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Re: Fuel smell from exhaust [Re: Fern] #3200819
12/30/23 12:50 AM
12/30/23 12:50 AM
Joined: Jan 2003
Posts: 31,018
Oregon
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AndyF Offline
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Oregon
Common problem when putting an aftermarket cam in a low compression engine with stock manifolds. I run into this kind of stuff all the time when doing Sniper installs. People put big cams in low compression engines and then the Sniper doesn't work very well. Timing is your friend, sometimes I set engines like this up with 25 to 30 degrees of timing. Raising the idle speed is also a good idea. Raise the idle speed as much as you can without causing other issues. Sometimes you need to change the converter in order to raise the idle speed enough to get the cam to work. The AFR at idle will also be important but can be a little tough to set with a carb. If the throttle blades are in the correct spot you can use the idle screws to tweak the AFR. If the throttle blades are too far open then you might need to drill them in order to get them back closed so the idle screws will work. Depending on the carb you might also have the ability to change the IFR and/or the idle air bleeds. Good luck and welcome to hot rod tuning 101.

Re: Fuel smell from exhaust [Re: madscientist] #3201085
12/31/23 10:25 AM
12/31/23 10:25 AM
Joined: Jan 2013
Posts: 103
M.A.
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Fern Offline OP
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Fern  Offline OP
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M.A.
Originally Posted by madscientist
Start with timing. You need to correct that first. Do not lock it out, or get it “all in” by some arbitrary RPM. The engine WANTS a curve.

You may end up using manifold vacuum to clean it up. I don’t read Vizard nor do I care but if you do do MVA you need to understand what happens to the curve when manifold vacuum goes away.

If I have to run more than 24ish initial then I know the engine is built wrong, or the engine is something like you have. An engine you want to make better. This is a sure sign the compression is down, which you already know. It is what it is.

The other issue is if you Jack a bunch of initial timing into it then add MVA on top of that you can and probably will run into tip in detonation. You have to balance your initial with MVA, plus you need to see how much MV you have at idle and get the vacuum can that can pull some timing with that low MV.

If it was me, I would set the initial at 24 and start there. See if you can get it close without MVA. I say 24-25 initial because if you get much over that you will absolutely KILL the middle of your power curve because these engines do not want max timing at peak torque. In fact, you’d be shocked what these engines want for timing at peak torque. It’s usually in the low to mid 20’s. So think that through.

Let’s just say your engine wants 26 degrees at peak torque (and I’m going to assume your engine has a peak RPM of about 5500 and peak torque is 1500 RPM below that…which is my best guess for now) and you start out with 28 initial. You may be ok IF you can hold that 28 degrees if timing all the way to peak torque and then have it gain 1.5-2.0 degrees per 1000 RPM. But that is damned hard to do unless you modify the advance weights (you have to make them very light to delay the start of advance) and even if you lighten the weights you may not be able to get a heavy enough spring. I know unless you wind your own springs you generally have to make your own.

At that point if you need more initial than 28 at the MOST, then you need to reduce the initial and add MVA and figure out what can you need or can modify to get the timing you want.

Once you get that sorted out, you can get into the carb. First mod is to lower the Idle Feed Restrictions to the lower position and see how much that cleans up the idle, if any. If it is still on the fat side you have some options. You can reduce the IFR. Or you can reduce the Main Air Bleed. Or a bit of both.

Once you sort all that out, you can start tuning the primary side of the carb, including the power valve.

The correct way to fix this is not easy or cheap but the end result is worth it. Sadly, many just lock the timing out and send it. Certainly is faster, easier and cheaper. But it’s not better.



A lot of great info here.
Thank you.
I will start with the timing and adjust the distributor curve and see how it goes from there.

Re: Fuel smell from exhaust [Re: Fern] #3201182
12/31/23 03:49 PM
12/31/23 03:49 PM
Joined: Aug 2003
Posts: 43,152
Bend,OR USA
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Cab_Burge Offline
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Joined: Aug 2003
Posts: 43,152
Bend,OR USA
Originally Posted by Fern
Originally Posted by madscientist
Start with timing. You need to correct that first. Do not lock it out, or get it “all in” by some arbitrary RPM. The engine WANTS a curve.

You may end up using manifold vacuum to clean it up. I don’t read Vizard nor do I care but if you do do MVA you need to understand what happens to the curve when manifold vacuum goes away.

If I have to run more than 24ish initial then I know the engine is built wrong, or the engine is something like you have. An engine you want to make better. This is a sure sign the compression is down, which you already know. It is what it is.

The other issue is if you Jack a bunch of initial timing into it then add MVA on top of that you can and probably will run into tip in detonation. You have to balance your initial with MVA, plus you need to see how much MV you have at idle and get the vacuum can that can pull some timing with that low MV.

If it was me, I would set the initial at 24 and start there. See if you can get it close without MVA. I say 24-25 initial because if you get much over that you will absolutely KILL the middle of your power curve because these engines do not want max timing at peak torque. In fact, you’d be shocked what these engines want for timing at peak torque. It’s usually in the low to mid 20’s. So think that through.

Let’s just say your engine wants 26 degrees at peak torque (and I’m going to assume your engine has a peak RPM of about 5500 and peak torque is 1500 RPM below that…which is my best guess for now) and you start out with 28 initial. You may be ok IF you can hold that 28 degrees if timing all the way to peak torque and then have it gain 1.5-2.0 degrees per 1000 RPM. But that is damned hard to do unless you modify the advance weights (you have to make them very light to delay the start of advance) and even if you lighten the weights you may not be able to get a heavy enough spring. I know unless you wind your own springs you generally have to make your own.

At that point if you need more initial than 28 at the MOST, then you need to reduce the initial and add MVA and figure out what can you need or can modify to get the timing you want.

Once you get that sorted out, you can get into the carb. First mod is to lower the Idle Feed Restrictions to the lower position and see how much that cleans up the idle, if any. If it is still on the fat side you have some options. You can reduce the IFR. Or you can reduce the Main Air Bleed. Or a bit of both.

Once you sort all that out, you can start tuning the primary side of the carb, including the power valve.

The correct way to fix this is not easy or cheap but the end result is worth it. Sadly, many just lock the timing out and send it. Certainly is faster, easier and cheaper. But it’s not better.



A lot of great info here.
Thank you.
I will start with the timing and adjust the distributor curve and see how it goes from there.
iagree All things mechanical needing fixing are "fixable" up wrench grin
Now, get to work and don't give up unntil YOUR very HAPPY with YOUR results grin wrench up

Last edited by Cab_Burge; 12/31/23 03:50 PM.

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
Re: Fuel smell from exhaust [Re: Cab_Burge] #3201199
12/31/23 05:05 PM
12/31/23 05:05 PM
Joined: Jan 2003
Posts: 1,232
Chicago, IL
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PLUM_72 Offline
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Joined: Jan 2003
Posts: 1,232
Chicago, IL
Not knowing where its at now, timing and carb adjustments may help a bit. However if there is any overlap on the cam, this is where you're going to get a large amount of fuel smell. Only way to tune that out is a new cam with grocery getter specs.


1972 Dodge Challenger
Re: Fuel smell from exhaust [Re: Fern] #3201243
12/31/23 09:35 PM
12/31/23 09:35 PM
Joined: May 2011
Posts: 4,489
northern,Ohio,USA
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Clanton Offline
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northern,Ohio,USA
You may have some work into the carb but I think you may need more to fine tune it along with the timing just pic away at it and it should get better.


GOTBOOST!New improved with Victor heads.
http://www.enginelabs.com/mopar-big-bloc...t-of-necessity/
Re: Fuel smell from exhaust [Re: PLUM_72] #3201252
12/31/23 10:04 PM
12/31/23 10:04 PM
Joined: Aug 2003
Posts: 43,152
Bend,OR USA
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Cab_Burge Offline
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Posts: 43,152
Bend,OR USA
Originally Posted by PLUM_72
Not knowing where its at now, timing and carb adjustments may help a bit. However if there is any overlap on the cam, this is where you're going to get a large amount of fuel smell. Only way to tune that out is a new cam with grocery getter specs.
WRONG tsk I've built, tuned and driven many hot street motors that didn't stink like raw unburnt fuel when idling
the stink is the unburnt fuel getting into the exhaust unburnt work shruggy
Lean it down and add some timing until you get it the ay you want it up scope twocents
EDIT, make sure the cam is installed properly and not retarded a bunch scope wrench work up

Last edited by Cab_Burge; 12/31/23 10:05 PM.

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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