Jetting for a 308/GT4 in New England

Hi all

Have read through a bunch of threads on here re jetting my 1975 GT4 and talked to a few folks...I think I have a plan.

My jetting is stock other than idle jet where I went from 55 to 65 (perhaps too much but I had a real dead spot around 3500 that I was told a bigger idle jet would help address).

Hesitation in that transition area is gone but now once the car is at temperature, I’m having much worse hesitation and bogging to the point of wanting to stall while on the idle jet - particularly from a full stop until around 3-4k when suddenly I get on the main and take off...needless to say, makes getting across intersections rather harrowing! Am considering going back to the 55 for idle and rest as follows:

130 main
F 36 tubes
200 AC
55 idle
Set timing @ 10*

Anyone else running a setup like this?

I’m in NYC so ethanol free isn’t an option and I typically go with 93 (also read that 91 could be preferable if anyone has an opinion there?)

Also imagine many of you are already familiar with this write up but for those who aren’t, I really learned a lot here:
http://dino246blog.blogspot.com/2014/08/carb-tuning-1-weber-dcnf-carbs-and.html?m=1
And this is great:
http://www.teglerizer.com/dcoe/inglese/inglesetuning.htm

Thanks

 

I just went threw a similar situation with my 308 GTB. Lots of hesitation low in the rpm range especially when trying to accelerate from a full stop. Every time I went anywhere near the accelerator I would also get a series of sneezes. I am now running 140 main jets, 185 air correctors, (200 made the situation worse, leaned it out too much), 70 idle jets, F36 emulsion tubes. The sneezes have completely gone away and the transition is smooth from idles to main jets. I am still getting a slight stumble from a full stop but nothing terrible. The floats were also adjusted, all four were at different setting.

 

Wow 70 for the idle jets seems big! I thought I had gone too high w 65. How do your plugs look?

 

I have only driven the car twice since these adjustments were made. So far so good! Oh, installed new extenders which helped as well.

 

70 idles is really huge, what's going on there!? I've set up a few early cam engines and they really like a 140 main with between 190-200 air corrector. Start with 140 mains see how she goes. Additionally, make sure you're getting full ignition timing at 6000rpm, should be about 37-38 degrees all-in.

 

Super helpful re timing thank you.

Any idea what you typically did for idles with the140 mains?

I had 135 mains, 220 air correctors and 55 idles before swapping the 65 idles...didn’t note the emulsion tubes but folks don’t seem to mention them so not sure how important they are or what priority they have. Wondering if maybe best to go to 55 idles and the 140 mains like you suggested before I go nuts w AC and emulsion tubes?

Like I said, with my old setup acceleration would kind of fall flat at around 3500. Was told a larger idle would help with the transition to the main jet and it worked to the extent that it pulls strong all the way through but now once warm, hesitation is at low rpms and when pulling away from a full stop.

thanks again

 

Not really sure, however, it would constantly sneeze just sitting there idling. While driving, every time you touched the gas pedal it would result in a flurry of sneezes. New extenders, adjusted floats and 70 idles and no more sneezing.

 

There's a chart "on here" with OEM jetting and these guys posting have good knowledge of updates for modern fuel.

I would also add in that ignition advance is HUGE.....did I say really important?....HUGE.
So a calibration and lube of both distributors is actually a 15k service on these cars.

 

A larger main brings the main circuit in a bit earlier, even more so with a small air corrector as well. I have always landed at no larger than a 60 idle jet, but as we can see above, 4right had to go considerably larger although that is the only circumstance I have heard of that. I'm wondering if he has DCNFs with different progression holes, as the US spec, Euro spec, and general retail spec all had slightly different idle circuit progression holes.

 

Erol- Good thread, link below is a good f-chat 308 Carb source (Post #4 has the chart BigTex mentions, and #12 is my compilation of good jetting threads):
https://www.ferrarichat.com/forum/threads/euro-carbs-conversion.634518/#post-147648091

I recently rebuilt and rejetted my 1980 Euro GTB carbs, and will be installing them in the coming two weeks, FWIW, I'm running 100LL AVGAS. Looking forward to road testing and reporting back.
This is the jetting:
210 Air Correction
F36 Emulsion Tubes
135 Main Jets
55 Idle Jets

 

I am running high compression pistons in a 76 Euro 308. Not sure if that plays a role in needing 70 idle jets to get the engine running correctly. The pistons are Daytona Comp equivalent, the one pictured on the left.

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I also found this link to be useful in describing the old-school method of idle mixture setting:
http://www.redlineweber.com/html/Tech/carburetor_set_up_and_lean_best_.htm#:~:text=The%20first%20thing%20to%20do,and%20begin%20to%20smooth%20out.

 

I'd be looking to put an O2 bung in each bank and connect a wide-band O2 sensor in each side. This is the only way you rare going to know where the lean/rich spots are through the RPM range for any given Main, ET and Air Correction. I'm not running stock cams and I have high-flow intake vales. I was really surprised to find a big lean spot between carb circuits.

 

Still having some issues. Did you ever manage to do this?

 

My understanding is that extra ethanol is what they (intentionally) add to raise the octane in 93 AKI gasoline. If you have stock 8.8:1 compression, I don't see why you need such high octane fuel. I believe it's also possible that a low octane fuel will have a lot of ethanol added in order to bring it up to 87 AKI -- so no guarantee that 87 AKI has less ethanol (but 93 won't usually have less):

https://www.rexenergy.com/ethanol-in-87-89-91-93-octane-gas/#:~:text=Yes.%2093%20octane%20does%20have%20more%20ethanol%20than,value%20of%20112%20when%20combined%20with%20premium%20gasoline.

"Yes. 93 octane does have more ethanol than 87. There is a common misunderstanding that ethanol is present in lower amounts in higher octane gas. In reality, it contains more. Although pure ethanol has a minimum octane rating of 100, it behaves as though it has a value of 112 when combined with premium gasoline."

 

Ethanol is added for EPA reasons. It is an oxygenate. Because it has a higher octane rating the refiners can use a lower octane base gasoline and use the ethanol to raise the octane. There are parts of the country with 93 octane with ethanol and other places with 93 octane without.

 

True, but I bet not in NYC .

 

Erol, there are 3 gas stations around us that have 91 octane ethanol free gas. That 35 gallon tank in my garage is fuel. You need to get a fuel caddy for your garage.

 

@Erol Any new symptoms, or does the GT4 still have a dead spot around 3,500 rpm? It may help to know if it's a Euro or USA model, factory or modified (e.g. exhaust, cams, etc.) Looking forward to the update.

 

ethanol free and high octane gas (sunoco branded) is available in Huntington, Long Island at Auto Tech - about 50ish minutes from NYC depending on LIE traffic...Good Ferrari tech there too:

116 SPRING ROAD HUNTINGTON, NY 11743
(631) 923-1675
https://autotechli.com/

 

I guess I'm with newto308 on this one.

Some thoughts on tuning carb cars. Data is king.

If possible, collect RPM, A/F data (Lambda is best as is compensates for different gas/alcohol blends whereas it is more complicated using A/F ratios and their different stoichiometric ratios) and also get absolute pressure data (tells you what is happening with the throttle vs time and plot all the data on an scatter chart in Excel. This data will tell you where any difficulties might be and give you a powerful tool to remedy them and measure the change.

You need to establish a baseline idle mixture such that all carbs have the same acceptable mixture. An initial condition if you will. It's important that all idle mixtures start out the same as this anchors the bottom end of the transition zone. The end of the transition zone happens with a successful hand off to the main circuit. I fabricated stainless sniffer tubes to sample the front (5-8) cylinder exhaust gas. I plan to set each idle mixture using a simple CO meter. The rear bank (1-4) can use removable sample pipes. Alternatively, a Colortune does a pretty good job on the 1-4 bank but really difficult use on the front bank. A Colortune won't tell you what Lamda or A/F ratio you have. It will give a burn color that can be compared between cylinders. Bunsen blue is getting very close to stochiometric, the yellow/blue boundary is a slightly rich condition and yellow is getting rich. In the not to distant future I'd like to corollate Colortune color to Lambda.

I synch up my carbs using a couple of 4 bank motorcycle manometers, so one column per cylinder. I repurposed the charcoal canister vacuum ports to be vacuum taps using small hose barbs. I cap them or gang them together using 3 mm silicone line and T-fittings for other purposes. When plumbed together, it provides an excellent, average pressure signal, that is fast responding, yet steady. The other thing with a multi-bank manometer is that it immediately identifies any misfiring cylinder as it's easy to see the rise and fall of the offending tube and visually compere it with the other cylinders. This device greatly speeds the balancing process. I hang it from the deck lid or other nearby support. Within each group of 4 columns, grouped for the 1-4 and 5-8 banks, the tubes are arranged to reflect the engine layout.

A good tune requires that the engine be in good order and that prerequisite systems like spark plug wires, ignition timing and valve lash setting are in good order.