The Stovebolt Site Forums The Shop Area Engine Shop In line 6, 250 Chevy

I put this rebuilt motor in & replaced my old head after having it serviced in 2024. Replaced the clutch too. It has 600 miles on it and still smokes when I first crank up or pat the gas.
It has sat up for the past 6 months while I worked on the sunken cab & foot / floorboard.
Cranked up & ran until warm today. I’d say the smoke was grayish & it initially smoked a lot.
So the rings should be seated by now. The head was serviced.
What could be wrong or what can I do? Bad head?
I know it has the wrong carburetor but that should not cause what I see??

Valve seal are likely dried up. Have you had it out on the road and run up pretty hard, it might help loosen rings if they are stuck or not seated. Give it a good 50 or 60 mile hard run, if it still smokes, pull the plugs and see if one or more are different from the rest. Also, check and see where the PCV valve is hooked up and if there is a baffle in the valve cover, it could be sucking oil, the line into the carburetor will be oily.

How many road miles do you have on it? Rings don't seat until they're loaded- - - -back in the dark ages Perfect Circle recommended 10 full throttle accelerations in high gear from 30 to 60 MPH, uphill if possible.
Jerry

X2

I've done the old PC recommended 10x full throttle accelerations from 30-60mph, and as a result none of my rebuilt engines ever used any oil. I would still do it today if I rebuilt another engine, because it's a good idea. Run engine around 2000RPM for the first half hour (after checking oil pressure, coolant etc) to break in the cam, then go on the road to do the rings. That's how I was taught.

The same break-in procedure can be done on a dyno with a series of full throttle pulls at different RPM settings. Everybody's shade tree shop has an engine dyno available like mine, right?
Jerry

600 miles.

I don’t think so Jerry. I’d like to see it!

I've got an elderly "Go-Power" water brake dyno that bolts onto a bell housing. It resembles a torque converter that's filled with water to put a load on the engine at whatever RPM I choose, and the throttle is advanced gradually until it's wide open at that speed. The torque the engine makes is recorded, and a fairly simple calculation is done to determine horsepower. Correction factors are included for barometric pressure, air temperature, altitude, and relative humidity. Then the RPM is raised by 500 RPM and the torque is measured again- - - -"rinse and repeat"! Then a HP and torque curve is plotted on graph paper. Changes in ignition timing, cam timing, and/or carburetor jetting will result in different HP and torque measurements. By the time an engine gets thoroughly tested, it's pretty much "broken in" (or just broken!) before it ever gets its first road mileage. Yes, I've blown up a few engines on the dyno!
Jerry

I have thought the motor runs rich mostly due to the odor & exhaust color. While working on the old bench seat I crank & warm up the motor every week.
It has been idling way too high until it gets very warm.
I’ve been told that I have the wrong carb anyway, that I need a 1 barrel. I’ll look into that next but I just realized that the carb has a tab for a wire that heats the choke coil, I never realized this before. There is no wire there so where would I run it from?
I was turning that part to open the choke butterfly valve and the motor cuts off soon as it open much. So it looks like I need to start from the beginning to adjust the carburetor & the choke.
Can I get some instructions on how to do this?

Yes, that's definitely the wrong carburetor. If you choose to continue to run it, adjust the choke cover to just barely close the plate with the engine stopped, cold and the throttle held halfway open, then wire 12 volts to the connector on the cover through an oil pressure sensing switch so the choke coil is only heated with the engine running. If the engine happens to stall with 12V applied to the choke, the valve will open prematurely.

Then make sure intake manifold vacuum is plumbed to the choke release canister, the round device visible at the lower left of your picture. That partially releases the choke plate once the engine starts, to prevent stalling from over-choking. Don't attempt to adjust idle speed or mixture until the engine is fully warmed up and the choke plate is completely vertical in the airstream.
Jerry

I have a couple of Monojet single barrel Rochesters off 1971 250's I'll make you a heck of deal on. Both are clean with kits, I will have to look and see what metering is in them as I was swapping stuff around awhile back. I should have the factory jet and rod, just not sure they are installed.

Joe

Thanks, would you like to send me a PM or whatever we do on this site?
Thank you.

PM would be fine, along with shipping address. Joe

I can only say that the head was fully serviced then stored for several months.

If the offenhauser intake is for a 2bbl carb, then that's the right carb for the intake. And it's also probably easier to find jets and etc. for it than for a Rochester B. You could also do the 2bbl version of the Sniper EFI.

Thanks, would you like to send me a PM or whatever we do on this site?
Thank you.thanks Jerry. So the 12 Volt heat source does stay on all the while the motor is running? Considering if it would be easier to connect to an ignition wire?
Why is the present carb not a good one to use? It is a 2 barrel.
Thank you!

I’d like to review my issues if I may. I had a difficult time replacing the headers ( they were bent in order to fit around the steering gear). So I put the new intake on in order to use a standard exhaust manifold. The intake is for a 4 barrel but the spacer makes it for a 2 barrel.
Adjusting the carburetor today (it was very rich). I turned the 2 adjuster screws in until the motor slowed down then enriched until it sped up & idled good. Should I connect a vacuum gauge to the carburetor to set it?

I connected the electric choke to the 12 volt from the the electronic ignition. It was running when I connected the electric choke. It wanted to stumble & choke down each time I connected it. I left it off.
As far as the bluish exhaust that seems to have changed. It’s cold & humid outside so I think I’m seeing that initial condensation coming out.
I’ll start over tomorrow and try to reset the choke & take another look at that exhaust.
I appreciate y’all’s indulgence!

Try connecting 12V from another source, even directly from a battery post, for a temporary test of the choke heater. You are probably loading down the ignition by grabbing voltage from there. If you have a multimeter, try checking the resistance of the choke heater from the flag terminal on the choke cover to the carburetor housing- - - - -it should read at least 10-20 ohms, so only a small amount of current draw is used to heat the choke coil. Yes, the voltage stays on the choke heater anytime the engine is running. Since you have an aftermarket 4 barrel intake, it's probably best to stay with the 2 barrel carb and adapter and try to work with it. I use a vacuum gauge connected directly to intake manifold vacuum when adjusting idle mixture- - - -turn each idle jet screw clockwise until you see a drop in vacuum, then back it out until the vacuum rises and stabilizes. Do the same thing for the other barrel mixture screw. You might have to do more than one adjustment sequence to find the "sweet spot". Once that's done, turn both idle screws about 1/8 turn rich (CCW) to assist in off-idle throttle response. The final adjustment is to the throttle stop screw to set the desired idle speed.
Jerry

Jerry:
Given the 4 barrel manifold probably has a common "chamber" where all fuel/air mixture passes is there a way to keep both barrels of his carb contributing to idle evenly and not having one rich and the other lean? Or am I overthinking the (non) issue?

Thank you Jerry!

I have the choke & carburetor functioning well now. The electric choke moves open pretty fast but it keeps running well.
Back to the bigger problem of burning oil. The exhaust smells & looks like oil if I rev up the motor or gun it. I won’t drive it again until I get the carpet & seat back in.
I certainly don’t feel like pulling the head off again but I may end up there😬
First I’m guessing I should pull the plugs just to see how they look. Then check the compressions?
If the compressions good & does not leak down it sounds like the head service was not done correct?
Anything else?

About 30 years ago, I was called in to mediate a dispute between a former student of my auto mechanics course and a machine shop that had built an engine for him- - - - -which became a chronic oil burner from the day it was installed. After doing a lot of "in the frame" diagnosis, they both agreed that the engine needed to be torn down. When I disassembled it, I discovered that one piston had been installed WITHOUT ITS OIL RINGS! Apparently the engine assembler was suffering from a bad case of Budweiser Fever when he put it together!
Jerry

Heaven forbid🙏 a compression test would not show anything wrong either!

Pull the plugs and lay them out side by side, you should see the oil burner(s) if it coming in on the intake side. If the exhaust guides are worn, you might not see the oil on the plugs.

I freshened up, a '85 Dodge 318, rings and a valve job over a weekend once. I used cast iron rings because of money and time. That thing used three quarts of oil in 20 miles, had to add oil just to get home from work. I drove it three days till new moly rings came in. After changing the rings, it still smoked for weeks till the exhaust was cleaned out! On the plus side, the exhaust lasted 200,000 miles.

It’s a strait 6. But I’ll pull them and look anyway.

Bad news, only 1 cylinder had adequate Compression! It was supposed to have been rebuilt by a machine shop in Va. you recall I’ve not been able to get the rings seated, been using oil 🤪🥸.
# 3 had 90 lbs, all others were 35 or more after revving up the motor.
I don’t suppose there are any other tests I can do?

Normally you don't do a compression test on a running motor.

Glenn, I feel your pain. Dealing with a stubborn engine issue, especially after pouring time and money into what you hope is a solid project, can be incredibly frustrating. I’ve been there, and I want to share my own experience in case it offers you some insight or comfort.

Back in 2019, I bought my ‘52 1/2 ton Chevy truck with a rebuilt 235 CU engine that came with all the paperwork but wasn’t running. At first, it seemed like a win. The engine was supposedly rebuilt by a reputable shop, so I assumed getting it running would just take some effort and a few parts. And you know what? Within a few weeks, that engine was purring. A quick test drive around the block assured me, I’d made a good deal.

But then… reality hit. By 2020, after spending a year fixing the mechanical issues that kept me from safely driving the truck, I discovered the real problem: oil pressure that was fine when cold but plummeted to near-zero when the engine warmed up. For almost a year, I refused to believe the machine shop might’ve messed up. I tried everything.

First, I discovered with allot of help from this forum, that the engine had the wrong rocker arm setup. I spent hours researching, bought the correct valve train from eBay. After cleaning the 60 year old, stiff rockers thoroughly, I found the rocker shaft was severely worn—hidden under a thick layer of rust. I was able to procure new replacement shafts, which improved the clearance, but not the oil pressure. I switched to a higher-viscosity oil, still no joy.

Finally, with no other options, I pulled the engine. And there it was: the real problem. Some of the main and rod bearings were over-bored, some by as much as .009 inches. It a miracle the engine had any oil pressure at all!

I took the engine to a trusted local machine shop. After a couple of months and a $2,000 investment, they brought it back alive. The truck runs strong now—and I learned a few lessons along the way.

I share this because I know how disheartening it can be to feel like you’re fighting a losing battle. But stubbornness, research, and persistence do pay off. If you haven’t already, consider having the engine inspected by a competent machine shop and be prepared for sticker shock, as good machine work doesn’t come cheap.

I hope this helps, Glenn. You’re not alone in this, and we the Stovebolt collective are here if you want to talk through ideas or solutions. Hang in there—these projects are tough, but the satisfaction of getting it right is worth every headache.

I wasn’t aware of that but each one held (did not leak down). One plug had a little oil on it (in a picture). Even with 1 plug out the motor cranks up & runs very smooth. Also the oil pressure stay up.

It cranks & runs great if a little rich still. I got a 1 barrel Manchester I could put on if needed.

I guess I’ll just drive it for a while. I can’t pull it out & apart again!

Yes sir that is a lot to take on but having drive & self motivation keeps one progressing. I’m glad you got her done even if it was expensive!
The oil pressure stays up after driving. So I guess it needs rings. I just can’t pull that motor again now so I guess I’ll just drive her a while. I just finished the floorboard & the seat. Waiting on the carpet to arrive.

Normally compression testing is done with the engine off, all the plugs removed, throttle held wide open, and "run" on the starter motor checking one cylinder at a time.

I suspect that the readings that you have are significantly influenced by the throttle being basically closed (reving the engine without load doesn't require much throttle opening). Try retesting and see what the numbers are before you condem the engine based on inaccurate numbers.

Be sure to pull the coil wire off so the plug wires do not arc against the engine or frame rails. A friend of mine pulled all the plugs and wires on a 440 MOPAR race car to clear the cylinders, he cranked it over and one of the wires arced by the head lighting off the fuel being blown out of the cylinders, they had a terrible time putting out the alcohol fire because they couldn't see where it burning!

Thank you for that information, I as always appreciate good input & advice! I think I have 600 miles on this motor & only one plug looked like burnt oil on it (in the picture).
So I need to have the plugs out, coil wire out. Turn the motor over say, 4 times & read each cylinder pressure?
While we R here, is there anything I can put in a cylinder that might help a problem here?

Do your first compression test with dry cylinders, then repeat after squirting a little oil in each one. If the compression comes up from the first test, that's an indication that the rings aren't sealing well on that cylinder. Although you could have good compression numbers and still consuming oil (that one plug sure looks oil fouled) but the oil ring broken and not doing it's job.

Instead of another compression test, let me suggest a cylinder leakdown test. Following the firing order by looking at the distributor rotor, get each cylinder to the top of its compression stroke, lock the crankshaft by putting the transmission in gear and chocking the wheels front and rear, or putting a couple of pairs of vise grips on the flywheel at the bell housing, put air pressure into the spark plug holes with an adapter made from a spark plug shell and an air hose quick coupler. Listen for air leaks at the carburetor (intake valve) the tailpipe (exhaust valve) and the oil fill cap (piston rings). If you use an air pressure regulator set to 100 PSI with no air flowing, the pressure drop will give you a reading of the percentage of leakdown for each cylinder. I've been using that test for 50+ years. For instance- - - -a pressure reading of 90 PSI on a cylinder with the air pressure in it would indicate a 10% leak rate. Much more accurate than a simple compression test.
Jerry

Gee, I don’t know about making an adapter from a spark plug. I understand the concept for sure. That’s how we do compression tests on my planes motor. Continental 85. I even have the gauge.

I assume the motor needs to be warm to get better compression readings?

Not necessarily- - - -all compression testing is simply a percentage comparison of compression pressures between individual cylinders. Less than 5% is good- - - -over 10% is bad. Warm, cold, or somewhere in between doesn't matter. Conventional compression tests are roughly equivalent to "Measure with a micrometer- - - -mark with chalk- - - -cut with an axe!" The adapter is simple- - - -knock the porcelain out of a junk spark plug, thread the steel shell with a 1/4" NPT tap, and screw an air line male quick connect fitting in. Then hook up an air hose from a regulator. So simple even a Marine can do it!
Jerry

Then I may be able to perform that deed!
I’ll look for my Dad’s compression tester gauge.
Otherwise set the pressure on the regulator at what, 125?
I went out & did another test with the plugs removed & turned the motor over a few times on each cylinder. Readings were much better for some reason. All were 120 to 122 lbs.
Helped me breathe easier anyway.
Doing a real compression test would tell me what exactly?

My engine runs rich for certain. Black plugs & exhaust pipe.I just tweeked that carburetor again. It may need that new carb put on it that I have. I think it’s a 1 barrel Rochester.

I’m pretty sure it’s bluish burnt oil coming out of the exhaust, only when I let off the gas after revving it up.

That's an indication of bad valve seals. When you let off the gas, vacuum in the intake goes up (sucks harder) and draws oil thru the valve seals. You may benefit from doing a valve job on the engine, since your compression readings are good at 120-122 psi. Like Jerry said, being within a few percentage points of the reading between highest and lowest cylinders is a good thing.