Holley 1850 idle screws turned in all the way, motor still idles

[The Ry Guy]

I've got a 1850 Holley that is giving me some trouble. I can screw the idle mix screws in all the way, but it still is too rich and the motor won't die. Engine runs great, starts fine, idles fine. Butt Dyno says it runs great, takes the throttle and cruises just fine.

I thought, perhaps the power valve was blown and it's getting extra fuel that way. It was, and I replaced it with a new one. Now the idle mix screws do change the RPM and Vaccum, but keeps increasing all the way up to where the idle mix screws are shut. I can also back the idle speed screw off all the way and still have it idle, albeit at a much too slow speed (high initial timing helps with this I think). Boosters do not drip, front bowl is level with bottom of plug, rear bowl just above the threads.

I have to be getting extra fuel from somewhere, but I'm not sure where that would be. Any ideas?

Specs:
302 Ford
Howards Cams Hydraulic Roller Adv. Dur. 270/278, Lift .528/.533 110 Lobe Sep installed at 106.5
9:1 Compression
18 deg. Initial timing, W/O vac adv, (55 deg with vac adv @ idle) Hooked up to manifold vac (15 inHg @ idle)
32 deg. Total timing, all in by 2900 rpm.

Holley 1850 Carb List #80457-2
Rear Metering block conversion with rear block from a 4777-2 carb.
66 jets all around
6.5 PV (Front Only)
.020 Transfer slot showing

[turbo camino]

Did you change the rear IAB to match the 4777 when you put that rear block on in place of the plate?

[aquartlow]

Sounds like the culprit could be too much of the primary idle transfer slots are uncovered

[My427stang]

Sounds like the culprit could be too much of the primary idle transfer slots are uncovered

Yep, give it some secondary idle, and readjust.

[Tuner]

If there is a piss hole below the secondary T-slot it is likely too large. Could also be the secondary is open far enough already that the secondary T-slot is discharging fuel. If is runs good, If you like the way it runs, why screw with it?

[mag2555]

Sounds like it's assembled with a wrong metering plate gasket.

[Thermionics]

If it has a functioning PCV system, check the engine oil level. If it is considerably over full, change the oil and adjust the carb.

I had a 327 SBC in the shop a few years ago with exactly the same symptoms, but it was all stock except the intake and Holley carb. Blown PV. After I fixed the carb, it too would idle the fastest with the mixture screws all the way in. At some point, I pulled the PCV valve out of the VC and the engine died. With mixtures screws out 1 1/2 turns it was perfect until I put the PCV valve back in. It had more than three quarts of gas in the oil.

[liqu]

Metering block bent or bad gasket

[The Ry Guy]

Thank you all for the replies. The reason I want to dig into this further, is that the idle vacuum keeps increasing, so I think I could go a little leaner at idle. Plus I'd like to know if I got a gasket on wrong or something.

Did you change the rear IAB to match the 4777 when you put that rear block on in place of the plate?

I did not change the IAB's. They are pressed in, and I am reluctant to drill them just yet.

The secondary's do have the piss hole, I have no idea if it's ever been modified. I can see most off the secondary T-slot, above the throttle bore when looking down the throat of the carb. The secondary stop doesn't appear to have ever been moved.

It does have a functioning PCV, i'll pull the hose off and see what happens. This a freshly built engine, so I've been keeping a close eye on the oil level, and it's definitely not increasing!

I'll check my metering block gaskets to make sure everything looks good there, and I'll report back with what I find.

[The Ry Guy]

Did some more fiddling with this yesterday.

What I ended up doing was switching from manifold to ported vacuum advance. I had 55+ deg. at idle (damper doesn't have marks after that!) when hooked up to manifold vac. This combined with the idle speed screw backed almost all the way off to get the idle speed low enough, wasn't helping either. I had 15 inHg of vacuum this way.

Switching to ported vacuum, I lost 3 inHg of vacuum, but now the highest reading is achieved with the idle mix screws at 3/4 of a turn out, instead of seated all the way. Turning them in all the way still does not kill the motor, but I do loose vacuum and the idle becomes less stable, so I'm headed in the right direction. I have 18 deg. initial timing, and the idle is slower, but more stable, and the exhaust is less smelly. Plus I was able to get the idle speed screw back to where the T-slot was more square, instead of almost shut. I may still have a metering block gasket issue, but i'll deal with that when I go to change the jets after a few runs down the drag strip to tune the WOT. It has all new gaskets, so perhaps I got something backwards or shifted just enough to cause an issue. Seems to run fine otherwise.

Additionally, I'm going to come up with a way to limit the max vacuum advance, with either a different canister or a stop for the pickup plate.

Thanks for the suggestions!