Idle problem on C15 261 CU IN

[TCLARK]

Maybe someone has had this problem also. I am having trouble getting the Chevy six to idle nice. It never quits but seems to go up and down about 150 rpm when idling.
I put in a carb kit (Rochester B) and added a pertronix ignition. Still not
idling nice.
The pertronix was quick to put in and the engine starts instantly now. Also the plugs had been black with poor combustion but I took it out for a spin and they
are all light brown. I think that it also has way more power than pre pertronix.

The distributor vacuum seems to work fine but this engine does not want to idle
smoothly.

[cletrac (RIP)]

Check for a vacuum leak.

[TCLARK]

Maybe the vac advance pot is leaking, I will check that. I heard that propane around the leak will increase rpm and can be used for finding leaks.
Thanks

[Bob Carriere]

Hi Terry

With the Pertronix I had to increase the gap from low 30 to 40 thou...

What kind of plugs are you using??.............. I am using AC C43

Have you tried a vacuum gauge yet..... directly to the manifold....... reading should be steady at 18 + or _ at about 500 rpm.

How slow are you trying to idle the engine...... the 150 rpm variation was it seen on a tachometer?

How does the increase come on and off........ regular intervals or odd interval.... at what RPM those the engine smooth down???

Do you have a see through old style glass gas filter just before the carburator???? do the fuel level vary a lot when idling....... bowl should be about 2/3 to 3/4 full......

The rebuilt Rochester B .... was it the original for that engine and does it have the larger throat opening... any sign of leakage around the carb top cover..... did you check against a flat glass to see if it might be warped.

If you were closer I would suggest swapping carb with mine to see if makes a difference...... currently using a YF carter which is a dream.

The increase in rpm as to be either more gas coming in or an air leak but a leak is a leak...... maybe just lack of fuel..... carb float too low...... anemic fuel pump not sending enough juice at low rpm....

Puzzling......

Cheers

[TCLARK]

I'm using AC plugs with an r in front of the number meaning they have built in resistance, that could be a problem.
I haven't yet hooked up a vac guage, my vacuum line is original steel and needs
to be tee'd.
The rpm flutters at idle at odd intervals about 2-5 seconds apart, I do have a new
plastic in line fuel filter, that is not plugged.

The carb is original to the 1959 261 engine,it has no gas leaks, but I didn't
check for warpage. I saw on you tube that warpage can be a problem.
Bob where did your carb come from?? What year an d model of vehicle,
I wouldn't mind finding one.
I will tackle this again tommorrow

[Phil Waterman]

Hi Terry

Everyone is hitting the most likely cause for lopping at idle, so you have a long list of things to work through. To add to the list what do you have for fuel pump? As Bob suggest fuel flow and pressure are also possible causes. Air leak in can be one of the hardest to track down. I had air leak at the fuel selector valve that took forever to find.

Reason for asking what do you have as fuel pump, is mechanical pumps can have problems at low RPM if the pivot points are worn, or valves in the pump are not sealing, fuel pressure gage should help ID those issues. Electric pumps seem to be more bothered by air leaks on the suction side. One quick check I've used is installing a section of clear tubing just before the carb, looking for air bubbles.

Keep us posted on what you find.

Cheers Phil

[Bob Carriere]

Hi Terry

The "R" plugs should not be a problem.... what is the heat range?? 42 being the coldest and 47 the hot ones for used for heavy trucks/firetrucks.... delivery trucks doing a lot of stop and go.

Make sure you have a proper connection for the vacuum gauge as once you use it you will be reconnecting again. Vacuum gauge can be used to set the timing without being double jointed with a timing light. and set the carburator idle screw. The aim is always to adjust until you get the highest steady vacuum reading.

My carb came form the Oaks Brothers in the West end of Toronto....... sons have taken over from Dad..... they do a lot of carbs for expensive restoration and calibrate hot rodder carbs using an old fashion flow bench with kerosene......huge contraption that actually flow/sucks air through the carb and allows them to adjust jets for engine cfm requirements. When you get your carb back they are ready to be isntalled and engine will start right away with just minor adjustments to the idle screw. Cost was just shy of $200.

I actually dealt with the Dad who only deals with the older stuff. He has a huge inventory of carb core all cleaned up, but not assembled until required.
He also made me a Zenith 22-228, which was used on the 270 GMC engine and has an external adjustment for the main jetting. Haven't tried it yet.
Another good source would be CarbKing on Stovebolt forum. There is a multitude of YF carbs out there some for Ford and some for Jeeps and AMC... you need to have the right carb number and jetted for the 261..... the Zenith is unique in having a removable inner throat passage that allows to tailor to various CFM requirements...... they were still being manufactured up to a few years ago in California for over $500 US.

I noticed that you have a steel cap fuel pump..... I love the original upside down model where you can see if fuel is moving...... but they are prone to drying up the cork gasket under the bowl and leaking badly...... last time that occurred Grant ordered a dozen gaskets for spare on Ebay.

Phil may be on to something with the fuel supply...... worn plunger running on a worn camshaft.... dirty fuel valves inside...... worn pressure spring.... air leaks...... I think a fuel issue is more likely to cause a cycling of RPM than a steady vacuum leak.... air leaks tend to be air leaks and steady.

What rpm idling speed are you aiming for???? and are you using a tachometer to get an accurate reading...... you might be trying to set it too low.

With the vaccum gauge connected you can adjust the timing by hand to get the highest reading....... than move to the idle screw mixture on the carb and repeat the process to get the highest vacuum reading..... then adjust the mechanical idle screw on the bell crank...... and repeat the last two process until it runs good.

Its always the piddly little stuff that takes the longest.

Cheers

[Bob Carriere]

The Oaks Brothers..... now seem to be operating from the same location under the name....

Carburetor Rebuilders Company Limited
60 Eugene St
North York, ON M6B 3Z4, Canada

The Dad was in is 80 when I last saw him a few years ago.....

They now seem to be offering machine shop services for milling heads, manifolds, ect.

cheers