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Old 10-13-2018, 07:52 PM   #21
heneste
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Default Re: Terriable backfiring

Similar to what Curtis has mentioned, I've run into fuel blockages due to rust (either in tank or sediment bowl) and with movement of the vehicle it can stir up the sediment and block the fuel flow. Likewise, any sediment that makes it into the carb can also cause this intermittent backfiring but periodic cleanings of these parts may correct it if it's a fuel issue.
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Old 10-13-2018, 07:59 PM   #22
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Default Re: Terriable backfiring

Check to make sure the spring on the moveable arm of the points is strong enough. It was mentioned in an earlier post. Hook a fingernail on it and pull it apart. It should give you a bit of a tussle.

A bad flexible wire under the upper plate can cause the backfire at high rpms too.

Did you recently have your carburetor apart? Leaving out the venturi will cause similar troubles.
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Old 10-13-2018, 09:41 PM   #23
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Default Re: Terriable backfiring

One test we did I failed to put forward was the fact we grounded each plug.
#4 showed a significant change in idle speed, #3 showed very little change, #2 showed a good change and #1 had NO change.

That was really the reason we have considered changing the plugs which have 6-7 thousand miles on them.
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Old 10-13-2018, 10:13 PM   #24
Marshall V. Daut
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Default Re: Terriable backfiring

'Sounds as if you are about ready to consider the weird causes of misfires since you've just about exhausted the list of usual culprits. So, odd though it may sound, try this: Take off the terminal box cover and wiggle the bolts sticking out. If either is loose or can be pushed in and out, it's possible that the heads are touching or arcing against the metal firewall and creating a misfire. This will be especially likely when the firewall gets warm and the metal expands to close the gap between itself and the stud heads. In the 1970's, a couple inferior and somewhat shady parts vendors (old-timers know whom I mean!) imported lousy terminal boxes that had bolts with the heads too big to be completely sunk into the plastic body. The holes were not countersunk deep enough, either. In any event, after some use and vibration, the bolts loosened up and moved ever so slightly (but enough!) towards the firewall, causing occasional arcing and misfiring. This happened to me personally in Germany in 1974 in my 1928 roadster. I swapped out EVERYTHING while sitting along side the Autobahn as you have done, but couldn't fix the misfiring. 'Drove me nuts!!! Cutting to the chase, it turned out to be one of those poorly-made reproduction off-shore terminal boxes with the too large head bolts and shallow countersunk holes. I have since diagnosed a couple other instances locally that perplexed the owners because they couldn't find the source of their Model A's misfire. They also had older terminal boxes that had been bought "on the cheap" from the same suppliers. There are probably still hundreds of them out there waiting to ruin some Model A owner's day.
I'm guessing if you have a car that was rebuilt or "restored" in the 1970's or 1980's, one of these timebomb terminal boxes may be at fault. If the two studs are not loose, back off the four mounting screws inside on the firewall, pull the terminal box out a little ways from the firewall and keep it separated by inserting a couple pieces of cardboard. Then start the engine and drive the car. If there is no miss, you can bet that one or both stud slot-head bolts is/are arcing against the firewall. It may be a long shot, but you've tried just about everything else.
Did I miss that you performed a compression check to ensure that a valve isn't stuck or burned out?
Marshall

Last edited by Marshall V. Daut; 10-14-2018 at 08:34 AM.
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Old 10-14-2018, 09:05 AM   #25
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Default Re: Terriable backfiring

Pull the side timing gear cover and check the large timing gear.
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Old 10-14-2018, 10:42 AM   #26
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Default Re: Terriable backfiring

Quote:
Originally Posted by Cape Codder View Post
One test we did I failed to put forward was the fact we grounded each plug.
#4 showed a significant change in idle speed, #3 showed very little change, #2 showed a good change and #1 had NO change.

That was really the reason we have considered changing the plugs which have 6-7 thousand miles on them.
Change the location of the plugs you have in the car and see if they have the same symptoms in different cylinders.
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Old 10-14-2018, 12:51 PM   #27
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Default Re: Terriable backfiring

You've checked a few likely culprits but not ALL.

Battery terminals tight?


Auxiliary ground strap needed?

Ammeter and connections thereto? check by attaching jumper to junction box lugs.

Pigtail in distributor?

Swap with a good carb?
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Old 10-14-2018, 01:10 PM   #28
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Default Re: Terriable backfiring

Mine was caused by trash in the carb.
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Old 10-14-2018, 05:49 PM   #29
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Default Re: Terriable backfiring

Go ahead and change the plugs, nothing to lose. Put in W-18's NOT 3X's, it will be cheaper to experiment this way.
Have you ck'd compression??
Paul in CT
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Old 10-14-2018, 08:19 PM   #30
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Default Re: Terriable backfiring

Soooo……many questions to answer so here goes:
Frank 55a As I have said I have changed out the Distributor body AND Cap along with the Coil wire. Ernie I had checked the nuts on the amp meter and ignition because I was in there to check amps on the back of the meter. Nothing lose at that time so I’m holding off taking the dash down again. As to the fuse box I don’t have one on my car. Synchro909 I’m thinking you meant the ignition switch, see above. Curtis in MA I had drained the gas tank at Gettysburg last year to check the filter and found nothing but a clean filter and not bragging but a very nice clean tank. I think I will hold off on checking this again for awhile. Heneste see previous answer. 1crosscut I have been in there to check the arm of points several times in adjusting points cause I want it perfect at .020” and never felt the spring to be bad. As to the flexible wire under the upper plate I plan on checking this again. No, I haven’t taken the carb. apart and I wouldn’t leave out any parts anyhow HA! HA!
Marshall V. Daut I have to say I had that box apart because the back part of the box was cracked OR I took it off for the same reason you mentioned about the bolts. I remember using RTV on the back of the bolts so I’m hoping it would be good for five years. But this is another suggestion I have to keep in mind if I don’t find anything else. NO, you didn’t check.at I had done a compression check . We actually had a suggestion from a non-fordbarner and non-Model A’er to see if we have a leaking head gasket BECAUSE when I went to short the plugs two of them didn’t really have any change in RPM. Katy now you are talking about something I don’t want to get into especially when I’m trying to find something simple, HOWEVER I will keep it in mind. 1crosscut I’m sorry but I don’t understand your suggestion or I guess what I don’t understand is what would it prove? steve s I actually disconnect my battery for the non-season and when getting it ready to get back on the road I always wire brush the posts, and cables, put electro (sp?) grease on each post then check the battery with a hydrometer done maybe 600-700 miles ago. The pigtail was screwed all the way in so I backed it out about two threads. I just had screwed it in with my fingers, no pliers. 1931 flamingo W18 Y’s are planned. No compression check but see above.

Thanks again to all who have responded and given me advice which I really appreciate. I’m just trying to get the info in some sort of order. By the way sorry for the rambling but trying to talk to everyone.
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Old 10-14-2018, 08:48 PM   #31
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Default Re: Terriable backfiring

a great case study in randomly trying things versus systematic trouble shooting, still playing out.
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Old 10-14-2018, 09:16 PM   #32
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Default Re: Terriable backfiring

What I meant by changing the positions of the plugs was to take
#1 plug to #2 cylinder
#2 plug to #3 cylinder
#3 plug to #4 cylinder
#4 plug to #1 cylinder

Then see if shorting out the plugs has the same result as your first test.
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Old 10-14-2018, 09:29 PM   #33
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Default Re: Terriable backfiring

I understand what you are saying BUT what does it prove? Please explain.
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Old 10-14-2018, 09:37 PM   #34
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Default Re: Terriable backfiring

Check all your electrical connections then do a load test on the battery. Sounds stupid but a friend had the same problem and it turned out to be a bad battery. Who would have thunk it!
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Old 10-14-2018, 09:47 PM   #35
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Default Re: Terriable backfiring

If you switch plug if they miss in the same ones they miss in now you know your plugs are good
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Old 10-14-2018, 10:35 PM   #36
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Default Re: Terriable backfiring

First - 1931flatheadthanks for the response and explanation. I will try that.
PalAl - also thanks for your response and suggestion.

Boy, do I have a lot of work ahead of me in the next couple of days with all the suggestions.

Thanks again!!
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Old 10-14-2018, 10:41 PM   #37
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Default Re: Terriable backfiring

When I get into something like this it sure can be frustrating, I like to eliminate many items at one time, just a thought have you considered "Hot Wiring" to eliminate the key switch, and all other electrical circuits other than the points, condenser and coil. A simple jumper from the starter terminal ...
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Old 10-14-2018, 10:47 PM   #38
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Default Re: Terriable backfiring

holdover Thought I said "used a jumper to check the ignition switch" in post 18 which eliminated the switch, OR so I think. Thanks for your response!
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Old 10-14-2018, 10:59 PM   #39
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Default Re: Terriable backfiring

Just fixed one like yours. Idled great, would run several quarters of a mile then sputter and backfire. Then start right up and repeat as you waited to get back into traffic.

Gas flow to carb was a dribble from disconnected fuel line. Blew air into tank from gas line and found high resistance (~15 psi to overcome). Checked screen in gas sediment bulb = clean. Disconnected interior gas line (was a repro fat one) and found it packed full of rust like coffee grounds.

About 12 weeks ago the leaky fuel shut off valve was replaced with an Ace Hardware ball valve complete with yellow natural gas teflon tape sealant and the stand-up in-tank filter was removed opening the floodgates for years worth of crud to descend into the fuel system and reduce flow to a trickle good enough for idle but not for go.

I have replaced a lot of corroded/dissolved/and as a result plugged in-tank filters and cursed them all the while but they do serve a useful purpose if they are installed AS A CONSUMABLE MAINTENANCE ITEM (around here 3-5 years is pushing it for trouble) and not a permanent fix.
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Old 10-15-2018, 06:46 PM   #40
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Default Re: Terriable backfiring

Quote:
Originally Posted by 1931flathead View Post
If you switch plug if they miss in the same ones they miss in now you know your plugs are good
Thanks, that is what I was getting at.
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