Ralph Pearce Posted April 17 Report Share Posted April 17 So had an immediate backfire on start-up, stopped by the store on the way home, so wondering if it would do it again. When about to start up again and head for home, I turned the key on then off cause I decided to call my wife. When I turned the key off, something kind of went "clunk" and the truck rocked a little. After my call, I went ahead and started, no trouble, and drove home. After I got home and turned off the engine, I decided to turn the key back on and again, "clunk" and the truck rocked. Thoughts? Quote Link to comment Share on other sites More sharing options...
Dave72dt Posted April 17 Report Share Posted April 17 By turning the ignition on and then off you did the same thing the points do when opening and closing, letting the coil fire a plug, and if a gas mixture is in that cylinder when it fires it'll rock the engine. That's my guess. 2 Quote Link to comment Share on other sites More sharing options...
Ralph Pearce Posted April 17 Author Report Share Posted April 17 Should also mention that I'm running Petronix electronic ignition (12v). Quote Link to comment Share on other sites More sharing options...
Ivan_B Posted April 17 Report Share Posted April 17 Never saw the points firing a plug when turning the key 🙄 Is this a stock setup, besides the ignition? Are you running an electric fuel pump or any other things connected to the ignition switch? 1 Quote Link to comment Share on other sites More sharing options...
Dave72dt Posted April 17 Report Share Posted April 17 2 hours ago, Ivan_B said: Never saw the points firing a plug when turning the key 🙄 Is this a stock setup, besides the ignition? Are you running an electric fuel pump or any other things connected to the ignition switch? I consider points a switc0h. Open, close, on, off. Ignition switch on, ign switch off. Logic tells me my previous post is a possible scenario and every electrical circuit in the truck is available when the key is on.. I don't know whether or not Pertronix would or could respond similarly. How repeatable is this condition? Now that it's sat for a while will it do it before starting the engine or does it have to be run for a while. Will it do it after sitting for ten minutes or a half hour? Will it do it a second time, turning the key on and off immediately after it does it? 1 Quote Link to comment Share on other sites More sharing options...
Ralph Pearce Posted April 17 Author Report Share Posted April 17 (edited) 36 minutes ago, Dave72dt said: I consider points a switc0h. Open, close, on, off. Ignition switch on, ign switch off. Logic tells me my previous post is a possible scenario and every electrical circuit in the truck is available when the key is on.. I don't know whether or not Pertronix would or could respond similarly. How repeatable is this condition? Now that it's sat for a while will it do it before starting the engine or does it have to be run for a while. Will it do it after sitting for ten minutes or a half hour? Will it do it a second time, turning the key on and off immediately after it does it? So yesterday I had a backfire on start-up after sitting for a couple hours. Then a while later at the store after sitting about a 1/2 hour I had the clunk after turning ignition key on and off. After driving home and turning the engine off, I had the clunk again when turning the key on and off. This morning (w/o starting the truck), I turned the ignition key on and off many times (sometimes letting is stay on for a bit), with no response. All stock except 12v with Petronix ignition. Edited April 17 by Ralph Pearce Quote Link to comment Share on other sites More sharing options...
Dave72dt Posted April 17 Report Share Posted April 17 Will it do after the truck has been started? Does the engine need to be brought up to temp or just started? Quote Link to comment Share on other sites More sharing options...
kencombs Posted April 18 Report Share Posted April 18 19 hours ago, Dave72dt said: I consider points a switc0h. Open, close, on, off. Ignition switch on, ign switch off. Logic tells me my previous post is a possible scenario and every electrical circuit in the truck is available when the key is on.. I don't know whether or not Pertronix would or could respond similarly. How repeatable is this condition? Now that it's sat for a while will it do it before starting the engine or does it have to be run for a while. Will it do it after sitting for ten minutes or a half hour? Will it do it a second time, turning the key on and off immediately after it does it? I agree with you that could happen, but only because it is a Pertronix. Points system won't do that because they fire on points opening. With the points open at rest there is no current flowing so it won't fire. Closed at rest there is current but the closed points don't interrupt the flow so spark. I have read that Pertronix fires on point closing, but have no personal experience with them. 1 Quote Link to comment Share on other sites More sharing options...
Dave72dt Posted April 18 Report Share Posted April 18 8 minutes ago, kencombs said: Closed at rest there is current but the closed points don't interrupt the flow so spark. And when you turn the key back off, the current flow is interrupted 1 Quote Link to comment Share on other sites More sharing options...
JBNeal Posted April 18 Report Share Posted April 18 22 hours ago, Ralph Pearce said: ... All stock except 12v with Petronix ignition. What is Petronix saying about this when ya ask'm about this prob? 1 Quote Link to comment Share on other sites More sharing options...
kencombs Posted April 19 Report Share Posted April 19 10 hours ago, Dave72dt said: And when you turn the key back off, the current flow is interrupted I was going by his last sentence which says it happened when he turned it on. Quote Link to comment Share on other sites More sharing options...
Ralph Pearce Posted April 19 Author Report Share Posted April 19 18 hours ago, kencombs said: I was going by his last sentence which says it happened when he turned it on. Thanks for all the responses. The backfire happened on start-up. The "clunk" happened when I turned the power on and off (without starting, but had been running earlier). Anyhow, will take the truck into the shop and see what they say. Quote Link to comment Share on other sites More sharing options...
Ralph Pearce Posted May 1 Author Report Share Posted May 1 UPDATE: So I found a reliable guy who does mobile classic car repair. He noticed that the carb leaked when I gave it a little gas before starting. He checked all the plugs and distributor operation (all looked good), and was of the opinion that the carb was the likely culprit. So the carb's being rebuilt, will get it back next week (last rebuild about 10 years ago). I'll update again when I'm confident that the issue's been resolved. Quote Link to comment Share on other sites More sharing options...
Ivan_B Posted May 1 Report Share Posted May 1 (edited) The likely culprit of what? The carb could cause a no-start, a flood, rich or lean running condition. What you've described in the first post does not seem to fit here 🤔 I might be in SJ in July, maybe we'll figure this out together 😅 Edited May 1 by Ivan_B Quote Link to comment Share on other sites More sharing options...
Ralph Pearce Posted May 13 Author Report Share Posted May 13 On 5/1/2024 at 10:02 AM, Ivan_B said: The likely culprit of what? The carb could cause a no-start, a flood, rich or lean running condition. What you've described in the first post does not seem to fit here 🤔 I might be in SJ in July, maybe we'll figure this out together 😅 The cause of the backfire. The guy who came over gave a convincing explanation of how the carb could be the cause and it def needed a rebuild. In any event, the carbs back in, I've put in new plugs, cap/rotor and she's starting right up and running smooth with no issues so far. 1 1 Quote Link to comment Share on other sites More sharing options...
Los_Control Posted May 13 Report Share Posted May 13 I agree, crap in the fuel entering the carburetor can plug up the jets and cause a lean running condition .... and a back fire simply because it needs more fuel. Good call on your mechanic, keep his phone number handy. I also am experiencing a similar condition ..... also I changed some wiring and need to troubleshoot which is causing the issue .... sadly I will not touch it for a couple months at this point. Quote Link to comment Share on other sites More sharing options...
Ivan_B Posted May 14 Report Share Posted May 14 4 hours ago, Ralph Pearce said: In any event, the carbs back in, I've put in new plugs, cap/rotor and she's starting right up and running smooth with no issues so far. I am glad you fixed it but, since several things seem to have been replaced at once, we'll probably never know what the issue was Quote Link to comment Share on other sites More sharing options...
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