intermittent misfire

[K20-Bowtie]

Hi folks,

I have an intermittent misfire that is puzzling me:
• 1972 Chevy K20 with 350 small block
• Delco HEI distributor
• Eldebrok 1406 carburetor with newly installed pressure regulator

Actions taken:
• Installed new plugs and wires last week. Old plugs were black and smelled rich
• Installed deadhead pressure regulator
• Timed engine at 12 degrees BTDC
• Adjusted jets on Eldebrok 1406 using vacuum gauge - vacuum around 17psi. Not any experience with 1406, but jets are within a turn from being totally closed (seems odd to me).

Observations:
• Intermittent misfire. Noticeable around 1200 rpm. I have dual exhaust with no cross over and observe misfire from both tailpipes.
• The misfire can be explained “runs along with intermittent, short surge”

I really don’t want to start throwing parts at the problem, so I am looking for additional troubleshooting ideas.

Thanks,
K20-Bowtie

[68 P.O.S.]

Your timing is in the right neighborhood. With new wires and plugs, how’s the HEI? Under the cap look good, lubed, moving freely, and coil and module good? Was the original power wired used for the HEI?

When’s the last time the carb was cleaned and rebuilt? Have you tuned the carb at all, other than adjusting the idle mixture screws? Adjust the floats to specs or else it will flood. Edelbrock recommends you set your fuel pressure regulator to 5.5 psi; anything more than 6psi will cause flooding. You’ll also need a heat insulating gasket to stop the carb from percolating fuel. Is the choke operating correctly?

[rockyrivermark]

Originally Posted by 68 P.O.S.

Your timing is in the right neighborhood. With new wires and plugs, how’s the HEI? Under the cap look good, lubed, moving freely, and coil and module good? Was the original power wired used for the HEI?

When’s the last time the carb was cleaned and rebuilt? Have you tuned the carb at all, other than adjusting the idle mixture screws? Adjust the floats to specs or else it will flood. Edelbrock recommends you set your fuel pressure regulator to 5.5 psi; anything more than 6psi will cause flooding. You’ll also need a heat insulati gasket to stop the carb from percolating fuel. Is the choke operating correctly?

What he said.
RRM

[K20-Bowtie]

Thanks for the replies. Just purchased the truck about 2 months ago, so I am unaware of the history.

The carb does look dirty looking down from the top. (pic attached). I understand it's not a big deal to remove, disassemble and clean the 1406. Noticed the Edelbrock recommends using Simple Green. Guess those days of using that caustic carb cleaner dip are over. Spilled that stuff on me one time - ouch!

I'll also pull the HEI cap off and inspect (pic attached). Good idea on looking at the power wire. I suspect the original 12v line was used. I can always jump a heavier gauge wire from the battery to see if that solves my problem.

Just ordered a Edelbrock carb heat insulator as well. Using a stock intake manifold and don't think that's a problem, but I am replacing anyway.

[Ziegelsteinfaust]

I had my ignition switch go bad giving me the similar issues.

[geezer#99]

Your 350 didn’t have an hei stock. Wiring would have been thru a resistance wire which was at 9v while running. I see a yellow wire by your distributor. That might be the original wire to the coil. It’s connected to the resistance wire.
Eddy carbs don’t bolt up to stock spreadbore intakes without a possible vacuum leak. Your misfire could be a vacuum leak. Your vac reading of 17 says vac leak too.

[maverickmk]

Originally Posted by geezer#99

Your 350 didn’t have an hei stock. Wiring would have been thru a resistance wire which was at 9v while running. I see a yellow wire by your distributor. That might be the original wire to the coil. It’s connected to the resistance wire.
Eddy carbs don’t bolt up to stock spreadbore intakes without a possible vacuum leak. Your misfire could be a vacuum leak. Your vac reading of 17 says vac leak too.

Like geezer said, your HEI needs 12 volts. I tapped directly into the fuse block and ran it straight to the dizzy. The pic below shows in the upper right portion where it says IGN unfused, I used one of those.
One other thing, did you look at the fuel filter?

[68 P.O.S.]

Now that I see the pics, geezer is correct. Here's a good how-to on replacing the stock dist wiring: https://67-72chevytrucks.com/vboard/...d.php?t=708975

I would rebuild the carb and tune it with a cal kit, get the spread bore to square bore adapter, and look for vac leaks.

[K20-Bowtie]

Thanks for the feedback - like this board. I've got some work to do.

- Clean carb
- Check voltage and wire to HEI
- Check HEI cap and internal works

Looking at my pics, it appears I have an adapter pate between the carb and the intake (mighty dirty carb).

I have previously (within a week) installed a fuel filter and fuel regulator.

[Steeveedee]

Originally Posted by K20-Bowtie

Thanks for the replies. Just purchased the truck about 2 months ago, so I am unaware of the history.

The carb does look dirty looking down from the top. (pic attached). I understand it's not a big deal to remove, disassemble and clean the 1406. Noticed the Edelbrock recommends using Simple Green. Guess those days of using that caustic carb cleaner dip are over. Spilled that stuff on me one time - ouch!

I'll also pull the HEI cap off and inspect (pic attached). Good idea on looking at the power wire. I suspect the original 12v line was used. I can always jump a heavier gauge wire from the battery to see if that solves my problem.

Just ordered a Edelbrock carb heat insulator as well. Using a stock intake manifold and don't think that's a problem, but I am replacing anyway.

Not that it is causing a problem right now, but the throttle return spring should be attached to the lever where the cable pulls it, and somewhere to the front of the intake manifold. With the spring installed as shown, you will see rapid wear of the throttle body because the shaft is pulled against the bore. It will even wear the bronze bushings, eventually.

[toolboxchev]

Originally Posted by K20-Bowtie

Thanks for the replies. Just purchased the truck about 2 months ago, so I am unaware of the history.

The carb does look dirty looking down from the top. (pic attached). I understand it's not a big deal to remove, disassemble and clean the 1406. Noticed the Edelbrock recommends using Simple Green. Guess those days of using that caustic carb cleaner dip are over. Spilled that stuff on me one time - ouch!

I'll also pull the HEI cap off and inspect (pic attached). Good idea on looking at the power wire. I suspect the original 12v line was used. I can always jump a heavier gauge wire from the battery to see if that solves my problem.

Just ordered a Edelbrock carb heat insulator as well. Using a stock intake manifold and don't think that's a problem, but I am replacing anyway.

I have a question first. Does your number one plug wire and rotor point to the number one cylinder?

[geezer#99]

Originally Posted by Steeveedee

Not that it is causing a problem right now, but the throttle return spring should be attached to the lever where the cable pulls it, and somewhere to the front of the intake manifold. With the spring installed as shown, you will see rapid wear of the throttle body because the shaft is pulled against the bore. It will even wear the bronze bushings, eventually.

Sure about that!
Have you ever seen a stock gm with the spring in front?
If so post a pic.

[K20-Bowtie]

Originally Posted by toolboxchev

I have a question first. Does your number one plug wire and rotor point to the number one cylinder?

I haven’t move the #1 cylinder to TDC & checked the rotor position. Would that verify the timing is in the ballpark?
Posted via Mobile Device

[Steeveedee]

Originally Posted by geezer#99

Sure about that!
Have you ever seen a stock gm with the spring in front?
If so post a pic.

It would appear that they did not, from the pics I've been able to find in the last few minutes. I stand by my advice, though. YMWV

[K20-Bowtie]

Thanks for all the input folks. I had some time last night:

- Disassembled the Delco HEI. Looked good inside, no play in the rotor, no signs of wear or burned parts.
- Ran a separate wire from the HEI to the battery, reassembled. Same slight miss.
- Just for kicks, I removed and cleaned the 1406 idle jets. Reassembled and attempted to adjust, but no change in the engine when adjusting jets. In fact, I bottomed out both screw with not change in engine

My plan this weekend is to remove the carb, clean and rebuild. Picking up a kit today. I will also run a permanent 12 ga wire from the HEI to the ignition spade on the fuse panel.

P.S. - I am sure this is a long standing complaint (my first 350 SB), but placement of the distributor way back against the fire wall? WTF? I am guessing that the engine bay is designed to accommodate a large block engine, but there is tons of room up front. Just an observation.

[geezer#99]

When your mix screws have no effect (even bottomed out) that tells you the throttle (idle screw) is adjusted too far open and you’ve uncovered the transition slot and are idling on the power circuit.
Info in here about the transition slot. It’s on a Holley but yours is the same.

http://www.crankshaftcoalition.com/w...EI_distributor

Usually the idle is adjusted like that just to get the motor to idle Well.
A cure is increased initial timing. Your timing of 12 btdc may not be accurate.
An easy test is to set your initial at 18 degrees. Vac pot to the hei plugged to set timing.
Then your idle will increase and then you can adjust the idle screw back out and see if your mix screws react better.
If the misfires go away you’ve found the problem.

[maverickmk]

Originally Posted by geezer#99

When your mix screws have no effect (even bottomed out) that tells you the throttle (idle screw) is adjusted too far open and you’ve uncovered the transition slot and are idling on the power circuit.
Info in here about the transition slot. It’s on a Holley but yours is the same.

http://www.crankshaftcoalition.com/w...EI_distributor

Usually the idle is adjusted like that just to get the motor to idle Well.
A cure is increased initial timing. Your timing of 12 btdc may not be accurate.
An easy test is to set your initial at 18 degrees. Vac pot to the hei plugged to set timing.
Then your idle will increase and then you can adjust the idle screw back out and see if your mix screws react better.
If the misfires go away you’ve found the problem.

Disregard comment, I found the answer in the article.

[Sheepdip]

P.S. - I am sure this is a long standing complaint (my first 350 SB), but placement of the distributor way back against the fire wall? WTF? I am guessing that the engine bay is designed to accommodate a large block engine, but there is tons of room up front. Just an observation.

Locating the distributor to the front where you could get to it actually was one of Fords Better Ideas!

[RichardJ]

>>I have a question first. Does your number one plug wire and rotor point to the number one cylinder?<<

When any reference says the rotor points to cylinder #1, they are not talking about cylinder #1 or the spark plug in #1.

They are talking about the wire tower on the distributor cap that the #1 plug wire is inserted into.

[RichardJ]

Many original filler necks had a tab spot welded to it that the spring connected to. I've also seen many engines that had a tab under the thermostat housing bolt that the spring was connected to.

[RichardJ]

Geezer says,
"Wiring would have been thru a resistance wire which was at 9v while running."

maverickmk says,
"Like geezer said, your HEI needs 12 volts."

If the alternator is working properly, there will be close to 14 volts, not 12 V at the fuse box. With 14 volts, the ballast resister wire will reduce the voltage for the HEI to 11 -12 volts, not 9.

Even if the voltage is a little low to the HEI, it will only effect the HEI output at higher rpm. This will not cause the miss K20 is talking about and he verified that himself.

>>- Ran a separate wire from the HEI to the battery, reassembled. Same slight miss.<<

Solve the problem you have now before possibly introducing a new problem.
Do you have a voltmeter to check the HEI voltage as you have it wired now?

Geezer ask,
"Eddy carbs don’t bolt up to stock spreadbore intakes without a possible vacuum leak."

You should start here and also check for other possible vacuum leaks as well.

[geezer#99]

Originally Posted by Steeveedee

It would appear that they did not, from the pics I've been able to find in the last few minutes. I stand by my advice, though. YMWV

There you be Stevee
I stand corrected.
Pic posted by Richard.
Apologies!

[Steeveedee]

Originally Posted by geezer#99

There you be Stevee
I stand corrected.
Pic posted by Richard.
Apologies!

No worries! It's all good.

[toolboxchev]

Originally Posted by K20-Bowtie

I haven’t move the #1 cylinder to TDC & checked the rotor position. Would that verify the timing is in the ballpark?
Posted via Mobile Device

I will not give a definitive timing indication, Some when installing an HEI will clock the distributor 1 cylinder backwards for the distributor to clear the intake manifold. Especially the Vac Advance can interfere with the intake manifold casting.

I am under the assumption it can affect the rotor phasing of the distributor. The following is a really great resource for the HEI

https://www.crankshaftcoalition.com/...EI_distributor

[Mike C]

Even if they look fine, your cap and rotor are as likely to be sources of misfires as the plugs and wires. Consider replacing them both or at minimum removing and cleaning the inside with brake klean. Carbon tracking is difficult to see on a black cap but years of use and junk accumulating in the cap from the saprk jump ionizing the air can cause them to fail.