carb gets rich when closing the throttle on hw

[Knebel]

Hi,

So, i have my carb (carter afb 4bbl) tuned in pretty okay with an o2 sensor, but i noticed something odd...

I cruise with 60mph on the highway at a good 15:1 AFR. This is, hard to say, about between 1/3 to 1/2 throttle opening, no load on the engine at about 2000 to 2100rpm. Now, when i CLOSE the throttle a little I see 12.5 to 13 AFR on my gauge at a certain spot and it stays there and wont go leaner. All going still 60mph on the highway. Steady increase of throttle is good at 14.7 to 15afr, slamming it richens it up. My Idle AFR is at about 13.5:1.

So, what causes the carb to go so rich when driving at speed and closing the throttle? I have the theory that the transistion slots are a bit too rich, driving around town i usually get a between 13 and 14:1 afr. And i get roughly 9.5 to 10.5 mpg with the 360...

How can I lean out the transistion slots on that carb?

 

[commando1]

Unless the engine is computerized and has FI, I'm thinking you're pretty much in the normal range for a NA engine.

 

[70bigblockdodge]

This is just a guess I have not used a air fuel gauge I need to get one gas is just to expensive to be throwing away on a good running but improper a/f. My guess is you need more timing at idle and if you are already at 33-34 total advance at 3300 rpm you probably need less mechanical advance in dist.. Quick check would be to put a vacuum gauge on it and then see how far you can advance at idle, keeping you same idle speed, i.e. advancing should raise idle speed, may need to adjust idle mixture screws also. Problem is stock chrysler dist. swings 20-22 degress so 34-22= 12degrees at idle car may idle better at lets say 14 btdc throttle plates closed more transfer slots covered up more air speed slowed through venturies all stacking up to drawing more fuel into engine at closed throttle causing your rich condition, this is the way I see it but I have never used a a/f/r gauge so ? leading me to more reason i need to get one of these thanks, good luck, let us know what you find.

 

[Knebel]

I have 18° idle timing, lower and i get no good idle vacuum and speed. Modified dizzy so its about roughly 34 total. I get about 11" vacuum at idle. Get more vacuum when I have over 30° idle timing. I have a vacuum can on the ported plug on the carb. Can says 6". 12° at idle dosent work good at all, very little vacuum and even with the holes in the throttle plateds i need to uncover too much transistion slot..

Previous owner said it had a cam in it but didnt know which... Im not so sure about that...judging by the sound of it. Idle sounds fairly smooth

 

[70bigblockdodge]

Sounds maybe like a vacuum leak? 11" at idle with a smooth idle seems low and your fuel mileage seems low also unless thats overall/ city, I got 13 mpg in a Challenger with 440, 2.94 rear 60series 14" tires, and a 292/.509 cam about 3100 at 70 mph. With that 18 degrees at idle it should be hard to start you have something out of whack, are you sure its on #1 and the dist is not clocked wrong I'm not trying to be a smart *** something is amiss in there.

 

[Knebel]

Sounds maybe like a vacuum leak? 11" at idle with a smooth idle seems low and your fuel mileage seems low also unless thats overall/ city, I got 13 mpg in a Challenger with 440, 2.94 rear 60series 14" tires, and a 292/.509 cam about 3100 at 70 mph. With that 18 degrees at idle it should be hard to start you have something out of whack, are you sure its on #1 and the dist is not clocked wrong I'm not trying to be a smart *** something is amiss in there.

Hey, yeah i agree with you tho. I recently regasketed the intake and there is no vacuum leak actually. I think my fuel milage is too low too. Yes, city driving and short trips. It starts cold after a couple spins and warm when it sees the key. Dosent seem to have a hard time starting. It could be that the balancer slipped but i havent gotten to turn the engine by hand yet to verify tdc. Yes i check #1 cylinder.

 

[70bigblockdodge]

I would be looking at that balancer make a piston stop tool out of a old spark plug and check that timing mark. That is great that the AFR meter/gauge is setting this off because if it runs okay other wise you may never find a problem and it could run better, I am definatly getting one of those no more guess work. What kind do you have BTW.

 

[Knebel]

I have an Innovate MTX-L. Decent little gauge to play with. I am uploading a Video right now so you can check it out. Its a Cold start, car sat overnight and choke is still a little rich for my taste...

 

[70bigblockdodge]

Cool thanks.

 

[Knebel]

Cool thanks.

Yw. One thing dosent work too well and thats balancing the idle. That took me forever to even get it close cause you see the number spike lean or go rich for a second... its also fairly distracting while driving. Im planning on getting everything set up right an then relocating it into the glovebox or so. Oh well... ill see that i can figure out a way how to hand- rotate that engine for tdc

 

[70bigblockdodge]

Front end up 1 1/4" socket and a breaker bar.

 

[1978 NYB]

I have an Innovate MTX-L. Decent little gauge to play with. I am uploading a Video right now so you can check it out. Its a Cold start, car sat overnight and choke is still a little rich for my taste...

Very nice. I'm going to think about getting one of those.

 

[Knebel]

So, I just found out that timing for a 1978 360 v8 is 20° btdc (that would be the engine I have)

18° isnt so far off then i think

 

[70bigblockdodge]

Where did that info come from?

 

[Knebel]

Where did that info come from?

came from here: http://ww2.justanswer.com/uploads/joecamel90/2010-02-07_102845_tune.pdf

when i got the car it had 40° at idle and I think it was 16 or 18 in the distributor. That was way too much i think. So I had done some research and limited the mechanical advance in the dizzy and set the timing to 18°. all with vacuum disconnected of course ;)

EDIT: but i have to say, retarding the initial timing a little seems to give it more crisp throttleresponse off idle.... so idk!?

 

[70bigblockdodge]

Something strange there it has to be in the dist or maybe a run of crankshafts that are machined wrong/different since your dist is different than stock you have to start at the begining check timing mark against zero at tdc then tune idle advance for highest vacuum level then set total timing to 34 btdc at 3000 rpm with a adjustable dizzy you can get this spot on. I wonder what the factory service manual list, maybe that is a misprint by mitchell because that is crazy advanced even for a big solid barely streetable cam. With yours no longer being totally stock go for highest vacuum at idle readjusting speed and mixture as needed, factory specs are rarely optimal usually emission setups/comprimize.

 

[Knebel]

Ok, will do that. All i know is that my fury had a 318 and this was replaced with a 360 from late 78. I didnt know this until after i bought the car and cleaned the gunk off the engine. Its got an edelbrock intake and a 4bbl. So... honestly, it could be anything in there. If i do the highest vacuum at idle and check with my timing light i end up with over 40° at idle!?!?

 

[70bigblockdodge]

Your dampener has to be slipped no way it would idle or even start at 40°btdc small block sweet spot is 34°btdc total timing at speed. If dampener is turned that far I would look into a new one, a 360 is external balance and it can't be doing your main bearing any good.

 

[moper]

You can see a lot more with that O2 sensor so you're getting ito the non-selfadjusting areas of the carb. It's one of the reasons EFI is so much better... My impression is it's a combination of the pressure drop above and bleo0w the blades vs the factory air bleed sizes. If you want to try to test that - try taking a small section of copper wire, removing a couple strands of the copper conductor, and inserting them down the cruise air bleeds. Leave them long so they hang out and bend that part that does hang out over where the air filter housing hits the carb neck. You only want to stick them in so they go down through the restriction - not all the way down into the passage below the restriction. You should see it go fairly rich but only at certain points in the throttle position. If it's the cruise air bleed there's not much you can do without getting seriously into the carb.

 

[Knebel]

You can see a lot more with that O2 sensor so you're getting ito the non-selfadjusting areas of the carb. It's one of the reasons EFI is so much better... My impression is it's a combination of the pressure drop above and bleo0w the blades vs the factory air bleed sizes. If you want to try to test that - try taking a small section of copper wire, removing a couple strands of the copper conductor, and inserting them down the cruise air bleeds. Leave them long so they hang out and bend that part that does hang out over where the air filter housing hits the carb neck. You only want to stick them in so they go down through the restriction - not all the way down into the passage below the restriction. You should see it go fairly rich but only at certain points in the throttle position. If it's the cruise air bleed there's not much you can do without getting seriously into the carb.

Hm. Well, that i could try. I got no issues getting seriously into the carb til I win the lottery and can do the efi conversion.