Tuning with an A/F gauge

[travis]

I’ve managed to get the idle and cruise much better and cleaner. But...in getting the cruise a/f in the 14.7-15.1 range, it seems I’ve created another problem.

Light throttle acceleration, staying out of the secondarys, it’s going very lean...like high 16’s/low 17’s afr. Same when driving down to road and lightly rolling into the throttle to maintain speed in hills, and the same thing trying to maintain speed in a headwind. No surging or pinging ever. I’ve got the 8” step up springs installed, which helped, but unless I really lay into the throttle to get into the secondarys or at least the power portion of the primary metering rods it goes scary lean. Is this normal? Should I be looking for something like a 10” step up spring?

Even though the drivability with this Edelbrock is pretty good, it is definitely down on power compared to my junk 1407 or the 670 Holley. Usually the power difference between the Edelbrock and Holley carbs isn’t that big, but in this case it definitely is.

[F-BIRD'88]

Which model carb? current jets rods set up?

[F-BIRD'88]

You may be able to reduce that lean condition at that driving point by modifying the airflow of the PCV valve.

This is why there are tons of unique calibrated OEM PCV valves.

[travis]

Edelbrock 1905 650 AVS2 carb, all as it comes out of the box except 8” step up springs (strongest I have on hand), and changed the primary metering rods from the stock 7037’s to 7547’s. These changes, plus some IMS tweaks, gets me the 14.7-15.1 cruise afr vs the 7037’s at 13.0-ish cruise. It’s much happier at cruise now.

No PCV valve

[steve cowan]

Travis,
Do those carbs have a transition slot like a holley carb??

[travis]

Travis,
Do those carbs have a transition slot like a Holley carb??

Yes it does. I've got the transition slot square, and the IMS's are very responsive. Idle is quite smooth, starts easily and idles perfectly hot or cold. It drives very well...but that hard hit from 2500+ rpms just isn't there like it had with the other 2 carbs I've got. It runs like a well tuned 300-325 hp engine.

The 750 edelbrock is I have is garbage...it peeled most of the carb jet threads out when I tried to dig into it further. It is also eroded badly inside...but in all fairness it is over 15 years old and has a LOT of miles on it. I have parts coming to try to fix the driveability issues with the 80670 Holley...I don't want to change too much on this 1905 Edelbrock as it is slated for another car. If that doesn't work, then I'm ordering another carb...or my wife will kill me. Whichever comes first

[BCjohnny]

Obsessively tuning to a' 'correct' number is not the way to go, you can tie yourself in knots ...... as said tune to what's 'required'

Tuning lean to save fuel often ends up with using more

Using a simple vacuum gauge can help, mostly at idle and low load cruise

A/F gauges served people reasonably well for many years ...... although establishing a baseline full load mixture is about 90% of their remit

Engines in general are imperfect creatures and one size doesn't fit all ...... carburettored engines are even worse

SUs/Strombergs etc can be got close to a full clean metering curve, due mostly to fuel atomisation I'd guess, but would always be down on absolute power

The Weber (Carter/Edel) 4 bbls IMO are probably the best overall compromise for road going V8s, not least where economy has some bearing

Just some general thoughts really .......

[travis]

I’m not looking for any specific numbers any more...I’m more using it as a warning that I’m going to far one way or the other. And in that regard it is helping. It would help more if I knew what the heck I was doing

[hoodeng]

Sounds like a plan Travis, at least you acknowledged you have more to learn with tuning. Nothing wrong with that at all, we all started somewhere, at least you have had a go, and seen where your limits are,,there are others that can tune an engine to a standstill and not see their finger prints still.

Is getting your car on a dyno an option? good operators get consistently good results, idle, cruse, accelerating and top number.

Cheers.

[Geoff2]

Travis,
[1] Comparing the 650 AVS to larger cfm carbs for power is not apples to apples. Maybe you need the 800 cfm AVS for max power.

[2] Leanness at cruise. More than likely you are running off the transfer slots at the speeds quoted. So you need to do two things. Make sure you have no more than 0.060" of transfer slot [ TS ] showing at idle [ less is better, as it leaves more of the TS for cruise & transition onto the main cct ]. This will require carb removal to check. This is the 'trick' I use to verify TS position WITHOUT continually removing the carb. I aim for 0.020-0.060" slot showing at idle. At 0.020", I measure from the idle speed screw head to the screw boss with calipers & also note whether the slot in the screw head is horiz or vert. Then I do the same for 0.060" slot showing & record the info. TS position can now be checked on the engine without carb removal.
If more than 0.060" TS is showing at idle, you need will need bypass air holes in the t/blades to admit more air. Start with 3/32" hole in each pri blade, & enlarge in 1/64th increments until the TS position is correct. The other thing you will need to do is increase idle fuel, because this is what you are cruising on. Unlike brand H & clones, you don't need a box of gaskets. In fact, no gaskets if you are careful. Remove the primary boosters. The idle feed restriction [ IFR ] is the small pinched end of the thin brass tube that hangs below the booster. It is probably going to be 0.031-0.035". Enlarge it in 0.002" increments & re-test to cure the surging.

[zwede]

Light throttle acceleration, staying out of the secondarys, it’s going very lean...like high 16’s/low 17’s afr.

Seeing a lean A/F spike at tip-in with a carb is normal and unless you feel a hesitation or miss is nothing to worry about.

If you have a "normal" street performance cam you should expect it to idle best in the 13-14 range, but having to idle in the high 12's is not unheard of.

For cruise I'd aim for 14:1 to give a little buffer for changing conditions. Temp/humidity/elevation all change the A/F so set it a little rich.

[F-BIRD'88]

Your carb is designed and calibrated to work with a functional pcv valve.. You are creating huge re calibration balance problems that cannot be simply acounted for by simple jet rod swapping.

Put the PCV valve back on. You'll go faster a lot farther.

The idle off idle and primary cruise jetting will be restored to a intended balance and thus very near out of the box primary jetting/rods will be very close to best.
There is nothing wrong with your 650 avs carb.

Yup the bigger 750 afb or 800 avs carbs will be a bit more powerfull at WOT than a 650 9 times out of 10.
They let in (a bit) more combustible air and fuel at WOT.

You can max the WOT power by dialing in the secondary jetting and air door on the 650.

All street carbs that have a 3/8" vacuum port on the base are designed and factory calibrated to WORK WITH a PCV VALVE.

[F-BIRD'88]

Further: By eliminating the PCV you have in effect made the carbs WOT airflow smaller.. The difference is not insignificant. Smaller carb at WOT equals less WOT power 9 out of 10 times.

Get a pair of those 650 s on a dual quad hirise manifold
and hang on tight.

[travis]

F’bird, you may have nailed it with the PCV valve. I put the original 7037 metering rods back in, still have the 8” step up springs, and hooked up the PCV valve setup from my other truck. It’s pretty good now...cruise is in the low-mid 14’s, WOT is fat at 12.0’s. Throttle response is better...I think a little more tuning and it’s going to be pretty good.

Then you lost me at dual quads on a 4900 pound truck

Now, here is something kind of interesting I thought. My other F150, with the nearly stock 351w, has a nearly brand new Holley 1850 on it. Factory jetting, and some minor tweaks to the idle speed and IMS’s and it runs great. I took that carb off and threw it on the hotter 351w, and it has the exact same (actually worse) drivability issues that the 80670 had. It idles super lean, and terrible response at anything under 2500 rpms. I piddled with it a bit...couldn’t find any vacuum leaks, and couldn’t get it to respond to any adjustments. Put it back on the milder engine and it runs just fine. Same cubic inches, and only 2” idle vacuum difference between the 2. Is the idle circuit just too small on these little street Holleys to work well on anything above 250-ish Hp?

[F-BIRD'88]

I might try a 1size smaller primary main jet now .
it should lean the WOT afr a hair. Then try a 1- to 2size leaner secondary jet on the 650eddy. Wether that resulting leaner wot afr is BETTER, overall is another story.
You can try it.. What matters is the engine is happy with the fuel calibration.