Logo
FULL SITE MENU

1999 Force 75HP 3-Cylinder Air/Fuel Carb Mixture Screw

P

Pshelfo

New member
I have rebuilt my (3) carberators and having trouble with the air/fuel mixture screws. Is there a factory setting and should all 3 carb screws be set the same? They were different prior. Tried setting by ear in water but can’t tell how to do. Setting to 1 and 1/2 seems to run better but noticed burning alot of fuel. 1 1/4 appears to have stall out issues in idle and when putting in gear.
 
1 1/2 is about right.
Fuel usage shouldn't be affected by the low speed air settings.
Excessive fuel use could be: wrong prop, overloaded, bad fuel pump and more.
The one I'd check first is the size of the prop: what RPM's at wide open?
 
What are the results of a compression test?-----Does spark jump a gap of 3/8" on each lead.-----Might be other issues causing the stalling.
 
Funny you should ask about WOT. For a few years I have been struggling to find why boat tops out at around 4500rpm at WOT. Used to run 5K which is spec. Compression tests yield right at about 120lb per cylinder. Spark jumps farther than 3/8 on tester. When I have screws set to 1 1/2 it holds ideal and does not stall. Was thinking that maybe air/fuel mixture was causing it not to reach top RPM. On top of carb rebuild i have replaced fuel pump, all full lines from gas pickup to cards, fuel primer bulb, voltage regulator and fuel filter. Prop has hit a few things over the years so maybe that? Not sure if a thrust washer has gone back as original one. Pitch is 21" which is factory spec. Going to set to 1 1/2 tomorrow and try things out. Sometimes at WOT it sounds like it is not running at that high pitch sound like you would think and needs a little more something. Have set the sync and link and the butterflys are wide open with no more to go. Thanks!
 
Does it take a long time to reach plane? If you're not reaching above 4,500 and it's slow to get on plane, you're "over propped" and you need to move down to lower pitched prop. However, if you used to reach 5,000 RPM's with the same prop, that's probably not the problem. The pitch of the prop is theoretically how many inches the boat will move forward with each revolution. If your pitch is too much (higher number), it over-works your motor. Like trying to take off in a car in second gear instead of first gear. I've attached a diagram of your carburetor. #14 is the main jet. You need to make sure you got that cleaned out really well. As mentioned above, the screws you're adjusting are actually "Idle mixture screws" and have no impact beyond making a smooth idle. Once you speed up, the main jets take over. If you're sure the carbs - specifically the main jets - are good and clean, and your compression is good and your spark is good and strong, a damaged, bent or in any way damaged prop may be causing some problems if it's damaged badly.
 
21p might be factory specs for another boat.
If you had 5000 before it was wrong before.
The 120 should hit 53-5500 at wide open.
Prop: if damaged could be part of the problem.
Overloading could be another.
Had a 21 with twin 88/85hp.17p from the factory.
Wanted MORE!! changed to 19p. RPM's dropped to 4900
but got 3 mph MORE!!
The fuel economy suffered, used almost 15-25% more.
 
Thanks for all the replies. I set the air/mixture to 1 1/2 and the boat appeared to run well when I took it out with no stalling. The pitch came on the boat (aluminum) of which I switched to SS many years back. The boat is a 1998 Nitro 640LX and it always ran around 5K RMP WOT and then dropped down but I cant' say what occurred around that time - think it dropped slowly. Good info on the air/mixture screw and I have cleaned the heck out of the carbs. Have taken the main jet out and sprayed with carb cleaner and pressured air. Also dipped carbs in Chem-DIP - think I took apart carbs 4 times to clean and adjust. The prop does has some damage and I have taken it to prop shops to fix before. Question - do thrust washers or any of the prop "gear" go bad over time? I might look into purchasing a new prop of equal size to check things out.
 
If you switched to a SS prop, if you still have the aluminum, I would try that one before purchasing a prop. Those things can be pretty expensive unless you can find a good used one.
 
Prop hardware doesn't go bad except the nut. If it's a Nylock the nylon insert can stretch and cause the prop to come off when used in reverse.
 
Back
Top