imq707s

I rebuilt the Holley 750 cfm vacume secondary carb on my 350 chevy that is in my boat. While I had it apart I used a .080 drill to enlarge the jet orfices on the stock metering plate that controls the mixture for the secondarys. The orfices were .077 and it was running a little lean at WOT so I thought enlarging them to .080 would richen it up a bit. I also milled the choke horn off of the carb since it doen't have a choke butterfly on it anyway. Other than that, I'll I did was soak all the part in carb cleaner, blow them all off, and install new gaskets and seals from a Holley rebuild kit. I didn't touch the primary jets and installed the stock 9.5" Power valve.

So after I put the carb back on the boat I cranked it a few times and it fired right up. But it didn't idle smooth like before, now it idles like it's running on less that all 8 cyclinders. If I rev it up and run the RPM to around 3000 it seems to run smoother, but it still pumps smoke out the exhasut and runs rough. I just don't understand how it can still idle at 800 rpm if it's that rich. I cranked the idle mixture screws all the way in and no change in RPM, still 800rpm and idleing ruff as hell. I turned off the motor and checked the plugs, they were black and wet with gas. So it's running way rich, but why?

Here are the only things that I can think of.

1. The Power Valve may have a leak?
2. Maybe driling the secondary metering plate to .080" scewed something up, but I don't see how that would effect the idle.
3. Some idle bleed orfice or air bleed is plugged up.

Please help, what do you think.

US1 Fountain

Did you replace the base gasket also? (gasket between throttle plate and main body) Using the wrong gasket will do just as you describe: will not run at idle, clears up some when reved up, then acts like it loads up real bad. The gasket looks close, but it actually leaves the channels/slots in the base plate exposed to outside= Major vacum leak.

Could also be the PV, or incorrect PV gasket

gmhdfan

Are you sure the float level is set right?

turbo2256

My guess would be the power valve. They can be over tighted easly some I would swear are bad from the get go.

imq707s

My rebuilt kit came with 3 or 4 different base gaskets, and I'm pretty sure that I put the correct one on there, but I'll pull it apart and check tonight. I'll also put in a different power valve. Here is the biggest question that I've got.




WOULD DRILLING OUT THE SECONDARY METERING PLATE ORFICES FROM .077 TO .080 AFFECT THE IDLE MIXTURE AT ALL?

MKast

You can drill the secondary jets all you want, they don't effect idle, for that matter, the idle circuit of the Holley is not designed to use the primary jets.

imq707s

So my best bet is to pull the carb apart and check the power valve and all the gaskets to make sure they are all on correct. Someone told me that the secondary metering plate has two small air bleed in it that efect idle on a 4160 carb. Is that correct. I'm thinking that drilling the orfices out to .080 might be the problem. What do yo think?

MKast

I would have to look at a holley book to confrim but as I remember the secondaries don't effect idle. I'm more concerned about the idle passages in the primaries being clean. they're very small, one tiny piece stuck inside will screw you. I use a air gun specially designed for carbs. It's pencil thin to blow out small orifices. Bottom line, the engine idled ok before carb tear down. The secondaries don't effect idle, there for I'd look in the idle circuit. Look for flatness with a straight edge on the base and body. Are the air screws straight and clean? Are you torqueing the base correctly? I know this stuff is obvious, but I'm not there, I'm thinking out loud.

turbo2256

It shouldnt have made a differance ... but what was the metering plate part no. The main metering restriction (refered to as hole "A") for any given size has several different idle feed restriction diameters. Your .077 dia has 6 different idle feed restriction diameters. holly has no .078 dia. But your power valve is the problem most likely. If it is blown idle screws can be turned in all the way with little or no effect on idle. The power valve enables you to run approx 7 to 10 sizes lower in main jet size for economy. When they blow its like having your main jet dump right in to the idle circut which might be about a .028 restriction a power valve rerstriction might be .040 ,.035 or such hence it runs rich. the secondaries dont even open untill about 3000 RPM. At this point you could still be rich on the primaries depending on engine load/vacuum . A 9.5 power valve idealy would open at 9.5 in of vacuum. Power valve are a sore point in Holleys one back fire and there done tighten to tight there done.

Other conciderations newer carbs have a check valve to prevent loss of PV due to back fire . Conversion to a meetering body on the secondary is more user friendly. On faster boats I dont like vacuum secondarys the secondarys shut done to fast and on occasion caused a boat go bow first into the lake ecect passengers and such.

MKast

Last time I ran into a bad power valve the engine was idling alot higher than 800 RPM. Just a thought.