I can't seem to figure this one out.

Specs: 250 straight 6, Offenhauser intake, Holley 390 CFM 4bbl, Split headers going into a single 2.5" exhaust, HEI, mild cam, powerglide.

Timing: 10 degrees advance
New-ish plugs and wires
No vacuum leaks
slight exhaust leak at the manifold/pipe flange (need to fix...)
Vac advance canister with 3 turns out
Carb is a bit rich. Need to re-jet. Have #54's in it now.
Vac is 18
Power valve is a 6.5
Floats are set properly
Don't know idle speed (no tach) but changing idle doesn't help.

Symptoms:

Big drop in idle when going into gear. In neutral I'd guesstemate the idle is around 900 RPM's. In gear it drops to 600 or so. I have to give it gas when shifting into gear to bring up the idle to about 1200 RPM's or it stalls out. Once in gear everything settles and it doesn't stall. Stays around 600 RPM with some slight wandering (50-100 RPM? Once again going by my ears...).

Any suggestions on what to try?

How did you adjust your idle mixture screws? With a timing light, did you visually inspect the timing to verify it advances when you rev it up and when you plug the advance in to the distributor?

Idle mix: with a vacuum gauge while in gear with the parking brake on.

Timing: it advances like it should when the canister is connected.

You may need more base timing. Gradually increase it a few degrees at a time and see if it gets better or worse.

I'll try it and get back to you.

Just to be sure you are adjusting the carb correctly.

For the idle mixture, are you turning the small screws on each side of the primary metering plate?

It could be that the stall of the convertor is too tight at idle?

Yes on the idle mix.

How would I adjust the stall? I thought that is whatever it is? As far as I know it's factory. I don't think the motor and trans have been pulled apart yet.

Does the engine do the same thing when going to reverse and drive?

I suggest rechecking for vacuum leaks. These intakes can be a son of a buck to seal up.
Once had trouble this summer with a carb adaptor leaking for me too.

Same thing in both forward and reverse.

I'm starting to lean towards the bottom of the carb and the spacer. I bought a new 1/2" spacer but I'm thinking I might go back to the old 1" and try it.

A couple more ideas:
1. I dont think thats too big of a drop. Typically that is what I see, about 900rpm neutral, 600rpm drive w stock converter. But, it should still be happy.
2. Is your vac advance ported to intake or spark port? In other words, I like spark port where you have NO vac advance at idle. Otherwise, when you drop it in gear, vacuum drops so you will lose advance, causing even more RPM drop.
3. To check for vac leaks, especially baseplate:
* Back the fast idle screw WAY out (count turns) so you can completely close choke plate without bumping the fast idle screw.
* Start & run, choke plate open, warm engine. Now slowly close choke plate. Engine should keep same speed, then slowly start to die out as the plate gets to be almost closed.
* If it speeds up when the plate is about 1/8" from shut, that is classic sign of a vacuum leak.
* Try both mix screws 1-1/2 turns out. That will likely be TOO rich (normal is 1 turn out or less) but just try it.

Thanks Deuce. Nice trick on figuring out if it's a vac leak.

The same choke plate trick will also detect clogged idle jets or feeds. That is fairly common too if the goo we call gasoline today dries in there multiple times, leaving crud.

Fresh rebuild on the carb...

So I had a chance to play around with it some on Saturday.

First I swapped over the vac advance line to manifold vacuum. It seemed a bit better but not a big difference there.

I think the real problem was the idle air screws were at vastly different settings. I played around with them a bit and got the engine to smooth out at idle. That seems to have made the biggest difference. It still wants to stall sometimes but not nearly as much.

I'll go back and re-set them with a vacuum gauge. I think that was the problem all along.

See:
https://www.holley.com/support/resources/
specifically,
click on: "A Look Inside Your Carburetor"
scroll to "Idle System"
Might be of help. Might be good to read the whole thing.
I've had to tweak base timing while doing this before so keep timing light handy.

Thanks. I already planned on doing just that next time I can slide out to the garage. Maybe tonight.

Also just ordered a pair of #49 jets. the 54's are way too rich.

What makes you think your main jets are too rich?

Plugs are wet.

My mechanic and somebody else from this forum recommended 51, fyi.

May end up at 51. I'll try the 49 and see what the plugs read. If it's too lean I'll go 51 next.

I'm sure I posted this before:

Main jet size has very little to do with idle mixture F/A ratio.
I.e., going down a main jet size will only change your F/A ratio a percent or two. But it will affect your WOT mixture lots more.

Look elsewhere for the problem other than main jet size.

What else could cause it to run rich? The squirter arm being adjusted too tightly? Power valve?

The idle mixture adjustment is directly related to your lean/rich at an idle. If it isn't adjusted correctly, many of your other tuning parameters cannot be accurately set either. If you have back fired the engine during your tuning stage, you could have popped the diaphragm in the power valve, causing an overrich condition, and it will need to be replaced before proceeding. With the engine idling, look down inside the top of the carb and see if you see gas pouring profusely out of the squirter.

If for instance he back fired through the carb least a couple of dozen times, what are the odds of the power valve being blown? Would it be easy for him to replace if the carb is on the bench? I have this friend...

New power valves have relief orifices built in. That's not really a problem anymore unless your friend backfired a whole lot.

To change the valve pop off the bowl, unscrew it, and pit a new one in. It's a 5 minute jib once you have the carb on the bench.

My 390 Holley runs a 52 primary jet, and .028" PVCR holes.
It has won numerous carb shootouts with the GTECH on bone stock 250 and 292 engines. I'd try 54 jets 1st if it had headers/cam but for now I think its gotta be near perfect.

For full throttle you need to know both the jet size and the PVCR hole size. I never even remember the vacuum rating ie 6.5 or 8.5 etc but I always measure those holes in the metering blocks. Don't know why Holley doesn't list those.

Glad the mix screws seemed to help. I also find that due to crud in the gas, you can set them, then suddenly after sitting for a year it will be "way too lean", cuz dried gas crud is clogging things up. you can turn the mix screws out as a crutch but that's a hint that the carb is dirty inside.

I'm running 54's now.

I'll also measure the PVCR holes. I think my caliper can do it.

I'm puffing smoke at full throttle and she smells. The plugs are also very wet. That's why I'm thinking the jets are too big.

I'll try the 49's that came in last night, run it a mile or two, and read the plugs again. I can clean them off with some carb cleaner in between.

I'm also running E3 plugs so I can't read the spark as well as I could if I were using regular plugs. I might pick up a cheap set of regular ones just to better read it then swap the E3's back in. The E3's do help it, though. The idle went from choppy to smooth with them when I swapped them in a year ago. Old plugs were new-ish too.

I have a plethora of power valves. When I set the vacuum screws I'll re-size it then.

Also considering changing the secondary spring to a lighter one.

I'll warm it up, before doing anything, and look into the carb while its running to see if my squirter is dumping fuel. That might be the case.

Probably do all this tomorrow morning. I'm sure my hung over neighbors will enjoy the Flowmaster 44 revving at 9 am...

https://youtu.be/mo17Lj3jqOs

She's running much better now.

I had to get out the vacuum gauge and set the idle air mix properly. Also set timing at exactly 10 degrees. Seems to like that.

After running it for a bit in my garage I pulled the #4 plug.

[img]http://imgur.com/VUmhUIf[/img]

Wetness is gone.

Didn't change the jets. Just played with the screws a bit. Also haven't been able to run her hard yet. I was out in the garage with my helper while my 2 year old was taking a nap. My wife was out so I couldn't take it for a drive yet. We'll probably take it out after my wife comes back and the boy wakes up.

Other pics while I was out there...

[img]http://imgur.com/a/FITQZ[/img]

[img]http://imgur.com/ESp2wVE[/img]

[img]http://imgur.com/Vk5rVrh[/img]

[img]http://imgur.com/WXZnbXS[/ing]

Still seems to be idled too high. You need to get a tach on it. Nice helper. Start them young.

I lowered the idle speed after that video. I need to reposition the return spring to get it to return all the way down now.

On a brightee note it can be turned down real low now. So low I fear starving the engine of oil.

We took her for a nice, long drive today. Did just a bit more fine tuning. Idle is down low like it should be, doesn't stall or stumble, and seems to be getting decent mileage. Power is pretty strong too.

Can't wait to do the 200r4 swap and really see what she'll do!