1990 L Drive 120HP-.080 Jets?!?!

[hismajst]

O.k., now I am officially confused! I would be led to understand that a larger jet (i.e. a .088 over a .086) would allow MORE fuel, correct? I just pulled the bowls on my carbs (1990 120HP Force L Drive) and found both the top and bottom jets to be at .080!?!? I confirmed this with some calipers and they were indeed .080. This thing should have been running leaner then all get out if my assumptions are correct. I cant even find .080 jets on the net.
I should also mention that at 5500 rpm my engine has an intermittent miss, kind of a low popping/detonation sound. Would this also be caused by the .080 jets??According to the reply by Roscoe, it should be at .086 and .088 for 6000+ feet above sea level. Would somebody who is carb. savvy please explain how the low idle needle adjustment relates to the main jets?? Does the main jet take over at some point ( I wouldn't believe there is any transition as ALL fuel in the bowl passes through the main jet-correct?)? Due to the small jets, would this explain why I had to screw out the idle needle adjustment so much to keep it from dying at 800rpm????

 

[jerryjerry05]

Re: 1990 L Drive 120HP-.080 Jets?!?!

The low idle needle is not an idle needle.It's an air adjustment needle.
Low speed jet is in the high speed jet, it's the small hole on the side of the mian jet.
The main jet takes over as soon as you move it past idle.
Turn air screw in till it bottoms and then out 1 and 1/8th turn.Then in the water in gear turn it in till it coughs/buck/slows down.Then out 1/2 turn.
The towershaft is what controls the idle.
Coughing at speed could be water in the fuel,a bad diaphram or a broken reed.J

 

[hismajst]

Re: 1990 L Drive 120HP-.080 Jets?!?!

I should add a side note that recently I have been going through fuel filters. I have an inline filter I installed and noticed a greenish mold looking buildup. I know this was from some fuel I have pumped at the gas station recently, and not junk/buildup in the fuel tank as I have flushed it recently..I recently rebuilt the fuel pump with all new poppets/screen/diaphram etc.. It has run a lot better since then but still has an intermittent miss at full throttle. I must add that I DO NOT have a water separater and will be adding one asap.

 

[jason32038]

Re: 1990 L Drive 120HP-.080 Jets?!?!

What elevation do you live or boat at? Stock jet sizes for that are .096 and .098. The jets are labeled on the face of the outside. Yes over 6000 feet would be .086. Smaller fuel jets are needed at higher elevations to reduce fuel and balance the air fuel ratio because air is thinner as you go up. You need proper jets asap before you cause serious damage. .080 is way too small. Scroll down to #40 here http://www.dougrussell.com/partscat...230,250,220,10,190,60,120,210,180,170,160,130 and figure out which jets you need to buy according to your elevation.

 

[Frank Acampora]

Re: 1990 L Drive 120HP-.080 Jets?!?!

The purpose of the carburetor is the mix fuel and air in the correct ratio for burning, which can vary from a rich 12 to 1 for power down to 16 to 1 for economy. Outside of this range gasoline will not burn correctly---remember a flooded engine? Too much gas and it will not burn at all.

When the butterfly is open less than say 1/4, there is not enough air flowing through it to atomize gasoline. Therefore the carb has a low speed circuit that delivers fuel through the narrow brass tube that runs from the bowl to the top of the carb. However, this is a bit of a misnomer since, being at manifold vacuum at all times, the low speed circuit delivers fuel at all times. It is just that the metered orifices are too small to supply the needs of the engine at higher RPM. The HIGH speed jet is sized to compensate for the fuel constantly delivered by the low speed circuit.

Since a venturi gets progressively richer as more air flows through it, the high speed jet functions to LIMIT the amount of fuel delivered only at full throttle. HOWEVER: Different carbs have different air bleeds and venting. (For example: If a given carb has larger metered orifices in the low speed circuit, said circuit always being at manifold vacuum will deliver more fuel at full throttle and the high speed jet needs to be smaller. Thus, a smaller jet may not necessarily mean less fuel delivery.)

Without a high speed jet different model carbs will deliver a different amount of fuel at full throttle. SO: The high speed jet is tailored to the carb, not the engine, and will be different for each carb model to ensure that a 14-16 to 1 ratio of air to fuel is delivered. And, of course, Jets are also smaller for increase in altitude where the engine is used.

Before you go messing with the jet, either contact Mercury or Tillotsen and find out if that particular jet is the correct one for the model carb you have.

BTW: I don't know your particular engine, but Force made the 120 (and 150) in a couple of different versions. One used the large 1 5/8 diameter venturi TC carbs which usually were jetted around the .090s. The other used the smaller 1 5/16 venturi WB carbs which were jetted anywhere from .066 up to .088 depending upon carb model and engine size. SO: .080 jets might very well be correct in your application.