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Johnson stalling/lack of power

tlea MOTOR: 1996 Johnson 175FS. Pretty low hours and not driven hard. Motor had a tendency to stall on acceleration at the carb transition point.(going thru 1800 RPM's) I found somewhere that the initial setting for the idle screws was 8 turns out. Screwed all the jets in and backed out 8 turns. Seems to run OK when cold, but as the day goes on the stalling problem gets worse. Just a bit of choke fixes it (temporarily). Yesterday (for the 1st time) I noticed a definate lack of power trying to get on plane. After it was on it ran fine. This only happened later in the PM (91 degrees F) and the engine was fully warmed up. Do I screw the jets out farthur? How far can I go? Idle's fine. WOT fine. Checked throttle sync-OK. Used an air flowmeter on each carb-all the same. New plugs. Good gas. Will do compression ck today, but have no reason to suspect a problem here.

Thanks
Tom Lea
2002-06-02
seahorse5 It sounds a bit like a vapor lock (heat soak) problem. Vacuum check your fuel system at all speeds and make sure it is less than 4" of restriction. Set the synchronization to have the carbs start opening 1 click after the spark advance. What is your top rpm? It should be close to 5500 rpm. Use Champion QL78YC plugs gapped at .030". See a knowlegable dealer to see if your motor was included in a service bulletin about changing nozzle vent sizes. 2002-06-02
tlea If it's heat soak, how's come ALL (or a LOT) of Johnsons don't do it?

Will do the fuel vacuum check tomorrow!

Top RPM right at 5500-5600

Will TRY this spark advance setting. Have a book and some knowledge and will give it a go.

Been using Champion QL77CJ4 plugs.Will try the QL78Yc's tomorrow too.
Will check with dealer tomorrow too.

Is there anywhere on the Internet to find if the motor was included in any Tech Service bulletin's?

Thanks for the help

Tom Lea
2002-06-02
oceanrunner I do not have an answer to your problem, but I noticed that you have a Johnson 175 that is one year newer than mine. I do not know if the fast strike is comaparable to the oceanrunner model that I have, but maybe you can help me. I have a couple look vacuum lines that I do not know where to hook them up at and a red switch on the fuel system that I do not know what position it should be in. Can you look at your motor and let me know where they go. I can be reached at [edited] 2002-06-02
tlea H-E-L-P
On the lake today: RPM's 5600. Fuel vacuum 4" with an occational jump to 6" under throttle burst. Spark plugs - QL78YC to .030"

**** thing ran great. Only a minor hesitation on initial acceleration and I think that could be carb idle adjustments.

THE DIFFERENCE: I did not have the cowling on today. There was no heat build-up under the cover while sitting in the sun! What's under there that is so affected by heat build-up? HOW do you find it?

Tom
2002-06-04
jegervais Where did you "tee" in your vacuum gauge? That engine should not pull that kind of vacuum unless there is a problem in the fuel delivery system.

What's under there, you ask?? Well a black poerhead (hot), a cover that's possibly a dark color, regardless, when it's on it holds the heat in. Fuel in the carb bowls will vaporize due to the higher temps (higher octane fuel makes it worse) and vaporized fuel won't go thru the system. The engine won't run well until the vapors are cleared out and liquid fuel get to the carbs...

-John
2002-06-04
tlea Well, I tied in the vacuum gauge in the line going to the vacuum switch that controls the warning light. 4" is fine. A momentary reading 6" on a throttle burst is to be expected. The warning horn is not set to go off till about 7" or so.
Thanks for the lesson on the contents under the cowl. If fuel vaporizes in the float bowl of one carburetor then logically speaking fuel will vaporize in all the carburetors in all the engines of the same color on the same lake on the same day. That doesn't happen. If it did, everyone in Florida and other southern states would be constantly having this problem, especially in the summer.
I more suspect something in the electrical arena that is affected my the high temperatures. I don't know if the spark advance on these engines is mechanical or electronic. SOMETHING is being affected, and it's NOT on ALL 1996 Johnson 175's.
Tom
2002-06-04
seahorse5 4" of vacuum is too high, that is the max. recommended by Johnson. Going even higher on acceleration, 6", is even worse.

A good fuel system will have about .5" to draw the fuel thru 8' of 3/8" hose, 1.5" for a good anti-siphon valve, and an addtional .5" for a spin-on fuel filter. That makes 2.5" which is attainable but most boats are just below 3". The more vacuum in your lines, the quicker the fuel "boils" or vaporizes.

There was a good article about a year ago in Bass and Walleye Boats about this and how to test it.
2002-06-04
tlea Vacuum is exacrtly the same for both tanks. Would vacuum be high if the tank switching valve was not completely switched. Factory didn't do so hot a job there.

What would be the down side of simply removing the anti-siphon valves from the tanks tops?

thanks
Tom
2002-06-07
jegervais Seahorse is absolutely correct. 4" vacuum is too much. Make up a tee with two 3/8" id fittings for the fuel supply lines and use whatever size fitting your vac gauge requires for the 3rd fitting. Connect it to the fuel filter inlet (on the engine) and re- 2002-06-07
tlea The test procedure makes perfect sense. Will try it tomorrow.
Prolly just replace the gas line as a matter of course. It's cheap and an easy job.

Thanks
Tom
2002-06-08

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