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Old 06-05-2011, 06:09 AM   #1
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Car: 1984 Trans Am
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Car running rough when under acceleration

Hey guys,

84 T/A, 350 and CC qJet. Car starts and runs up to high idle just fine. The problem i'm having is unless I drive over 2K RPM, it runs really rough, studders, shakes and is altogether unhappy. because I'm in England and I'm currently waiting for a new set of plugwires and cap/rotor to make sure it's not that (should be here by the end of the week), I was wondering if there was anything else i could check?

The car runs fine at speed if I keep the RPM's over 2000 but if I don't, it studders pretty bad. Meaning if I'm driving from a stop, even if I hammer on it, until the RPM's hit 2k, it runs...rough. Still seems to have good power, but runs rough. Smooths out over 2K. If driving and the car kicks into overdrive at 35, it really struggles, stuttering and being not cooperative. I drop it into 3rd, bring the RPM's up, I can still feel it stutter but it's "better". Stutter may be cylinder missing (Spark plug/plug wire?) Dwell is set to 30* and it osscilates so the MCS is working just fine. No Check engine lights. I was thinking one of my plug wires may be shorting to the headers (My number 3 plug has been melting away for some reason--three wires since November on that plug alone) so it could just be that. Turns all white and flakes, exposing the wire. It's not gone that far yet, but the boot end has started to crack and pit.

Also, when cold, unless I let it warm way up at high idle, it stuggles to stay running and likes to stall out on me, even when still on the high idle if I put it in gear. That just a fact of life for a carbed car? I remember it working very well on the old 305, so I don't know exactly what changed other than there is no heat tube coming up off the headers now.

Anyway, just looking for a sounding board for ideas until my plugwires and cap/rotor come in. could be it's all the same cause from the wire going bad?
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Old 06-05-2011, 04:06 PM   #2
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Car: 86 T/A, 83 Z/28
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Re: Car running rough when under acceleration

Well heres my thoughts on your issue. As far as your sparkplug wire is concerned how much clearance do you have from your header? I'm assuming youíve probably already checked that but if youíre still burning plug wires on that cylinder you may be running lean on that cylinder causing that cylinders exhaust port to overheat burning the plug wire. This can be checked with an infrared thermometer if you have access to one. Just compare that exhaust ports temp with all the others. They should be similar. Itís kind of difficult to troubleshoot everything else when you know you may have a bad plug wire that could be causing your misfire.

Second as far as how the car runs cold... Carbs are naturally temperamental in the cold weather. However your problem doesnít sound like a fact of life type issue. What Iím thinking is this. You had a 305 swapped to a 350. You cars ECM still thinks you have a 305. When the cars warm this isnít such a big issue because you have the O2 sensor telling your ECM to make adjustments. However when the cars cold itís in open loop mode and your ECM doesnít have any feedback as to how the cars running. What it will do is it will guess settings for things like the timing and fuel mixture that were tested my GM to work well.... on a stock 305. After swapping in a 350 these settings may no longer work so well. Also you mention you removed the pipe coming off the exhaust manifold. The reason that was there was to help the carb run well when the cars cold by giving it access to hot air until the car warmed up a bit. May want to consider putting that back. Lastly Iím guessing itís an original carb. It may be time for a rebuild. I would at least check out the operation of the choke.
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Old 06-06-2011, 11:10 AM   #3
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Re: Car running rough when under acceleration

Carb is new. With NAF and Aperion's help I was able to tune it pretty good and it was runnign awsome till about a month ago when this hesitation/rough running started. For the hot air tube, I'm running headers now so that's not really an option. I'll see about trying to get one of those infared thermostats, can normally pick them up cheap but for me, it's a week or two shipping for anything from the states.
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Old 08-26-2012, 11:41 AM   #4
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Re: Car running rough when under acceleration

I hate to resurrect a dead thread, but I've been doing searches to try to figure out what's going on with my Z. What ended up being the cause of your problem? I have the same (or extremely similar) problem, minus the spark plug wire melting, with my CCC camaro.
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Old 08-26-2012, 01:27 PM   #5
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Re: Car running rough when under acceleration

Quote:
Originally Posted by GOT_CAMARO? View Post
I hate to resurrect a dead thread, but I've been doing searches to try to figure out what's going on with my Z. What ended up being the cause of your problem? I have the same (or extremely similar) problem, minus the spark plug wire melting, with my CCC camaro.
Ok, mine turned out to be a couple of issues. Yes, my carb wasn't tuned properly, however I had an underlying problem that was harder to diagnose and didn't find out until I re-tore my carb apart. My car was running super-lean and we couldn't figure out why (I broke down and took it to a specialist and worked with him for a couple days, he let me hang out and help him tear it down). The diaphram in my charcole canister went bad and the carb was sucking charcoal back up through the vacuum lines into the carb, choking out all the little tubes inside the carb. We kept finding these little black bits inside the carb. We blasted out the carb good, removed the line going to the charcoal cannister and problem solved. the line to the vacuum cannister in question was the larger diameter line going to the middle-top of the carb on the front. It was also full of fuel down to the diverter so I think fuel got in there, then down to the cannister which allowed the charcoal particles to float up into the carb.
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Old 08-26-2012, 04:43 PM   #6
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Re: Car running rough when under acceleration

Thanks for the reply Ozz! I did some checking around the evap system, turns out the top diaphragm of the control valve does not hold vacuum. I plugged the tube coming from the port on the carb and drove the car, there was minimal improvement. I can hear a faint whistling noise in that general area too. Not sure if it's the carb gulping air or another vac leak. I'll have to wait for the control valve to arrive before I dig any deeper.
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Old 09-13-2012, 10:28 PM   #7
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Re: Car running rough when under acceleration

Maybe this is my problem. Ours show up above 40 mph or if I run it through first and it shifts into second. It will start to die like it's starving for fuel. Pumping the gas, letting off, or sticking it in neutral does not help and it usually dies and is hard to get started. When I was replacing hoses I noticed black grains in the fuel purge valve (?) that has four hoses in front of the carb. I replaced the valve. Also when replacing the hoses, I removed one on the vapor canister and it was leaking fuel. I thought they were all just vapor lines.

I am rebuilding the carb this weekend. I will look into the vapor canister. I see they don't stock new canisters in town, so I may need to temp plug hose to carb also. Any suggestions on troubleshooting?
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Old 09-14-2012, 12:03 AM   #8
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Re: Car running rough when under acceleration

Quote:
Originally Posted by dlinger View Post
Maybe this is my problem. Ours show up above 40 mph or if I run it through first and it shifts into second. It will start to die like it's starving for fuel. Pumping the gas, letting off, or sticking it in neutral does not help and it usually dies and is hard to get started. When I was replacing hoses I noticed black grains in the fuel purge valve (?) that has four hoses in front of the carb. I replaced the valve. Also when replacing the hoses, I removed one on the vapor canister and it was leaking fuel. I thought they were all just vapor lines.

I am rebuilding the carb this weekend. I will look into the vapor canister. I see they don't stock new canisters in town, so I may need to temp plug hose to carb also. Any suggestions on troubleshooting?
Rebuild the carb, and completely disconnect the charcoal cannister from the system, except for the PCV valve (IIRC they are connected). I think it also means I was pushing too much fuel and had flooded the carb to the point where it pushed fuel out the vacuum ports on it. This may have happened to you as well as one time. Right now, my charcoal cannister is almost completely disconnected as I don't need emissions here.
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Old 09-15-2012, 05:02 PM   #9
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Re: Car running rough when under acceleration

I don't need to really worry about emissions either. I need to research how it all works. I thing the gas tank needs the vent of the vapor canister unless you go to a vented gas cap??? I disconnected the hose at the canister to the purge valve and tried to blow through it to the tank. I could not get air to go through it so I used the air compressor. We could hear it at the tank filler. On the rebuild I found a couple of problems. Put it all back together and it runs smooth at highway speeds. Hopefully we cured the problem. Thanks for your help.
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