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Rebuilt 400 HO suddenly runs rough
I posted this over in the ‘67 GTO forum’ but I think this is probably the better place. My 67 400 HO has only 250 miles on it since a full rebuild. It is .040 over with a roller cam and ported 670 (Gessler) heads. Everything on car (tank, lines, pump, filter) is brand new.
I drove it today and about an hour into the drive I noticed it suddenly stumbled. It had been running perfectly before this. I thought maybe my fuel gauge might be inaccurate (but doubted that since this entire car just went through a 7-year frame-off resto). I stopped in gas station but it only took about 8 gallons, so that wasn’t the problem. After that, the stumble got markedly worse. Car would kick, stutter, jerk and backfire. I nursed it back hone and the first thing I checked was timing. The distributor was not loose and the initial timing looked like it was at about 8 degrees. But the timing was very erratic as I could never get engine to smooth out using the throttle. I checked the rotor and it wasn’t cracked or loose. I also put the timing light lead on each wire as the car idled (roughly) just to make sure that there wasn’t a miss. Each wire would trigger the light. I thought I might try swapping on a different carb, just to rule that out as a possible issue. When I went to pull the fuel inlet line, no fuel leaked out of the carb. I seem to remember every time I pulled a carb in the past, some fuel would always dribble out. Any suggestions would be appreciated. https://uploads.tapatalk-cdn.com/202...e2a174f2dc.jpg Sent from my iPhone using Tapatalk |
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Ok. I’ll give that a try. I tried running it with no filter, but then had same problem. Then I swapped on a carb I know works well, and again same problem. So I know the issue is not the carb or filter.
I also noticed that asi rev the engine and you feel it miss, the timing light strobe gets intermittent. It flashed faster as i rev, but then you’ll see the blinking misses a beat or so. Could that still happen even if it’s a fuel problem? What about the points? Again, they are brand new but I know that is no guarantee on the quality (sadly). Sent from my iPhone using Tapatalk |
You did not talk about what you have for an ignition system. Stock GM points and coil, aftermarket electronic like Petronix? Points triggering an ignition box like MSD or the dozens of others? You didn't really specify. In general, a quality timing light will flash each time the #1 plug fires and not miss any cycles. That's assuming good clean hook-ups, good timing light and good plug wires.
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Stock distributor, stock coil and points. Everything new/rebuilt. Using an Innova digital timing light.
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Fuel delivery is at least part of the issue.
The way that we check the fuel system is to get a small funnel and fill the quadrajet with gasoline through the bowl vent. The fuel line needs to be hooked up to do this.
Pour in several ounces, even to the point that it overflows a little. Start the engine. It should run without missing for ten or fifteen seconds if there are no other problems (like ignition issues). If it is flooded a little, it may sputter, then clear out and run smoothly and then, basically, run out of gas. If it runs clean for a moment or two before it dies, check your fuel pressure and fuel delivery volume. My guess is that either your fuel pump crapped out or there is a leak in the fuel line somewhere between the pump and the fuel tank allowing it to suck air rather than gasoline. Either will cause the symptoms you describe. It could also be the sock inside the tank over your pickup line. The ethanol and other additives in the fuel basically destroy them and they collapse. Good luck! |
bad condenser ....
are you using delco remy points n condenser ?? |
I bet that in your distributor the wire that runs under the breaker plate has totally dry rotted insulation with a lot of it missing off the wire, and when the plate move as you give the motor throttle the power to the points gets shorted out!
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Thanks Joe. I’ll try that this morning. The fuel pick up in the tank is new. Well, it’s probably 3-4 years old, but it was replaced with the tank when Steve Barr was doing the restoration, so I’m assuming that should be ok. Sent from my iPhone using Tapatalk |
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I’m not sure. I think so. I sent the distributor out to be completely restored (may have sent it to Pete Serio) and it came back ready to drop in. How do I check if condenser is bad? Sent from my iPhone using Tapatalk |
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Thanks Steve. I popped off the cap to check the rotor, but maybe I should also pop off the rotor to check the breaker plate. I’ll take a photo and post it. Sent from my iPhone using Tapatalk |
To test the condenser fully which is a capacitor you need a meter that can test for ESR ( effective series resistance) because a cap should have very very low resistance to it.
In place of that tester if you have a ohm meter hook it up to the lead and the body of the condenser. When you do this the meter will flash a resistance reading and then go to infinity, or reading open. If you get any reading once the condenser charges up of even 10,000 ohms then it’s bad. |
I pulled the rotor and everything appears in order. No wires have any worn insulation.
https://uploads.tapatalk-cdn.com/202...be8cfec254.jpg https://uploads.tapatalk-cdn.com/202...300191d8ed.jpg Sent from my iPhone using Tapatalk |
If your timing light is going intermittent and then that’s when you drop cylinders, then you 100% have a ignition issue and not a fuel supply issue.
With this being said just to be sure you should test that timing light on another car! How do the connections on the coil look in regards to the plus and minus wires and there crimped on terminals? |
Ok. I also have another issue that is fuel related. There is all kinds of junk in the fuel inlet in the body. After I put the correct carb back on, I noticed it was leaking a little bit. I went to tighten it and it seemed that the big 1” fitting was loose. I removed the fitting altogether ant there was all this ‘grit’ in the inlet and on the little bronze filter. Almost seems like the threads were ground up. I wiped my finger in the inlet and pulled out even more of it. I don’t think this was what caused the rough idle because when I swapped carbs last night, it still ran rough.
I think the fuel inlet needs a helicoil repair. You can see some of the grit in this photo: https://uploads.tapatalk-cdn.com/202...7f64f2dee0.jpg Here it is after I wiped the inlet threads with my finger: https://uploads.tapatalk-cdn.com/202...0767a5f40f.jpg https://uploads.tapatalk-cdn.com/202...e118901a1f.jpg I’m assuming I should no try to do anything until this gets repaired. Do I have to send this out to someone like Cliff Ruffles? Sent from my iPhone using Tapatalk |
Ouch.
Yes, that needs to be repaired before you try to run that carb any more. Otherwise the grit will work its way thru everything in the carb....
Sorry to see that happen, but these are fifty-year-old parts. :rolleyes: We've had three qjets do this and EVERY ONE was a Carter-manufactured quadrajet. One of them even stripped out the needle and seat threads. I presume lesser quality metal was used when they were made. When you get it fixed, I'd suggest losing the bronze filter and installed a good paper one instead. My 2 cents. |
Ok, thanks. I still have my other Q-jet that I can use to at least track down this rough idle. I’ll use a paper filter next time.
I found a site (quadrajetparts.com) that offers the service of re-machining the threads. I’m going to call them tomorrow. Sent from my iPhone using Tapatalk |
I don't use the in carb filter. One, no reason to remove those fittings and two, those filters can plug up easily. I use a robbmc big body filter. It sounds like it was starving for gas if carb needle was blocked with debris. The timing light behavior makes me think fouled plug or bad wire.
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Thanks chip. I get that the grit may have blocked the carb that was in the car, but I swapped on another carb and I still have the rough engine. I will pull each plug later.
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