Rich 3000 rpm cruise

[PRNDL]

Rich 3000 RPM cruise at 70 mph at 20 hg of vacuum with my Holley 750 Street HP 82750SA vacuum secondary. 13 AFR. Everything else is in range and clean.

This is a 4150 with 4 corner idle. Downleg boosters.

This per my AEM UEGO AFR gauge.



The carb is set up as follows:

Front curb idle at 3/4 turn.
Transition at .030
Secondary curb idle at 1/8.
No transition showing

All 4 idle mixture screws at 3/4 turns. Very responsive and will stall the engine if one is turned in.

Due to my cam, idle AFR in neutral at 950 rpm is 13.5 AFR at 13hg vacuum. In drive about 13.8 AFR at 11 hg vacuum. It’s very happy there. It gives me the highest vac and RPM readings along with the strongest gear engagement. I’m happy with the idle.

All 4 IFR’s are .029 and all 4 IAB’s are .071. These are like this from the factory.

I’m very happy with my off idle and transition. Anything under 2500 rpm and under 50 MPH, is super clean and crisp. 14.5 to 15 AFR. Car feels super crisp and strong. Vacuum in this range is about 17 hg vacuum.


The problem starts as I near 3000 rpm in third (TH350, 3000 stall, no overdrive), 3.73 gears, 28 inch tires. About 50 to 70 mph, my AFR’s shoot to 13 AFR at 20 HG vacuum. If I let off the gas or give it more gas, it leans out. Cruising on the highway, I’m at 13 AFR. 70 mph, 3000 rpm, 20 hg.

This carb came with 77 primary jets and 36 HSAB. Transition was great but cruise on mains was 12.5 at 3000 rpm then leaned out hard as rpm’s increased to where I got a stumble. Even with 77 jets. And that’s with a 10.5 PV.

I slowly dropped the primary HSAB’s down to .025to flatten the curve, then jetted to 73. This gives me the best compromise. However, I’m rich on the low end of the main circuit, the l lean out to almost 16.5. I have to run a 10.5 PV and had to drill out my PVCR’s to .067 just to make it all work.

My secondaries are spot on. 84 jet with .036 secondary HSAB. Purple spring and adjustable servo top to 1.5 turns. That’s all factory.

Now, if I go up on my primary HSAB, say from .025 to just .028, I lean out on the top end of my primary main circuit and get a stumble just before my secondaries kick in. Plus a lean AFR spike. The same happens if I jet down from 73 to 72 or less.

Wot is great at 12.5.

Not sure what to do. I almost feel this primary metering block is defective. Like it has an air leak. It acts like I’m running massive primary HSAB.

I have tried all kinds of combinations with the primary idle T-slot positions vs idle mixture screws and IAB’s.

Not sure what to do. It runs great but my plugs are black and I burn through fuel a lot.


Any ideas? I was thinking of running a plain Jane old school 750 3310 primary block.

383 stroker with 10.5 compression. AFR 195 heads. Comp Cams xr282hr 230/236@50 110 lsa 550 lift with 1.6 rockers. Edelbrock performer rpm air gap and 1 inch open spacer.

Any ideas of fixing that rich highway cruise?

[1980RS]

I have one of those 750 on a street car, I have 68's in the front with 25 high speed bleeds. Cruised around great, has a slight flat spot at 1st but the Thomason Power plate fixed the problem.

[PRNDL]

One more thing. This carb has regular Holley metering blocks. Only 2 .028 emulsions with IFR’s on the bottom.


Not messing with too many emulsions or high up IFR’s fancy metering blocks anymore.

[PRNDL]

Yeah, I have the new version of that carb. The 82750SA. Mine came with 77 front jets and all 4 HSAB’s at .036.

The old version, the 82750, came with 75 jets and .025 primary HSAB. I copied the old one set up and it worked far better. Still rich at 3000 but not as bad.

[PRH]

Sounds like you’ve tried a fair amount of stuff.

My suggestion at this point would be some transfer slot restrictors on the primary side.

[PRNDL]

I was leaning in that direction.

Will it affect my transition cruise? It’s dead on right now.

What size should I start with?

Could transition a main jet be overlapping?

[PRH]

My thought is, at 20” of vacuum, the TS is def a player.

Might some other circuits need adjustment after restricting the TS?

I’d say that’s highly probable.

I’d make the TS restriction easily adjustable, and start experimenting.

[PRNDL]

Is that an 8-32 tap? Can anyone advise the size I need? What size TSR to start?

[PRNDL]

Can we revisit the way the primary jet / circuit curves from rich to lean? Maybe a defective metering block?

Check this out.
In its factory form, 77 p-jets, 84 s-jets, and .036 HSAB’s all the way around. Plus only two .028 emulsion bleeds, full throttle was 14.5. Cruise at 3000 rpm was 12.5. Made no sense. As I gave it more gas, it would shoot up to like 17 AFR on light highway acceleration. Like to pass someone. It made no sense. Rich cruise and lean WOT. I was confused. Jet up or down?

So I focused on cruise and jetted down to 75 p-jet. Highway acceleration and wot at that point were not possible. That only brought cruise AFR to 13.

Slowly brought the .036 p-hsab down to .025. Fixed my crazy leaning curve, a lot, but cruise got more rich. Dropped down to 73 jet and found the most tolerable balance. 3000 rpm cruise was 13.5 but still went to like 16 aft on hard (non wot) acceleration. Put in a 10.5 PV abd drilled the PVCR’s from .063 to .067 and ended up where I am now.

13.5 3000 rpm cruise
13.5 primary only acceleration.
12.5 wot

If I accelerate lightly on the highway, say to pass soneone, at like 4000 rpm, it goes up to about 15 AFR. No stumbles.

[BillK]

Could it be something other than the carburetor ? What distributor are you running ?

[Tuner]

I would use 10-32 for T-slot jet. You may need a larger orifice than a 8-32 can provide. If you use 8-32 you can always go to 10-32 later.

Calculate T-slot L x W = Area and start with about 10% less in the jet than the slot.

Example: if slot is .025" x .200 = .00500" Area equivalent to near .080" diameter x .9 = .004500 equivalent to near .076" diameter. Most old school OEM Holleys that had a T-slot restrictor were in the .070" to .078" range.

Keep in mind that light load at high vacuum like 20" more EGR is occurring because the high vacuum is sucking "reversion" back into the intake, so a richer A/F than is ideal at higher load may be required to avoid misfire. The higher percentage of EGR is the same reason rich A/F is required for idle at low RPM.

[PRNDL]

Progression programmable ignition with MSD box. The TDC pointer has been confirmed three different times. 36 degrees all in at 3000 rpm. 12 degrees of vacuum via full manifold vacuum. 24 degrees at idle. 36 with vacuum. It loves that much advance at idle.

[PRNDL]

I tell myself to leave it alone because it runs great and hauls ass, but I blacken plugs bad.

[PRNDL]

Something else. If I cruise at 80 mph, just 10 mph over, I’m at like 14.5. If I slow down to about 50 mph, I’m at 14.5. It’s that 70 mph 3000 rpm 20hg that drives me crazy!

[steve cowan]

Rich 3000 RPM cruise at 70 mph at 20 hg of vacuum with my Holley 750 Street HP 82750SA vacuum secondary. 13 AFR. Everything else is in range and clean.

This is a 4150 with 4 corner idle. Downleg boosters.

This per my AEM UEGO AFR gauge.



The carb is set up as follows:

Front curb idle at 3/4 turn.
Transition at .030
Secondary curb idle at 1/8.
No transition showing

All 4 idle mixture screws at 3/4 turns. Very responsive and will stall the engine if one is turned in.

Due to my cam, idle AFR in neutral at 950 rpm is 13.5 AFR at 13hg vacuum. In drive about 13.8 AFR at 11 hg vacuum. It’s very happy there. It gives me the highest vac and RPM readings along with the strongest gear engagement. I’m happy with the idle.

All 4 IFR’s are .029 and all 4 IAB’s are .071. These are like this from the factory.

I’m very happy with my off idle and transition. Anything under 2500 rpm and under 50 MPH, is super clean and crisp. 14.5 to 15 AFR. Car feels super crisp and strong. Vacuum in this range is about 17 hg vacuum.


The problem starts as I near 3000 rpm in third (TH350, 3000 stall, no overdrive), 3.73 gears, 28 inch tires. About 50 to 70 mph, my AFR’s shoot to 13 AFR at 20 HG vacuum. If I let off the gas or give it more gas, it leans out. Cruising on the highway, I’m at 13 AFR. 70 mph, 3000 rpm, 20 hg.

This carb came with 77 primary jets and 36 HSAB. Transition was great but cruise on mains was 12.5 at 3000 rpm then leaned out hard as rpm’s increased to where I got a stumble. Even with 77 jets. And that’s with a 10.5 PV.

I slowly dropped the primary HSAB’s down to .025to flatten the curve, then jetted to 73. This gives me the best compromise. However, I’m rich on the low end of the main circuit, the l lean out to almost 16.5. I have to run a 10.5 PV and had to drill out my PVCR’s to .067 just to make it all work.

My secondaries are spot on. 84 jet with .036 secondary HSAB. Purple spring and adjustable servo top to 1.5 turns. That’s all factory.

Now, if I go up on my primary HSAB, say from .025 to just .028, I lean out on the top end of my primary main circuit and get a stumble just before my secondaries kick in. Plus a lean AFR spike. The same happens if I jet down from 73 to 72 or less.

Wot is great at 12.5.

Not sure what to do. I almost feel this primary metering block is defective. Like it has an air leak. It acts like I’m running massive primary HSAB.

I have tried all kinds of combinations with the primary idle T-slot positions vs idle mixture screws and IAB’s.

Not sure what to do. It runs great but my plugs are black and I burn through fuel a lot.


Any ideas? I was thinking of running a plain Jane old school 750 3310 primary block.

383 stroker with 10.5 compression. AFR 195 heads. Comp Cams xr282hr 230/236@50 110 lsa 550 lift with 1.6 rockers. Edelbrock performer rpm air gap and 1 inch open spacer.

Any ideas of fixing that rich highway cruise?

Have you tried setting TFS to 0.020" front and rear?
I find it a bit odd it needs a 25" HSAB,this is usually when you have to much emulsion in the blocks.then going the other way (36) HSAB the mains will come in earlier and probably go lean up high due to to much air.
28 - 30 is what I usually see in HSAB.
I personally don't like vac sec carbs as we really don't know what's going on compared to mech secondary carb but that's my opinion.
When I do the transfer slot jet I use a 6/32" set screw and run for the street 0.060" - 0.068"

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