Carb running rich

[Paul_G]

No they dont. He already determined that the PV was not blown out. He also determined that he had to open the throttle plates in to the transition circuit to get a high enough idle. At that point the carb is getting out of the idle circuit in to the transition circuit.

 

[Paul_G]

Study a power valve, it does not allow fuel into the vacuum chamber. That’s the situation you would have if you removed it. The float bowl would empty into the vacuum chamber and into the engine

The engine was idling around 900 RPM in the transition circuit. Thats high enough to be pulling fuel through the main circuits I would think. Ether way the power valve needs to be changed to a smaller number valve.

 

[68 HEMI GTS]

If you look at a metering block you will see the PVC restrictions lead to the same main wells as your main jets. Only thing connecting the idle and transition circuit to your main wells is the IFR. So unless your boosters are feeding fuel at idle or off idle, (not good or normal Holley operation) the PV will have no effect on the idle or off idle AF ratio. Providing it’s not leaking into that vacuum chamber. And soon as the engine revs up a little bit what happens? Vacuum comes up above the Pv rating (unless your hard on the throttle). I actually run 6.5 PV on my 511 that has 5.5-6” vacuum at idle. Runs super clean at low rpm operation because as stated the PV has no effect on the idle transition circuit. I like the higher number PV to provide faster enrichment as I’m going toward WOT.

This dominator has a 6.5 PV, it’s idling below that vacuum rating. I’ve logged a lot of miles on this car with several different carbs I’ve built and a A/F meter over the last few years. I can tell you I’ve never had the PV vacuum rating being around the same as the idle vacuum to ever be an issue. And I’ve also learned a Ton over the last few years. I’ve built and tried stuff purely for the quest of knowledge. See my latest thread where I’m working on some mechanical six pack carbs. I’m not baseing this on what I’ve read but, rather what I’ve learned and or tried.

 

[68 HEMI GTS]

The engine was idling around 900 RPM in the transition circuit. Thats high enough to be pulling fuel through the main circuits I would think. Ether way the power valve needs to be changed to a smaller number valve.

If your mains are on at 900 RPM you have issues. You can look at the carb and see if that’s happening. It’s flooding out if that’s the case.

 

[Paul_G]

If you look at a metering block you will see the PVC restrictions lead to the same main wells as your main jets. Only thing connecting the idle and transition circuit to your main wells is the IFR. So unless your boosters are feeding fuel at idle or off idle, (not good or normal Holley operation) the PV will have no effect on the idle or off idle AF ratio. Providing it’s not leaking into that vacuum chamber. And soon as the engine revs up a little bit what happens? Vacuum comes up above the Pv rating (unless your hard on the throttle). I actually run 6.5 PV on my 511 that has 5.5-6” vacuum at idle. Runs super clean at low rpm operation because as stated the PV has no effect on the idle transition circuit. I like the higher number PV to provide faster enrichment as I’m going toward WOT.

I will concede that you are right about the PV and Main jets feeding fuel together through the main circuit. I will also say that my comment about the PV dribbling fuel was inacurate. The PV will not effect idle, but will be open as soon as the throttle is opened. Not the best situation for a street car. You want the main jets to be solely supplying fuel during cruise. In his situatuion with a 6.5 PV it will be supplying fuel all the time along with the main jets. Not a good situation for a street car, causing the engine to be running pig rich at cruise.

 

[Paul_G]

Let me define cruise. Understand that at cruise the engine is basically on the idle circuit. Light acceleration will transition to the main circuit. We drive in and out of the main circuit and idle circuits on the street. You want the main jets supplying that fuel at light load and a moderate AF ratio for economy. As vacuum drops, at more engine load, more open throttle, we want the PV to assist the main jets with more fuel.

It is much harder to tune a high performance engine to run well on the street than it is to run a high performance engine tuned for the track to be at idle or WOT with no in between.

 

[68 HEMI GTS]

How many carbs have you built? How many race engines have you tuned? You don’t appear to really understand what your taking about. His carb is obviously screwed up but, it’s not because of the PV rating. His car surly cruises down the road at above 6.5” of vacuum. If it doesn’t it has serious issues. I’m speaking from My own experience. My dominator cruses down the road at 14.5:1 with that darn 6.5” PV. That’s not by mistake.

 

[Paul_G]

I have been keeping junk running on the street for 40 years. I am not a racer, nor an engine builder. But I am pretty good at keeping a Holley in pretty good tune on the street using a vacuum gauge and an AF ratio gauge. I tune for good economy and good performance, on the street.

 

[dcala84141]

So what was the final outcome? I was having similar issues with my quick fuel 830 diamond series carb.I couldn’t get the engine to idle on the idle circuit and at cruising speed about 2,800 rpm 355 gears in 4th my air fuel guage reading was 11.3 and plugs were black.I have since changed my IFR from .037 to .035 and main jets from 76 to 71 also went to a hotter plugs my AFG now reads 13.8 at idle and 13.5 at cruising speed. I still have a ways to go but I feel i’m heading in the rite direction