[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