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 Pilot Circuit Help 
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Post Pilot Circuit Help
Hello,

I have been getting lots of information from here but everything I’ve tried isn’t helping my idle.

I have a mud buddy mini 23, with a Mikuni Vm 34 and bps exhaust. I can’t get the engine idle circuit perfect. My problem is that it idles rich. There is black smoke, and sooty plugs. I get my idle set to about 1100 rpms then I go to the air screw but it does nothing. I actually removed the screw and the engine continued to run. So I thought my pilot jet was too large. I went from a 25 down to 17.5, same thing engine runs even with the screw out. I turn it in and nothing happens until the screw seats and the engine will still run just rough. I guess my next move is to start increasing the pilot jet size until I see an affect with the air screw.

From all of the research I’ve done the air screw should make noticeable differences with slight turns. That is not the case for me. Any help is appreciated!


Sun Aug 29, 2021 12:28 am
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Post Pilot Circuit Help
From what I’ve read the vm carbs are tough to tune on the low end because of the round slide.

If you’re not already, be sure to do your testing/tuning with a load such as idling in the water. You won’t get an accurate result doing this on the trailer. Get it close then fine tune the screw by feel. Err on the side of slightly rich at idle for best acceleration. If you’re getting black smoke, keep dropping the pilot jet size.

Wideband o2 sensor is a good way to take the guess work out of it all.

I think you need to keep dropping pilot size. It runs worse with screw all the way in (less air), runs better with screw out (more air). Need less fuel to be able to see a difference with air changes.

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Mon Aug 30, 2021 4:19 pm
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Location: Gladewater TX
Post Re: Pilot Circuit Help
make sure valves are set, a leakdown test would be nice too. is the carb from bps? if so it will have a "pump seat" in it. thats a good thing. if it came from anywhere else the seat will need to be swapped. on a stock ish motor i haven't had issues with the pilot circuit. also put eyes on intake gaskets where they bolt to heads


Mon Aug 30, 2021 5:44 pm
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Post Re: Pilot Circuit Help
Hey fellas, I really appreciate the replies. After that tough night I went back to the basics and purchased a compression gauge. 170 on one cylinder and 120 on the other.

I’ve decided I will run it this waterfowl season and make the repairs next season.

I really like the tip of fine tuning with a load. Next spring I will be giving that a try. I will keep reducing pilot size, if needed after the repair.

I’m the 3rd owner and it has a bps exhaust so I assume the carb is from bps. What is a pump seat?


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Mon Aug 30, 2021 6:35 pm
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Post Pilot Circuit Help
if so it will have a "pump seat" in it. thats a good thing. also put eyes on intake gaskets where they bolt to heads






Will, I did some research and I don’t have a pump seat. That is a seat for the float needle right? I am running a fuel pump and had to add a regulator because it was overflowing the carb.

Keep any eye on the intake gasket for sucking air or leaking oil or both?


Mon Aug 30, 2021 8:30 pm
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Post Re: Pilot Circuit Help
How did you run the compression test? That typically offers less insight to the condition of the motor than a leakdown which is why Will recommended a leakdown


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Mon Aug 30, 2021 9:01 pm
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Post Re: Pilot Circuit Help
the seat u can buy, what if the compression is low just from a popped headgasket? just a thought


Mon Aug 30, 2021 11:41 pm
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Post Re: Pilot Circuit Help
Should be the 1.5mm seat for the pulse pumps, right?


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Tue Aug 31, 2021 11:51 am
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Post Re: Pilot Circuit Help


Sun Sep 05, 2021 4:42 pm
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Post Pilot Circuit Help


Sun Sep 05, 2021 5:34 pm
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Post Re: Pilot Circuit Help


Sun Sep 05, 2021 11:29 pm
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Post Re: Pilot Circuit Help
It was not choked


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Sun Sep 05, 2021 11:29 pm
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Post Re: Pilot Circuit Help
Hey all,

I’m back with a lot more knowledge and info. After your advice I purchased a leak down. I found out I had leaky valves and head gaskets. Sooo what do I do in that situation? Tear the entire engine down and put it back together with new gaskets, lapped valves, deck the heads etc. I put it all back together with new rings. And my leak down on cylinder 1 is 0% and 5% on cylinder 2. Not bad for a first timer. I took it out to the lake this weekend (with my wideband) and was thoroughly impressed. Not with the out of tune carb, but how much more reliable my engine was and the felt increase in power. Now on to the jetting. I was running a 20 pilot jet and went down to a 15. It is still a little rich (12 afr at 1/8 throttle). So I’m gonna drop to a 12.5 and see what that does. The mid range was pretty close, but I was surprised by wot. I had a 165 main and at 3/4 throttle it ran great (13.5 afr) but when I hit wot the afr gauge would go super lean and engine would die. The only other main jet I had was a 190 so I threw it in there thinking it was too big. Nope. The engine at wot now doesn’t die but is running about 17 afr. My surprise is that I am in need of a bigger main and that small of a pilot jet. Everyone I see on here run mains in the 160’s.

This isn’t much of a question but I wanted to give you all an update and also see if that sounds crazy to anyone else. If I didn’t have the wideband sensor I wouldn’t believe myself.


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Mon Apr 11, 2022 8:01 pm
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Location: Trinity Bay - Texas
Post Re: Pilot Circuit Help
Hey - sounds good...

Here is my personal 2 cents.... Bad gas is a PITA....

Long ago I was good at working on carbs till a mechanic told me something
that changed my "boating life" - "be damn picky about your fuel" he said...

I asked - "why is that" - to which he replyed.... "Fuel is designed for cars
that use the fuel right away" - but in boats you have weeks and months where
they sit without fuel use.... "OK" says me - WTF at you telling me ???

Basically he says - "only run 93 octane - and always use additives" --- In my
confusion I asked - "why"....

He says Federal law requires clean burn additives - most of which are a
"white powder" added to fuel which drops out of suspension after time and
exposure to humid air - like what a boat is exposed to... "Dang" I say - is
that why I see all that white "goo" in the bottom of a carb each time I chemical
dip it then fully clean and rebuild it ? ? ?

SO - naturally I ask what should I buy - he says "Premium 93 like Chevron
and put a bit of Stabil in it unless you are running all the time then additives
like Yamaha RingFree when you are hard at it all the time".... Also Chevron
does not use the "white powder" they use a liquid called Techron which never
drops out of suspension after time...

Those words - changed my boating life - I have never since worked on a carb
in my boats - lawnmowers - chainsaws - generators.... Because I only fed
them Chevron 93 with appropriate additives to add fuel life or clean the engine...

2 cents worth...

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Mon Apr 11, 2022 8:17 pm
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Post Re: Pilot Circuit Help


Mon Apr 11, 2022 8:26 pm
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Post Re: Pilot Circuit Help
Even non-ethanol gas has federally required clean burn additives...




If all those little carb fuel passages are clogged up - it don't matter - does it ? ? ?

What I was getting at was - you have been down for a while - give carb a good
chemical dip overnight - rebuild it and then run it hard before making such
jetting changes to see where you are....

You have done the hardest part - but often it is the little details that matter
like good fuel and good additives...

Not to mention - new fuel lines of at least 5/8" a new bulb and a new filter...

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it is the journey that matters, in the end."


Mon Apr 11, 2022 8:33 pm
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Post Re: Pilot Circuit Help
As to jets...

The plugs tell the story if you know how to read, sniff, and taste it - LOL...

A motor runs great when too lean - but burns off cylinder wall and valve metals
when there is unconsumed oxygen present....

Always err on the side of too rich and the motor will last longer...

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Mon Apr 11, 2022 8:40 pm
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Post Re: Pilot Circuit Help


Mon Apr 11, 2022 8:50 pm
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Post Re: Pilot Circuit Help
Keep going bigger on the main. I have a 260 in my 23. Went from a 270 with carb heads and exhaust to a 260 when I added a cam to the mix. Running a 17.5 pilot. Still pretty rich on the low end. I need to step it down to a 15 or smaller but it works for now. I’d rely on the wideband readings. Shoot for 12.6-13.2. 13.2 being the max. Also I recommend testing with a typical load in your boat.


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Tue Apr 12, 2022 6:22 am
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Post Re: Pilot Circuit Help


Tue Apr 12, 2022 10:24 am
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Post Re: Pilot Circuit Help
Assuming you have your wideband clipped into your exhaust outlet and not to a header pipe, you’re actually getting an average afr of both cylinders. One cylinder, believe the left when looking at motor from front, usually runs leaner than the other. Shooting for a rich average ensures you don’t run too hot on the lean cylinder. richer means cooler in these motors. Slightly rich allows for the motor to be okay when it leans out during cold hunting weather. A hair rich also ensures you maximize power but at the expense of fuel economy. On top of all that, I’ve had two people who build these engines give me that range as targets. I think If you shoot for 13.5 you run the risk of running too hot and breaking down oil and parts sooner. Just my .02.

If you want to get real technical on best tuning practices, change your wideband from afr to lambda and shoot for .85-.89. Google can explain this concept better than I can.


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Tue Apr 12, 2022 12:05 pm
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Post Re: Pilot Circuit Help


Tue Apr 12, 2022 8:26 pm
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Post Re: Pilot Circuit Help
So my afr bung is on the right cylinder when looking at it from the front. So from what you said that would be the richer side. So I should really be tuning that to 12.6 and the left side will be a touch leaner. Makes sense to me.


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Tue Apr 12, 2022 8:45 pm
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