rich exhaust smell

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wireweld

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I have been tuning my car and I have a rich exhaust smell and I was wondering if a large duration cam will cause this. My cam is a hughes 3742al,237/242 @50. This is in a stroked 340 with eddy heads and a 650 mighty demon.
I have been trying to tune out the rich smell with no success.
 
I had that problem too.I thought it was the edelbrock carb.Turned out to be plugs.
 
Idle vacuum? Power valve?

Beyond that, a cam with a lot of overlap like that will tend to be inefficient at low RPM.
 
I agree with the cheif there. The larger the cam, the more of a pain it is to get it to NOT to smell at idle. This is where a 4 corner idle system and adjustable bleeds etc..... come in.

Keep at it. Your cam isn't that large. A helpful device is a 02 sensor. I do realize not everyone can afford a nice brand new dual sensor LMN or whatever they call it. But even a realitively inexpensive unit like an Edelbrock or K&N 02 sensor can help alot.
There retailing under $150 bucks.

A quick glance at any rpm, hopefulllly by your passenger co-poilt will tell you where you stand on the balance of air and fuel.
 
i think chief hit it. check vacuum @ idle in gear(auto), neutral(manual) and get vacuum reading.
 
You could bump up the initial timing 2 deg more and back-off the main idle speed. This will lean out the idle circuit a bit, David.
 
Okay, I've been thinking and I have some other things to try. Not that we want to swamp you with advice.

One of the pitfalls of running a slightly small carb on an aggressively cammed large (stroked 340) engine is that the primary throttle blades may be out of the idle range at idle. That is, to provide the airflow demanded by the engine, the throttle blades (primary) will be open far enough to totally uncover the "idle transfer slot" in the throttle body. So now, your carb is actually running on the main circuit at idle, (with the power valve seated) but the is insufficient airflow for the main circuit to meter or atomize the fuel efficiently. If your mixture screws "make no difference" like you can screw them in all the way and the engine doesn't die, this may be you problem. Consequently, if the power valve is unseating at idle, the power circuit is coming into play. Either way, rich idle.

So, what to do? First check idle vacuum and ensure your PV is at least 2 numbers lower. So if you have say 9" Hg at idle you want a 6.5 PV. Thats what comes in a lot of Holley's. Another quick check, look at the primary booster venturis, are they dripping at idle? Classic blown/ incorrect PV. So now asuming your PV good, FLOAT LEVELS ARE CORRECT, and your timing is reasonable, do the idle mixture screw test. You should, by turning each screw in 1/2 turn alternately, lean the engine to the point it dies. If the engine seems unaffected, your primary throttle blades may be open too far. On a classic old school Holley, to fix this you unbolted the carb, flipped it over and gave the secondary throttle stop screw a half turn and tried again. If your carb didn't have one of these (AFB, AVS, Q-Jet) you started by drilling 3/32" holes in the secondary blades. Either way, you got really good at unbolting the carb.

BUT with the Mighty Demon, it looks like the secondary throttle position is adjustable without unbolting the carb. Nice touch! How did I figure this out? I went to the Barry Grant website and downloaded the manual. There is even a nice picture of how it should and should not look on page 3. Try opening the secondaries 1/2 turn and closing the primaries. See if that helps.

http://www.barrygrant.com/fromBarryGrant/Demon Instruction Manuals/MIGHTY Demon Manual.pdf
 
A lean condition will also smell of gas so I would check the plugs to see what they look like.


Chuck
 
340mopar said:
A lean condition will also smell of gas so I would check the plugs to see what they look like.


Chuck
Click to expand...

Exactly. Alot of people confuse stinky exhaust for rich exhaust when it can actually be lean. That's why you have to do a plug read or use an exhaust analyzer like Rumblefish said to tell for sure.
 
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