Best jetting for webers for 78 308 GTB

This happened to me, but not sure if it was for the same reason.
Just my unproven opinion, but I think that sometimes the engine compartment is not as well vented as it should be, and as it heats up and there are some issues with vapors in the fuel system. When it gets warm, be sure you have an operating continuous loop fuel system (i.e. has a return line to the tank), the tank is on the full side and venting properly. Since I opened up my starboard vent to cool the engine compartment, changed my fuel lines and re-vented my tank with new hoses, I have not had a problem.
Good luck

 

It could very well be rich. Higher ambient temps mean less dense air - less oxygen per unit volume of atmosphere - while your carbs are delivering the same amt of fuel per unit volume. Since I live in an area of the country where driving weather occurs year round, I feel and hear the difference weather can make on driveablility. I've considered rejetting my car a bit leaner in the summer and returning it to it's original jets in the winter.

 

I took a look at my jets last night, looked nice and clean, fresh O-rings on the idle jets, but it's no wonder I'm not entirely satisfied with how it runs....

idle jets - 60
main - 130
a/c - 190

 

I have 60 Idle, I believe 135 main, and I think 200 a/c. It's a 79, and runs great. I also have original ignition.

If you're having a flat spot, it might be your ignition advance sticking and not necessarily your jetting.

 

A little off topic but I'm curious...Would there but any significant change in gas mileage between 55 and 60 idle jets??? I'm getting 13-15 mi/gal.

 

yes.

 

I, too, have original ignition. I don't have a flat spot but I get an occasional cough through the carbs (the carb that coughs is random) at mid throttle, mid rpms going up or coming down through the rpm range. I'm thinking it's due to the main being a bit lean, or my floats being off. There is the Memphis Exotic Italian Car Show this weekend so I won't jump into the carbs again for a while. Certainly wish I had A/C in my garage!

 

Just bought a 75 euro GT/4. Everything is stock and its smog exempt. Would the recommended jetting be diffrent for a euro? There is a flat spot at mid throttle. Gosh its been years since i played around with Webers!
Thanks for any help.
Dave

 

If your ignition is working well, go up one size on your main jets and try again.

 

Yuk.
first would try:
idles: 55
main 135

then:
idle 53
main 140
(my bet this will be good)

 

and keep the 190 A/C ?

 

yes - safe place to start

 

Can someone please help me out here. When you go up in size on the idle jets say .55 to .60 you increase the amount of fuel available to the idle circuit. Next you adjust the A/F screws to the ultimate mixture so then both size jets would have the same A/F ratio...yes/no??? What's the difference? Also does changing the idle jets size affect the idle rpms or the ability to get a correct idle speed??? (idle down).

One more Q...I had .55 and 1.25 and a flat spot in transition. Changed to .60 and 1.35 and flat spot went away but somewhat rich. After reading this post I changed the idle jet back to .55 and the flat spot came back. Will changing the main to 1.40 and keeping the .55 idle jet correct this?? Just need some encouragement as a 308 requires removing the rear bonnet and air cleaner to access the jets. No easy task when working alone. Thanks.

 

While you are setting the idle (throttles closed) mixture with the idle set screws, the idle jets are also dynamically feeding the transition circuit as well, which is a component of your flat spot. To Wit: the flat spot is the lean area where the transition circuit is going away and the main circuit hasn't fully come in. Within a window, you will be typically be able to achieve idle speed within a range of jets.

Yes...probably. What you are doing is balancing how the main and transition circuits overlap. One way to think about it is you have gone as rich as practical on the idle jets, "raising" the rpm level it will support, and with the larger main you are "reaching down the rpm band" where the main jet comes in covering and lean swap-over flat spot. Every car is a wee bit different and unpredictable -- it does sound like you are doing great and making all the right changes for the right reasons (as I understand them).

Have fun and good luck!!!!!!!!!!! You are becoming a master of a lost art!