More information is needed to give a better idea of the issue/causal factor(s). Taking your statement that "the engine doesn't mis-fire - the tach doesn't jump" then it cannot be an issue with the torque convertor locking/unlocking (assuming it is an automatic; we need to know manual or auto also) as any misfire/lean burn issue or convertor lock/unlock would be noted by a change in engine RPM. It's hard to diagnose also as a better description of what you mean by a "bump bump" and then another "bump" is in order. Here is what you need to do to enable a better understanding of the issue and what may be at fault: (1). does the "bump bump" occur when on a straight and level road surface driving at a constant engine load torque/speed (i.e. steady state drive with minimal engine torque to maintain a constant speed) on a smooth road or (2). does the "bump bump" occur ONLY when the vehicle hhits a tar strip or other transient road condition? (3). is the "bump bump" associated with a condition like the tires hitting a tar strip on the road - i.e. there is a tactile vibration as well as audible noise occurring with the issue? (3) does the "bump bump" occur when you to thru a torque reversal - that is, when you just back off the throttle slightly (when travelling at a constant speed on a level surface) and then tip back into the throttle? This condition is most likely to occur on a manual when in top gear (5 assuming a 5 speed; what is the make/model of the vehicle also). (4) what road speed and under what road conditions does this issue occur in? Can you get this issue at the same engine RPM regardless of the gear it is in or does it occur at a given road speed regardless of the engine RPM? If you can better describe the issue with regard to when/where it occurs, what make/model and powertrain (engine and transmission) it has, many more and accurate diagnostics can be made. Look forward to your reply.
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Today's Featured Article - The Cletrac General GG and the BF Avery A - A Bit of History - by Mike Ballash. This article is a summary of what I have gathered up from various sources on the Gletrac General GG and the B. F. Avery model A tractors. I am quite sure that most of it is accurate. The General GG was made by the Cleveland Tractor Company (Cletrac) of Cleveland, Ohio. Originally the company was called the Cleveland Motor Plow Company which began in 1912, then the Cleveland Tractor Company (1917) and finally Cletrac.
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