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Lean backfire Holley Street Avenger 870 vs 489 BBC

16K views 23 replies 9 participants last post by  brownnote 
#1 ·
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.
 
#2 ·
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.
 
#6 ·
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.
 
#7 ·
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.
 
#10 ·
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.:beers:
 
#11 ·
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.
 
#12 ·
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..
 
#13 ·
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.
 
#14 · (Edited)
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)
 
#15 ·
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.
 
#17 ·
I would go to the smaller primaries and see how that works for you. You have a wideband so you will know right away how it likes/dislikes the change with respect to afr.

Also, on the secondaries, you could go to a softer spring and see if you notice a difference or the "bog".... if it does bog then pick up the spring is too light. Otherwise you could try to rig some sort of metal arm to the secondary shaft that will tell you how much it opened by how much it bends etc.
 
#20 ·
I still have a stumble and lean condition on throttle but I discovered yesterday that one of my plug wires had contacted a header and burned and that was the cause of my backfire. I had one of those heat resistant covers on it and it burned through anyways so I ended up have to route it differently. Thanks for the help so far. Im still working on getting this carb dialed in!
 
#23 ·
Im still running rich, so much so I think I messed up my O2 sensor. Its coated in carbon :mad:. I went down 3 jet sizes with little result, but the o2 sensor doesnt seem reliable any more either. Is it possible to clean a O2 sensor?

I still go lean and get a small backfire on accelleration (much rarer now), and I noticed smoke when I was shifting into second when I was getting on the throttle. I had my intake off to fix a oil leak and I noticed the intake valves look dirty.

Im going to try a holley 750 next. I wish I could afford EFI but unless I get a new better paying job thats a year or more away.

I found a holley 750 dp on craigslist, do you think its safe to buy a used carb ?
 
#24 ·
Well, a couple things. The 850 or 870 is the recommended size carb for your app up to 6000 rpms (only one I checked), so I don't think the carb is too big.

I think if I were you, I would go ahead and pull the 870 apart, ensure all the passages are clear, and put her back together if you already haven't done so.

As for the o2... are you sure it is toast? you could always get a replacement, but I have run some really rich setups and not fried or messed up a wideband sensor. Have you free-air calibrated the thing lately? If it is running 12 or 13:1 on the highway, def. go leaner on the primary.... I go one size at a time, but that is me. found my car wanted 14.2-14.7, and surged at 14.6-15.2... every car is different, but seems like you have room to fix there. As for the lean part throttle, are you sure the secondaries are set properly? Can you ensure that the spring isn't too light and that they aren't opening too early? It doesn't seem like that is the case, but thought I would throw it out there.

Does the lean portion stay lean if you hold the throttle in that area, or is it momentary? Maybe the previously mentioned idea of larger pump shot might help depending.

Float levels set ok? I ran my secondary float level way too low once, and had the same thing happen... car would cruise etc but as soon as the gas was down too low, it bucked and hated me!

I think I would just get another sensor if you think it is truly bad.

Oh, and you could try the other carb, although I wouldn't.... in my experience every used carb I have had I needed to fully dissassemble, clean, and readjust or they would have never run right!
 
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