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Old 12-03-2010, 12:06 PM
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Chevy 350 power loss/backfire

Alright all, after being redirected to the Hotrodders forum many, many times by google, I figured I'd register on the forums to get more detailed information. I've got a 350 Chevrolet, 5.7L, recently rebuilt (So I was told) with an Edelbrock performer Manifold, and from what I can decipher, an Edelbrock 1406 carb. (Calibrated to be specifically lean) Bought the vehicle a few months ago, and it runs fine, idles well but every time you hit a specific point on the gas pedal, the acceleration flatlines and begins to stumble, with the occasion gunshot *pop* noise. (Guessing that's a backfire, and I'm assuming that the hesitation begins whenever I hit the secondary jets/blades) As soon as you let off the gas and fall behind that specific point on the throttle, power is restored and everything is peachy again. The timing has been set by hand (I know this is instantly a huge no-go and more than likely the source of the problem, but I don't have access to a timing light, and the timing scale on the engine is in an awful place.), and the vacuum advance is connected to manifold vacuum; you can hear the diaphragm suck into place when its connected to the vacuum port. Any suggestions, or experience related to the matter would be greatly appreciated. Thanks in advance, -J

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Old 12-03-2010, 12:12 PM
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In addition, I seem to recall while having the distributor off, seeing some rust inside it (the advance module itself is a tad rusty), perhaps the mechanical advance weights are corroded and locked up. I'll look into that as soon as I nab a timing light, so that I can get everything done in one chilly winter sitting.
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Old 12-03-2010, 01:37 PM
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A lean condition can cause some of the issues you are having. Also check your fuel pressure and fuel filter, the eddy carb likes about 5 or 5.5 psi constant.

You may want to go through the carb and check out the jet and rod sizes, as you noted the factory 1406 jets and rods are on the lean side as compared to say the 1405. A calibration kit that can help you tune the 1406:

http://www.summitracing.com/parts/EDL-1487/

The 1406 info is in here:

http://www.edelbrock.com/automotive_...406_manual.pdf

As you can see from the calibration chart of the 1406 you don't need a full kit to richen it up some, the 1455 rod alone will get you some what richer as long as the jets are the factory ones. It would be good to know what you have reqardless so pulling off the top plate of the carb and checking jets sizes is a good idea.

As far as timing goes,,,It will help the entire process to first find TDC and get a calibrated timing tape on the damper first. I know its cold but yeah these things pay off.

http://www.crankshaftcoalition.com/w...op_dead_center
http://www.crankshaftcoalition.com/w..._a_timing_tape

The 1406 and the dist may need some work. Along with the timing light get yourself a good automotive vacuum gage. You will want to take some vacuum readings to see about leaks etc, to set up the carb and to get an idea about the setting up of the distributor.

Is this a HEI dist? if so a good idea would be to clean it up and check over the various items listed:

http://www.rustynutscarclub.com/HEI.htm

What cam is in the motor? the timing can be adjusted to suit. A limit on the vacuum advance and the mechanical adv may be needed. A performance cam will want higher base timing, if you set it up higher then you need to limit the mechanical for sure. The vacuum advance should be in the range of adding approx another 12 deg adv. Most stock HEI dist will add around 20 which is too much.
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Old 12-03-2010, 04:20 PM
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Update: So today, after fidgeting around with my distributor and setting it back to where it was before I started messing with it, (Idle hands are the devil's playthings) I decided to go for a drive.This was somewhat less than enjoyable, because at all ranges, I'd get nothing but stumble and hesitation, and I dared not push it farther for fear of backfiring. Only in park what the stumble mostly cease. So, I chugged my way to the local parts store and picked up a timing light and vacuum gauge to kill two birds with one shot. Vacuum readings are good, I get 20 inHg at idle from all manifold ports. (Despite the 3 way tree that never seems to screw all the way in.) Moving on to timing, I diconnected the Vac advance and plugged the port, hooked up my light and shot some readings. (Chevy placed that timing tab in an awful spot, way behind the water pump ) and was rewarded with an initial timing reading somewhere off to the left of the scale. I bumped it back to 8 (the only number I could read on the scale, hope its really an 8, lol), cranked the distributor back down, re-connected the vac advance, and the timing drifted back off to the side, assuming each notch is 4 degrees, and each notch was in the ballpark of 1/4 inch, I'd say the new timing is somewhere around 20. As for total advance, I don't have a tachometer or any way to judge rpm's, but when I applied throttle and held consistent (No clue as to the rpm's), the timing only became more retarded, I'm assuming due to the drop in manifold vacuum reducing the vacuum advance. If I were to punch the throttle and let off, the timing would actually jump past top dead center for a split second, not sure if that means anything. From my observations, it sounds like the mechanical advance isn't operating at all, I'll check in the morning; its freezing and I've got a cold. Thanks again for your time, guys.
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Old 12-04-2010, 09:14 AM
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Quote:
Originally Posted by 8-bit Dominus
Update: So today, after fidgeting around with my distributor and setting it back to where it was before I started messing with it, (Idle hands are the devil's playthings) I decided to go for a drive.This was somewhat less than enjoyable, because at all ranges, I'd get nothing but stumble and hesitation, and I dared not push it farther for fear of backfiring. Only in park what the stumble mostly cease. So, I chugged my way to the local parts store and picked up a timing light and vacuum gauge to kill two birds with one shot. Vacuum readings are good, I get 20 inHg at idle from all manifold ports. (Despite the 3 way tree that never seems to screw all the way in.) Moving on to timing, I diconnected the Vac advance and plugged the port, hooked up my light and shot some readings. (Chevy placed that timing tab in an awful spot, way behind the water pump ) and was rewarded with an initial timing reading somewhere off to the left of the scale. I bumped it back to 8 (the only number I could read on the scale, hope its really an 8, lol), cranked the distributor back down, re-connected the vac advance, and the timing drifted back off to the side, assuming each notch is 4 degrees, and each notch was in the ballpark of 1/4 inch, I'd say the new timing is somewhere around 20. As for total advance, I don't have a tachometer or any way to judge rpm's, but when I applied throttle and held consistent (No clue as to the rpm's), the timing only became more retarded, I'm assuming due to the drop in manifold vacuum reducing the vacuum advance. If I were to punch the throttle and let off, the timing would actually jump past top dead center for a split second, not sure if that means anything. From my observations, it sounds like the mechanical advance isn't operating at all, I'll check in the morning; its freezing and I've got a cold. Thanks again for your time, guys.
Ayuh,... Dickin' with the timing ain't gonna help a Lean condition...
A vacuum fitting that don't tighten is 1 place to look,.. Aka; Fix it...
And,...
I gotta believe you're confused about the timing, in that it Advances, not goin' farther retarded...

The symptoms you state are clearly Screaming a Lean condition....
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Old 12-04-2010, 11:49 AM
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I had a similar problem with my 350. Screwed with everything, then took it to a buddy who knew more than me. He took a water bottle and squirted the header tubes. The number 7 tube let the water sit there longer than any of the others. When hot, water should immediately burn off. That told him there was something wrong with that cylinder. I pulled the plugs next and did a compression test. Sure enough, number 7 was lower than the others. I pulled the head and found one of my intake valves had bent and wasn't able to seat correctly, so it would leak. When the truck backfired, it would backfire violently through the carb, so my carb was covered in soot. The water test is an easy little old timer trick that pays big in finding problems.

Good Luck
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Old 12-04-2010, 02:52 PM
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First off, I've got to say, thank you for your comments so far, I really appreciate the help. On another note, now that I decided to tackle an issue, my car felt it would really benefit to malfunction somewhere else. On to the carb now... the car feels as if its running extremely lean, I thought my fuel pump was going to go kick the can while putting around town yesterday and today. I get home, hook up my nifty vacuum/fuel pressure gauge up to the fuel line, after the flter, right before the hookup to the carb, cranked the engine over and got a consistent 6lbs of pressure. Reattach everything, start the car, turn it off, and I hear a gurgling noise coming from the carb. Take the air cleaner off, and fuel is babbling out of the left primary venturi booster. (It did this once 2 months ago) After taking the top of the carby (Figured I'd attack while it was still showing symptoms) , I notice air is bubbling back into the fuel in the fuel bowl through the left primary jet. Air's not supposed to go that way, is it??? I removed both front jets and venturis, cleaned them as best I could with my quarter can of c-cleaner ( couldn't aim the damn can down and still get spray ) and I'm going to take it for a drive tonight, most likely for the worst. On a plus note, I learned what a jet was today, I always assumed "jet" was the whole venturi assembly. Oh well, that's what happens when the technology you work with is older than you are.
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Old 12-10-2010, 03:49 PM
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Alright, fixed my own problems here, turns out, my new found power loss was attributed to the loose distributor button that I found today. One side wasn't screwed down and was causing it to hop about while spinning, causing erratic spark, etc, etc. Also solved my high end backfire/power loss, like I thought before, my mechanical advance was rusted past the point of dislodging/repair, causing no additional advance at high rpm. When my new distributor ships and I reset the timing, my Caballero should be back to crunching pavement in no time.
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Old 12-10-2010, 11:54 PM
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Quote:
Originally Posted by 8-bit Dominus
Alright all, after being redirected to the Hotrodders forum many, many times by google, I figured I'd register on the forums to get more detailed information. I've got a 350 Chevrolet, 5.7L, recently rebuilt (So I was told) with an Edelbrock performer Manifold, and from what I can decipher, an Edelbrock 1406 carb. (Calibrated to be specifically lean) Bought the vehicle a few months ago, and it runs fine, idles well but every time you hit a specific point on the gas pedal, the acceleration flatlines and begins to stumble, with the occasion gunshot *pop* noise. (Guessing that's a backfire, and I'm assuming that the hesitation begins whenever I hit the secondary jets/blades) As soon as you let off the gas and fall behind that specific point on the throttle, power is restored and everything is peachy again. The timing has been set by hand (I know this is instantly a huge no-go and more than likely the source of the problem, but I don't have access to a timing light, and the timing scale on the engine is in an awful place.), and the vacuum advance is connected to manifold vacuum; you can hear the diaphragm suck into place when its connected to the vacuum port. Any suggestions, or experience related to the matter would be greatly appreciated. Thanks in advance, -J
To start with less sat the time by bring number piston all the way up on TDC (top dead center), number one shpuld be the right front hole on the dist. this will give you a good idea where timing should start at if you have stock cam then your timing will be 6 degrees BTDC, you are going to need a timing light or the problems are going to remain. The other question is how many miles does this motor have on it, if you have as many as 110K miles on it you need to change to timing set. The last thing I want to say is that not having a good ground from motor to frame, now this is not the one from your battery this nis the one from the back of the motor to the frame.
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Old 12-11-2010, 01:22 AM
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Originally Posted by mustang7065
The last thing I want to say is that not having a good ground from motor to frame, now this is not the one from your battery this nis the one from the back of the motor to the frame.
Good thought. You also need a new cable from the frame to the body. Grind all the paint off in a spot on the firewall. Check behind the spot and make sure you aren't going to drill through something expensive. Drill a hole and attach a new ground cable from the firewall to the frame. Go inside the car, pull the carpet back and put a nut on the bolt that you have installed from the motor side of the firewall. Tighten it. Now, mush silicone all over the connection on the inside of the firewall and the outside of the firewall to prevent oxidation of the connection. Now, you should have the body grounded to the frame and the frame grounded to the motor and all should be right with the world.
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Old 12-11-2010, 03:26 AM
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Might try hooking your vacuum advance up to ported vacuum on the carb.
Some engines like it, some don't. Only takes a minute and if you don't like it, put it back on manifold.
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Old 12-11-2010, 02:26 PM
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Hmmmm, again, thanks a ton for the input. As I stated previously, the biggest hunk of the problem is all but gone now, and the car and I are happy as a clam. But, out of sheer curiosity, what do you mean, ground the frame to the body? I'm no electrician, and I never knew that they did such a thing. So I simply want to connect a spot on the frame to a spot on the body with a nice, clean, heavy gauge connection? Would that be sufficient?
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Old 12-11-2010, 02:38 PM
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Quote:
Originally Posted by 8-bit Dominus
Hmmmm, again, thanks a ton for the input. As I stated previously, the biggest hunk of the problem is all but gone now, and the car and I are happy as a clam. But, out of sheer curiosity, what do you mean, ground the frame to the body? I'm no electrician, and I never knew that they did such a thing. So I simply want to connect a spot on the frame to a spot on the body with a nice, clean, heavy gauge connection? Would that be sufficient?
Yep, just make sure you're down to bare metal, then cover the connection with silicone. If you'll get up under the dash, you'll see that some of the electrical components are grounded to the underside of the dash. That's the body. If the body is not grounded to the negative side of the battery somehow, then whatever the component is that is grounded to the dash ain't gonna work so well. Ground battery to engine. Ground engine to frame. Ground frame to body. Done.
Use all new cables. Sometimes a cable will look OK on the outside, but be corroded into two pieces on the inside. I like to use #4 welding cable from a welding supply store. They have nice copper lugs there also, that you can solder to the wire and make a really bulletproof cable. Use shrink wrap too, available at Radio Shack and other suppliers.
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Old 12-11-2010, 08:23 PM
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Originally Posted by 8-bit Dominus
Hmmmm, again, thanks a ton for the input. As I stated previously, the biggest hunk of the problem is all but gone now, and the car and I are happy as a clam. But, out of sheer curiosity, what do you mean, ground the frame to the body? I'm no electrician, and I never knew that they did such a thing. So I simply want to connect a spot on the frame to a spot on the body with a nice, clean, heavy gauge connection? Would that be sufficient?
Cars/trucks need to be grounded, the motor sits on a rubber mounts and the only way to ground the motor and to the frame is by attacting a wire from the firewall tso the engine so that you have positive current flow for you electronics. Sometimes a vehicle will short fuses, run rough at mid to high RPM or your gauges may act crazy. If don't have one I would advise putting one in your car will like you for it.
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