lean mixture damage

[offroadrolls]

SBC 400 with an aluminum intake and Edelbrock 750 4 barrel. The engine's got about 14,000 since rebuild. I've known for a while something was not right at about 4,000 rpm and above. To me it felt like the A/F mixture was too lean. Recently I had some time to check it out. I have a new in the box Edelbrock 850, so I decided to bolt in on and take it for a spin.......Man did it run nice. So I popped the metering rods and main jets out of the 750 and took a look see. Come to find out the guy that owned the car and did the engine before I bought it had gone 18% leaner on the primaries and had gone about 8 steps leaner on the secondaries which totally explained why it ran like crap above 4,000 WOT. So I re-jetted the primaries to 4% leaner and went 1 step richer on the secondaries to make up for it @ WOT. The car runs much better..... not perfect but a lot better.
So here's my question. What if any damage will a super lean mixture do to the engine. This thing is using 1 quart of oil per 500 miles and will oil foul #3 plug every 2000 miles. #1 plug also shows signs of oil consumption but not as bad as #3. All the other plugs are fine and the V seals and guides are all new. So for sure it's rings. Did the lean mixture cause it?

 

[Sam_Fear]

I can't answer your question. But...

So who actually built the engine? Are you sure it was a good build? Has it always been burning oil? Did the guy have the block machined or just yahoo'd it and threw new rings in?

 

[von-zipper]

I'd run a compression test on it first! That ought to give you a better idea of what might be going on. If your plugs were fouled, that never helps! Run them for a little while then pull and check again.

Zipper

 

[jmlcolorado]

I'm not an expert in this field, but I don't think oil consumption or an oil foiled plug would be caused by a lean mixture.
Usually a lean mixture (not overly lean) will produce more power, since it burns faster and hotter, all at risk of burning a hole through pistons.

A freshly rebuild engine, however, should not be fouling plugs and eating oil.
If you pull the plug, what does the top of the piston look like? Leaky rings?
Compression test might show #3 cylinder not up to par, which would point to rings.

 

[offroadrolls]

I can't answer your question. But...

So who actually built the engine? Are you sure it was a good build? Has it always been burning oil? Did the guy have the block machined or just yahoo'd it and threw new rings in?

I'm not sure "who" built the motor but, I have no reason to believe the build was on the cheap. The guy who built the car that the motor is in is an interesting character. Real nice guy, grew up in Bakersfield CA in the 50's. The car is my 72 xj6, he painted it had the interior done and did the sbc conversion all about 15 years ago. Then it sat until he sold it to me about 6 years ago. After many years of hot rodding, it was the last car he ever built, only because he had always wanted to do a sbc jag. When I first got the car it had about 6000 miles on the build and used no oil. It started using it after I drove it to LA and back the first summer I had it. About 2000 miles round trip going 80 the whole way.

 

[Studebaker]

Rings

Id for shure say it was ur rings, an oil burner means u got blow by-that is both ways by the piston. Pushing oil up past the "bad" rings that are not doing there job

 

[Sam_Fear]

So for sure it's rings. Did the lean mixture cause it?

I think he already knows it's the rings. So could a long running lean out cause ring problems?

 

[bob w]

Did it ever overheat? Experience preignition or detonation? Do the heads have steam holes?

 

[offroadrolls]

"Usually a lean mixture (not overly lean) will produce more power, since it burns faster and hotter, all at risk of burning a hole through pistons."

Ya that's what I thought I remembered and perhaps that extra heat caused a ring to break?

Doesn't really matter. I was going to list the motor and tranny on CL as a hear it run and sell it. I have a roller 400 sbc that's been stroked and injected for the car and am currently stock piling parts for a 5 spd clutch conversion too.

Was just curious if anyone knew how dangerous that super lean mixture was.

 

[offroadrolls]

Did it ever overheat? Experience preignition or detonation? Do the heads have steam holes?

It's run hot, 220 but never over heated or boiled over. No to the preignition / detonation.
Yes it has the steam holes. Ole Bob even called them the Smokey Yunick steam holes.

 

[Dr Crankenstein]

It's possible the hotter combustion temps have damaged the piston and its ring lands, especially the top one. If that's the case, it may have broken rings.

 

[sneakysnake]

The heat can stick the rings,burn a hole in pistons and eat up exaust valves.When I was younger I put a carb from a 302 on a 351 to get better gas milage, It burned all the exaust valves.

 

[offroadrolls]

It's possible the hotter combustion temps have damaged the piston and its ring lands, especially the top one. If that's the case, it may have broken rings.

Thanks Doc, it's a real shame too. This thing dynoed 260 @ the rear wheels and with the 700r4 / 3.73:1 granted it's not a monster but when the plugs are clean it runs great, a lot of torque.