Topic: Newly Built Engine with White Smoke Coming from Exhaust and Spark Plug Holes

[Finnigan]

Fired up the rebuilt motor today for the first time, came alive on second kick!

After about 30 seconds of idle I noticed white smoke coming from around the spark plug/fin area of cylinder 1 and also out of carb #3 (wtf?).  Shut it down but couldn't find the source, pulled the spark plugs to check for oil and there was white smoke in each chamber but the plugs were clean (didn't expect much after a minute of idle).

White smoke is also coming from exhaust and there is no oil anywhere to be found.

This is a 1970 or 71 head without guide seals on the exhaust values but I didn't expect this much smoke.

Doing a compression test shortly, the oil is not overfilled (PSI is about 70).

Not 100% sure where the smoke is even coming from...a little nervous to let it idle and watch if it goes away on its own.

[Finnigan]

Short update,

No oil on spark plugs
Even compression on all cylinders
The #1 cylinder seemed to be the culprit so I decided to replace the exhaust gasket and found it bolts were torqued down but gasket was partially squished, there was some oily residue on the non-compressed side.  Took extra care to try and evenly torque the bolts this time, if this persists I can probably use 2 copper crush gaskets to achieve a better seal.

I also looked inside a few tappet covers to make sure there was oil around the cam, all looks good.

My hope is with a motor that still needs to go through break-in this might go away.

[HondaMan]

I've done a lot (a LOT) of post-build startups of the 750. When I've had the white smoke issue, it has always been due to some amount of oil somewhere getting into the cylinder(s). I run the engines for 10-15 minutes (about 1 quart of fuel) to see if they straighten up: all but 2 have done so as the oil rings sorted out their contact with the cylinders walls.

The 2 that did not were:
1. The oil rings were apparently faulty from the supplier. This is EXTREMELY rare, and the only time it ever happened to me: it was with Honda's own 3-piece oil rings on 2nd-oversize Honda pistons (0.5mm oversize) in a near-sandcast K0 engine. After the owner re-changed the oil rings, they seated almost immediately. The symptoms included also some spitting of oil from the open rear engine crankcase vent that had not yet been connected. Later on I discovered that there are 2 possible widths of these 3-piece oil rings, and the 'faulty' ones were most likely the thinner ones. How they came to be in Honda boxes as a ring set is still a mystery at this point, about 4 years later.
2. I attempted (against my experience) to 'help' someone out once by simply having his bores de-glazed (at a machine shop that does all my work) for $25 instead of being bored for new pistons, and then installed new rings, with 3-piece oil rings. This one spewed white smoke from startup and filled my garage with it, almost solidly after 10 minutes. I re-pulled the top end and installed some 1-piece oil rings I also happened to have (this was all in stock bore sizes) and ran it about 25 minutes, getting less and less smoke as the time went on. He had to pay for the 2nd head gasket, but rode the bike for the 3 remaining college years he had. I don't know where he, or that bike, are now, but it finally stopped smoking with those 1-piece oil rings. It was a 750K4.

I don't often see white smoke from loose exhaust valve guides, and those K1 guides that have the pointy tops and no valve seals on them can last over 100,000 miles (mine went past 126k, just started weeping a bit of oil then). They are made from Stellite, a cast iron metal that is nearly hard as glass! If the exhaust guides have a place for the oil seal (shorter, and with the groove) then you may want to install some fresh seals. Believe it or not, this can be done on the 750 (engine out of the bike) without pulling the head: I've done it myself. Gotta pull the rockers to get to the valve springs, and use a lever-compress type valve spring tool (I had one in my shop back then) and bring that cylinder to TDC so the valve won't fall down inside the engine, but it's possible. 

[newday777]

Short update,

No oil on spark plugs
Even compression on all cylinders
The #1 cylinder seemed to be the culprit so I decided to replace the exhaust gasket and found it bolts were torqued down but gasket was partially squished, there was some oily residue on the non-compressed side.  Took extra care to try and evenly torque the bolts this time, if this persists I can probably use 2 copper crush gaskets to achieve a better seal.

I also looked inside a few tappet covers to make sure there was oil around the cam, all looks good.

My hope is with a motor that still needs to go through break-in this might go away.

Using 2 exhaust donuts doesn't work. It won't seal.
Use a fresh donut.

[Finnigan]

I've done a lot (a LOT) of post-build startups of the 750. When I've had the white smoke issue, it has always been due to some amount of oil somewhere getting into the cylinder(s). I run the engines for 10-15 minutes (about 1 quart of fuel) to see if they straighten up: all but 2 have done so as the oil rings sorted out their contact with the cylinders walls.

The 2 that did not were:
1. The oil rings were apparently faulty from the supplier. This is EXTREMELY rare, and the only time it ever happened to me: it was with Honda's own 3-piece oil rings on 2nd-oversize Honda pistons (0.5mm oversize) in a near-sandcast K0 engine. After the owner re-changed the oil rings, they seated almost immediately. The symptoms included also some spitting of oil from the open rear engine crankcase vent that had not yet been connected. Later one I discovered that there are 2 possible widths of these 3-piece oil rings, and the 'faulty' ones were most likely the thinner ones. How they came to be in Honda boxes as a ring set is still a mystery at this point, about 4 years later.
2. I attempted (against my experience) to 'help' someone out once by simply having his bores de-glazed (at a machine shop that does all my work) for $25 instead of being bored for new pistons, and then installed new rings, with 3-piece oil rings. This one spewed white smoke from startup and filled my garage with it, almost solidly after 10 minutes. I re-pulled the top end and installed some 1-piece oil rings I also happened to have (this was all in stock bore sizes) and ran it about 25 minutes, getting less and less smoke as the time went on. He had to pay for the 2nd head gasket, but rode the bike for the 3 remaining college years he had. I don't know where he, or that bike, are now, but it finally stopped smoking with those 1-piece oil rings. It was a 750K4.

I don't often see white smoke from loose exhaust valve guides, and those K1 guides that have the pointy tops and no valve seals on them can last over 100,000 miles (mine went past 126k, just started weeping a bit of oil then). They are made from Stellite, a cast iron metal that is nearly hard as glass! If the exhaust guides have a place for the oil seal (shorter, and with the groove) then you may want to install some fresh seals. Believe it or not, this can be done on the 750 (engine out of the bike) without pulling the head: I've done it myself. Gotta pull the rockers to get to the valve springs, and use a lever-compress type valve spring tool (I had one in my shop back then) and bring that cylinder to TDC so the valve won't fall down inside the engine, but it's possible. 

Thanks HondaMan, this is encouraging.  I'll let it run for about 10 min to see if this goes away and update soon.

[Tracksnblades1]

I've done a lot (a LOT) of post-build startups of the 750. When I've had the white smoke issue, it has always been due to some amount of oil somewhere getting into the cylinder(s). I run the engines for 10-15 minutes (about 1 quart of fuel) to see if they straighten up: all but 2 have done so as the oil rings sorted out their contact with the cylinders walls.

The 2 that did not were:
1. The oil rings were apparently faulty from the supplier. This is EXTREMELY rare, and the only time it ever happened to me: it was with Honda's own 3-piece oil rings on 2nd-oversize Honda pistons (0.5mm oversize) in a near-sandcast K0 engine. After the owner re-changed the oil rings, they seated almost immediately. The symptoms included also some spitting of oil from the open rear engine crankcase vent that had not yet been connected. Later one I discovered that there are 2 possible widths of these 3-piece oil rings, and the 'faulty' ones were most likely the thinner ones. How they came to be in Honda boxes as a ring set is still a mystery at this point, about 4 years later.
2. I attempted (against my experience) to 'help' someone out once by simply having his bores de-glazed (at a machine shop that does all my work) for $25 instead of being bored for new pistons, and then installed new rings, with 3-piece oil rings. This one spewed white smoke from startup and filled my garage with it, almost solidly after 10 minutes. I re-pulled the top end and installed some 1-piece oil rings I also happened to have (this was all in stock bore sizes) and ran it about 25 minutes, getting less and less smoke as the time went on. He had to pay for the 2nd head gasket, but rode the bike for the 3 remaining college years he had. I don't know where he, or that bike, are now, but it finally stopped smoking with those 1-piece oil rings. It was a 750K4.

I don't often see white smoke from loose exhaust valve guides, and those K1 guides that have the pointy tops and no valve seals on them can last over 100,000 miles (mine went past 126k, just started weeping a bit of oil then). They are made from Stellite, a cast iron metal that is nearly hard as glass! If the exhaust guides have a place for the oil seal (shorter, and with the groove) then you may want to install some fresh seals. Believe it or not, this can be done on the 750 (engine out of the bike) without pulling the head: I've done it myself. Gotta pull the rockers to get to the valve springs, and use a lever-compress type valve spring tool (I had one in my shop back then) and bring that cylinder to TDC so the valve won't fall down inside the engine, but it's possible. 

Thanks HondaMan, this is encouraging.  I'll let it run for about 10 min to see if this goes away and update soon.

How did it turn out..?

[HondaMan]

...yeah, what he said ^^^ ?

[seanbarney41]

Just about any time I start an engine that has not ran in a long time smoke starts coming from unexpected places.  Its just all the residue of oil, cleaners, degreasers, paint, dirt, etc. burning off.  Yeah, the insides of your pipes will often have this too...even brand new ones.  Dont over think it

[Finnigan]

The extreme smoking out of the tailpipe is gone!

I’m still getting white vapor out of the crankcase but there is only water vapor inside (seems odd that after 20 mile ride it’s still coming out..)

My exhaust still isn’t sealing great after putting a new single copper crush gasket.

The exhaust is a NOS 4-1 motorad

[HondaMan]

The extreme smoking out of the tailpipe is gone!

I’m still getting white vapor out of the crankcase but there is only water vapor inside (seems odd that after 20 mile ride it’s still coming out..)

My exhaust still isn’t sealing great after putting a new single copper crush gasket.

The exhaust is a NOS 4-1 motorad

Is it just one header pipe leaking, or...?

[Finnigan]

Hard to tell but I think its number 1 OR a combo of 1 and 2

[HondaMan]

Maybe try this: loosen the rear (footpeg) pipe mount, then loosen the exhaust clamp's 2 bolts/nuts, wiggle the pipe assembly (firmly...) and then retighten the head end, followed by the footpeg end afterward. I've sometimes seen the aftermarket 1-2 side be obstinate about seating, most likely because the #2 pipes are always 'off' just a bit. This has fixed 2 bikes (here) with the aftermarket 4-4 pipes, so far. I didn't have to replace the exhaust donuts, either.