# How to check vacuum to fuel pump johnson 90hp



## My Demeyes (Aug 2, 2010)

I'm trying to get my new to me 1992 johnson 90 hp going again. It was running in the test tank, I took it to the lake to run it and it wouldn't hold fuel pressure. I did discover some air leaks at the tank fittings, fixed those. Now it has good fuel to the new fuel pump, but zero fuel out of the pump. I'm confused as to why the fuel pump won't push fuel to the carbs.


----------



## bountyhunter (Apr 28, 2004)

does the squeeze bulb stay hard?


----------



## My Demeyes (Aug 2, 2010)

It does, but I can't even get it to fire, because the fuel won't pass through the fuel pump.


----------



## My Demeyes (Aug 2, 2010)

It's a new pump, I've taken it apart to check for problems, but all is good. I'm suspect of a low vacuum problem that won't operate the fuel pump. I'm just not sure how much vacuum it should produce.


----------



## fishingguy (Jan 5, 2006)

I've learned when fuel is the problem, replace it all. Every fuel line, vacuum hose, clamp, filter, pull the carbs and rebuild them. Do it all once and be done with it. It's really not that hard, carbs may intimidate you, but it's really quite easy. There are a lot of great driveway running motors that seem to revolt when they get put under a load. Save yourself some head aches, do it all.


----------



## Uncle Paul (Jul 10, 2004)

fishingguy said:


> I've learned when fuel is the problem, replace it all. Every fuel line, vacuum hose, clamp, filter, pull the carbs and rebuild them. Do it all once and be done with it. It's really not that hard, carbs may intimidate you, but it's really quite easy. There are a lot of great driveway running motors that seem to revolt when they get put under a load. Save yourself some head aches, do it all.


Something to look at most carbed engines around the time frame that engine was produced used both vacuum and pressure from the engine to drive the fuel pump. The pump was nothing more than a diaphragm vibrating back and forth with two check valves, to operate the pump it would have a hose called a pulse line to the engine, take this hose off at the engine and crank the engine you will feel both vacuum and pressure as the engine cranks. Replace the hose checking the fittings for leaks the moving up the hose some engines had what is called a pulse dampener it smoothed out the pulses to the pump. Check fittings to this unit and remove the hose to the pump side and crank the engine again checking for pressure and vacuum if not this needs replacement if its good replace hoses check fittings.If all is good to this point you have a gas delivery problem not the pulse line


----------



## Uncle Paul (Jul 10, 2004)

Uncle Paul said:


> Something to look at most carbed engines around the time frame that engine was produced used both vacuum and pressure from the engine to drive the fuel pump. The pump was nothing more than a diaphragm vibrating back and forth with two check valves, to operate the pump it would have a hose called a pulse line to the engine, take this hose off at the engine and crank the engine you will feel both vacuum and pressure as the engine cranks. Replace the hose checking the fittings for leaks the moving up the hose some engines had what is called a pulse dampener it smoothed out the pulses to the pump. Check fittings to this unit and remove the hose to the pump side and crank the engine again checking for pressure and vacuum if not this needs replacement if its good replace hoses check fittings.If all is good to this point you have a gas delivery problem not the pulse line


----------



## Uncle Paul (Jul 10, 2004)

Just thought of another thing you asked about how much vacuum is required I’m sure there is a spec somewhere but if the engine has enough compression to run it will have more than enough vacuum to drive the pump. Also most pumps if the diaphragm has a hole you will see a gas leak out of it


----------



## My Demeyes (Aug 2, 2010)

Well, I had a DAA and had the inlet and outlet lines switched to the fuel pump. I got em switched when I was replacing the fuel lines. Got it started up but it runs rough, has a bad stumble at idle. This is a motor that sat for a long time. Has low hours, but sat many years. I've cleaned carbs in ultrasonic cleaner, put in carb kits, new fuel pump. I have a service manual, and everything is set per the manual. Just cant get it to smooth out. I've not had it on the water yet to run it hard to clear it out. I did do the seafoam decarb thing, but I just can't get it to stabilize to an acceptable idle before I take it out.


----------



## My Demeyes (Aug 2, 2010)

Also has 125-130 lbs compression after warmed up.


----------



## bountyhunter (Apr 28, 2004)

great, pull the plugs see if they,ll tell you which cyc is giving you the problem.


----------



## My Demeyes (Aug 2, 2010)

bountyhunter said:


> great, pull the plugs see if they,ll tell you which cyc is giving you the problem.


I pulled em and checked when I did the compression check, they all looked the same, black and wet


----------



## My Demeyes (Aug 2, 2010)

I put new plugs in and ran for 20 minutes at high idle, 3 plugs were black, 1 was fairly clean. All 4 cylinders have spark. This was with a seafoam cocktail for fuel. The suspect cylinder might have a weaker spark, hard to tell.


----------



## firemanmike2127 (Mar 17, 2013)

A couple of questions. Did you purchase this engine from a credible individual that gave you an accurate history of use & was the engine properly fogged with oil prior to the extended period of storage? I'm going to assume that you've picked up a decent piece of equipment that just needs some routine servicing & proper tune-up adjustments, That being said, we're discussing an outboard motor that is 25 years old, & an engine that has glazed cylinder walls from many hours of run time or previous overheating damage will idle poorly, if at all. Something that I always look at first is the idle stop screw. If someone has 'used up' most of the available adjustment (idle stop screw screwed most or all of the way in) that's a pretty reliable indicator that the motor is has internal issues & is 'tired'. First, were the cold compression readings the same as the when the motor was warm ? 125-130 is a pretty average reading for a 100 cu. in. V-4 of that age. Secondly, have you checked the spark with a proper tester (one that has an adjustable gap setting & allows you to watch all the cylinders simultaneously). The spark should be bright blue, consistent, & 'snap' every time each cylinder fires. Set the gap to 1/2" prior to performing the spark test & crank the engine for 15-20 seconds while evaluating the ignition performance. Lets say that all is well with the condition of the cylinders/rings & the ignition is firing properly. The next area to look at is the synchronization. Ensure that the throttle plates are fully closed & also that they open simultaneously (the linkage connecting the carbs is adjustable). Now, on to the timing portion. First, there is a procedure outlining how to set the adjustable timing pointer (on some engines this index mark is not adjustable). Follow the procedure in your manual to adjust the timing pointer (it's hard for me to explain this step & the pics help a lot). One of the real critical steps is next, & that is setting the pick-up timing. That's the point where an embossed mark on the throttle cam aligns with the throttle linkage roller & the carburetors are just starting to open. There will be a specified timing setting (in degrees) & you'll definitely need to look at the pictures in your manual for a reference so you can see how the embossed mark & roller should be aligned AFTER the ignition pick-up timing has been set. This is a very important step & it has to be done correctly so the engine had the proper amount of timing right when the throttle plates are starting to open. Wide open throttle timing has to be set last, & it's also very important to make that adjustment when the motor is running very close to near maximum RPM. It's absolutely essential that the timing & synchronization adjustment procedure is done correctly for an older carbureted motor to idle well & accellerate smoothly Let us know how this works out for you


----------



## firemanmike2127 (Mar 17, 2013)

The lower plug looks like it's not firing properly......Mike


----------



## My Demeyes (Aug 2, 2010)

firemanmike2127 said:


> The lower plug looks like it's not firing properly......Mike


Thank you for the detailed reply. I bought this motor and boat, just for the motor, before I pulled it off the donor boat I had it running fairly good without doing much to it. The motor probably only has 100 hours or less on it. Knox marine did the last service on it in 2010, and the boat was last used in 2012. Prior to the 2010 service it sat for over 10 years. I am assuming it wasn't fogged before sitting as the owner passed, and his widow let it sit in the yard for many years. So the story goes. I have checked the synchronization of the carbs per the manual, but I don't have the test propeller to properly check timing. Nothing appears to have been moved from the factory settings as the orange thread sealer on the threaded rods all seems to be in its original position. I was thinking it just needed to be run to clean it out good, but haven't had it on the water yet. After the seafoam treatment it is draining some black residue from the exhaust.


----------



## Uncle Paul (Jul 10, 2004)

firemanmike2127 said:


> The lower plug looks like it's not firing properly......Mike


My Demeyes: your compression numbers look good but got a question about you plug picture, was the picture taken after the seafoam treatment before you ran the engine on the normal fuel mix. If it is you are looking at a false reading because the treatment will make the plugs look very dark and wet, in my experience with the seafoam treatment it will take some time running on good fuel and getting the engine good and hot before you can get a good plug reading. If the picture is after the treatment and

after some running time on good fuel the picture look like the upper plug has water on it(the small bead at 6 o’clock just below the center electrode)It also could be gas because it looks like its running way to rich. The bottom plug looks like that cylinder has no spark but it’s hard to see from a picture


----------



## Uncle Paul (Jul 10, 2004)

Uncle Paul said:


> My Demeyes: your compression numbers look good but got a question about you plug picture, was the picture taken after the seafoam treatment before you ran the engine on the normal fuel mix. If it is you are looking at a false reading because the treatment will make the plugs look very dark and wet, in my experience with the seafoam treatment it will take some time running on good fuel and getting the engine good and hot before you can get a good plug reading. If the picture is after the treatment and
> 
> after some running time on good fuel the picture look like the upper plug has water on it(the small bead at 6 o’clock just below the center electrode)It also could be gas because it looks like its running way to rich. The bottom plug looks like that cylinder has no spark but it’s hard to see from a picture


 Mr Demeyes just read your last posting about dripping seafoam an because I’m not sure if you have ever done the seafoam treatment a suggestion You need the run the engine hard after the treatment to blow all of the loosened crap out of the engine if its dripping it’s still inside the engine and you won’t get a good plug reading


----------



## My Demeyes (Aug 2, 2010)

Well, after much head scratching, I pulled the carbs again to find the idle circuit feeder tube was completely blocked in the cylinder with the clean plug. I think that's the problem. I'll try it again tomorrow. I got to test every ignition component on it just for fun, lol.


----------



## My Demeyes (Aug 2, 2010)

Got it running fairly smooth at idle, it still has a little burp once in a while at idle. I'm hoping to put it on the water Saturday to blow out the cobwebs. I have a kicker as a backup if I need it to get back in. Probably go out of catawba or mazuriks Saturday morning.


----------

