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Old 27-11-2021, 01:44   #1
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Outboard carburetor - question on Pilot Jets

I would appreciate some insights into the nitty-gritty of Pilot jets in small outboard carburetors.

Photos attached below show the Main jet (large) and Pilot jet (small) of my 15hp Yamaha. But my question is really generic - I am not looking for a fix, I am trying to understand the technicalities.

My understanding has always been that:
The Pilot Jet only supports combustion at IDLE and low RPMs.
The Main Jet supports combustion at high RPMs and WOT.

By that reasoning, a blocked Pilot Jet should not prevent an engine from running at WOT (or high RMP).

But this past month, my experience has been that an obstructed Pilot jet seems to disable the Main jet. And the engine won't run at any RPM - low or high or WOT.

Why is that?
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Old 27-11-2021, 05:08   #2
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Re: Outboard carburetor - question on Pilot Jets

My best guess is either an imbalance in air pressure in the bowl, or a mixture that won’t combust
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Old 27-11-2021, 05:14   #3
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Re: Outboard carburetor - question on Pilot Jets

Let me try. Based on similar motorcycle carbs, in a simple carburetor like this the pilot jet remains active throughout the RPM range - its venturi is in the throat and nothing blocks it. Your mixture is going lean on you if the pilot jet is blocked.
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Old 27-11-2021, 05:55   #4
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Re: Outboard carburetor - question on Pilot Jets

Thanks for these suggestions; further ones are welcome.

@Sailmonkey: I would think the air pressures in the float bowl and venturi should not be affected by whether the pilot jet is open or not.

@tkeithlu: I get what you say, though the diameter of the main jet is really so much larger than the pilot jet that you would expect the absence of the minor fuel flow contribution from the pilot jet should not be able to put a blanket stop to the whole show.
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Old 27-11-2021, 06:02   #5
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Re: Outboard carburetor - question on Pilot Jets

Quote:
Originally Posted by tkeithlu View Post
Let me try. Based on similar motorcycle carbs, in a simple carburetor like this the pilot jet remains active throughout the RPM range - its venturi is in the throat and nothing blocks it. Your mixture is going lean on you if the pilot jet is blocked.
Indeed.
As an engine needs more fuel at higher speeds, there are two jets, one for low speed [idle], and one for high speed.
The low speed jet feeds fuel into the air stream at low speeds.
At higher speeds, they both do.

Here’s a good basic generic Outboard Manual:
OUTBOARD MOTORS FOR PACIFIC ISLAND MARINERS
Learner’s Guide

https://spccfpstore1.blob.core.windo...oard_lg.pdf%22

Your question goes beyond the above manual; but tkeithlu and Sailmonkey have provided good answers.
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Old 27-11-2021, 06:06   #6
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Re: Outboard carburetor - question on Pilot Jets

The pressure in the bowl is controlled by venting that works on the supposition that both jets are delivering fuel. The fuel mixture is also controlled by the supposition that both jets are providing fuel.

I’m thinking also that the keys are tied to progressive vents…..but can’t be sure.

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Old 27-11-2021, 08:04   #7
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Re: Outboard carburetor - question on Pilot Jets

Quote:
Originally Posted by Sailmonkey View Post
The pressure in the bowl is controlled by venting that works on the supposition that both jets are delivering fuel.
Attachment 248921
I thought the air in the float bowl is simply normal air pressure.
I read somewhere that (some?) bowls have overflow ports to the outside where excess fuel can escape if the bowl overfills; such ports would equalize the bowl with atmospheric pressure.
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Old 27-11-2021, 10:22   #8
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Re: Outboard carburetor - question on Pilot Jets

Quote:
Originally Posted by Marqus View Post
I thought the air in the float bowl is simply normal air pressure.

I read somewhere that (some?) bowls have overflow ports to the outside where excess fuel can escape if the bowl overfills; such ports would equalize the bowl with atmospheric pressure.


The bowl vent in my 5hp mariner OB are in the carb bore. So they’re subject to the whims of the Venturi through there.
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Old 27-11-2021, 17:43   #9
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Re: Outboard carburetor - question on Pilot Jets

Quote:
Originally Posted by Sailmonkey View Post
The bowl vent in my 5hp mariner OB are in the carb bore. So they’re subject to the whims of the Venturi through there.
Ok, that would mean the lower-than-atmospheric air pressure in the venturi gets down to the float bowl via that vent.

Can you nudge me a bit more to the point where I understand why, at WOT, the main jet does not pass any fuel from that bowl when the pilot jet happens to be obstructed?
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Old 27-11-2021, 18:01   #10
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Re: Outboard carburetor - question on Pilot Jets

I don't think all outboard carbs are the same.

Now, I'm certainly not an expert, but my 2 stroke Mercury is a bit different than the carb pictured. For one, there is no choke shutter. Secondly, the idle jet doesn't really exist either. it seems that the fuel for slow speed operation passes though a small passage from the bowl, through the carb body, and into the engine block, where it exits into the throat.

I think I've seen some Yamahas and other small engines with similar arrangments.

On my engine and others like that if the idle passage is blocked the engine won't run at high speed, even if it will sort of run at low speed.

The reason? When the throttle is opened the accellerator pump in the bowl injects raw fuel into the throat through that little passage. The engine stumbles and dies when you open the throttle if that passage is blocked.

Maybe something like that is in play in other engines too.
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Old 27-11-2021, 19:46   #11
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Re: Outboard carburetor - question on Pilot Jets

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Originally Posted by wingssail View Post
On my engine and others like that if the idle passage is blocked the engine won't run at high speed, even if it will sort of run at low speed.
Yes, and doesn't that seem to be unexpected - keeps going at low speed but not high?

I can actually get my engine to hit high revs, but it dies down in about five seconds despite the main jet being crystal clear.
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Old 28-11-2021, 05:44   #12
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Re: Outboard carburetor - question on Pilot Jets

Question, are 2 and 4 stroke carburetors similar? My 2 stroke 15 Yamaha went for a swim. On taking the carb apart I found lots of small channels, some interconnected. I found this out as I pressurized the channels with WD-40. I'm not sure if they are directly connected to the jets, but until I cleaned the channels the engine would not run, of if it did, it was erratic.
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Old 28-11-2021, 07:26   #13
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Re: Outboard carburetor - question on Pilot Jets

Following. My 15hp Yamaha 2-stroke runs well, but is often hard starting. anyone know any tricks for separating the lower unit, to change the impeller....evidence of Cr makes separating really difficult.
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Old 28-11-2021, 08:06   #14
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Re: Outboard carburetor - question on Pilot Jets

I did have a similar problem on my 4hp 4 stroke Yamaha. It would start fine, idle ok, go to WOT and the engine would die. I spent some time trying to figure this one out.
The carb people were smart, they put a drain in the bowl and after checking to see if everything was in order on the engine, I came back to drain the bowl of the carburetor and drained it into a ziplock bag only to find a few drops water. Water doesn't float it sinks to the bottom, you start your engine the pilot jet for some reason doesn't have the suction to move that water, but when you go to
WOT it goes down that main jet and then the engine stalls. This is probably an over simplification of the problem, I have found that symptoms such as yours usually are a result of contaminated fuel.
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Old 28-11-2021, 08:22   #15
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Outboard carburetor - question on Pilot Jets

Quote:
Originally Posted by sailcrazy View Post
Following. My 15hp Yamaha 2-stroke runs well, but is often hard starting. anyone know any tricks for separating the lower unit, to change the impeller....evidence of Cr makes separating really difficult.


What brand?
If you have an extender is just an extra step
If not is just like the video
https://youtu.be/Dgi_epY7HNY
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