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Discussion Starter · #1 ·
Hi, everyone. Please help me with tuning my 390 engine. My problem is that I get a backfire when I push the pedal down more than about half way at once. It'll backfire through the exhaust, but mostly the carburetor. I've messed around with it quite a bit with little progress. My setup is a basic holley 600 cfm, with only a front metering block, on a cast iron 4 barrel manifold. I have 64 size jets, a 8.5 high flow power valve that fits my vacuum reading. My timing is set at 20 initial with manifold vacuum, about 10 without, it reaches about 33 at 3000 rpm. It's an HEI distributor with the lightest springs on it. I've gone through a couple squirters, I started drilling them out just to avoid ordering a bunch. I left at a 31 and then drilled a squirter out to 1/16th of an inch on both holes. It made an improvement, but I dont want to try and drill out any higher. I'll start messing around with my mechanical advance next, but any thoughts would be greatly appreciated. Thanks
 

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I can't stand running a vacuum advance to manifold vacuum where it has vacuum and any advance at idle with throttle plates basically closed.
Quite a few factory cars came this way and I always get better smoother performance changing it around.
I use ported vacuum above the throttle plates so that basically limits it to one of the few carb ports in most cases. I would rather use no vacuum advance and just have mechanical than to have advance at idle.
I would try even plugging vacuum advance and bump up timing to 12-14 (not sure what a 390 should be at. I only had one in a 70 something truck we called War Wagon), then adjust timing a little to see if it stops popping. I didn't see where you said you tried any other timing changes.
I would think the best running would be with timing set to a good base level maybe 2-3 degrees more than a stock engine spec and vacuum from carb port so it only has vacuum and advance when you slowly open the throttle.
Good luck and post back what you figure out.
 

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Change the vacuum advance signal as suggested, then try richening the idle mixtures some more.

When you drive it, can you slowly roll into the throttle from idle up to the main jets without a big flat spot? I once found a 600 Holly with the wrong metering block gasket. It had the hole to let the idle mixture through, but no hole for the transfer slots. Both holes are side by side, 2 for each side. You had to basically goose it from an idle to get up on to the mains. He had driven it that way for years. Without the transfer slots the engine runs lean on transition.

390 intakes are hard to install without getting a vacuum leak, especially the 40# cast iron ones. Swapping to an aluminum one is a win win sinario, less weight on the front end and easier to handle when installing. Your back will appreciate it.

Sent from my moto x4 using Tapatalk
 

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Discussion Starter · #4 ·
I just got back from making changes. I first went back to ported vacuum and about 12 degrees of initial, I went to heavier springs and richened up idle mixture a bit. It ran ok, better than before, I still couldnt get past 1/2 throttle at once but it didn't backfire nearly as bad as it was, just hesitation and a pop. Then I got rid of the vacuum advance because it looked like it was getting way to much, like 40 degrees of advance at 2000 rpm. It still ran ok, I didn't see much change. Then It backfired again and started running like crap, surging and barely accelerating. I limped it home and saw a lot of fuel coming from the boosters past idle, I might have blown the power valve is my guess. Any more thoughts on what I should do next? I'm gonna open up the front of my carb and see what happened and maybe go up a jet size while I'm at it. For now I might go back to manifold vacuum and see how it works with the heavier springs. I liked how it cruised off of manifold before.
 

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What size camshaft is in the engine?? What is your compression ratio??

Along with making squirter size changes, you often have to make pump cam size changes also. Often the 2nd hole in the linkage and pump cam work better, as this provides a bigger setting for the pump cam. Pump cam assortments are available, they are color coded.
What color is the current pump cam??
 

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My setup is a basic holley 600 cfm, with only a front metering block, on a cast iron 4 barrel manifold. I have 64 size jets, a 8.5 high flow power valve that fits my vacuum reading. My timing is set at 20 initial with manifold vacuum, about 10 without, it reaches about 33 at 3000 rpm.
JMHO, 600cfm seems a little small for this engine and #64 jets seems on the lean side to me. Also 8.5 power valve is way too stiff for 10hg, try a 5 or 4.5 pv.
 

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Hi, everyone. Please help me with tuning my 390 engine. My problem is that I get a backfire when I push the pedal down more than about half way at once. It'll backfire through the exhaust, but mostly the carburetor. I've messed around with it quite a bit with little progress. My setup is a basic holley 600 cfm, with only a front metering block, on a cast iron 4 barrel manifold. I have 64 size jets, a 8.5 high flow power valve that fits my vacuum reading. My timing is set at 20 initial with manifold vacuum, about 10 without, it reaches about 33 at 3000 rpm. It's an HEI distributor with the lightest springs on it. I've gone through a couple squirters, I started drilling them out just to avoid ordering a bunch. I left at a 31 and then drilled a squirter out to 1/16th of an inch on both holes. It made an improvement, but I dont want to try and drill out any higher. I'll start messing around with my mechanical advance next, but any thoughts would be greatly appreciated. Thanks
You can verify other conditions to make sure they are not affecting your tune.

Top dead center with rotor pointing at #1. Yes, no, almost ?

While the cap is off, check and measure timing chain slop. If this is a 1972 or later block there will be a retarded timing set installed if not already changed out. A 1972 or newer truck motor will be rated around 200 horsepower. The assembled date should be near or onto the pad where the oil filter mounting attaches to the block.

Let it idle in the dark and look for faulty spark plug wires and cracked spark plug insulators.

I've had the best results adjusting the screws on this 600 holley with a vacuum gauge. Go for the highest and then slightly richened.

It runs like a couple of ignition wires are crossed...from your description.

Good Luck
 

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Sounds like you might have several issues going on and with overlapping problems. As Eric asked what is stock static compression ratio and then the cam and Dynamic compression ratio? Vacuum can should be hooked up to ported vacuum. The idea is base timing then timing increases as rpm increases by vacuum actuator and then centrifugal (weights & springs). After the EPA got a hold of the engines in 1972 and then cats added in 1974 base compression went from 10:1 or so to down to 8.5 because of knock issues with the lower octane of unleaded, then they also added EGR and then lean burn, increasing the idle setting just to keep the engines running, and other games by the factory.

A stock engine around 1970 -1972 would have base timing around 4* BTDC. A 1974 engine running unleaded with EGR and Lean Burn would be set at 12 to 14* BTDC. The factory was using the advance in timing to increase the compression pressure to get the engines to start and idle.

Try to set your timing at 4 to 8* before with the vacuum can unhooked and see if starts running better. Total advance on SBC is usually about 36, not sure about yours.

If carb needs rebuilt, need to do that too then get it adjusted correctly. Idle to cruise 600cfm should be OK but if you want more rpm or wind it up your probably looking to go 650 to 750. I suspect as mentioned the #64 jets are too small.
 

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Discussion Starter · #9 ·
Ok, I'm back. My 390 is stock besides the carb and hei distributor, It came out of a 1971 ltd so I think the compression ratio is 8.6:1. The pump cam is also the stock red one, it's in position one but I can't figure out how to fit it into any other position, honestly. I have an 8.5 power valve because I get 18 vacuum reading idling in drive. I'll check out my plug wires next.
 

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Discussion Starter · #13 ·
it's a Holley 0-80457SA 600 cfm street carb. And speaking of the secondaries, the linkage that opens them at wide open is binding and opening them at quarter throttle. I'm gonna disconnect the linkage and see how it runs now.
 

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it's a Holley 0-80457SA 600 cfm street carb. And speaking of the secondaries, the linkage that opens them at wide open is binding and opening them at quarter throttle. I'm gonna disconnect the linkage and see how it runs now.
There's your problem, send it back.
 

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There's your problem, send it back.
Yep! Do it. Hopefully this solves your issues.

On the manifold vs. ported, I have to disagree with people recommending ported vacuum. I'm pretty sure most of the carb manufacturers, if not all, also agree with using manifold vacuum (I'm speaking from memory -- do your research). There is no logical reason to 1. Not want the timing in at idle for a smoother, higher vacuum signal and 2. Want a short 'spike' in timing at tip in.
 

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Discussion Starter · #16 ·
Fixed the linkage so that it doesn't mess with the secondaries, I thought it was supposed to open them, but it wasn't supposed to affect them at all. News to me. After fixing it, I dropped the primary float a bit and now I can floor it without the engine complaining. Thanks everyone!
 

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Ok, I'm back. My 390 is stock besides the carb and hei distributor, It came out of a 1971 ltd so I think the compression ratio is 8.6:1. The pump cam is also the stock red one, it's in position one but I can't figure out how to fit it into any other position, honestly. I have an 8.5 power valve because I get 18 vacuum reading idling in drive. I'll check out my plug wires next.

If it's all stock then the original plastic covered crank gear is still in there on the timing set. A new set for say 1964-1969 would be a smart improvement. More so than spending money on a performer intake manifold that is not better than the original "S" intake, just lighter. :evil:

I like some good news once in a while. Good news that all is fixed now.
 

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There's your problem, send it back.
I agree.
Strikes me that someone may have messed with this carb then sent it back to Jegs.

The Red pump cam is not stock, stock would be a white cam....red is the one of the smallest Holley pump cam,and this carb comes with a Black vacuum secondary spring, which is Holley's stiffest, slowest rate spring that is listed as never fully opening even at 6000 rpm on a motor of your size.

If this is a Street Avenger carb, there is a sticky thread here at Hotrodders about how to correct their notorious off idle stumble that is due to the configuration of the Street Avenger metering block.
You might want to give that a read. https://www.hotrodders.com/forum/holley-670-vs-street-avenger-hesitation-fix-507033.html

The Street Avenger carbs have lots of dissatisfied users reported around the various internet car forums ever since they were introduced. Typically it is a lean stumble or backfire pop just off idle that is the chief complaint.
 

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Yep! Do it. Hopefully this solves your issues.

On the manifold vs. ported, I have to disagree with people recommending ported vacuum. I'm pretty sure most of the carb manufacturers, if not all, also agree with using manifold vacuum (I'm speaking from memory -- do your research). There is no logical reason to 1. Not want the timing in at idle for a smoother, higher vacuum signal and 2. Want a short 'spike' in timing at tip in.

I will respectfully disagree. Ported vacuum above the throttle plates so there is no vacuum at idle and you get back vacuum as you slowly crack open the throttle is the smoothest best way to go. This is almost always how they were set up until the late 70s early 80s when they were worried about admissions. They cranked the timing up for lower emissions and better fuel mileage at idle. The problem with manifold vacuum is you have your highest advance and your highest timing usually at idle. At least it's very high. When you stomp the pedal or accelerate quite hard you lose the vacuum so you lose degrees of timing. It takes awhile for the RPMs to catch up and for the distributor advance to kick in and bring the timing up which usually isn't as far anyways. Kind of like turbo lag.
As the RPMs go up from a normal take off the engine will handle more timing but it doesn't get more timing until the distributor Advance kicks in which is quite a ways up. I find the smooth acceleration is when you have the vacuum advance starting to advance as you crack open the throttle just slowly accelerate. Then by the time you get up to 15 or 20 miles per hour or so or up in the to 2500 to 3500 RPM range the advanced weights can kick in. I don't know of many if any of the older cars in the sixties and seventies that had vacuum advance with full vacuum all the time but later on especially in the GM's they loved to run manifold vacuum to the vacuum advance. 305 in the conversion vans did it 307 olds Motors did it. I think it's over all kind of a waste. There would be no point to design an engine with a vacuum advance if it was going to be Advanced all the time at idle. Just take the hose off and throw it away and wire the vacuum advance all the way Advanced. This will pretty much make it run 98% the same darn way.
You should see what I used to do to each egr valves....
 

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I will respectfully disagree. Ported vacuum above the throttle plates so there is no vacuum at idle and you get back vacuum as you slowly crack open the throttle is the smoothest best way to go. This is almost always how they were set up until the late 70s early 80s when they were worried about admissions. They cranked the timing up for lower emissions and better fuel mileage at idle. The problem with manifold vacuum is you have your highest advance and your highest timing usually at idle. At least it's very high. When you stomp the pedal or accelerate quite hard you lose the vacuum so you lose degrees of timing. It takes awhile for the RPMs to catch up and for the distributor advance to kick in and bring the timing up which usually isn't as far anyways. Kind of like turbo lag.
As the RPMs go up from a normal take off the engine will handle more timing but it doesn't get more timing until the distributor Advance kicks in which is quite a ways up. I find the smooth acceleration is when you have the vacuum advance starting to advance as you crack open the throttle just slowly accelerate. Then by the time you get up to 15 or 20 miles per hour or so or up in the to 2500 to 3500 RPM range the advanced weights can kick in. I don't know of many if any of the older cars in the sixties and seventies that had vacuum advance with full vacuum all the time but later on especially in the GM's they loved to run manifold vacuum to the vacuum advance. 305 in the conversion vans did it 307 olds Motors did it. I think it's over all kind of a waste. There would be no point to design an engine with a vacuum advance if it was going to be Advanced all the time at idle. Just take the hose off and throw it away and wire the vacuum advance all the way Advanced. This will pretty much make it run 98% the same darn way.
You should see what I used to do to each egr valves....
You have your highest *vacuum* advance at idle, not your highest advance, when hooked to full manifold. This is a good thing -- it provides a cooler engine and a smoother idle due to a better vacuum signal. The loss of this vacuum advance with throttle changes is not a bad thing.

You contradict your own thinking with this statement:
"When you stomp the pedal or accelerate quite hard you lose the vacuum so you lose degrees of timing."

This statement is true in both ported and manifold vacuum scenarios, so it does nothing to pad your side of the 'debate.' You are not getting any of your so-called 'benefit' if you mash the throttle because vacuum is ~0 in that case.

What you are calling "distributor" advance is mechanical advance, and it should increase ~gradually up to the RPM you are quoting. Your wording makes it sound like it has to reach that RPM before it "kicks in" and that you need to somehow help it with vacuum advance during that time.

Vacuum advance should be in effect during low load/high vacuum situations (i.e. idle). There is absolutely no reason you should want the vacuum advance to "kick in" before mechanical advance at throttle tip in before RPMs are up. If the extra vacuum advance is beneficial in this scenario, it means that your base timing is too low.

Respectfully disagreeing on my end as well, but also respectfully pointing out that you have some inconsistent logic within your post and that you are, IMO, perpetuating BAD info.
 
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