help with carburetor choice

Discussion in 'Classic Mustang Specific Tech' started by bdepedro, Apr 16, 2013.

  1. I've been having some carbonation issues and could use some help. The engine in my car is a modified 289. It's got 10.5:1 pistons, 351 heads, 0.10 cut from block deck and heads, MSD ignition, Edlebrock Performer intake manifold, and headers. I used to have a Holley 675 Street Avenger carb on it but the carb had been modified and ran very rich. I tried rejeting it but couldn't leak it out any. It was suggested to me that the carb was too big and I mistakenly (I think) switched to a 570 Street Avenger. The engine runs well under normal conditions but leans out when I get into it even fairly hard. I don't know enough about carbs (clearly) to solve this on my own so I'm hoping I can get your advice. I've been told by a guy that runs a local shop and races that the Street Avengers are good for stock engines and that I should use an HP carb for a modified engine. He suggested a 650. Someone else suggested that the mechanical secondary setup is not great for street use and a 750 vacuum secondary carb would be better. What do you think? How can I determine which would be best without wasting a bunch of $?
  2. Im no carby expert but have tried a few different carbies and found the Holley 650 with vacuum secondaries is a very well suited carby for a mild 289. Unless you have a very worked race type motor running high lift cam , & worked heads, the 650 Holley with vac secondaries will do the job perfectly.

  3. I haven't ever had good luck with holly in general, I always have great performance from autolite 4100's and edelbrock carbs that use a similar design. If you look on line for a cfm calculator that determines carb size by motor dimensions and rpm you may find that 650 may still be on the big side.

    The gt 350 used a 715 and those could spin to a much higher rpm than most people spin these things today.
  4. Just rejet the secondaries of the carb you have now. They will have a number stamped on them. Just go up one number and try it, if its still too lean, then go up another number.
  5. I've had really good luck with Holley's, and currently run that same 670 on my 351.
    In your post, you said "carbonation". Do you mean detonation, or carbon build up? Are you saying the plugs are really black?
    I believe the CFM is good for your engine, it's just jetted wrong. Also, do you think you have any fuel in the oil? Could there be a float issue as well?
  6. Sorry, that was supposed to be "carburation". I can't remember the exact lift and duration of my cam but in the "street/strip range. I've tried rejeting both primaries and secondaries. I'll have to check what's in there now when I get home. Horse power is estimated at around 350 but never dynoed. According to Holley guidelines in Summit and Jegs, a 570 is appropriate for my displacement but. 650/670 for that HP. I would be happy if I can work with the 570 but IDK. A friend has a 750 on his 347 w/ 425 HP.

    With the Old 670, it was getting enough fuel that it wasn't burning it all. Unburnt gas out exhaust and ran very rich. With 570, it pings badly and grey smoke when I get on throttle quickly.
  7. AED 650 HO
  8. Not sure what that is.
  9. Ive been daily driving carb cars for 10 years and been around this block a few times.

    At the end of the day if you aren't using a wideband with an o2 sensor in your exhaust near the flange, or at least a narrow band o2 with a voltmeter I think you are taking shots in the dark as far as A/F ratio. Getting a wideband can run $200 - but you are considering spending $3-400 on a new carb when you have already bought one.

    Either the 570 or 670 you already own would both work great if you put some time into tuning them (unless the 670 just has a problem as you are figuring). Do you have an in-line fuel pressure gauge or can you hook one up? If you started with a fuel pressure gauge and a wideband you could see where you are at and what is happening at different speeds. If you already have a wideband, and your 570 won't respond to richer jets in the secondaries as suggested above, you may need to go to a bigger accelerator pump squirter, or a power valve that activates at a higher vacuum reading. You didn't mention what cam you are running. If your engine pulls a lot of vacuum, and your street avenger shipped with a low power valve (like a 6.5), you may not be getting into the power valve quick enough.

    get rid of the holleys and get a real carb. you can play with a/f and changing jets/bleeds and even messing with metering and never get it right. I have seen cars pick up .4 in the 1/8 mile bolting on AED and other custom carbs over tuned stock HP holleys that were as good as they could get without buying different metering blocks, ect. I like AED because they happen to have carbs on the shelf at summit for under $550. the 650 HO is a bargin at $505
  11. for your application you need a carb in the 650-750 cfm range. edelbrock and holley make good ones for most engines. summit also has a good carb as does barry grant. the BG carbs are going to be a bit better than off the shelf holleys and edelbrocks though as BG tends to eliminate some of the foibles of the holley and edelbrock designed.
  12. So, rather than adjusting the timing to deal with the pre-ignition you have determined to throw more raw fuel at the issue?

    Run the smaller carb and tune the motor properly. Even a 351 only calls for a 680CFM @ 6700 RPM and much less CFM than that at 5300 RPM! (Do you know what happens at 5252 rpm?)

    Most people think bigger is better, we call those people wrong.
  13. if it goes dead lean with one carb and not another it isnt a timing issue. the thing i hate about holleys (even the HP series) is that the metering is never right. we ran a good 650 HP, a new 750 HP and a good 830 DP on a 351w with cleveland heads. it was fat on the secondary side with all of them. the car actually went quickest with the 830 when it had the back barrels unhooked. like that it went high 7.20s in the 1/8 (at 93 mph). with all 4 barrels hooked up it went 7.30s at 96 with the 830 and the two HPs. it didnt make a difference. put an AED 750 HO modified on it out of the box from summit and that car went [email protected]. with tuning it went 6.80s. all the rich stumble of the holleys was gone.

    venturi size depends on things other than ci and rpm. on a given cylinder the larger the intake, heads and cam are generally the smaller carb it will want. generally small heads, cam and intake will take a larger carb due to having better booster signal. with everyone elses calculations the cleveland above should have run best with a 650, yet ran great with a real 750. so a built carb can be big and work well.
  14. The HP was recommended to me because of the mechanical secondary linkage. The cam has .512 lift/292 duration and we're thinking that the current vacuum linkage is opening up the secondaries too late due to a lack of vacuum. I'm not sure how much vacuum the engine is producing or how much it takes to activate the linkage but I do know that I do have problems building enough vacuum for my power brakes. I'm not sure how this plays into the carb size equation as the vacuum secondaries on the 670 didn't seem to be an issue but do on the 570. (?)
  15. AED 650 HO. you will not believe the difference. depending on how mild the cam is it might work great with a 750 HO, which is also mech secondary. there is too much to this stuff to go into in a single post, but it is not about the cfm rating. its venturi size and metering, carb signal from the cam and intake/port cross section. even if you ran a vac secondary and played with the springs, you still have all the other problems that holleys have. if you need a choked carb quickfuel has some.