427 Side Oiler Carburator

[mrmustang]

Originally Posted by BAsque1

Good morning folks:
Merry Christmas and Happy New Year to all and Happy Holidays for the upcoming feasts!!
I have a question regarding carburation for my 427 Side oiler, my car came with a Holley 770 CFM carb and I have been having issues with it. Rather than try to rebuild and go through frustrating trials and errors I decided to replace the carb with a new one of exactly the same specs; 770 CFM. I have been told after the fact that a Holley 650 CFM would be enough for I am not racing this engine. I have no idea what carb came from the factory with these engines, van anyone shed some light on the matter??
Your input is always appreciated
Regards
Lou

Lou,

Depending on your specs, a 600-650cfm double pumper, 4 corner idle circuit carburetor is recommended.....For when it comes to fuel consumption, bigger is not always better. Chassis (not engine) dyno does not lie


Perhaps post some specs on your engine and we can go from there.


Bill S

[blykins]

You can think of carb sizing in extremes.

A very small carburetor would offer great throttle response but would be really down on power. The engine would be able to create a great signal but the carb itself would be a restriction.

A very large carburetor would have horrible throttle response and you would find yourself adding jet to try and compensate for the lack of signal. It would probably make more horsepower/torque, but it would be at the top of the rpm range.

Your goal for a street car is to hit the middle of that spec. Great throttle response with the ability to feed the engine it's on.

[BAsque1]

Originally Posted by mrmustang

Lou,

Depending on your specs, a 600-650cfm double pumper, 4 corner idle circuit carburetor is recommended.....For when it comes to fuel consumption, bigger is not always better. Chassis (not engine) dyno does not lie


Perhaps post some specs on your engine and we can go from there.


Bill S

Bill, unfortunately I don't have the specs other than it is a 427 Side Oiler, I know it has a 770 Holley Street avenger, and that is what I purchased to replace the old one, the rebuilding was quoted at about $300.00 the new one is $367.00 so that was a no brainer.

The present carb is too rich and I have not been able to adjust it to a leaner mixture--no effect when we turn the screws. the bowls were checked and that is fine, but I am still getting a very rich mixture.

My gas mileage went down from 11-12 to about 8 MPG, not that this will break me but, it tells me that there is something wrong. I removed the electric gas pump and left the OEM mechanical so I get between 5- 5.5 lbs as it should vs 9 lb that I was getting before.

I use very good gas BP 93 octane or Shell 93. I used a bottle of Techron a while back without much change, the in line gas filter is new and now I installed a new Fram filter when I removed the electric gas pump.

Before it gets to normal temp (180 F ) the mixture seems very rich. The manual choke is working fine and the secondaries are also working fine. Long story short I decided that I would change it.
Regards
Lou

[saltshaker]

I have a 428 w/750dp... Carb has been worked on and I get 15mpg....my 428 was chassi dyno'd @411rwhp @6k rpm
Dyno's wont tell you how it will respond on the street. A good carb tunner can get rid of a hesitation! My vote is a 750dp....
Jon

[DanEC]

I'm not sure a Holley Street Avenger is overall a good solution considering some of the issues you mentioned. Assuming you have a fairly healthy cam in the engine - and your comment that turning the idle mixture screws did not affect it - the primary throttle blade is probably opened too far at idle to allow the idle circuit to work properly.

At hot, curb idle the primary throttle blade should rest in the middle of the little transfer slots in the side wall of the bore. That creates the vacuum signal that pulls fuel through the idle passages. If blade rest at the top of the slot or above it, it loses it's vacuum signal and basically you have no control over the idle mixture.

A possibly work around is to look at the underside of the base plate and see if there is a small adjustment screw that's a stop for the secondaries to close. Usually Holley has one to avoid the secondaries from closing completely and possibly sticking in the bore. That screw can be adjusted to open the rest position of the secondaries slightly to pass a bit more air at idle. That will allow closing the primaries down at curb idle so the blades rest in the proper position. You should get a lean roll off at hot idle when turning the mixture screw all the way in and some rich effect when turning it way out (fall off in rpm, engine vacuum, rich smell).

But the idle mixture circuit only works at closed throttle. As soon as the throttle is opened the transfer cirucuit takes over and in most Holleys (and I'm pretty sure the Street Avenger) it isn't adjustable. It's controlled by air bleeds and while it can be modified through trial and error process, it's not a user-friendly process. That is probably where your car is running rich and a new carb is unlikely to do anything about that. That is where going to something like a Quick Fuel Holley-pattern carb becomes a good idea - because the transfer system is tunable. Holley may have some newer models in the higher price range that are now tunable - I don't know.

After the transition circuit, as the throttle opens further the carb picks up the load through the primary metering system and it's tunable by the primary side jets. But it's usually that idle and transition circuit that create a lot of the richness issues that everyone struggles with.

Originally Posted by BAsque1

Bill, unfortunately I don't have the specs other than it is a 427 Side Oiler, I know it has a 770 Holley Street avenger, and that is what I purchased to replace the old one, the rebuilding was quoted at about $300.00 the new one is $367.00 so that was a no brainer.

The present carb is too rich and I have not been able to adjust it to a leaner mixture--no effect when we turn the screws. the bowls were checked and that is fine, but I am still getting a very rich mixture.

My gas mileage went down from 11-12 to about 8 MPG, not that this will break me but, it tells me that there is something wrong. I removed the electric gas pump and left the OEM mechanical so I get between 5- 5.5 lbs as it should vs 9 lb that I was getting before.

I use very good gas BP 93 octane or Shell 93. I used a bottle of Techron a while back without much change, the in line gas filter is new and now I installed a new Fram filter when I removed the electric gas pump.

Before it gets to normal temp (180 F ) the mixture seems very rich. The manual choke is working fine and the secondaries are also working fine. Long story short I decided that I would change it.
Regards
Lou