Never would have guessed

[BSB67]

Just a story I thought I'd share.

So I've started playing a little with the GTX I picked up recently. Its got the MP 484 cam, M1 single plane intake and a Holley 950 (80496). Starts okay, idles fine once warmed-up. Drives pretty well. No bog or stumble, no off-idle or light throttle sag/hesitation. No popping. Doesn't stink. Honestly, nothing to complain about at all, nothing short term to fix based on the driving experience. So carb tuning drops a few levels on my priority list.

Before my last drive I took the air cleaner off just to look around. I found it odd that all four throttle blades had holes drilled in them. I've had a lot of different carbs on motors with cams this size or larger and never felt that even two hole were needed, let alone four. Okay. Next, I crack the throttle open. Nothing, nothing from the primary squirter. What?? How does that work? Pump cam installed backwards. Check the idle mixture screws next. Driver side primary and passenger side seconary - 1/2 turn out. The other two, 2 turns out.

I keep wondering what path exactly did this carb take to get from the out-of-the-box settings/condition to this.

The 80496 is pig fat in the IFR/transition, but still hard to imagine that it could run this well without a squirter. Am I off track thinking this is crazy sh**?

Comments/thoughts welcome.

 

[pnora]

The throttle plate holes are normal on that model. Might be to much carb for your engine. Big carb and secondary squirters most likely helped it start and run somewhat decent. No doubt from the box to someones hands that did not get it right.

 

[RR Fan Dan]

The throttle plate holes are normal on that model. Might be to much carb for your engine. Big carb and secondary squirters most likely helped it start and run somewhat decent. No doubt from the box to someones hands that did not get it right.

That’s the first thing I thought when I read 950cfm carb. That’s a lot of carb for most engine builds.

 

[RemCharger]

That’s the first thing I thought when I read 950cfm carb. That’s a lot of carb for most engine builds.

Have to remember it's only 950cfm at 6000 rpm WOT. It should be able to run well with enough vacuum from any combo. We run a 930 on a 383 for bracket racing but it starts and runs beautifully.

But ya the previous carb tuner on that 440 just found the sweet spot where " it seems to work"

 

[hunt2elk]

I bet it won't take you long to get it running even better than it is now....

 

[RemCharger]

I bet it won't take you long to get it running even better than it is now....

I assumed that was already done

 

[Geoff 2]

Is the pri & sec shaft ratio 1:1?
If so, activating the throttle will give a fuel squirt from the sec squirter.

 

[BSB67]

The throttle plate holes are normal on that model. Might be to much carb for your engine. Big carb and secondary squirters most likely helped it start and run somewhat decent. No doubt from the box to someones hands that did not get it right.

Yep. I think your right on the throttle plate holes. I've got another "950" from Biggs with the 80496 body, but different base plate w/o plate holes.

This "950" has the 1.375" venturi, which is the same size as the Holley 750 (4779) but throttle bores (1.75") of the 850. This small venturi 950 has an actual flow of 830-840 cfm.

Yes, pedal to the floor a couple times to get it to start., Now it's clears as to why.

 

[BSB67]

But ya the previous carb tuner on that 440 just found the sweet spot where " it seems to work"

Initially, in trying to keep an open mind, my thought was that maybe this guy was a genius. But based on the rest of the car, more likely as you say.

 

[BSB67]

I bet it won't take you long to get it running even better than it is now....

I assumed that was already done

Thanks guys,

Well, I did re-install the pump cam correctly, just because. But this never-ending brake job is slowing me down. Ugh.

I have a new AEM O2 gauge sitting on the bench. Need the bungs installed. Thinking about buying the BLP metering blocks. The fuel needed for break away and for tip in without the primary squirter needs to be coming from somewhere. Probably take it apart and measure the IFRs and IABs before I go to far.

 

[BSB67]

Is the pri & sec shaft ratio 1:1?
If so, activating the throttle will give a fuel squirt from the sec squirter.

It is not one to one. But the linkage has been modified. For what purpose, it is not yet clear. But might be playing a role in this.

 

[BSB67]

Okay, back at it.

Had O2 bungs welded in. Installed the AEM meter and went for a drive. Just as suspected - Very fat in all cruise modes, from off idle to 70 mph. I guesed that it would be in the low 11. Wrong, low-mid 10s. Idle was okay at the one turn out that I had set earlier. WOT was good for now too - high 12s.

Got some fully adjustable metering blocks. Took apart the 80496. Nothing inside was altered by the previous owner. Factory set up:

Primary jet 79
Secondary jet 79
MABs (P&S) .032
IABs (P&S) .074
IFR (P&S) .039 (I think the Factory spec is 0.040)
PVCR (P&S) .052
Idle mixture screws 1 turn out

Changed to:

Primary jet 74
Secondary jet 85
MABs (P&S) .032
IABs (P&S) .074
IFR (P&S) .033
PVCR (P only) .067
Block Rear PV
Idle mixture screws 1 1/2 turns out

Basically dropped the IFR and primary jets a bunch.

Test drive - Car ran noticably better. Particularly at high speed cruise. Idle A/F at mid to high 13s. Low speed cruise 1700 - 2200 (transition) in the high 11s. High speed cruise was in the high 11s and low 12s. Part throttle acceleration in high gear (Primary Jet plus PVCR) was high 12s. Basically everything in the cruise is still on the rich side.

Up next. Drop the primary jet to 72, open the PVCR to .073. Drop the IFR to .030.

Also, the holes in all 4 throttle plates might be to much for this cam. Idle at 1000 rpm, the throttle blade are almost completely closed w/ almost no transfer slot exposed. Maybe just holes in the secondary plates.

I do believe that this carb is a really good carb for a high perfomance based 440. Its the best of both the 750 and 850 Holley DPs, IMO. I just don't know what Holley was tuning it for when they set it up.