why cant I get over 4200 rpm ?

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shadowrider

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Feb 19, 2009
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I have touched on this before, but I am still in a rut with this problem. I have a 73 merc 850 with a jet pump. I have rebuilt carbs, new fuel pump, new fuel lines, link n sync done. I installed a new tach and verified accuracy with a dwell tach. 4200 rpm is all it will do period. It will plane out the boat and idle pretty well. last time out it seemed to smoke a lot. Last compression check was 140 psi on all 4 cyls. I have inspected the exhaust cavity and no restrictions found. Spark is bright blue and jumps a huge gap. I was thinking about leaning the carbs out a hair to see if that will help. Some one on this forum mentioned lowering the float level 1/8 inch but I am leary of that idea. Sure would like the tach reach 5000 any way. Thanks for your input. Shadow
 
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mr 88

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Re: why cant I get over 4200 rpm ?

Did this problem occur after your rebuild or is this a "new" engine for you ? What size boat and prop pitch ? You could be way over pitched, was the speed the same etc before you installed tach ? need more info to help you.
 
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shadowrider

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Re: why cant I get over 4200 rpm ?

boat size does not matter, has a jet pump, I called manufacturer of jet pump and they verified I had the right pump for my serial #. Problem has remained the same before and after carb rebuild. There is only one impeller available for the pump. Motor seems to be running rich, was going to lean idle screw like 1/8 turn to see if that will help. I pulled the cap and found 2 small fingers from the vent laying in bottom of cap, but doubt if that will change a thing. I polished the contacts while I had it out.
 
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oldman570

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Re: why cant I get over 4200 rpm ?

With the motor shut off and the throtal at WOT are the carbs open all the way? Might need a link & sunc done if not. Dtsconect the throtal cable at WOT and see if you can push the throtal arm open father and have the carbs open up father as the cable and distrubutor might need lubed and greased to get the throtal plate on the distrubtor to move father and open up the carbs more.JMO.
Oldman570
 
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mr 88

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Re: why cant I get over 4200 rpm ?

Since you know it all why post on here. The boat size and prop do have a bearing on your max rpms but you know it all! good luck with your carb's haha.
shadowrider said:
boat size does not matter, has a jet pump, I called manufacturer of jet pump and they verified I had the right pump for my serial #. Problem has remained the same before and after carb rebuild. There is only one impeller available for the pump. Motor seems to be running rich, was going to lean idle screw like 1/8 turn to see if that will help. I pulled the cap and found 2 small fingers from the vent laying in bottom of cap, but doubt if that will change a thing. I polished the contacts while I had it out.
Click to expand...
 
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shadowrider

Petty Officer 2nd Class
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Messages
195
Re: why cant I get over 4200 rpm ?

I have done a link and sync, carbs are open etc. All the obvious things have been checked 3 times.
It has a San Leandro jet pump. Their website states that a jetpump developes full power at a standstill.
(http://www.outboardjets.com/faqs.php#4),(Look at #4 and#11.) thererfore boat size does not matter, that is a fact. Thanks for your reply Oldman570.
Mr88 maybe you can learn something.
 
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schematic

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1,102
Re: why cant I get over 4200 rpm ?

Have you checked the fuel pump for pressure as well as diaphragm leakage into crankcase?

Hows the fuel? Fresh and 50:1 ?
 
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shadowrider

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Re: why cant I get over 4200 rpm ?

I replaced the fuel pump and gas is fresh midgrade 50:1 with mercury outboard oil. Fuel pump change made no difference. I did not pressure test it.
 
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mr 88

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Re: why cant I get over 4200 rpm ?

Sorry but I own 3 towers that tach out at 6 grand along with a 67- 650 / 73- 500 and built one short leg tower over the winter,so I won't learn anything .You start out and ask for help then question my questions and of course never answer them.How did it run before you installed the San Leandro jet pump? Did you acquire this problem as is or was the setup running fine last season,then you decided to install the jet pump .Your main jets and float control mid to high speed performance not your idle jets so adjusting them will not affect your top end.Even if the questions seem stupid to you they are asked for a reason,just try answering them or don't get on here and ask for help.
shadowrider said:
I have done a link and sync, carbs are open etc. All the obvious things have been checked 3 times.
It has a San Leandro jet pump. Their website states that a jetpump developes full power at a standstill.
(http://www.outboardjets.com/faqs.php#4),(Look at #4 and#11.) thererfore boat size does not matter, that is a fact. Thanks for your reply Oldman570.
Mr88 maybe you can learn something.
Click to expand...
 
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wired247

Lieutenant Commander
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Oct 8, 2011
Messages
1,557
Re: why cant I get over 4200 rpm ?

You don't have enough power to run the pump past 4200 RPM. Its an 85HP 4 cyl. You need more power to run that pump faster than 4200. Theres your answer.
 
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shadowrider

Petty Officer 2nd Class
Joined
Feb 19, 2009
Messages
195
Re: why cant I get over 4200 rpm ?

Wired247, that is a tempting possibility, As I stated earlier, I called the manufacturer and he verified that I had the right pump and it should spin up to 5500 rpm. I have a G pump which is the proper one for that motor.
Mr88 you are not bright enough to be on here, My first statement was that I had a jetpump, then you write "What size boat and prop pitch ? You could be way over pitched, was the speed the same etc before you installed tach ?", Do you know that a jetpump has no prop?
Learn the facts before you get your panties in a knot !!! Again you could learn something here unless you are too stupid.
 
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