I'm reaching out for some help on a problem I've apparently created for myself on my '65 300HP, 4SP, Carter AFB carb, no PS, PB, or AC. The background is that I had been bothered for some time by a slight oil leak at the ends of the intake manifold and an overall ratty appearance of the intake; however, the car ran fine. So this is probably one of those cases where if something isn't REALLY broke, don't try to FIX it!
I decided to remove the intake, clean it up, replace with proper gaskets, etc. At the same time I also changed coolant and replaced all hoses. Everything came apart and went back without a hitch. I did have a concern when I removed the intake because I noticed that whoever change the gaskets previously was extremely liberal with gasket sealer everywhere on the gaskets, particularly between the gaskets and the heads. I decide to assume this was done as a matter of course and not to compensate for some mating problem known to exist between the manifold and the heads; thus, I decided to install the new gaskets (original design with tab) using sealer only around the water passages and of course on the end seals. I did not remove any of the plug wires from the distributor - only tied the distributor cap up and out of the way. I had the distributor position marked and since the engine was not turned, I was able to replace the distributor back to the exact location removed. The only thing I did need to do was slightly turn the oil pump shaft. This concerned me since it wasn't clear why the shaft should have turned but it seems to me the few times I've a distributor out in the past I also had to adjust this shaft. I had the carburetor off but did not disassemble anything on the carb. I only sprayed the exterior with carb cleaner to clean things up. The carb manifold gasket was replaced but I used the original insulator and stainless top plate. Cosmetically, everything looked great to me after reassembly, the engine started up normally and I had no water or gasoline leaks. The dwell seemed a little off at 28 degrees so I reset to 30. I set the timing with vac advance plugged to 8 degrees BTDC and did a little adjusting to the idle mixture screws. Overall the idle didn't seem bad set at 600 RPM hot although at 500 RPM it didn't seem quite as smooth as it had been. It used to always run around 500 to 550 RPM. The BIG problem occurred when I took it for a run. I immediately noticed much more of a problem trying to engage the clutch smoothly from a start. I was getting some hesitation and roughness on initial acceleration and then at a steady speed in any gear (maybe about 1500RPM) I had a constant miss or bucking from the engine about once every second or two.
After getting over the initial shock and frustration, I checked a few things related to those items I had apart. My immediate thought was I had some sort of serious vacuum leak either external or internal at the intake or at the carb base. To check for an external leak, I ran some propane along the intake edge and the carb base with NO noticeable change in idle; however, my confidence in this test was a little low since I'm not used to doing this test and maybe I didn't have a strong enough gas flow. I was resisting hitting the same areas with carb cleaner since I hated treating my pristine manifold to any cleaner residue.
To check for an internal leak, I hooked a vacuum gage on the dip stick tube and sealed off the crankcase air flow intake tube going to the rear of the engine. At idle, I had a steady 3" of vacuum. I then sealed off the crankcase air output tube on the oil filler tube and measured NO vacuum on the gage. To me, this seems to indicate no internal leak. Am I right about this?
So at this point I'm stuck. If I don't have a vacuum leak at the areas noted, is it possible I screwed something up with the carb just taking it off and cleaning it up externally. Are these AFB carbs prone to something getting screwed up thru mishandling? I did have the carb upside down on my bench to check for a clean mounting surface prior to reinstalling.
I checked a few other things that I thought might come into play although I had not done anything to affect them. The centrifugal advance weights are completely free and not binding. Placing a vacuum pump on the vac advance can, I find that the advance begins at 8 in vacuum and is completely advanced at 10 in vacuum. The actual vacuum measurement at idle on the vac advance can is 11 in vacuum with about 1/2" flutter. The connection is at the front base of the carb. This vacuum reading seems low if it is suppose to be about the same as manifold vacuum but I don't know what it was previously. I did not get an actual intake manifold vacuum reading because I don't have a easy measurement point. The plug in the top of manifold looks like has been there forever. I was concerned about trying to break it free. Do you think an actual manifold vacuum reading would be useful?
I really appreciate any comments on the things I've checked so far and where to turn next.
Thanks,
Dave Brown
I decided to remove the intake, clean it up, replace with proper gaskets, etc. At the same time I also changed coolant and replaced all hoses. Everything came apart and went back without a hitch. I did have a concern when I removed the intake because I noticed that whoever change the gaskets previously was extremely liberal with gasket sealer everywhere on the gaskets, particularly between the gaskets and the heads. I decide to assume this was done as a matter of course and not to compensate for some mating problem known to exist between the manifold and the heads; thus, I decided to install the new gaskets (original design with tab) using sealer only around the water passages and of course on the end seals. I did not remove any of the plug wires from the distributor - only tied the distributor cap up and out of the way. I had the distributor position marked and since the engine was not turned, I was able to replace the distributor back to the exact location removed. The only thing I did need to do was slightly turn the oil pump shaft. This concerned me since it wasn't clear why the shaft should have turned but it seems to me the few times I've a distributor out in the past I also had to adjust this shaft. I had the carburetor off but did not disassemble anything on the carb. I only sprayed the exterior with carb cleaner to clean things up. The carb manifold gasket was replaced but I used the original insulator and stainless top plate. Cosmetically, everything looked great to me after reassembly, the engine started up normally and I had no water or gasoline leaks. The dwell seemed a little off at 28 degrees so I reset to 30. I set the timing with vac advance plugged to 8 degrees BTDC and did a little adjusting to the idle mixture screws. Overall the idle didn't seem bad set at 600 RPM hot although at 500 RPM it didn't seem quite as smooth as it had been. It used to always run around 500 to 550 RPM. The BIG problem occurred when I took it for a run. I immediately noticed much more of a problem trying to engage the clutch smoothly from a start. I was getting some hesitation and roughness on initial acceleration and then at a steady speed in any gear (maybe about 1500RPM) I had a constant miss or bucking from the engine about once every second or two.
After getting over the initial shock and frustration, I checked a few things related to those items I had apart. My immediate thought was I had some sort of serious vacuum leak either external or internal at the intake or at the carb base. To check for an external leak, I ran some propane along the intake edge and the carb base with NO noticeable change in idle; however, my confidence in this test was a little low since I'm not used to doing this test and maybe I didn't have a strong enough gas flow. I was resisting hitting the same areas with carb cleaner since I hated treating my pristine manifold to any cleaner residue.
To check for an internal leak, I hooked a vacuum gage on the dip stick tube and sealed off the crankcase air flow intake tube going to the rear of the engine. At idle, I had a steady 3" of vacuum. I then sealed off the crankcase air output tube on the oil filler tube and measured NO vacuum on the gage. To me, this seems to indicate no internal leak. Am I right about this?
So at this point I'm stuck. If I don't have a vacuum leak at the areas noted, is it possible I screwed something up with the carb just taking it off and cleaning it up externally. Are these AFB carbs prone to something getting screwed up thru mishandling? I did have the carb upside down on my bench to check for a clean mounting surface prior to reinstalling.
I checked a few other things that I thought might come into play although I had not done anything to affect them. The centrifugal advance weights are completely free and not binding. Placing a vacuum pump on the vac advance can, I find that the advance begins at 8 in vacuum and is completely advanced at 10 in vacuum. The actual vacuum measurement at idle on the vac advance can is 11 in vacuum with about 1/2" flutter. The connection is at the front base of the carb. This vacuum reading seems low if it is suppose to be about the same as manifold vacuum but I don't know what it was previously. I did not get an actual intake manifold vacuum reading because I don't have a easy measurement point. The plug in the top of manifold looks like has been there forever. I was concerned about trying to break it free. Do you think an actual manifold vacuum reading would be useful?
I really appreciate any comments on the things I've checked so far and where to turn next.
Thanks,
Dave Brown
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