[StriperLover]

Re: 1973 mercury 850 not getting over 4200RPM @WOT

i am certain that isnt the issue as this motor has power trim and tilt and i do adjust the trim to increase the RPM's. if at full down on plane it only revs up to about 3800-3900 rpm then when trimmed up it gets to about 4100 with the 17P prop.

 

[StriperLover]

Re: 1973 mercury 850 not getting over 4200RPM @WOT

anyone have any ideas? please help

 

[StriperLover]

Re: 1973 mercury 850 not getting over 4200RPM @WOT

does anyone have any ideas at all? also the manual i have is a seloc and it really doesnt have crap as far as troubleshooting anything. what are the best manuals people like around here?

 

[StriperLover]

Re: 1973 mercury 850 not getting over 4200RPM @WOT

please somebody have any ideas on this? i really need some help on this as all the mechanics around here wont touch older mercs.

 

[diaric]

Re: 1973 mercury 850 not getting over 4200RPM @WOT

its probably not a prop issue by the sounds of it. you should check that the carbs are opening fully at wide open throttle. if not linkages would need adjusting. could also be an old fuel hose collapsing on itself. could try to pump bulb when u got it maxed out and see if it picks up.could be a blocked tank vent so it can only suck out so much fuel at a time. the other possibility is that it might need a good decarb with deepcreep, espiecially if it did a lot of low idle trolling. hopefully someone else will pipe in with ideas

 

[j_martin]

Re: 1973 mercury 850 not getting over 4200RPM @WOT

Check the motor mounting height.

Could it have a high speed miss. Try running it with one wire off at a time. Be sure you ground the off wire so you don't overstress the spark system. If it's a one cylinder problem, that would at least locate the hole.

Does it crawl up to speed, or get there quick and quit pulling?

Don't look like I have an answer, but lots of questions.

John

 

[StriperLover]

Re: 1973 mercury 850 not getting over 4200RPM @WOT

thanks for the input but unfortunately i have to rule out most of those things and this is why.

1st I just rebuilt the carbs,
2nd it has a brand new permanent fuel tank will all brand new fuel lines
3rd i dont troll with it it does not idle well very long with out loading up(maybe a clue) as to i dont know how long this old 2 stroke should idle without loading up
4th i pulled one plug at a time already and the loss is even with all 4 so 1 bad cyl already ruled out
5th as to how long it takes to get up to speed to me it seams normal as to how long it takes to get up there but again i get like 3600-3800 before trimming out on plane then 4100-4200 after trimmed out
6th i have checked like 5 times for the butterfly position @ WOT and it wasnt the first time i checked it was something like 3/4 but after adjusting the cables to make the butterflies open all the way it didnt even make any difference.

for some reason i think it might be timing but i set everything to what the SELOC book says it should be. but from prior experience on cars it seams like a what would be a bad vacuum advance solenoid on a V8 4 barrel. hope this helps with the possible trouble shooting hints

 

[diaric]

Re: 1973 mercury 850 not getting over 4200RPM @WOT

you stated before that the motor was new to you, so you don't know its operating history. a lot of trolling and low idle use the 2 strokes can carbon right up. the exhaust ports can get so plugged with carbon that the engine won't WOT. its worth a shot to do a decarb on it, and deep creep is good to have for when you winterize it. you also don't say anything about touching the fuel pump.

 

[Plainsman]

Re: 1973 mercury 850 not getting over 4200RPM @WOT

I bet folks stop helping because it looks like you have a bad attitude towards their suggestions. It might be better to bit ones lip, then the hand that is trying to help you. Just my perception.

Have you looked at this:
http://forums.iboats.com/showthread.php?t=168855

 

[redjmp]

Re: 1973 mercury 850 not getting over 4200RPM @WOT

Ok try this. Clean your plugs or install new ones. Run at wot <under load> and test spark if you have a tester. Pull all plugs right away and verify that they are all hot and showing signs of deposits forming. Any that are colder/cleaner would indicate a problem with that cylinder.
If all good then pump ball at wot and see if that helps. Could be pump failing.
Disregard seloc and verify timing pointer is at exactly TDC by putting something like a long socket extention in the #1 spark plug hole and rocking flywheel back and forth and marking where the extention just starts to move in either direction and then set pointer exactly in between those marks.
Then go and find the link and sync thread by clams canino in the FAQ section here and follow that.
If still no good, do a deep creep as suggested. If still no good, verify float levels and jet sizes in carbs. Might have smaller jets installed for running at altitude. Did you blow out all passages with compressed air during rebuild and verify correct float levels?
Invest in a small screw in 90* fitting and transparent line for your carbs to see exactly where the gas level is under wot. Usually fits in the plug hole to change out main jets. Works just like a water level on a garden hose.
If still no good, verify compression with a different tester. May have been run without oil and all holes are evenly low and your tester is out of whack.
Is there something I missed? LOL

 

[diaric]

Re: 1973 mercury 850 not getting over 4200RPM @WOT

another important question that has been missed. does your boat have foam under the floor or is it just the aluminium bottom. you could have 500 lbs of water soaked foam down there which will slow you right down

 

[StriperLover]

Re: 1973 mercury 850 not getting over 4200RPM @WOT

I am sorry to have come off with a bad attitude. I was very irritated and i just took it the wrong way when people are suggesting things i have already stated i have checked.

next last night i took her out with a buddy set her up on plane @WOT AT the 4200 RPM and messed around with the timing screw and got no change what so ever in the RPMs i have tested the spark with a tester and all CYLs fire with a 25mm gap in the tester. i like the idea about the jets and will pursue that avenue next thanks again for all your help.