Hi To The Nailhead experts I need a little advise on My 425V8 Cat. I have driven my car around some this year. What I notice is that every once in a while. If I step on her, I have a backfire through carb. Mostly going up a hill. I tuned her up, set timing etc. Idles good, dont over heat, good vacumn advance, It will do this only once. then even out, dont miss and runs smooth. Carb? timing is right on, cant figure it out. loose chain? Any advise will be greatly appreciated.:beer Thanks In Advance Gary
My car is in the identical situation as yours. Same backfire under a load then cleans itself up. Compression check on mine was good.
Usually when they get like that it's a loose chain. The chain will "curl" on the initial load-up and then catch up, which is when things smooth out. Problem is this is a long term problem, coming on gradually as the chain stretches past it's limits, and you would know it's coming from a long time back. If this was something that came on suddenly, then I'd start looking at ignition and carburation. Old mechanic's axiom; if the problem is long term and gradually getting worse, then it's going to cost a fortune because it has to be rebuilt. If it's something that just happened, it's something easy and fast that usually can be corrected without having to tear the motor down. So start looking for something simple. Try power-timing the car, see if your vacuum advance is working right first before taking the front housing apart. Just thinking; - did you check the vacuum profile? This could also be a vacuum leak at the manifold; - my 60 Invicta did the same thing to me once, and what it was was a bad intake manifold gasket. If you haven't ever had the carb off it could be that the throttle plate on the carb finally corroded through and started a massive vacuum leak.
Thanks for the input. :TU: I am beginning to think that the chain is starting to stretch. I replaced carb manifold gasket, good vacuum to dist. 30 degree dwell, 2.5 timing, carb redone last yr, new tune. All to specs per shop manual. Motor has not been apart in the 30 years I`ve owned her except waterpump. Any good way to test the timing chain w/o pulling it apart? I need a timing cover seal anyway. But I had low back fusion in march, so I cant do it for a while. Hope I cant make to the monsoon season:beers2: gary
Well after a 3 day trip to Ocean Shores because it was getting over 90 degrees here. thats to damm hot to work on a car. It was nice Today is cool in the 60`s. So I did a compression test cold. pulled all plugs All are even around 153lbs to 155lbs. Shop Man. says 180lbs normal. Carb gasket is new. plugs are new, wires are new. Timing chain? Whats the best way to test if its bad, w/o tearing it apart. Any good ideas? :TU: Thanks Gary
First Guess: engine pops on acceleration because the carb isn't supplying fuel to match the increased air. Common as dirt. Often traces back to a defective accelerator pump or the (vacuum or velocity-actuated) secondaries flopping open too quickly. Can't say I've traced an acceleration pop back to a stretched timing chain--but it's easy enough to test. Turn the crank clockwise as viewed from the front bumper until the timing mark on the damper is aligned with the "0" mark of the timing pointer. Pull off the distributor cap, and watch the rotor as you SLOWLY turn the crankshaft backwards. The instant the rotor starts to turn, stop turning the crank and look at the timing mark. Rule of Thumb: 10 degrees of slop means you NEED to replace the timing chain and gears. 5 degrees of slop means you PROBABLY SHOULD replace the chain 'n' gears 2 degrees of slop means they're probably just fine for most uses.
Thank You I know what the next thing to try tommorrow. The Crank test. By the way I have maybe 1000miles on a rebuild of the carb. I put a kit in it last season. Its the original carb. Whats the cure on the secondaries. Is there a way to control them. They open velocity-actuate. Sounds like I`m getting closer. Thanks Again
Schurkey, is right on....but first I would ck out the accellerator pump, and set it at its richest setting if it is squirting all right.....
Thanks Guys Its sounds alot easier than a chain. For the record the gas mileage is worse than ever. 8-10 used to get 13-14. Ive heard that new acc. pumps sometimes leak, dont fit properly, the neoprene style. last one was leather. Gary:beers2:
Old mechanics trick,,,, for the leather ones.... take the new one and skin it back toward the top end and then fold it back like it was and then soak it in trans fluid or motor oil for a few minutes and then install it.... works much better...
If the milage has dropped off and the carb is in good shape, ck the vac advance system for vac leaks.... simple test; just put a vac hose on the vac advance and then suck the air out and use your tongue to plug the hole in the hose and see if the canister holds the vacume.... The vac advance is your gas milage......
I believe firmly in the KISS (Keep It Simple, Stupid) philosophy of diagnosis. Your chain may be loose, but I doubt that it's the problem - a couple of degrees of slack increase is not going to cause a 30-40% decrease in gas mileage. I would go for the simpler stuff - fuel, air, spark (in correct proportion, at the right time). BTW: you can check the chain w/o removing the distributor cap. Bring the timing mark to zero on the index. Put a wrench on the crank and move the crank back and forth (Clockwise and counterclockwise); you will feel it when the chain slack is taken up and the cam starts to move the lifters. More then 5 degrees and it's worth replacing the chain for the performance gain. Good luck!
Thanks to all that answered my distress call. I bite the bullet and took the car to my local wrench that works on most of the old cars in town. He said the carb. is where to go. I rebuilt it myself last year but w/o the proper equipment to boil out the inside he says I have a plugged passage or ?. So I will know more when I get it back.ou: Thats why I wish I had a shop for the tinkering I like to do. And having my low back fusion 2 mo`s ago kind of cuts back on my projects. I need to have the car operating to make the Open on the 20th plus a few others this summer. ray: Its sure good to know that there are people like you guys to try and help us with our problems. If I can ever help I will Gary:beers2:
:af: Just an update.. I had the carb rebuilt. Get the car back, $300 bill. Drive home and get ready to head to our club meeting. Start out with wife. Car starts acting up and dies driving along. Stop at daughters house. Pop the hood and see gas leaking out of right rear of carb. :af: dumping gas on manifold, a good amount. Got pi##ed. Called shop had it towed back. I`ll pick her up today. He said a piece of rubber hose stuck in jet??? causing the gas to overflow. Hell, I could have done the carb better than that. It`s hard after low back surgery to crawl under the hood I`m so thankful that I wasnt on the interstate in rush hour.ray: I could have torched the car, myself and the wife. If the gas would have reached the exhaust. Bye Bye to us That happened to my 70 GS 455 at a mall when the ex wife parked it and gas line leaked and torched the engine comp. I get real leary of things like that. Hopefully it will be right this time.. Gary
Yeah, I'd be a little PO'ed...A piece of rubber hose...right! How would that get in there? Chances are he goofed the float setting. It's easy enough to do, get a little careless, bend the tab the wrong way, or more common, assume that the new needle and seat are the same height as the old one was and don't even check the float setting. Maybe you should consider beating that mechanic with a rubber hose, or tell him where to put one... Hope it all works out for you; - carb problems can be frustrating. What kind of a carb was it anyway? I used to be able to build a stock AFB in my sleep, they aren't that complicated. A Rochester 4GC, well, then I'll cut your mechanic some slack, those things are a bit trickier.
Thanks Marc and all Thats what I felt like doing, I`d have done more than that if the car caught fire and one of us or both would have been hurt. He put a new fuel filter and hoses on it. Just to make the story believeable. I went a picked her upp today and the girl just gave me the keys. I wanted to have a chat with the mech. but he wasnt around. Ha:spank: Another ?. she drives great at lower speed but when I punch the gas down to the floor. It start to choke a bit, kind of like starving for more gas. But I`m not sure about this. any good idea`s. I dont want to go back to this guy unless its in the carb. Its almost feels like bad plug wire or coil. The wires are new last year. By the way its an AFB. and I`ve went through it myself 4-5 times in 30 years. I appeciate any info from an old naillhead pro Gary
Don't be so quick to cast blame. I've run into this problem many times in the recent past. The problem is with this "New" fuel we are getting. Contains alcohol & that will eat away at the "Old" rubber hoses as well as "Clean" out any debris that may be in the tanks/lines or elsewhere. Now, I only use the high pressure fuel injection hose for any replacements. More costly but doesn't seem to be affected by the fuel.
Gary, I had a similar stumbling problem. My mechanics simply tuned the carb and the problem went away.
