Surging question

[70Custom10Nut]

It's been a while since I've been on. Got to pay the bills you know. Here's my question
Even after replacing all plugs, wires, filters and even carb, I'm getting hesitation from my engine. Starts fine and even runs well, but at idle the rpms will drop approximately 1-200, and then return to normal. There's no set pattern to it though. 10 years ago I put an electronic conversion kit in my stock dizzy, and never had any issues, until now. Anyone else out there had an experience like this? Am I looking in the right direction? My ears are wide open.
BTW, Happy Memorial Day to all!

[1969K10]

just stabbing in the dark, but fuel filter or pump? or even vacuum advance module?

[70Custom10Nut]

That's what I'm lookin at. I just don't need to spend a bunch of money on "it might be". I have a clear filter and the fuel bubbles while comes into it. Is this an indicator of a bad pump? thanks

[burke117]

i am having the same pulsing problem, but mine is at cruising speed (35-45 mph). I also have replaced plugs, filters, fuel pump, rebuilt carb. Any ideas?????

[D-Rat]

It's epidemic....I'm about to do plugs and wires to see if it helps. I get the intermittent backfire around 3k rpm under acceleration too. Carb is brand new. No vacuum leaks....

I do hear "check the timing before you do anything" here a lot, but I have no way of checking it at 3k rpm, as there's no timing mark reference to check that from.

[mclairmo]

Buy a timing tape, they are cheap. That will allow you to check timing at 3K rpm.

[silverstreak]

Most surging situations I've run into have been related to a lean mixture. Try richening the idle mixture a bit and see if that helps. Gas companies have been constantly messing around with gas formulas recently, winter blends, enthanol blends, nitrogen enriched, so who knows.

[70Custom10Nut]

Update info. The condition feels almost like "points flutter". of course I don't have points. The idle circuit shuts off when under power, so adjusting the idle mixture settings wouldn't have an effect. Again, it's almost a skip in the ignition at random. One of the first things I did to try to fix it was make sure the timing was at 4 deg BTDC. When I get the chance, I'm going to make sure the wires and control module are all tight with good grounds. Thanks for all the input up to this point

[D-Rat]

Sharing this thread. my timing is 10 degrees at 800rpm. I just replaced the plugs. Plug #7 was fouled a bit - the rest look good. The truck isn't running any better on new plugs. I have new wires and I'll replace those tomorrow after the truck is cool again (too bad it's 100 degrees outside!)

My father-in-law says to replace the vacuum modulator thingy on the transmission...says that's what's fouling #7 - getting trans fluid in the engine. Anyone ever heard of this? He also suggested taking the timing down to about 6 degrees. Would love to hear everyone's thoughts.

[silverstreak]

" it's almost a skip in the ignition at random."

I understand what you meant now. I think you're on the right trail by checking the grounds and module. I had a wire break off the pickup coil in my distributor that caused an intermitent no start problem. It was just barely making contact.

[mdwilf]

I've found that surges are almost always caused by either irregular fuel or spark.

Fuel caused by: bad pump, plugged filter, hole in fuel line, or carb issues

Spark caused by: bad wires, issues with dizzy or rotor, messed up advance (vacuum or mech - such as sticky advance, missing or broken springs) or a very loose timing chain that allowed the cam to "rock" back & forth at steady throttle pressure.

You might also have a loose wire that is shorting the system out.

Don't you love specific answers?

[JRBECK64]

How old is your carb? The accelerator pump diaphram may be leaking fuel causing the engine to surge. My 69 was doing the same thing and thats what another member told me was probably the problem. I actually gave that Carb away to another member with a rebuild kit so Im not sure how it worked out for him.

JRBECK64

[mr48chev]

Where do you buy your gas? No need to name the brand but I have found out that my truck runs a lot better if I go to newer name brand stations or Costco for gas. I won't put AMPM gas in anything I own no matter what it is and I don't like to go to stations that have old tanks.

I've got about the same condition with my truck right now but I'm pretty sure it's in the low speed circuit but haven't taken the time to decide what to do with it.

I would check the wires and connections in the distributor though especially on the pickup coil where the wires hook to it as that seems to be a common problem.

[gonzo6869]

You said that you see the fuel bubbling through your clear filter. Is the filter almost empty or is it full, I going to assume that it is near empty, if so you have fuel delivery problem. Some things to look at are: fuel pump, look for any other fuel filter a pervious owner might have installed and last take your sending unit out and check the sock, make sure it is not clogged up with 40yrs worth of junk.

[70Custom10Nut]

the bubbles are there at idle, no matter the fuel level. the filter is almost empty except when the rpms are up. the carb is a new edelbrock 1406.

[chris91]

I had recent surging problems with mine, A new PCV valve fixed the problem. I also changed the valve cover gaskets.

[gonzo6869]

Originally Posted by 70Custom10Nut

the bubbles are there at idle, no matter the fuel level. the filter is almost empty except when the rpms are up. the carb is a new edelbrock 1406.

It sounds like you have a fuel delivery problem. Check your pressure and everything else I mentioned earlier. I had that same problem, I thought it was the fuel pump, but even after I changed it, I could still see the fuel bubbling up through the glass. It turned out to be a hidden fuel filter the P.O. installed had clogged up causing the carb to starve.