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Greaser2
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Joined: June 03, 2021
Posts: 34
Location: Houston
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 Posted: Sun Sep 19, 2021 1:18 pm Post subject: Carb syncing problem |
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| I have dual Weber 40 DCNF in my square. Did a rebuild kit and having issues syncing. Each carb is synced to it self but one to the other is a problem when setting the rpm to what I think is 900, air flow on one side is 3.5 while the other side is 7.5. I can’t adjust the 7.5 down as idle speed screw is backed off all the way. If I adjust the 3.5 up to 7.5 the idle is probably 1500 plus. Thinking the 7.5 side is too high so how can I adjust this down closer to 3.5. I tried smaller idle jets but no impact. |
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W1K1
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Joined: March 04, 2004
Posts: 4919
Location: Southern AB
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Greaser2
Samba Member
Joined: June 03, 2021
Posts: 34
Location: Houston
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| Posted: Mon Sep 20, 2021 2:30 pm Post subject: Re: Carb syncing problem |
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| Yes |
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W1K1
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Joined: March 04, 2004
Posts: 4919
Location: Southern AB
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ataraxia
Samba Member
Joined: March 19, 2010
Posts: 4504
Location: Illinois
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| Posted: Thu Sep 23, 2021 8:02 pm Post subject: Re: Carb syncing problem |
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| What's your fuel pressure? |
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Greaser2
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Joined: June 03, 2021
Posts: 34
Location: Houston
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| Posted: Fri Sep 24, 2021 1:55 pm Post subject: Re: Carb syncing problem |
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I did both sides and seemed fine. Plus, if there was a vacuum leak, wouldn't the air flow on top be lowerer? If not, I will deffenently look harder. |
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Greaser2
Samba Member
Joined: June 03, 2021
Posts: 34
Location: Houston
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| Posted: Fri Sep 24, 2021 1:55 pm Post subject: Re: Carb syncing problem |
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| ataraxia wrote: |
| What's your fuel pressure? |
Thinking it was 3-4. |
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ataraxia
Samba Member
Joined: March 19, 2010
Posts: 4504
Location: Illinois
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| Posted: Fri Sep 24, 2021 6:52 pm Post subject: Re: Carb syncing problem |
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| Greaser2 wrote: |
| ataraxia wrote: |
| What's your fuel pressure? |
Thinking it was 3-4. |
You really need to know - Weber carbs like lower fuel pressure. I was running a fuel pump that put out *a hair* over 4psi and I couldn't get the carbs tuned. Swapped it for another pump I had on hand that's putting out 3 psi and they're much happier. I bet I could knock it down to 2.5 psi and they'd be fine.
Fuel pressure is one of those simple things that you have to get right before you waste a bunch of time chasing carb issues that might not otherwise exist.
Last edited by ataraxia on Sat Oct 02, 2021 7:23 am; edited 1 time in total |
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Greaser2
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Joined: June 03, 2021
Posts: 34
Location: Houston
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| Posted: Sun Sep 26, 2021 11:33 am Post subject: Re: Carb syncing problem |
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| The fuel pressure is a hair over 4. Even with the idle speed screw all the way off, my idle is at least 900. I will look into a new pump and/or regulator. Since this engine is modified betting the fuel to turned up on purpose. |
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ataraxia
Samba Member
Joined: March 19, 2010
Posts: 4504
Location: Illinois
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| Posted: Sun Sep 26, 2021 1:59 pm Post subject: Re: Carb syncing problem |
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| Greaser2 wrote: |
| The fuel pressure is a hair over 4. Even with the idle speed screw all the way off, my idle is at least 900. I will look into a new pump and/or regulator. Since this engine is modified betting the fuel to turned up on purpose. |
An idle of 900 isn't bad but if the carbs are getting too much fuel pressure, they're going to be hard to tune and run rich no matter what you do. You'll spend a lot of time scratching your head wondering why the car isn't running right if you don't start with fuel pressure.
Start by getting a fuel pressure gauge and a regulator to get the pressure down near 2.5 - 3psi. (EDITED to change psi) Once you've got that set - you'll then want to sync the two sides, then set the idle mixture. You may have to go back and re-sync the carbs after the mixture is sorted out.
Last edited by ataraxia on Sat Oct 02, 2021 7:24 am; edited 1 time in total |
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Erik G
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Joined: October 16, 2002
Posts: 13270
Location: Tejas!
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| Posted: Mon Sep 27, 2021 7:42 am Post subject: Re: Carb syncing problem |
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redline says 2.5 to 3psi
you can run higher if you know what you are doing but most don't. float level and fuel pressure first |
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ibldrc
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Joined: September 23, 2015
Posts: 156
Location: Savannah, GA
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| Posted: Tue Sep 28, 2021 11:39 am Post subject: Re: Carb syncing problem |
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| Greaser2 wrote: |
| The fuel pressure is a hair over 4. Even with the idle speed screw all the way off, my idle is at least 900. I will look into a new pump and/or regulator. Since this engine is modified betting the fuel to turned up on purpose. |
With idle screw all the way off, the engine should die. Really sounds like at least a vacuum leak. Make sure you butterfly shaft bushings are good, could be leaking there.
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Greaser2
Samba Member
Joined: June 03, 2021
Posts: 34
Location: Houston
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| Posted: Mon Oct 11, 2021 7:47 am Post subject: Re: Carb syncing problem |
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| With the new regulator on I moved the psi down to about 2.75. This did reduce my high side airflow from 7.5 to 5.0, not bad. When I adjust the low side up to match my idle is lower but still probably 1500-2000. Solid move in the right direction and actually runs and starts better but not quite there. Ordered two bearings to replace on the high side. |
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W1K1
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Joined: March 04, 2004
Posts: 4919
Location: Southern AB
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ataraxia
Samba Member
Joined: March 19, 2010
Posts: 4504
Location: Illinois
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| Posted: Mon Oct 11, 2021 9:13 am Post subject: Re: Carb syncing problem |
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| If you get your fuel pressure down to 2.5psi, check/adjust the timing and adjust the higher carb to the lower one, your idle should come down more. |
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Greaser2
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Joined: June 03, 2021
Posts: 34
Location: Houston
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| Posted: Tue Oct 12, 2021 8:39 am Post subject: Re: Carb syncing problem |
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| ataraxia wrote: |
| If you get your fuel pressure down to 2.5psi, check/adjust the timing and adjust the higher carb to the lower one, your idle should come down more. |
I can't adjust the higher carb down, idle screw is backed off all the way. This is the problem I'm trying to fix. Other carb is fine, or at least adjustable up and down. |
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ataraxia
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Joined: March 19, 2010
Posts: 4504
Location: Illinois
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| Posted: Tue Oct 12, 2021 9:42 am Post subject: Re: Carb syncing problem |
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If you mean the 'idle speed' screw is backed all the way off on one and the idle is still too high - your throttle linkage needs to be adjusted - both sides should open/close at the same time. This adjustment is done with the linkage completely disconnected from both carburetors. Once they're the same, you reconnect and adjust the linkage so that both carbs open/close at the same time.
If you're talking about the 'idle mixture' screw - you may need to change the idle jets or you have a leak in the carb that doesn't adjust. You shouldn't be more than (IIRC) 1.5 turns open from lightly seated on the idle mixture screws. Ideally, it's less than 1.
You usually adjust the 'high' carb down to the low one to get the idle adjusted - and knowing that your timing is good (verified, not assumed) and the engine is in otherwise good working order. If you have other issues - the carbs are not the place to start because they're adjusted after all other issues are fixed. |
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Greaser2
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Joined: June 03, 2021
Posts: 34
Location: Houston
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| Posted: Wed Oct 13, 2021 2:48 pm Post subject: Re: Carb syncing problem |
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| ataraxia wrote: |
If you mean the 'idle speed' screw is backed all the way off on one and the idle is still too high - your throttle linkage needs to be adjusted - both sides should open/close at the same time. This adjustment is done with the linkage completely disconnected from both carburetors. Once they're the same, you reconnect and adjust the linkage so that both carbs open/close at the same time.
The throttle linkage is disconnected, high side with idle speed backed completely out, lowest air flow I can get is 5.5 and will not stall. Other side I can get air flow down to about 2.5/stall and by adjusting the idle speed screw up to 5.5, matching the high side, then my idle is 1500 plus. Need to get the high side down.
If you're talking about the 'idle mixture' screw - you may need to change the idle jets or you have a leak in the carb that doesn't adjust. You shouldn't be more than (IIRC) 1.5 turns open from lightly seated on the idle mixture screws. Ideally, it's less than 1.
Nope, these are fine, adjusted the idle mixtures slightly to get each barrel matched per carb. I switched idle jets from 45 to 40, this did lower the air flow slightly on both carbs but then bogged under low rpm acceleration. Switched back to 45's
You usually adjust the 'high' carb down to the low one to get the idle adjusted - and knowing that your timing is good (verified, not assumed) and the engine is in otherwise good working order. If you have other issues - the carbs are not the place to start because they're adjusted after all other issues are fixed. |
I agree, i should be able to adjust the high side down but cannot. If timing was an issue i would think both carbs would be impacted in the same way. I have adjusted the timing a bit and can get small improvements/lowering air flow equally on both sides for example but still no where near enough to get the high side down to where it should stall.
Lowering the fuel pressure to 2.75psi has made the biggest impact to get the high side down from about 8.5 to 5.5. It is actually running better and easier to start but I have noticed I don't have the power I did with fuel pressure at 4.1psi |
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ataraxia
Samba Member
Joined: March 19, 2010
Posts: 4504
Location: Illinois
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| Posted: Wed Oct 13, 2021 3:08 pm Post subject: Re: Carb syncing problem |
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| Greaser2 wrote: |
| ataraxia wrote: |
If you mean the 'idle speed' screw is backed all the way off on one and the idle is still too high - your throttle linkage needs to be adjusted - both sides should open/close at the same time. This adjustment is done with the linkage completely disconnected from both carburetors. Once they're the same, you reconnect and adjust the linkage so that both carbs open/close at the same time.
The throttle linkage is disconnected, high side with idle speed backed completely out, lowest air flow I can get is 5.5 and will not stall. Other side I can get air flow down to about 2.5/stall and by adjusting the idle speed screw up to 5.5, matching the high side, then my idle is 1500 plus. Need to get the high side down.
If you're talking about the 'idle mixture' screw - you may need to change the idle jets or you have a leak in the carb that doesn't adjust. You shouldn't be more than (IIRC) 1.5 turns open from lightly seated on the idle mixture screws. Ideally, it's less than 1.
Nope, these are fine, adjusted the idle mixtures slightly to get each barrel matched per carb. I switched idle jets from 45 to 40, this did lower the air flow slightly on both carbs but then bogged under low rpm acceleration. Switched back to 45's
You usually adjust the 'high' carb down to the low one to get the idle adjusted - and knowing that your timing is good (verified, not assumed) and the engine is in otherwise good working order. If you have other issues - the carbs are not the place to start because they're adjusted after all other issues are fixed. |
I agree, i should be able to adjust the high side down but cannot. If timing was an issue i would think both carbs would be impacted in the same way. I have adjusted the timing a bit and can get small improvements/lowering air flow equally on both sides for example but still no where near enough to get the high side down to where it should stall.
Lowering the fuel pressure to 2.75psi has made the biggest impact to get the high side down from about 8.5 to 5.5. It is actually running better and easier to start but I have noticed I don't have the power I did with fuel pressure at 4.1psi |
The power will come back once the carbs are properly sync'd and tuned correctly.
Sync the carbs (no linkage on them) and adjust for LBI and you'll be a lot closer. |
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Greaser2
Samba Member
Joined: June 03, 2021
Posts: 34
Location: Houston
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| Posted: Sun Oct 17, 2021 1:30 pm Post subject: Re: Carb syncing problem |
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| So got the new bearings in and doing this think I found the problem on the high carb. The butterfly valves didn’t seem to close 100%. They visually looked right but just not aligned right. Put everything back and got the valves to close 100%. Got the carbs synced and my idle is were I like it and runs really well. Probably need to confirm the timing is correct. So what should I have my timing at, my engine is not stock as it has performance parts but not positive how it’s set up. |
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