Yay! I've got a 4 cylinder Olds 307!!!

[deathwishracin1]

As the title states, it's a v8 Olds, but only runs on 4. During the last race (August enduro) the car suddenly lost power, and since then, barely runs. Compression is fairly even on 7 cylinders, the 8th is dead (exhaust valve spring has no tension). I changed the cap, rotor, plugs, coil, wires, ignition module, fuel filter, timing chain (old one was very stretched). The car started out computer controlled, so thinking that was the problem, I changed the carb and rebuilt the distributor using a non-computer controlled one I had laying around as parts (wrong housing and didn't have the money to buy one). All parts installed were working when removed from previous vehicles. This has me completely stumped :confused: any ideas??

[Hollywood]

Oh boy, which cylinders are firing?

[deathwishracin1]

1, 4, 6, and 7. 3 has no compression. I neglected to say the fuel filter and carb were changed before the last race. The fuel filter looks fine and the carb appears to be pumping gas from both primaries, but I haven't checked the secondaries...

[art11758]

First check your firing order. (Chiltons at the Library should have the correct order) If that's all good, check for spark at each plug. You already checked compression. (Chiltons can give you target numbers there, too) Then with the valve covers off and the plugs out, 'bar' the engine over a quarter of a crankshaft rotation at a time. It takes two full rotations of the crank to go through each cylinder's four cycles. Follow the firing order and try to note each cylinders valve action particularly the one with no compression. Even with a lame spring, you should have valve action that is fairly even across the board. If you don't, possible troubles are: broken valve, collapsed pushrod or lifter, or a lobe went away off of the cam. Let me know how you do.

[richardpetey]

Usually, most GM motors are even #cyls. on one bank and odd #cyls. on the opposite bank................... :rolleyes:
If you look at the cyls. that you mentioned are firing they are the two inside cyls. on one side and the two outside cyls. on the opposite side of the manifold............ :help:
Because the manifold is of a dual plane design, one side of the carb. feeds the two inside cyls. on one side and the two outside cyls. on the opposite side and visa-versa, so I would look for a carb. flow problem, vacuum leak or even an EGR valve or orifice leak common to that side of the manifold/carb............................. :confused:
The chances that 4 lobes wiped out on the camshaft without any loss of oil pressure are kinda nil, also................ :p
This is all based on the assumption that you have a baseline compression of 150lbs. and all cyls. are within 20% of each other, except #3 cyl. of course ............ :D
UNCLE PETEY.......................... :wave: :wave: :wave: :wave: :wave:

[woot lawrence]

Where did #3's compression go? Try putting compressed air into the cylinder to see where the air leaks out. If you have a bent or burnt intake valve, that could be the vacuum leak that Uncle Petey is talking about on that side of the manifold/carb.