have i made a terrible mistake?

[juanbenae]

like the title states, I'm wondering if ive made the wrong cam choice for my recent street hotrod 3.oLmotor build. I went with the webcam 120/104 grind with 95mm 9.5-1 JE pistons that volume measured out to around 9.3-1, big port early SC heads, ARP rod bolts, single plug, SSI's, 2in single out sport muffler, 40mm weber carb equipped.

the motor since inception 3-4 hundred miles ago has run very rich at idle & freeway cruz speeds. I monitor the driver's side bank with an AF meter seeing mid 10's to low 11's during idle and until the progression to where the main circuit comes in where I get very good AF from mid 12's to mid 13's all the way to redline. ive have played around with idle jets trying as low as 47's & 50's where the AF improved some but still rich low and the car barely ran!! ive settled on 55s after trying 60's at the outset where the rich numbers remain as does the good power through the entire range with good AF up top. I have worked in AC at 170, 175, and currently have 180s in place (have 185s in hand yet to try). it remains rich on the idle circuit. The rest of the story is 160 mains from the outset with F3 ET, and 36mm chokes on a fine set of webers that I reacquired after having them on my spec911 racecar for a couple seasons some years ago. dyno's them at the outset with them running & preforming beautifully.

this is where my cam concern comes in. admittedly, as I posted in another thread yesterday in hindsight the 120/104 may not have been the best choice of cam but the "hot street/race" description seemed right up my alley. it had been many years since I streeted an aircooled 911 and frankly forgot how little you stay in the high rev range, duh!. at this point I am not in a position to swap out cams and would like to make this work as part of the hobbie is solving these problems,, right?

the motor is fresh, as stated runs very strong, the floats in the carbs have been set right on the money, the dizzy was recurved by JWE so timing is at mid to high 30's of advance so I believe all the I's are dotted and T's crossed. what I'm wondering is if the significant overlap of the webcam grind I'm using is going to play havoc with the carbs throughout the idle circuit with the carbs? as noted ive played around with idles and airs but have not got it close enough yet to even consider a colortune lean best set up effort. I am going to try a set of F20 emulsion tubes in place of the F3's as weber literature states they may help with leaning out the idle circuit, but they also state the F3 might help here...?

I'm also considering a change from the 36mm chokes to a set of 34's. all ive read at paul's performance oriented site says that a smaller choke might actually enrich the AF in some cases. what I'm considering, right or wrong (?)is the added velocity of the 34mm venturi might offset the cam overlap forces enough with jetting adjustments to get the idle circuit livable?

please advise, and any thoughts would help.

t

[faapgar]

Everyone complicates a simple matter.You have 36mm venturis.Basic rule of thumb is the main jet is 4 times the size of the venturi.They do not sell a 144 main so start with a 145 main,55 idle & 180 air correction.Take a run & see how it is.Exhaust I.D.might be a little small but try this setup.Total advance should not exceed 30 degrees with premium fuel.Fred

[Gordo2]

Recommend re-posting your issue with a title related to you carburetors (i.e. Weber Carbs - Rich Idle/Progressing Circuit) - you will get a lot of know how from the board if the community see's you are working on your Webers...

That said, have you considered modifying your carbs so you have an air correction jet in you idle progression circuit?

I have PMO's and ran into a similar situation like what you are experiencing - I tried various idle jets, but leaned out too much if I went below 55's and finally ended up modifying (enlarging) the idle air correction jets.

Good luck, Gordo

[juanbenae]

Originally Posted by Gordo2

Recommend re-posting your issue with a title related to you carburetors (i.e. Weber Carbs - Rich Idle/Progressing Circuit) - you will get a lot of know how from the board if the community see's you are working on your Webers...

That said, have you considered modifying your carbs so you have an air correction jet in you idle progression circuit?

I have PMO's and ran into a similar situation like what you are experiencing - I tried various idle jets, but leaned out too much if I went below 55's and finally ended up modifying (enlarging) the idle air correction jets.

Good luck, Gordo

don, I have many pages of your "GORDO" carb tuning thread saved & renamed for specific things I like to revisit from time to time as I move through this. I would hope that a thread title where I thro myself upon the mercy of the court might garner less attention than some 3rd person title. I felt crummy enough coming to the ERBF for this topic, but the other current thread regarding cam choice might coincide. I did not want to highjack that point.

any thoughts on the f20 ET's I have coming vs. the f3' ive been running? thoughts on my theory of down sizing the chokes to lean out a hot cam at idle for street use by jetting & air correction?? am I off my rocker?

I found this in my extensive quest for information. Weber Emulsion Tube Tuning • Speed Talk not IDA or flat six specific, but a good read none the less.

[Gordo2]

I don't think an emulsion tube change will fix your rich idle / progression circuit issue.

E-tubes can change when (RPM range) your main jets begin to contribute - but if you are rich from idle to main transition, an E-tube change probably won't address this.

Check Paul's website mid-way down regarding the modifications for a tunable idle air corrector:
Performance Oriented

If your carbs are clean / tight and everything is in working order - this may be an option to reign in your idle mixture.

Meanwhile, I can't comment on how the E-tube swap (F3 to F20) would influence performance without digging into the design differences between the 2.

Good luck,

Gordo

[Trackrash]

My carbs were running rich on upper progression. Under light acceleration or on the freeway at around 70 -75 I was down around 10.5 to 1 AF.

I changed from my F3 imulsions to some F26s and it made NO difference.

I then opened up my idle air corrections from 110 to 120. That helped a little, not sure if it was really necessary, however.

I suspect there is something else going on with your carbs. I would look, with a mirror, down the carb with the motor at around 1200 rpm. Look for fuel dripping down from the main nozzles or accelerator pumps. Also look to see if you are getting a lot of fuel stand off.

What air cleaners are you running?

[juanbenae]

thread participation is Gordon centric! ! I'm good with that.

I have K&N filters that are very light on oil currently so they are flowing well. I'm sure you guys have seen this chart before:::




this is what ive based my hope on if you look at the description of the ET characteristics for the F20 will lean out the idle circuit, and if you look at the F3 it shows they have a richening effect on idle. the F3's have no holes up high which in my quest for data found are more likely promote the rich on idle circuit symptoms ive got. i had a set of F78's in hand that i never installed and they had all the holes up top, big and many. no holes below the top .5" on the tube with differing out/inside diameters from the F3. found little data on the F78 so I'm exchanging them for the F20s. i will again hold them next to the F3 i have out of the car currently when they arrive to see the port layout and ID/OD of the two units. will post some pics of them for you guys to digest with me.

id still really like to hear thoughts on going from the 36 to 34mm chokes to offset the effects of the cam overlap down low with the greater velocity the 34's would provide. I'm somewhat aware the smaller choke will promote a richer mixture by nature.

will share the ET pics when they arrive early this week. thanks for the time Gordon x2

[faapgar]

Hi,I gave you a very simple solution to get you back on track.I was a factory trained Porsche mechanic.I think they are called technicians now.I started in 1971 and had a shop for many years.Theory is just that.I have repaired or rebuilt several thousand 911 motors.Might take you 20 minutes to install my advice for your problem.I can only imagine the ratchetjaw thought process that has taken much longer.Oh,I am not a fan of the duck unless it in underglass with a nice glaze.Good luck with the guys with the charts.Fred

[Eagledriver]

I don't think there's a fix for this. My 46 PMO's and my 46 Webers have exactly the same mixture results as you are having. 55 idles will barely work with 60s running much better. At low throttle settings (cruising) mixture is about 10 to 1. Idle can be tuned fine, and WOT can be tuned fine.

I think most of us just drive it that way. It runs fine and if you don't look at the AFR guage, you don't notice it. I wonder if the change of progression holes from 3 to 4 made this problem worse.

-Andy

[juanbenae]

Originally Posted by faapgar

Hi,I gave you a very simple solution to get you back on track.I was a factory trained Porsche mechanic.I think they are called technicians now.I started in 1971 and had a shop for many years.Theory is just that.I have repaired or rebuilt several thousand 911 motors.Might take you 20 minutes to install my advice for your problem.I can only imagine the ratchetjaw thought process that has taken much longer.Oh,I am not a fan of the duck unless it in underglass with a nice glaze.Good luck with the guys with the charts.Fred

fred, not doubting your Porsche credentials by any means, but I do not anticipate a main jet will influence a rich at idle & cruse situation. or am I missing something in your insight? I do have some 145 mains in hand so if ET change does not improve it I will take the 15 minutes to place those in to see if this improves at idle AF's.

with the extensive experience humor me with your thoughts on downsizing the chokes to offset the hot cams.

[faapgar]

Hi,smaller venturis is less air.What is your timing set at idle.Does your distributor use vacuum advance?I looked at the article of the addition of idle air jet & it makes sense.You have a 2 in and 1 out muffler which may be a bit restrictive at idle.Do you have a less restrictive muffler to install for a test to see if would help your AFR at idle.I would be most concerned with good transition and driveability as you do not spend that much time at idle.Is the car twin plugged?Ciao

[juanbenae]

Originally Posted by Eagledriver

I don't think there's a fix for this. My 46 PMO's and my 46 Webers have exactly the same mixture results as you are having. 55 idles will barely work with 60s running much better. At low throttle settings (cruising) mixture is about 10 to 1. Idle can be tuned fine, and WOT can be tuned fine.

I think most of us just drive it that way. It runs fine and if you don't look at the AFR guage, you don't notice it. I wonder if the change of progression holes from 3 to 4 made this problem worse.

-Andy

I wanna keep tinkering though to see if the ET suggested to provide a leaner mixture on the idle circuit has any influence. the F20's will be here tomorrow and I will share some pics of them side by side with the F3's to see the port layout differences & ID v. OD of the two units.

hoped to make it to sears this weekend but with father's day & all I likely won't have time for the trek. been watching race results and it seems like you're picking & choosing your events more carefully this year. September for sure! be fast andy!

don't know how to do double quotes but faapgar says

Hi,smaller venturis is less air.What is your timing set at idle.Does your distributor use vacuum advance?I looked at the article of the addition of idle air jet & it makes sense.You have a 2 in and 1 out muffler which may be a bit restrictive at idle.Do you have a less restrictive muffler to install for a test to see if would help your AFR at idle.I would be most concerned with good transition and driveability as you do not spend that much time at idle.Is the car twin plugged?Ciao

appreciate your continued interest fred. recurved dizzy & single plug as stated above, no vac advance with the carbs, running about 33* BTDC at idle a around 37 at 3k rpm. I did read at performance oriented, paul abbott's site the 2-N-1 muffler could be something to look into. ive got a damaged 2 out M&K that used to be on my spec car that I should look into repairing.

regarding less air with the smaller venturi. what my consideration is that the greater velocity of the 34mm venturi might better atomize the fuel to offset the cam overlap at cruse & idle. where the current 36mm units are pulling air volume the greater velocity of the 34's & jetting "trys" might obtain a better used of less volume?? if this is crazy talk them someone stand up and say so, please? I'm here to absorb the process.

now to watch the dub-nation!!

[Gordo2]

To me - it was worth my time playing with different configurations to get a first hand understanding of how each component influenced performance.

Try to modify one thing at a time, and think through and study what the component does - you will end up spending some $$ learning, but if you take the time to understand what you changed, and how it works - the understanding you come away with will be much greater than what you learn from simply reading about the carbs.

Study those e-tubes - design differences and how they influence performances. In the end though, I still think you will find that they influence performance from transition up (RPM range, i.e. ~ 2,700 RPM to redline) and unfortunately don't impact your idle (~800 RPM to ~ 2,700 RPM) to transition.

Good learning, good fun - enjoy.

Gordo

[juanbenae]

Originally Posted by Gordo2

To me - it was worth my time playing with different configurations to get a first hand understanding of how each component influenced performance.

Try to modify one thing at a time, and think through and study what the component does - you will end up spending some $$ learning, but if you take the time to understand what you changed, and how it works - the understanding you come away with will be much greater than what you learn from simply reading about the carbs.

Study those e-tubes - design differences and how they influence performances. In the end though, I still think you will find that they influence performance from transition up (RPM range, i.e. ~ 2,700 RPM to redline) and unfortunately don't impact your idle (~800 RPM to ~ 2,700 RPM) to transition.

Good learning, good fun - enjoy.

Gordo

exactly! like I said previously your in depth thread has been a very good resource thus far.

[juanbenae]

so, got the F20 ET's today and thought id share the differences. the newer looking brass is the F20 and as you will notice there are bigger holes throughout. two sets of four higher than any holes on the F3 & bigger too boot. the inside diameter of the F20 is larger and it is prolly 3/16" shorter and has chamfering at the tip. I was not prepared for them to be different lengths as the previous in hand F78s were the same as the shown F3's. also the F3's have a widened section for just a portion of the tube where as the F20 is the thicker dimension it's entire length. looks like the shoulder of the F3 is the same diameter of the F20. the F20 also has a chamfered inlet at the top where the F3 does not??

I think I will be down sizing the AC back to a 175 from the current 180's. it's run a high RPM with good power from 170 up the current 180 in 5 spot increments.

[juanbenae]

ooops, photos here. will not try them for a couple days, but will surely report back on what I find in regards to influence on the idle circuit.

[juanbenae]

OK. dropped the F20's in today with a 175 AC and initial readings are that they have leaned it out some. seated all idle screws and went out two turns to start and had to richen up 4 of 6 but no more than half a turn to get the popping stacks settled. AF got better at idle & 3k RPM under no load than ive seen thus far. only traffic where I live at this, the last 3 & next two hours so no sense in going for a drive.

it's hot as balls here currently, weather id never drive in so the thin air will be under consideration. will report back, but the new ET's appear to be very promising in getting idle & cruz numbers I feel I can live long term with.

t

[juanbenae]

well, to circle back on this the F20 leaned out the top end & highway cruz nicely. still was not where I wanted it to be so in pouring over paul's site and what faapgar suggested above I moved from the dansk single out to a double out this week with wonderful results. at sitting idle it runs in the mid to high 11's AF (from low 10's)and at highway cruz I'm seeing mid 12's (from low 11's). I was able after the new muffler to really lean up the mixture via the idle mixture screws more so than ever before experiencing no lean popping from any of the stacks.

it will jump lean for just a moment to high 14's off throttle, but everywhere else I'm very pleased. may go back to the 170 AC, but it being so hot today I'm thinking the 175's are the ticket for now. power is good throughout so I feel I'm close. close enough that I will again administer a color tune effort to get things dialed in.

miss the sound of the single out as the double drowns the intake sound of the carbs I covet, but it's what works so ahead we move.

have acquired a second 02 sensor that I will place temp in the currently unmonitored passenger side bank during the color tune efforts.

sits like this now

160 mains, 60 idles, 36mm venturi, 175 AC, F20 ET & tall secondary venturi.

[VFR750]

I'm tuning a set of zeniths with weber IDA transition ports. Idle and cruise are only affected by the idle jet and air correction.

60 idle jets and 130 air corrections work for me. Dc20/19 cam in mine. Light throttle cruise 10.7-11. Idle 12.5. Highway cruise 12-12.5

[VFR750]

Btw, it has taken a lot of tweaking to get to this point.

Trying to get a reasonable cruise was challenging. I was at 10 as well. Better acceleration jet actuation timing with my hatchet style bellcrank and larger acceleration jets helped me lean out the bottom without suffering stumble.

Emulsion tubes can influence stumble, but they can't influence cruise.