I have a 1977 motor 440. It was a dog so I had my mechanic install a Hughes whiplash cam made for these low compression motors.
I have a holley 770 avenger carb with vacuum secondaries, with edelbrockrpm I take (I think). And stock manifolds.
The engine runs really well with a noticeable lopy cam. The only issue is the unburnt gas smell from the exhaust. I'm assuming from the cam specs.
Any way to stop it from smelling so bad?
Timing is at 17 initial and 37 all in.
I'm going to rebuild the motor at some point, but I would like to drive the car and not smell like gas.
Thanks

Sounds like the air/fuel mixture is too rich.
Was the gas smell present before the cam change, and was anything else changed ?

Sometimes cam timing kills the idle vacuum, in which case the carb adds more fuel via an enrichment circuit, like a power valve.

The gas smell was minimal, if at all, before the cam swap.
Vacuum is bad at idle, barely enough for brakes.

Are you running a vacuum advance off manifold vacuum? If not you should. Sounds like you need more idle timing.

You may need to lean down the idle fuel feed circuits, both sides of that carb. to get rid of that smell.
You could try slowly screwing in the idle mixture screws first to see if that helps stop the stink or not, you might be able to fix it that way, maybe not.
I don't have any real hands on experience with that line of carbs.

Try more initial timing. 17* is not nearly enough in my experience. A bumpy cam often requires 25* or so initial and all in as soon as the engine can tolerate it at cruise. By 2,000rpm if possible.

Of course all that depends on other factors, too; gears, weight, trans, converter, etc. But those timing events are good targets to shoot for.

Unfortunately, those low compression engines can be more prone to detonation than higher compression units that have good quench because they don't have quench going for them.

Experiment with that timing and see what happens.

If you install a performance cam in a low compression engine it is going to stink out the exhaust. Any performance camshaft changes the rpm range the engine operates efficiently at to a higher rpm range. It also reduces cranking compression. The smell is unburned fuel, because the engine has enough overlap that it is not burning the fuel charge. The loping at idle that everyone likes is because the engine is not efficient at that low rpm range anymore. Yes, you can advance the timing, play with the idle circuits of the carb, and other patches but bottom line is if you install a camshaft in a low compression engine it will make hydrocarbon emissions at idle and stink. First thing I would do is take a cranking compression reading, if it is below 130 lbs no patches are going to remove the stink. Best bet would be to advance the cam timing to help the issue. Cam timing is different than ignition timing.

Good info. Thanks.
The whiplash cam is designed for low comp engines.

Thank you good info. Will try timing.

i run in to that same idel smell couple times before--- i enriched the ildel fuel restrictons until i got my best results with the mixtures screws around i turn out,plus vaccum advance plug into manifold vac?

Using idle manifold vacuum is a band aid, but does tell you that the engine likes/wants more initial timing. The biggest problem with using manifold vac to help the idle timing is that it drops off then with throttle.

There are numerous videos on going inside our distributors. All that needs done (usually) is for the slots to be shortened and much lighter springs to be installed. Not really hard at all and can be done with a Phillips screw driver, needle nose pliers and some JB Weld.

Of course, on some low compression engines, you may be able to just lock the advance out and set the timing at it's best total advance. As long as it doesn't hit the starter or ping, it's all good.

I guess David Vizard doesn't know anything.

Do you believe everything you read?
You shouldn't, make sure what you except as the truth is the truth, especially in all things mechanical

Cab, do you know who David Vizard is? If he states that ALL street cars should run manifold VA then I believe him.

Chevy use to use manifold vacuum advance and then switch to ported vacuum for smog rules, I don't use vacuum advance on any of my hot street builds, especially with todays electronics EFI set ups
BB Mopars on pump gas seem to like 14 to 18 degrees advance BTDC at or under 1200 RPM and then all the advance in by 1800 RPM, 32 to 36 degrees BTDC total with the correct tune and correct heat range spark plugs

I have read Vizard's book as well as Emanuel's and would like to make a few points:

David Vizard is a revered engine builder and an acknowledged Chevy big-block engine expert. Enough so that GM contracted with him. And most of his brand specific books center on GM engines. So that is where his experience lies. Not with BB Mopars.

Vizard's book was written a long time ago. And although some things are the same today, some things have changed since it was written.

I have not read Vizard's manifold porting recommendation in any other Holley tuning book. And certainly not for ALL engine applications.

Which brings me to a few questions:

Have you tried re-curving big block Mopar distributors to see if that if preferable to Vizard's recommendation?

What advantage do YOU believe manifold vac has over a properly curved distributor in our big blocks?

Do you not see the disadvantage of vacuum dropping off with throttle opening and therefore dropping timing out?

I agree with you, Cab. The only thing that I might have a little different experience with is the initial; the bumpier the cam, the more initial they typically like. All the way to locked out if it doesn't hit the starter too much when cranking. But most of the time, my engines (both BB and Hemi) seem to do well with initial in the mid 20s then with the advance in as soon as possible like you recommend. I use that even on engines with milder cams. The caution comes with load in high gear. You have to watch for detonation when cruising with total in that early. Gearing, weight, converter, etc all come into play with that.

These are my experiences with primarily street driven cars with more cam than stock.

@DaveRS23, everyone...


So for the record, I am of the same opinion you are, that being: choose carefully, the individual components have to 'interlock/complement' each other, and that's not just parts fitting but equally important to their configuration. No matter how "esteemed" the author of a particular publication/book is, he/she may simply not have had the required experience with MY particular combo for their recommendations to be valid.

Having said that, I've been thinking about the manifold vacum option recently and simply ran out of time this past season to try it out. The reason being: I've done pretty much everything I could to make the carb ported option work, and because of the cam (hydraulic roller, 240/248 @0.050") I simply need to run more advance at idle. But as you stated above, when dealing with the limitations of the mechanical setups you eventually run into the brick wall: can NOT give it more INITIAL advance b/c the started will kick back.

In my current setup (W2 408 stroker, 10:1 static CR, 195 psi cranking, 91 octane pump gas) I use 21 deg initial, followed by a fairly quick curve with additional 15 deg all in by 2000 RPM, with the final 15 deg for cruising purposes. The 4K stall converter and 4.10 gearing helps with this on my street ride (engine does not really see much load that would cause it to labour at low RPM thus promoting detonation).

However, that's still not enough because for testing purposes bumping up the INITIAL to around 25-26 nets me an even steadier burn, and it's not even the 'smooth idle' that I'm after here, rather it is the more complete burn of the mixture (the sniff test and AFR tell that story).

So the theory at least is that by running off of manifold vac signal I may be able to:

1) keep the INITIAL mechanical advance low enough to prevent the started kick back
2) immediately bump up the idle & off idle advance to around that 25-28 mark where my combo seems to like it most
3) as soon as I get on it (we are talking cruising here, not WOT) the drop in the manifold vac signal is countered by mechanical advance coming in pretty quickly

#3 may in fact be controlled a bit more so as not to rely on the mechanical taking over all the ignition advance chores, after all there is a certain amount of adjustability in the vac can to either bring it in earlier or later, too bad you can't control the amount of the advance itself...lol

Anyways...worth a try, and that's probably going to be my first spring time experiment.

BTW: all of the above tuning could be eliminated by running one of the newer eletronic distributors where you can program the curve, that thought has certainly crossed my mind, but the prices on these units are still steep so in the meantime I'm relying on my Mallory YH Advance distributor that provides a fairly comfortable & quick adjustability (this is the mechanical advance setup that MP went to in later years - Mallory has a nifty spring tuning kit that provides a good mix of advance springs).

So to @carnut68: there are ways to make manifold vac work, but it's not a 'black or white' kind of a thing!

Okay. What you are really talking about is a sort of hybrid system of manifold vac AND a re-curved mechanical advance. I can see where that could be an advantage on some particular applications. But without the re-curve in the mechanical advance, that would still leave the timing drop off with throttle. Which I would think would be noticeable in at least some instances.

Maybe the biggest hurdle in dialing in your 'hybrid' system is in trying to control the amount of advance that the 'can' brings in. There used to be cans with different amounts of advance, but I don't know of many choices for them nowadays. And for a complete burn, light load cruise may want a different amount of advance than the idle wants forcing yet another compromise.

For my part, I set the initial as much as possible to the point that it does hit the starter. Just not enough to hurt the starter or to keep it from starting at all. A big capacity battery allows a few more degrees of initial than lesser ones. With one of the newer smaller starters and a 930cca battery, I can usually get about 24* initial and keep the cruise advance where the engine likes.

Keep us posted on your system, it will be interesting to see how it works out. Come on spring time!

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.