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Old 01-30-2019, 08:51 AM   #1
1934
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Default spark at the plugs

('37 FH motor)
When I short out the plugs with the motor idling there are 4 cylinders that seem to be doing all the work and 4 that seem to be doing nothing, I.E. when I short out one of the 4 good ones the motor bogs down considerably and when I short one of the other 4 there is no difference in the idle. I thought it might be the plugs so I switched them up and no difference- always the same 4 cylinders not firing properly.
Does this sound like a timing issue or.... Any ideas appreciated.
Doug.
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Old 01-30-2019, 09:28 AM   #2
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Default Re: spark at the plugs

Are the weak cylinders fed from the same side of the carburetor? That would suggest a fuel problem.
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Old 01-30-2019, 09:41 AM   #3
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Default Re: spark at the plugs

Check for a hole between the heat riser port and the intake runner. Friend had this problem, drove him and many Flathead friends crazy until he found the hole. E mail me at springgrange@personainternet.com or give me a call at 705 439 2567 evenings(eastern time) if you want more info.
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Old 01-30-2019, 09:49 AM   #4
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Default Re: spark at the plugs

Are the plugs all on one side? Or staggered if that’s the case ck compression poss valve problem just a thought
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Old 01-30-2019, 10:04 AM   #5
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Default Re: spark at the plugs

Staggered; 2 on each side of the motor.
Thanks.
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Old 01-30-2019, 10:07 AM   #6
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Default Re: spark at the plugs

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Staggered; 2 on each side of the motor.
Thanks.
If this is the case, I agree with "rick r". I have heard of that exact problem before.
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Old 04-13-2019, 08:12 AM   #7
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Default Re: spark at the plugs

Update;
Finally warm enough here to get the car out and put a few miles on it. Still having the same issue as described above. I sprayed a can of carb cleaner into the carb at fast idle and that did absolutely nothing. As per Rick r's advice I examined the intake and could find no holes or cracks anywhere. Just to be sure, I changed the intake manifold and the problem is still there.
I'm pretty sure it's firing on all 8 when I'm driving because there doesn't seem to be any loss of power or sluggishness and I can drive 50 or 60 mph with no problem.
Thanks for any input, Doug.
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Old 04-13-2019, 08:31 AM   #8
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Default Re: spark at the plugs

Hummm. "Always the same cylinders" acting up. Tell us which 4 cylinder numbers seem good and which numbers seem bad so we can compare to the firing order. Sounds like something off inside distributor cap(s). How are your plug wires and their connectors ? Clean inside of distributor cap ? OR if you have a spare distributor cap and/or rotor try changing them. (may be arcing at low rpm but doesn't show at high rpm)
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Old 04-13-2019, 08:36 AM   #9
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Default Re: spark at the plugs

Have you ever given any consideration that the plug wires may be bad?
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Old 04-13-2019, 09:07 AM   #10
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Default Re: spark at the plugs

If it is a carb problem, spraying the carb cleaner in at a fast idle won't do anything. At the very least you need to spray it with the engine off. Really you need to take the carb apart and really clean out all the passages. Has the car been sitting long enough to get the carb gummed up?
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Old 04-13-2019, 11:37 AM   #11
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Default Re: spark at the plugs

On the right side the 2 middle cylinders are not firing, on the left side it's the first and last that are not firing. I think the plug wires are good because there is a strong spark when I short out the plugs.
Last fall I had the distributor out and put a NOS rotor in and new points. Maybe something got screwed up when I reinstalled it. But I don't know why the spark is good at the plugs if it is shorting somewhere? Guess I'll take the dist. off again and see if I can find anything.
Thanks.
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Old 04-13-2019, 12:13 PM   #12
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Default Re: spark at the plugs

Maybe one of the idle circuits in the carb is fouled with debri. Try removing the idle adjustment screws, give the holes a good shot of carb cleaner followed by compressed air and do this a couple of times, put the idle screws back in and adjust - see if that solves the problem. My thinking is that an idle circuit of the two is fouled for the motor runs okay on the main circuits.
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Old 04-13-2019, 04:14 PM   #13
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Default Re: spark at the plugs

Quote:
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On the right side the 2 middle cylinders are not firing, on the left side it's the first and last that are not firing. I think the plug wires are good because there is a strong spark when I short out the plugs.
Last fall I had the distributor out and put a NOS rotor in and new points. Maybe something got screwed up when I reinstalled it. But I don't know why the spark is good at the plugs if it is shorting somewhere? Guess I'll take the dist. off again and see if I can find anything.
Thanks.

I'd check the interior distributor caps to make sure all the wires are properly seated.
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Old 04-13-2019, 05:17 PM   #14
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Default Re: spark at the plugs

Put a vacuum gauge on wiper line and adjust mixture jets on carb for max vac,,,had the same thing last week,,,,,,worth a try,,,
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Old 04-14-2019, 11:07 AM   #15
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Default Re: spark at the plugs

Thanks to all who posted. I'll let you know when I get this sorted out.
Doug.
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Old 04-15-2019, 02:26 PM   #16
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Default Re: spark at the plugs

What Don P said X2 start with the simple stuff first.
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Old 04-16-2019, 09:27 AM   #17
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Default Re: spark at the plugs

The front part of the distributor rotor fires cylinders 2,3,5 & 8 Looks like the front part of the rotor is not working properly. Sometimes the brass conductor from the center of the rotor, where the coil output is connected, can crack as it goes to the front or the rear sections of the rotor.
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Old 04-17-2019, 05:55 AM   #18
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Default Re: spark at the plugs

I've ordered a rebuilt distributor from Third gen auto. Not sure if it will fix the problem but will soon find out. Tired of messing around with old distributors and 'hoping for the best'. Now I can feel confident about timing etc. when driving the car. I will keep you posted.
Thanks, Doug.
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Old 04-18-2019, 12:53 AM   #19
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Default Re: spark at the plugs

Quote:
Originally Posted by 1934 View Post
On the right side the 2 middle cylinders are not firing, on the left side it's the first and last that are not firing. I think the plug wires are good because there is a strong spark when I short out the plugs.
Last fall I had the distributor out and put a NOS rotor in and new points. Maybe something got screwed up when I reinstalled it. But I don't know why the spark is good at the plugs if it is shorting somewhere? Guess I'll take the dist. off again and see if I can find anything.
Thanks.
Since there is a strong spark to those four cylinders if everything else in the cylinders are good the spark must not be arriving at the correct time. You have done exactly what I would have done testing this engine.

You do not have an ignition scope to verify the firing pattern so all you can do is kill the cylinders and see if each cylinder drops the rpm, which they did not. You sprayed carb spray in the intake. Very good idea, that is the second thing I would have done. Had the carb not been providing fuel to those four cylinders the addition of the carb spray would have straighten the engine out and the rpm would have come up. Its not a fuel delivery issue. You swapped the intake, that did not fix it. Its not an air delivery issue.

That leaves compression valve problems or the spark not arriving at the correct time. I hate to say this but is there anyway this is as simple as the firing order getting mixed up on those four cylinders? The cylinders are numbered like most V8s. On every single V8, if you look down at the top of the engine you will see that one head is further forward than the other. That furthest forward cylinder is always number #1. Except on a flathead V8. A lot of people make that mistake.

My dad replaced his flathead plug wires with the correct original lacquered, color coded, cloth wires. When he got done it would barely run. He swore to me that he replaced them one at a time and that was not the problem. I drove down there and he had them all screwed up? Sometimes you can mix a couple wires up and an engine can still run pretty decent, especially a V8. You would think it would be bucking ,coughing and shaking drastically but sometimes they just feel like they are down on power. It depends on which ones in the firing order got mixed up I would double check and then check them again to make sure they are correct. Remember that these new V8s can kill four cylinders at an idle to save fuel and you cannot tell that it is not running on all eight. I have one and its smooth as glass at an idle.
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Old 04-18-2019, 07:55 AM   #20
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Default Re: spark at the plugs

Thank you Flathead Fever, I appreciate your input. I'm just waiting to get the distributor I ordered and am anxious to see how the car runs once I install it.
Doug.
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