Hey all,

Just bought a 68 elco.  #s on block per Mortech reveal 307, and it's got an elect ingnition, eddy carb (600 I think), beefy cam (specs unknown), non-camel hump heads (valve size unknown), headers w/2.5 exh, and feels like highway gears and it's a 350 TH auto.  thermostat is 180 and it runs no hotter than 190.  timing advance is unknown.

when I first got it there was no problem.  Now, the problem is that after it starts cold and runs fine for a little while, it then runs very rough after warm up to the point where it's hard to even keep it idling, although on the gas it will run, but sputters.  It has autolite A26 plugs, which are new.  i thought it was poor idle mixture adjustment, but an eddy site indicates that even if that were the case, there should be no problems off idle and certainly none at 1/2 or more throttle.  I have adjusted the 2 mixture screws, but they are back to baseline.  the crazy thing is how well it runs prior to the problem starting.

any thoughts on the potential culprit(s) before I go swapping the cheapest parts first?  Thanks a lot!

Brian  

blown head gasket? check the radiator for oil and oil for water 307s a notorious for burning lobes off cams they are 1.94 heads the standard bore is not big enuff for 2.02 valves  

thanks for the tip.  water and oil are both un-infiltrated.  

Brian  

Doesn't sound like you have a carb problem, though I'd check the idle circuit and make sure it's working properly. From there, I'd take a good look at your fuel system.

1. Is the fuel pump vapor locking when the engine gets up to temp?

2. Are any of the fuel lines near any heat sources that can cause vapor locking.

3. Are you getting trash in the gas? Have you checked the fuel filter to see how it looks?

4. Maybe drop the gas tank down and check to see if there's any garbage in the tank.

If that's not the problem, start checking out the ignition system. You said the motor has an electronic ignition, is it a stock HEI or an aftermakret HEI? Is it an aftermarket points conversion system from either Crane or Pertronix? How're the plugs and plug wires?


The 307's bore is 3.875 (same as the 283) and a stroke of 3.250 (same as 327), so it's not quite the small bore like the 305, a 307 will take 1.94 intake valves. Super Chevy did a 307 build-up back in the April '06 issue. Here's a link to the story online:

http://www.www.superchevy.com/technical/engines_drivetrain/cams_heads_valvetrain/sucp_0604_chevy_engine_performance  

thanks for the help, you may be onto something with the vapor lock, since the problem happens after warmup.  I will check these thing out.  the plugs are new and they don't seem to be fouling, but no idea on the wires, filter.  The gas tank actually has a minor leak where the tube meets the tank, so I need to drop that anyway.

i'll let you know about the filter and I may just changes the wires regardless

thanks again,
Brian  

You have fixed it by nowprobably,  But your choke is stuck closed.