Logo
FULL SITE MENU

Johnson 120hk -86 bogs down at 4500+ RPMs

F

Forsete

Member
The engine runs fine up to 4000 RPMs but occasionally starts to bog down and up again at 4500 and above.
Sometimes I can run in WOT for approx 1 minute w/o issues.

This have been done to try and solve it:
Changed the primer bulb.
Different tank of gas.
Rebuilt carbs and blew them with compressed air.
Rebuilt fuel pump**

**I am a bit concerned that the pumps diaphragm might be worn out since its so stretched on the "pumping" areas?
Picture of the diaphragm in fuel pump:
1.jpg
 
Yes, I'd suggest replacing that diaphragm as long as you've got it all apart anyhow.

If pumping the fuel primer bulb corrects the problem, I suspect that diaphragm. However if the primer bulb has a tendency to be drawn flat... suspect a fuel restriction such as what follows.

(Fuel Anti Siphon Valve)
(J. Reeves)

Many of the later OMC V/6 engines incorporate a fuel restriction warning via a vacuum device attached to the powerhead. If the engine overheats, or if you have a fuel restriction, the warning is the same.... a steady constant beep.

NOTE... Only the V/6 & V/8 engines have the above "Fuel Restriction Warning". The warning horn will not sound on the other models.

The fact that a engine is not overheating, but the warning horn sounds off with a constant steady beep, and that the rpms drop drastically would indicate that the engine is starving for fuel due to a fuel restriction. Check the built in fuel tank where the rubber fuel line attaches to the tank fitting. That fitting is in all probability a "Anti Siphon" valve which is notorious for sticking in a semi closed position. It will be aluminum, about 2" long, and the insides of it will consist of a spring, a ball, and a ball seat. If this valve exists, remove it, knock out those inner components which will convert it to a straight through fitting, then re-install it. Hopefully that cures the problem.

The above procedure will cure a restriction problem with the anti siphon valve as stated. BUT, it may also allow fuel to drain backwards to the fuel tank when the engine is not running (siphoning backwards) due to the fact that the carburetors/fuel pump etc are higher than the fuel tank. This condition is not an absolute as the valves in the fuel primer bulb usually prevent this backwards siphoning problem. However.... if this does take place, the cure would be to install a new anti siphon valve.

NOTE: There has been cases when the output valve in the fuel primer bulb would come apart, and the inner portion of the valve would actually reverse itself and be drawn back into the primer bulb's output valve body. This in effect would create a shut off valve and result in a fuel restriction. If this is the case, you should be able to feel something laying in the bottom of the primer bulb when held horizontally.

Thousands of parts in my remaining stock. Not able to list them all. Let me know what you need and I'll look it up for you. Visit my eBay auction at:

http://shop.ebay.com/Joe_OMC32/m.html?_dmd=1&_ipg=50&_sop=12&_rdc=1
 
Thanks for the response Joe,

I find it hard to keep up with the priming when running fast, feels a bit like the primer bulb cant keep up with the consumption (same feeling with the old and new bulb and the two different portable tanks I use). When I press the bulb it doest flex back out quick enough for me to be sure that I can push/provide more gas than consumed.. at speeds at around 3000RPMs its no problem to keep the bulb hard (but there are no issues at this range).

I will try and find a replacement diaphragm.
I will also replace the hose from tank to pump.

I just hope this isn't an expensive electrical issue or mechanic for that matter...
 
If the fuel primer bulb does not flex back immediately, that indicates a fuel restriction... strange however that this would take place on separate portable tanks UNLESS the air vent is not open.
 
Hi Joe,
I was very thorough with the vent on both tanks.

- I will try and bypass the anti siphon valve
- Get a new diaphragm to pump
- New hoses
- Add carb cleaning additive to fresh fuel from a different gas station
..this weekend.

Thanks
 
Last edited:
One thing..... Between the heavy duty carburetor face plate and the carburetors, there is a thick rubber gasket (part of the face plate). If that gasket is missing or damaged, it will have an affect on the engine similar to what you've mentioned.

However, the situation with the fuel primer bulb still indicates a fuel restriction ie..... won't inflate quickly etc.
 
I have now tested the engine again. Unfortunately the repair kit for the fuel pump was not correct so I had to use the same diaphragm.
I replaced all hoses and added carb cleaning fuel additive to fresh gas.
I did not remove the siphon valve but I doubt that is the issue any more (read below).

The engine ran perfect for at least 20min of revs above 4500k and approx 5min of WOT, I thought it was fixed so I cruised at 3000 rpms for a period 2min and then increased throttle once more and the issue came back.. It started to quickly bog down and up again if I tried to push above 3500 rpms, I felt the bulb and it was firm, pressing on it did not help. I also tested to choke with the key but that just made it loose more power.
Feels a bit like it could be ignition issues, its a lot like the issue that I had with a poor ignition cable last season, but then I was constantly REALLY short on power and not "intermittent" like this.
Seems like the issue come after the engine is really warmed up, because it newer happened within the first 10 minutes.
 
I am having a fuel starvation issue with my v4 Johnson 125hp 2000. I have never had any problems with it. One trip out 6 seconds after getting up on plane it bogged down. Pumping bulb helped. After third failed trip it would die at idle in the tank. I bypassed the fuel system to the engine with another tank, changed fuel filter and plugs. When I rebuilt the fuel pump it ran in the tank in neutral to 5000 rpm fantastic. Today I took it out and it went into fuel starvation idling away from the dock. When I rebuilt the vro fuel pump the kit did not have a new o ring for the diaphram shaft or vacumm port. I would appreciate any advice
 
Last edited:
It was run at 1000_2000 just up to 5 for a couple seconds to see if it starved. I have been running the boat hundreds of miles the past two years without issues
 
Forsete.... Carefully and closely, visually inspect the stator under the flywheel to see if it may be melting down... a sticky looking substance dripping down upon the timer base and the powerhead. If this condition exists, that would resulkt in a voltage drop from the stator to the powerpack which in turn would cause erratic., weak, and eventually no ignition when the stator heats up.
 
Back
Top