'65 300HP miss and bucking at steady speed - NCRS Discussion Boards

'65 300HP miss and bucking at steady speed

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  • Dave B.
    Expired
    • May 31, 2006
    • 52

    '65 300HP miss and bucking at steady speed

    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
  • Timothy B.
    Extremely Frequent Poster
    • April 30, 1983
    • 5177

    #2
    Re: '65 300HP miss and bucking at steady speed

    Dave,

    The vacuum port at the front base of the 3721 AFB should read full manifold vacuum, on a stock 300hp engine there should be approx 18" vacuum at that port. Check for leak in the rubber hose and if the line is not leaking vacuum, test the vacuum advance control for a leaks. Report idle vacuum taken from a tee in that line with the vacuum advance connected and engine warm at curb idle. That port will provide the full manifold reading so no need to fool with the plug in the intake runner.

    Could you have damaged a spark plug wire moving the distributor cap, I assume the valve adjustment was not touched.

    Comment

    • Edward J.
      Extremely Frequent Poster
      • September 15, 2008
      • 6940

      #3
      Re: '65 300HP miss and bucking at steady speed

      Dave, Do you believe the car is misfiring on a cylinder(spark plug)? If so I would be looking for a spark plug wire that maybe grounding out. or maybe one of the terminal ends not all the way plugged in the cap? another thing did you spray any cleaners around the ports on cylinder head. If so sometimes this could foul a plug out if to much spary cleaner was used (I.E. carb. or brake cleaner). just food for thought Ed
      New England chapter member, 63 Convert. 327/340- Chapter/Regional/national Top Flight, 72 coupe- chapter and regional Top Flight.

      Comment

      • Joe M.
        Very Frequent User
        • February 1, 2005
        • 589

        #4
        Re: '65 300HP miss and bucking at steady speed

        Retorque the intake and the carb hold down nuts.

        Comment

        • Duke W.
          Beyond Control Poster
          • January 1, 1993
          • 15610

          #5
          Re: '65 300HP miss and bucking at steady speed

          Teeing in a vacuum gage to the vacuum advance signal line yields manifold vacuum, and as stated, it should be about 18" Hg. assuming the engine has the original cam or something very close. Your reading of 11" at 600 is strong evidence of a vacuum leak. The '65 VAC should start at 6" and provide a maximum of 22 deg. crankshaft advance at 12". This is a one year only VAC for the 300 HP engine, and I'm not fond of the specs.

          What is the ID number on the installed VAC?

          You should be able to pump it down to 20" and the vacuum should hold. Does it?

          If the VAC is original to the car a leak due to a broken diaphragm is not unknown after nearly 50 years. If the VAC is malfunctioning replace it with a B22 (assuming you have a manual trans), which is 0 @ 8", 16 @ 15". The max centrifugal is 26 @ 4100, so I recommend you increase the initial timing to 12 as long as the engine will tolerate this amount without detonation.

          If you removed the carb from the manifold, did you get it back on with the correct heat shield, insulator, and gasket stack?

          Duke

          Comment

          • John H.
            Beyond Control Poster
            • December 1, 1997
            • 16513

            #6
            Re: '65 300HP miss and bucking at steady speed

            Dave -

            If you have Lectric Limited dated reproduction plug wires and you had the distributor cap tied up out of the way for any length of time, I'd bet on several wires that have gone south; those LL wires do NOT like being disturbed to that extent (mine sure didn't).

            Comment

            • Dave B.
              Expired
              • May 31, 2006
              • 52

              #7
              Re: '65 300HP miss and bucking at steady speed

              John,

              Thanks for the info on the wires. Those are exactly the wires I have. I worked on this project intermittently over about two weeks, so they were tied back for that length of time. What symptoms did you have when you had the problem? Where you able to confirm a bad wire just by measuring resistance or were they just failing under load? I still have the old set of wires - no known problems - just not dated. So this looks like another path I need to pursue, although I still have the low vacuum reading issue.

              Thanks,

              Dave Brown

              Comment

              • George J.
                Very Frequent User
                • March 1, 1999
                • 774

                #8
                Re: '65 300HP miss and bucking at steady speed

                I had the same problems you have when I had the dated wires. They either fail outright, or come loose from the plugs. I would try a different set of wires and see if that solves the problem before going any further.

                George

                Comment

                • John H.
                  Beyond Control Poster
                  • December 1, 1997
                  • 16513

                  #9
                  Re: '65 300HP miss and bucking at steady speed

                  Originally posted by Dave Brown (45938)
                  John,

                  Thanks for the info on the wires. Those are exactly the wires I have. I worked on this project intermittently over about two weeks, so they were tied back for that length of time. What symptoms did you have when you had the problem? Where you able to confirm a bad wire just by measuring resistance or were they just failing under load? I still have the old set of wires - no known problems - just not dated. So this looks like another path I need to pursue, although I still have the low vacuum reading issue.

                  Thanks,

                  Dave Brown
                  Dave -

                  When I buttoned it back up and started it, I had a 4-cylinder V-8 engine. If you want some GOOD wires, talk to Ken Anderson (www.KandBCorvetteProducts.com); you don't want to do that job more than once.

                  Comment

                  • Stuart F.
                    Expired
                    • August 31, 1996
                    • 4676

                    #10
                    Re: '65 300HP miss and bucking at steady speed

                    My LL wires have been on and functioning perfectly since about 1998 (routinely to red line).

                    Could you have possibly put your distributor back in one tooth off? Did it time up at the normal physical position relative of say the VAC in relation to the intake manifold? You might want to do another TDC set up on #1 if you're not sure. If you need to move it, you can walk the oil pump shaft around by just pulling the distributor up enough to go (turn CW) to the next tooth one at a time and the pump shaft will tag along until you get it back to the right position. Takes a little finesse, but us old farts do it all the time.

                    AFB's aren't exceptionally fussy, but you may have dislodged some dirt into one of the air bleeds of a primary cluster, but I doubt it and I've never seen it happen.

                    I'd bet on a super vacuum leak being as how you mentioned the excess of gasket sealer on removal of the manifold (? mismatched ports due to milled heads? or badly warped manifold?).

                    Stu Fox

                    Comment

                    • Dave B.
                      Expired
                      • May 31, 2006
                      • 52

                      #11
                      Re: '65 300HP miss and bucking at steady speed

                      Stu,

                      Thanks for the comments. I was confident I had the distributor back in the original position. I took photos of the distributor and vac advance can relative to the manifold prior to removal. I had the rotor position marked relative to the base along with the photos. It dropped back into the exact position. When I started the engine the initial timing with vac advance plugged was right where it had been.

                      My immediate concern was the same as yours regarding a manifold leak and specifically an internal leak. I thought the check I described regarding vacuum readings on the crankcase eliminated the chance of an internal leak but maybe I'm mistaken. It seems to me that with the crankcase ventilation sealed off, any internal vacuum leak should result in a vacuum reading at the dipstick tube. Could you comment on the test I did?

                      Thanks,

                      Dave

                      Comment

                      • Timothy B.
                        Extremely Frequent Poster
                        • April 30, 1983
                        • 5177

                        #12
                        Re: '65 300HP miss and bucking at steady speed

                        Dave,

                        Your test tells me there is no internal leak at the manifold/head interface. The 3" vacuum is the pull of the pcv restriction at the carburetor and the second test blocking that restriction would result in seeing a slight pressure on the vacuum gauge from blowby into the crankcase provided the pressure does not leak out gaskets.

                        Report back with the vacuum reading at the vacuum advance port in the front of the carburetor, you almost have to start there.

                        Comment

                        • Wayne W.
                          Extremely Frequent Poster
                          • April 30, 1982
                          • 3605

                          #13
                          Re: '65 300HP miss and bucking at steady speed

                          I know you don't think its a carb problem, but the most common problem I ever had with an AFB back in the day was the power valve pistons. They would stick in the bore and cause exactly the same symptom, surge or buck at steady speed. Pull the caps on the valves, remove the pistons, and clean with very fine sand paper. Re-install and the thing will run smooth as silk.

                          Comment

                          • Stuart F.
                            Expired
                            • August 31, 1996
                            • 4676

                            #14
                            Re: '65 300HP miss and bucking at steady speed

                            Wayne;

                            Can't say I have ever seen the pistons stick enough to cause the off idle hesitation he describes, he'd be mostly on accelerator pump and low speed circuit. The L-75 with exhaust heat in the phenolic block below the SS plate though would concern me. AFB's are very sensitive to heat and/or vacuum/exhaust leaks at their base. I'd want to be very certain of a good and correct base seal, maybe even to the point of blocking off the exhaust heat passages in the manifold. I'd also want to make certain the choke hot air tube is pulling just air and not exhaust due to a leaking heat tube in the exhaust manifold.

                            Stu Fox

                            Comment

                            • Dave B.
                              Expired
                              • May 31, 2006
                              • 52

                              #15
                              Re: '65 300HP miss and bucking at steady speed

                              Thanks for all the feedback from all of you. I continue to be frustrated with this issue. I'd like to summarize everything I did as a result of the suggestions on this thread:

                              1. The vacuum advance can holds perfectly pumped down to 20". This was measured by disconnecting the line at the carb and pulling vacuum thru the metal and rubber tubing to the can. The can is a replacement unit from LIC that I installed a couple years ago. The only number I see on it is#239 on the support bar. This may not be the ideal unit for the car but it did not exhibit an issue prior this project.

                              2. The date coded plug wires were replaced with my old set. The old set had been functioning fine - I only replaced them since I wanted the date coded wires. The problem was still the same with the old set of wires. The resistance on the date coded wires is as expected and as follows: #1-17.7K, #2-16.9K, #3-17.3K, #4-19.8K, #5-15.0K, #6-14.6K, #7-13.4K, #8-11.9K, Cap to Coil-6.6K.

                              3. One thing I noticed after changing wires was an increase in vacuum reading from 11" to 12". This may be normal day to day variation - I don't know. I also tried another vacuum gauge and the reading went to 13". I decided to use this vacuum gauge from this point on for reference. It's actually the gauge on the vacuum pump. All vacuum readings were taken from a T on the vacuum advance line at 600RPM.

                              4. At this point I decided to pursue the seal between the carb and manifold even though I could not detect a leak with propane of carb spray. When I removed the carb, stainless plate, insulator and gasket, I noticed a residue of droplets between the insulator and stainless plate. I guess it was gasoline - don't know what else it would have been. I noticed that I had installed the insulator upside down. This had the effect of partially closing off the opening for the exhaust passage since the opening is tapered to align with the exhaust openings on each side. The only escape for the exhaust would have been thru the opening equal to the thickness of the bottom gasket. I corrected this on reinstall but didn't really think is was an issue since other threads on this Board have recommended closing these openings entirely. I chose not to take anything apart on the carb at this point since I didn't want to change multiple things at once. I also cleaned up the stainless plate better than it had been. It now looks like a mirror! Reassembly with a new bottom gasket yielded no change in the problem. I did notice the idle vacuum was now 14". Maybe there is a problem with this seal that I didn't eliminate but affected somewhat with the gasket change?

                              5. I then switched my attention to the timing even though I had the distributor back in it's original location and idle timing was good at 8 degrees BTDC with vacuum advance line plugged. I read so any threads on setting engine timing, I thought it was worth exploring. After reading this stuff I got to wondering if maybe my timing mark had slipped on the damper. I verified I had the timing mark on the #1 compression stroke. I didn't really want to break the seal I have on the valve cover to check valve position so I inserted a fiber optic scope I have into the plug opening to observe the piston movement. I convinced myself the piston was moving thru TDC when the timing mark was on 0 degrees so I don't think timing mark was off. I always thought the positioning of the distributor should be a little better; i.e., the vacuum can should be better centered. I pulled the distributor again and observed that the dimple was not aligned to the rotor. This must have been changed by a previous owner and I'm thinking the engine may have a replacement cam - I don't know much about the history of this car except a previous owner did a lot of work on it around 1980. I flipped the gear 180 degrees and reinstalled the distributor. The rotor aligned pretty well with #1 wire position, timing mark set at 10 degrees, and the distributor rotated to the point where the points just open. So it's not clear to me why the distributor had been flipped at some point. For some reason on startup, the idle timing was higher than I expected at about 15 degrees so I reset it to 12 degrees - a little higher than the previous 8 degrees. At this setting the vacuum advance can is perfectly centered. The vacuum is hanging around 14 to 15" at this point. All this was a good educational exercise in tinkering with the distributor but (as you probably assumed) it had no impact on the PROBLEM! Actually, the idle even seemed a little more uneven today.

                              At this time, I think I will remove the carb, take it apart, and check for any debris or maybe some problem I created when taking it off initially. If that has no impact, I'm really starting to think the worst. I'm wondering if I have some internal damage to the manifold. Suppose I had an internal crack between the intake passage and the exhaust passage? This would have not been revealed with the check I did for an internal vacuum leak. I'm about ready to pull the manifold again and have a shop verify it's integrity if that's possible. The other thing I thought about doing is pulling all the plugs and checking color, appearance, etc. The #1 plug I had out was a perfect tan color. I could also do a compression check although I don't have any compression data prior to this project.

                              As always, any further help and suggestions are greatly appreciated.

                              Dave

                              Comment

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