Engine specs are in sig...

I currently have a 6.5" PV and want to try a 10.5" but there's a single stage standard flow, single stage high flow, and a dual stage that's specifically for 4160 model carbs, which is what I have. Which do I want?

 

Single stage standard flow.

How did you decide on the 10.5?

 

I borrowed an Edelbrock air/fuel ratio meter from work and what it told me was if I'm just cruising at say 50mph with my foot off the gas I'm a tad rich but as soon as I give it a little gas it goes way lean and will stay there until I push the gas further at which point I can feel the power kick in which I think is the secondaries opening. I hooked up a vac gage a couple weeks ago and I can't remember exactly what it read now but I think at cruise it was 21" and would drop to about 10" under load. I also sort of suspect my primary jets but right now I have 67s in there and it's rich enough that I can smell fuel in the exhaust. The other possibility is that I have a blockage somewhere. I just rebuilt the carb and maybe I dropped a piece of old gasket somewhere.

 

Check the vacuum under "various cruise conditions", is Holley's recommendation. Pick a PV below the lowest observed cruise vacuum.

Vacuum at 55 mph cruise could be much higher than at 40 mph cruise, or vice versa. A PV opening at too high a vacuum is a receipe for surging.

 

50mph with your foot off the gas--so you are testing during decel? If you are light/moderate throttle lean, try bigger primary jets. Some 70's. Nothing in your sig. looks like you would need anything but the usual 6.5 powervalve.

 

I had 68 primaries when I first started tuning and with those it would almost instantly foul the plugs out til they were pitch black. Maybe I need to tear this thing down again cuz something's not adding up. I'll get the vac gage on it again and will report back with what it is exactly.

 

67's and 68's are close enough that I can't imagine how 68's would foul a plug and 67's wouldn't--everything else being equal. Dig in deeper, I don't think a powervalve swap is going to help. How is the ignition system?

 

Well I got a vac gage on it. 17" at idle, at ~50mph with my foot off the gas it's at about 21" and according to the AF meter it goes lean at 15" and the power valve kicks in right at about 6.5" which is what's in there now. So, I ordered a 10.5" PV which is the highest they make. Damn thing was $20! It should be in tomorrow, we'll see what happens with that. I also ordered a pump cam kit which was another $30 for five pieces of plastic. Ridiculous.

 

well did the 10.5 make any difference?

 

It didn't really help too much. It still has the bog at part throttle, it just comes out of it a little earlier due to the PV. I'm at a loss right now lol.

 

Im having some of the same problems. I hope to get it fixed soon.

 

try a larger squirter, worked for me. Some tuning of the pump cam may help as well

Eric

 

Eric, do you mean the accelerator pump squirter? Changing the pump cam was also on my list but I'm skeptical to this being the problem. My rationale is that the accelerator pump only provides a boost while the throttle is in motion but the problem I'm experiencing is continuous at about 10% throttle so the pump would only fix it for the first few seconds. In other words, if I hold the throttle at 10% then the engine will stutter as long as I hold the throttle in that position. It almost feels like the problem is in the primary jetting as the stutter starts at such a light amount of throttle but I know the primaries aren't blocked because changing jets will affect the color of my plugs.