Converting Tri-Power center carb for manifold vacuum advance
The problem that initiated this carburator modification was a poor and inconsistant idle on a 1967 L-71 engine caused by the ported vacuum advance system that was used on the three years that the Tri-Power carburator setup was available. See this posting for more details on the original idle problems: https://www.forums.ncrs.org/showthre...847#post714847
This idle problem could be fixed by converting the distributor vacuum advance from ported vacuum to direct manifold vacuum. Several posts on the TDB have been posted discussing doing this conversion from ported to manifold vacuum. They all center on switching the distributor VAC from what was installed at the factory to a new VAC that pulls in full vacuum advance on the distributor at about 2" of vacuum below the idle vacuum. For this engine a "B-26" VAC that pulls in full advance at 12" HG was chosen and installed on the distributor. Now all that's needed is to connect the "B-26" to direct manifold vacuum.
Three ways to do this have been suggested on the TDB. One is to install a "tee" in the vacuum hose that runs to the choke pull-off on the center carb and connect a vacuum hose from there to the "B-26" VAC. Another way was to install a vacuum fitting in the intake manifold that has one more port than what was already there if one was installed such as for power brakes. The third one mentioned in the TDB was to modify the carb to provide manifold vacuum instead of ported vacuum to the base of the center carb where the vacuum advance hose to the distributor is normally attached so the engine still looks stock. This third solution involved drilling a small hole in the throat of the carb below the throttle plates and plugging the existing hole just above the throttle plates with a smear of epoxy.
Another way to provide manifold vacuum to the original ported vacuum outlet at the base of the carb is discussed below. This way is simple, looks stock, and is reversable without carb disassembly if reversal is desired at any time. This method requires a 6-32 threaded allen head set screw that is 3/16" long and can be bought at any Home Depot store. The one used in this modification was made of stainless steel so any long term corrosion with the carb base is minimized. This set screw takes a 1/16" allen wrench to install.
The existing hole in the carb base for the ported vacuum will be tapped for the 6-32 threaded set screw and then the set screw is installed to the bottom of the new threads as depicted in the pictures. This hole is .120" in diameter (equivalent to a #31 drill bit) as opposed to the recommended hole size for tapping 6-32 threads which is a #36 drill bit (.1065" diameter) but there's virtually no load on this set screw and it works perfectly.
A new hole using a #31 drill bit or a 7/64" drill bit if you don't have a numbered drill bit set, is then drilled from the inside end of the old ported vacuum channel in the base of the carb into the manifold vacuum channel where the choke pull-off receives it's manifold vacuum. This new hole is also tapped for a 6-32 thread so at any time the 6-32 set screw can be taken out of the old ported vacuum hole and placed in the new hole to completely reverse this modification if a return to ported vacuum advance is desired. But, once you go to manifold vacuum advance, you'll never want go back. If the four pictures that were uploaded for this post came through, it should be very evident how to make this modification to the center carb base plate. Hope this post helps someone with their idle problem. If the pictures didn't come through then let me know and I'll email them to yopu. my email address is edhampshire@bellsouth.net.
This idle problem could be fixed by converting the distributor vacuum advance from ported vacuum to direct manifold vacuum. Several posts on the TDB have been posted discussing doing this conversion from ported to manifold vacuum. They all center on switching the distributor VAC from what was installed at the factory to a new VAC that pulls in full vacuum advance on the distributor at about 2" of vacuum below the idle vacuum. For this engine a "B-26" VAC that pulls in full advance at 12" HG was chosen and installed on the distributor. Now all that's needed is to connect the "B-26" to direct manifold vacuum.
Three ways to do this have been suggested on the TDB. One is to install a "tee" in the vacuum hose that runs to the choke pull-off on the center carb and connect a vacuum hose from there to the "B-26" VAC. Another way was to install a vacuum fitting in the intake manifold that has one more port than what was already there if one was installed such as for power brakes. The third one mentioned in the TDB was to modify the carb to provide manifold vacuum instead of ported vacuum to the base of the center carb where the vacuum advance hose to the distributor is normally attached so the engine still looks stock. This third solution involved drilling a small hole in the throat of the carb below the throttle plates and plugging the existing hole just above the throttle plates with a smear of epoxy.
Another way to provide manifold vacuum to the original ported vacuum outlet at the base of the carb is discussed below. This way is simple, looks stock, and is reversable without carb disassembly if reversal is desired at any time. This method requires a 6-32 threaded allen head set screw that is 3/16" long and can be bought at any Home Depot store. The one used in this modification was made of stainless steel so any long term corrosion with the carb base is minimized. This set screw takes a 1/16" allen wrench to install.
The existing hole in the carb base for the ported vacuum will be tapped for the 6-32 threaded set screw and then the set screw is installed to the bottom of the new threads as depicted in the pictures. This hole is .120" in diameter (equivalent to a #31 drill bit) as opposed to the recommended hole size for tapping 6-32 threads which is a #36 drill bit (.1065" diameter) but there's virtually no load on this set screw and it works perfectly.
A new hole using a #31 drill bit or a 7/64" drill bit if you don't have a numbered drill bit set, is then drilled from the inside end of the old ported vacuum channel in the base of the carb into the manifold vacuum channel where the choke pull-off receives it's manifold vacuum. This new hole is also tapped for a 6-32 thread so at any time the 6-32 set screw can be taken out of the old ported vacuum hole and placed in the new hole to completely reverse this modification if a return to ported vacuum advance is desired. But, once you go to manifold vacuum advance, you'll never want go back. If the four pictures that were uploaded for this post came through, it should be very evident how to make this modification to the center carb base plate. Hope this post helps someone with their idle problem. If the pictures didn't come through then let me know and I'll email them to yopu. my email address is edhampshire@bellsouth.net.
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