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  Ideas wanted for Johnso 40 fuel starvation

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Author Topic:   Ideas wanted for Johnso 40 fuel starvation
msullivan006 posted 09-30-2002 10:13 PM ET (US)   Profile for msullivan006   Send Email to msullivan006  
Sorry for the long winded description, but ..here goes: I've got a very low hours 1987 Johnson 40 on my 13' Sport. It has just started with a problem that has me stumped so far. It starts fine, idles fine, and initially accelerates pretty well up onto plane. Not quite its normal power, but it is smooth and reasonable powerful. Anyway, it will run like this for say 10-15 minutes at partial throttle after cold start, holding plane and sounding good. Then it starts to loose power slowly, and speed sags off until it falls off plane -even on full throttle. Then it continues at this stage of pushing along without quite enough power to get it onto plane, but not getting any worse. There is some sense of surging, or as if it is cutting in and out on one cylinder, but this is fairly subtle. If it is run quite slowly for a few minutes, it will then accelerate better, and get onto plane for a minute or two, and then revert to the not-quite-enough to plane level. After a while more, running it slow has less effect, only allowing it to do a very short burst when throttled up. Squeezing the primer bulb has no effect, nor does squeezing the oil bulb. However, hitting the electric choke gives an immediate burst of power, allowing it to jump back to plane in its old style (but only so long as the choke is used to keep adding brief shots of fuel).

Because of this effect from the choke, I believe that it is a fuel flow problem. Since squeezing the primer bulb has no effect, I believe that the fuel pump diaphram is not a likely culprit. One further idea - I think that we got a good bit of fresh water into the tank just as this first appeared (through a leaking seal around the gas guage) When the problem first appeared, the fuel filter ended up about 1/3 full of water. Seeing that, we pulled the tank and completely emptied it, added dry gas and tried again - no difference at all, although filter stayed clear. Then I drained both carb float chambers through the drain screws on lower fronts and got out some cruddy looking stuff, but again no effect on problem. Carbs were rebuilt this past summer (say 10 hours ago)and were running great until this problem suddenly appeared.

On one hand, it has all of the feel of the fuel flow being restricted by an increasing vacuum building up somewhere along the fuel flow. However, opening the tank cap has no effect, and I don't know of anywhere else to check for something which could block and allow a vacuum to build.

On the other hand, it happened so closely to getting the water in gas that it seems likely that the two are related. but if so, where could there be more water in the system?

Anyway, any gurus out there willing to throw out some ideas?

Thanks to all,

Mike Sullivan

Jiles posted 10-01-2002 08:21 AM ET (US)     Profile for Jiles    
Were the carbs. rebuilt before or after the water contamination? Did you do the work? I would drain the carbs again and if no difference is seen, I would clean them again and concentrate on the main jets. Be sure and check the float level. If possible, I would try this before any work is done----- When you are having the problem, stop the motor and install new plugs. Run the engine "IMMEDIATLY", at wide open throttle, for several minutes. Shut down the engine and remove the plugs, look at the color of the center ceramic insulators of each plug. Most likely the problen cylinder will be lighter in color. They should both be light tan to grey, NOT white.---Good luck--
ShrimpBurrito posted 10-01-2002 12:57 PM ET (US)     Profile for ShrimpBurrito  Send Email to ShrimpBurrito     
MS - I have the same year, make, and power of your engine and had the same problem. The culprit in my situation was the in-line fuel primer bulb, so I removed the pesky thing. I can't tell any difference in starting the engine without it.
msullivan006 posted 10-01-2002 09:15 PM ET (US)     Profile for msullivan006  Send Email to msullivan006     
Great thoughts so far - keep 'em comming. The problem developed after the carb rebuilds, which ran great for 10-15 hours (several weeks). The primer bulb is an excellent candidate, since I replaced the hose from tank to motor a couple of years ago, but kept the old bulb.

My latest thought is that the junk that I got out of the carb bowls may be the real key - it was sort of watery, but also had some black bits. I know that the filter is fine, clean and would catch anything like this junk on the way through. However, the filter only had water and no dirt. Therefore this stuff must have come from somewhere downstream of the filter. Since the carbs were apart and completely clean so recently, I know it is not a residue from years of little use. Therefore, the bulb goes off- must be something breaking down in the short fuel lines inside the engine cover, or inside the fuel pump itself. Looks like replacing all those short lines is just a matter of being careful to not reroute anything, so that's now first on the list (with perhaps a new bulb for luck).

Does anyone know if the fuel pump (VRO on '87 J-40) has a diaphram that I can acquire separately? I got a quote for the pump at $300+. so that's going to be the very last thing I replace.

A couple of years ago, I had almost the same problem with a yard tractor ('86). after throwing a few pieces like a fuel pump at it, I finally took everything apart. The culprit was some stringy stuff that had accumulated in the tank - as it ran, the junk would be pulled tighter and tighter into the pick-up tube and eventually stop the engine. First symptoms were slow loss of power which could be overcome by choke, but eventually just too plugged to run at all. Then, while it sat wherever it was in the yard at the time, the vacuum would release and the junk was springy enough to spread out again and let gas flow. Re-start after a while and same routine - good power, then loose power, then eventually die. John Deere dealer said he had seen that only once before and thought it had to do with spider webs or some such stuff. Anyway, sounds close enough to the current problem to try the same fixes.

Thanks to all, mike

msullivan006 posted 10-01-2002 09:15 PM ET (US)     Profile for msullivan006  Send Email to msullivan006     
Great thoughts so far - keep 'em comming. The problem developed after the carb rebuilds, which ran great for 10-15 hours (several weeks). The primer bulb is an excellent candidate, since I replaced the hose from tank to motor a couple of years ago, but kept the old bulb.

My latest thought is that the junk that I got out of the carb bowls may be the real key - it was sort of watery, but also had some black bits. I know that the filter is fine, clean and would catch anything like this junk on the way through. However, the filter only had water and no dirt. Therefore this stuff must have come from somewhere downstream of the filter. Since the carbs were apart and completely clean so recently, I know it is not a residue from years of little use. Therefore, the bulb goes off- must be something breaking down in the short fuel lines inside the engine cover, or inside the fuel pump itself. Looks like replacing all those short lines is just a matter of being careful to not reroute anything, so that's now first on the list (with perhaps a new bulb for luck).

Does anyone know if the fuel pump (VRO on '87 J-40) has a diaphram that I can acquire separately? I got a quote for the pump at $300+. so that's going to be the very last thing I replace.

A couple of years ago, I had almost the same problem with a yard tractor ('86). after throwing a few pieces like a fuel pump at it, I finally took everything apart. The culprit was some stringy stuff that had accumulated in the tank - as it ran, the junk would be pulled tighter and tighter into the pick-up tube and eventually stop the engine. First symptoms were slow loss of power which could be overcome by choke, but eventually just too plugged to run at all. Then, while it sat wherever it was in the yard at the time, the vacuum would release and the junk was springy enough to spread out again and let gas flow. Re-start after a while and same routine - good power, then loose power, then eventually die. John Deere dealer said he had seen that only once before and thought it had to do with spider webs or some such stuff. Anyway, sounds close enough to the current problem to try the same fixes.

Thanks to all, mike

Cicada posted 10-02-2002 09:28 AM ET (US)     Profile for Cicada  Send Email to Cicada     
Does the Johnson 40 have a power pack? Sounds very similar to problem I had with my '78 70hp.
Bigshot posted 10-02-2002 10:15 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Borrow another fuel tank and hose....that will eliminate that issue. Then remove the fuel bowl drains and drain carbs. You may have water in the bowls. I personally think you have a stuck needle or float. If the lines were bad or pump then squeezing the bulb would bypass it and you would see leaks. The pressing of the enrichner(not a choke) is directly shooting gas into the engine so you obviously are starving of gas. WHich is not bad as long as you are getting oil. No oil....no lubrication....no bearings....no engine. Do a compression test and get those carbs straightened out asap.
grandmufti posted 10-10-2002 12:04 AM ET (US)     Profile for grandmufti  Send Email to grandmufti     
I just fixed a buddies motor that had the problem you describe.It ran like crap all summer long.There was a tiny sliver of rubber that was floating at the metering jet just past the fuel inlet pipe.When he rebuilt the carburetor and slid the old hose back on the barbed fitting he inadvertently shaved a sliver of rubber that would block the metering jet.The boat woulb run fine till you gave it WOT then would starve for fuel.When you backed off throttle the sliver would float off the jet and let the bowl refill with gas.Than motor ran fine till the sliver blocked the jet again.It was just kind of floating around in there.

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