Missing at 3500 RPM and Above

[oldbugger]

Plugs?

[drolston]

Originally Posted by oldbugger

Plugs?

Please elaborate. Will plugs that fire okay at 5,000 rpm no-load misfire at that RPM under load? Does the pressure on the compression stroke go up that much? I don't think so; just on the power stroke.

[oldbugger]

Whenever i have misfire at higher RPM thats the ist place i check, easy to do.

[drolston]

Originally Posted by oldbugger

Whenever i have misfire at higher RPM thats the ist place i check, easy to do.

What do you actually look for on the plugs? Or, do you mean that you just put in new plugs and problem is fixed? My plugs are NGK B-6L with under 3,000 miles, properly gapped (.025), and look fine (light gray, no deposits).

[tubman]

"Light gray"? When I was into reading plugs (back in my snowmobiling days), we liked to have them a light chocolate brown; like a freshly broken open Hershey Bar.

[drolston]

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Originally Posted by tubman

"Light gray"? When I was into reading plugs (back in my snowmobiling days), we liked to have them a light chocolate brown; like a freshly broken open Hershey Bar.

That light brown is what I liked on my flatheads in the olden days, but does not seem to be the case with modern alcohol blend fuels. I am already running a colder plug than is usually recommended. That is, NGK B-6L versus the hotter NGK B-4L.



Am I the only one wondering where the light brown went?

[cadillac512]

The light brown went the way of unleaded fuel and modern additives. Most plugs in a good running engine will now stay almost white for quite a while, eventually getting some color. Same with the light gray tailpipes....all you see is either black or almost no soot at all.

[cadillac512]

Originally Posted by drolston

Please elaborate. Will plugs that fire okay at 5,000 rpm no-load misfire at that RPM under load? Does the pressure on the compression stroke go up that much? I don't think so; just on the power stroke.

Yes, compression pressure does go up that much but it's throttle opening that does it rather than load per se'. 5000 no load takes very little throttle opening and the cylinders don't get a full air charge but full load wide open does for sure. Lots more resistance to the arc jumping the plug gap under load at high rpm.

[oldbugger]

Issue resolved?

[drolston]

After getting my broken door glass replaced I finally got around to checking to see if fuel flow was restricted. The electric pump pushed a nice steady stream through the filters, mechanical fuel pump, and pressure regulator. The mechanical pump also pumped a decent amount, through turned-off electric pump when cranking the engine.

So, back to the ignition as primary suspect. I replaced the Bosch blue coil with the Pertronix Flamethrower I had left from when I removed the Pertronix Ignition. I took the opportunity to check both coils with my trusty Simpson 260 meter. The Pertronix measured 1.5 Ohms in the primary windings, as labeled. The Bosch measured 2.6 Ohms; not right. I have not taken it for a test run yet, but hoping the high resistance of the Bosch coil could be the problem. But why would the Bosch coil work okay no load and break down when winding up in second gear? Maybe because the no-load test was with a cold coil and the street test it was hot?

[JayChicago]

If you are 12 volt, the Bosche Blue at around 3 ohms is the correct coil. The 1.5 ohm coil is for 6 volt, no resistor. If you run that on 12 volt, current (amperage) will be double what it should be. It may work fine and will give a hot spark. Petronix used it. Then again, points or coil may fail in time due to over-current.

[drolston]

I had the dash resistor bypassed with the Bosch Blue. I will put that back in for the 1.5 Ohm Pertronix coil and give it a road test.

[Flathead Fever]

Originally Posted by rockfla

Michael
I am getting my 504 back the end of the week.....how do you tell on the Sun that the points are floating? On the degree wheel lights???

Here is a good video on the Sun 504 Distributer Machines. I used one on a 289 to setup the timing curve on a Mallory distributor 45 years ago. Probably ten years ago I had a chance to buy one cheap, so I brought home. It took me a while to find the flathead distributer adapter and I bought it. I haven't used it yet. I paid as much for the flathead distributor adaptor as I did the entire 504 machine. When the distributer is working correctly all eight lights will be flashing at the same spots. If the points are bad the lights will start jumping all over the place. It lets you check to make sure each cylinder is firing exactly when it's supposed to be. Checks for worn bushings or a distributor cam. Checks the advance. I worked in a fleet with 400 vehicles for 30-years, and we didn't have a distributer machine, we did have a sun scope but no distributer machine. You don't really need one, except for checking the alignment of each cam lobe, but they are fun to play with if you have the distributer off of the engine. Mostly, it makes a nice-looking cabinet for storing my flathead distributor parts in.

https://www.youtube.com/watch?v=8EqUH2ZnSSY

This one is not a flathead but it' still good.

https://www.youtube.com/watch?v=CWbPeWZtAxg

[drolston]

Issue resolved.
I mis-remembered having the resistor bypassed with the 2.6 Ohm Bosch blue. It was in the circuit, probably resulting in too low voltage at the coil with the points closed.

With the 1.5 Ohm Pertronix flamethrower coil and the resistor in the circuit, the engine wound right up to 5,000 rpm without losing power. Volt meter shows 13 volts at the coil wit the points open and ~8 volts with the points closed.

Thanks to all for your help.