Help tuning carb 870 Street Avenger on 468

[Yellow73SB]

468, ~9.6:1, 049 2.19/1.88 ported, 231*/236*, .591/.615, old torker manifold, 1.75 long tube headers

I have an 870 vacuum secondaries Street Avenger right now and it really doesn't run that good

A few questions to start off

When I just the idle mixture screws it doesn't do anything

Also after I rev it it does not go back to idle quickly it takes a long time for it to eventually hit idle

I have a few other questions but I want to solve these two first

I seems like a vacuum leak almost, but I don't have a clue

 

[MYBAD79]

sounds like a leak, it seems to be sucking air somewhere.... try spraying carb cleaner around the carb base and intake.
Do you have the calibration kit ? Try larger jets....or check what's currently installed first.... what color are the plugs ???

 

[TimAT]

I'll go with a vacuum leak too. Try choking the carb with your hand- if it smooths out or really doesn't change then it's a leak for sure.

If the base idle is too high (throttle plates open too far) then the idle mix won't have much effect because it's off the transition slots and into main metering.

 

[mrvette]

I dunno jack about Holley, BUT, every damn carb I seen has some strange things going on when mounted on some intake mani....they no two ever seem to line up right on gasket sealing....dunno why it's gotta be so much of a freeking mystery meat, but it is....

then make sure you not sucking any air on the bottom of the intake passages, ie that the plane of the heads as installed agrees with the intake mani flange plane....you maybe sucking oil/air up from the crankcase area....

don't ask.....:suicide:

 

[Yellow73SB]

sounds like a leak, it seems to be sucking air somewhere.... try spraying carb cleaner around the carb base and intake.
Do you have the calibration kit ? Try larger jets....or check what's currently installed first.... what color are the plugs ???

I may have to try the carb cleaner, or ether. I don't have a calibration kit

Currently it has 75 and I think 80s in the back with 6.5 power valves on both metering blocks. And a 30 or 35 squirter. These things need to changed also but I need to solve these problems first

I'll go with a vacuum leak too. Try choking the carb with your hand- if it smooths out or really doesn't change then it's a leak for sure.

If the base idle is too high (throttle plates open too far) then the idle mix won't have much effect because it's off the transition slots and into main metering.

I did try choking it as another person suggested it and it did die down.

I think that may be my problem, my dad took this off his big block which ran like crap and he adjusted and messed with everything.

I think that possibly the rear butterflys aren't open enough so I have to over compensate with the front ones

I'll take the carb of adjust those

Another thing is that I took the electric choke off, I just screwed a screw into the hole, maybe I should just epoxy it to make sure it's completely sealed

I dunno jack about Holley, BUT, every damn carb I seen has some strange things going on when mounted on some intake mani....they no two ever seem to line up right on gasket sealing....dunno why it's gotta be so much of a freeking mystery meat, but it is....

then make sure you not sucking any air on the bottom of the intake passages, ie that the plane of the heads as installed agrees with the intake mani flange plane....you maybe sucking oil/air up from the crankcase area....

don't ask.....:suicide:

I don't know jack either :lol:

I sure hope the intake gaskets aren't leaking

 

[MYBAD79]

If you're not sure if it's the intake gasket or the carb you're pretty much shooting in the dark. Do you have another carb that you know is fine ? Maybe borrow one ???

Pulling the intake, cleaning and re-sealing is no fun...

 

[Yellow73SB]

I'm fairley certain they aren't I had a small 600 on it for a short time and it ran fine

 

[TimAT]

Let's go back to the basic stuff- Idle mix screws lightly seated, then back out 1-1/2 turns. Look at both the primary and secondary throttle plates- make sure they're on the bottom of the transition slots. The slot should look like a square when the plates are correctly positioned. Check float levels. Hook a vacuum gauge to full vacuum and see what it reads at idle. Use that number to determine the correct power valve.

Not coming back to idle kind of sounds like the throttle plates aren't coming back real quick- return spring stiff enough and no binding in the linkage anyplace?

When you look for a vacuum leak, check carefully around the throttle shafts- too much return spring can wear the shafts/carb base plate.

See if you can prove the secondary plates are staying closed until throttle position and engine demand (load) ask for them to open. If there's an internal vacuum leak thats getting to the diaphragm, it may be opening the secondaries when you don't want or need them.

 

[Yellow73SB]

All fixed

Put on a Holley 3310, runs great

Starts without pumping the throttle