all has been good with my truck until last night. it idles fine but once you give it some gas it sounds like it is missing and backfires a little bit. it wasnt doing this a few days ago it just started. i know the timeing is set right because i set it back in september when i had my heads off to replace the head gaskets. i havent done a tune up on it in a while, maybe a year. i am going to do this and replace the fuel filter this weekend. hopefully it will cure my problem. dos anyone have any suggestions on what it could be?? or could i be right on the money by givin her a tune up.???? i have a 390 with a 600 cfm 4 barrel edelbrock if that helps any.

 

Recheck the timing. Check the accelerator pump. Check the ignition module and distributor pickup. If it has points check the condenser. Check the fuel filter. If the ignition system has been splashed with antifreeze replace the secondary ignition components.

Just some places to start...

 

She's goin lean on ya. Check the carb for blocked fuel passages, bad accelerator pump along with the fuel filter change. The fuel bowl level may be low also. When an engine is lean under a load it will cough (so to speak) and stumble when pulling away from a dead stop. Some engines that are lean will just simply die when put into drive (auto) or when the clutch pedal is released (manual). The choke not working on a cold engine will creat a problem similar, but clears up after the engine warms up .It isn't impossible that your engine may be lean from a vacuum leak too. A bad PCV valve may be effecting all around engine performance and is a sort of controlled vacuum leak, so give it a close look.

During cold weather a carb can ice up and cause some really undesireable engine performance. The only help is to have a carb, intake and breather combo that introduces warm air/exhaust into or under the carburetor. I think there may be an aftermarket carburetor base that flows warm coolant to help prevent carb icing.

 

I had the same problem with a Dodge I used to own, it ended up being the spark plug wires. It idled fine, but you could still hear a miss, and it wouldn't go any faster than 35mph.

 

I'd start with the plug wires. They may be breaking down under a load especially if they are carbon core.

 

well tomorow i am going to start by getting new wires and fuel filter because i have never changed the wires. i am also going to check the mixture screws on the carb, actually i was thinking about that last night after i posted this message. the spark plug wires have been on the truck since i got it 2 years ago. i have heard about the carb iceing in cold weather and about it running rough until it warms up. it did do it last week when i drove the truck. even after it was at opperating temperature. i had also thought that tthe mechanical fuel pump was going out on me. could that be my problem??? what is the accelerator pump??? isnt that internal to the carb???? if so how can i tell if it is bad??? btw i have a edelbrock 600 cfm if that helps. might be do for a rebuild and some bigger jets maybe even the edelbrock off-road needle and seats.

 

The accelerator pump is easy to understand and locate. There is a screwed on cover underneath the front fuel bowl of most holley carbs. It has a lever that moves when the throttle moves. This lever moves through the action applied to it by a plastic cam near the end of the throttle shaft. Look down into the throat of the primary side of the carb and manually move the throttle. Only do this with the engine off and not hot. As you move the throttle/carb linkage you will see gas squirting down into the carb. If you don't see the gas then you have a bad accelerator pump. If the pump lever is adjusted properly the gas will squirt as soon as you move the carb linkage.

Something I forgot to mention that is also a very important part of engine performance. The vacuum advance on the distributor. If it isn't working properly it will cause poor acceleration. Also underneath the points advance plate are centriugal weights and springs that allow them to work after certain rpms are reached. If these weights are stuck or sticking at times then you have poor acceleration performance again.

 

i got the new fuel filter installed and new pcv valve. and she was still missing. not as bad but still doing it. i checked the accelerator pump the way you suggested and it seems to be doing its job, fuel is suirted onec the linkage moves. i started her up and took the air cleanner off and had a look. it was all full of carbon in there and alover the choke plate. sprayed about 1/2 a can of B-12 carb cleaner down the primarys and all over the choke linkage. now she runs fine. i was gettin worrid i dont have much funds this week so it is good that was the problem. i also discovered that my electric choke was set up wrong, so i adjusted it the right way and now the choke plate actually closes once the engine is warm. seems to idle a lot better too. she is still back-fireing a bit. i checked the timeing and it was set at 10 degrees BTC. is this where it should be????? if not what could be causeing a mis-fire???? i am going to get new plugs and wires on payday hopefully that will help. what do yall think?????

 

Sounds like you are getting closer to fixing the problem. Did you check to see if the vacuum advance is working?

 

no didnt. i am going to see if just ckeaniing it and puttin the new fuel filet fixes the problem. how would i test it to see if it works??? it has been a while for me

 

To test the vacuum advance just use a vacuum pump hooked to the front port. with the engine off and the dist cap off apply some vacuum and watch the points plate. It will move as the vacuum builds up. It should also hold the vacuum. If the diphragm is leaking the vacuum will drop on the vacuum pump gauge. If for some reason you have the vacuum for this hooked up wrong it will not function properly. The vacuum line for the advance port should be hooked into the port of the carb that delivers vacuum at just off idle to part throttle. After the rpms are high enough the centrifugal weights take over and the vacuum for the advance is mostly if not all gone by half throttle and is certainly gone by WOT. Please don't go and hold an engine at WOT just to see if the vacuum drops for the advance port! ;-) Believe me it does.

To check and see if the centrifugal advance is working just unhook and plug the vacuum line for the vacuum advance. With the engine running at idle have someone check the timing. Then increase the engines rpms slowly and watch for the timng to advance. You shouldn't need to rev it very high to see the timing change. If the timing advances then it is working if it doesn't then you have something wrong under the points plate. You also want to see the timing return to the proper idle timing as the rpms drop down to idle.

 

hmmmm i guess i will have to borrow a vacume guage and stuff from my buddy. i have a breakerless distributor. so i dont think i have the weights to worry about. do you know anything about how the vacume advance works with a breakerless ignition?? it has been 7 years since i took auto class in highschool so it has been at least that long since i have messed with a distributor.
just an update as of my earlier post this afternoon........ i took the truck for a drive to the store to get some stuff and it is still missing when you give it gas. it is weird it revs fine when you just sit there with the truck running and the tranny in park, you can gas it all you want and it doesnt miss a beat. it only seems to be when it is in gearand when you are driving. i dont have a vacume leak anywhere on the engine or any vacume lines unless i am missing one somewhere. i even checked the vacume tree and the line going to the tranny. all was good no leaks. this is turning into a problem that i dont know about. just like my battery going dead once a week even after changing all related components ( i kow thats a wireing problem). i need to get this thing figured out befor the wife starts complaining about it and me drivin her car to work.
what is the timeing soposed to be set at anyways. i cant find it in any of my books??????????

 

Yes even the solid state ignition has the centrifugal advance system under that plate. This system is made up of weights and springs in which the total amount of advance is regulated by a set of reluctor arms.
I understand the need to get it fixed. It is hard to eliminate every aspect of ignition problems in a short time when relying on someones help online. There is a chance you have some inductance of the #7 and #8 spark plug wires, so keep them away from each other where possible. I don't know how much you are familiar with so I am just throwing everything I know that causes ignition problems. There are even symptoms of missing when there is low compression in one cylinder caused by a worn cam lobe/lobes, lifter that collapse, valves not sealing and sticking valves.
I hope I have helped you and not confused you.

 

Almost forgot the timing answer. I would start at 10 degrees and go from there. Being that this engine may be a little different than when new and the gas today is different it will be hard to follow factory specs.

 

well all the info is apreciated. you learn something new everyday. i just wish the haynes manual had the breakdown of the distributor in it it would make it a heck of a lot better. not sure if my chiltons does or not ill have to check when i get home for lunch. i am going to get that vacume guage and test the vacume the engine is putting out. and if i can sell some of my stuff i will buy the new spark plugs and wires. i am also gonna check my fuel pump and ignition module. who know i might get lucky. and for the timeing i might try movin it to 12 BTC and see how it drives from there. thanks agiain ill let ya know what i figuer out as soon as i get a chance to look at it.