MTFCA Forum

Hello. I was driving my 1926 roadster last week and it started making this bup bup sound at idle and i could feel a slight miss every time it did it. It was really making the sound while going down a hill. (The sound was coming from the tailpipe) I checked the compression and it was good. I noticed that #1 and #2 spark plugs were black while #3 and # 4 were a nice light gray. I checked the spark while running and each plug WIRE was firing good. I'm running a CLEAN New Day timer. I pulled the head suspecting a blown head gasket but everything was sealed. The entire tops and valves of 1 and 2 cylinder heads were black with soot. I've got a new head gasket coming from Lang's. When I put it back together, where should i look next? Thanks, bobt

In my experience it is common for the number one and two cylinders to be a more sooty than three and four. If you look at how the water circulates in the block the back cylinders run hotter and don’t tend to soot up whereas the front two run cooler because of the placement of the water inlet from the radiator and combustion is not quite as good. Other opinions to follow…..

All sounds normal to me.

Robert ;
That is normal for a model T .
Look at the Book :THE MODEL T FORD OWNER ,
page 303 - 305 .
Sorry for your Head Gasket and your work .

Toon

Is your head gasket on backwards?

No. Head gasket is not on backwards. Thanks for the help. I will re-assemble and try something else. Thanks again, bobt

Hi Robert,
You may want to look for a vacuum leak on #3 & 4 ? Have you had to run a little more fuel than usual?
Craig.

If you are running a water pump without a thermostat, your coolant will take the shortest route from the intake to the outlet. This would cool the front of the engine more than the back side. With the weather cooling down, your first two cylinders might not be heating up enough to get thorough combustion of the fuel.
Norm

No. Not running a water pump or thermostat. My engine has always ran cool. ( new flat tube radiator) bobt

Assuming the car is not using oil, I'd look for a vacuum leak at the joint between the rear intake port on the block and the rear branch of the intake manifold. A crack in the intake manifold is another, less likely possibility. A slight leak at the exhaust valve in # 3 or # 4 cylinder is a possibility. Such a leak could mimic an intake manifold gasket leak at low engine speeds and loads. A weak valve spring at # 3 or 4 is another possibility. A sticky valve is yet another.

What was your "good" compression and why did you pull the head if it was OK? Do you have an external oil line kit installed? That might over oil those 2 front cyls. causing blackening of the plugs. Now: was it soot or oil on those plugs? Soot is running rich & oil is, well, oil. 2 different problems.

I have lost count of the times that people have gotten curious/concerned/worried over sooty plugs on #1 & #2 cylinders. If the car otherwise runs fine, then just leave the plugs alone and have fun. The popping, (not to be confused with backfiring), through the exhaust on deceleration or going down a hill while engine braking is normal. These are old, inefficient engines. Either the design of the intake, or the cooling system causing the front 2 cylinders to run cooler than the back 2, or both, seem to contribute to this fairly normal phenomenon.

That said, it still wouldn't hurt to look for a vacuum leak at the "3-4" intake port.