If you open the IAB more then you can probably remove the wire from the IFR, correct?

I mean the total flow is controlled by the screws, the mix is controlled by the IFR and IAB, right?

Been studying a bit, probably going to attempt your wire trick on my carb shortly.

 

It depends on the RPM range of interest from what I'm seeing. Based on looking over the data from the logger, the wires in the IFRs had the most impact between ~1500 and 2000. While opening up the IABs impacts 600 to ~1500. From what I understand (and have seen through the logger), the mixture screws only impact idle. Once the transfer slot is further exposed, the fuel mixture from the IFRs and the IABs is then mixed with air flowing through the primaries.

What evidence to I have of the above? Well I opened up the IABs and saw the mixture go much leaner below 1500 (but not at idle) yet saw not change above 1500. Now I think I could take the wires out to counter this, but then I'd be too rich between 1500 and 2000 (until the main jets come into play). Plus right now, I'm between 1 1/2 and 1 3/4 out on the idle mixture screws which makes me think I've got a reasonable Ihttps://www.ford-trucks.com/forums/newreply.php?do=newreply&noquote=1&p=14537596AB and IFR combination.

Since going from the #46 to the #45 drill is a small change, I used a small punch to open my brass set screws a tad more. With changes at the 0.001" level, I checked the drill bits and holes with my micrometer. Long story short, the #46 holes ended up increasing the area by 7% while the ones I made starting with the #45 ended up increasing the area by 14%.



Going larger with the IABs didn't help. There was a minor reduction in the MPG, but more concerning was being quite lean between 1000 and 1500 RPM during certain engine loads. From the data logs, after the shift to 4th and continued slight acceleration from ~1000 RPM, the AFR would frequently hit 17. With the #46 size IABs, the equivalent situation would produce ~15.5. However, very light load conditions (such as just off idle) still show the AFR being rich ~12 - 13. Hhmmm, maybe an effect of the accelerator pump . . .

I'm starting to think one has to target carb adjustments for a specific objective such as mileage or performance and engine loading. For example, trying to achieve an ideal 14.7 (or something close) across a large range of driving scenarios may be too much to ask. But a typical cruising RPM or RPM range seems fairly straightforward. Now I'm starting to appreciate why many people are sold on fuel injection.

 

Yeah, me too, but you've made quite a bit of progress. Even if you don't go any further you've done some pretty good work.

Have you checked the AF ratio while pulling anything, heavy load in the bed, etc?

I'd assume that a heavy enough load would pull the power valve into play, but just before that I wonder if it would lean out due to the modifications and drop in vacuum.

 

Did a little reading on some various carburetor parts. I found that the accelerator discharge nozzle is a .31 on the 600 and a .25 on the 750 I have. I've got a stumble when I hit the gas, if I barely roll into the throttle it runs fine. If I stomp it, again, fine. I think it is running rich at idle and the accel pump.

Not sure if your 1850 is the same or not. What are the numbers on the air horn?

 

No, I haven't. The closest I've come is climbing a long, fairly steep grade. As I recall, it was a tad bit leaner going up, and richer (relative to 14.7) coming down. It would be great to map the space of AFR versus RPM for a couple of different loading conditions.

This morning I was puzzling about the impact of vacuum. Mostly I was trying to understand why I still run rich slowing ramping the engine in the driveway, but things look much better under load. So far, I'm thinking the change in vacuum is the source, but this also impacts the timing. On my list might be a vacuum gauge hooked into the data logger.

 

I've got a 31 on my 1850. You might also look at the accelerator cam - mine is orange. From the AFR, I'll see 10 for a second or two when I hit the gas. So far, none of my changes to date have impacted this. I need to do some more reading to see whether I want try a smaller nozzle or different cam. But first, I'm going to experiment with adjusting the timing now that I'm content with the IFRs and IABs.

Thanks for the dialogue!

 

I swapped the 25 in tonight, I'll let you know how that does. Took about 5 minutes to swap from one carb to the other. Considering I saw black smoke after a brief wot I figured it couldn't hurt. I think I have the white cam on mine.

I need to just go ahead and order a wide band.

The smaller nozzle should make for a smaller intensity but longer shot of fuel. The orange and white cams are both on the middle range, the orange being slightly hotter. If this doesn't cure my part throttle stumble I'm thinking about trying a pink cam.

http://www.holley.com/data/Products/...mpCamGraph.pdf

 

I still have a part throttle hesitation at low RPM, but the 25 shooter seemed to help.

Last check on mine, mostly city driving, 9.09 mpg.

I'm slowly catching up with you. Still need to dial in my timing better, but I'm essentially starting from scratch. The old carb wasn't behaving and the timing was screwed when I got it.

 

That's encouraging. Maybe experiment with the cam positions and see if it impacts the hesitation? Out of curiosity, do you have a vacuum gauge hooked up? I was thinking about how to separate impact of the accelerator pump from the power valve. The last time I had my vacuum gauge on, I recall it dropping enough with rapidly opening the throttle that I remember thinking I wanted to look more closely at this in the future. Even if it opens briefly, maybe the amount of fuel is small relative to that introduced by the accelerator pump.

 

I've got one I used for tuning, but it isn't mounted yet. Been thinking about running a vacuum line and a tach sometime soon.

Not certain whether the hesitation is rich or lean, either way the 25 shooter doesn't hurt it, but I wonder if I don't need an 8.5 or 10.5 power valve. Light acceleration works great, hard acceleration works fine, but when I push the throttle like normal non-super-gas-mileage driving I get a hesitation then a strange, very light surging. I think it has to come down to the vacuum advance or the power valve. I think I may have had two different problems, too big shooters for moderate throttle response and too tight of a power valve for mid range acceleration.

Maybe.

lol

I think the power valve is probably worth 10-20 points jet increase, but that is just an estimation.

 

Anyway you can look at the exhaust when this happens? A couple of places I read commented on checking for a puff of black smoke as a means to discern the accelerator pump introducing too much fuel. Seems an AFR gauge would be the best way to sort this out. In the interest of isolating timing versus the carb, I wonder if disconnecting the vacuum advance would tell you anything helpful?

 

Smaller acceleration doesn't produce black smoke that I can tell, but WOT does. The O2 is looking more and more like a worthwhile investment.

I have disconnected the vacuum advance to eliminate that variable, especially since tinkering with it improved the problem. Didn't get rid of it.

 

While experimenting with the timing tonight, I thought I'd start looking at the accelerator pump. Here's a comparison between my orange cam in the #2 position (the way it came from Holley) and in the #1 position. Either way the AFR is far too rich and the engine starts to stumble so here's a definite future change. I was expecting the #1 position to help, but it's worse and pushes the AFR gauge off-scale. More to learn before I decide on which cam to try next.



I also put my previous drilled IABs back in and advanced the timing 4 degrees. So now I'm at 14 initial which is 27 with the vacuum advance hooked to manifold vacuum. Getting a smooth idle required dropping the idle AFR a tad to about 13.5 from the 14 or so I had at 10 degrees initial. In either case, I'm still idling about 600 - 625 RPM. Since I've leaned out the mixture, I'm expecting the more advanced timing to help improve the MPG further. The first data points in my table also suggest the engine wants more advance.

 

I drove around a bit today, got it good and warm, hit the gas and watched out the mirror and saw a black puff at part throttle acceleration. Still terribly rich. I guess there is a benefit to pipes sticking out the sides rather than straight out the back. I'm going to disable the secondaries to make sure they're not leaking as they have done in the past.

I've got my timing pushed about as far as I can. It is right at the point that it loads the starter when warm. I still can't make it ping. I need to swap my springs and possibly lean it out some more. I'm a little leery about that. Guess I need to quit stalling and get my O2.

Every single change I make to it picks up power. This thing is crazy. I slung rocks in my dad's driveway this afternoon completely by accident.

One thing that is kind of bothering me for my project and yours, the 25 is the smallest shooter they have. The orange cam is fairly low end of the mid range, same as white, which is what I think I have. Pink isn't significantly smaller. Mine is running rich on acceleration with a 600CFM carb and 25 shooters on a 400 CID FE. I think the accelerator pump setup is going to have to be modified beyond Holley's parts.

 

Re:air to fuel at idle with more timing, what are your plugs gapped to?