134 Fhead Surge While At Idle

Why would my motor surge when I'm idle? RPM's go up and down.
I have a new electronic distributor, recently rebuilt the carburetor, only use ethanol free fuel. what else could it be?

 

I'm sure those with more knowledge will chime in. A couple of items to check. Check for vacuum leaks. Check the pcv system for proper operation and leaks. Another possible culprit could be fuel pressure coming from the fuel pump. Hope this helps.

 

No pcv system.

 

Constant up and down rpm change. Very noticable.

 

Did it ever have a pcv system? When I did my current restoration my engine didn't have a pcv system. Once I acquired the parts and installed it I noticed my engine settled down. Wasn't experiencing a surge per se but the idle wasn't steady before I installed the pcv system. I have an electronic distributor on my F134 as well.

You stated that the carb was rebuilt. I'm sure the idle screw and air mixture screw are set correctly. I would double check the air mixture screw setting just to rule it out. Improperly set it can cause the engine to run lean or rich. If to lean then this could cause the engine to surge. Also, check the rest of the fuel system, fuel pump and filter (either the small screen, if you have fuel pump with glass bowl, or if one was installed)

 

Thanks Glenn. Good questions, I forget that need to ask questions to get a better picture of what is going on instead of just jumping right in with possible fixes.

 

It ran like a top. Took it out for a 2 hour cruise. "Maiden voyage". First time without a breakdown.

Now though, im stumped. Ive got the guy that rebuilt my carb coming over tomorrow after work. I will keep you posted. Any and all suggestions i can present to him are welcome.

 

Take the air horn off the carb and look down the throat. Can you see fuel dumping in anywhere while idling and surging?

 

Check the lines for bends and kinks. Replace any badly worn hose. Dirt, or water in the fuel. Clean out the gas tank and change the fuel filters.Main jets may the the wrong size. Vacuum leak. Check the carburetor throttle to make sure it isn't loose in the throttle body. Might be a leak at the intake manifold, or perhaps where the carburetor mounts to the manifold. Fuel level is too high, or too low. Check the float level.

 

new gas tank/lines/fuel filter. also put in an inline filter before the fuel pump. I only use non ethanol fuel, and I get it from the same place, every time, for all of my small engines, jeep, boat, and tractor.
it runs great at higher RPM's, i.e. when driving. but when I slow down for a corner, or when I took it through the trail behind my house, the lower speeds suffer due to the rpm surging.
It is a new electronic distributor, so however it advances, is beyond me.

 

Here's a note about surging, or what I've always called 'hunting' (the vrooom, vrooom, over and over, seen on the tach going from about 600 to 1500 rpm. up and down). My description is probably a 1 in 100 chance for helping anyone but I'll nonetheless say that my recent 'hunting' occurrence was because the thin wires connecting the distributor to the coil were loose. The nut that fastens two leads to the coil terminal had loosened. I only discovered this by a chance glance at the coil and touching of its leads while fiddling with the fuel mixture screw trying to remedy the surging at the carb. Like most everyone commenting here, I believed that the surging was fuel related -- but lucky for me it was a quick wire-connection fix.

 

this I can try on my own, as soon as I get home

 

'70 should have a PCV system on it.
They were on JEEP motors from 1960 at least.

 

Mine does not. I will have to ask my carb guy what it is, and why i dont have one

 

Every electrical wire is connected and tight. All 7 of them