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73-87 Chevy _ GMC Trucks => Performance => Topic started by: 75_chevy on March 10, 2014, 10:56:16 pm
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I have a goodwrench 350 rated at 260hp with about 50k miles on it. recently I've put headers 2.5" pipes glasspacks and turndowns. was running pretty good. also have anew 1406 600cfm carb.
the other day I replaced the spark plug wires and took it for a test drive. on the test drive my truck completely died on the freeway. get towed home and we find the fuel pump had basically fallen apart. so we put the old gm one back on. one that works and we mistakenly replaced about a year ago.
now when I drive it it drives terrible. it's fine from idle up to about 2100 rpm.
if you rev it up it immediately starts backfiring extremely bad and the whole motor loses power. it sounds like a fully automatic gun of some sort. very obnoxious and not making me happy to have this happen. what could possibly cause this? I adjusted the timing and I've eliminated the charcoal canister but it was popping before that. anyways adjusted timing it's at 12' btdc give or take a degree. still popping. I gradually gassed up to 2300 rpm and it started surging and dieing. please help. this is my daily driver and I am only 19 and learning as I go. I can limp it around town to work for the upcoming days but I need to fix it as soon as possible in case I need to use the freeway. thanks everybody(http://img.tapatalk.com/d/14/03/11/avu8epes.jpg)
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also I've adjusted my idle screws and got them pretty dialed in. my regular idle is around 1k which is ok with me I may tune it down to less, but when I shift into gear it drops to about 400 rpm which is way too low usually it's around 6
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mechanical fuel pump is pushing 8 to 8.5 psi
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I'm really stuck what other info can I give you guys
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Look for vacuum leaks, burned spark plug wires, wires not plugged in all the way. Don't crank them down, but see if the carburetor bolts have loosened by snugging them a little. Using insulated wire pullers or something to protect you from getting shocked, you can pull the spark plug wires off the plugs one at a time with the engine idling. If one doesn't cause an rpm drop then that cylinder is a problem for some reason. If you suspect fuel then you can get a few feet of fuel hose and use it to extend the fuel line from where it hooks into your carb. into a bucket. Have someone crank the engine and see if you get a good flow. A trickle won't cut it.
Have you done anything to your truck just before the problems? I always start with the last thing I worked on just in case I screwed something up or knocked something loose.
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Edelbrock 1406 carburetor has a maxim fuel pressure of 6.0 at idle. Please see page 8 of the following manual. owners manual (http://www.edelbrock.com/automotive_new/misc/tech_center/install/1000/1404_manual.pdf)
Under Fuel Pumps and Pressure, it says to set the fuel-pressure regulator (if equipped) to 5.5 psi. Minimum pressure is at WOT which should not drop below 2.0 psi, if it does you will need a higher flow fuel pump.
Continue reading and it will give instances of what happens with more than 6.0 psi of fuel pressure and what to do to correct it. Good luck and keep us informed.
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Another thing to check is if there is water in the gas tank... Remove the line from the carb and pump some gas into a jar and let it settle...
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jaredts: only things I did before it started was 1put the old fuel pump on2 replaced plug wires 3removed charcoal canister.
bake: thank you I just printed out that manual and I'm going to get a regulator tomor.
capt: I'll check that out tomorrow, but if there was water in the gas wouldn't it backfire during idle or at any rpm? my real problem area is high acceleration around 2200 rpm. also I was Reving while idling and it didn't pop at high rpm, but it backfired as soon as I hit high rpm while in drive.. thanks guys !!
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If there is water in the gas it will probably run out of gear at high rpm (relative) but once under load it will start missing and backfiring.
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ok gotcha. and to clarify it is not like one or two cylinders missing, at the high rpm or wide open throttle every cylinder starts popping and backfiring badly. which I think rules out bad wires, especially since I just got new ones
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hows the fuel filter?
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it had a small amount of black crud floatin around last time I saw it (within the past week) I saw it right before work and didn't have time to clean it and forgot
about it
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i would check that it might not be clogged but may be restrictive and causing you to run out of fuel.
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I'll go check that out right now
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k it was worse than I thought but I don't think they're was enough stuff in it to block the gas too bad. I also found out my fuel line is shredded where it meets the filter on both ends. I will get new line tomor and for now I put the plastic inline filter that came with the carb on. thank you Irish
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some could of passed by the filter and clogged up the carb. but i would fix the filter problem and try it out
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ok gotcha. and to clarify it is not like one or two cylinders missing, at the high rpm or wide open throttle every cylinder starts popping and backfiring badly. which I think rules out bad wires, especially since I just got new ones
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Just a question here, how did you determine it was the cylinders popping and not the carburetor ?
k it was worse than I thought but I don't think they're was enough stuff in it to block the gas too bad.
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Paper fuel filters are a funny creature. What happens is they collect containment's and before they plug up enough to become blocked they will slow the fuel flow down. So when you first fire up your truck it will run great, a couple of minutes later it will be running rough because the containment's in the filter settled when you took off the pressure, now the pressure is on and they flow back into the paper filter elements and slow the fuel flow. Eventually they will plug all together.
This doesn't sound like your issue but when they show signs of containment's you should always change the filter, I keep one in my truck along with a 1/4" nut driver just in case.
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actually it could be the carb I suppose I just thought it was cylinders because I was hearing it through the exhaust.
the filter I was using is not a paper filter it was a metal round one (http://img.tapatalk.com/d/14/03/12/apu3aje9.jpg) I'm sure the same thing could happen though ?
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ok new filter on.. reinstalled charcoal canister.. adjusted timing a couple degrees.. adjust air fuel ratio screws.. better.. still idles low in gear , also pops around 3k now not so much 2100.. gonna keep timing an tuning screws
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The idle in gear concerns me, your carb. has an adjustment for WIDE-OPEN THROTTLE, CALIBRATING THE PART THROTTLE (Cruise Mode, Power Mode), and CALIBRATING THE POWER MODE STAGING. Make sure you read the manual you downloaded for these settings, it is where I looked to get the info and adjustments the last time I adjusted mine.
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I dialed mine in with an A/FR meter. The tune is on here somewheres. It would probably work for your setup; your close to mine. Also, you can probably dial that into 16* BTDC as since you are running manifold vacuum and not ported vacuum.
Ah, here it is. Read reply #12 in this thread:
http://forum.73-87chevytrucks.com/smforum/index.php?topic=26985.msg225757#msg225757
Also, have you pulled the step-up pistons out and cleaned them lately, or at least shot some carb cleaner down the bores? They get gummed up. If they don't move up and down freely, weird stuff can happen.
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Since you said it ran alright before you replaced the wires, double check the firing order. Be especially sure about #5 and #7 being correct- they're most often the two that will be wrong. Easy to get the wires crossed even by people who know what they're doing. I have caught myself a few times.