Porsche 2.7 Weber Jetting

[cveres]

Trying to jet my 2.7 motor after a refresh. Its been some time since I had it sorted out and running well. I cant find my old notes. Its a 2 hour old 2.7 motor 10.5 comp. web cam 120 / 104s with 40ida 36 intake ports PMO manifolds, headers and 2 - 2 chamber mufflers, MSD box with the distributor @ 30deg total after 3000rpm @ about 2050 lbs total weight - I can't recall what I was running in the way of main jets so I started at 150 on the mains leaving the following in place: f3 tubes, 185 Air Correction and 55 idle jets with 36 mm vents / Chokes installed. It idled nice at 900 rpm but sputtered after 5K. After rechecking everything I started carb tuning - I have worked my way up to the largest jets I have - 200s and its clean now to 7500 rpm and the plugs are still very clean (bosch range 8 conventional)

This seems like a lot of jet based on what I have read lately - does this sound right at all ? Does anyone have a similar set up? Based on the plugs and I feel I could still go higher. I am reading the plugs after a few minute drive back to my house at no more than 3000 rpm.

[KTL]

I think if you're needing to go that high on the main jets then you need to rethink your air correctors. Downsize your air correctors and you shouldn't need to use so much main jet. Looking at Paul Abbott's page, his 2.7 race spec listing near the bottom of the page is something in the neighborhood of what you've got

Performance Oriented

[roblav]

People seem to endlessly debate these sorts of things... last time I ran a high performance 2.7 (9:1 compression w/ S cams) I used 36 degrees advance and 34mm venturi.

[cveres]

I need to get out a magnifying glass and read that Air correction jet # 185 is huge based on that chart.

[cveres]

I correct myself - I see the id schedule of the numbers above the chart. Maybe not too far off

[zedsn]

I have used 34 and 36 venturi's on my 2.7 RS spec motor with PMO's along with web cams and didn't notice any difference. Wish I had my notes. I sold my car and the new owner got all my imfo.

[eastbay]

You did not mention a wideband, so assume you don't have one. Seems crazy in this day and age to read plugs and guess when widebands are so cheap and informative.

[Jack Stands]

My 2.7 with RS pistons and S cams ended up with these jets on the 40IDA carbs: 34 ventures, 155 mains, 70 idle, F26 emulsion tubes, 180 air correctors, and 150 idle bleeds. Timing is 33BTDC at 3000rpm.

[zedsn]

Nothing wrong with reading plugs in my opinion.

[juanbenae]

Originally Posted by eastbay

You did not mention a wideband, so assume you don't have one. Seems crazy in this day and age to read plugs and guess when widebands are so cheap and informative.

+1 if you have a bung in either or both headers it's foolish not to have one of the AEM gauges on board if your tuning carbs sans dyno. sure, reading plugs is good in telling how it's running cylinder to cylinder, but plug can change from moment to moment under varying running conditions so they can fib some. I have bungs in both SSI's and go from bank to bank, left to right every four or five months to check in on that three......



I have nearly the same set up. 9.5-1, early 3.0L, the same 120/104 cams, 40mm webers with 36mm venturi. ive fooled around with a ton of jetting combinations. ended up with 160 mains that were consistently best through out the set up process. 60 idles after going up and down with varying AC efforts. ive gone from 170 to 185 in increments of five and am currently running 175air correction.

thing I found with the webcam grind we've chosen that it likes to run rich on the idle circuit no mater what you do. with F3 ETs it ran rich across the mains too on pump gas no matter what jets I implemented. I went to n F8 ET and it leaned me out across the entire range. it's still high 10,s low 11,s air fuel per my gauge at idle, spikes a bit lean for a second if I get on the mains too quickly, and is mid to high 12,s n freeway and high rpm.

im surprised with thw 10.5 CR a twin plug was not mandatory on pump gas.

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[Trackrash]

Seariously, the AEM AF gauge is the best investment I ever made. With the gas nowadays reading plugs will tell you little. You could be rich on the idles and lean on the mains and your plugs will look gray. AHIK.

One more data point. With my 40 webers, I'm running 36mm vents, 160 mains, 170 airs, F3 imulsions, and 65 idles (120 idle airs). Under power from 4 - 6 k rpm I see 12.5 - 1 AFM. See my garage for motor details.

[juanbenae]

Originally Posted by Trackrash

Seariously, the AEM AF gauge is the best investment I ever made. With the gas nowadays reading plugs will tell you little. You could be rich on the idles and lean on the mains and your plugs will look gray. AHIK.

One more data point. With my 40 webers, I'm running 36mm vents, 160 mains, 170 airs, F3 imulsions, and 65 idles (120 idle airs). Under power from 4 - 6 k rpm I see 12.5 - 1 AFM. See my garage for motor details.

what are your cams like mr January? im mr july!!


I find with CA gas from winter to summer grades, yes there is such a thing.... makes a significant difference in how my carbed motor runs from season to season. im going to be moving from sea level to 4000 feet in the next couple years and im wondering if carbs will be serving me best in the thinner air.??


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[Trackrash]

My motor is a '78 SC with JE pistons running 9.8-1 compression and Dougherty custom GT2-102 cams. IOW GT2 cams with only a 102 deg lobe center. I chose these cams since I want response NOW and good midrange torque for autoXing the car. Good power from ~3500 rpm to over 6500. Max RPM 7500.

Yea, the gas is crap. The problem is when the carbs get heat soaked after sitting for a while after shut off, they will go lean during idle upon restarting. Seems worse the hotter the weather, but not sure how the different gas formulation works into that.

[1QuickS]

Trackrash: I assume PMO "Anti-percolation" modification has been performed? If so then this partially explains your hard, hot re-starts.

Jack Stands: F26 emulsion tubes want larger main jets than F3s do which helps explain the large main jets for your application.

Juanbenae: Adjusting idle air bleeds (smaller) will allow tailoring progression/transition to use smaller idle jets while maintaining mixture strength for transition.

A 2.7 with compression & cams like yours would typically want 36mm venturis, 155 to 160 mains, F3 emulsions, 170 air corrections & 55 idles.

Double check your emulsion tubes are clean, fuel pressure is 3 to 4 psi, float levels have been set with the car on level ground and with engine running and finally check that side-to-side air flow balance is checked at 3000 RPM; have a helper hold throttles open or use a turnbuckle rod to replace the long link from bell crank @ #3 to throttle cross bar to hold throttles open.

Invest in AFR sensor to verify seat of pants performance and jetting.

I apologize for suggesting this since I do not know the symptoms of detonation vs. ability to rev: If compression is 10.5:1 and you are using pump gas with a single plug ignition then possibly you are richening the mixture with large main jets to overcome detonation issues.

Bottom line is best described by John Passini: "Suck it and see." You can postulate jetting but give the engine what makes it happy.

[cveres]

Thanks all for the input. I have a auto meter AF gauge and a bung in the collector (disconnected now). 15 Years ago it seemed like a novelty toy. Have the sensors or gauges gotten better ? I have some 175 Air correction jets and will start there (vs the 185s in now). Last night the sputtering after 5500 rpm returned - I have and MSD with New Bosch Coil (Brazilian) and non resistor rotor. I have the MSD High vibration coil on order. This motor worked so well for so many years! The only change is an aluminum case. The mag case in spite of being shuffle pinned and time serted moved around quite a bit. Stumped for now on this sputtering.

[Jack Stands]

Thanks for your suggestions, Paul.

[1QuickS]

Jack Stands: Just a comment regarding your jetting. All the other posts in this thread offered jetting with F3 E-tubes so I thought your jetting showing large mains with 34mm venturis needed a little explanation.

cveres: So, your engine ran fine as configured before rebuilding with aluminum case? Cams & displacement & compression ratio & exhaust & ignition & carburetion & fuel delivery as before rebuild? If not then what changed?

[Trackrash]

Originally Posted by 1QuickS

Trackrash: I assume PMO "Anti-percolation" modification has been performed? If so then this partially explains your hard, hot re-starts.
.

Yes and no. I had originally done that mod. However one of my carbs, at #1, was leaking out upon shutdown. There was no peculation, just gas dribbling out of the throttle plate and fouling the plug upon restart.

I enlarged the extra holes that I had drilled there, with no improvement. I then plugged those extra holes completely, but only on the left carburetor. None of that helped. No difference between carbs.

What helped is lowering the float levels slightly to the very bottom of the lower line on the glass vile. I have not noticed any fuel leaking or fouled plugs since.

The other thing I did was go to 65 idle jets. Funny it doesn't run any richer with those jets, but runs much smoother.

The problem I was describing isn't hard starting. It will start up just fine. Problem is it won't idle, without my foot on the throttle. The AF on my gauge will read 15 or 16 -1. After a few minutes of driving things return to normal with the AF around 12 -1 and a smooth idle.

The carbs get heat soaked after shut off and that just causes them to run lean until they cool off from the air being drawn through them.

Yes, I have the insulators under the manifolds.

[1QuickS]

Trackrash: With longer studs you can install two or more insulators. I recall Grady Clay used a three insulator stack on his track car.

Your 120 idle air bleeds offset the large idle jets is my guess.

If percolation isn't causing fuel to drain onto throttle plates then there is a siphon issue that is implied. Idle air bleeds emulsify the fuel delivered to the idle/progression circuit. Upon engine shut-down the idle air bleeds allow atmospheric air to enter this fuel supply circuit and break the siphon action that would otherwise occur. Fuel siphoned in this method will hit the top of the closed throttle valve & drain out through the throttle shaft bushings.

Another fuel path into the cylinder is via percolation passing up the emulsion tube well.

I think you may want to investigate the cleanliness of your galleries feeding the idle jets. If dirty/corroded then they will inhibit fuel flow to the idle jets, regardless of its size. I have seen this in my work and now I routinely open & clean these out. I developed a method for using compressed air to isolate these galleries to forcefully blow through them but this has proven ineffective compared to mechanical cleaning. Also, some internal restriction may explain the siphoning although I am not sure. Regardless, the cleaning of the galleries is a good thing since these carbs are quite old.

[juanbenae]

Originally Posted by 1QuickS

Juanbenae: Adjusting idle air bleeds (smaller) will allow tailoring progression/transition to use smaller idle jets while maintaining mixture strength for transition.

Bottom line is best described by John Passini: "Suck it and see."

the idle air bleeds are the only factor ive not experimented with to this point. can I find what size are currently in place by removing one and looking for numbering like other jets?

ive also checked volume of the accelerator pump squirt and I am only getting around .4cc which is lower than the prescribed .55 summer or .65cc winter spec per some Porsche literature that was shared with me at my local shop. ive got the rebuild kits but have yet to pull the carbs to change the diaphragms out.

jerry woods is fond of saying "suck it and see" when you are asking for free advise and he's not in the mood to share it... jerry likes my pops better than me.