I agree with your logic on the jetting - a larger venturi will yield a weaker vacuum signal at the same air flow/RPM, so you'll have less pull on the fuel and, therefore, a leaner mix. And it would be true all across the flow range, not just at WOT.

Speaking of WOT, once you settle in on the jets you need to tune the power valve. Since power valves open with low vacuum and you are increasing your vacuum at any given speed/load your PV will open later and later as you enrichen the AFR with the jets. But, that's what you want as the richer you go the less you need the PV.

So, you may be fine with whatever PV you have in. But, that's the last step in the tuning.

 

Thanks Gary, I'll post on how the new jets are working out and, if I decide to move up further in jet size, I'll report back with the new vacuum readings at cruise.

 

Very interesting! I appreciate you taking the time to do this and report the results. Finally got around to rigging up a vacuum gauge in the cabin in my effie and have been waiting for warmer weather to take it for a spin.

One post on the HAMB talked a little bit about using a vacuum gauge for jetting, and when I read this post went and dug it up again. For some reason was thinking that smaller jets meant higher cruise vacuum - this is not the case or, this guy is just suggesting starting out on the high side and working down.
So - does the vacuum decrease at both ends of the jetting sizes, with a narrow peak at optimum?

"Actually a vacuum gauge is great for cruise jetting. Pick a flat piece of road and little wind start with rich jetting drive 60 down the road check vacuum go down a jet size and repeat checking reading keep repeating until you have reached the highest reading."

Have read the hot ticket is to choose a power valve that is two points lower than the minimum cruise vacuum observed at cruise, as Gary mentions this would be the last thing to change, since jetting changes vacuum results.

 

This is the first I have heard of using a vac gauge for jetting size. I was told and I do pull the plugs to see what they look like and adjust from there.

I have and do use the vac gauge for PV. This is on Holley carbs and even the books I have never seen the gauge for jetting.

Now along this same line would this work for any carb be it a v1, v2 or v4 carbs. Oh and Holley made a v3 carb also.

As for starting rich and working lean it is a lot safer doing it this way. Run too lean and you can hurt pistons.
Dave ----

 

I figure it's just another tool in the toolbox to aid in proper jetting tho not as a substitute or replacement It's remarkable the number of diagnostics that can be made with manifold vacuum. Even when using a wideband O2 it's still very important to look at plug coloring because some cylinders will run a bit leaner compared to others, the A/F ratio indicated is an average. But with modern fuels plug reading is a lot more difficult.

 

The only way a vacuum gauge will dial in the AFR in is if you want it to be 14.7:1. That's because that is the point at which the engine will give max power and, therefore, max vacuum. However, that won't give max MPG, which will be at a leaner AFR.

 

That's an interesting point Gary. EFI engines routinely run very high A/F ratios at highway cruise, carbureted engines, I'm told, cannot run as lean a mixture but supposedly 15 to 16:1 is considered normal. I'd still be worried about a lean cylinder but the claim is, that excessively lean mixtures only cause damage on heavy acceleration. Lightly loaded, an engine will tolerate lean mixtures past stoich.

 

I've read, but don't know for sure, that EFI systems target 14.7:1 AFR. And I've seen that several places but, as said, don't know if that is true.

As for carb'd vehicles, one reason they can't run as lean is that each cylinder is different due to the intake manifold and carb placement. So, I just tune leaner and leaner and check the MPG and compare that to the driveability. For instance, on Dad's truck I got it so lean that I was getting almost 16 MPG, but the driveability was poor as it didn't run well until hot and even then it didn't like to accelerate w/o giving it a lot of throttle. So, I went back down a jet size and lived with it.

 

When I jetted down, it seemed the engine ran "ragged" at level cruise, it will be interesting to see what the afr shows. Saturday is supposed to warm up a bit so I may try it out on my rig.

 

As noted in my other thread (Carb/fuel problem, I think), replacing the plugs, rotor and dist. cap solved my bog problem.

To continue on my rejetting quest, I have a question on reading spark plugs. I pulled the original plugs in trying to solve my bogging on hard pull issue. The insulators were white. After changing to Autolite 25 copper plugs, I took the truck out, drove it around to warm it up and then took it for another hard uphill run. I came back home, shut it down immediately and pulled one plug on each side. Insulators are are still white. Can I assume from this one test that the jetting is still lean? If so I will up-jet to 57.

 

When you say "I came back home, shut it down immediately" how many miles were involved? It doesn't take much running lean at part throttle with a hot plug to bake off any color from the pull.

 

Remember jetting is almost exclusively a steady, level cruise deal, above say 45 mph. Idle, part throttle, and wide open throttle are totally different circuits. The ceramic color that everybody usually focuses on is just one thing to look at. The ground strap for example shows total ignition timing and heat range.

For wide open throttle afr look deep into the plug, where the ceramic transitions to the metal plug shell. Should be a tan ring. Can go lean to the point of misfire on jets for testing and back up rich a little because the engine load is low at cruise. 16.5 AFR is considered best economy for a carbureted engine. Too lean on wide open throttle under load though, will nuke pistons or roast valves.

If you're like me eventually you'll get tired of (more or less) guessing and pick up a wideband O2, they are really slick!

 

I live at the bottom of a very steep hill. I ran up the hill twice at hard acceleration, turning around at the top of the hill each time and came back down, went up my steep driveway and shut down. The hill is probably 1/4 mile long, so I just came back down the 1/4 mile, up my driveway (150 feet) and parked and turned the motor off right away.

So 2 1/4 mile uphill runs hard and back down twice at normal speed, then up the driveway and off.

 

Your plugs probably are showing your AFR from the acceleration run then. But, as Tedster9 said, that means they are showing the jet + power valve AFR, not just jetting.

 

The above is true. The problem they started running into, and you can too, is at cruise the rpms are so low with the overdrive transmissions and lock-up converters, they were starting to cruise on the part throttle circuit(sometimes called the transition circuit). The carb was barely open, which is what they want for fuel mileage and emissions.

You can adjust this somewhat on a holley carb if you mess with the stop screw on the secondaries.