My old 327 has a lot of camshaft. It's 300 advertised, 232 @ .050. 110 degree LSA Oddly enough, my issue isn't with idle...it is choppy but consistent at about 700, and pulls about 10 inches vacuum with a A/F ratio near 14:1. No power brakes here. The issue is that ANY throttle application drops the vacuum and richens up the mixture. Just easily stepping away from a stop sign, it jumps to 12:1 A/F. At cruise, I am doing 3000 rpm at what feels like light throttle opening...and the vacuum is about 14 while the mixture is somewhere between 11 to 12:1. This is with a 4:11 rear. It feels to me like there is minimal load on the engine. I am trying to get something better then the 10 mpg I am getting now. Should I try recalibrating the carb to deliver a leaner mixture at cruise? Seems like the carb thinks I am at a greater throttle opening than I am and is activating the enrichment circuit. Thoughts?
You're a tad on the rich side, the 4:11 gear isn't doing you any favor's either, unless you have O/D.
Stepping away from a stop sign the mixture should richen because of the pump shot. Like Squirrel mentioned what carb? I have a Holley 950HP that I converted to ported vacuum, restricted the idle circuit and manage around 16-17 : 1 on my AFR gauge at a light cruise. At idle it's 14-14.5 and at wide open throttle it's 13.0-13.2 which is a tick lean. My camshaft is 234- 244 @ .050 .600 lift on a 514 BBF.
If it’s a Holley, what power valve do you have. At 10” I’d have 45 at the max, maybe a 35 or 25. Possibly none on the rear if your using a block.
You have a lot of factors involved besides the carb or how it is jetted. The vacuum gauge sometimes referred to as an "economy gauge" in the 70's is going to give it's best reading when you find the engine's happy cruise speed. That may or may not be 3000 rpm. That was The idea when these gauges were popular in cars like mid 70's Pontiac Grad Prix. Drive at the road speed where you got the vacuum gauge to read the best reading and be gentle on the gas pedal on acceleration. This chart is primarily for tuning up the rig at the shop or checking engine condition with the gauge and used to be in every Motor Manual back years ago. At least a similar one was.
Well, when the engine is running the vacuum is steady. At no load in neutral, it will pull 20 inches at higher speed. The accelerator pump shot helps explain why it suddenly goes rich from a stop sign, but it never leans out once I am moving. It stays at 11 or 12:1 forever, until I stop and the engine returns to idle. I have not tried full throttle mixture checks yet. I'm focused on getting the cruise condition under control. I am thinking that maybe my metering rods are lifting and activating the enrichment circuit at cruise, but the standard spring is supposed to be set to do that at 5 inches vacuum. I am hoping that if I can get my mixture right, it might make the engine happier and bring up the vacuum as well. Right now the poor thing feels sluggish across the board. I fear that the prior owner threw a big cam in an otherwise stock engine, and the compression isn't high enough. If that's the case, I can't go with a smaller cam or a bigger piston right now, but will have to live with it for a while. I'm just trying to make the best of it.
Did you check to see if the pistons and needles were drawn all the way down in the chamber at idle and not bouncing up and down? You might just need a weaker spring under the piston. Fuel pressure is a big factor too. Edelbrock AFB's don't like much more than 5.5 psi.
This is an example of a cordless vacuum. If you feel the need to vacuum while at a cruise, this will work perfectly. Stores easily behind seats or in the trunk
Try swapping to the lightest spring and see what happens. Since you have a afr gauge , those carbs are easy to play with. With a big cam and 411s , I think 10 mpg is about right.
Yeah for a street driven engine you want to jet for flat ground cruise first. Well let me take that back - first you want to make sure the Ignition is squared away with a good HOT spark, and no defective plug wires or spark plugs that are 3 heat range steps off from stock, the distributor has a good ignition timing advance curve, the vacuum advance has plenty of lead at cruise, there aren't any vacuum leaks, fuel pump pressure and float level/fuel level is correct. All that stuff has to be done before getting serious about AFR. And this is important - the power circuit must be completely offline at steady flat ground cruise. With a vacuum gauge and a wideband AFR this is easy to see. What you're doing in effect is tuning or adjusting the fuel curve by focusing on each circuit individually in isolation. Spark plugs are fouled real bad with numbers like that I expect. If you've got good ignition and a good distributor curve just about any OHV engine will happily low-load cruise at 16-1. This is where you make yer money in terms of both good power and engine efficiency. Jet down until you find that point where "lean surge" or misfire occurs at steady cruise. Then jet back up 2 sizes. If you could cruise down the highway at 20" of manifold vacuum at 19-1 on the idle circuit, that would be what you want, but you can't because carburetors and intakes aren't going to allow for that. Once the cruise mixture is straight then start looking at wide open throttle numbers and tune the power circuit. Don't mess with the jetting anymore. Again for emphasis if the ignition spark is defective or the distributor curve is defective, "hot" plugs installed, it simply won't respond the way you think it should. Needs a good fat spark with about 50° of timing or more at cruise. Vacuum advance is different in that the engine will start to rattle when you let off the gas, and then go away on acceleration. The optimum vacuum advance adjustment point is just short of that. Don't worry about what it shows with a timing light in the garage, give the engine what it wants.
I spent days driving around staring at a vacuum and AF gauge trying to tune my edelbrock 1406. After awhile of changing swapping out jets, rods, springs, and squirters I started to get a handle on what changed what, when. What I recommend is finding the longest, straightest, least traffic road and accelerate as slowly as you can, watch the gauges, the vacuum will decrease the more you open the throttle, you should be able to clearly see when the spring overcomes the vacuum and pushes the metering rod out of the jet. You can adjust when this transition happens by swapping out springs. If it is always too rich, put in a smaller jet. If it is too rich then goes too lean or visa-versa then change rods (smaller tip for richer full throttle, smaller shaft for richer at cruise). You could also try disconnecting your accelerator pump (for testing purpose) so you can see what the jets and rods are doing without the pump. Honestly the conclusion I came to was the AF gauge will make sure you are in the ballpark and help you see what is happening when, but after that its just about how it feels and drives. Carbs are not precision instruments and even if you had every jet and needle combination you'd never find the perfect combo for all situations.
I don't know if I agree with that. They are excellent at precisely metering fuel if that's what you want. The AFR gauges showed me that. I couldn't believe it. They are also really good at wasting huge amounts of gas, too.
After owning maybe 40 hot rods in over 60yrs of hot rodding, I can honestly report that I have never calculated what MPG I was getting...but, sometimes that was because I didn't have a gas gauge either.
Yeah, I don't drive a 60 year old car for fuel economy, but if it's getting 9 mpg and it's capable of 18 that means my tune is fucked up bad. See the difference?
Snotty cams love a lot of initial timing! (Like stated above...20* initial ain't too much.) Limit your total advance to around 36-40*. IF it has low CR no worries about pinging it to death. Lotsa initial will really clean up the idle. Tune carb to get the highest vacuum reading. Lightest metering rod springs are for snotty cams too. Honestly....even if you get the perfect tune and perfect AFR readings....a 4.11:1 gear is gonna spin that 327 up so 10 mpg is about it. 4.11:1 gears and big cams were made for 1960's gasoline@29.9per for 100 octane. That's just how it was/is! 6sally6
I mean they aren't fuel injection. As a whole they do an amazing job but watching an AF gauge can make you feel like its all over the map.
Well they're supposed to be all over the map, depending on load and RPM. I get your point but it's probably more difficult to tune a carburetor to run really well on the street and highway than it is to tune for a single mode like drag racing.
Yes, all my ignition parts are new and dialed in. I am running about 19-20 degrees initial, with 35 total, then 10 to 12 added with vacuum can running on manifold vacuum. My vacuum gauge is teed in to the line that runs the vacuum advance can. Edelbrock EPS dual plane intake. (one of the few that has the oil fill tube) Oh...I have 2 cars with A/F fuel gauges, and one is a digital, the other is analog. I can recommend the analog. The digital however...it is constantly in motion and very difficult to decipher what it's telling me, so if anybody is thinking of getting an A/F gauge, the models with a pointer are far easier to read than the models with flashing LEDs and digital displays. (FYI rant) If I choose to drive this thing to work, I have a 70 mile trip each way. With a 15 gallon tank, I have to fill-up every day. I don't expect that to change much, but more is better, and I want it to run as right as I can. I have not verified the fuel pressure or float level. That could be an issue...it runs an electric pump with no regulator. Anyway, thanks for the helpful tips. I have a calibration kit ordered, but Edelbrock is out of stock on that item right now so I will have to wait. At least I have direction for a plan.
Verify fuel pressure and float level ASAP. Check Summit or Jeg's for the calibration kit's, they might be in stock.
Well, if you use the chart that Edelbrock supplies with their carbs, and online, you can see what rods and springs you need, (jets, too) to lean it the way you want. Ask around, someone in your area might have the pieces you need. I know, we share that kind of stuff in our circle of friends.
A/F meters are not an absolute value, the reading is variable on the where the sensor location is placed. A carbureted engine at light throttle openings might like 15 to 1 mixture for best use of fuel. 12 to 1 is for high power loading .
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