Running Rich
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Logistics Pro
Joined: Apr 2010
Posts: 4,446
Likes: 25
From: Panama City, FL
I will do the other checks first and try and get that dialed in. I wont be able to get to it till sunday so i'll post up then on how it runs. But I did notice that on the left top right in the middle of the carb there is another port, looks to be 3/8 that is just open. What could this be for? Its the same size as the fuel port but the fuel port is behind it. I will take a pic tomorrow and post it so you know what im talking about.
Postmaster
Joined: Dec 2010
Posts: 2,951
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From: Gary,Indiana
Ignition timing does not affect the actual ratio of air to fuel that is presented to the combustion chamber; this is solely the job of the carburetor. The only reason ported vs. manifold vacuum and timing got brought up was with regard to where to plug in a vacuum gauge to set the mixture. It's still possible to have a strong vacuum signal in the presence of a rich condition. Also note that the vacuum gauge is only used to tune the mixture at idle. Off idle, at cruise and under acceleration use different metering circuits.
I had this same problem with my boat. 355ci Mercruiser/SBC . I tried a few different carbs, different coils, different timing setting and it still would run rich at low rpms and once you got it off idle curcuit it ran nice and clean. Alot of no wake zones. Then I went to a MSD system with multi spark at lower rpms just for marine. 10*BTDC and total timing of 31*BTDC and runs nice and clean. Oh and I'm running a Edelbrock AFB with spacer plate and intake.
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
I agree with you about timing not affecting air to fuel ratio and I understand and agree 100% with the rest of your statement. But the closer to TDC the ignition fires there is more of a chance that not all the fuel will get burned as well as the timing being BTDC, that and I hope you agree with me that too much timing BTDC will create a lean condition that will burn a whole in a piston. So finding the sweet spot is important.
Logistics Pro
Joined: Feb 2000
Posts: 3,955
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From: Mesa
I think it's best that you have an understanding of how your carb works. The Carter/Edelbrock carbs are so simple to work on. Read this: http://www.edelbrock.com/automotive_...404_manual.pdf
The idle screws will only adjust the mixture at idle. Once you're on the way the primary jets/rods control the mix. By changing rods/jets you can lean or richen the mixture.
The idle screws will only adjust the mixture at idle. Once you're on the way the primary jets/rods control the mix. By changing rods/jets you can lean or richen the mixture.
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Logistics Pro
Joined: Apr 2010
Posts: 4,446
Likes: 25
From: Panama City, FL
Ok, I was wrong on how my distributor was hooked to the carb. Distributor is hooked to the upper 1/4" port and the lower one is capped. So today I went ahead and made sure my timing was at 6° BTDC and screwed the idle mixture screws all the way in and backed them off 1 1/2 turns. Took the cap off the lower port and hooked my vacuum gauge to it. Started the engine and turned each mixture screw 1 turn at a time till they were all the way out. RPMS came up a bunch but the vacuum gauge stayed at zero unless I reved it up then the vacuum gauge came up.
So now does that indicate I have a vacuum leak somewhere? All the hoses are new. I just had the intake off last week to replace the seals that go between the heads and intake. Sealed everything up good and torqued everything down. I didn't pull the heads. Could it mean my valve seals are bad? Carburetor was just rebuilt in august and has all new parts. BTW, the engine is a 390 if that helps any.
So now does that indicate I have a vacuum leak somewhere? All the hoses are new. I just had the intake off last week to replace the seals that go between the heads and intake. Sealed everything up good and torqued everything down. I didn't pull the heads. Could it mean my valve seals are bad? Carburetor was just rebuilt in august and has all new parts. BTW, the engine is a 390 if that helps any.
Logistics Pro
Joined: Feb 2000
Posts: 3,955
Likes: 50
From: Mesa
As long as it idles it's fine. If you haven't changed any jets you won't burn a piston. If your vac guage only moves with the throttle then that is the wrong port. I tuned my truck using the rpm drop method. Starting at 1 1/2 turns then adjust IN until the rpms start dropping. Once that happens, back the screw out about 1/4 turn. Then do the other one. Kinda trial and error. Running rich is still a function of the jets/rods NOT the idle screws.
How about a pic of that carb? You want the vac advance on the higher port on the left front. The lower port on the right is full vac.
How about a pic of that carb? You want the vac advance on the higher port on the left front. The lower port on the right is full vac.
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Logistics Pro
Joined: Apr 2010
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Likes: 25
From: Panama City, FL
Ok, here are 2 pics. The first one I took the cap off on the right lower port, the one im suppose to use for checking manifold vacuum. The second pic shows the vacuum gauge hooked up and from that port I read zero at idle unless I move the throttle.
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MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
Nothing you've described would make me think you have a vacuum leak.
That sure is strange. You've got it hooked up correctly, for sure. I thought the AFBs are the exact same as the Edelbrocks, but maybe there are some slight differences. You can find another manifold vacuum source elsewhere on the engine if there is a 1/4-inch tap on the manifold, or a free spot on the vacuum tree that feeds the power brakes.
Logistics Pro
Joined: Feb 2000
Posts: 3,955
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From: Mesa
Do you get vac just as the throttle is opened? Wonder if the throttle plate is closed beyond the port opening. I'd cap it and tap off the T in the manifold. On the choke, is there a 1/4" tube comming off the bottom? If so where does it go? Should go to a heat stove on the exhaust manifold. If not you can run a hose from there to the hose I see comming of the top, back left of the carb. The Carters use vac thru the choke to draw heated air off the exhaust to heat the choke(also have an elec coil) The idea is the air comes thru the aircleaner via that port in the back, to the heat stove then to the choke. That way it's filtered. This addd air should already be figured into the propper mix.
On Edit, looks like that hose is connected like it should be.
On Edit, looks like that hose is connected like it should be.
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Logistics Pro
Joined: Apr 2010
Posts: 4,446
Likes: 25
From: Panama City, FL
Do you get vac just as the throttle is opened? Wonder if the throttle plate is closed beyond the port opening. I'd cap it and tap off the T in the manifold. On the choke, is there a 1/4" tube comming off the bottom? If so where does it go? Should go to a heat stove on the exhaust manifold. If not you can run a hose from there to the hose I see comming of the top, back left of the carb. The Carters use vac thru the choke to draw heated air off the exhaust to heat the choke(also have an elec coil) The idea is the air comes thru the aircleaner via that port in the back, to the heat stove then to the choke. That way it's filtered. This addd air should already be figured into the propper mix.
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