Carb Needle condition / function

YOBE

Cadet
Joined
Mar 12, 2019
Messages
22
Hi,
just started with the inspection of my carb GM 2 Jet 7040110. I'm not an expert in carbs but the 2 "Needles" look weird to me. See Picture.
Questions:
1. Is this a normal condition of the needle?
2. Are the left and right needle supposed to be similar or different?
3. Can someone explain if there is a difference between the right and the left needle OR do they have the same function?
4. Can I close 1 of the 2 and just use 1 needle? As I see that many similar carbs only have 1 needle. (also in the rebuild kits I mainly see 1 needle)

Thank you, Yorick
 

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04fxdwgi25

Chief Petty Officer
Joined
Mar 25, 2022
Messages
543
Yup, looks like someone that shouldn't own tool over tightened them.

And to answer your other question, no, you can't run on just one idle screw. Before doing too much on that carb, suggest you start reading and gaining some knowledge on how a carb works.
 

Scott06

Admiral
Joined
Apr 20, 2014
Messages
6,475
Hi,
just started with the inspection of my carb GM 2 Jet 7040110. I'm not an expert in carbs but the 2 "Needles" look weird to me. See Picture.
Questions:
1. Is this a normal condition of the needle?
2. Are the left and right needle supposed to be similar or different?
3. Can someone explain if there is a difference between the right and the left needle OR do they have the same function?
4. Can I close 1 of the 2 and just use 1 needle? As I see that many similar carbs only have 1 needle. (also in the rebuild kits I mainly see 1 needle)

Thank you, Yorick
As mentioned those are the idle mixture screws one for each bore. someone hammered them down tight damaging them - get replacements from a place like mikes carb parts.

Are you rebuilding the entire carb including needle seat and float? The engine would not run with the mix screws closed off. Would assume needle and seat was overfilling the bowl dumping fuel in and someone cranked down on these trying to lean out the mixture. Who knows , who ever did this didnt know chit about carbs...
 

kenny nunez

Captain
Joined
Jun 20, 2017
Messages
3,305
Check that where the needles protrude into the throttle body that the holes are not raised from over tightening, with the screws out the bore must be smooth. If the area around the small holes are raised you will have a hard time getting the idle smooth. In other words, the throttle body is ruined.
If every thing is correct the screws should be 1.5 turns out from lightly seated for a smooth idle.
 

YOBE

Cadet
Joined
Mar 12, 2019
Messages
22
Ok, thanks for the reply. As I guessed, the needles are ruined. And I found some spares at Mikes already đź‘Ť. Lucky the bores are not raised, so I guess this is still ok.
 

YOBE

Cadet
Joined
Mar 12, 2019
Messages
22
As mentioned those are the idle mixture screws one for each bore. someone hammered them down tight damaging them - get replacements from a place like mikes carb parts.

Are you rebuilding the entire carb including needle seat and float? The engine would not run with the mix screws closed off. Would assume needle and seat was overfilling the bowl dumping fuel in and someone cranked down on these trying to lean out the mixture. Who knows , who ever did this didnt know chit about carbs...
I checked the float (was rebuild) and cleaned the needle that stops flow to the bowl. Also based on visual inspection, needle looked fine. But is there a simple way to check if I really closes the fuel flow in the right manner?
 

YOBE

Cadet
Joined
Mar 12, 2019
Messages
22
Yup, looks like someone that shouldn't own tool over tightened them.

And to answer your other question, no, you can't run on just one idle screw. Before doing too much on that carb, suggest you start reading and gaining some knowledge on how a carb works.
Hi, I did quite some efforts into understanding how a carb works. But the 2 barrel concept didn’t ring a bell. I thought it was purely related to overall capacity of the carb versus engine air flow. But now it’s clear that the 2 barrels serve 2 DIFFERENT sets of cylinders. 👍 And of course 2 idle screws would be required. Thx for the input.
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
13,457
Think of this kind of 2bbl as 2 Carbs, that share a Fuel Bowl, Throttle and Choke Shafts.
 

Grub54891

Admiral
Joined
Jun 17, 2012
Messages
6,090
is there a simple way to check if I really closes the fuel flow in the right manner?
Once you get the float height set, tip the carb over, apply pressure, I use the mighty vac that does pressure also. It should hold at 5-7 lbs.
 

Scott06

Admiral
Joined
Apr 20, 2014
Messages
6,475
I checked the float (was rebuild) and cleaned the needle that stops flow to the bowl. Also based on visual inspection, needle looked fine. But is there a simple way to check if I really closes the fuel flow in the right manner?
As mentioned above you can test with a little air pressure. Biggest issue folks run into is the amount of junk parts being used by rebuilders and sold on amazon etc.

Looks are one thing the cheap ones can get hung up or leak where it threads into bowl top. Make sure float is in good condition. When it back together real test is looking down bores, venturis should not be dripping fuel at idle if they are float is too high or needle and seat is not shutting flow off. If there is an issue and you are sure needle seat and float is working then check fuel pressure. Too high fuel pressure can overcome the needle and seat.

Mikes carb parts has good parts and videos if you need tech support.
 

Candy Gramformongo

Seaman Apprentice
Joined
Sep 14, 2024
Messages
33
Also, O'Reilly Auto Parts has Walker Products brand carb rebuild kits. Usually not in stock at your neighborhood store, but available within a day or with a shortish drive to a regional "mother ship" store.

I successfully rebuilt a 2Jet last summer with one.
 

todhunter

Canoeist
Joined
Sep 15, 2020
Messages
1,316
When it back together real test is looking down bores, venturis should not be dripping fuel at idle if they are float is too high or needle and seat is not shutting flow off. If there is an issue and you are sure needle seat and float is working then check fuel pressure. Too high fuel pressure can overcome the needle and seat.
This. My carb is a 4-barrel, but I was having major running issues (stranded me 3 times requiring a tow at the beginning of the 2023 season), and it turned out to be my fuel pump putting out too much pressure, blowing the needle off the seat. I ended up having to install a fuel pressure regulator and a gauge to dial fuel pressure back to a level that the carb can handle. Since I've done that, I've had a season and a half of trouble free boating.
 

tank1949

Lieutenant Commander
Joined
Apr 4, 2013
Messages
1,896
For peace of mind and safety concerns, I'd replace the carb. Especially, if boat has been consistently used in saltwater, I'd replace it. Condensation is a nightmare for boaters.
 

kenny nunez

Captain
Joined
Jun 20, 2017
Messages
3,305
I bet that your fuel pump is not a genuine marine version. You should look into the correct pump for your engine. Does it have the Tygon hose from the side of the pump to the flame arrestor?
Which engine do you have?
 
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