Wilsons Sandbox

A gold star to the person that solves this problem. My TA, with an XPEG engine has the following problem. The engine exhibits, what seems like a misfiring at about 3,500 RPM. This occurs whether the engine is under load, ie, pulling a hill, or if the engine reaches 3,500 RPM going downhill, under no load. The problem does not occur below that engine speed and only when the engine is above about 65C. The engine runs normally at 75-80 C The problem does not occur when car is not in motion. The engine is configured as: overbore 0.060", Crane 3/4 grind cam, freeflow exhaust, standard 1500 head ( no milling ). The following things have been done to determine the problem: Replaced coil (twice), points/condenser (twice), distributor cap and rotor, replaced entire distributor with NOS unit (including, cap and rotor), replaced all high tension wiring, including all connections, new N5C spark plugs, engine runs at 4-5 % CO thru the entire range, floats adjusted, up and down, replaced both carbs with rebuilt units, replaced head with reconditioned head ( same specifications ). Valve clearance 0.016". Timing set at 5-6 degrees static ( we are above 6000 elevation ) 25 degrees advance at 3,100 RPM ( these settings have been standard for this engine since installation, approx 25,000 ago ) The engine lower end has about 25,000 miles since rebuild and this problem has been occurring for approximately the last 1,500 miles. Sometimes, if the engine speed is held at the problem point ( approx 3,500 RPM ) it will clear and the RPM will increase normally to 5,000. ( not unlike a plug fouling ). So, any thoughts? Thanks and cheers....Bill Bollendonk, Littleton, Colorado

Bill What color are the plugs when they come out. Anything unique about the tips after they have been in a while? Would a broken valve spring do this? Joe Potter Eugene, Oregon USA [Non-text portions of this message have been removed]

Bill: It might be useful to confirm that the fuel system is, in fact, delivering enough fuel consistently. If the pump was marginal or intermittent or the filter was clogged, as rpm and quantity needed to keep up with engine demand increased, the float levels would not be maintained and misfiring from temporarily leaning out might be the problem. For example, upon accelerating from rest, the pump would need to replace the fuel that had just been used in the float bowls. If fuel delivery was weak and slow in doing so, stumbling could occur until it caught up, at which time, the engine would begin running normally again until the next significant acceleration occurred. Even a partially clogged fuel filter could slow the fuel delivery to the float bowls enough even if the pump was operating normally. Bob Johnson [Non-text portions of this message have been removed]