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Posted By Discussion Topic: 47 Ford high rpm miss -- page: 1 2 3 4     Sgstepp@yahoo.com 09-21-2016 @ 5:30 PM reply profile send p.m. Member Posts: 11 Joined: Sep 2016 My 47 Ford half ton pick up with flathead engine is experiencing a high rpm miss. It idles great but when the rpm is brought up to approx 800 rpm and higher it has a severe miss. This happens when cold or hot. I have replaced the plugs, points and condenser. The points only had about 100 miles on them since I changed them last but they showed some burned points so I replaced them. I adjusted them to .015 gap. The rotor and cap looked great and clean so I kept them. Could the plug wires be a culprit, especially since they are so close together inside the wiring tube. How much of a job is it to install new wires. Also what kind of connection is on the cap end of the plug wires. Do they simply pop out when disconnecting from the distributor cap. Should I install a new distributer cap at the same time or am I totally off base with these thoughts. I thought about the carburetor being the culprit but I can't wrap my head around that being the cause. I have compression on all cylinders between 80 and 90 lbs so sticky valves do not appear to be an issue. Does anyone have any thoughts that would help me out? Thanks for any feedback. ford38v8 09-21-2016 @ 6:43 PM reply profile send p.m. Senior Posts: 2734 Joined: Oct 2009 I would leave the plug wires alone for now, and concentrate on fuel delivery as the possible cause. A restriction in the lines caused by rust debris, or within the fuel filter can reduce the flow to the carburetor, thereby starving the engine at higher rpms. The fuel pump can be the cause, for much the same quality of fuel issues. The carburetor may itself have an accumulation of debris in the bowl, preventing a full flow through the main jets. I suggest a simple test, run the engine up to the point of the heavy miss you describe, then shut it down. Remove the airhorn to discover the level of fuel in the carb, as well as any debris in the bottom of the bowl. If that test shows a low level of fuel, a second test can be done with the pump, by removing the line at the carb and directing it into a large container. With the ignition OFF, crank the engine over continuously to see the capability of the pump to deliver. It should be a strong pulsating stream. If not, look to the pump and filter next. You should have a glass sediment bowl on your fuel pump, which may show an excess of sediment, or air bubbles, which would indicate a dried up cork gasket. Here's hoping you won't need further ideas, and please get back to us with what you discover. Alan This message was edited by ford38v8 on 9-21-16 @ 6:47 PM TomO 09-23-2016 @ 6:55 AM reply profile send p.m. Senior Posts: 7243 Joined: Oct 2009 Check for a strong spark at all RPMs. Do a cylinder balance test, by shorting out one plug wire at a time with a screwdriver. The drop in RPMs should be the same with all cylinders. If one or more do not have an effect on the RPMs, concentrate on that cylinder. If you have a plugged fuel line, sometimes pulling out the choke will get enough gas to the carb to improve the running. Tom Sgstepp@yahoo.com 09-25-2016 @ 2:27 PM reply profile send p.m. Member Posts: 11 Joined: Sep 2016 Hello Alan, I finally got time to perform the tests that you described. After running the engine up to the point of noticing the miss, I held it there for about 10 seconds and then shut it down without changing the throttle position. I removed the air horn and looked into the float chamber to see the fuel chamber at half full. I saw no sediment in the chamber so I reassembled the air horn. Also the fuel pump is performing as expected it should. I did get a small burst of fuel from the carb fuel connection when I started to remove it from the carb showing some pressure in the line. After reassembling the air horn, I restarted the engine and it still has that dreaded miss. I took it for a short drive and took it up to 40 mph which is really quite fast for this setup and it had the miss all the way from approx 800 rpm to a very high rpm and it had the miss the entire time. What are your thoughts? ford38v8 09-25-2016 @ 3:04 PM reply profile send p.m. Senior Posts: 2734 Joined: Oct 2009 I think TomO's advice is good, and does appear to be better than mine considering the outcome of your tests so far. Always do the simple tests before progressing to the more complicated and expensive ones, by the way! In rereading your first post, you say the points were burnt after only 100 miles, which I should have caught at the previous reading! With ignition, my first guess would be that your new condenser may be at fault, as even new ones may fail in short order. Check the mounting first, as it must have a good solid connection to ground. Tell us about your ignition setup: Is it as original to your year 1947, and the coil you are running? With or without the original resistor mounted on the coil bracket? Have you converted to 12 volt? I think these are some of the questions TomO would ask, as he is more into ignition issues than I am. Alan ken ct. 09-25-2016 @ 3:54 PM reply profile send p.m. Senior Posts: 1513 Joined: Jan 2010 If you have more than normal pressure at the gas line when you removed the line,check and run car W/O the gas cap your vent may be blocked or wrong cap,you may need a VENTED cap on there. Ive seen it happen before. ken ct. Sgstepp@yahoo.com 09-28-2016 @ 4:34 PM reply profile send p.m. Member Posts: 11 Joined: Sep 2016 I failed to mention before that I have replaced the condenser on the distributor at the same time as the points. The elec system is the original 6 volt system with positive ground. The coil has been changed once with a six volt coil from the parts store after the original gave up when it got hot. The plugs are new. I cannot see a resistor on the coil bracket. It must be located elsewhere. I did the old trick of pulling a single plug wire and listening to the engine idle. The right bank tested good with definite engine rpm drop when each wire was pulled. The left bank was different. The front cylinder had no engine drop, the second cylinder had no drop, the third and fourth cylinder had about 50% engine rpm drop. Does this explanation or test help to diagnose the engine miss? The only other thing that I ran into this weekend is a notice on a gas pump to use non-oxygenated regular gas in all collector vehicles. I have regular gas in this pick up. Also I must use the choke to start the engine when cold, not so much when warm. I hope this information helps. cliftford 09-28-2016 @ 9:05 PM reply profile send p.m. Senior Posts: 845 Joined: Jan 2014 Do a compression check and see how they compare. The cylinders should be within a 20 lb. range. If not, you have some serious problems, probably rings and/or valves. ford38v8 09-28-2016 @ 9:30 PM reply profile send p.m. Senior Posts: 2734 Joined: Oct 2009 As you pulled the plug wires to test the individual cylinders, did you also check the spark itself at the plugs? Hold the plug wires near a head bolt to check color and strength? And have you inspected the condition of the spark plugs? Pay particular attention to differences between plugs and spark at the good firing cylinders as opposed to the poor firing cylinders. The lack of a resistor at the coil may also offer a clue. Some coils have internal resistors, and would be identified as such on the packaging. Also, your year Ford is very close to the point at which the location of the resistor changed from earlier models above the steering column under the dash, to the location at the coil mount on the engine. I'm not positive one way or the other, but there must be a resistor in the circuit somewhere. Your burned points are another clue that either you have no resistor in the circuit, or that your condenser is faulty. Condensers do sometimes go bad right out of the box, and must also be connected solidly to ground to work correctly. Alan This message was edited by ford38v8 on 9-28-16 @ 9:32 PM ken ct. 09-28-2016 @ 10:47 PM reply profile send p.m. Senior Posts: 1513 Joined: Jan 2010 Use a NAPA cond. # ( IH-200 ) these are very reliable. Been using these in rebuilding for yrs. ken ct omo

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