why would this happen • The Dodge Challenger Message Board

by **david340** » 31 Mar 2008 23:38

i have a 72 challenger 340 six pack,i was out for a drive resently and as allways the car was running perfect with no issues.i pulled into my local petrol station to fuel up at the car filled it up at the same pump i allways do.ready to proceed with my journey i started the car and as i moved onto the road and gave the car some gas it ran bad,was down on engine power and was missing and back fireing a bit.i am just a bit confused why it ran fine before i stopped and then all of a sudden it ran bad.has anyone any ideas to my problem,thanks david.

by **Jon** » 01 Apr 2008 0:56

How long did you run it before the backfiring? If more then a carb full then possibly bad gas. Was there a delivery truck there recently as some times the filling of the underground tank stirs up the crap from the bottom.

by **david340** » 01 Apr 2008 6:53

hi jon,i drove the car for about 10 miles before i fueled up and as allways the car was running great i thought myself it was bad fuel.as bad as the car was i drove it for the day as our club was out for a run and i thought the car would clear out,put 50 miles or so up on the car and made sure the car was running on air before i refueled put fresh fuel in but no diffrence,i also changed the coil,ecu,fuel filter and spark plugs the only diffrence was when i changed the plugs the car ran that bit better so maybe i need a new set of plugs the car was useing champion RN14YC plugs i had to reuse the old ones until a new set arrives.maybe i have carb problems an area that i have ni idea about.thanks for replying david

by **dave-r** » 01 Apr 2008 7:57

Could also be something to do with heat soak.

When you stop the car at full operating temp and switch off the engine bay gets very much hotter than normal.

This can make problems in the electrical system (ignition leads/coil etc) show up. So there might be a problem there. The coil in particular does not like heat. Make sure it is mounted somewhere cool.

Also the heat can boil the fuel in the fuel line and carb(s). This can make the car hard to start or cause the problems you mention.

If you switch the engine off and hear bubbling in the carbs or see any bubbles in the filter then you need to see if you can keep your fuel and carbs cooler.

If you think fuel is boiling in the carbs try blocking off the heat riser through the intake or use an insulating gasket between carbs and intake.

If you are using headers these problems usually crop up in a car that ran fine with stock iron manifolds.

by **david340** » 01 Apr 2008 11:39

thank you dave for your reply i will see if what you said is the problem and hope fully ill get it sorted as its annoying when up to this the car was ok,if it was a motorbike ill have it sorted in know time but not haveing much of a clue bout these yanks makes it harder for me to fix a problem when one accurs,thanks again :thumbsup:

by **dave-r** » 01 Apr 2008 12:10

No problem.

But bare in mind I am just giving you more options.
It could just be bad fuel as first suggested. In fact if this has never happened before and you have not changed anything recently it is the most likely cause.

If muck in the fuel stops the fuel bowl valve sealing then the fuel bowls overflow into the carb and it could be raw fuel in the engine making it run rough and fuel in the exhaust causing backfires. The plugs would get fowled up this way.

by **david340** » 01 Apr 2008 15:24

thanks dave,i keep thinking its bad fuel allright but from the second i started the car after refueling it ran bad surely it would have burned up the remaining good fuel that was in the carb and fuel line frist and then started running bad when the new fuel arrived to the carb.one other thing i disconnected the fuel line at the fuel pump to see what the flow was like and its not great its flowing but not fast maybe there is a small bolckage in the tank.iam home from work now and i am going to spend the evening trying to solve the problem,thanks :thumbsup:

by **drewcrane** » 01 Apr 2008 19:32

one of the things i have done overhere in the states, cause we dont have gasoline anymore they call it "motorfuel", i have had some really bad problems with heatsoak, stumbling , cutout of power, i do know this that we have a wintertime blend, and a summertime blend , not sure what they do in ireland, but they could have just added some new fuel to there supply tank and stirred up some sediment , and you got it, i digress, i have just installed a fuel return line,just by using another stock fuel line , next to the old one , in this case i installed a 3/8 inch line for a hemi. , then i used the old 5/16 line for my return, im sure somebody else has done this, but is does keep a fresh supply of fuel to the carb,for me in the wintertime blend will boil off at 50 degrees f, what happens is the stuff that lights the gas gets boiled off, and the car wont start,the return line fixes this problem, hope this helps , from a yank who has issues with these cars too! sorry for the long paragraph

by **david340** » 01 Apr 2008 20:20

thanks drew for your reply,i have tryed a number of things today to see would the car return to normal and no difference i changed the coil,ecu,fuel filter and pump i even got a diffrent brand of petrol (fuel) ran the carbs dry and then started the car with the new fuel and still no change.it must be a problem within the carbs would they be woreout or would there be any particular part on the carbs that might need repalceing.you mentioned you ran a return line on my car there is just a single line from the tank to the carbs would this be right as the car allways ran perfect this way.thanks for all the help pitty i was more experienced with these cars :thumbsup:

by **drewcrane** » 01 Apr 2008 22:54

yes there is only a single line on most stock challys , your carb may havr a stuck float, or something clogging a metring rod, if its a carter, if a holly a clogged jet, or a blown power valve but i dont want to confuse you to much, as dont get carried away to much , as you and dave said the car ran fine before this tank of gas,so lets back up a little and start with the tank , the pick up may have sucked up something but you said fuel ran through the system well ok, try some carb cleaner if you havent already and spray the carb down including the linkage,also i use whats called bk 44 its a fuel cleaner, it will clean out deposits,and then if that doesnt work you gonna have to remove the carb and have it looked at by someone, if you can i realize your resources are limited, but try that, try to keep your diagnostics simple and 1 thing at a time , dont go to far as you might create another problem as i often have done myself, good luck you will find it!

by **david340** » 01 Apr 2008 23:08

thanks very much drew for all your help,they are a holley sixpack set up on my car with a 340 motor. tomorrow i will center my attention to the carbs and hopefully get it fixed.the kind of person iam i would just go out and buy a new set of carbs and be done with it ,but knowing my luck if i did that the car would still run bad.ill keep you up dated on my progress :thumbsup:

by **drewcrane** » 01 Apr 2008 23:11

after talking to a buddie that worked at an emmission shop, has said that most fuel pumps at the fuel depots have good filters on them and most likley didnt have crud in them , however it is a six pac and it is a holly and the power valve went bad is there alot of fuel in carb? ya know like to much fuel ? can you smell alot of fuel at idle?,also maybe you will have to do a compression check, hope fully no valve have broken, it will tell right away keep it simple to replace the power valves is a simple task, so is a compression test.

by **david340** » 01 Apr 2008 23:18

sorry for this silly question but where are the power valves located.i couldnt imagine that i have a broken valve in the motor as i only drove the car for about 10 miles at about 40-50 mph with no probs.there is no smell of fuel the car starts ok idles ok but not as smooth as before but rev's badly when you gas it.

by **drewcrane** » 01 Apr 2008 23:43

no silly questions , it is in the carbs, since u have 3 of them i think they are located in the front and the back carb, i have a 4 bbl and the are located in the float bowl, the big case on the front of the carb held on by 4 screws, remove those and it is threaded in the metering block , ask dave maybe he has a diagram as im not sure exactly as to if you have 2 or 3,but you didnt break anything in the engine, i think you blew a power valve, i have a demon and they have fixed the powervalves, as well as the later hollys,i didnt mean to scare you about the valve spring , but i wanted to point out to the worst case scenario, it happend to a mate of mine, so look for a diagram of your carb,those powervalves blow alot,especially after a back fire, if you had a true backfire you need to replace those valves! if you cant find a diagram i will look for one, cheers drew

by **Jon** » 02 Apr 2008 1:10

I think the jets and or orifices are clogged with either sediment or water. I would not fill the tank again until this is determined. Otherwise, you will need to find a disposal site for the entire 18 gallon tank.

Also, another easy way too find out about bad fuel would be talking to the locals that frequent the fuel depot. A popular fuel station in our town had a delivery of bad fuel that screwed up our Yukon and probably a few others. Coincidentally the local GMC dealer fixed it for free. We were out of warrenty, and no questions asked. Talk about community.

by **drewcrane** » 02 Apr 2008 1:14

yes water is something a filter cant seem to clear out of the fuel , nice community not like the old days where you were stuck wondering :tears:

by **david340** » 02 Apr 2008 7:21

[quote="drewcrane"]no silly questions , it is in the carbs, since u have 3 of them i think they are located in the front and the back carb, i have a 4 bbl and the are located in the float bowl, the big case on the front of the carb held on by 4 screws, remove those and it is threaded in the metering block , ask dave maybe he has a diagram as im not sure exactly as to if you have 2 or 3,but you didnt break anything in the engine, i think you blew a power valve, i have a demon and they have fixed the powervalves, as well as the later hollys,i didnt mean to scare you about the valve spring , but i wanted to point out to the worst case scenario, it happend to a mate of mine, so look for a diagram of your carb,those powervalves blow alot,especially after a back fire, if you had a true backfire you need to replace those valves! if you cant find a diagram i will look for one, cheers drew[/quote]

thanks drew,my biggest problem here in ireland is not having anyone near buy that could fix these kind of problems at least in the uk there is a number of people and places that would get me going again.even if there was notting wrong with the carbs i would at this stage have them rebuilt i wish i could take the carbs of and send them off some where to be rebuilt just for peice of mind,summit sell the six pack carbs i know there expensive but i do have to get the car going again,i looked at jacks auto parts for the rebuild kit but i couldnt find them.many thanks for all your help david.

by **dave-r** » 02 Apr 2008 7:51

I can rebuild the carbs for you David but it is very expensive to do on a six pack.

The power valve is in the center carb metering block. That is the bit with the idle mixture screws on. A back-fire up through the carb can damage it but one in the exhaust wouldn't.

Did it backfire up though the carb as soon as you tried to start it at the petrol station? If so then the problem might be something daft like the timing altered due to a loose distributor hold down clamp.

You do have electronic ignition and not points don't you? :shock:

Otherwise all this could just be your points and it was just chance it happened at the petrol station.

If it drove fine to the petrol station then something happened when you were there or on the way there.

by **drewcrane** » 02 Apr 2008 11:12

ahh i knew there was a powervalve somewhere,i didnt ask if it were a points car, those carbs can be a problem tuning, but it loooks like you know how to do em, i hope you guys can get it going! good luck, im sure you will :thumbsup:

by **david340** » 02 Apr 2008 11:20

hi dave,that would be great if you could re-build the carbs as i am sure at somr stage they would need doing.i no you would have to get the parts frist but approx how long would it take to do them and roughly the cost.

just an update on what i did today so far,i took all 3 carbs off to give them a good cleaning inside and out stating with the center carb and being very carefull dismantled the carb at the bowl frist which inside was nice and clean then there was the block peice which i presume it was the power valve and gave it all a good cleaning.do these carbs have two main jets,what i seen were like motorcycle main jets, these had 61 stamped on both of them but one of them was blocked with a peice of dirt would this be the cause of the problem.the car starts the same ticks over fairly good but wont rev out good this is before i dismantled the carbs.

the car has an electronic ignition i changed the ecu and coil already and the plugs.there was no problems with the car before i stopped i had a quarter tank of fuel the car drove perfect i pulled up to the pump as normal there was no backfire turned it off filled up started the car to head of started grand and sounded the same no backfireing but when i got on the the road and gave the car some gas it ran bad.while driveing every so often there would be a quick backfire but this only happened twice on a 50 mile journey.

sorry for going on about this and thanks :thumbsup:

by **dave-r** » 02 Apr 2008 12:26

If you supply all the parts and pay the postage i will do them for free. Check out Year One for three rebuild kits and the two rubber diaphragms (sold separately) that you need.

The center carb has two jets in the metering block with the power valve as you described. So if one was blocked that would cause problems and maybe the source of your problems.

The two end carbs just have what are called metering plates. These have two holes in the bottom edge of each plate that act the same as jets.

This link has some diagrams of the parts.

http://www.mopar1.us/6barreltips.html

by **david340** » 02 Apr 2008 12:57

i couldnt expect you to do it for notting dave id have to pay you for doing them.
ill finish off cleaning the carbs today and ill let you know how i get on thanks david.with all this help i am getting form you guys over there ill have a lots of beers to buy at the nats this year :thumbsup:

by **drewcrane** » 02 Apr 2008 17:09

man you cant beat free labor in less ur like kin folk ,nice dave ,yea good luck looks like you are on your way to 6 pac heaven again! :biggrin:

by **dave-r** » 02 Apr 2008 18:41

Honestly. If you just want new gaskets and it checking over I will do it for nothing. No big deal.

by **david340** » 02 Apr 2008 20:02

thanks drew and dave but i got the car sorted and its even better than before,its more responsive, starts better and is even quicker off the mark when i gas the pedel.it must have being that peice of dirt that was glogging the air jet.i gave the carbs a good cleaning and it must have done the job.i have another six pack car on the way so i now have a bit more experience for that one if it ever gives trouble like this.many thanks for all the help :thumbsup:

by **drewcrane** » 02 Apr 2008 21:11

excellent, i learned a thing or two my self thanks dave! :nod: