I recently resurrected a 71 Dart with a 225. I rebuilt the carb, installed a new distributor, set timing to 5* BTDC, new coil, ballistic resistor, gaped the spark plugs, lashed the valves. The car fires right up and will idle all day no problem. If it is revved up it will back fire intermittently, if it is driven it doesn't back fire as much but shortly after driving it begins to idle very low and often stalls. When it idles low it also makes a strange sound that I would describe as a pinging squeal. It sounds like the fan belt is slipping and something is hitting the fan blade, but I assure you it is not. It is coming from inside the engine.
intermittent backfire/low idle
- Thread starter fireman4
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twayne24365
Well-Known Member
With it running did you check timing with a timing light?
5* is not a lot of timing. Try up around 12 or 15.
nm9stheham
Well-Known Member
The /6's have pretty soft valve springs and the valves tend to stick in the guides, especially after they get old and lots of old oil crud and varnish inside. So sticking vavles are often an issue. This is one possible explanation for the noise form inside the engine.
Another possibility is that the water pump impeller is loose and hitting, scraping inside the water pump cavity in the block. It would be easy to pull the water pump and look for signs on the impeller and block cavity. If this is an auto trans car, another possibility is that a torque converter bolt has backed out. The 2 above problems would obviously be on opposite ends of the engine!
But neither would necessarily be connected with a low idle. Sometimes carbs and air cleaners will squeal/squeak at certain low air speeds, but that would not explain the pinging.
The drop in idle could well be it is just normally dropping off of the fast idle step; the fast idle cam on the carb rotates into position to raise the idle speed when the engine is cold and the choke activates the fast idle cam on the carb. If you have not done so, look again at the curb idle setting. It ought to idle OK with 5 degrees of timing.
Another possibility is that the water pump impeller is loose and hitting, scraping inside the water pump cavity in the block. It would be easy to pull the water pump and look for signs on the impeller and block cavity. If this is an auto trans car, another possibility is that a torque converter bolt has backed out. The 2 above problems would obviously be on opposite ends of the engine!
But neither would necessarily be connected with a low idle. Sometimes carbs and air cleaners will squeal/squeak at certain low air speeds, but that would not explain the pinging.
The drop in idle could well be it is just normally dropping off of the fast idle step; the fast idle cam on the carb rotates into position to raise the idle speed when the engine is cold and the choke activates the fast idle cam on the carb. If you have not done so, look again at the curb idle setting. It ought to idle OK with 5 degrees of timing.
mguner
How many is too many?
Valve spring, rocker arm, push rod.... something in that area.
Good suggestions and a few I hadn’t thought of.
Advance timing to 12*or 15*– I tried this but is seems to backfire more if it is advanced more than 5* or retarded more than 5* BTDC
Water pump – a good possibility will pull it to see.
Valve springs – also a good possibility I will keep running it trying to burn off the varnish.
The fast idle is set properly and it runs fine after the set screw drops off the cam. It gets the death idle that stalls it only after driving for a little awhile. It seems too lean but turning the fuel mixture screw out only helps to a certain point and the idle no longer increases.??????????????
Thanks for the comments
Advance timing to 12*or 15*– I tried this but is seems to backfire more if it is advanced more than 5* or retarded more than 5* BTDC
Water pump – a good possibility will pull it to see.
Valve springs – also a good possibility I will keep running it trying to burn off the varnish.
The fast idle is set properly and it runs fine after the set screw drops off the cam. It gets the death idle that stalls it only after driving for a little awhile. It seems too lean but turning the fuel mixture screw out only helps to a certain point and the idle no longer increases.??????????????
Thanks for the comments
Breadburner
Well-Known Member
Check the plugs maybe you can isolate it to a cylinder.....
mguner
How many is too many?
Possibly a cam lobe gone south..... Check the travel of all the rocker arms.
mguner
How many is too many?
If the exhaust valve doesn't open or barely opens you will get stuff feeding back into the intake when the intake valve opens. Acting much like a malfunctioning EGR system but really acting up under load.
nm9stheham
Well-Known Member
Just running it will not likely free up a sticky valve; it will take some treatment. Sloooowly pour half a can of Seafoam or Rislone down the carb while using the other hand to the help keep the revs up to a fast idle. This will get on the intake valve stems and be pulled up into and clean the intake guides some. Put the other half int eh crankcase and see if that cleans the exhaust side. Pull the valve cover and start it an let it idle. Watch the rockers to see if any look 'lazy', indicating valve sticking. You can work some carb cleaner down the guides from the top by spraying a bit through the springs.
This wil not help worn guides, which is common.
As for falling of of a good idle..... I almost suspect in the intake area. If richening the mixture does help, then here is one of the best tricks I have learned on FABO: lay a shop rag or other loose weave rag over the carb while running at idle and see if it smooths out. If so that, indicates a lean mixture. It could well a vacuum leak that opens up. Also, hook up a vacuum gauge and read manifold vacuum when cold, when idling Ok, and when idling badly and let's see what that shows.
This wil not help worn guides, which is common.
As for falling of of a good idle..... I almost suspect in the intake area. If richening the mixture does help, then here is one of the best tricks I have learned on FABO: lay a shop rag or other loose weave rag over the carb while running at idle and see if it smooths out. If so that, indicates a lean mixture. It could well a vacuum leak that opens up. Also, hook up a vacuum gauge and read manifold vacuum when cold, when idling Ok, and when idling badly and let's see what that shows.
I previously treated the valves with sea foam before starting and after I got it running treated them with Marvel Mystery Oil. I am also running Marvel in the oil. I lashed the valves again yesterday (i do it while its running) everything looked good. I still need to try and isolate to one cylinder. I will definitely try the shop towel trick and I will check for leaks again. I used the spray bottle method (thinking I may also try the carb cleaner method).
Pulled plug wires one at a time, when I pulled #5 it did not backfire. I lashed the valves to .010 and .012. ( which I belive are the specks for the comp cam that is in it) this raised the idle and I was able to adjust it down with the fuel mixture screw. I also discovered that the throttle return spring bracket was lose and about to fall off, I adjusted it and the idle is stable now. The screeching ping noise is also gone for now and only occurred when the idle was so low that it was about to stall. It also only backfires now when I let off the gas. When I connect the vacuum advance (on the brand new distributor) it runs pretty shitty under load and backfires all over the place. So for now I am running with the vacuum advance off. Getting close any other suggestions?????
nm9stheham
Well-Known Member
Vacuum diaphragm in the advance dashpot ruptured? (Vacumm leak...)
So backfires under load..... with vacuum advance. Have you checked the timing marks against true TDC so see if the marks are accurate?
Put a separate vacumm hose on the advance dashpot and suck on it to see if the mechanism is working right with the cap off. Then see what that does while running.
So backfires under load..... with vacuum advance. Have you checked the timing marks against true TDC so see if the marks are accurate?
Put a separate vacumm hose on the advance dashpot and suck on it to see if the mechanism is working right with the cap off. Then see what that does while running.
It is a brand new distributor with the vacuum pre-installed. I put a vacuum on it to test it before I installed it to insure that it advanced the rotor. I had heard somewhere you should do that before you install a new distributor. The timing mark is probably off a bit since it has two marks. I do not have a positive stop so I cant check for exact TDC, but I did confirm the mark is on the compression stroke of the #1 cylinder. I tuned it by ear to the spot that it runs the best. Smooth idle, no backfiring under load which is 5* BTDC on the new timing mark. But it does backfire when I let off the throttle. I think that might be because of the exhaust leak????? I know there is an adjustment for the vacuum pod but have yet to find which way to turn it, probably just trial and error. I will try two turns clockwise if that doesn't work then 4 turns counter clockwise?????
Inertia
Well-Known Member
I was going to ask if you had an exhaust leak, at the start of the thread..
Repair the exhaust leak and the backfire will likely go away..
It's quite common.. lol
cheers
Repair the exhaust leak and the backfire will likely go away..
It's quite common.. lol
cheers
I installed a new gasket, actually did a double gasket. The leak is 99% resolved, it only backfired once on the test drive and was barely audible. thx
Inertia
Well-Known Member
Glad it worked out..
cheers
cheers
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