Rejetting carbs

I am sure at least part of their reasoning for using 36mm venturi was because they knew the car was going to be driven pretty much flat out the entire time in upper rpms. In those days they did have means of determing a/f ratios and dynos, but I do not know how accurate. Ferrari did pretty well back then so one can assume they were at least somewhat accurate

 

plug cuts.

drive into the pits flat out, cut the motor and coast to a stop...pull the plugs and check.

 

Corsa 308

I think that we have exactly the same engine and similar problems. I have a 1976 dry sump 308 engine with Borgo high (er) comp pistons, P6 cams, and some mild head work (don't know if it is good or bad-done by an old school Ferrari guy). Had the engine running in my 79 GTS for 5 years now and have never gotten it to run satisfactorily due to a low rpm stumble off idle to 2500 rpm, ie from a standing start. Pulling away from a stop light or stop sign. Not trying to drag race just want to be able to pull out in traffic and have the car go instead of stumbling. I have tried everything including the 36mm (ran like a dog) 34mm (better) and 32mm (best but not good enough) venturis. From what I have read and personal experience, I would not use anything more than the 32mm venturis as two respected sources (Carobu Engineering and Pierce Manifolds) state that the 32mm venturis will support all of the HP that these 3 liter engines can make unless the displacement has been increased or the heads flow substantially better than stock.

Even with the 32 mm venturis I had the stumble and for a while was covering it up with 22-25* static timing and 40* overall. Never had any detonation at 40* but did not get rid of the stumble entirely and I was never comfortable with that much advance in the engine. Made it a bear to get it to run below 1000 rpm at idle, among other things. Was able to do this because I have the Electromotive ignition in the car which allows one to adjust the spark advance curve at three points: idle, 3000rpm and then retard it until it hits redline. Adjustable redline set at 7500 rpm.

So, on a last ditch try before I tore into the engine and started replacing expensive and hard to get to parts (cams, higher comp pistons etc) I decided to try one last thing. A tuner had put F25 (not F24) Emulsion tubes (replaced the stock F36 ET's which are junk) in it for me years back and it got marginally better but still had the problem. Some time ago, I had been told by someone at Pierce that there was not that much difference between F24 and F25 and it probably was not worth trying them. As a last try, I ordered a set of F24 emulsion tubes ($16 each from Top End Performance, Pierce is around that amount also) and put them in yesterday. A REVELATION!!!!. These F24's are the hot ticket for this engine. Noticed a substantial difference and low end stumble is much better but not totally eliminated. Pulled the plugs and replaced them with new NGK plugs BP6ES gapped at .035 (electromotive ignition needs this bigger gap) . With bigger cams and higher comp pistons, everyone seems to want to overjet these engines, and the engines run very rich as a result. Right now the car runs great with 32 mm venturis, 45 idles, 125 mains, F24 ET, 145 pumps, 180 AirCorrectors. I have 9* static timing and 35* at 3000 rpm dropping down as the engine runs to redline. The plugs have a greyish white residue on them and the porcelain (white part) is clean all the way to the bottom. Never had that happen on a 308 before, even on the stock engine that came with the car. I might be a bit lean but it is now time for an Air Fuel meter to see if this is true. I have always in the past had mostly black plugs with just mostly chocolate brown on the porcelain. Bottom line, the F24 ETs are amazing. Throw out your F36 as they are junk with these cams, I am very surprised that you can run the car at all down low with F36 and 34mm venturis. The P6 cams seem to be able to make any combination of jetting work at high rpm, it is just down low where there is a problem. Am now hopeful that I can get this thing really running right without tearing it apart. I will keep you posted on how the AF ratio turns out and would be happy to hear how things go with your engine. Hope this helps.


Don

 

great post! looking forward to your a/f numbers. still running 40 advance 6k+?

 

Don although you are running 180 air correctors,I still think that the idles are a bit to small which will also be the cause of the stumble at transition.The A/F meter will tell you whats what though

 

a 45 idle sounds small. i have 50 idles and already have to wind out the mixture screws 4 turns compared to less than 2 with 55s. i'm amazed it runs!

 

I agree that a 45 idle is too small for a hot cam; Would expect at least 53-55 and maybe bigger.

Also a good time to remind some of the newer carb guys that emulsion tubes have no rhyme or relation to each other based on their name, e.g., an F24 is not even close to an F25.

 

With all the usual caveats about 'opinions', based on others experience and mine, my suggestion as a starting point for P-6 cams, regardless of CR, NO head mods:

32 mm venturi for street with stock heads; 34mm if you have head work

55 idle, 140 F24 190

This should be a **safe** and workable jetting to lean from.

<edit>
wrt accell pumps, as long as you have the std black mega-mongo pump cam it just doesn't seem to matter much what accell jet you have

 

I have all those jets and will try them tomorrow. I tried 40 idles with 135 mains but this was worse than 45 with 125 mains. I put the 125 mains back in and have 47 idles set up for another round of testing tomorrow. When I put the F24's in I had 50 idles and it ran OK tried 45 idles and it ran better so I tried smaller idles. 40s were too small. So, let's see how it runs tomorrow. After years of trying I have a pretty complete range of all the jets. The F24's were definitely game changers!

I will advise of progress tomorrow evening.


Don

 

All good stuff, thanks guys.
I sent those Dyno charts to Mike @ Pierce and had a chat with him late last week, but he hadn't had time to review the graphs in depth.
He did have a quick look however and was very intrigued with the figures. In particular the horsepower curve.
He thought it looked nearly too perfect. Just keeps climbing and climbing and doesn't seem to be affected by the richness unlike the torque which does get affected.
Hopefully next week he will give some thought to it.
What I reckon I will do, working on the theory of not changing everything is change the ET's and see what that does.
It will probably involve changing the mains and/or the A/C as well.
I will leave the 36mm venturi's in there for the moment as changing them is more expensive and they seem to of been in this car for a while and in theory for a reason.
It's always seemed strange that with these Venturi's, mains and P6 cams etc that apart from the stumble @ 2600 the car does run like a dream (AF ratio aside).
It does start and idle beautifully and winds out like a rocket. In reviewing some of my GPS data from the track, I am driving the car in between 4,000 and 8,000.
So a 2600 stumble doen't matter to me. Keeping in mind that I just want it to go well on the track and if it is cranky getting to and from, so be it.
A friend has suggested that a four gas analyse would be more enlightening and to watch out for the placement of the gas sensor and make sure it is well up the tail pipe, I wish I had've been more knowledgeable on the day of the dyno but so be it.
I will ask the Dyno guy tomorrow if he has some four gas graphs of my dyno, plus I wish I had more figures of AF ratio at particular sustained revs and not just an accelerate.
So, I will talk more with Pierce and most likely source some F24's from them as a starting point.

Don, good to see that you reckon the F24's are looking good for you.
My plugs have always been "black as" also.

ps, I did check all my jets etc yesterday and they all are the same and as I expected.

 

Don, from your info above am I right that you say you have your advance @ 35 degrees @ 3,000, but decrease it (less advance) as you go towards redline?
If so, what are the figures, and why do you do this?
From my reckoning you always had to have a lot of advance up high just to combat the speed that all the interanls are running to make sure the charge gets fired at the right time.

Steve

 

what did you use to measure them?

 

Firstly I just wanted to check that they were all the same, or at least had all the same markings.
They did.
Although I reckon you are suggesting that doesn't mean someone hasn't drilled them out in the past. I agree.
As far as that goes I compared the ET's with pix people have posted here and they looked the same although that doesn't mean the holes weren't larger than they should be. I used the shank of a 1.5mm drill to see if it would go into the main jet holes. It didn't so i assumed that they hadn't been drilled out past 1.45.

Steve

 

i just found some mains marked as 135 that were actually 140 and previously some 125s SOME of which were drilled to 130. all still with the original markings on them....so pays to check. just buy a jet gauge....it's worth it if you're going to muck around with this stuff.

 

Digging up an old thread because there is some GREAT info here. Hope some of the more knowledgeable of you are still around to comment...

So I started pulling my carbs apart. Started with checking the float heights. No surprise the two I have always had issues getting adjusted properly were both off by several mm's. Not much but probably enough to make a difference.

I have complete rebuild kits and planned on replacing the float valves while there. The new ones were quite a bit different though. They were considerably taller, were marked 200 (versus 175 existing), and had holes in the body where the existing ones don't.

Fired the car up after finishing the floats and it did help some. Still have more tweaking to do though.

 

Those float valves are actually calibrated. I have never steered from using anything but the stock size, but if your car is going fine then more power to you. Glad you're getting hands dirty

 

I didn't put the new ones in. They were quite a bit different and would have required pretty large adjustments of the tabs on the floats to make it work.

 

I bought an exhaust gas analyzer but had to order a lighter socket adapter. In the meantime I noticed the front and rear carbs don't seem to be synched up very well. That's tonights project. Will start from scratch and get them all set properly.

That would probably explain the large stumble I had just off idle!! More fun and work to follow!!

 

I am having a similar problem. car starts easily and idles smooth, but stumbles just off idle from a standing start and also when using the throttle. Once it get over the stumble it performs well at all rpms....

 

Was everything OK, and then this "problem" suddenly appeared, or has it been an issue for a long time (like ever since the stock US 308 ignition retarding-at-idle system was removed or the ignition system was replaced with something else)?

 

Could be something is interfering with flow if everything was fine before.

 

Perhaps your throttle plates are too closed at idle causing a transitional condition. Try a simple test where you add 1/4 turn at a time to the throttle stop speed screw on the carbs. You don't want to over do it or raise the idle too much but I had the exact same problem and 1/2 turn to the speed screw still kept me under 1000 rpms at idle and totally cured a small stumble I was having when pulling away.

 

This is my ground up restoration 74 Euro car. Was working perfectly. I have been playing in the band last 2 years, finally getting around to working on the two GT4's
Thank you for your suggestions.

 

What is the difference by going up one main jet size vs going down one AC size?

I'm currently using 140/220 ET24 and the car runs to rich at WOT. Going to try 130/220 or 130/210.

 

As a rule of thumb (not exact but approximate)

1 air corrector size change = 1/2 of a jet change

Also a LARGER air corrector delivers LESS fuel so you would use a larger air corrector if you want to lean the mixture.