Weber tuning help, please

[Guest 12lbsofbeans]

I have a 78 with a fairly built F54/E88 combo 10.5:1 compression .495 lift 282/272 duration cam, head is ported, larger valves, MSD ignition, timing is at 16 degrees initial - goes to 36 all in by 2400 rpm. Fuel is 93 octane 3.5psi seems to hold steady under load. Runs real strong with triple webers.

 

Carbs are 40 DCOE's

36mm chokes

50F6 idles

140 mains

55 pumps with .05 return valve

195 air correctors

F16 emulsion tubes

 

With this setup it runs strong, pulls smooth and consistent to 7400 rpm, very little if any hesitation. Low end feels soft, once it hits 3800 rpm it takes off. Its been somewhat down on power though since I moved 2500 ft higher elevation. I was told to run smaller chokes to increase air velocity through the carbs.

 

I tried running 34mm chokes with the exact same jets as listed above; It runs and idles fine, revs fast and strong with no load but take it out on the road and it bogs bad. If you barely give it gas it pulls to probably 3000 rpm then wants to be shifted, if you give it gas it just goes BLAH until you let off then it comes back to life. I thought I'd have to jet leaner with the smaller chokes. But going to 135 mains didn't help seemed to bog worse, so I went to a 40 pump same problem.

 

Well after reading some I swapped in 170 air correctors and this let it rev a little farther before it wanted to be shifted maybe 4500 rpm. Still bogs when you go over 1/ 2 throttle suddenly.

 

I advanced timing from 10 to 16, with the 36 mm chokes it was lean low/mid range but advancing the timing lead to detonation off idle, increasing the idle jet didn't help this is why I was told to switch to smaller chokes - air velocity at lower rpms. Anyway I can run more timing without pinging with the 34's and it feels smoother/stronger I just can't get rid of the bog.

 

Right now its running pretty good. It pulls off idle really smooth as long as you don't give it too much gas. If you floor it, you get a bog then it takes off and pulls harder than before to 7000 rpm. If your rolling partial throttle and try to floor it, it bogs and unless you feather the gas or downshift you have to let off, even if you downshift it bogs then goes this is around 3800 rpm. Under heavy load like climbing hills it starts stumbling feels like its running out of gas. Taking off full throttle its quick and pulls real hard but when you shift at the bottom of the next gear there is a slight bog. The problem seems to be in the transition from partial to full throttle.

 

This is what I'm running right now

34mm chokes

50F6 idles

140 mains

40 pumps with .05 return valve

170 air correctors

F11 emulsion tubes

 

Anyone out their with 34mm chokes on a L28 mind telling me what jets your running or maybe point me through my confusion? I thought I'd have to lean it out from the 36's but it seems like the 34's need alot more gas. I'm confused

 

Thanks

[Lockjaw]

I believe I ran 140 mains on my 40's with 33 mm chokes, and about a 190 or 195 AC. I found my car ran much better with the F11 emulsion tube, the F16 was to fat to suit me.

 

I wonder if you are getting any black smoke from the tail pipe when it bogs. Usually a bog means you are lean, but also when lean, you should hear the carb spit too.

 

If you have another set of pump jets, I would try bumping up to the next size. I would think that you should not have to make a radical change to move up in altitude, maybe increasing the size of the AC, or dropping the main jet one size.

 

One thing that will really help you nail down your problems is and air/fuel mixture guage, or whatever they call them. Plumb in an O2 sensor, hook up the gauge and go for a drive. It should be able to tell you immediately where and what your issue is, and then you can take steps to address it. If weber parts are still as expensive as when I had them, the gauge is money well spent.