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Old 12-02-2012, 08:07 AM
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New Holley carb "pops" under acceleration

Hey guys, I'm trying to tune up a carb ffor a buddy of mine, it's a Holley truck avenger 670 on a chevy 350 in a jeep with 44 boggers. It had an edelbrock on before the Holley that didn't pop like this so it's not the motor itself. But if your in neutral and start revving it, around 2000 rpm it starts popping out the carb. Not a backfire. There's no flames, just a pop and maybe a little puff of smoke. And under hard acceleration it does it even worse. So far iv adjusted timing, rejetted, adjusted mixture screws with vac gauge. Checked acel pump operation and arm adjustment, everythin iv tried has got it to run better, but still pops. Any suggestions??? Thanks

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Old 12-02-2012, 11:31 AM
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Is the idle circut adjustable maybe it air screw is turned out to far or a bad seat on air bleeds. Sounds like maybe an air leak or leaky valve. But thats a WAG.

I would call the manuafacture and ask for help walking thro install and setup. Would not be suprise to find something is not right and a new carb is in order. New does not always mean right. Sometimes the factory makes mistakes. Best to get them on the phone and see if they will take it back after they determine the problem.
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Old 12-02-2012, 12:49 PM
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350cruiser,
If I'm not mistaken, a pop out of the carb is an idication of a lean mixture or improperly adjusted valves. Since the engine was ok before the carb swap, and you think the timing is correct I'd check the float level and fuel pressure before anything else. adjust the float so that the fuel just starts to trickle out of the sight hole after removing the plug with the engine at idle.
FWIW
ssmonty

Last edited by ssmonty; 12-02-2012 at 01:07 PM.
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Old 12-02-2012, 01:29 PM
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If the fuel level and pressure(4-1/2 to 5-1/2lbs +/-1) are ok, then check for vacuum leaks, (gasket,pvc valve,vacuum hoses,ect.)
FWIW
ssmonty
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Old 12-02-2012, 07:09 PM
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Alright guys, thanks to everyone for the replies. So far iv done everything possible to fix the problem. But what helped the most was rejetting, factory this carb comes with a 68 primary and an 89 secondary which seems massive to me, I cut them down to a 65/68 and now it runs 100% better. but still a pop every now and then under a hard acceleration or reving up past 1500 in the drive way. After further inspection of the carb... It also has a 2.5 power valve. According to my vac gauge I need a 6.5. Could this cause my problem??? I will indeed change it first thing in the morning, but what happened when you have too low of a power valve? Thanks
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Old 12-02-2012, 07:57 PM
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Quote:
Originally Posted by 350cruiser View Post
Alright guys, thanks to everyone for the replies. So far iv done everything possible to fix the problem. But what helped the most was rejetting, factory this carb comes with a 68 primary and an 89 secondary which seems massive to me, I cut them down to a 65/68 and now it runs 100% better. but still a pop every now and then under a hard acceleration or reving up past 1500 in the drive way. After further inspection of the carb... It also has a 2.5 power valve. According to my vac gauge I need a 6.5. Could this cause my problem??? I will indeed change it first thing in the morning, but what happened when you have too low of a power valve? Thanks
WTF 68/89 w/a 2.5 PV???

That pos would be going right back where it came from- on their dime- and a replacement that hadn't been already sold and returned demanded in return!

There's no earthly way- even w/bad QC- that the carb would have left the factory like that!

The 2.5 PV wouldn't cause it to run bad sitting in the driveway w/o a load on the engine. But God only knows what else might have been done to this carb by whoever was "tuning" it before you. If it was me I wouldn't waste any more time on it. If you do want to work on it despite all that, baseline it back to stock specs and work from there.

Holley numerical listing here.
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Old 12-02-2012, 08:37 PM
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Look up the instruction sheet on the holley site for pn 93670. Scroll all the way to the bottom and look at the factory specs for the 670. All that oversize non sence is factory. It was ordered brand new so it has the proper jets in it according to the paper work. Why such big jets with a 2.5 valve on an off road carb has got me wondering, but I got it where it needs to be now. But all and all it's a bad a** carb after it's been jetted and tuned properly. Almost rolled the jeep over earlier and it sat there at idle purring like a kitten like nothing was going on. Iv done everything suggested above plus some and can't seem to get rid of the last few "pops". Let me know what y'all think about it. Thanks
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Old 12-02-2012, 10:24 PM
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Quote:
Originally Posted by 350cruiser View Post
Look up the instruction sheet on the holley site for pn 93670. Scroll all the way to the bottom and look at the factory specs for the 670. All that oversize non sence is factory. It was ordered brand new so it has the proper jets in it according to the paper work. Why such big jets with a 2.5 valve on an off road carb has got me wondering, but I got it where it needs to be now. But all and all it's a bad a** carb after it's been jetted and tuned properly. Almost rolled the jeep over earlier and it sat there at idle purring like a kitten like nothing was going on. Iv done everything suggested above plus some and can't seem to get rid of the last few "pops". Let me know what y'all think about it. Thanks
Thanks for correcting me- I'd have bet a dollar that was nowhere even near to stock- that's why I didn't even bother to look it up which is on me.
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Old 12-03-2012, 06:19 AM
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I'd have bet more than a dollar that a 68/89 2.5 wasn't stock. That's outrageous to me. Especially for an off road carb, what kind of motor do they plan on putting a 670 on to do some mud and rock crawling?? I'll call holley in a couple hours and see what they recommend for fixing it. Thanks for the help.
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Old 12-03-2012, 07:26 AM
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I'm wondering the same thing. I guess they think it's going to be used on something that's WFO all the time.

I could see something like a sand or mud drag racer or tractor puller w/a small displacement engine maybe needing something like a 670 cfm carb w/a 2.5 PV, but the jetting still makes no sense to me. It'll be interesting to hear what Holley has to say.
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Old 12-03-2012, 10:09 AM
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It's no wonder why the carb is backfiring. You suppose to run a 6.5 PV not a 2.5 PV. Carb is leaning out you. Did you check the secondary spring. It may be opening too early. The plugs are fouling out because of the large jets.
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Old 12-03-2012, 10:45 AM
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Well according to the guy at Holley which iv talked to twice. That carb has been tested and proven to run out of the box like that. The primary is set up different than any other carb to run at steep angles, and the secondaries are set up normal, but all he would say about the huge spacing between the primary and secondary valves is that's how it's designed and needs to be ran.... After I argued that I already put the smallest jet in my kit (64) primary, it still smokes, and makes your eyes water it runs so rich. The "puff" out the carb I believe is a little smoke and tiny gas droplets. So it's still rich. I haven't messed with the power valve because Im sure that will have nothing to do with it at idle. So he suggested upping the pump squirter from a 28 to a 35 which I did and it made it worse. He was determined it was running too lean until I told him it helped putting smaller jets. So he said put an even smaller one to see, so I went from a 65 to a 64 with no change. Under normal driving and even slamming it to the floor it only pops a few times now. But in 4 low in a muddy incline it sounds like it blowing up under the hood it pops so bad. His next best thing was to send it back and get a new one... Which is the step I'm taking now, I'll keep ya up to date on how all this goes... thanks to everyone
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Old 12-03-2012, 11:28 AM
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Quote:
Originally Posted by 350cruiser View Post
Well according to the guy at Holley which iv talked to twice. That carb has been tested and proven to run out of the box like that. The primary is set up different than any other carb to run at steep angles, and the secondaries are set up normal, but all he would say about the huge spacing between the primary and secondary valves is that's how it's designed and needs to be ran.... After I argued that I already put the smallest jet in my kit (64) primary, it still smokes, and makes your eyes water it runs so rich. The "puff" out the carb I believe is a little smoke and tiny gas droplets. So it's still rich. I haven't messed with the power valve because Im sure that will have nothing to do with it at idle. So he suggested upping the pump squirter from a 28 to a 35 which I did and it made it worse. He was determined it was running too lean until I told him it helped putting smaller jets. So he said put an even smaller one to see, so I went from a 65 to a 64 with no change. Under normal driving and even slamming it to the floor it only pops a few times now. But in 4 low in a muddy incline it sounds like it blowing up under the hood it pops so bad. His next best thing was to send it back and get a new one... Which is the step I'm taking now, I'll keep ya up to date on how all this goes... thanks to everyone
One way to run more secondary jet would be to use a very small power valve channel restriction and large air bleeds. But still...

Sometimes that noxious smell you get is from unburned fuel, but not necessarily from being too rich. Instead, it's because the timing isn't sufficiently advanced to burn the relatively lean air/fuel mixture that's sometimes seen w/a cam that has a lot of reversion/lot of overlap/narrow LSA/late intake closing point.

Do you know the cam specs? Does the CR of the engine match the cam?

Last edited by cobalt327; 12-03-2012 at 11:34 AM.
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Old 12-03-2012, 01:29 PM
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I understand the overlap and big cam. But it's on a stock 350, only mod is headers... So im lost on the whole thing. I talked to holley again and did everything that he advised with no difference. So it's boxed up and another one is on the way...
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Old 12-03-2012, 02:43 PM
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Change the PV because it's ruptured from the backfire. That is why your eyes are watery and stinging.
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