Rebuilt RS spec jetting issues
 

I had my 2.7 rebuild finished today.We went with the Bruce Anderson book specs for a RS type motor ,Mahle Euro RS 90mm P&C's with E-cams and ssi's and sport exhaust and 40 IDA webers.The book says to jet this carb with 34 venturi's,F3 tubes,135 main jets,145 air correction and thats what we went with.I can't find any other source that recommends air corrections that small.All of the posts I have looked at and charts I have seen all call for 180 air correction.The engine only has a 100 miles on it so far so this may be premature but here goes.Idle is quite rough like a idle jet is plugged,transition is okay and running on the main circuit seems good although I have not gone past 5000rpm yet.Could the small air correction be causing this or should I check filters ect.The carbs were balanced ,floats checked ,acc pump set up ect.I have cleaned idle jets twice already just to be sure but nothing was found.http://forums.pelicanparts.com/uploa...1225161879.jpg

My 2.7L RS Spec w/40IDAs has 55 idles, F26 emulsions, 145 mains and 180 air corrections. I have S Cams not E Cams. Can't remember the venturis but I think they are 36s. The intakes were ported to 36mm. Not sure if that means anything to your application.

Good transition, pulls like a freight train once over 3500 revs all the way to 7 grand. The only issue I have is some gurgling/popping on decel in the 2000-3000 rpm range.

Funny that there are carburetor specs for a 2.7 RS;)

My 2.7 .... RS p/c...intake ports opened to 36mm with tall intake manifolds, 40IDA with 34 mm chokes...55 idles...f3 tubes...150 mains...180 air correction...solex cams with MSD ignition and Magncor wires.

Starts right up hot or cold, no spiting or popping has lots of low end and explosive power in the upper RPM range. I idle at 1100RPM......

I have almost this same setup with the exception of 60 idles and 34mm venturis and it runs fantastic. Even gets 22mpg at 80mph cruising.

Solex cams, 10.5:1, 90 mm Mahles.....race gas...prior to that, 9.7:1 Shastas & pump primo.
I use Anderson's specs with the inclusion of 60 idles which he doesn't supply.

I also run the MSD set-up. After 6 months of fouled plugs due to the rich running, the MDS cured it.....that was 8 years back. Tho rich (& safe), it is sweet.....except for the black smoke.:D ......It's best not to be back there when I clear its throat. .....http://forums.pelicanparts.com/uploa...1225209741.jpg

My question is do you think the air correction numbers are right or maybe they were a misprint in the book.All other info I can get point to 180 airs and everything else is pretty close.

I think that has to be a misprint as it's the only source that has that spec. Mine was way too rich at 3k rpm cruise with the 140s. I used the 180s and it cured it immediately.

Pretty easy to try the 180's and see!

about how many turns of idle a/f mix screws?

do all idle mix screws respond?

is there more than a 1/4 turn difference between most and least?

Do you know how much the syncrometer is reading?

Did you blow air through all idle passages?



needless to ask but have you checked all spark plugs to make sure none are loose?

Seems to be all in spec,I didn't do the rebuild myself but I have checked and cleaned idle jets.The car pulls fine above 3000,but below and especially in transition from idle circuit to main circuit it does not seem happy.

so you have an idle problem, a running problem up to 3k rpm and a transition problem?

Try 180 airs and see if that works. I have nearly the same setup and use 57 idles, F28, 150 mains and 180 airs. My LC1 shows its spot on 11.8-12.5 all the way up.

Idle is poor after warming up and after some load has been put on motor like going uphill,transition rpm's like from 2500-2900 are not good some missing and farting.Full acceleration is good but have not taken it over 5000 due to it just being broken in.

do you have an infared temperture gun to see what exhaust tube gets noticablely colder when the idle miss happens.

Getting a new motor to run good is a challenge...or it is for me, particularly a hot rod motor. It took me months to get my 2.7L to an acceptable state and I had a 18 months of prior Weber experience.

The toughest part to figure out is weather you have a carb problem or an ignition problem.

Carbs:
You either have to learn the Weber dance or take it to someone that knows exactly what they are doing, then watch them closely. There is no "in spec" with a hot rod. There are adjustments which lead to the best compromise.
The 2 most frequent problem areas are float settings (there is a spec here) and synchronizing side to side....this is a challenge as it is all in proper adjustment.
When you are sure that these are right you get to....
Ignition:
The sad fact is carbs are stupid. For proper response and piston health they will run rich. Too rich for an "in spec" ignition system. They will cause rough running till your hotrod cams come on the bump. The motor will run herkey jerkey at low rpms and plugs will foul before their time...making for a dead cylinder & then you think you have a plugged idle jet.
Time for a band-aid.
You need the "burn through" capability of a high energy ignition system.
An MSD set-up will supply this. This consists of the following.
The MSD unit...like a 6AL....but pick one.
A high voltage coil...MSD's hi vibration coil seems to work. It will live in the stock mount.
Well insulated wires....8.5mm Magnecores are good.
The proper heat range plug gapped to .045-.050.

This is my .02 on the subject.....formed by experience & input from guys that know better than I.

Thanks for the reply from all,I took the car back to the mechanic that rebuilt the motor and he wasn't VERY receptive to the idea that the jets could be wrong.He did change plugs,they were totally black and #1 was fouled and changed air correction back to 180's but no new balance job.This has changed how the car behaves,there is a slight flat spot from 1500 to 2000 but no big deal but the carbs do sputter a bit at 3200 now instead of 2800 or so.I am now thinking my mains at 135 may be a little lean.He also changed plugs to Bosch 9's that are very hot.I think I will check balance tomorrow and then just drive the car until it is broken in and then fuss with the carbs.I have 300 miles since rebuild,what would be considered a good mileage to consider car broken in.

that sounds like great news to me. It's a case of new carb&eng'itus.


I ran the hottest NGK BPR5ES during initial dialing in. Sometimes I'd clean the plugs 3x/week. Gotta have somewhat clean plugs for dialing in. I kept all idle A/F screws even going down from rich to lean.

When you get that far where you're not fouling plugs every tank of gas you have found step 1.

I don't have any experience with Webers except owning a set of IDA & IDS
But I believe how lean you can go will depend on transistion.

The specs are probably a good starting point, but the chances of getting it spot- on, in custom built motor after motor, are slim. Most will need adjustment of some sort. I think there are too many variables that can make a standard jetting chart anything more than a good starting point. Just another 2 cents

If idle is crappy, look at the idle circuit (idle jets and passages, idle mixture setting, balance between carb throats).

How were the floats set? Did your wrench measure it statically (mechanical measurement) or dynamically (engine running)? The latter is more accurate. However, this would mostly affect mid-range and high speed performance.

Don't use hot plugs to compensate for an over-rich symptom.

You should not need a high-energy ignition box at this point to "fix" these symptoms. The factory engines w/Webers did fine without.

Sherwood

I had the webers on before the rebuild and they ran great so I don't believe the ignition is not up to snuff.It is just fine tuning,they are not running terrible or anything just not perfect the way they were before.The only real changes to the motor are the euro pistons and the cams,hence the change in jetting.

To add to the confusion. I just had my 2.8 built by Ralph Meaney, well know to the NW guys. He dyno the engine in the car, and this is what he set it up with. No gurgles, flat spots, NO issues.
AC - 160
Main - 150
Venturi - 36mm
Idle - 60
Emul. - F7

The dyno shows it goes a little lean from 5800 to 6500. I am going to step it up to 155 mains.

It dyno's at rear wheels, max, 229 HP @ 7,000 rpm. Max torque 196 @ 5800 rpm.

5800 rpm is 218 HP

PMO got back to me and suggested starting with 145 mains and 180 airs or stay with my 135 mains and go to 170 airs.I will try the 145 mains to start with and go from there.

I have the exact engine with different carbs. These carbs are stupid in a way. There is only 3 simple circuits. What you do with each circuit is dialing in.

my engine pulls pretty good from 4k-5.5k in 5th gear with 135 mains & 190 air correctors. Can't say above 5.5k for any distance.

at 3.8-4k rpm is where I see the mains taking over EGTs. My EGT's when fully on the mains is great.

hot plugs is not to substitute for a rich mix. It's step 1. Forget about acceleration. Pick different rpms and gears and hold steady looking for lean pops. If not it's rich. I currently use NGK BPR8ES and plugs stay clean. It's easier to clean hot plugs occasionally that a colder plug 5x as much while you're finding out how lean you can go on the idle A/F mix. I read Sherwood's post about the A/F mix screws responding to very small adjustments. He's right and I can prove it with EGT temps.

all above crap is a prejudicial rant

The idle is now good,around 950rpm,a tiny flat spot from 2000-3000 and then at 3500 seems to come onto the cam.The only spitting and popping I have is below 2000 in a higher gear than I should be and light cruising around 2900 or so.Just the slighest push on the gas pedal eliminates all popping so it really is more when on the decel or under no load cruise.I have turned the mixture screws out 1/8 turn on all barrels and will try that.It is pouring rain right now so I may be chasing my tail with the drastic change in atmospheric pressure.I don't have the 145 mains to try out yet so that will have to wait.

the 2000-3000 flat spot is when accelerating?

"popping around 2900" is with a full 10 mile warmed up engine with very clean spark plugs at a steady throttle?

Does the popping sound like a group of cylinders going lean or just a single pop every few seconds? If it's a single pop every few seconds it's probably a single or 2 cylinders going lean, not necessarily all cylinders. What I did on that single pop every few seconds is track down which cylinder was going lean by turning only one cylinder out 1/2 turn. If it didn't stop that pop I returned that 1/2 turn rich to the all equal turns baseline and try another cylinder, etc till I found which was the culprit. Once I found the lean cylinder I reduced the 1/2 turn rich to 1/4 rich and looked for it to pop or not. Etc.

My #4 cylinder is the only one richer than all the other equal turns. Once I completed that exercise I changed the hot NGK BPR5ES to new BPR7ES and reduced my new baseline turns another 1/8 turn. At that point I had another lean pop occasionally. At that point I went to a colder BPR8ES. The colder plug eliminated the pop. My BPR8ES runs clean.

Generally going from one NGK heat range to the next is about 100 degrees F change in tip temperature.

above may not be exactly what happened to me but it's a good explain.

After the idle mixes were set I went after the acceleration circuit.

Combustion heat is power. I sought as much heat as possible with nice looking spark plugs.

Brent

Here is how I have mine setup.


34mm venturies
60 Idle jets
145 Main
180 Air Corrector
F3 Tubes
BP6ES Plugs

You can read more here:
http://forums.pelicanparts.com/porsche-911-technical-forum/348641-weber-idle-jet-question-you-weber-experts-2.html

Left this morning with no popping at all even cold.There still is a very flat spot from 2-3k's,probably just a different feel from my old ciis pistons and cams.I will leave this setup until motor totally broken in and then will try to dial it in perfectly.

I tuned my accelerator pump flows by adjusting flow lean till it fell flat when driving then slightly adjusted more flow till the flat spot was gone.

If acceleration flow is rich you cool the EGTs noticeably and it'll bog, wait a second and then zoom away.. Better is max power high EGTs so it's an even max acceleration.

again.. just my routine