1976 caddy 500 cu in V8 engine misfire at temp and speed. OK idle

I Have a 1951 Ford COE on a 76 GM 1 tom dually with a 76 caddy 500 cu in big block. It has a HEI distributor, external mech fuel pump and filter. No eng problems until a few weeks ago whern it started to occasionally misfire at speed. Problem seems to worsen has the engine reaches temp. Idles OK cold and warm. Have checked timing ok at 4 BTDC steady and steady advance with increase of RPM. Engine vac ~17" and steady. Have replaced plugs, points, coil, cap, rotor, fuel filter and pump. Have installed voltmeter on hei coil voltage lead to check for steady supply during next road test. Getting very fustrated on the cause. Any suggestions welcome! Thanks, Johnny52

if you have HEI you don't have points? Could be ignition module. Have you checked vac. while driving it does sound like vac. leak

"....I Have a 1951 Ford COE on a 76 GM 1 tom dually with a 76 caddy 500 cu in big block....."

Neat would love to see pictures. I had a 51 F6 COE at one point and also a liking for the 500 CI Caddys.....have one in my 83 El Camino.

Kind of hard to diagnose long distance. A couple of quick checks that might or might not help.


".....external mech fuel pump and filter....." Not sure I've ever seen an external mechanical fuel pump. External pumps are usually electric and engine mounted pumps are usually mechanical.

If you are running a factory style engine mounted mechanical pump you might want to check and see if it's using the internal fuel filter. Looking at the bottom of the pump you may find a big "nut" the output line connects to. Remove the line and fitting and you may find a long paper filter (like the ones used in some Quadrajets). Yeah I personally found out about that one the hard way.

Cadillac HEI distributors also occasionally would have intermittent spark issues due to the wires that connect to the module breaking inside the wire insulation (caused by the wires being flexed back and forth as the breaker plate moves due to the vacuum advance). A quick check is to disconnect and plug the vacuum line to the distributor then due a test drive) If it clears the problem chances are that is your problem.

Hope this helps.


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The resistance in electrical conductors increases with temperature. It could be something as simple as a plug wire connection at either the distributor or where the wire is crimped to the plug terminal under your plug boot. I would look for something simple first.

standard mechanical fuel pump (new); replaced ignition pack (called em points - sorry), vacuum steady around 17". Last test showed steady coil voltage supply also. Last test seems to be getting worse with noticeable fluctuations (40-60) in oil pressure with slight rpm change, more than normal. wouldn't run steady at ~ 1000rpm and had a carb backfire as well. Think it might be time to think about removing cab and pulling engine.

Trying to upload photos but having difficulty, need to find 10 year old..

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Originally Posted by johnny52

Trying to upload photos but having difficulty, need to find 10 year old..

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Note that you may have to reduce the size of your pictures if you took high resolution shots, something less than a megabyte. This list was put together by someone else, copied and pasted here.

1. Find the pictures you want to post your "pictures folder" and note the file names and file size.

2. Log on to the thread you want to post on and click on the little blue button "post reply" at the lower left of the page or start a new thread.

3. Type in any text you want. You can edit or add more after adding your photos.

4. Click on "Go Advanced" at the lower right of the page.

5. Scroll down and click on "manage attachments".

6. A small box will appear in the upper right. Click on "Add files"

7. Click on "Select files" at the bottom of the box and a box will appear refering to your computer. Now navigate in your disc files till you get to the exact folder where the picture that you want to post is.

8. Select the file name of the first picture you want to add, and click "Open". The name should come up in the "add files" window. Click "Upload" and that file will appear at the bottom of the box. You can do this for five pictures in a row.

9. If you already added text, scroll up and select your cursor either at the start or finish of the text, depending on where you want the photos. Scroll back down to the attachments box and click "Insert Inline", then "Done".

10. Click on "preview post" to check the order of the pictures, and drag & drop the files to revise as you wish. When happy click "submit".

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Originally Posted by johnny52

standard mechanical fuel pump (new); replaced ignition pack (called em points - sorry), vacuum steady around 17". Last test showed steady coil voltage supply also. Last test seems to be getting worse with noticeable fluctuations (40-60) in oil pressure with slight rpm change, more than normal. wouldn't run steady at ~ 1000rpm and had a carb backfire as well. Think it might be time to think about removing cab and pulling engine.

Trying to upload photos but having difficulty, need to find 10 year old..

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hope this works..2 images uploaded

Wow neat looking 51's.

Again it's hard to tell long distance. Did you try pulling and plugging the vacuum advance line at the distributor and seeing if that cured the fluctuation goes away?

Have you checked any vacuum nines and caps that might be on the carburetor to make sure you don;t have a vacuum leak.


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Check the timing chain for wear. Use a timing light as you slowly increase rpm and see if the timing walks/moves around at speedy rpm stops along the way as you increase slowly.

vacuum and timing look OK last check. Waiting on more knowledgeable friend to stop over and help troubleshoot. Hope I don't have to remove cab to pull engine...

new holley carb on order. will recheck timing and vacuum again before swap out late next week. If new carb doesn't do the trick, it is pull cab snd engine in fall. (HOPE NOT!)

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Originally Posted by johnny52

new holley carb on order. will recheck timing and vacuum again before swap out late next week. If new carb doesn't do the trick, it is pull cab snd engine in fall. (HOPE NOT!)

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Just curious, why not get a kit for the Holley carb you've got and rebuild it? Gotta be cheaper than a new one!

My experience with Holley carbs is that anytime there's a backfire it probably took out the power valve!

Beautiful trucks! I'm just going to mention that I had a keep I worked on with a similar problem. It can find them after shut off it would crank and not start. It turned out to be the ignition cables! And they looked very recent! You might want to text for resistance mine would work for a after moving them around but still DOA! Just throwing this out there because I'm following this thread. Good luck Amigo!

Just experienced a similar condition, it was a venturi (ported) vacuum cap deteriorated. Changed the rubber cap and all is good. Sometimes it is the simple that baffles.