48 hp Johnson surging

roccos

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Feb 14, 2008
Messages
9
Help!
I recently bought a small RIB with a 1994 Johnson 48hp. Motor runs fine up to full throttle but when I go to full throttlle (in the 4500 to 5000 rpm range)it surges until I back off a bit.
The fuel is fresh, tank is clean, hoses look good. spark plugs are very good, compression is good. I checked the screen in the fuel pump it's good. Carbs were rebuilt a few years ago.
Any ideas??????? Thanks in advance.
 

tashasdaddy

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51,019
Re: 48 hp Johnson surging

could be fuel pump not keeping up, restricted vent on fuel tank. you need to get it turning 5500.
 

ezeke

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Sep 19, 2003
Messages
12,532
Re: 48 hp Johnson surging

Most often, surging at high speed is the result of an air leak somewhere in the sytem. A fuel restriction can produce similar lean condition.

Check the float bowl gasket, hoses, tank vent, etc. Make sure the carburetors and intake manifold have good gaskets and that the bolts are torqued to spec.

Check that the fuel filter is right for your motor and clean.
 

ICEMAN

Petty Officer 1st Class
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Dec 11, 2000
Messages
292
Re: 48 hp Johnson surging

I notice you indicated that the motor is running in the 4500-5000 range. Is that the tach reading? That motor may have a power pack with a rev limiter, which will give you the symptom you indicated. You may want to check the tach to see if it is accurate. the rev limiter kicks in around 6200 rpm. you could also have a misfiring pack.
 

roccos

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Feb 14, 2008
Messages
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Re: 48 hp Johnson surging

I was just guessing on the RPm's. I'll recheck it next time out and repost.

EZEKE, I did not see a fuel filter, only the screen inside the pump. Am I missing something?
 

tashasdaddy

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Re: 48 hp Johnson surging

most have an white inline fuel filter before the fuel pump.
 

ezeke

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Re: 48 hp Johnson surging

If you don't have a fuel filter, I would strongly suggest that you get one.

The full-sized water separating filters mounted near the tank are the best.
 

pete6464

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Apr 26, 2008
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21
Re: 48 hp Johnson surging at 2900 rpm

Re: 48 hp Johnson surging at 2900 rpm

i noticed my fuel bulb was deflating and that when you opened the fuel tank cap air rushed in.tank was in vaccum.how high an rpm should 48spl run at?
 

ezeke

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12,532
Re: 48 hp Johnson surging

When the bulb deflates like that your fuel pump cannot keep the float bowl full of gas to power the engine. The engine can start to run lean (too much air or not enough gas) which can do a lot of damage to the pistons.

A lot of portable tanks need to have the cap open a little whenever you are running. Some have a smaller cap on the top of the fill cap that needs to be opened. Whatever you have, you have to be sure that the tank has adequate ventilation to compensate for the fuel that is being drawn out.
 

ezeke

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Sep 19, 2003
Messages
12,532
Re: 48 hp Johnson surging

The 1990 models had a full throttle operating range of 4500 to 5500 RPM, which generally means you would prop to run slightly higher wide open, say 5800. I think the 1994 model has the same range.
 
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