1987 Force 35 low compression- MAYBE?

[jreyn006]

Need a little help here- I recently did a compression check on my motor model# 357F7A and both cylinders are right around 90 PSI. The motor starts and idles?when cold" but its hunting a bit from 900-1200 and sometimes it'll randomly shut-off. It runs great a WOT on the river and idles just ok when it's cold but when you come back from running it hard for awhile--- she wont run at all without the fast idle up on the throttle quadrant and as soon as you pull it back to go into gear it shuts off immediately. I also checked fuel pressure at idle "cold" and its bouncing around a bit- sometimes steady- then bounces- all around 4-6 PSI. It looks like the needle is following the hunting engine so I don't think its a concern. I guess what concerns me is the compression? My Clymer says minimum compression should be around 120-130 PSI? I've super cleaned the carb all ready and can 100% tell you no jets are clogged. Am I fighting a low compression problem? I'm not familiar with 2 strokes- how do I isolate the problem?? Thanks in advance

 

[foodfisher]

Try a decarb with seafoam then clean your plug/s.

 

[Nordin]

90 PSI seams a little bit low BUT not alarming low. Most importend is that both cyl. are equal or not more than 10% differens. After you cleaned the carb, how did you set the air/fuel mix screw? It should be 1-1 1/4 turn out from lightly seated.
If it is set to lean it will stall when you put into gear.
Never set it leaner then 1 turn out even if you get a better idle.
Check the air/fuel screw and also the idle screw. It should idle 700-800 rpm in water and in gear.

 

[jreyn006]

Thanks Nordin- the mix screw was set to one turn out but by the book it says to set it by RPM which seems a little difficult in my situation due to the erratic idle speeds so I don't feel comfortable "fine tuning" when she's hunting like she is. As well it runs real decent when its cold- this is strictly a problem after its hot? I feel like i can knock out any mechanical blockages just because it runs real decent cold- no issues idling, starting, neutral to forward gear goes smooth and she stays running fine. I just cant expect to take the boat down river and turn it off because it wont restart unless i slam the throttle forward and even then as soon as i bring it back to neutral to put in forward gear it will stall immediately. How in the heck does something run so good when cold and wont run at all when hot???

 

[jerryjerry05]

The air screw is set when the motors warm.
It will run a bit different hot compared to cold.
The actual compression with a good gauge is about 145-150#
Most manuals don't post a "standard" way to many variables.
Like Nordin said the 90 on both cylinders is the GOOD part.
As long as both cyl. are close or even.
My gauge reads different from yours.
I have a set bought in 1985 160$ a gauge a friend gave me free.
The free one reads 30 less than the other????

Make sure the compression #s are done with a different gauge.
Then make sure the fuel pump diaphragm is ok.
Remove the air screw and check for damage.
Any cuts or grooves on the taper? No? Continue.
Then set the air screw set it at 1 and 1/2 turns out..
Still running rough then turn out the air screw another 1/2 turn.
The motor must be warm to adjust.
Start(warm?) then as it's running turn the air screw in until it stutters or stalls.
Then out 1/2 turn.

 

[Jiggz]

". . . the mix screw was set to one turn out but by the book it says to set it by RPM which seems a little difficult in my situation due to the erratic idle speeds so I don't feel comfortable "fine tuning" when she's hunting like she is."

This is the very reason the initial setting of the air fuel mix screw is pretty much standard, i.e., 1~1-1/8 turn out from lightly seated. Although you can deviate from this initial setting when tuning up, it is most likely you will end up very close to this setting after your done as long as you did it correctly. But never, as mentioned, go below 1 turn out even if the motor sounds and idles better at a lower setting. I believe what is being referred to by the book to set based on RPM is the idle screw, which is totally different from the air-fuel mixture screw.

There are revealing symptoms with your post. The most obvious one is, it has a hard time starting after running it hard but it will do so if given a throttle (fast idle) which would mean the idle screw (below the timing tower) is not properly set or because it runs low on fuel in the carb's bowl due to improperly set or sticky float.

The proper idling but hunting idle rpm is also telling, which means intermittent flow of fuel into the carbs or electrical problem. Using a spark tester or timing light, you should be able to tell if it is electrical by watching the spark's consistency. I use either but I prefer the timing light and deflected to a dark colored paper watch the consistency of the light. If the light inconsistency matches the engine rpm, then you know you have electrical problem. If none on either cylinder, then you know it is fuel related. Then the next step is to install a G2 clear fuel filter (less than $5 from W-mart) between the fuel pump outlet and the carb input. After priming and starting, the fuel filter should stay at least 3/4 full. If not then the problem is upstream of the fuel filter. But if it does stay full, then the problem is with the carbs.

 

[jerryjerry05]

The book says start at 1 turn out but a lot of times that isn't enough.
If I'm working on it I start at 1 and 1/2 cause I know it will start there.
One customer I have has an 85/85hp and the air screws needed to be turned out almost 2 full turns just to start it.
Mine are set at 1 turn out. The factory came out with an updated service bulletin and said the 87-89 85/125 air screw should be set at 1 turn out and left there.
Bad thing is some don't run like that.

 

[Jiggz]

Remember the keyword is "initial" setting. This means it is either you end up with this setting or maybe more but never less. It is the recommended "initial" setting because it has been found to be safe to ensure proper lube and not ran the motor on lean settings. This setting is also found to make sure you get the motor to start meaning this "initial" setting will provide enough fuel oil mix to provide fuel for combustion and lubrication. The final setting will be determined as soon as you set your idle rpm with the idle screw.

So what is the final setting you may ask? Well, that is a very subjective question but the procedures for setting the idle fuel air mixture screw details it to be the mid point between too rich and too lean settings as discussed in the "Link and Synch" sticky post and also in the service manual.

From my personal experience I ended up with 1 1/8 from lightly seated. Does that mean this is the universal setting? NO, but this is the mid-point for my motor setting.