1970 4hp Johnson low compression

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ugmold

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saltchuckmatt said:
I mean the motor might be done. Fuel pump works off of the crankcase, with your low compression it may no longer work. Spit balling really.
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So if there is not enough compression the pump won't work? I guess that could be it.
 
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Crosbyman

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post 17 !!!

the pump does not involve cylinder compression it works from the pressure build up in the crankcase when the associated piston is on a downstroke. pressured & vacum pulses to the pump come from the crankcase ....not the burn chamber of the piston.

for starters...to eleminate the pump as a prime suspect just feed gas directly DOWN into the carb inlet with a small can held above the the carb. (blood transfusion style) if the engine works then you can backtrack and find out why the pump is not pumping properly to fill the carb. if not fix the carb.

make certain the rear end of the pump lines up with the side hole where it mounts .. look for anything hampering the crankcase pulses from reaching the rear end of the pump.

I have used cheap chineese pumps with success. those 4hp are hard to kill if oiled properly in their life. .


by the way.... have you done anything electrically to this kicker ... like checking the coils for cracks , cleaning points, replacing the condensers (probably bad after 50 years) measuring ohms from sparkplug boots to engine frame ...you need 3-8 Kohms approx. WHile you take this steady measurement......bend-flex the plug wires to make certain no wires are cracked in a bend somewhere. IF you do change the coils change the wires & boots

 
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saltchuckmatt

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Crosbyman said:
post 17 !!!

the pump does not involve cylinder compression it works from the pressure build up in the crankcase when the associated piston is on a downstroke. pressured & vacum pulses to the pump come from the crankcase ....not the burn chamber of the piston.

for starters...to eleminate the pump as a prime suspect just feed gas directly DOWN into the carb inlet with a small can held above the the carb. (blood transfusion style) if the engine works then you can backtrack and find out why the pump is not pumping properly to fill the carb. if not fix the carb.

make certain the rear end of the pump lines up with the side hole where it mounts .. look for anything hampering the crankcase pulses from reaching the rear end of the pump.

I have used cheap chineese pumps with success. those 4hp are hard to kill if oiled properly in their life. .


by the way.... have you done anything electrically to this kicker ... like checking the coils for cracks , cleaning points, replacing the condensers (probably bad after 50 years) measuring ohms from sparkplug boots to engine frame ...you need 3-8 Kohms approx. WHile you take this steady measurement......bend-flex the plug wires to make certain no wires are cracked in a bend somewhere. IF you do change the coils change the wires & boots

Click to expand...
Whoa!!! Crosbyman on the war path!

I know compression doesn't affect the pump but tells a story of how worn the motor is. (Rings, bearing, seals) All of those things could affect how the pump gets it power from.

If you read the OP is not getting fuel out of his fuel pump and I just wanted to let him know it could be more than just a pump.

Did you see his compression numbers?
 
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Crosbyman

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no wars for me... :)

I have faith in that little kicker. the compression readings are low yet I have this feeling the measurement process or meter quality is suspect at this point. just me I guess.

if the reeds are ok pressure build up in the crankcase should build up enough to work that small pump.

who knows as you mention, maybe the crankshaft seals are shot and no pressure build up is possible . Some suction only should at least be felt on the carb face .

even the side pulse port should produce "some" pressure pulsing on a fingertip. If none... well RIP it used 4hp are cheap. :) .

Come to think of it rigging up a small pressure/vacum meter on the pulse port should should ...some needle movement ! If none... well RIP it

as to compression I am no mecanic but my 9.5hp works wonderfully at 65psi
 
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ugmold

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racerone said:
There is very little that can go wrong with those pumps !!!------A new diaphragm fixes them in most cases.-----Yes the diaphragm is available all day long.
Click to expand...
If there is not enough compression will they work?
 
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saltchuckmatt

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Crosbyman said:
no wars for me... :)

I have faith in that little kicker. the compression readings are low yet I have this feeling the measurement process or meter quality is suspect at this point. just me I guess.

if the reeds are ok pressure build up in the crankcase should build up enough to work that small pump.

who knows as you mention, maybe the crankshaft seals are shot and no pressure build up is possible . Some suction only should at least be felt on the carb face .

even the side pulse port should produce "some" pressure pulsing on a fingertip. If none... well RIP it used 4hp are cheap. :) .

Come to think of it rigging up a small pressure/vacum meter on the pulse port should should ...some needle movement ! If none... well RIP it

as to compression I am no mecanic but my 9.5hp works wonderfully at 65psi
Click to expand...
Funny how many compression testers test incorrectly.

Interesting that your 9.5 runs that low.... When working on outboards or chainsaws what have you, after 80 to 90 lbs I haven't had much luck.

Maybe I'm to picky or have YOU checked your tester!! ; )

PS I don't think he should give up just know what he might be up against.
 
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racerone

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I can not see or hear your motor.------So what happens when you operate the manual fuel pump.-------If crankcase compression is low than pump may not work.
 
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ugmold

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Is it ok to run this engine (or try to) without a prop? Being engaged all the time makes it tough to work on.
 
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ugmold

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9-15-2022
Last night I took apart the fuel pump, I also found the old parts of the fuel pump I kept when I rebuilt it. Watched a few non specific videos etc. Felt confident on how I put it back together. Diaphragm was a bit saggy/stretched, but didn't look awful.
Put it on the boat, starts up pretty quick 3-5, sometimes 1 pull, runs, dies. I got it running and kept squeezing the bulb to keep it going.
Comparatively speaking it feels as powerful as my "good" 4hp.

• Points/condensers are new aftermarket, installed years ago, rechecked gap.
• I tried the IV gas down the line and it will run like that, hard to keep filling with the prop engaged on the water. (I have no barrel)
• I think it is the fuel pump, that saggy diaphragm might be it (although it has hardly been used).
• The Cheap Chinese Pumps look exactly like what I have, no assembly required. I have read that most rebuild kits don't have diagrams of how to re-assemble, so you still have to make a semi-educated guess.
 
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racerone

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It be OK to run without a prop.-----Just don't open the throttle too much.----And make sure water is well above the impeller.----Like 6 to 8" above.----The factory repair kit ( yes it is available ) has comprehensive instructions.----Nothing to it.
 
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ugmold

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racerone said:
It be OK to run without a prop.-----Just don't open the throttle too much.----And make sure water is well above the impeller.----Like 6 to 8" above.----The factory repair kit ( yes it is available ) has comprehensive instructions.----Nothing to it.
Click to expand...
I found an OEM and like you said it has instruction, and cheap. Thanks
 
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ugmold

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The OEM Kit worked great, took some fussing with the carb (could use a kit, I sub'd some o-rings I had) but now starts on first pull and purrs, smoother than my other one.
This one has a "weedless" prop, and there is some damage in the plastic clutch. It looks like a shorter "pin" was used.
But regardless Things are good and appreciate the help, racerone you've helped me a bunch of times, thanks again.
 
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