Popping through carb
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Popping through carb
I been trying to get my truck to run smooth. When I get on it it lugs then pops through the carb. I tried setting the mixture on the carb advancing the timming. A while back I posted about my truck having a funny idle. well I changed the air screws since one was flatten and I replaced the fuel pump since it was leaking. My truck sounds like one big vaccum leak. but i think it is sucking through the carb. there is so much vaccum lines that i dont know where to plug the dizzy in. the people before me had on a line that had no vaccum but when i hooked it up to vaccum the truck reacts the same. and whe I am trying to set my timing the mark bounces up and alot i cant get it to stay in one spot. Any ideas as what it could be?
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A badly worn cam can cause this. How many miles on the engine? The 305 chevys were famous for this problem and poping back thru the carb was the first indicator. You have to find out where ALL of those lines go first.
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From: Murray
It would help if you gave us details as far as engine size...2 or 4 bbl carb....ect. But from what youre describing..my guess would have to be that you need to work on the timing some more...if the timing mark is bouncing around just tune it in by ear to begin w/ so that it doesnt idle too fast or idle so solw it wants to die...then bolt the dizzy back down drive it and see if it is a little better..repeat untill you have it dialed in.
Now thats assuming that you dont have a problem with the timing chain which it could also be. If theres too much play in it due to wear it could have skipped a tooth and thrown things off. Could also be a symptom of a bad EGR valve if yours was equipped with one and still has it..if not it could be a worn out gasket on a carb spacer thats causing it as well.
Basically youre gonna have to do a process of elimination to figure out whats up with it check the things that you can inspect visually first and then move on to things that may require taking parts off.
Hope it helps,
-Chris
Now thats assuming that you dont have a problem with the timing chain which it could also be. If theres too much play in it due to wear it could have skipped a tooth and thrown things off. Could also be a symptom of a bad EGR valve if yours was equipped with one and still has it..if not it could be a worn out gasket on a carb spacer thats causing it as well.
Basically youre gonna have to do a process of elimination to figure out whats up with it check the things that you can inspect visually first and then move on to things that may require taking parts off.
Hope it helps,
-Chris
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From: Crownsville, MD
Could be something simple
I had the same problem on my 390 back in 1989. I took it to a mechanic and he said it was a wiped cam and wanted to start tearing into it. I couldn't afford that and didn't think it was a good way to approach the problem. I told him to start looking at the little things first (we both thought ignition) and then we can move to the bigger ($$$) things if necessary. After picking at it for a little while, he found a bad plug wire. Burned right through the core.
Installed a new set of wires and been running good ever since.
Installed a new set of wires and been running good ever since.
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From: Island Southeast Alaska
Try everything above. When looking for vacuum leaks I use staring fluid. Not Either. Brake clean and sometimes WD40. What ever is handy. While running spray around the carb (Check for play in throttle bore). Spray intake to head gasket areas. If engine chances tone. Speeds up or slows down than ya have a vacuum leak. I know ya have a hundred vacuum lines in the way. But the wash down method will still work. Where I live we have no enforced emission laws so I remove everything that is not needed. And that leaves only the brake booster, PCV and Dizzy. If the big diafram sticking out from under carb bothers ya than build or buy a spacer to keep the carb high enough so linkage works. I am going with a vacuum leak, jumped time, or the wrong ported vacuum to dizzy. And maybe a combo of all three. I am assuming from your Sig that we are talking about a 78 351m/400?
PS: According to your profile ya have a 78 bronco and live in Washington state. I was raised there and spend a lot of time in the Shelton area. Strip the smog stuff of and run it. I have never had a rig inspected in that state that related to emissions.
PS: According to your profile ya have a 78 bronco and live in Washington state. I was raised there and spend a lot of time in the Shelton area. Strip the smog stuff of and run it. I have never had a rig inspected in that state that related to emissions.
Last edited by arctic y block; Jan 31, 2007 at 12:31 PM.
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Post Fiend
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From: Island Southeast Alaska
Also if a rig is over 25 years old you can get historical plates. And regardless what they say as far as only using it in parades and an occasional Sunday drive. I licence all mine that way and use them as daily drivers. Never been stopped except for the gear head Officer to check out my ride. As everyone dose when I am cruising my UNI.
Check out my gallery. Thous pic's were taken in Washington. Note the original Colorado plates.
Check out my gallery. Thous pic's were taken in Washington. Note the original Colorado plates.
Last edited by arctic y block; Jan 31, 2007 at 12:46 PM.
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Well first off, you don't time the engine with avacuum line arttached to the distributor.
You disconnect the vacuum line to it and plug it with a golf tee or something.
Next you need to check for timing gear to chain slop.
Grab a breaker bar and a socket to fit the crank bolt .
Remove dist cap
Put socket on crank bolt and turn crank one direction ..stop
Now before you turn it back the other direction..keep your eye on the Dist rotor.
Turn the crank the other way and observe how many degrees you turn the breaker bar before the Rotor STARTS to move....STOP
Now ..turn it back the other way again and observe when the rotor STARTS to turn to verify this degree movement.
Come back here with the results.
Back-firing out the carb means timing is off
The Timing mark jumping around a lot is usually a bad Chain to gears situation which effects timing.
If this is about the Bronco,FTE does have a Big Bronco forum..... but we can help you here
You disconnect the vacuum line to it and plug it with a golf tee or something.
Next you need to check for timing gear to chain slop.
Grab a breaker bar and a socket to fit the crank bolt .
Remove dist cap
Put socket on crank bolt and turn crank one direction ..stop
Now before you turn it back the other direction..keep your eye on the Dist rotor.
Turn the crank the other way and observe how many degrees you turn the breaker bar before the Rotor STARTS to move....STOP
Now ..turn it back the other way again and observe when the rotor STARTS to turn to verify this degree movement.
Come back here with the results.
Back-firing out the carb means timing is off
The Timing mark jumping around a lot is usually a bad Chain to gears situation which effects timing.
If this is about the Bronco,FTE does have a Big Bronco forum..... but we can help you here
Last edited by Mil1ion; Jan 31, 2007 at 02:18 PM.
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Yeah its a 78 351m the 400 threw a rod so the guy I bought it from put a 351min it. 77 I do beleave it has all the smog but I dont have smog checks and it is used for back roads mainly and a trip to town to get bigger things that my wifes car cant handle. The motor is stock and came from a wrecker so the miles I dont knowand it holds decent compresiion but the cam or numbers of thing could be wrong sounds like I am hoping to build a 400 this this summer if my pocket allows it. but if I need to change the cam and any internall parts then I am going to wait to put it into my 400. Do I need the vaccum lines for the egr? what about the plate? also what about the soup cans for the gas tank? they run into the lines too. I know when I build my 400 it will have weiend intake no egr and a eldlebrock performer. I also adjusted the timing by ear that is the only way I can get it close but it still pops through the carb any less and it bogs really bad any more and it wont start.
Last edited by 78fordranger; Jan 31, 2007 at 03:11 PM.
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The EGR will help prevent pinging or pre-detonation.
Of course the vacuum lines make a difference.
There are thermal switches that turn vacuum switches on and off.
Do the test I mentioned ..it will help with peace of mind.
Of course the vacuum lines make a difference.
There are thermal switches that turn vacuum switches on and off.
Do the test I mentioned ..it will help with peace of mind.
Post Fiend
Joined: Sep 2006
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From: Island Southeast Alaska
Like I said(in post #7) ya can strip all the Vacuum stuff off. and run the brake booster. dist. and pvc. And it will run fine. Get an aftermarket plate to replace the one under the carb or not. It will run the same either way. The timing slop test that Dennis said. needs to be done first. to tell if the timing set is worn.
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From: Murray
Well,
As far as the EGR goes....mine wore out then siezed up...so seeing as i didnt have to smog it anymore I just took off the carb and removed the EGR and EGR plate as one unit and blocked the small hole in the intake and I replced it with a carb spacer. Then capped off its vac. connection at the tree and tightened it all up.
Never had a problem w/ any pinging whatsoever and its been near two years now. Its only purpose is for pollution control it re-circulates un-burnt gas back in to be fully combusted (Exhaust Gas Recirulation valve). So if you remove it and block the vac. connections off correctly it shouldnt be a problem...heck you might even free up a couple ponies by doing it!
The only other thing you will want to check is for excessive play in the dizzy itself...the bearings at the bottom of the shaft can wear out and cause the rotor to wonder around in there giving you an erratic point gap...just pop off the rotor cap and see if you can wiggle the rotor from side to side...if you can then it may be time for a new dizzy alltogether which would give you a good excuse to siwtch over to one ofthe non point type units if you dont already have one.
-Chris
As far as the EGR goes....mine wore out then siezed up...so seeing as i didnt have to smog it anymore I just took off the carb and removed the EGR and EGR plate as one unit and blocked the small hole in the intake and I replced it with a carb spacer. Then capped off its vac. connection at the tree and tightened it all up.
Never had a problem w/ any pinging whatsoever and its been near two years now. Its only purpose is for pollution control it re-circulates un-burnt gas back in to be fully combusted (Exhaust Gas Recirulation valve). So if you remove it and block the vac. connections off correctly it shouldnt be a problem...heck you might even free up a couple ponies by doing it!
The only other thing you will want to check is for excessive play in the dizzy itself...the bearings at the bottom of the shaft can wear out and cause the rotor to wonder around in there giving you an erratic point gap...just pop off the rotor cap and see if you can wiggle the rotor from side to side...if you can then it may be time for a new dizzy alltogether which would give you a good excuse to siwtch over to one ofthe non point type units if you dont already have one.
-Chris