Lean carburetor but black spark plugs

[josehf34]

Some time ago replaced my carter AFB for a Street Demon 625 CFM in my 360 SB due to a very rich or very lean condition, now I'm stucked with a similar problem with my new carb

My engine has a comp cams 262H XE camshaft, new timing set, stock pistons, stock heads and stock exhaust without any emission control system

I'm running the smaller main jet and the biggest metering rod that holley makes for this carb but my car is still running rich! I did the smoke test two weeks ago (smoke test is mandatory for any car in my country, doesn't matter how old it is) and the guy told me that the car was running rich while idling (9.7% CO2 @750 RPM) but is fine at cruising (3.2% CO2 @2250 RPM). Checked my spark plugs and all of them are black or dark brown but yesterday went to a travel and after 90 minutes going up hill the car started overheating, removed three spark plugs and all of them were white (very lean condition I suspect) I drove for another 20 minutes out of the hill and once I arrived to my destination checked the spark plugs again and 6 were black but the electrode was white and the remaining two were completely black

So I don't know what else should I check, my initial advance is 12º BTDC, I don't know what is my total advance (my timing marks only goes from 10ºATDC to 10ºBTDC), checked for electrical problems and tried GM HEI ignition system and mopar ignition system without any difference between them

 

[65sporty]

I don't think it's a timing issue. I would start back at the carb. The street demon looks like a thermoquad.

 

[josehf34]

I don't think it's a timing issue. I would start back at the carb. The street demon looks like a thermoquad.

yep, the street demon is like the result of pairing a Carter AFB and a Carter Thermoquad, is a very simple carb with an awful design that makes any change like metering rods or jets a nightmare but it works really really well!

I have a question: The front idle mixture screws what control? I've read that them controls the volume of the air flow while idling and also I've read that them control the volume of the air/fuel mixture that the carburetor uses while idling. What's the true?

 

[detmatt]

The idle mixture screws control the volume of fuel being introduced to the volume of air that is controlled by the idle speed screw located on your linkage. As soon as you come off idle none of these things make a difference.

 

[fury fan]

I disagree on the idle screws being involved only at idle. They are the component to adjust idle AF, but they will contribute during transition to cruise, possibly across the board.

FWIW - there's an old adage that 90% of carb problems will be found in the ignition.

 

[detmatt]

From an article.
Keep in mind, these are IDLE bleeds only. In other words, the second you crack-open your throttle, these idle bleeds quit working. Some people think they can lean out or richen a carb by simply adjusting these screws, and that is SO wrong. The only thing you are adjusting with these bleed screws is the IDLE mixture. Yes, they DO affect an "off-idle" mixture a tiny bit, so if your engine has a slight stumble right when you start to crack-open the throttle and you know you are getting an ample squirt from the accelerator pump, then sometimes cracking-open the idle mixture screws another quarter to half turn "can" make that stumble go away, but not in all incidences. The ONLY way you can "really" lean-out or richen-up a carb from the outside is if you have a customized carb with a "Dial-a-Jet" system, on it, otherwise you have to open-up the carb and swap the main jets out to either a larger or smaller size.

 

[saylor]

something else about the idle circuit: I know on the stock carb I was messing with, there was a .035~ measurement on how far the idle bleed ports should be uncovered when the carb is at rest. meaning, when the butterfly is closed fully, only .035 of the idle transfer slots should be exposed. I had a bent butterfly, and more than .035 of the idle transfer slot was exposed, and I never was on the idle circuit at all. i was already in to the main circuit. thus I could never get the carb set properly. it took me literally 1 month to figure it out :/ and only with the help of a more experienced person. all he said was ' something is wrong with this carb I cant get it set'... then I had to swap out for another carb. it bolted on and flew.

find another carb from somewhere - even if its only for an hour or so. hell buy one then return it. something. just as a test.

try not to die -

- saylor

 

[fury fan]

From an article.
Keep in mind, these are IDLE bleeds only. In other words, the second you crack-open your throttle, these idle bleeds quit working. Some people think they can lean out or richen a carb by simply adjusting these screws, and that is SO wrong. The only thing you are adjusting with these bleed screws is the IDLE mixture. Yes, they DO affect an "off-idle" mixture a tiny bit, so if your engine has a slight stumble right when you start to crack-open the throttle and you know you are getting an ample squirt from the accelerator pump, then sometimes cracking-open the idle mixture screws another quarter to half turn "can" make that stumble go away, but not in all incidences. The ONLY way you can "really" lean-out or richen-up a carb from the outside is if you have a customized carb with a "Dial-a-Jet" system, on it, otherwise you have to open-up the carb and swap the main jets out to either a larger or smaller size.

What article is that from, and what carb is it for? My comment earlier was geared toward AFB/AVS, but we should probably make sure whatever we say (meaning me) is geared correctly to the carb at hand.

 

[Knebel]

Could you post a picture of the spark plugs?

 

[detmatt]

What article is that from, and what carb is it for? My comment earlier was geared toward AFB/AVS, but we should probably make sure whatever we say (meaning me) is geared correctly to the carb at hand.

It was a general article not any particular carb specifically.

 

[Landyacht]

Could it be oil fouling due to bad valve seals?

 

[josehf34]

Could you post a picture of the spark plugs?

Sure I'll do it this night

Could it be oil fouling due to bad valve seals?

I'm not sure, I put new valve seals less than six months ago but I've never been 100% sure if them were properly installed because i read that some seals needs to be pressed in the valve guide and I didn't did it

 

[Landyacht]

Sure I'll do it this night



I'm not sure, I put new valve seals less than six months ago but I've never been 100% sure if them were properly installed because i read that some seals needs to be pressed in the valve guide and I didn't did it

I good indicator of bad valve seals is if your exhaust gives off blue smoke when the car is first started after sitting for a long time and then disappears a minute or two later.

 

[fury fan]

Totally unrelated, but needs to be pointed out.
Look at Saylor's avatar (post #7), then look at mine (#8). Same car. Then look at our stats. We joined 1 day apart!
Percentage-wise he gets liked a lot more than I do, though.

 

[detmatt]

Totally unrelated, but needs to be pointed out.
Look at Saylor's avatar (post #7), then look at mine (#8). Same car. Then look at our stats. We joined 1 day apart!
Percentage-wise he gets liked a lot more than I do, though.

That just means you need to post more.

 

[fury fan]

No-can-do'sville, Matt. It takes effort to seem to be nice and helpful, so I'm afraid if I posted more my façade would come down and my true colors would show. It would smell like Moparts in here...