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  #1  
Old Oct 16th, 10, 08:51 PM
Microgiant Microgiant is offline
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Default Lean backfire Holley Street Avenger 870 vs 489 BBC

Im trying to tune a carb for my first time. Im using a Innovative Wideband O2 meter to help. On acceleration I get a lean spike up to 17:1 or 18:1 and (lately) often a backfire.

On cruise I get 12:1 to 13:1 and when I get home I smell like gas all over my clothes. From what Ive read I need smaller jets, my primaries are stock 78s now.

For the throttle issue Im not sure if I need to change the accelerator cam, the squirters or the pump. I would sure appreciate some help from those more experienced than myself to get this dialed in.

I should mention my plugs have about 300+ miles on them during the rich condition. Im not sure if they should be changed.
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  #2  
Old Oct 17th, 10, 07:16 AM
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Default Re: Lean backfire Holley Street Avenger 870 vs 489 BBC

Tim, are you using a Holley Street Avenger 870 on a 489 BBC ?? The subject title is confusing to me?? I have the Holley Street Avenger 870 on my 396 with NO problems. Didn't have to do anything really to carb except adjust idle. I would change the plugs and check timing. Air fuel ratio of 14.7 is optimal.
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  #3  
Old Oct 17th, 10, 09:15 AM
Microgiant Microgiant is offline
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Default Re: Lean backfire Holley Street Avenger 870 vs 489 BBC

Yes this is a 870 Street Avenger on a 489 BBC. I will get some new plugs and double check timing. I think its at 12 degree before TDC now. But that shouldnt affect what my O2 meter is reading right?
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Old Oct 17th, 10, 02:48 PM
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Default Re: Lean backfire Holley Street Avenger 870 vs 489 BBC

Yes the timing could effect the O2 reading.
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  #5  
Old Oct 17th, 10, 03:01 PM
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Default Re: Lean backfire Holley Street Avenger 870 vs 489 BBC

That spike at 17: and 18: is way to lean... At WOT you should see NO leaner then 13: on cruise 14 to 15 and idle at 14:5
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Old Oct 17th, 10, 05:29 PM
SIDEWAYS SIDEWAYS is offline
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Default Re: Lean backfire Holley Street Avenger 870 vs 489 BBC

17-18:1 is likely a missfire...missfires show us lean cause of the excess (unburnt) air going through the system. 12* of timing is likely causing the backfire. Try around 20* initial. I'm sure you have a good sized cam in there, and will like all the timing you can give it.
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Old Oct 17th, 10, 07:25 PM
Microgiant Microgiant is offline
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Default Re: Lean backfire Holley Street Avenger 870 vs 489 BBC

I was wrong about 12 deg timing.

Right now Im worried about cruise and part throttle. Once I get that sorted I hope to tackle full throttle. My distributor was loose and timing was about 20deg, I backed it off to 18 and that helped with the backfiring, I tried 17 and it also helped a little more. I thought I was supposed to run 18 deg, Should I keep going lower?

When under heavy load throttle still gets me a lean spike. Less timing is also giving me a richer cruise. I got some new plugs but havnt changed them yet. The old ones didnt look too bad.

The big change most recently was sticky tires. The car sat for 6 months also. But current gas is fresh. The motor is still very fresh I just clicked over 1k miles.
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Old Oct 18th, 10, 12:10 PM
Microgiant Microgiant is offline
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Default Re: Lean backfire Holley Street Avenger 870 vs 489 BBC

After talking to the guys at my local speed shop I am going to try swapping my power valve. I did have a big backfire a while ago when my alternator was 180 out. I will keep you posted.
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Old Oct 18th, 10, 05:41 PM
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Default Re: Lean backfire Holley Street Avenger 870 vs 489 BBC

Quote:
Originally Posted by microgiant View Post
after talking to the guys at my local speed shop i am going to try swapping my power valve. I did have a big backfire a while ago when my alternator was 180 out. I will keep you posted.
distributor 180 out.
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  #10  
Old Oct 21st, 10, 05:01 AM
mark191 mark191 is offline
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Default Re: Lean backfire Holley Street Avenger 870 vs 489 BBC

Quote:
Originally Posted by Microgiant View Post
After talking to the guys at my local speed shop I am going to try swapping my power valve. I did have a big backfire a while ago when my alternator was 180 out. I will keep you posted.
Changing the power valve is only going to change the point at which it opens and is not going to give you more fuel at wot. If you determine you dont have a miss under load then i would bump the main jets up a couple of sizes and see if that helps.
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Old Oct 21st, 10, 08:32 AM
brownnote brownnote is offline
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Default Re: Lean backfire Holley Street Avenger 870 vs 489 BBC

Quote:
Originally Posted by mark191 View Post
Changing the power valve is only going to change the point at which it opens and is not going to give you more fuel at wot. If you determine you dont have a miss under load then i would bump the main jets up a couple of sizes and see if that helps.
I think he is worried that the pv got damaged from the backfire.

And he is having lean part throttle, not wot if I am reading it right.

So, I would think you need to lean the primary jets a couple #'s (that is if the pv is fine), then if you still have the lean part throttle, it could be pumpshot.

I would be interested to see what # your pv is, what your vacuum is at idle, and if there is a certain vac. # that on part throttle the lean condition arrises. If you are going lean at say 8" and you have a 6.5 pv, you could go to an 8.5 depending on the idle vacuum and cruise vacuum. But, It could be the pump shot isn't big enough too.

A little more specific info would be good on the issues, say vacuum etc.

I am not sure about timing on a BB. Small blocks tend to like 18 at idle, about 36 up top, maybe 34 for some, but a BB? I don't know.
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  #12  
Old Oct 23rd, 10, 06:54 AM
BlackoutSteve BlackoutSteve is offline
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Default Re: Lean backfire Holley Street Avenger 870 vs 489 BBC

A lean backfire through the carb on acceleration is fixed with a richer accelerator pump-shot.
It's not timing, it's not jetting, it's not a vacuum leak, it's not the power valve etc.
Simply increase the shooter size.
ie: if you have a 28, next size up is a 31 and you may need to go up more than one size depending mainly on the size of the intake's plenum volume.

Also, don't read too much into your A/F gauge. although it is correctly showing you a lean condition where you say the backfire is occuring..
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  #13  
Old Oct 24th, 10, 11:38 AM
ssenay72 ssenay72 is offline
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Default Re: Lean backfire Holley Street Avenger 870 vs 489 BBC

I just went through some tuning on my holley, not so much backfires but just lean and off idle bogs etc. I did find a wire that was burnt through the boot, not fully but i suspected it might be part of the problem. I have a street hp holley however. I pulled it apart changed both jets sizes, power valve , squirters and Pump cams a few times to get it right. I dont have a dyno but just going by feel. Its alot better from " what i feel " . So experiment with jets too. I fouled my share of plugs for sure. And your timing and the timing curve could be part of the problem. MSD dizzies come mostly with the really heavy springs in them and i found swapping one out helped.
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  #14  
Old Oct 29th, 10, 01:55 PM
Microgiant Microgiant is offline
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Default Re: Lean backfire Holley Street Avenger 870 vs 489 BBC

I havnt had a chance to work on this due to stomach flu

I get 12" of vacuum at 900 rpm. I got rid of some of the backfire problems by leaning out the cruise and changing from a 45pv to a 65pv. PV was NOT blown. I still have an occasional backfire out of the exhaust when I change gears and accellerate. At partial throttle, not full. I am getting average 13:1 AFR during cruise and I smell gas. Not as much since I leaned out the idle.

Primary jets are 78s. I have some 75s laying around. For the throttle problem I will try moving the cam down to the next hole.

I dont plan to take the motor past 6500 rpm. The guys at my local speed shop think my carb is too big. What do you think? Should I get a 750-770?

edit: (I have to say that when accellerating I dont know if the secondaries are opening some or not)

Last edited by Microgiant; Oct 29th, 10 at 02:15 PM.
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  #15  
Old Oct 29th, 10, 02:07 PM
SIDEWAYS SIDEWAYS is offline
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Default Re: Lean backfire Holley Street Avenger 870 vs 489 BBC

There is an inherent problem with the 870 Avengers - for some reason, the secondaries dont come it right. Lotta times, its just delayed - you can feel them come in, but its too late and another gear change is coming. I took one to the track, and the car just about stalled and had a slight backfire. Couple friends of mine had teh same issue, and got other carbs. No amount of tuning with nozzles, springs, 50cc pump kits could fit it. All the other vac sec carbs including the 770 Avenger did not exihibit this issue - they run great.

A regular 850 DP will fork great for your 489.
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Team Camaro Tech > First Generation Camaro Info and Tech - 1967- 1969 "Where it all started" > Engine      Current Topic: Lean backfire Holley Street Avenger 870 vs 489 BBC
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