sdfsd

Hello I recently bought an Oldsmobile Delta 88 1980. I has been originally a diesel but someone has swapped olds 307 engine there. Engine is from late 80s Caprice Classic. My engine has Edelbrock 1405 carb in it and vacuum operated HEI distributor.


My car had terrific fuel economy, 9mpg hwy. I checked float level and adjusted it according to manual. That helped a bit but fuel economy is still bad.

I installed an narrow band oxygen sensor into my exhaust into place where it has originally been. I plugged volt-meter into oxygen sensor.

I as getting 50-80mV when driving 50mph or slower. If I drive 55mph or faster, it gives me 900mV-950mV.

First I thought I had wrong kind of step up spring in my carb and my carb was running on power mode all the time. I tried different ones and I even tried it without any springs. Still it run rich.

Next I plugged fuel pressure meter into fuel line. Fuel pressure was little high, 5.8PSI. But it was still under 6.5 PSI which is max for this carb according to Edelbrock websites.

Next I lowered fuel level a bit. Everything else went more lean but still in speeds faster than 55mph, my car ran very rich.

I pretty lost right now, what to measure/adjust next? Could my high fuel pressure be problem?

Smitty275

Check everything else. Ignition timing, EGR valve, plugs, cap/rotor/wires, etc.

sdfsd

I have checked all basic things. My car doesn't have EGR. My ignition doesn't have any problems, timing is good and previous owner has changed cap and rotor.

joe_padavano

I will admit that you've done most of the things I would have suggested. Did you install the carb? Was it new or used? Bottom line is, do you know what the secondary jetting is? How about air valve adjustment?

sdfsd

Some previous owner has installed the carb. I don't know has it been new or used, it looks pretty clean and it still has Edelbrock sticker init so there is a possibility it has been new.

I have disconnected secondaries, only primaries work now. I did this because I wanted to see if my problems has something to do with secodaries. Disconnectin secondaries didn't help.

I haven't adjusted air valves and I have done anything to secondary jetting.

I have changed needles in primaries. Jets are stuck in there so I don't change them if it's not necessary.

One of my friend said this possibly has something to do with main well bleeds which are non-changeable. Has anyone drilled them to get more leaner A/F ratio on higher rpm cruising?

M-14

Tell use your timing numbers, initial, mechanical, & vacuum advance. You can run up around 50 degrees btdc (at cruise) for best fuel economy.

olds 307 and 403

I would throw that carb in the bush, your mileage is half what it should be. Get either a Qjet from a late 70's 350 rebuilt or get a 625 Street Demon. Set your base timing at 20, if it already isn't there.

sdfsd

Initial advance is 20, max vacuum advance is 28 degrees, and mechanical advance is 11 degrees at 1400; 14 degrees at 1600.

Now I have disconnected vacuum from vacuum advance because it's not adjustable and gives way too much advance. My advance is now in mid 30s when cruising.

I forgot to say I don't have EGR in my car. That affect's my spark advance too.

I think I'll get a vacuum can with 10 to 12 degrees vacuum advance.


I'll try with this carb first and if it doesn't start to work, I will get Q-jet.

M-14

Well vacuum advance does help mpg, you can try limiting the vacuum advance with a little metal tab. I never had much luck with those adjustable vacuum advance canisters.

olds 307 and 403

I have ran 50 at idle and around 60 total at cruise on low compression Olds V8's, like your 307. They like a lot of timing.