Powerhead Advice

[chris-p]

Ive exhausted my backyard mechanical abilities, need some help.

Working on a 1975 25hp Johnson.

Dropped it in the test tank, and she would start lean sneezing pretty badly, so pulled it out and figured I would start with the carb.

Cleaned and rebuilt, then tried again. After 10 secs or so, starts lean sneezing again, I have to back out the low speed needle about 4 turns just to get it to run, and will only run for about a minute until the fuel supply runs out…..

Figured fuel pump was shot, so tried another one. Same thing. Tried a third, same thing.

Power head leaking so bad it cannot run the pump? I dunno, Im stumped. I cannot see any obvious signs of crankcase leak.

Need some ideas here.

[outboard315]

Let’s try the easy ones first, tank vent open? Or does it matter location of tank height wise. Does that tank work on other motors?

[johnyrude200]

Make sure the packing nut/plastic bearing for the low speed needle is there. Easy thing to overlook on those motors (early 70’s thru ’76 models of 18-20-25). It will present in the same way you are seeing. If it is missing or you don’t have one, you can smear some grease or perhaps some vasoline (that will incinerate inside the motor since it’s petroleum based, and you can get it out with carb cleaner if necessary) in there temporarily to see if it helps, but will need one eventually.

Make sure your float is set correct. Level with the upper carb casing, and make sure it wasn’t accidentally install it upside down – the motor will just plain do weird things with no rhyme or reason. Of course, make sure the carb is clean without obstructions through various orifices.

Have you noticed that after it stalls, the carb needs to be re-primed? If it does, then you know it’s a tank or pump issue. If it is flooding as you prime it, well then your float, needle, or boss gasket is bad (or maybe all of them).

You may want to try running it with the choke closed to confirm it is simply a fuel metering issue at the carb, not delivery. If it runs longer before stalling, or at least runs albeit incorrectly, then it’s a metering issue with the air/fuel mixture.

Otherwise you have an air leak somewhere of course. Just be advised that the low-speed needle is just about useless for adjusting things on those motors. I’ve seen them range from 1.5 to 2.5 turns out to get them to idle better without a cough, but they will smoke like hell and blubber badly no matter what you do. Run it too lean and they spit back badly; basically all you can do is tighten the needle to the point where there cough, then back it out enough so it doesn’t but also doesn’t totally blubber out. There’s almost no leeway between the two extremes, either. (thank you very much, stubborn OMC engineers…not listening to the service reps 😀 ).

[slim60]

A while back there was a post about using plumbers rope for packing washers. I’ve used it and it works well.

[chris-p]

I have cleaned and rebuilt the carb. I have done quite a lot of these motors, it is not a tuning issue.

Ill put a fuel pressure gauge in line from pump to carb and see what I get through the various RPM stages.

If memory serves, it should produce 1 psi at ide, up to about 2 psi at high throttle.

I was hoping this would be a quick go over for my friend. Never the case.

[weedlessdrive]

Sounds like a tank/ line issue check the bulb to make sure it’s good. I’ve had them go bad inside and not really leak.

[fleetwin]

OK, so this is not your engine, and you probably don’t know any history on it. But, ask your friend anyway.
The fuel pump may not be working just because of all the strange crankcase pulses while sneezing/coughing. But, it is more likely that that there is an airleak/fuel restriction somewhere. And again, this may be an issue of a low fuel tank height due to the test tank.
Ask your friend how the engine ran the last time he used it. You said you had the carb apart, was it gummed up inside?
Are you SURE the throttle plate is closing/not jammed/synced properly? These engines are cold blooded on their best day. Have you checked compression and spark? This engine uses the beloved low tension ignition, perhaps a weak ignition is causing all the fuss, have you tried new plugs, even though the ones you are using might be "new"?

[crosbyman]

tried with the tank above the motor ?? to see if it runs bettr
if so it seems you ahve a fuel pumping issue

Joining AOMCI has priviledges 🙂

[chris-p]

Yeah, I tried lifting the tank, even though it is the same tank/line I use on all motors I service. Didn’t help.

I tried a few combinations of J6s, J4s. Havent tried another brand as of yet. It came to me with original NGKs in it, and did the same thing with them.

This is the first time I have used this outdoor test tank/fuel tank combo this season. Perhaps the tank is the issue restricting fuel. Had not considered that as I use it so much. Hopefully that is it.

My friend bought the motor and has never used it. I restore all his motors for him. It apparently ran, but we all know how that goes.

Don I hadn’t considered a weak ignition, as all my instincts were telling me the fuel pump was bad, but after 3 pumps I have no ideas other than the motor is not creating pulses to power the pump. I have never encountered that before.

I only added the info about the lean sneezing as I wasn’t sure if it was related to the issue at hand.

Once I finish my after work beer, Im going to try to hook up the fuel pressure gauge and see what is going on there.

Thanks all.

[chris-p]

Compression is almost unbelievable, 175 psi per hole (have to verify gauge!), strong spark.

Will try another fuel tank and hose.

Checked out the linc n sync Don, The cam is pulled out as far as it will go, and still the throttle butterfly does not start to open until it goes in gear and hits the quick slope of the cam. If I remember correctly on these models, the throttle should be closed when the roller is between the hash marks, and just start to open when the roller passes the second mark. The throttle does not start to open until well past ( a few inches along the cam) the second mark.

That still wouldn’t explain the no pumping scenario, but as I type Im putting a fresh rebuild kit into the fuel pump to totally rule that out.

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