urban_cowboy

What exactly does increasing or decreasing the IFR affect? Does it affect the cruise AFR?

I have a situation with my Pro Systems carb where idle AFR is good (high 13s), but while in cruise when I crack the throttle the AFR goes to high 12s. If I open the throttle a more than cruise (i.e. I accelerated on the primaries) the AFR climbs back to mid 14s. I have tried increasing the idle air bleed (started at 76) all the way up to a 99 with little affect other than I kept having to open the idle screws to maintain idle AFR. If I jet down, the cruise AFR comes up some, but the AFR while running on the primaries climbs to 15+ which caused stumbles. I basically cannot jet down anymore and have the primaries work right. Would decreasing the IFR help? The IFR in my 11180 metering block is currently .035. Patrick at Pro Systems has been great, but he do not seem to be enthusiastic about helping me tune out this small issue since my WOT performance is good.

Another question. My metering blocks have glass sight glasses. Pro Systems recommended running the primary fuel level at at 25-40% and the secondaries at 25% or less. I currently have both set to about 25%, but it is hard to make small changes in fuel bowl fuel level when the engine shakes around . Now my question...will lowering my fuel bowl level affect the AFR due to emulsion port locations?

Scott H.

Increasing the IFR rating would allow you to fly in more severe inclement weather, within the boundries of the safe operation of that specific air craft.
Decreasing the IFR, would be a huge savings in money, and probably life expectancy, as flying in limited visability can be deadly. Just ask the Kennedy's.
Sorry man, the devil made me do it

Without knowing you HCI and timing, it's a tough call. Definitely reset your front bowl, it will make a difference. Everything starts with correct float heights.
The other important item is where your WBO2 is located. How far away from the exhaust valves, makes a difference. It's possible that you could be trying to correct for something that isn't what it seems... if that makes any sense.
This may sound weird, but you may need to start with your secondaries (plate opening), even though I understand you are only accelerating on your primaries. IIRC, your carb has 4 port idle circuits and dual metering blocks. so the secondary throttle valve opening can really jack with things under medium to high vacuum.

I had read that Pro Systems has a good tech line. Might be worth a call. Be ready to have your HCI, timing, and vacuum readings (along with AFRs) ready for them if you call.

Good luck!

Gun Jam

Scott thinks BVR is great but its time to have some fun because targets insight VFR, Darkstar, Judy Judy Twenty Mike-Mike going in for guns

67mustang302

IFR's affect idle AFR, cruise AFR and part throttle light accel AFR when the primaries are either not active, or barely active. You could try 33's and see if there's any change, or even 31's and run a smaller air bleed.

kalli

well i'll join in. probably getting smacked about but hell I want to learn as well.
it seems like it's leaning out on light accelleration, wouldn't it be an idea to maybe adjust the PV? measure the vacuum at this light accelleration and if that vacuum is numeric higher than the PV, adjust the PV to that number? and if it is numeric less or equal then adjust the PV restrictors (open them up a tad?) of course making sure that the vacuum on light accelleration is numeric lower than in idle/cruise

or do I have this wrong and you don't want any PV during light accelleration? i thought I would ... ;-)

67mustang302

The power valve is for the main circuit only. Under light accel in low gear on level ground you may only be running off the idle circuit(it's going to depend on how the whole vehicle is set up), so the PV will have no effect. PV rating effects WHEN the extra fuel is added, so if your main circuit is too lean under light-medium accel but with more throttle becomes correct, then the PV needs to open earlier. Actual AFR from the PV is determined by the PVCR's, which on most carburetors are NOT adjustable.

But again, it depends on whether the main circuit is being used or not. If the cruise and idle AFR are correct and you're leaning out under light accel, then that means that the main circuit should be active and it's not, or that the main circuit is active but too lean. In that case if the main circuit IS active, then a PV that opens sooner may be needed. It might also need a smaller HSAB to either bring the mains online or to increase the booster sensitivity on the mains so it draws more fuel.

Also keep in mind that leaning out under very light accel isn't necessarily unwanted. Under very light accel in low gear with minimal throttle the engine may tolerate mixtures as lean as 15:1, sometimes more. It depends on the vehicle/engine setup. If it surges or runs rough under a lean accel but it smooth under a richer accel like 13:1, then it's too lean. But under very light accel and cruise, the cylinder pressure is far too low to hurt anything, so you generally want to get it as lean as it will tolerate without running rough, surging or doing anything else undesirable. Lean accel under light-medium throttle can result in slight pinging and start getting things hot(like exhaust valves), but again, it depends on how tolerant the individual setup is.

Keep in mind though that most performance engines do not tolerate leaner mixtures at low rpm and part throttle. Remember too that 14.7:1 gives best emissions, leaner gives better mileage but makes more NOx.

I've found on my setup, 90% of the normal driving in the city is entirely off the idle circuit, and the mains almost never come on. If I leave a stop hard enough for the mains to activate, it's barking the tires or spinning.

kalli

thx for that. i just assumed all prosystems have PVCRs. Well this opened my eyes a bit here. I have the ever so slightes hesitation right after just opening the throttle (basically the least you can give power driving from a dead stop in 1st gear), or when in 4th completely off the accellerator and then just touching the pedal to speed up a little bit. I'm quite sure I can hold it at this lean surge if I don't change throttle position (i just don't want to try longer than a second, it's disturbing). as soon as I give more pedal all is great.
From how I understand your post I should only look at idle and transfer circuit, chances are that I just have to richen up the idle a bit.

during cruise I was running 15.2:1 with the holley no problem whatsoever and car was very good on fuel. I just have to spend another full day with the Demon to get it right.

back to Urban, what I don't get is when the AFR is lean at stages, why would you increase the air-bleeds. bigger airbleeds add more air, leaning it out even further.

and 67m302, his IFR is currently 35, why decrease to 33 to allow less fuel in there?

urban_cowboy

I am running a 8.5 PV which does not open under light acceleration or cruise but will open when I get more fully on the primaries. I run 10" of vacuum at idle at 950-1000rpm and 15" during cruise, so the 8.5 works better than the stock 6.5.

My Wide Band O2s (one in each pipe running true duals right now) are located about even with the gear shifter so they are probably 3' as the pipes run from the exhaust valve. Innovative Motorsports warns about getting them too close to the exhaust valve because of heat and too close to the end of the pipe because of fresh air incursion.

The car runs pretty well under this current config, but my fuel economy is definitely worse than when I was running low 14s AFR at cruise. This engine will not tolerate AFRs above 15 (noticeable stumble), so I stay below 14.7 pretty much all the time.

I installed my new Dr J tweaked intake last night and while the carb was off, I took a look at the metering block on the primary side. It is a 11180 (stamped on the outside) with a modified brass jet pressed into the IFR and the 2nd emulsion port down. It looks like there are three emulsion ports with a fourth in a separate location right below the top three. I have seen 3 and 5 emulsion ports, but an cannot find much info on 11180 metering blocks. The IFR is not a screw out type bleed like the billet blocks have, but it looks to have been modified by Pro Systems.

I have not driven the car since changing out the intake, only heat cycled and checked for leaks. Based on my unloaded testing, it definitely goes from 13:1 at idle to 12:1 under slight throttle back to high 13s as the primary throttle opens more. I have the primary and secondary throttle plates even at this point, but played with them a while back and made no difference in the cruise AFR.

The issue is not performance rather than I just do not need quite this much fuel under light cruise.

Question: Does the idle circuit pull fuel through the main jets or straight from the fuel bowl? I am trying to figure out why jetting down the primary was the only thing to affect this cruise AFR. FYI, in an unloaded condition, the throttle position I am talking about results in 2000-2500rpm. In gear cruising down the road, it is just a little open from off the throttle or idle position.

67mustang302

I idle circuit pulls fuel through the main jets and into the idle well. But jetting may change the cruise AFR if you're cruising with the primaries. It's going to depend on the setup, some cars use the primaries and others don't during cruise. If you're even using the primaries a little bit during cruise, then jetting will effect cruise AFR. But the IFR's are much smaller than the main jets, so changing jet size won't affect the idle circuit.

You could actually prevent the mains from coming on during cruise by using a larger HSAB, but it may require rejetting and could possibly mess up the AFR curve if you go too far.

urban_cowboy

I am currently running a 36HSAB. I have tried a 40 and did not notice much change and was scared to go much higher away from the preset. I guess where I am scratching my head is if the tip-in/slight throttle rich condition is the primaries, then I wonder why it is rich and then raises by 1.5AFR with just a little more throttle and stays there through the rest of the throttle position movement...until I hit the secondaires. The primary fuel curve is correct except for the initial starting point.

What I though was happening was the transfer slot was too rich, but idle and primaries were right on. A I off on this? If the rich condition is the primary circuit, it goes from 12:1 to 13.5:1 with just a slight throttle change, so the curve would be correct for the most part with the exception of the initial few degrees of throttle movement. That is why I was thinking the transfer slot circuit.