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Jul 21st, 2025 | | | | Joined: Mar 2000 Posts: 784 'Bolter | | 'Bolter Joined: Mar 2000 Posts: 784 | I need help diagnosing a problem. This weekend I went on a cruse with my club, about 80 mi round trip. The truck ran fine untill we were almost home. After we got off the freeway (about 30mi trip) each time I came to a stop the engine would rev so low the engine would die. If I kept my foot on the gas it would stay running, but if I let off it would slow down and die.
Keep in mind that I have recently changed out my dual Rochesters for a pair of Carter/Webbers, and up until now everything was fine. When we got home I thought maybe I had the idle set too slow. So I increased the idle but the engine still seemed to run rough. Then I thought maybe I have a plug that is fouled. I pulled the plugs, none were fouled but all were covered with black soot. No oil, just dry black soot. Isn't this a sign of running rich??
I need some help with this situation. I can fix things that are broke, but diagnosing what the problem is is where I fall short. I need some suggestions on what to check to get this thing running smooth again.
Thanks
George
Engine background:
57 235
fenton headers
offy dual intake with Carter/Weber 2brls
Howard M4F cam
pertronics ignition
| | | | | Joined: May 2007 Posts: 95 Wrench Fetcher | | Wrench Fetcher Joined: May 2007 Posts: 95 | check your fuel pressure ?? | | | | | Joined: Feb 2004 Posts: 28,675 Kettle Custodian (pot stirrer) | | Kettle Custodian (pot stirrer) Joined: Feb 2004 Posts: 28,675 | The carbs are either waaaaay too rich, or you've got a slight flooding situation. The float level(s) could be set a little too high, allowing fuel to trickle into the venturis at idle. That symptom would probably be more noticeable with the engine up to normal running temperature as it would be after an extended road trip.
Look for fuel percolating into the air stream (into the venturis) at a hot idle. If it is, lower the float level slightly. You might also consider going to a slightly leaner high speed jet to prevent sooting up the plugs under cruise conditions.
Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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| | | | | Joined: Mar 2000 Posts: 784 'Bolter | | 'Bolter Joined: Mar 2000 Posts: 784 | underdog, I've got a variable pressure reducer on the line now set at 3.5lbs. Granted I have not checked to see if it is accurate with a pressure gauge. I'll check that out.
Jerry, I'll lean out the carbs a little and see if that helps.
Thanks Guys, Keep the suggestions comming. | | | | | Joined: Oct 2005 Posts: 1,317 Shop Shark | | Shop Shark Joined: Oct 2005 Posts: 1,317 | T. Langdon recommends reducing the fuel pressure to 3.5 psi. I've got basically the same setup with an AutoZone Mr. Gasket $30.00 adjustable regulator. It's acturally more like an adjustable orifice as opposed to a genuine "regulator" but it does the job. I also have a gauge setup and 3.5 on the little dial gives you 3.5 at the carb.
At one of the shows some 8-ball cranked it down to 1.5 psi and the truck ran fine at normal highway speeds until I noticed it. I think that part of the Rochester "flooding" issues could be cured by dropping down the pressure to the carb. With stock gearing and running up at 3K I could see where the extra vibration might make it difficult for the needle to seat properly on a 50 year old carb. May be all wet on this "theory" but it might be worth a shot.
Dave | | | | | Joined: Dec 2007 Posts: 198 Wrench Fetcher | | Wrench Fetcher Joined: Dec 2007 Posts: 198 | Whewn I had the 261 with dual Carter Webers on it still instaleld in Red, and took it to the dyno and included air/fuel monitoring in the dyno run, the dyno operator remarked that it was running way rich everywhere except at wide open throttle (sort of the reverse of what you'd want!), and that it was running (quote) "pig rich" at idle.
So, I agree with the above comments posted.
Jim G | | | | | Joined: Mar 2000 Posts: 784 'Bolter | | 'Bolter Joined: Mar 2000 Posts: 784 | Thanks everyone. It looks like we have the problem pretty nailed down. I'm going to take over to my favorite old-time mechanic and have him do his magic on it. This is a little out of the range of my expertise.
I'll let you know what the outcome is.
Thanks again
George | | | | | Joined: Mar 2000 Posts: 784 'Bolter | | 'Bolter Joined: Mar 2000 Posts: 784 | Well just got the truck back from my machanic. Turns out that the carbs had crud in the idle circut. I thought this was odd because I have a fuel filter before the fuel pump and a glass bowl filter just before the carbs. He said that the carbs (even though new) may have come with some crud in them and just got worked loose on a longer drive. Everything is cleaned out, readjusted and balanced. Seems to run good now.  The machanic was impressed the way it drove with the T5 and the Carter/Webers. He is looking forward to adding a T5 and Carter/Webers to his Model T. Thanks for all the advise George | | |
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