[Formula jg]

I’ll post all the details this weekend, keep in mind it won’t be apples to apples comparison because I have mechanical secondaries vs your vac. secondaries. In the meantime you may want to provide us more information on your carb, eng., trans, etc.

I should mention I did upgrade my HEI (after throwing a bunch of new parts at it while chasing the stab and stumble issue) with an aftermarket blue tooth HEI with fully adjustable timing curve, someone of the best money ever spent. Will have to check if the stumble was resolved before or after the new HEI install.

[ta man]

An AFR spike will always show for a split second be it from off idle to full throttle or from cruise to full throttle. What you want to see is instant recovery of the AFR after the quick AFR spike.
It is interesting how many carb issues show up in non Holley carbs or modified carbs.

[Formula jg]

Dido ta man, thats exactly what shows up on my data log chart a momentary lean spike, instant AFR recovery with no sign of any stumble or hesitation.

[Brentco]

Originally Posted by ta man

An AFR spike will always show for a split second be it from off idle to full throttle or from cruise to full throttle. What you want to see is instant recovery of the AFR after the quick AFR spike.

Yeah I’ve found that as well. But I’m not concerned with split-second spikes that just show up on the graph. I’m talking about spikes over 16 AFR that last at least 1 second or longer, and sometimes never recover during the whole period of acceleration. When it spikes up to approx 17-20 AFR you can feel it hitch or stumble as well.

I’m able to tune around most of the issues. My biggest hang up right now is stabbing the pedal to full throttle (or sometimes 1/2 throttle). The AFR will spike up to 19-20 and the car will just hang. I usually have to back off the pedal at that point, and the car will pick up and go. Anything less than a rapid stab — ie, if if I roll into full throttle just a bit — it roars through the full rpm just fine.

This seems like the area that should be covered by the accelerator shot. But I have maxed out the pump shot (up to 50cc with .047 shooter and big yellow cam) and it doesn’t overcome the lean spike. You can see the AFR dive initially for a split second but then it shoots up lean.

Here are some examples from logged runs.

Interestingly, my Edelbrock 800 AVS2 never had lean spikes at initial throttle opening, on the graph or in the seat (not saying it’s a better carb, just an interesting note).

[Brentco]

Originally Posted by Formula jg

I’ll post all the details this weekend, keep in mind it won’t be apples to apples comparison because I have mechanical secondaries vs your vac. secondaries. In the meantime you may want to provide us more information on your carb, eng., trans, etc.

I should mention I did upgrade my HEI (after throwing a bunch of new parts at it while chasing the stab and stumble issue) with an aftermarket blue tooth HEI with fully adjustable timing curve, someone of the best money ever spent. Will have to check if the stumble was resolved before or after the new HEI install.

Much appreciated! Yeah I’m not expecting to copy your configuration, it just always helps to have reference points from similar builds.

Your bluetooth HEI sounds cool. I sometimes wonder why we pay $500+ bucks for MSD distributors that give you just a handful of advance curve options with springs and weights, when this could easily be handled electronically with 100% adjustability. But then again I also just recently . . . . paid $500+ bucks for an MSD distributor that gives you just a handful of advance curve options with springs and weights.

Here is my current build:

1976 468 CID; 10:5 CR
Forged 4.25 crank; ICON forged flat-top pistons 4.3cc relief
HR 230/236 @ .050; 282/288 adv.; ground on 112 LSA
.561/.573 lift w/ HS 1.65 rockers
87cc Round Port E-Heads
Performer RPM
850 Speed Demon VS
MSD ignition system
Doug’s Headers
TH350
3,000 stall PTC converter
3.73 axle ratio
Pypes exhaust w/ X crossover

[AG]

Originally Posted by Brentco

Yeah I’ve found that as well. But I’m not concerned with split-second spikes that just show up on the graph. I’m talking about spikes over 16 AFR that last at least 1 second or longer, and sometimes never recover during the whole period of acceleration. When it spikes up to approx 17-20 AFR you can feel it hitch or stumble as well.

I’m able to tune around most of the issues. My biggest hang up right now is stabbing the pedal to full throttle (or sometimes 1/2 throttle). The AFR will spike up to 19-20 and the car will just hang. I usually have to back off the pedal at that point, and the car will pick up and go. Anything less than a rapid stab — ie, if if I roll into full throttle just a bit — it roars through the full rpm just fine.

This seems like the area that should be covered by the accelerator shot. But I have maxed out the pump shot (up to 50cc with .047 shooter and big yellow cam) and it doesn’t overcome the lean spike. You can see the AFR dive initially for a split second but then it shoots up lean.

Here are some examples from logged runs.

Interestingly, my Edelbrock 800 AVS2 never had lean spikes at initial throttle opening, on the graph or in the seat (not saying it’s a better carb, just an interesting note).

That seems like a lot of pump shot with the 50 cc pump and 47 shooter. I have 37 shooters on my race motor with a Victor/Dominator and don't get any stumble. Maybe try a smaller shooter to extend the duration of the shot.

[Formula jg]

Brentco, any carb spacer on that RPM intake?

Also, if you have timing specs please also provide that info.

[Brentco]

Originally Posted by AG

That seems like a lot of pump shot with the 50 cc pump and 47 shooter. I have 37 shooters on my race motor with a Victor/Dominator and don't get any stumble. Maybe try a smaller shooter to extend the duration of the shot.

I agree it’s a big shot. To get into more detail, after installing the 50cc pump I started with the 47 nozzle, and it made a richer initial AFR dive than necessary — sometimes down to the 10-11 range. So I worked back up to a smaller nozzle to extend the bigger shot out over a longer period. There is a 37 nozzle on there now, which (usually) only drops the AFR to around the 12 range during that initial rich dive. So going any smaller on the nozzle would not give enough shot up front.

[Schurkey]

Consider testing with a CO and HC meter instead of, or in addition to an O2 sensor. Perhaps a full "4- or 5-gas" analyzer.

CO is an enormously more helpful guide to rich/lean than measuring O2. HC plus high O2 is a characteristic of misfire.

[johnta1]

So I worked back up to a smaller nozzle to extend the bigger shot out over a longer period.

Try a different accel. cam. Needs to have a longer ramp so it will spread the shot out until the RPM's of the engine can catch up to the squirter. (or change position of the cam)

[Formulajones]

Since it wasn't mentioned, keep in mind when getting into squirters that big you really need the hollow screw. The channel through the body is only so big, so when the squirter reaches a certain size, around .035 or so, it's really not accomplishing much to go any bigger unless the hollow screw is installed.

I find pump cams affecting intensity and duration to have as much or more affect than just squirter changes alone. Personally I haven't had an engine I couldn't tune with just using the pink squirter in the #1 position on various style intake manifolds.

Just as big of a player is initial timing, how much you have, and how fast it ramps in off idle. Vacuum advance is another big player.

Just some ideas if not already mentioned.

[Brentco]

Originally Posted by johnta1

Try a different accel. cam. Needs to have a longer ramp so it will spread the shot out until the RPM's of the engine can catch up to the squirter. (or change position of the cam)

Oh, I’ve tried them all Only the brown and the yellow cams give you the full capacity of the 50cc pump shot, at least as I understand it. But I have tried a variety of cams, ramp profiles, and hole positions, and none fared any better.

It seems like what I’m seeing is the pump shot not being able to keep up with the RPMs of the engine, rather than the other way around, no? If the RPMs were not able to keep up with the pump shot, it would cause a rich condition right?

[Formulajones]

The pump shot is only there to cover up the hole you make when the throttle blades open. After that they have nothing to do with the fuel curve once that shot burns off.

So if I'm reading your statement correctly, and you're having a lean condition as RPM rises, after your initial pump shot, then you probably need to look at your transition circuits.

[Brentco]

Thanks Formulajones, all good points. I do have the hollow screw. For timing, right now I have 17 initial, 21 mechanical, and about 9 vacuum on manifold. Advance starts around 1100-1200 and all in by 3000. Have tried different initial settings and ported vacuum (although not extensively).

Originally Posted by Formulajones

Personally I haven't had an engine I couldn't tune with just using the pink squirter in the #1 position on various style intake manifolds.

It does seem like it’s more pump shot than it should need. I’m very much open to ideas about what might be going on.

And before anyone says it, I have squared the transfer slots, etc. Carb is very sensitive to throttle blade position. Right now primary t-slot is square at .020” and secondary blade is at the very bottom of the slot with barely a sliver showing. I have tried all configurations and that seems to work best.

[Formulajones]

Where are you located Brentco?

[74Grandville]

You’ll probably eventually get rid of the lean spot by dumping tons of fuel, but a better way to go is find a vacuum secondary carb when running on the street. Just my experience/2 cents.


Sent from my iPhone using Tapatalk

[Brentco]

Originally Posted by Formulajones

The pump shot is only there to cover up the hole you make when the throttle blades open. After that they have nothing to do with the fuel curve once that shot burns off.

So if I'm reading your statement correctly, and you're having a lean condition as RPM rises, after your initial pump shot, then you probably need to look at your transition circuits.

The lean spike seems to be right in that hole that’s supposed to be covered by the pump shot. You can see here the pump shot only gets me .25 to maybe .5 seconds of enrichment right after stabbing the pedal to WOT but then the increasing engine RPMs eat it up quickly and spike lean. This is with a 50cc shot through a 37 nozzle, which will discharge for at least a full second, so it doesn’t seem to be an issue of the shot running out early.

You are 100% correct though that the transfer slot seems to be a major player. It’s strange how much of an effect that has on WOT, but that certainly seems to be the case. I’ve tried different IFR and IAB sizes, and have gotten to a point where now the lean spike only happens with a very quick stab to WOT. But I’m fighting the balance between that and an overly rich cruise. I’ve gone down from the .035 stock IFR to a .031 (in the lower position), and now I’m back up to a .033. The .033 basically eliminated the lean spike everywhere but stab to WOT. Now, however, my freeway cruise AFR is back down to high 11s. I suppose I could live with that if it got my WOT to the right place, but I can’t really live with both a rich cruise and a bog launching to WOT.

[Brentco]

Originally Posted by Formulajones

Where are you located Brentco?

SoCal — Orange County.

[Brentco]

Originally Posted by 74Grandville

but a better way to go is find a vacuum secondary carb when running on the street. Just my experience/2 cents.

This was kind of buried in my description, but it is a vacuum secondary carb (and that’s one of the reasons I got a vac sec in the first place, supposed to be more tunable for the street).

[Brentco]

Originally Posted by Formula jg

Brentco, any carb spacer on that RPM intake?

Also, if you have timing specs please also provide that info.

No spacer — just a plate, maybe 1/8” thick, to provide clearance for the 50cc pump arm.