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Old 07-27-2010, 01:59 PM
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Carb or timing?

I've read through some of the posts and havent found the answer I've been looking for. I'm new to this site and it appears to be rather productive. I have been working on cars my whole life and have never had to pay a mechanic so it's rare I have to turn to others. That being said, here goes...
I have an 84 GMC 1/2 ton Pick up, stock 305, 4 bbl carb. Nothing special, all stock. I've had the truck about 2 and a half years with no problems other than a dead spot in the pedal. I've always just ignored it as it wasnt bad. A friend had an 84 GMC van with a brand new (rebuilt) carb. He junked it due to a blown head gasket and gave me the carb. In effort to repair the dead spot, I changed them. I should probably also mention I had a small engine fire that Im sure damaged the float in the old carb. The new carb was working great so I junked the old one. Big mistake I think. After about 2 weeks, the new carb started giving me trouble. At high speeds, 55-60, it would act like it was running out of fuel until you let up on the gass pedal. Since then, its gotten worse as I can't get up to 40 without this problem. If I punch it, the engine studders and acts intermittently like its almost out of fuel, occassionally backfiring through the exhaust. I pulled the carb, checked and cleaned the filter, cleaned all the carb ports. Gasket is brand new, not damaged. Most of the vac lines are capped off as they always were. In park at an idle it runs rough but well. Bump the key and it starts right up. When I pull the throttle to about 2500rpm's, the carb and exhaust backfire. This is gradually pulling the throttle. If I "snap" the throttle, No hesitation, no backfire. Also, I have a new knock in the driver side valve cover. Assuming a rocker. Sounds like a Dodge. I've tried adjusting the mixture, no different. Cap and rotor are in good condition but I have a spare set to try. Any suggestions would be greatly appreciated. Again, I am quite experienced in auto mechanics and look forward to helping others here. Thanks!

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Old 07-27-2010, 02:27 PM
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You said it sounds like it's not getting any fuel. Did you check the simple things like a clogged fuel filter or a bad fuel pump? Check the simple things first before you go and spend money on new parts that you might not even need. Good luck
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Old 07-27-2010, 02:37 PM
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Two things come to mind;

Fuel pressure. Plugged filter/s, weak pump, anything that could effect fuel pressure.

Pickup coil in the distributor, module, coil, etc.

Well maybe three things, flat cam lobes. (noise in the valve cover)

Good luck.
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Old 07-27-2010, 02:57 PM
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When you suspect more than 1 thing is wrong, use elimination process.
1 put a tee in line going to carb from fuel pump with a long hose so you can look at pressure during a test drive should be about 4 to 7 psi or so.
2 notice at what rpm this happens bring it up to same rpm with a timing lite hooked up n check timing at that rpm
remember if you have low pressure it could also be in suction side of fuel pump
3 check float bowl level
4 make sure top of air filter is not sucking down due to air demand of motor
5 make sure there is adequate clearance betwwen air filter and fuel bowl vent
I really think this a fuel starvation(leanout)issue,last part you played with was carb and seemed like it made it worse.
I hope this helps its at least a starting point.
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Old 07-27-2010, 02:59 PM
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done

I did the simple stuff. filter is brand new, free of dirt. Fuel pump is pumping plenty, its fairly new. Fuel and pressure are good up to the carb. Is it possible the clacking and poor performance is due to a bad lifter/pushrod? I also pulled, cleaned and gapped all the plugs. All looked normal except one. It had oil sludge on it, but I know it has a bad valve guide as it blows a little smoke when it is first started.
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Old 07-27-2010, 03:00 PM
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I almost forgot i have also seen this happen due to bad ign.coil
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Old 07-27-2010, 03:08 PM
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could be lifter take valve cover off n bump engine over with ign off & measure valve lift at top of spring, this is not a dead nuts method but it will tell you if there is problem with cam,pushrod, start motor with valve cover off and if you have hyd.cam use the eng. run method to adj valves. could be a number of other things like burnt out xhaust valve timing off in cam due to chain malfunction etc. but i really would eliminate fuel issues first.
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Old 07-27-2010, 03:11 PM
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i just sent you private message,read it.
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Old 07-27-2010, 05:39 PM
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duh, lol

I dont know why I didnt check this before. I noticed that when I push the throttle, the secondary bottom plates dont open at all. When the primary is opened about 50%, shouldnt the secondary start to open? I know the top plates are vac operated, but the bottom is controlled by a rod on the pass side near the choke. Does this sound right? I'll be checking that out tomorrow.
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Old 07-27-2010, 06:57 PM
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what kind of carb? q-jet?
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Old 07-27-2010, 07:01 PM
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quad

Yes. I call em "quadrajunks"
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Old 07-27-2010, 07:09 PM
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thats funny, the guy i bought my engine from called them that, and he was in fl too. but the qjet is a very good carb. if i remember corectly the secondaries dont open unless you are driving, not when you're in park.
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Old 07-27-2010, 07:29 PM
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Quote:
Originally Posted by jstuhl
I dont know why I didnt check this before. I noticed that when I push the throttle, the secondary bottom plates dont open at all. When the primary is opened about 50%, shouldnt the secondary start to open? I know the top plates are vac operated, but the bottom is controlled by a rod on the pass side near the choke. Does this sound right? I'll be checking that out tomorrow.
The secondary throttle blades are locked out if the choke is closed.

Use a can of flammable-type carb spray on it (not that water-based crap)- just follow the can directions. Cost: <$2.
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Old 07-28-2010, 12:52 PM
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timing chain

Thanks for all your input, guys. I found out the hard way what the problem is. I had the truck running ok, so took it to the store. When I came out it wouldnt start. It cranks once or twice then bumps. Im quite certain now the timing chain jumped. I guess the symptoms Ive been experiencing was an indication of a stretched chain and I pushed it too far. Damn, thats the first time she ever left me stranded. Wheres my tow strap, lol.
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