The Stovebolt Site Forums The Shop Area Engine Shop Timing Question 250 Chevy

I have most of the bugs worked out and major items like brakes ready to go on my 66 Chevy truck, but want to make sure I have all right with the engine before the maiden voyage.

The engine is a rebuilt 250 with a 1 barrel Rochester carb. I have tried setting the timing by the book with not much success. I'm assuming the marks on the tab are BTDC on the side farthest from the pavement and those closer to the ground are after.

There is no way I can get the timing to any less than 8 degrees before TDC without killing the motor. The truck seems to start best with the timing even more before TDc than that. When I get the thing where it starts really well, I get backfiring thru the carb when I hold the throttle open, but it runs ok when idling.

If I get the timing where I have the most vacuum...(around 18)the thing is really hard to start, but runs smoothe.

I have a HEI installed. Will that make some difference in the timing?

I have to wonder about the dist. being 180 out, but I didn't think it would run if that was the case.

I may be wrong, but I am under the impression that the TDC mark is the one closest to the driver's side of the engine. Could that be causing your problem?

Drew,

That would be how I have it interpreted. My first explanation wasn't very good! I got the timing to the range using a light, but it just seems the motor is under a little load. I don't have much vacuum with it set at this mark either.

It does start and seems to idle and rev good though.

SixPac,

I don't know what the problem is, but being a HEI should no difference in the timing, and if you were 180&deg out it wouldn't run!

Does a 250 with a HEI have a vacuum advance can? Are you disconnecting this and plugging the carburetor port when you time it?

Stuart

Yes, has vacuum advance and plugged it. I went thru whole procedure again a little while ago...set idle, pulled vacuum line and plugged, adjusted idle mixture, set timing, reconnected vacuum..starts great, idles ok, backfires thru carb on steady throttle. Do you think I could have something wrong electronically? I also double checked the plug wires and firing order.

I am not familiar with the 250, but could the harmonic balancer rubber have delaminated and the timing mark be inaccurate? One way to check the accuracy of the TDC mark is to install a stop in number one spark plug hole and rotate the engine til it hits the stop, mark the spot on the balancer, rotate the engine in the opposite direction until it hits the stop and mark the balancer at that spot. Halfway between the two marks on the balancer it TDC.

Baldy,

I have heard of that happening, but the balancer is new.

SixPac,

153624....right! The backfire off idle on a steady throttle sounds like a valve issue to me.

Stuart

Looks like I might be in for a lesson on adjusting valves. Never have done that, but suppose I could.

when you get the most vacuum, but its hard to start, just back off the timing a little, not much..that should be really close......

I,m with Darin. You are just going to have to experament to get it where it works best. When it surges & runs rough at part throttle it,s usually too much advance. Try it with the vacuum advance unhooked. You may have to taylor the advance systems to suit your engine. There are some kits available for that.