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Old 08-27-2017, 04:54 PM   #1
Chevyman1987
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1985 chevy c10 rough running

I got a 1985 chevy c10 305 it's running rough I just got done doing a compression check and all where 130 and put a new timing chain in and timed but stills runs rough and backfires so what could be the deal with it also I checked all the pushrods and all where good.
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Old 08-27-2017, 04:56 PM   #2
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Re: 1985 chevy c10 rough running

could be all sorts of things...carb,electrical is a place to start..did you check the cam for wear?
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Old 08-27-2017, 05:16 PM   #3
Chevyman1987
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Carb is a 6 months old was having the problem back then and after i done the carb and timing I just walked away from it and didnt check the cam. I just would not think that it would be it the truck only has 88xxx miles. Not saying it couldn't be. That's where I'm stuck at don't really know what to look for as far as eletric parts goes
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Old 08-27-2017, 05:25 PM   #4
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Re: 1985 chevy c10 rough running

Could be a vacuum leak around the carb mounting plate, start the truck and spray carb cleaner around the carburetor, if the rpm increases or smooths out you have a leak
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Old 08-27-2017, 06:21 PM   #5
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Re: 1985 chevy c10 rough running

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Old 08-27-2017, 06:45 PM   #6
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Re: 1985 chevy c10 rough running

backfire is usually a timing issue...
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Old 08-29-2017, 02:18 PM   #7
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Re: 1985 chevy c10 rough running

gotta be a carb issue. I have exact same set up. was also running rough so i put a rebuild kit and it seemed to do the trick.
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Old 08-29-2017, 04:13 PM   #8
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Re: 1985 chevy c10 rough running

Just got done tuning the carb and runs whole lot better. But still backfires putting a timing light on it tomorrow. I put the carb and timing chain in myself but paid someone to time it as I had no timing light at the time.
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Old 08-29-2017, 04:16 PM   #9
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Re: 1985 chevy c10 rough running

But thank everybody for there input. Does anyone know of a good video shoeing how to tune the carb just to make sure I'm doing it right. Don't really wanna make it worse then what it is. But it can sorta drive right now lol
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Old 08-29-2017, 11:33 PM   #10
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Re: 1985 chevy c10 rough running

Ok 1st things 1st.
What is the fuel psi at the carb?
Have you checked to see that power enrichment piston & needles are free & not stuck?
Have you checked the vac advance diaphragm for being ruptured?
Have you checked the fly weights in the distributor for being stuck?
What manifold vacuum does it idle at?
Are you sure the EGR isn't stuck open?
What is the base timing set at?
Check these 1st & report back.
My Paw N Law used to brag on his '73 that it had only had 1 oil change in it's life. Bro N Law was putting new cam in at 35K miles...
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Old 08-30-2017, 09:02 AM   #11
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Re: 1985 chevy c10 rough running

I put a new fuel pump on it at the beginning of the year. Before that it wasn't getting any gas so I think that helped lol. The EGR is gone has the bypass plate. My vac. Has new lines and when you unplug you can tell it's working. As far as the fly Weights didn't think to check that. I'm gonna look today and let you know.
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Old 08-30-2017, 12:01 PM   #12
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Re: 1985 chevy c10 rough running

I had a backfire when I first started up my V8 Pontiac Firefly - turned out I had #6 and #8 swapped.

#8 was firing on the compression stroke, which explained the rough running. #6 was firing on the exhaust stroke - igniting the unburned fuel in the exhaust.
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Old 08-30-2017, 12:06 PM   #13
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Re: 1985 chevy c10 rough running

Wires are all hooked up right. I did that before on my 74 I had took me an hour to find out what I did wrong lol. And the weights are not stuck. Now just waiting on my timing light to show up.
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Old 08-30-2017, 04:00 PM   #14
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Re: 1985 chevy c10 rough running

Ok so got my timing light in and the line is on the 2nd notch. I adjust back and forth on all the marks. And still backfire. I pulled the cap and made sure it's lining up on #1 plug and it was. Checked to make sure #1 piston was up and it was. What else can I check
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Old 08-30-2017, 06:54 PM   #15
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Re: 1985 chevy c10 rough running

The fuel pump psi is 7. I set both carb screws all the way in now for the most part no backfire from the talipaipe there still a little but can barely tell it but now it's coming from my carb. And when I try to adjust out it starts back to the pipe.... So just to recap I have checked the compression for the piston all came back at 130.... I checked the timing and it's right on the 2nd mark... Fuel psi is 7.. And fly weights are not stuck. I have removed my vavle cover and checked my pushrods all seemed and look fine. I have replaced my vac. Line that goes to the disbutor and made sure it's working.
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Old 08-30-2017, 06:55 PM   #16
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Re: 1985 chevy c10 rough running

Open the hood when its dark and look for crossfiring on plug wires. Sounds dumb but its free and easy...
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Old 08-30-2017, 08:12 PM   #17
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Re: 1985 chevy c10 rough running

Is your engine giving a Poot Poot Poot noise out the exhaust at idle?
Which carb do you have? A Qjet?
Does the exhaust SMELL rich enough to burn you eyes when idling?
Still need to check the manifold vacuum at idle. See what value it is & whether it fluctuates up & down.
Not got a lumpy cam does it?
Do you have twin pipes?
Kinda related I had a 302 Ford once that had an exhaust noise at idle. With the Vacuum advance hooked to manifold vacuum it made the noise, but hooked to ported vacuum it quit.
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Old 08-30-2017, 08:45 PM   #18
Chevyman1987
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Re: 1985 chevy c10 rough running

Yes it has a qjet on it. And no bad smell no more I turned the carb screws all the way in and no more smell. But now when I put it in drive it backfire through the carb now lol. I been adjusting the carb more and more. I can go down the road but when I push the gas a little hard you guessed it pow. So when I adjust the carb any further whee it's at backfire from tail pipe and not much power. Also I put a elderbrock on it and had the same issues and it was new
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Old 08-30-2017, 09:11 PM   #19
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Re: 1985 chevy c10 rough running

Did you plug off the vac line to the vac pot on the dizzy when you checked your timing?
Sounds like lack of initial timing.
THe second notch would be 6 degrees.
You need at least 14.

If you can run it with the mix screws all the way in that tells you you're idling on the power circuit.
You need to get the throttle closed more to get the mix screws working.
More timing will help you do that.
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Old 08-30-2017, 09:29 PM   #20
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Re: 1985 chevy c10 rough running

Vacuum can on distributor could be bad also. If it has one
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Old 08-31-2017, 08:55 AM   #21
Chevyman1987
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Re: 1985 chevy c10 rough running

Yes it was pluged off when I checked timing and I had both screws all the way in and it ran good that way well at idel once I gave it some gas poping in the carb now. I backed out half a turn. Anymore then that pop out the tail pipe.
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Old 08-31-2017, 01:01 PM   #22
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Re: 1985 chevy c10 rough running

Quote:
Originally Posted by Chevyman1987 View Post
Yes it was pluged off when I checked timing and I had both screws all the way in and it ran good that way well at idel once I gave it some gas poping in the carb now. I backed out half a turn. Anymore then that pop out the tail pipe.
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That tells you you're idling on the power circuit. Running rich. That's why it pops out the pipe. Incomplete combustion due to not enough initial timing combined with the over rich mixture.
You need to get your primary throttle closed so you cover the transition slot.
There's some info here about the transfer slot. It says for a holley but all carbs are the same.
http://www.crankshaftcoalition.com/w...EI_distributor

Put 10 degrees more initial into it. That'll speed up your idle. Then turn your carb back down and get your mix screws working right with the transfer slot.
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Old 08-31-2017, 01:14 PM   #23
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Re: 1985 chevy c10 rough running

Plug wire grounding out? That was my problem...
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Old 08-31-2017, 03:09 PM   #24
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Re: 1985 chevy c10 rough running

Like geezer said- Look into carb at boosters & see if they are dripping. Not seen Manifold Vac reading yet. If low & steady advance initial timing & close idle screw on carb. If fluctuating could be valves leaking.
Note: Just because timing light is on the line don't mean it's right. Damper rings on old Chevys are notorious for turning on the hub & you may have to set initial time "By Ear".
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Old 08-31-2017, 04:21 PM   #25
Chevyman1987
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Re: 1985 chevy c10 rough running

Thanks geezer for the info. I advance timing to 16 and did a little more adjusting on my carb as much as I could do and we went down the road there is very slights poping at idel. And can't give it all it's got it will bog down after about 3/4 peddel down. But for the most part it's back running like it use to hell it's better then it use to run. Got it non running lol and when you say manifold vac where is that located. I'm not very incline on this kinda stuff but love monkeying with it.
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