1991 force 120 decelerates after hitting top speed

[leej]

i have been working on this boat for most of the summer....new impellor.....new fuel lines...new diaphram in fuel pump...carbs have been completely gone through with new parts and gaskets....floats have been set properly(11/32 i believe was the setpoint for the floats) as well as the idle screw....new primer bulb...see through in line fuel filters one between tank and water seperator and the other between fuel pump and carbs...we have had the boat out about 6 -7 times after all the work and it has run like a champ 35mph top and never gave any trouble...the other morning i was out in it just at day break....i was running at wot and hit a patch of floating milfoil but was out of it quick...just after that i noticed the motor bogged slightly and dropped to 32mph (tach does not work) and i noticed a slight missing in the motor but not too noticeable...checked to make sure nothing was wrapped in prop.....took it out today and it went up on plane just fine and then as soon as I hit top speed it bogged down again and would only run 25 mph...but i could back out of the throttle and and hit it again and again come up on plane with same effect....you can here the motor try to push the boat but then bog again...not sure what it may be since we have been through just about everything....it has had a compression test which was good...good gas and oil in tank....new rectifier...please help any advise will be appreciated...i can give more info if needed

 

[pnwboat]

Re: 1991 force 120 decelerates after hitting top speed

Try squeezing the primer bulb to see if that has any affect. If it helps then I would suspect that you have a fuel delivery problem. If you have any "quick connect" fittings on the fuel lines, try by-passing or replacing them. May even have a bad primer bulb. There is a one way valve in the primer bulb that can go bad and cause a restriction.

If everything checks out with the fuel lines and quick connects (if you have them) and squeezing the primer bulb has no affect, look for vacuum leaks around the gaskets under the carburetors and carburetors adapters/intake manifold. Also check all the vacuum hoses. There is a vacuum balance hose that goes between the carburetor adapters. Make sure it is in good shape and secured properly on both ends.

 

[jerryjerry05]

Re: 1991 force 120 decelerates after hitting top speed

The tank vent could be pinched or clogged?
A NEW diaphragm can go bad with just 1 turn of the crank.
Like pnw suggests the squeezie can go bad.Even the good ones.
Check the filters for water?

 

[leej]

Re: 1991 force 120 decelerates after hitting top speed

yesterday evening we took it back out and squeezing the primer bulb had no effect on it...the primer bulb is only about 3 days old it is an attwood brand....there are no quick connects on any of the lines....i will check for vacuum leaks today...the tank hose is clear have checked it and even tried running with gas cap off but nothing changed there either....from what i can tell there is no water in my in line filters....here is a dumb question but gotta ask...when I first started on the boat and replaced all the fuel lines i replaced the water seperator but there was alot of old gas in the tank that i ran through it due to how difficult it was to get the tank out....could the water seperator need to be changed ...like i said before i have a fuel filter in front of it to keep debris out of it but didnt know if that seperator could clog if it has absorbed too much water....i am going to go check the vacuums and gaskets for leaks and possibly check diaphragm for tears..i will post this evening with my findings....another question could this possibly be electrical for any chance....my thoughts were gas supply problem but not sure..thanks

 

[pnwboat]

Re: 1991 force 120 decelerates after hitting top speed

The symptoms you describe sure sound like fuel starvation. Having said that, you shouldn't rule out anything else but start with the fuel delivery first. It's possible to be an electrical problem, but based on your symptoms I wouldn't think so. Is it possible to temporarily run off of a portable tank?

 

[leej]

Re: 1991 force 120 decelerates after hitting top speed

just come in from checking it...i did find the vacuum lime between carbs was loose on the top side and someone had put silicone around it and the silicone has started coming off....the lower side is tight and does not move but the top just slides in and out....no not very possible to run off of portable tank..

 

[leej]

Re: 1991 force 120 decelerates after hitting top speed



what would be the proper fix for this metal nipple to be secured to the intake manifold

 

[pnwboat]

Re: 1991 force 120 decelerates after hitting top speed

Remove the hose from the metal nipple. You may have to cut the hose off but the hose is a standard size so not a big deal to replace. Any automotive parts store should have the correct size.

The nipple is a press fit into the adapter/intake manifold. You can push the nipple into place and take a piece of wood or anything else of similar material to prevent damage to the end of the nipple and place on the end of the nipple and gently tap it with a hammer to seat it firmly into place. If the hole is wallowed out and the nipple will not stay in place, you can use some epoxy such as JB Weld, or PC-7 (available at Ace hardware, Walmart) and epoxy it in place.

 

[leej]

Re: 1991 force 120 decelerates after hitting top speed

the metal pipe goes in but is loose so i will try to epoxy it...can this hose be the cause of my symptoms....i will post again once i have taken it out for a test run...thanks for all the advice

 

[pnwboat]

Re: 1991 force 120 decelerates after hitting top speed

I would say that there is a chance that this may be the cause. I've seen on more than one occasion where one of the adapter plate/intake manifold bolts was about 1 or 2 turns loose on a 120 HP motor and it caused similar symptoms. Out on the water the motor would rev up to about 5500 at WOT and start to bog. Pull the throttle back to 4500 - 5000 and it was fine. Tightened the bolt and the problem was fixed.

 

[leej]

Re: 1991 force 120 decelerates after hitting top speed

i will check the bolts as well...my rpm guage does not work it is pegged out at all times even with boat turned off...thanks for all your help...I will repost prolly tuesday since that will be my next day to take it out for a test...mine never bogged until that one morning and seems to be getting worse each time it runs so i need to fix it before the motor gets damaged....thanks again

 

[leej]

Re: 1991 force 120 decelerates after hitting top speed

got the jb weld quick set formula...found a couple of loose bolts but they may have only turned bout 1/4 - 1/2 turn to tighten....i did however find one bolt missing in the lower carb intake plate so i replaced it could not find the one that was there..now all bolts are in and tightened thoroughly on both plates...just waiting to take it for a test....thanks again

 

[leej]

Re: 1991 force 120 decelerates after hitting top speed

took the boat out today....No change in the way it runs...all my fuel filters stay full and never come close to running dry...even the one between my fuel pump and carbs....could it be a possible sheared key in the flywheel...i am at a loss and cant figure this out since it was running fine then all of a sudden started running like this...any ideas would be appreciated...after i hit WO and it drops to 25 mph i can pull the throttle back to idle and hit it again and it will plane back to WO then drop off again...

 

[jerryjerry05]

Re: 1991 force 120 decelerates after hitting top speed

Check the fuel pump.
Check the gaskets around the pump.
Ethanol can melt the inside of the old hoses.
Replace ALL the hoses,even the hard ones.
The floats.The level and drop?
You have a manual?
The carb,upside down,the float should be level with the base.
The drop,should be almost touching the post.
Get some sparkies and test the plugs as you run.the motor will run with the sparkies in the system.
Just be careful not to drop one in the water
Run it and spray some starting fluid around the carbs and the intake and the pump.
Not directly into the carb.If it's sucking air it will make the motor stumble.
Where you located?Profile? You might be close to someone who can help?
If your close? Bring it by.

 

[pnwboat]

Re: 1991 force 120 decelerates after hitting top speed

A sheared flywheel key would most likely cause it to run poorly at all speeds not just WOT. Like Jerry suggested, check the fuel delivery. Fuel pump, carburetors etc.

 

[leej]

Re: 1991 force 120 decelerates after hitting top speed

All fuel hoses were replaced 2 months ago and 2 fuel filters were put in them as well as a new water seperator....i run non ethonal fuel through them....my carbs were set at the same time with the carb upside down it was set to 11/32 which is paralell to the base and the float was set to almost touching the nozzle ....new main nozzles were put in carbs as well as carb kits with new gaskets....i will checkk the fuel pump to verify that the new diaphram has not torn...all plugs are firing and coil packs work and ohm within specs when tested...i have a previous thread on this motor that I started when I was working on it....You can read it if you would like to see everything that I have done and checked....http://forums.iboats.com/force-chrysler-outboards/force-120-1-cylinder-not-firing-626239.html thatis the web address of it...I have no manual so i have had to find the documentation on the motor from the web....I have sparyed carb cleaner around all gaskets and cannot locate any intake leaks...thats why this baffles me...like i said though i will check the fuel pump again to verify since i know they can rip easily....I know it happened when I hit the patch of floating mill-foil....I am open to any ideas and i appreciate all of u guys knowledge and help....I will check these things this evening wheni get home

 

[pnwboat]

Re: 1991 force 120 decelerates after hitting top speed

How far out from lightly seated are the idle mixture screws on the carbs set?

 

[leej]

Re: 1991 force 120 decelerates after hitting top speed

about a turn

 

[pnwboat]

Re: 1991 force 120 decelerates after hitting top speed

That should be about right. 1 turn out is usually the minimum for the motor to run right.

Do you have a Fuel enrichment solenoid or a butterfly plate in each carb for your choke?

 

[BlueFishCrisis]

Re: 1991 force 120 decelerates after hitting top speed

If you haven't already, I would temporarily bypass all of the filters and separators you have installed. There may be too much restriction added by all the devices that your OEM fuel pump can't overcome at WOT. The fuel pump should still have a mesh filter integral in the pump housing to provide filtration. If no change, put them back....