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Old 10-07-2010, 10:07 PM
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am i running lean or just right?

chevy 350, rv cam, quadrajet carb, stock everything else. i pulled a plug to see how the fuel mix was to see if i could lean it out a bit and get better gas mileage. but it looks like its running pretty lean already. maybe too lean? i have the mixture screws out 4.5 turns each so i would think its running really rich. maybe i have the wrong plugs?
each pic is the same plug. but all the spark plugs look the same. kind of a blue color on the tip and white ceramic.

sorry about the bad pix. if there not good enough i might be able to get better ones.

thanks for the help
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Old 10-07-2010, 11:04 PM
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By my eye sight it`s too lean. Blue on the tips indicate too much heat. The correct mixture will show a tan color on the plug. However, you can`t put a lot of faith into plug viewing, especially in pictures but if the tips are as white as they look and you see blue undoubltably means it`s getting too hot.
Are you sure it doesn`t have any vacuum leaks? This maybe why the idle screws are turned so far out. Is the vacuum advance connected to a full time manifold source or a ported source? what`s the timing set at? Did the Quadrajet come on your engine or did you just pick one up?
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Old 10-07-2010, 11:10 PM
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I think it looks pretty good, todays pump fuels won't darken a plug up like they used to. I wouldn't change a thing if it feels like it runs fine, might be a little on the rich side.

How many miles on that plug??
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Old 10-07-2010, 11:28 PM
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The mix screws only affect idle. 95% of the time that entire circuit is irrelevant to the mixture.

I would say you're a bit lean but those plugs don't look too bad. Any drivability issues?
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Old 10-08-2010, 10:00 AM
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i took it out for a drive today and pulled a plug when i got back. looks the same as before. then about 4 hours later i started it and just let it idle for half an hour. pulled a plug and it looks the same. so im guessing ive got a vacuum leak somewhere. 1977 chevy 350 that i took all the emissions control crap off of. capped 3 vacuum ports on the carb.

about 20 hours on the plug. its an off road only truck so i dont have a speedometer.

would the pvc cause a engine to run lean if it was sucking a lot of air?


ill check the timing and where the vacuum advance is hooked up when i get off work.


thanks
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Old 10-08-2010, 12:34 PM
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Quote:
Originally Posted by jak327
chevy 350, rv cam, quadrajet carb, stock everything else. i pulled a plug to see how the fuel mix was to see if i could lean it out a bit and get better gas mileage. but it looks like its running pretty lean already. maybe too lean? i have the mixture screws out 4.5 turns each so i would think its running really rich. maybe i have the wrong plugs?
each pic is the same plug. but all the spark plugs look the same. kind of a blue color on the tip and white ceramic.

sorry about the bad pix. if there not good enough i might be able to get better ones.

thanks for the help
These look fine, the insulator is slightly brown, not stark white, there is a little deposit on the ground electrode. All of this looks as normal as you'd see without specific tests on a new plug. As with many things, the color of the plug is telling what could be several stories, it's up to the mechanic to identify which is the correct story. A hot heat range plug in an otherwise normally set up engine will develop insulator colors that make you think the mixture is lean as the plug will look fried white. The reverse with a too cold plug, all other things being correct, the plug will become dirty and make the mixture look rich when it isn't. There are many other combinations and permutations of plug colors that can fool you as to cause. So the rules of colors is just a guide to a data point, they don't necessarily lead you a specific problem as several types of errors in engine set up can lead to similar looking plug coloration's.

Bogie
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Old 10-08-2010, 12:50 PM
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Quote:
Originally Posted by jak327
chevy 350, rv cam, quadrajet carb, stock everything else. i pulled a plug to see how the fuel mix was to see if i could lean it out a bit and get better gas mileage. but it looks like its running pretty lean already. maybe too lean? i have the mixture screws out 4.5 turns each so i would think its running really rich. maybe i have the wrong plugs?
each pic is the same plug. but all the spark plugs look the same. kind of a blue color on the tip and white ceramic.

sorry about the bad pix. if there not good enough i might be able to get better ones.

thanks for the help
4-1/2 turns on a Q-jet isn't too far out. Plugs look OK, I would suggest using a projected tip plug.

If you are not getting any preignition, and the plugs do not look like they're getting any hotter than what you show here, the heat range is close- but try a cooler plug if you feel there's a case for it- there's no cut and dried way to know other than to try various ranges/types and compare the results, IMO.
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Old 10-08-2010, 01:06 PM
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To tell if the primary side of the Q-jet is metering, do a tip-in test.

Run the engine until up to full operating temp. Using a vacuum gauge, set the idle mixture screws. This assumes the float isn't 'heavy' and the float level is correctly set.

Remove the air cleaner, set the idle speed screw to give you about 2000 RPM, then using your hand or the choke flapper, close off the air flow through the primaries just a little bit until you see if the engine wants to pick up speed or drop down some.

If the speed picks up, you're on the lean side, slowing is on the rich side. Right at the lean cusp is fine, i.e. a slight increase in RPM when choked a little.

You can tailor this w/the APT. If there's not enough adjustment for the APT to bring it in, you need to work with the rods/jets.

What rods are you using on the secondary side?
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Old 10-09-2010, 10:58 PM
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the carb is all stock. i only rebuilt it. then took it apart again because the needle and seat had some dirt in it. got it back together just fine.

when i rebuilt it i put the mix screws back where they were at 4 turns each. now im at almost 5 turns out and im just starting to get black in the exhaust pipes. straight piped. no mufflers or cats. only about 4 feet of 2.5" pipe.

when doing a vacuum test where do you hook the vacuum gauge to? sorry if thats a dumb question but ive never done it before. i dont have a vacuum gauge but ive needed one before so ill buy one this time.
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Old 10-10-2010, 12:42 AM
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You connect it to a manifold port which is a port that pulls vacuum full time, including idle.
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Old 10-10-2010, 08:49 AM
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looks a bit lean to me, also, looks like it could use a bit more timing (ill explain that if someone wants me to)
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Old 10-10-2010, 08:52 AM
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Plugs look fine to me also.


Cole
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