Cause's for Carb. backfire

[Ed Griffin]

Could ya'll post some reasons why you would get backfire through the carberator?

I've finally been back at trying to figure out what caused my D-24 to quit on me last year and have replaced wires, plugs, differant distributer and a new coil. Last night I was able to get the TDC mark to line up with the pointer but to do so I had to move the distributer so far clockwise that the vacuum is pointed straight up, 12:00 o'clock.

I did have to flip the distributer to make sure I was getting TDC with the rotor at 7:00 o'clock but this was only because I had put it in wrong earlier. It had the older one in when it started acting up and not been tuned in at least 2 yrs according to the previous owner.

Now with all this done the carb gives out a little pop backfire, not very loud but it is backfiring after so many rotations of cranking the engine over. From what I understand if you get backfire through the carb it is timing, if through the exhaust it is fuel symptoms. This is why I'm asking because I'm running out of ideas other than the timing chain jumped a tooth or something along that line.

This is how it all started. I was driving down the hwy and the car began to lose power. I found a ramp to get off at to turn around and as it was coming to a stop I was having to really pump the gas to keep her from dieing. While pumping the gas and coming off the hwy it backfired through the carb. only a couple of times. Once i saw the car was not going to die I turned around and headed back home. Now and then it would backfire through the carb but not constantly yet it was like it was only running on 5 cylinders, very sluggish.

I limped home which was a good 5 miles from where it all started. Would a timing chain that's jumped allow the engine to run at all much less that well enough to drive that far?

Something else to consider is that the exhaust manifold had a small crack under it yet it ran fine with it like this. Could this possibly have burnt a valve and be causing these symptoms?

Plans are to replace the manifold but i'd like to find the source of the problem before removing anything that doesnt pertain to a tune up. The carb. is getting a good flow of fuel but even a shot of starting fluid did not help last night. The fact that the carb. is backfiring tells me to follow that route of troubleshooting but I could really use your feedback.

Thank you in advance, Ed Griffin

[Woody Branham]

Just a thought, My DeSoto did the same thing. Thought I wasnt going to make it home, check your vacuum advance. Woody

[Jim Saraceno]

If you are concerned about a burnt valve you need to do a compression test, that will tell you everything. The other test, if you can get it running is the vacuum gauge reading, that will also tell you much about your engine.

Did you look at your sparkplugs when you pulled them? That should have told you if it was one cylinder not running or if they were all affected. They could have also told you a lot about what was going on.

If you are concerned that your timing chain jumped a notch, pull the plug that is over #6 cylinder, put a stiff wire in the hole and turn the engine over by hand until that wire tops out, your timing mark should line up with your pointer.

I'm not sure what you mean when you say... "Last night I was able to get the TDC mark to line up with the pointer but to do so I had to move the distributer so far clockwise that the vacuum is pointed straight up, 12:00 o'clock."... The timing mark is on the crank pulley and has nothing to do with the distributor. Are you saying that at TDC you had to turn the distributor to get the timing light to fire? If that is the case, could your oil pump been the one that jumped a notch?

Do the simple tests first, they'll save you a lot of time.

[Ed Griffin]

If you are concerned about a burnt valve you need to do a compression test, that will tell you everything. The other test, if you can get it running is the vacuum gauge reading, that will also tell you much about your engine.

Did you look at your sparkplugs when you pulled them? That should have told you if it was one cylinder not running or if they were all affected. They could have also told you a lot about what was going on.

If you are concerned that your timing chain jumped a notch, pull the plug that is over #6 cylinder, put a stiff wire in the hole and turn the engine over by hand until that wire tops out, your timing mark should line up with your pointer.

I'm not sure what you mean when you say... "Last night I was able to get the TDC mark to line up with the pointer but to do so I had to move the distributer so far clockwise that the vacuum is pointed straight up, 12:00 o'clock."... The timing mark is on the crank pulley and has nothing to do with the distributor. Are you saying that at TDC you had to turn the distributor to get the timing light to fire? If that is the case, could your oil pump been the one that jumped a notch?

Do the simple tests first, they'll save you a lot of time.

Jim, I havent done a compression test so far but only for this reason. When this all happened I thought that it was just time to do a tune up on the car since I hadnt done one yet and finding out from the previous owner hadnt in 2 yrs. I figured this would be the first step to take and see what came out of it and/or would cure the problem. Once I began to go through this I did'nt think twice about the new spark plugs and put them in with the new wires. Once I got all of that taken care of I cranked the engine to see if that helped and found that all of the new spark plugs were the wrong length, too long. I ended up with 6 new plugs flattened as well as 1, possible two messed up spark plug holes in the head. It boogered up the threads so badly on one i had to put a heli coil in night before last. Trying to remove another plug, cyl. #6, I noticed it acted as if it were cross threaded so I wrenched it back in for the time being.

I honestly dont know if i have the old plugs anymore. I usually take the old ones and put them back in the new box's and mark which cylinder they came from just for the reason you mentioned. It's been abt a yr ago now and so I'm stuck with out that chance to look and see what condition they were in. I do know that before this happened there was no indication that the motor was having any problems. Oil pressure was good, heat was where it was supposed to be... .

What i meant about getting it to TDC was I took the vacuum loose from the vacuum modulator and plugged it at the distributor. Then with the harmonic balancer marked on the TDC line I hooked up the timing light on #1 cylinder and began to crank the engine and move the distributor until the mark reached the TDC mark on the harmonic balancer. Before hand the chalk mark was on the passenger side of the TDC mark.

To get it to move to TDC while watching the timing light I had to move the distributor clockwise. Once it was at TDC the distributor had been moved so much that the vacuum modulater on the distributor was pointing straight up.

The distributor and all it's components along with the vacum mod. are all in good working order. I pulled it from my engne that I "refurbished" that was tested and running last time I tinkered with it.

[bob westphal]

From the way your story goes, I would say that the timing chain is bad. It doesn't have to jump to give problems. It could be just stretched. This would give some strange symptoms including popping through the carb and still run similar to what you mention. If you have a burned intake valve, the engine would run 'OK' with just a miss and the popping. Doing the check that Jim S. mentions won't give any indication that the chain is bad because the crank pulley is connected directly to the crank and therefore the timming mark will always line up when #6 is at TDC. The location of the distributor rotor is the indicator. You mention that the original direction that the vacuum advance points is now changed.This gives a huge clue as to what has happened. The normal direction of the advance is at about 10:30 on the clock. Put #1 piston at the top of it's stroke on the compression stroke. You can tell if you are coming up on the compression stroke by sticking your finger over the plug hole. When you feel the air rushing out, finish bringing the piston to the top of it's stroke. Note where #1 plug wire is on the distributor cap. Pull the cap and note where the rotor is pointing. It should be pointing to where #1 wire would be. If it isn't you have now found the problem. You need a new timing chain set. Good luck

Bob

[Plymouthy Adams]

I agree with Bob, first thing that popped in my mind was a jumped timing chain..I have seen this happen..at best you can get it to electrically match the crank/piston again but the valves are now opening and closing out of sequence, thus the snap crackle pop....you may have to pull the valve adjust side covers off to verify this along with a TDC guage set up in number six slug as you reposition your distributor at correct 7 o;'clock position. The cam will still be a bit hard to tell by looking here if out a tooth or so..you need the specs and a degree wheel..else removal of the timing chain cover would be best.

[Merle Coggins]

What engine are you running? On my 218 the distributor drives off of the oil pump, which is driven by the crankshaft. So even if the timing chain were to jump a tooth or two, it wouldn't effect the distributor. If you have to turn the distributor to a different location to get your timing light to show TDC, than I'd be checking out the vacuum advance mechanism or the centrifugal advance unit. There's a possibility that something there has gone to full advance or possibly greatly retarded the timing on you.

However, the backfire in the carb would suggest a faulty intake valve. Maybe one is sticking partially open. This would allow the combustion to get back into the intake manifold and back up through the carb.

That's my thoughts on the subject anyway,

Merle

[Plymouthy Adams]

Merle..the pump is driven off a gear on the center of the cam shaft that is in turn operated by the timing chain as it connects the crank and cam gear..that is why a chain can get enough slop in it and have a direct inverse effect on theelectrical timing. Slop will allow running but a jumped tooth results in mechanical/electrical out of sync...

[D25_Steve]

How bad is a backfire through the carb as a symptom? It is something that I wanted to get figured out, but not something I thought I needed to do before driving it more.

[Merle Coggins]

Ok, I stand corrected. I looked at pics of my engine and the oil pump location. It would be driven from the cam. For some reason I was thinking it sat lower and was driven off of the crank. The jumped timing chain would make sense now.

Merle

[james curl]

Does it do it when accelerating while cold without the choke? If so it is not getting enough fuel or too much air for the manifold temperature. If it is doing it while hot under accelerating it is possible your accelerator pump is not delivering enough fuel. On my 55 chev 235 engine in my pick up I have a 390 cfm Holley four barrel that did not have a choke and would backfire when cold. I installed an electric choke and have not had any more trouble with it. When I first installed my rebuilt engine in my car I had dual Rochester carburetors on it and kept blowing the rear air cleaner off of the car when cold, once warm it was ok. Have since installed dual Carters with no chokes and do not have any problems. The Rochesters give very little accelerator pump action at low throttle movement causing the engine to go lean and back fire. The off idle circuit does not deliver enough fuel volume with the reduced air flow through the dual carburetors.

[Ed Griffin]

The engine is not running. When all this happened last year and I limped home and parked it thats the last time it would run. Right after that is when i tried giving it a tune up and when the long plugs bottomed out.

Last night I took the plug bolt out and put a stiff wire inside and watched the travel of the #6 piston. When it reached the top i stopped and then checked the TDC notch's and it could not be seen at all. Then I removed the dist. cap and the rotor was at abt 01:00 o'clock so this told me I had the distributor 180 degrees off. Pulled the ditributor back, moved the rotor to 07:00 o'clock and then hooked the timing light back up to #1 wire and turned the distrubutor as i cranked the motor while watching the timing light.

I was able to get the mark at TDC but the distributor was as far clockwise as I could turn it and the vacuum modulator was still pointing striaght up/ 12:00 o:clock. This was after loosening the back side adjustment bolt as well to be able to move the distributor over that far.

Funny thing is that after i re-did all this last night I was'nt getting the puff-puff backfire through the carb. anymore when I was cranking it but it still did not act as if it even wanted to start.

There doesnt seem to be any fuel problem because the carb is getting enough fuel and with a kick of starting fluid the engine would at least stumble trying to crank. Even if it were not getting proper fuel the motor would stumble and not just keep turning over with a shot of the starting fluid.

I'm going to triple and quadroople check the firing order and the rotor position again. Also try moving the wires over. With it turned that far the wires may need to be moved so the firing order is correct with the distributor being so far off now.

I was reminded that this also needs to be done if the distributor has been moved this much as well as it is not uncommon for the distrubtor to be off this much on a motor as it wears down over years.

I really appreciate everyones feedback on this and will have it all in hand to review as I'm working on it more tonight. I went through this before with the motor i refurbished and once I got the firing order correct the motor cranked right up so I'm hoping this is the case again and not a streatched timing chain or valve trouble.

If I think I've gotton everything in order and it still does'nt do anything then I'll do a compression check and post the results. Keep those ideas coming and thanks again guys, Ed

[Don Coatney]

Ed;

You really need to do a compression check. Good possibility the long spark plugs bent one or more valves. This would cause backfiring through the carb. I have never seen a Mopar flat 6 timing chain jump a tooth. Have heard tell of them being installed one tooth off but not jumping a tooth. If you find no compression in one or more cylinders pull the head.

[Jim Saraceno]

Ok, someone help me here. I'm confused. The fan belt pulley or harmonic balancer is mounted on the crankshaft, not the camshaft. If Ed's #6 piston is at top dead center, the timing mark should line up with the pointer regardless of the timing chain, correct? If it is not then the key is missing and the pulley has shifted.

I had been thinking that when the #6 cylinder was at TDC, if the timing mark didn't line up then the timing chain was off, but now I'm thinking I was mistaken and the timing mark should be aligned with the #6 (and #1) piston regardless of the the chain. If the chain was off then the valves would be out of time.

Anyway, as Don said, Ed you need to do a compression test. If you can't get the spark plugs out, you need to pull the head (it's a fairly easy project) so you can drill the peened plugs out without getting the shavings inside the cylinders, then insert any helicoils that are needed. While the head is off, you can see if there is a problem with the valves and the valve timing.

Something is just not making sense to me so I may just be way off track here.

[Plymouthy Adams]

You have to stop and think of three things..I think you have a handle on it.

1 The piston and crank and harmonic balancer is the same point and locked togehter by keyway on the balancer and the journal on the crank.

2 The timing chain times the crank/piston to the relationship of the valves with the camshaft

3 The camshaft drives the oil pump that in turn drives the distributor.

That is how you can have the damper, slug the same, tooth off on the chain and the cam is out of whack..tooth off with the oil gear to the cam throws off the electrical timing of the didtributor.

[Normspeed]

Jim's right about the crank pulley/timing mark and the piston position. Those won't change no matter what happens to the timing chain, cam, oil pump and distributor positions. I think some valves hit some plugs and got bent. Not too big a problem but the compression check is a must, and maybe pull the head after you see what the compression says.

I don't understand, though, why Ed's crank pulley timing mark would not be visible when #6 (and #1) are at top dead center.

[hankstermsp]

I had the same problem with my 1950 Dodge truck 6 cyclinder. Water in the gas was causing my problem. I drained and flushed out the gas tank and gas line. Removed the glass filter bowls and cleaned them. Put additives in gas. Started right up with no more backfiring. Sounds too simple but worked for me. I was lucky there is a screw in plug on the gas tank for draining. You would not believe the junk and water that came out. Good luck!

[Ed Griffin]

Thanks guys for the extra input. My biggest fear from the start has been that I bent a valve or more when the long plugs bottomed out. Now that I've finally gotton the car in the garage I'm able to go through it to locate the problem. One thing that I have to remember is even before I tried to tune up the car and bottomed the plugs out that there was an stumbling problem before that happened. So now I'm most likely dealing with two problems other than the original one.

I've been reading up on the Motors manual and the 1952 Chiltons manual to make sure I'm going through "the basic's" correctly first. My next move will be a compression check and removal of the valve covers to get a visual on them as I turn the motor over with the remote starter.

If compression is good then I can take advantage of seeing the valves and knowing for sure when both are closed on # 1 cyl. and I'll know for sure if I have it set at TDC correctly or not. Most likely I've bent a valve, broken a spring etc..then as suggested I'll have to remove the head.

If that's the case then I may as well replace the cracked exhaust manifold and the head with the extra's I have but first things first. If the problem goes even further into the engine then it's time to pull it and replace it with the one I refurbished awhile back and see just how well I did on that one.

hankstermsp- funny that you mention the moisture problem. In the early 90's I did have that same problem only with a 81 Bronco. Whats funny to me about it is that it happened just as I was about to reach Stevensville just south of St Joe, your area. Worked at the nuke plant there and visited St Joe a few times. We were there for 3 months so got to see what winter and spring is like there. Spring was nice, winters there are brutal. Not only did I learn about moisture problems there but also to never put your emergency brake on over night. They tend to freeze in place Welcome to the forum....Ed

[Jim Saraceno]

For what it is worth... the first time I did a compression test only my '35, I used an extension that screwed too deeply into the spark plug hole. The valves came up and peened it over so I could not get the tester out. I pulled the head so I could drill the tester out without letting the shavings fall into the cylinders. I fully expected to see a bent valve or two. The valves didn't even have a scratch on them. Spark plugs may be harder that the compression tester, but with any luck, the valves are still harder.

Let's hope you find your valves in good condition.

[hankstermsp]

Ed, You're right about the park brake freezing up. I learned the hard way. I've known quite a few guys that have passed through the Cook Plant. They're still coming and going.

[Ed Griffin]

I hope so too Jim. Started to do a compression check last night with #1 cyl and once I pulled the spark plug I noticed that the threads on the plug were somewhat flat where I had previously put all the new plugs in. This was when I found that I may need more heli coil collers. I tested the loose fitted plug hole and came up with 35lbs. Then on #2 cyl came up with 16 or 17psi. I tried pipe thread tape and all it did was skin it off as I screwed the tester in. Also tried an O-ring but the gap between the 14mm threads on the tester gave the O-ring no place to back into to make a good seal.

Thought about this as I was typing but I may be able to use one of the 3 collers that came with the heli coil, thread it all the way up on the tester and hope that it will but into the top, cover the gap and still have enough on the bottom of the coller that the 14mm threads can garb the plug hole and tighten down for a good seal.

The heads going to have to come off but if I can get a good compression check then will at least know where i stand before replacing head or to leave off and work on whatever it may be.