Carb tuning

[delco1946]

I’m trying to tune my choke and carb. I think I got the choke taken care of due to some great YouTube videos. I’m less certain about general carb tuning. I hooked up my dwell / rpm meter and a vacuum gauge.

The carb was quite lean from recent deq adjustments, after which I had all new fuel systems installed (tank, pump, etc).

I did some reading that basically stated to enrichen the mixture 1/4 turn at a time as the vacuum increased. I did this and at about 600 rpm am getting approx 20 inches of vacuum. I didn’t bother to keep going as I’m doing “baby steps” to avoid screwing anything up. However I noticed when I disconnected the vacuum gauge from the stem at the back of the engine, this extra air actually caused my rpm’s to increase. I’ve always thought that if your engine is tuned correctly it should stutter and run rougher and generally slow. Does this means I screwed up? I’m not sure how to interpret this…..

[Gary Lewis]

While I agree with your logic that on a perfectly-tuned engine the extra air from an open vacuum port should slow it down, my experience has always been that it speeds up.  I can't explain that, but it happens.

But I don't like to tune for max RPM.  That always seems to leave the engine slightly lean.  So I find max RPM and then open the screws 1/8 - 1/4 more to give just a bit more fuel.  That helps on the transition from idle to acceleration.

[mat in tn]

that is correct. once your tune is "perfect" , highest vacuum for the desired rpm. when introducing air without fuel it will lean and speed up slightly before stumbling. however, that is one of the fine-tuning points to keep in mind. Gary you are right to find high vac then go a hair richer. that aids in smoothing out the engine while in gear and on tip in.
on a carburetor, once the settings and tune are set, the driving is the biggest variable aside from the environment. temp, humidity, altitude, etc. the first thing you do when you step on the gas is you give it air. the accelerator pump squirts a fine stream of liquid in, but it has to vaporize to burn. most of it lays in the intake while it evaporates. so, if it were spot on lean it would be super lean upon tip in and stumble or possibly even stall. i find it more important to have both barrels tuned as closely matched to be more important than a set rpm. 50rpm higher with a good even tune will drive much better

[delco1946]

Thanks for the tips. I’m curious since I didn’t continue turning my idle mixture screws richer, how much higher folks have gotten a stock 351m? Is 20.5 inches “good enough”. I also don’t want to make it so rich that it worsens my already not great mpg, but before (at 17 inches) I noticed I was struggling up real steep hills). It’s behaving much better now :)

[Gary Lewis]

20 1/2" is pretty good for any engine, but a lot of that depends on the idle RPM.  IOW, you can get the vacuum right on up there by running the engine faster.  But if you have it idling normally then you might get it to 21".

Having said that, I have three vacuum gauges and they all read differently.  So the key is to turn the screws out & in until you find the highest vacuum, and then richen it up just a bit more.

[BigBrother-84]

Hi gentlemen!

Where is the right place to connect vacuum gauge on a 351w with a stock carb?

I suppose directly on the manifold, but can someone precise the placement?
Maybe a pic?

Thanks!

[Gary Lewis]

Yes, you want the vacuum gauge connected to manifold vacuum. Some port that has full vacuum all the time.

[rcarlisle]

Not perfect, but I am connected to manifold vacuum up at the heater tree on firewall.   Just because it is manifold vacuum and I could reach it without having to go into contortions to get to the tree it connects to on the intake.  I have a plan to get some vac line and maybe run a capped line to the side of compartment.  I have several points that have been blocked off by previous users.  See pic - the middle nipple where the plug boot with bolt in it, I think, is where I connected.  Mine is 302 but that vac tree should be there on the truck to run the heater controld.  I can see the supply running down to the manifold.

I can hang the vac gauge on the hood reinforcment to read underhood. For test driving, I run the gauge up to the windshield wiper arm and hook and prop it under the blade to view while driving.