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What causes stinky idle with performance cam?
Posted By: Mebsuta
What causes stinky idle with performance cam? - 07/22/12 05:16 AM
What is the biggest culprit of stinky idle with a performance cam? You know, where it smells like a big lawn mower and gets all over you if you walk behind it. Is it overlap, late exhaust valve closing, or something else?
I don't care too much but some gurlz won't ride in a stinky car.
Posted By: Prince_Valiant
Re: What causes stinky idle with performance cam? - 07/22/12 06:02 AM
Overlap causing reversion.
The reversion is basically spent exhaust doesn't fully exit, and a good fresh charge of fuel and exhaust then don't fully enter, and leads to a poor combustion (which also might be occurring from low-dynamic compression due to very low intake velocity from the big cam). Add it all together and there you have it...poor idle...
Posted By: Secret Chimp
Re: What causes stinky idle with performance cam? - 07/22/12 08:01 AM
Supposedly a four hole or split spacer will help cure this from a standpoint of the exhaust interfering with carb signaling, unless you're running a single-plane intake or a dual-plane with the divider cut down.
Posted By: RobX4406
Re: What causes stinky idle with performance cam? - 07/22/12 08:08 AM
People get pretty clean idles with 250+ at .050 cams and others can't get a clean idle with a cam with 20-30 degrees less duration and overlap.
Maybe it's got more to do with the tune up than the camshaft. The time it using total method crowd are usually pretty good at providing a stinky idle most of the time, unless they get lucky.
JMO
Posted By: ahy
Re: What causes stinky idle with performance cam? - 07/22/12 02:56 PM
Agree its high overlap (long duration + tight lobe seperation) that is the main factor. I suspect a single plane manifold makes it worse also.
Tuning can help clear it up... at least partly. Adequate initial advance plus the leanest AF that will hold a steady idle. Mine likes 20 degrees initial advance and around 14.5 air/fuel. Getting those two dialed in helped both vacuum and "fumes".
Posted By: topside
Re: What causes stinky idle with performance cam? - 07/22/12 03:34 PM
Unburnt fuel. Depending on the carb, can be weak vacuum signal leaving AFB/AVS metering rods at full enrichment.
Posted By: GreenGlow
Re: What causes stinky idle with performance cam? - 07/22/12 05:04 PM
Put some C-12 race fuel in it and when you find a girl that loves the smell..... Marry her.
Summit has some stuff you add to your fuel that will make it smell like cherries or flowers or something like that...... But I would go with the race fuel idea
Posted By: RSNOMO
Re: What causes stinky idle with performance cam? - 07/22/12 05:14 PM
'Put some C-12 race fuel in it and when you find a girl that loves the smell..... Marry her.'
There's some righteous advice...
Posted By: Dodgem
Re: What causes stinky idle with performance cam? - 07/22/12 05:23 PM
Posted By: dustergirl340
Re: What causes stinky idle with performance cam? - 07/22/12 05:28 PM
I love the smell of Torco 110. Heck, even 93 octane is good.
Posted By: Triggerfish
Re: What causes stinky idle with performance cam? - 07/22/12 05:45 PM
Quote:
'Put some C-12 race fuel in it and when you find a girl that loves the smell..... Marry her.'
There's some righteous advice...
I agree & get a lot of thumbs up from female truck lovers, but until I swap the huge cam in my 383 mag for the Lunati Truck Avenger cam I ordered, I'm in the same boat. Not only fuel smell & wet tail pipes, but my low stall verter, tall tires & gears makes for lousy low end performance & poor fuel economy. Installing a 1" 4 hole spacer on the Weiand dual plane & 600 eddy carb + bumping initial to 20 improved low end manners a little, but still not acceptable for street performance.
Thought of Rhodes lifters to tame the overlap, but heard they're not that predictable. Think a cam swap is your best solution to not only fix the smell issue, but better performance & fuel economy.
Posted By: fox
Re: What causes stinky idle with performance cam? - 07/22/12 07:07 PM
The reversion caused by overlap/big exhaust and big intake is pulsing air both ways thru the carb. It will meter fuel either way! This make idle/low rpm extra rich. Figure out how to lean it down ( and the idle screws are only part of it) and you can clean it up.
A good carb tuner should know how.
Using a good wide band A/F meter is an easier way.
Posted By: RSNOMO
Re: What causes stinky idle with performance cam? - 07/22/12 10:06 PM
Call me old-school, but I just can't get cozy to the idea of my exhaust smelling like Hires...
Or Frosty...
Posted By: jbc426
Re: What causes stinky idle with performance cam? - 07/23/12 07:23 AM
What are your initial and total timing numbers?
Are you running vacuum advance plumbed to manifold vacuum or straight mechanical advance?
How much vacuum does it have at idle(in gear if an automatic)?
Posted By: Mebsuta
Re: What causes stinky idle with performance cam? - 07/23/12 10:53 AM
Right now I use the 383 HP cam in a 383 Roadrunner. It has 46 degrees of overlap. Ignition timing is 8 degrees BTDC, by the book. Don't know what the total is. I use ported vacuum advance. Manifold vacuum at idle is 16 inches at 700-750pm. Runs good, just smells like a big Briggs and Stratton. Always has.
Thing is, I might try something with a tighter LSA and close the intake valve earlier. That gives you more overlap and bumpty hot-for-teacher idle, which gurlz like, but if it's going to be more stinky, they don't like that. Gurlz want it all don't they? Luv them anyway.
Posted By: OhioMopar
Re: What causes stinky idle with performance cam? - 07/23/12 11:12 AM
Quote:
I love the smell of Torco 110. Heck, even 93 octane is good.
Yes, but you're already married. So that means the search continues... 
Posted By: ireland383
Re: What causes stinky idle with performance cam? - 07/23/12 12:08 PM
I would up your initial timing and than adjust your idle. This should clear it up some. You need to get a timing light on there to figure total as well.
Posted By: Challenger 1
Re: What causes stinky idle with performance cam? - 07/23/12 12:19 PM
Quote:
People get pretty clean idles with 250+ at .050 cams and others can't get a clean idle with a cam with 20-30 degrees less duration and overlap.
Maybe it's got more to do with the tune up than the camshaft. The time it using total method crowd are usually pretty good at providing a stinky idle most of the time, unless they get lucky.
JMO
Put a good tune in her and she won't stink.
Posted By: Challenger 1
Re: What causes stinky idle with performance cam? - 07/23/12 12:27 PM
Quote:
Agree its high overlap (long duration + tight lobe seperation) that is the main factor. I suspect a single plane manifold makes it worse also.
Tuning can help clear it up... at least partly. Adequate initial advance plus the leanest AF that will hold a steady idle. Mine likes 20 degrees initial advance and around 14.5 air/fuel. Getting those two dialed in helped both vacuum and "fumes".
I use a single plane manifold and my car don't stink at idle. I run 22 degrees intial, I don't know why guys are afriad to run plenty of intial timing. 
Posted By: bonefish
Re: What causes stinky idle with performance cam? - 07/23/12 01:32 PM
Quote:
Quote:
People get pretty clean idles with 250+ at .050 cams and others can't get a clean idle with a cam with 20-30 degrees less duration and overlap.
Maybe it's got more to do with the tune up than the camshaft. The time it using total method crowd are usually pretty good at providing a stinky idle most of the time, unless they get lucky.
JMO
Put a good tune in her and she won't stink.
mine was so bad i couldnt even sit at a lite, it would gag people for a country block.i couldnt even tune the car unless i had it facin down wind on a breezy day and i aint jokin.timming and carb fixed it.im runnin a fairly radical street cam.
Posted By: 451Mopar
Re: What causes stinky idle with performance cam? - 07/23/12 04:21 PM
You did not say what engine combination, but recurving the dist for more initial timing, and making sure you are not in the carbs transition circuit are two tuning items that help.
On my 505" stroker with a mild cam, I am running 18-degrees initial, and 35 total timing. I also had to drill the throttle blades so I could close the throttle out of the transition circuit at idle.
Posted By: RobX4406
Re: What causes stinky idle with performance cam? - 07/23/12 04:50 PM
46 degrees of overlap and it has a stinky idle?
That's not a very large cam or have much overlap.
Here's what I might do. Double your initial and reset the carb using a vacuum gauge to highest reading, leanest mix setting. Then maybe out 1/8 turn. As long as it starts when warm without dragging the starter, that's a good starting point.
I don't even run stock 318's at 8 BTDC, nothing I have has less than 14 initial. Throw the book in the trash because the timing events from an FSM were not to optimize performance. They had emission profiles to meet, even back then. I'd figure out the total and tailor it after setting initial before you hurt the engine.
If you don't have a timing tape or marked balancer, find a post with "hillbilly timing tape" and do that so you can determine total. Just don't drive around with an excessive amount of total, bad things happen...
It takes some work to get ignition timing right, but, the benefits of a clean running engine are worth the time.
Posted By: Mebsuta
Re: What causes stinky idle with performance cam? - 07/25/12 05:14 AM
Okay, the tab on the timing cover goes up to 15, so I guesstimated and set it to 18 degrees initial BTDC. Set the idle back down to 700 rpm. Now getting 17 inches manifold vacuum and idle is cleaner. Took it for a little ride and it is happy; no castanets and easy starting. Will revisit in more detail later and will not be afraid of a more bumpty cam in the future. Thanks.
Posted By: ireland383
Re: What causes stinky idle with performance cam? - 07/25/12 12:52 PM
Glad it's working out better. I'm at 18 initial also with a cam 235/482.
Posted By: RobX4406
Re: What causes stinky idle with performance cam? - 07/25/12 10:30 PM
Quote:
Okay, the tab on the timing cover goes up to 15, so I guesstimated and set it to 18 degrees initial BTDC. Set the idle back down to 700 rpm. Now getting 17 inches manifold vacuum and idle is cleaner. Took it for a little ride and it is happy; no castanets and easy starting. Will revisit in more detail later and will not be afraid of a more bumpty cam in the future. Thanks.
Cool you got it sorted out.
Posted By: jbc426
Re: What causes stinky idle with performance cam? - 07/26/12 12:26 AM
Quote:
Okay, the tab on the timing cover goes up to 15, so I guesstimated and set it to 18 degrees initial BTDC. Set the idle back down to 700 rpm. Now getting 17 inches manifold vacuum and idle is cleaner. Took it for a little ride and it is happy; no castanets and easy starting. Will revisit in more detail later and will not be afraid of a more bumpty cam in the future. Thanks.
Before you call it done, check to see how much total advance you have with the vacuum disconnected. 36 degrees total is about the maximum you want. If you haven't welded up the slots in your distributor, you are probably well past that if you're running 18 initial on an otherwise stock distributor.
Posted By: Mebsuta
Re: What causes stinky idle with performance cam? - 07/26/12 06:40 AM
Allright I'll look at that. Need to get a hillbilly timing tape on the balancer.