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Discussion Starter · #1 ·
My ZZ 502 came with a Holley 870 Street Avenger with vacuum secondaries.


The motor runs slightly rich and the secondaries show up occasionally at wot . I was thinking of downsizing to the Holley 850 Double Pumper for a better mixture and performance.


This is a street car and so far economy has been reasonable, driving around on the 2 barrel so I would probably adjust the secondaries to come in late.


Any feedback would be great. Thanks
 

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You would not be downsizing by going to an 850 double pumper. The 870 vs 850 rating is really a marketing thing. The 870 is basically a vacuum secondary version of the 850. The major difference is that the 850 will be calibrated richer. Also if you have a heavy car with an automatic trans, numerically low rear gear, and or low stall speed converter, a double pumper may not be the best choice.

Have you tried tuning what you have?
 

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If you are using a double pumper for the performance then an 850 or even 950 is a good choice. What type of transmission you use or gear ratio you have does not affect the carb size,,,
 

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Discussion Starter · #4 ·
ZZ 502 Carb

I have a replica 34 Ford, with 300 gear 9 inch ford and 6 speed manual Tremec.

basically all I have done with the carb was put in the lightest spring for the vacuum pot. That really did not make a difference. The secondaries rarely open, but when they do I can feel the difference.

i have just tried a set of ngk plugs, but haven't driven enough to see if they would burn cleaner than the ac delco.

There is an article about drilling out the vacuum port to the pot and putting a tube through to the venturi, but I would rather have mechanical secondaries.
 

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in first gear when you stomp the throttle the tires should break loose and try to catch on fire. The street avenger carbs are not very popular as out of the box they dont work well. After you change everything they are ok. I worked on one 770 all day and eventually gave up. Replaced it with an 850 "HP" series . Not saying your 870 is not worth working on, just expect to spend time on it,,,,,
 

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Discussion Starter · #6 ·
ZZ 502 Carbuerator

So after talking to Holley & AED who assured me that the 750 DP was good for up to 600hsp. Torn between that and the forums that said it would be too small I decided on the AED 750 DP. I purchased trough Summit, knowing if it didn't work right I could send it back. The AED is basically a Holley reworked by AED using their own metering blocks, jetted and flow tested to their own specs. Out of the box it started right up and it seemed to idle fine, but I ended up turning in the idle screws in a bit to reach max vacuum. So they are 1 turn out now.


The 870 Avenger had been running rich and vac. secondaries rarely opened, when they did it was partially.


I live on a 45 mph Island and we have had a lot of rain. So I have only been able to make a single test drive in a week, I have to pick low traffic time also. The gas pedal seems a bit touchier, so I have to get used to it. I was able to run it up to 5000 rpm in 1st and second with the 300 gear puts me about 70mph so it doesn't seem to be lacking for cfm. Also chirped the tires when I hit second, the 870 Avenger didn't do that. When it cooled down I pulled the plugs and the black soot was gone.


So I need to put some miles on it and will try to see how the new carb does all around and if my mpg changes one way or the other, and will update this post. So far I'm happy. FYI, the AED Holley was a few dollars cheaper than the Holley.
 
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