Another TBI running rich

evilunclegrimace said:

And, all things being equal you get a slight bump in compression ratio when you increase the bore size

Click to expand...

The real-life issue is that it's unlikely that all things are going to be equal.

Replacement pistons are often sabotaged by reducing the compression height; AND the replacement head gaskets are generally thicker than the originals.

Unfortunately, "destroked" pistons are frightfully common in the aftermarket. You're lucky if you don't end up with LESS compression ratio; and what dicking with the compression height does to quench/squish should be a crime.

Now, if the oversized pistons have correct compression height, AND we're not talking about a Heron-head engine (a SBC is not Heron-headed; although the Chevy "W" block was) AND the head gaskets aren't thick as a feather pillow, then you're absolutely right--an overbored engine would have slightly higher compression ratio.

It's all dependent on the replacement parts selected.

Schurkey said:

The real-life issue is that it's unlikely that all things are going to be equal.

Replacement pistons are often sabotaged by reducing the compression height; AND the replacement head gaskets are generally thicker than the originals.

Unfortunately, "destroked" pistons are frightfully common in the aftermarket. You're lucky if you don't end up with LESS compression ratio; and what dicking with the compression height does to quench/squish should be a crime.

Now, if the oversized pistons have correct compression height, AND we're not talking about a Heron-head engine (a SBC is not Heron-headed; although the Chevy "W" block was) AND the head gaskets aren't thick as a feather pillow, then you're absolutely right--an overbored engine would have slightly higher compression ratio.

It's all dependent on the replacement parts selected.

Click to expand...

Well, I guarantee you this is cookie cutter parts, nothing great at all.

If I could get in an edit the fuel tables I bet that would solve it but I've never been involved in that so I am pretty much clueless.

No amount of tuning can totally overcome a mechanical issue, it may mask it but it'll still have the same problem.

It all started when I put the new engine in. I've been doing this for 40 years so I am aware of paying attention to details on swaps.

I'm sorry you have this issue and hope you fix it.

Ok...
Been watching this thread in hopes a solution would be found..
My 94 k1500 5.7 ecsb with 294k orig miles started doing the same thing.
Cold start starts instantly and cold high idles perfectly for about 15-30 seconds, then goes rich and rough idle for about 1 minute, then clears up and runs fine.

Only does it on first cold start of the day and never when warm.

Just passed emmisions with great numbers.

If I disconnect the brown timing wire, it does not do the rich stumble at all.

Recent services have been:
TB rebuild, (GM kit)
New distributor
New ICM, (Delco)
New cap, rotor, wires and plugs, (All Delco)
New TPS, (Delphi)
New IAC, (Delphi)
New MAP and Solenoid, (Delco)
New EGR, (GM)
New O2 Sensor, (Bosch)

I'm thinking the Bosch O2 might be my issue as it was the last thing I replaced.

I don't recall the truck had this cold start issue before the new O2... and I've heard in some applications the 1 wire Bosch units don't light off fast enough.

My buddy scanned it and said the O2 readings were correct during the problem times but my gut says try a different O2 sensor...

I forget now which is which but there is an o2 sensor you don't want to use. Maybe it's the Bosch. Either way, I changed mine and it didn't matter.

If I remember right, unplugging this timing light eliminates my problem too.

I don't know how it could be an O2 sensor if the engine isn't in closed-loop. What does the scan tool show?

Schurkey said:

I don't know how it could be an O2 sensor if the engine isn't in closed-loop. What does the scan tool show?

Click to expand...

https://docs.google.com/spreadsheets/d/1-ZuNLTi5wAIa6wHoWYgAKCl0L-tR7CCnhOxiydSjjJw/edit?usp=sharing

I don't Google. They're evil. Just tell me if the problem occurs during closed-loop, or open-loop operation