AFR slowly richens at idle

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DrCharles

DrCharles

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I have a new strange problem with my big cam 451. Holley blue pump with regulator. Normal behavior: Idles at about 13.5:1 AFR on the gauge at a fairly steady 1200 rpm, 8" Hg. If the speed ever drops below 1000, the vacuum signal also drops and there is not enough fuel to maintain a decent idle, so it spirals in and dies. The Contour fan load kicking in will drop the idle 100 rpm, which is why I keep it at 1200 (fans off) so it won't fall off the cliff below 1000 when they kick on.

It has now started idling as above for 10 seconds or so, but then starts to actually get richer on the gauge, and by the time it reaches 12.5:1 the idle is slowing down as expected when too-rich, and as before, once it drops enough it dies in 5 more seconds. If I give it some throttle and rev it up to 2000 or 3000 for a few seconds, it clears out and will idle normally again... until the cycle repeats. I can hold it at 2000 and it won't slow down and die. The other night it died for the first time at a stoplight and I couldn't get it to restart for several minutes - also thinking it's flooding.

The wideband AFR is actually measuring O2 in the exhaust - so a misfire (e.g. plugs loading up) will cause the gauge to read falsely lean which it's not doing. That's why I think this has to be in the carb.

I've been racking my brain trying to think of anything other than a slow leak in the needle/seat causing the bowl level to rise. Fuel pressure is a steady 4.5 psi at the carb. I don't see gas pouring out of the boosters when it dies. A ruptured PV diaphragm would do it all the time, not intermittently. Likewise a piece of debris in the IAB's (this is an old 4780-2 Holley 800 DP that I converted to adjustable brass).

Any thoughts? Pulling my hair out!
thanks.
 
Whats your timing / curve like ?
Could it be you are into the mechanical advance at 1000 and if it drops below that you lose timing and vacuum signal to the carb ?

And what is thecam profile ?
 
Too much cam

Drop in your cam Lift, Duration, Duration at .050 ths, and lobe center numbers here so we can see where your cam operating rpm numbers fall into.

Guessing you have a cam that operates in the 2000 to 7000 rpm range.

A cam in the 1000 to 6000 rpm operating range would be a lot easier to get a better idle out of for street driving applications.
 
Cam is [email protected], .652 gross lift. (Slightly reground Mini-Express mushroom). Four-speed, 3.91 gears, 26.7" tires.

The cam is not the problem here - the idle is manageable as I noted in the first paragraph of my post. No plans to replace it, thanks. :p

I will double-check the timing, but I don't see how that could be richening the mixture with decreasing vacuum signal. Also why would it idle for 10 seconds and then start richening without the RPM changing?
It's set up with one very light spring and one heavy spring with loop, so it cranks around 15 degrees for easy starting, idles at 27, and slowly increases with RPM to 35@4000 (there's a little more advance still to come, but I will not stand next to it and rev it any higher while holding a timing light close to the engine) :rolleyes:

The point of my post is that something CHANGED on Friday and I could use some help figuring out what it was that changed.
 
DrCharles said:
I have a new strange problem with my big cam 451. Holley blue pump with regulator. Normal behavior: Idles at about 13.5:1 AFR on the gauge at a fairly steady 1200 rpm, 8" Hg. If the speed ever drops below 1000, the vacuum signal also drops and there is not enough fuel to maintain a decent idle, so it spirals in and dies. The Contour fan load kicking in will drop the idle 100 rpm, which is why I keep it at 1200 (fans off) so it won't fall off the cliff below 1000 when they kick on.

It has now started idling as above for 10 seconds or so, but then starts to actually get richer on the gauge, and by the time it reaches 12.5:1 the idle is slowing down as expected when too-rich, and as before, once it drops enough it dies in 5 more seconds. If I give it some throttle and rev it up to 2000 or 3000 for a few seconds, it clears out and will idle normally again... until the cycle repeats. I can hold it at 2000 and it won't slow down and die. The other night it died for the first time at a stoplight and I couldn't get it to restart for several minutes - also thinking it's flooding.

The wideband AFR is actually measuring O2 in the exhaust - so a misfire (e.g. plugs loading up) will cause the gauge to read falsely lean which it's not doing. That's why I think this has to be in the carb.

I've been racking my brain trying to think of anything other than a slow leak in the needle/seat causing the bowl level to rise. Fuel pressure is a steady 4.5 psi at the carb. I don't see gas pouring out of the boosters when it dies. A ruptured PV diaphragm would do it all the time, not intermittently. Likewise a piece of debris in the IAB's (this is an old 4780-2 Holley 800 DP that I converted to adjustable brass).

Any thoughts? Pulling my hair out!
thanks.
Click to expand...
What are you running for fuel pressure?
 
If it's nothing else - check the carb baseplate to main body gasket, or the screws. I had a Holley that was warped there on the main body. Heat soak made it richer after several minutes. A file and new gasket cured it. But that's an unusual problem.
 
If you or one of your buddies have another carb, swap it on for a test run in your garage. See if it straightens up that idle.

Even a 1406 Edelbrock 600 cfm would work for the diagnostic test.
 
Demonic said:
If it's nothing else - check the carb baseplate to main body gasket, or the screws. I had a Holley that was warped there on the main body. Heat soak made it richer after several minutes. A file and new gasket cured it. But that's an unusual problem.
Click to expand...

Hmmm... the only thing different (other than the bizarre behavior) is the recent addition of an Edelbrock 1/2" wood spacer. But I've had the car out since then and it didn't act up when hot. That spacer is very hard, and I don't tighten the mounting bolts to 100 ft-lb either ;) so I doubt I warped the baseplate...

I don't have enough car friends who'd have a carb I could borrow, but thanks for the suggestion.
 
Sounds like you have an electric fuel pump, here is a test for you.

Fire it up as normal, warm it up a little, now pull the fuse or power lead to the electric fuel pump so it quits pumping with the engine running yet.

Now you will be running with only the fuel left in the fuel bowls, without the added pressure of the pump.

See if this helps straighten out the idle, and see if the AFR stays around 13.5

The engine will shut down when the fuel bowls empty.

This will be a test, if it straightens out then your existing needles and seats sound like they are letting extra fuel by with the electric pump running, adding to your rich idle condition. They could be seeping just a very little bit, instead of sealing off properly shutting off the fuel when the fuel bowls are full.

Also could be your float levels could be set too low, not shutting off the fuel when the fuel bowls are full.

You were saying this is an old Holley.
Ethanol Fuels are hard on carb internal parts too if it has ever been run with ethanol ?
 
George Jets said:
Sounds like you have an electric fuel pump, here is a test for you.<snip>
See if this helps straighten out the idle, and see if the AFR stays around 13.5
Click to expand...
Thanks! I didn't think of that, but I should have ;) I have a toggle switch on the dash so I can turn it off. No need to disconnect anything.

Also could be your float levels could be set too low, not shutting off the fuel when the fuel bowls are full.
Click to expand...
Shouldn't that be "float level set too high"? The fuel will still shut off when the float rises to the set point. (With a running engine, there is just a trickle always coming in that exactly matches the trickle going out) :) Next thing to double-check: fuel level not rising continuously when the engine is off but the pump on!

You were saying this is an old Holley.
Ethanol Fuels are hard on carb internal parts too if it has ever been run with ethanol ?
Click to expand...

I did a complete rebuild this winter, converted to 4-corner idle (Quick Fuel baseplate) and adjustable brass (drilled setscrews). Everything's clean inside. Other than one tankful recently, it's always had ethanol-free pump premium. It's only got 149 miles on it, too.
 
Yes, you are correct if float level is too high that will keep letting the fuel in.
 
DrCharles said:
Cam is [email protected], .652 gross lift. (Slightly reground Mini-Express mushroom). Four-speed, 3.91 gears, 26.7" tires.

The cam is not the problem here - the idle is manageable as I noted in the first paragraph of my post. No plans to replace it, thanks. :p

I will double-check the timing, but I don't see how that could be richening the mixture with decreasing vacuum signal. Also why would it idle for 10 seconds and then start richening without the RPM changing?
It's set up with one very light spring and one heavy spring with loop, so it cranks around 15 degrees for easy starting, idles at 27, and slowly increases with RPM to 35@4000 (there's a little more advance still to come, but I will not stand next to it and rev it any higher while holding a timing light close to the engine) :rolleyes:

The point of my post is that something CHANGED on Friday and I could use some help figuring out what it was that changed.
Click to expand...
Are you running Vac advance off the manifold port? If you are the advance start point is on the edge, or the Vcan could be leaking. [email protected] may need drilled throttle plates to prevent T slot over exposure. Check the PCV if you are using one. Make sure the power valve or plug has enough clearance, blocks to main body may not be sealing.
 
Yes, my t-slots are correct and I have additional bypass air (via the center stud). My ignition timing is not causing the AFR to get slowly richer while idling until it dies. Let's keep focused on what has changed since Friday...

It's also very unlikely that power valve interference to the metering blocks would have suddenly started - that would have been obvious on assembly. Also using a genuine Holley PV which is "supposed" to clear. But an internal gasket leak is a possibility (reusable blue gaskets).

The most likely suspect is a creeping fuel level from a dirty or sticking needle & seat. I'll check that this morning, then start looking for zebras instead of horses :)
 
DrCharles said:
Yes, my t-slots are correct and I have additional bypass air (via the center stud). My ignition timing is not causing the AFR to get slowly richer while idling until it dies. Let's keep focused on what has changed since Friday...

It's also very unlikely that power valve interference to the metering blocks would have suddenly started - that would have been obvious on assembly. Also using a genuine Holley PV which is "supposed" to clear. But an internal gasket leak is a possibility (reusable blue gaskets).

The most likely suspect is a creeping fuel level from a dirty or sticking needle & seat. I'll check that this morning, then start looking for zebras instead of horses :)
Click to expand...

mderoy makes a good point. I’ve seen the power valve hit the main body so many times I can’t count them.

Even had some power valve plugs hit the main body.

I always dry fit the metering block with the PV installed and no gasket. If it doesn’t hit you’re golden. If it does, then you need to clearance them main body or turn down the face of the PV plug in a lathe if that’s what’s hitting. Hell, you can even grind down the PV plug if you had to.

I have noticed that the PV gaskets have been getting thicker. If used with thin metering block gaskets, you can get into trouble.

Just an FYI FWIW
 
yellow rose said:
mderoy makes a good point. I’ve seen the power valve hit the main body so many times I can’t count them
Click to expand...
Thanks for the tips, both of you :) But that wasn't the problem.

It is now fixed. I removed the level plugs and turned the electric pump on. I promptly discovered that the secondary level was too high - but the primary was pouring out of the hole and I know that's not where I set it.
Removed and cleaned both needle/seats. There were a couple of tiny shards of what I think is an old level nut gasket... anyway I reinstalled with three threads showing, and carefully backed the nut counterclockwise on each until I could just get fuel dribbling out the hole. Then I turned them back 1 flat because the engine was cold. Warmed it up and went for a drive - back to normal, idling in the high 13's as usual. No more AFR "creep" or flooding!
:thumbsup:
 
I've looked into it... but I'm cheap :) Maybe I'll just build my own, I have a well-stocked junkbox.

Besides, the load will still be all there once it's fully on - so the idle rpm still has to be higher than I want it when the fan is off. Maybe not quite so high, since as you mention it won't be a sudden full-on (~500W electrical or about 1 hp at the crank, allowing for inefficiencies. The starting surge is probably twice that for a couple of seconds). On the other hand, since it won't always be running at max, it might be easier for the engine to deal with.

I have tried everything in carb tuning (and ignition timing) but this cam just has so little idle torque and vacuum signal to the carb that it has trouble handling an instant 2 hp load from the fan, let alone letting the clutch in below 13-1400. It wasn't designed for low-rpm street use, of course. Oh well, some compromises are necessary to run a BFC on the street!

Another possibility is to install an a/c idle solenoid and power it from the fan circuit :rolleyes: since it's old school anyway. :realcrazy:
 
Dont want to create new thread so I will try to get some help here.
My spec is similar in a way:
470ci 11:1, 4.10 rear, 29” tire, mvb 727, cam 260/[email protected], .700 net lift solid roller with 108lsa. Max wedge port heads, 1050 dominator. Timing is 34 degrees locked. Vacuum at cruise is 12, at idle in gear 6, idle in park around 10. Normally I like to have lean idle and lean cruise, but my setup gives me hard time getting there. Also time of the year is problematic because it’s cold out and cold starts cause fouled plugs so I can’t really use them as mixture indicator. I drive too short distances for plugs to clean up so I need to rely on afr gauge. This cam has around 45 degrees overlap so I assume afr reading is a bit off meaning it shows leaner than it really is? Yet it idles best at 13.2-13.4in Park and 14.0-14.2 in gear at 1250ish in Park and 950ish in gear.
I prefer to have it lean at 15 in gear but then my vacuum drops to 5 max and I need to have almost 1400rpm in park to maintain around 900rpm in gear. I have no problem crusing at around 15afr with these settings.
Should I just leave it as is till I can actually drive the thing of perhaps some air bleeds tuning is an option for me?

Can too tight converter be an issue here? I plan to change it before summer, it stalls around 3300, wanted to go to 3800+

With smaller cam lean idle was not an issue.
 
When you run big cams with low comp you don't actually compress much of a mixture at idle. You actually need to be rich at idle to sustain the load. Your cam has much more than 45 degrees of overlap so you bleed off a lot of cylinder pressure which doesn't help. When you run 34 degrees locked timing you need to have the mixture lean to work as lean mixtures burn slow. Try lowering the timing and richen the mixture and try and get the idle rpm to drop as little as possible when you go from park to drive.
 
Cam has 46 degrees overlap exactly. I can't run less advance, its already a comprimise from 36. As stated in my initial post, the lowest rpm drop is with 13.2-13.4 in park and 14.0-14.2 in gear. I decided to get new converter, something that will be future proof, went with dynamic 9" 4800 stall witch mechanical diode. My old converer was not nitrous ready anyway. I will update whats the situation after I get parts. Since I'm in Europe, its probably going to take some time.
 
I run a 246/250 @ .050 with .650 lift on a 108 in my 408 . Timing is 16* intial and 34 total . Idles at 875-900
 
mbaird said:
I run a 246/250 @ .050 with .650 lift on a 108 in my 408 . Timing is 16* intial and 34 total . Idles at 875-900
Click to expand...
My previous 245/253 .640 cam idled like that no problem with very lean afr (around 15 in gear).
 
For a point of reference;
Back in year 2000, I ran the 292/292/108 cam in my 11.3Scr 367 which is [email protected]; with a 4-speed and 3.55s
With 1.6 arms the lift maths to .542 , and the Timing was;
14* Idle-Timing,
32/34* PowerTiming at 3200rpm (alloy heads)
22* in the Vcan, making CruiseTiming of 48* at 2600rpm
A ran an old 750DP, with quite a bit of bypass air, to get the blades down on the transfer slots. I have no idea what the AFR was, Nor what the vacuum was.
She idled at 700; or less (with less Idle-Timing) if/when I wanted it to.

I have not tuned a cam as big as yours, so IDK how much of what follows will be helpful, but here goes anyway.

At idle, your engine likes a lot of timing. It will like an awful lot of timing. That doesn't mean you should give it to her.
By 2200rpm, you cannot give it enough timing,with the factory D, to cruise with, and still be able to start it, without kickback on the starter.
If you run a lot of idle timing, your idle-rpm will rise up.
In response to that you will idle it down with the speed screw, closing the transfer slot exposure underneath the blades, probably too far. I mean IDK, because the biggest cams I usually tune are the 292 or smaller. So now it has gone lean.
In response to that, you might max out the mixture screws.
This is a bad tune.
Yeah it idles just fine.
But when you put it into gear, there is not enough fuel coming out of the transfers, and/or, the fuel is slow to get moving, and/or, the sudden application of load, slows the air down even more.
So it stalls.
And the exhaust burns your eyes and nose.
So instead, you crank up the idlespeed until it no longer stalls. But now your mixture screws are not doing much of anything, so your engine is at the mercy of whatever is spewing from the transfers. And now it usually bangs hard going into gear.And the exhaust still burns your eyes and nose.
This is a bad tune.

The right tune, is to put the mixture screws in the center of their adjustment range, and set the idle Afr with the speed screw, while simultaneously taking timing out of it, to where the combination of Mixture-screw fuel, plus transfer slot fuel, combined with a retarded timing; lets the engine idle at a reasonable speed that does not bang into gear nor stall.
To achieve this, the engine will want some bypass air.
That 292 cam in my 367 wanted more air than could be supplied by, two of 3/32 holes, one in each primary throttle blade, to make it happen; (no 4-corner idle)
My guess is that your engine will want more TOTAL bypass air.
When you get this right, the engine will no longer stall/ bang going into gear, and it will tic over nicely in gear, and it will not stink up your garage, and bonus is, it will have some tip-in power.

If your 1050 has a 4-corner idle (IDK, I have never see one up close) then I suppose you have already tried everything. But the important take-away here is that;
All your Idle-fuel sources (ports and slots) entering the engine, need to be balanced with
All the air that is entering the engine, and
They all have to get mixed together BEFORE entering the chambers.
And let the IdleTiming be what it will be.
At WOT, the first time your engine cares about timing is at stall-rpm. And the second time it cares is at somewhere around 3400rpm. If you can hit both points, hooray!
But at your power level, in a streeter, I wouldn't be overly concerned with finding ALL the power, cuz more than likely the chassis can't handle it all anyway.
At Part Throttle, and from off-idle to ~3400rpm, The Vcan should be supplying the difference.
I'm just trying to be helpful...

Now; lets talk about your 3300TC
For a streeter this is about the right stall.

Your engine, with a starter gear of 4.10 x2.45=10.04 should pickup nicely.
But when you are blasting thru the gears, that TC is only working for you, just one time, as it gets the car moving. By 3300/.59=5600 on the tach, your engine will never fall below stall again. (.59 being 1.45/2.45, your 1-2 shift split).
On the other hand, a 3800 will hit the same split at 3800/.59=6440. So if you short shift at say 5600 the theoretical drop-in rpm is to 3300, but the 3800 cannot fall that far. This is good, but it happens at 25mph at WOT, with 4.10s, and 27s; and your tires should already be on fire,lol.
If your tires are not on fire at WOT, at Warsaws elevation of 330ft; something is wrong.
 
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