new motor, new carb config help
 

looking for some advice as my new motor is coming together and I hoping to find a good baseline for break in & beyond when it comes to carb jetting.

the motor quickly is an early big ported 3.oL that will have JE 9.5's @95mm, single plug, with the webcam 120/104 grind. the carbs are a reacquired set of 40mm webers that used to top my 3.oL spec motor back in the day. the spec motor was also a big ported 3.oL with stock SC pistons & cams. the car ran wonderfully & was dyno'd at one point with the AF right on. the carbs are currently configured the same at::

145 mains
170 AC
F3emulsion tubes
50 idles
36mm venturi
4.5mm Aux venturi


id expect with the hotter cams & compression increase I could go larger on all the jets, but to where? id think the 36mm venturi would remain as adequate? the car is going to be a street hotrod mostly, but may track it a couple times a year.

a friend I put a ton of faith in suggested what learned & called the "potter theory" for considering main jet sizes. he suggested venturi size X aux venturi size equals a good main jet jump off point. using this theory my 36mmX4.5mm would equal 162, or a 160 or 165 main jet. apparently potter's theory did not trickle down to AC, E tubes or idles because my friend's advice ended there..


I'm very interested in what the brain trusts recommendations are..* thank you,

t

I should have included this earlier. the manifolds are a bit of a mystery. I recalled them being PMO and still may be?? could this be a Richard Parr of PMO's early work? see photo-- the REP seems to be too coincidental.

http://forums.pelicanparts.com/uploa...1485647228.jpg

additionally, the bottom exit port of the manifold is a mere 35mm. is this going to hamper performance, or is this choking down to help with intake velocity?

sorry for the stupid questions, but I am clamoring to understand instead of just bolting them on like last time & getting lucky:D

Carb Tuning
 

Juan,

If you really want to dial them in - I highly recommend investing in a wideband.

I used one to help get my PMO's properly configured:http://forums.pelicanparts.com/911-engine-rebuilding-forum/821976-gordos-pmo-carb-tuning.html

I'm fairly certain those are PMO manifolds - I recommend you measure (or lookup) your intake port size on your heads, and see if the manifolds are the right size.

I believe the bottom opening of the PMO manifold should be the same size as the intake port opening on the head.

BTW - I was told that the PMO manifolds can be ported / ground (opened up) to some extent to mate to the intake port size (if needed).

Good luck,

Gordo

The cams will obviously flow quite a bit more in the upper rpm range which is where the air corrector makes a difference. As a starting point I would bump the main jet size to 150 and air corrector to 160. You can try a 38mm choke as well but I think in the end a 36 will serve you best. Try not to make such major jumps in jetting until more data is available. And absolutely tune it with a wideband or on a dyno. Preferably both

You will regret not matching the manifolds to the ports. I have the same manifolds and carbs on my just finished 3,0. I spent some time porting the manifolds to match my 39mm ports.

Or you can by new PMO manifolds that are correct for your motor.

I am running a GT2-102 cam, it's probably a little milder than yours.

I'm starting with 155 main jets and 36mm venturis.

According to Bruce Anderson, in his book, 155 is the correct starting point. He also suggests 60 or 65 Idle jets. Let us know if your 50s work.

What are you running for a distributor?

Hopefully I will have my first drive tomorrow.
http://forums.pelicanparts.com/uploa...1485743183.jpg

R.E.P. = Richard Parr

Yup those are PMO manifolds like Al said.

Completely agree that a good AFR gauge and/or a colortune tool are the best way to get you dialed in. Takes some time but it pays off.

Paul Abbott (1QuickS here on the forum) is a wealth of knowledge on carbs and he is a fan of the colortune. So it must work. I haven't used it yet but I do have one you're welcome to borrow.

Paul could certainly dial in the sizing of your carbs if you need to blend it all together. He does some nice modifications to them.

Performance Oriented

Click on the two tech headings on the main page and prepare to be blown away with technical knowledge.

There's some graphs and text in this post to compare against what you've calculated so far.

http://forums.pelicanparts.com/porsche-911-technical-forum/598453-urgent-need-help-setup-weber-46-911-3-2-engine.html

jleutjen shared a nice image to give you some starting points as well

http://forums.pelicanparts.com/porsche-911-technical-forum/708652-my-40-ida-carbs-slowing-me-down.html


I also have this collection of jets linked below you're welcome to use in dialing in your setup. I have PMO carbs (for an engine nowhere close to ever running) and the jets are different from Weber 40/46. PMOs use jets that are Weber 48 IDF. So I don't have a need for the Weber 40/46 stuff I have.

http://forums.pelicanparts.com/porsche-911-used-parts-sale-wanted/794775-weber-40-46-jets-emulsion-tubes.html

ive sourced some jets locally, but will likely need some additional options. here's what I have coming-- what I think I would like to have on hand in addition to this is designated (xxx)

air correction- 175 & 180's (feel I'm set here with the two next increased sized from where I sit now @ 170's)
idles- 60's (possibly a set of 55's to compliment the 50's in place. although Gordon trackrash above sems to indicate the 60's are a good option.)
mains-160's (would like to have a set of 150 & 155's on hand. the 160's may be a bit too big???)

considering the 36mm venture & F3 emulsions should be fit per my research here.

the exhaust will be a set of SSI's and a dansk 2 in 1 out. had the stock SC distributer recurved to allow for more advance and lost the vac advance. will be using a Mallory fuel pump with a PMO or other type return capable pressure regulator. ignition will be an MSD 6AL & blaster coil. I will be installing an on board AF meter/gauge prior to any start up.

what I'm trying to work toward is a decent start up base line to get the initial break in of the motor accomplished, then working to tune for performance after the break in. I will be having a shop help with the initial tuning/break in & then work on additional tuning myself if required. I want to have a handful of jet options in hand for this start up break in period.

the manifold opening up?? Gordon, when you state you opened your set up please explain. was this an effort by actually removing material from the lower side of the manifold? how far up the lower manifold surface did this reach? did you simply remove material with a drum type sanding fixture? I would think that it would be very important to have each port to be exactly the same in regards to material removed both at height with in the manifold & in diameter. how did you monitor this?

KT, looked over your jet selection and it might appear your mains are entirely too big for my needs, but thanks for you continued input both here & privately.

am I heading in the right direction?


t

Jet Reamer Set
 

Another option for jetting - I purchased a jet reamer kit, and used the same set of jets to dial mine in.

You can solder, drill and ream the openings to make them smaller, or just ream to make bigger.

Once dialed in, I bought a Weber set at the sizes that worked best.

I played around with a lot of configurations and would have had a major investment in jets if I had tried by buying / swapping each config.

Good luck - BTW, read Mr. Abbott's gouge. It's as good as it gets.

Gordo

I just drove my car for the first time today and the carbs were fine. Only had to adjust the idle screws and the car ran fine. I may try some 55 idles at some point, however my adjustment screws are about two turns out, so my 60s seem about right. Over all the motor runs fine, but obviously there is some tuning to be done.

Here are my tools. First you carefully open up the bottom of the manifolds, checking often to make sure they match the ports, without going bigger than the ports in the heads.
Then you continue enlarging the inside of the manifold, being careful to maintain the original shape and smooth curves inside. I used the disk on a bolt to check to make sure the size I wanted was maintained. Once to the size, I used the sandpaper to smooth the surface. I only went to 37.5 mm in the middle of the manifold. So the top where the carb bolts is 40mm, the bottom is 39, and the middle is 37.5. That will give me a slight venturi effect to help with the midrange. I found that this configuration gives good throttle response and midrange. I might be giving up a couple of HP at the top end however. I have the option of enlarging them up in the future if I feel that my top end is lacking.
Keep in mind you will need the special heat insulators designed for carbs to be used on an SC motor. Get those first before you begin.
http://forums.pelicanparts.com/uploa...1485842189.jpg

very good rash! happy to hear the monster roars.

after doing exhaustive research here & in the tech forum I found it was less concerning to have the head port bigger than the manifold port. not that it is something I'm less than interested in pursuing, but in my world id like a junk manifold to practice some on B4 I did for real;). I am leaning at this point to beveled heat isolators, but if my heads continue to be frozen in out of state I may have the time. anyone have an old manifold I could get my ream on with for practice?


when given three try's I usually nail !:D

t

I would run your current set-up for break in. Your current set-up will likely be a bit lean for your mission, but IMO it will be better for your engine. Too rich a mixture will wash the oil from the cylinders and dilute your oil. Both of these are bad when breaking in a new engine. Run 26 degrees or less on the advance until you get the carbs dialed in and you won't have to worry about detonation.

-Andy

hey andy! hope you & the mrs. are doing well. interesting take, so by retarding the timing some at the outset may help mask a lean condition some? i will be taking AF reading during the initial start up & 20-30 standing run. if i recognize a lean condition id prefer to swap in jets at that point, recheck the AF before taking it for an excel/decell ride. i was under the impression a motor being under load is going to increase a lean condition??

i guess I'm saying i don't want it fat or lean.... all good info I'm absorbing like a sponge. hope to have it running by your march event to share with y'all in the garages.. t