96 200hp Johnson Venom loosing power at times?

[dwaynelo]

I have a 96 200hp Johnson carb motor on a 20' Gulf Coast that has about 225 hours on a new power head. WOT is about 5300 rpm with a shallow water cup in the prop. A couple of weeks ago, it would only run about 4600-4800 rpm's and I noticed it had lost power. I changed the spin on fuel filter, plugs and put some 92 octane gas in it (mixed with what was in the tank) and it went back to 5300rpm's. Today, I was running WOT and it dropped to 4800 and then to 4500 and then to 4300 and you could feel and hear the lose of power. All the sudden it got a burst of energy (lol) and hit 5300 rpm,s again. You can feel it surging at times like it has a plugged filter or bad gas. When I got home today, I pulled the spin on filter off and poured it in a glass and let it set, to see if I had any water in the fuel and no water. I changed the in line plastic filter under the cowling and it was easy to blow through with no resistance. I changed it today anyway. Can a fuel pump be going bad or be weak? Is the fuel pump a diaphragm style pump? Aren't they usually good or bad? It seems to be fuel related. Any suggestions or thoughts??????

 

[flabum]

Re: 96 200hp Johnson Venom loosing power at times?

It could be a few things. Get a vacuum gauge and hook it inline on the fuel hose before the engine. Put a piece of clear hose in as well. Take it for a spin and note the readings and monitor the fuel flow through the clear hose. Vacuum of less than 2 inches and air bubbles in the clear hose indicates an airleak into the fuel system ( no air bubbles and less than 2 inches indicates a bad fuel pump). Vacuum over 4 inches indicates a restriction.

 

[HighTrim]

Re: 96 200hp Johnson Venom loosing power at times?

Welcome to iboats. First off, there is no need for the high octane fuel, it only wastes your money, 87 octane is what you need. The surge may be fuel delivery related. When were the carbs last rebuilt? Have you tried hooking up a remote tank with fresh gas to isolate the built in tank for issues or fuel lines for leaks? I would then inspect the plugs. What type are you running, what is their condition? Then check spark. You should get the spark to jump 7/16 of an inch with a bright blue ZAP!! If spark is good, check the timing with a timing light as per your manual.

 

[dwaynelo]

Re: 96 200hp Johnson Venom loosing power at times?

Flabum, do you recomend installing the clear hose and guage outside the cowling so it is easier to see or just run it with the cowling off? Do I need to run it at WOT or will the problem show up at any rpm?

 

[flabum]

Re: 96 200hp Johnson Venom loosing power at times?

Flabum, do you recomend installing the clear hose and guage outside the cowling so it is easier to see or just run it with the cowling off? Do I need to run it at WOT or will the problem show up at any rpm?

Yes and yes to WOT

 

[HighTrim]

Re: 96 200hp Johnson Venom loosing power at times?

Type of setup Flabum is talking about.

"http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&sspagename=STRK%3AMEBTOX%3AIT&viewitem=&item=4610113711&_trksid=p3984.cTODAY.m238.lVI"

 

[dwaynelo]

Re: 96 200hp Johnson Venom loosing power at times?

Hightrim, I copied the info you sent and iy did not pick up anything. I take it that it is something sold through e-bay? Vacume guage setup?

 

[HighTrim]

Re: 96 200hp Johnson Venom loosing power at times?

yes, should work, copy without the " " then paste in your browser window.

 

[dwaynelo]

Re: 96 200hp Johnson Venom loosing power at times?

Thanks, HighTrim, Got it.

 

[gregr1971]

Re: 96 200hp Johnson Venom loosing power at times?

i have that guage set up if you need to borrow it.

 

[dwaynelo]

Re: 96 200hp Johnson Venom loosing power at times?

Flabum, does it matter how long the vacume line will be? Can I run the 1/4" vacume line up by my console and put the guage on so I can see it while I run or does it need to be near the tee?

 

[dwaynelo]

Re: 96 200hp Johnson Venom loosing power at times?

Greg, thanks for the offer. I may take you up on it. Thanks for all the assistance form all you guys and I hope ya'll have a great 08! Happy New Year!!!!!!!!!!!!!

 

[dwaynelo]

Re: 96 200hp Johnson Venom loosing power at times?

Hightrim, see my response to your questions below.

Welcome to iboats. First off, there is no need for the high octane fuel, it only wastes your money, 87 octane is what you need. The surge may be fuel delivery related. When were the carbs last rebuilt? (2 years ago when the new power head was put on. I run the boat every weekend at least once if not twice.) Have you tried hooking up a remote tank with fresh gas to isolate the built in tank for issues or fuel lines for leaks? (Not yet, it just started it again today) I would then inspect the plugs. What type are you running, what is their condition? (I changed them last week. They looked OK but changed them anyway. QL77's; I may change to QL78's) Then check spark. (I will) You should get the spark to jump 7/16 of an inch with a bright blue ZAP!! If spark is good, check the timing with a timing light as per your manual.

 

[tashasdaddy]

Re: 96 200hp Johnson Venom loosing power at times?

it also sounds like a classic case of a clogged fuel tank vent. dirt dobbers.

 

[HighTrim]

Re: 96 200hp Johnson Venom loosing power at times?

I agree TD, that was one of my first thoughts and why I asked if he hooked up to a remote tank to rule that out. You cant suck fuel out of a vaccuum. Dwayne your anti syphon valve is located where the rubber fuel line attaches to the tank fitting. It will be aluminum, about 2" long, and the insides of it will consist of a spring, a ball, and a ball seat. You can remove this, possibly solving your problem, or replace it. They are notorious for sticking in a semi closed position, causing a fuel restriction, hence dropping the rpms as you say is happening. If you do remove it, there is a chance that you will get fuel running back to the tank since it is lower in comparison to the carbs, motor etc... The fuel bulb should prevent this, due to its inner valves, but if this does happen, you will need to replace the anti syphon valve instead of just removing it.

 

[dwaynelo]

Re: 96 200hp Johnson Venom loosing power at times?

Hightrim, I will check the valve. It seems to be working because when I am fueling it, you can hear and feel the the fuel vapor coming out of it when I am pumping fuel into the tank.