Secondary fuel starvation on Holley 770

[8literbeater]

I have a stroked 489 Chevy that I built back in 03-04, in my 59 pickup. I used a Holley 770 truck avenger carb, and had the engine tuned on a dyno before installing it. We got 490 hp at 5200, and 560 pound feet of torque, I think at 4100. I'd have to check the sheet. That really doesn't matter that much though.

After all this time, and about 9000 miles, I pulled the heads, and upgraded from the hydraulic flat lifter cam to a hydraulic roller cam. The specs are similar, with just a few more degrees duration, a little more lift, and a little more area under the curve, since it's a roller cam.

The carb is now going extremely lean when the secondaries open. I think I've tried everything, so this is almost a last resort.

Here's the info:
With the old cam, from the dyno tune, I had 69 primary jets, 89 secondary jets, and a 6.5 power valve.

I dropped the compression just a touch with this cam from 10 down to 9.94. I always had problems with the higher compression when the air temp out here gets over 110 degrees.

At about 800 rpm idle, I get 15 inches of vacuum. At 65 mph cruise, it's about 12.

Initial timing is at 15 degrees, no vacuum advance. I've never run one on this.
Max timing is 36 degrees.

The secondaries start opening just over 2000 rpm. Between 2100 and 2400.

Fuel pressure always runs right at 4 psi.

I run ngk plugs, heat range 7.

When the secondaries open, acceleration stops, and the pinging/detonation is scary.

Here's what I've done:
Checked and tested the power valve.
Replaced the fuel filter. I use a large spin-on filter with a water separator drain cock.
Replaced the fuel pump. The old pump that I've always run was maybe just a little low on pressure. Maybe my gauge is off. The new one holds pressure when the secondaries open though.
I've jetted up from 89 to 91 to 95 to 99. No improvement.
Tested everything for vacuum leaks, including the intake gaskets.
Pulled the metering blocks and sprayed out everything there is to check. The thing is as clean as a whistle.
Hooked up a narrow band O2 sensor to my multimeter so I can see the fuel ratio while driving. It's normally always at 800 millivolts, until the secondaries open and it goes to zero.

Maybe some other things that I'm forgetting.*

Why is it perpetually going lean?

 

[Old Iron]

Do you have steel or aluminum heads?
A big block with 9.94 requires 91 or higher octane with aluminum or steel heads.
Pinging/detonation doesn't mean the carb is going lean.
4 psi is on the low side. I like 6 psi
Holley 770 calls for
74 primary jets
99 secondary jets
2.5 on the power valve
Ultimately you need to read the color of the spark plugs to get the true jetting down.
Put the vacuum gauge on, start the engine, kick it wide open (don't hold it wide open [ddd) while looking at the gauge and install a power valve 1 point higher than the lowest vacuum you see.
Have you checked the the level of the floats?

 

[8literbeater]

Ah yes, the floats. I've checked and triple checked them.

They're iron heads, and I've always run only 91 octane.

The carb as it came out of the box was horribly bad on the dyno. Just adjusting the carb and timing gained 125 horsepower. Most of that was in the carb. I do remember that it originally had 99s on the secondaries.

The O2 sensor is telling me that it's going way lean right when the noise starts. I can't even run it like that long enough to get a look at the plugs. It sounds like it's going to come apart.

I agree on the fuel pressure. I'm thinking I might need a better power wire to help gets more volts to the pump.

Also the quench distance is right at 59-60 thousandths.

What I don't get, is that it ran great for all this time. A minor cam change and 0.07 drop in compression shouldn't completely screw it up I wouldn't think.

I'll try bumping the primaries up to 74 to see if it improves.

I'll also see about getting more juice to the fuel pump to see if I can get it over 5 psi. I should probably suspect my gauge too. It's a cheapo. The pump has a 72 gph rating.

If the 6.5 power valve is wrong, wouldn't it just introduce fuel earlier than a 2.5? I thought about that. The power valve might be compensating for the smaller primary jets.

 

[Old Iron]

I figured you had all the bases covered.
Have you tried another carb? Like an 850 or 1050 cfm
Just thought of this, I had an MSD 6AL box that would advance the timing to over 50 degrees, on its own and only on two cylinders. I thought the engine was coming apart every time it did this.
Took me a while to figure that one out.

 

[studebaker59]

Did you upgrade the valve springs to match the cam?

 

[BlueNorther]

How do you know the secondary side is opening that soon. 2100 is way too low of an opening point. did your carb guru remove the black spring for a lighter one?

also this carb needs at least 7psi to provide the fuel needed.

I've run a similar sized Holley vac secondary carb on an engine in the HP level you describe and we used 84 jets all around and the power valve fuel restrictions were drilled out from .040" to .060" primary and secondary side. both power valves were 2.5 hg.

since the pinging stuff only happens when the secondaries start to open, there might be a restriction in the secondary fuel inlet tube or the needle and seat.

all the pictures of this carb I've seen indicate it is a single feed carb, I would find a good set of dual feed bowls and properly plumb in dual fuel lines and use at least .110" needle and seats.

 

[8literbeater]

I figured you had all the bases covered.
Have you tried another carb? Like an 850 or 1050 cfm
Just thought of this, I had an MSD 6AL box that would advance the timing to over 50 degrees, on its own and only on two cylinders. I thought the engine was coming apart every time it did this.
Took me a while to figure that one out.

I only have a 625 Road Demon as an alternative. There's a guy selling a Barry Grant Silver Claw 850 locally on Craigslist for $300. I'd like to think that this one would still work, but I'm sure an 850 would work better at this point.

Did you upgrade the valve springs to match the cam?

Yes sir. I used a Howards cam, and the lifters and springs that Howards recommended when I called them.

How do you know the secondary side is opening that soon. 2100 is way too low of an opening point. did your carb guru remove the black spring for a lighter one?

also this carb needs at least 7psi to provide the fuel needed.

I've run a similar sized Holley vac secondary carb on an engine in the HP level you describe and we used 84 jets all around and the power valve fuel restrictions were drilled out from .040" to .060" primary and secondary side. both power valves were 2.5 hg.

since the pinging stuff only happens when the secondaries start to open, there might be a restriction in the secondary fuel inlet tube or the needle and seat.

all the pictures of this carb I've seen indicate it is a single feed carb, I would find a good set of dual feed bowls and properly plumb in dual fuel lines and use at least .110" needle and seats.

I can't be totally certain that the secondaries are opening, except that if I go to a lesser throttle angle, the power comes back. I know the spring got changed, but I can't remember what it has in it. It may not be optimal yet, but it shouldn't affect the mixture. Holley says 5-7 psi, and I agree. I'm going to get a better quality gauge today, so I can know for sure what's going on. My only other gauges are fuel injection gauges.

I also suspect that the primaries and power valve could be ill-matched, but regardless, the primaries are now running a bit rich at around 12 to 1 according to my rudimentary narrow band O2 sensor. The 2.5 power valve would probably help with that. It only has one power valve.

I agree that a dual feed setup would be preferred, but they run almost $300 for just two fuel bowls. That's insane. I have a friend that is going to see if he has a carb that I can get the bowls off of for free.

I may be a little too optimistic in thinking that Holley would build a 770 cfm carb that will flow the proper amount of fuel without modification.
Maybe the 770 just simply is not big enough anymore, unless I do something like add the dual feed and bigger seats.


I went back to 74 primaries, and finally got an actual reading on the meter when the secondaries opened. Still around 18 to 1, but it was off the chart before.
I then tried 79 primaries, and saw a negligible difference. Of course now the primaries are rich.

I'm going to get a better quality gauge, and pull the rear needle and seat to check for blockage.

Gotta run for now. Thanks for the advice, guys.

 

[BlueNorther]

check out swap meets and look for old 3310's they have the dual feed bowls and rear jet block with a power valve hole, and I bought 3 of them not so long ago for $50. a piece.
look for ones with damaged throttle bodies, they usually sell cheap.

the way you describe your problem, it's all in the secondary side, 1st thing put back the black spring in the vac chamber, and take a small circle of cardboard, like from a oil filter box, cut a slit in the middle of it and slide it on the rod coming out of the chamber.

when testing the car, push the cardboard piece up against the bottom of the chamber, then drive it, stomp on the gas and when the secondary opens the cardboard will slide down the rod the amount of the rod travels and stick there till you pull over and take look at it. you will have an accurate measurement of the travel.

on a vac secondary carb you should never know when the secondaries open, if you can feel when it happens, the spring is too light, it is basically causing a bog in acceleration curve.

 

[Dr Crankenstein]

Just curious... did you change gaskets? Wrong gasket on the secondary metering block?

.

 

[8literbeater]

Just thought of this, I had an MSD 6AL box that would advance the timing to over 50 degrees, on its own and only on two cylinders. I thought the engine was coming apart every time it did this.
Took me a while to figure that one out.

How do you even find that?

check out swap meets and look for old 3310's they have the dual feed bowls and rear jet block with a power valve hole, and I bought 3 of them not so long ago for $50. a piece.
look for ones with damaged throttle bodies, they usually sell cheap.

the way you describe your problem, it's all in the secondary side, 1st thing put back the black spring in the vac chamber, and take a small circle of cardboard, like from a oil filter box, cut a slit in the middle of it and slide it on the rod coming out of the chamber.

when testing the car, push the cardboard piece up against the bottom of the chamber, then drive it, stomp on the gas and when the secondary opens the cardboard will slide down the rod the amount of the rod travels and stick there till you pull over and take look at it. you will have an accurate measurement of the travel.

on a vac secondary carb you should never know when the secondaries open, if you can feel when it happens, the spring is too light, it is basically causing a bog in acceleration curve.

Good ideas. I'll work on it. In the past, I could never tell when the secondaries opened.

Just curious... did you change gaskets? Wrong gasket on the secondary metering block?
.

Nope. Same gaskets.

 

[Old Iron]

Beater, I found it just by reading the color of the plugs.
All the plugs were almost black from over jetting while chasing the severe vibration. I was running 110 octane so no detonation.
The 3 and 5 plugs porcelain was so bright white it'd make a movie star jealous [ddd
When I sent it to MSD they confirmed what I had expected.
When I timed the engine at 41 degrees those two plugs were at 55 degrees.

 

[8literbeater]

I bought a new Marshall/Russell/Edelbrock brand pressure gauge. It's only getting three and a half pounds.

New pump, new filter, new gauge, 12.75 volts at the pump while running, and still can't get gas.

I tried blowing the line out from the pump back to the tank. The tank is inside the front of the bed, up high. It's not like the pump has to lift the gas hardly at all. It has a ventilated cap, and I even checked the pressure with the cap off.

So, it's likely that the new pump is weak. I wonder if Napa will take it back. I guess I need to buy a "real" pump, maybe with a regulator.

I now have a rattle coming from the number four cylinder area. So things might have just gotten really expensive.

That efi is looking cheaper every day.