Catch can smoke - normal?

[TobyB]

Oh, heck, I'd forgotten about that.

 

Yes, it helps!

 

t

 

[Einspritz]

First things first for my curiosity; how many miles on the engine?

 

I had a weber with a K&N on the vent and it never smoked in 100K miles.

 

You can try changing the oil, adjusting the valves, and running it a high RPM (to free up the rings) but I suspect it is blowby from wear.

 

So, if you can find someone local to you to do not just a compression test, but a leak down test as well.

 

If you can't do a proper test, then at least put each cylinder at TDC (on the flats of both cams) an use a fitment (could be a rubber plug or threaded part) and compressed air into the cylinder. Put your ear up to the tail pipe, if you hear hissing, you may have a bad exhaust valve. Then listen in the throat of the carburetor, if you hear hissing there, it can be a bad intake valve. If you hear hissing from the valve cover area, through the dipstick or your catch can, then it is blowby past the rings. It can be one or all on high mileage engines, high mileage  is relative upon how it was driven and if the maintenance was followed.

 

If you  have air, perhaps you can borrow the tools from a big box parts house, at least a compression gauge.....and if you do a compression test, be sure to peg open the carb butterflies to let the air in.

 

IIRC on the older cars, usual oil consumption was 1 qt. every coupla thousand miles.

 

 

[backeis]

Thanks! Engine has approx 100k miles. Went back through PO's notes and he noted lots of blowby. For now it runs ok (at most the smoke is annoying), but I guess I should go search for a decent mechanic here in NYC to do these tests with.

In regards to vent hose diameter, since that can up earlier: It's about 3/8", from what I can see/feel.



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[Einspritz]

OK, so if you have a stock air cleaner, why can't you just reattach it? If you have a K&N, some used to have a hole in the bottom tray of the filter assembly to attach the hose, or made a piece that was held between the top and bottom and directed the hose to the air filter. In that way, the fumes could be redirected back into the engine to be reburned.

[backeis]

I no longer have the stock air cleaner. I guess I could attach the hose to the weber - but isn't the catch can theoretically the better solution? Not spewing oily shit back into the carb and getting it dirty?


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[Simeon]

Compromise is to get rid of the dinky filter on the can and add a hose back over to the filter housing. That way you will catch (some) gunk and get some negative pressure applied to the breather. 

[backeis]

Would you say hosing back to carb is generally better than catch can then?


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[Simeon]

Nope. The ideal system has a number of elements:

 

Efficient extraction of gases to prevent pressurisation 

 

Removal of oil and water to keep the inlet charge clean and undiluted. 

 

With an engine in good condition the second is less less important but when it gets worn and the blow by increases the quantity of crap from the breather can make a mess of the carb and cause charge dilution (hence the catch can). 

 

If you go from your engine, to a catch can and then on to the carb you can, in theory, get the best of both worlds. 

 

I would stuff your catch can with some stainless wire wool / pot scrubbers to give some surface area for the oil to coalesce on. This will improve the performance of the can in filtering out the crap and minimise the amount that makes it back out of the can to the carb / filter. 

[Mark92131]

15 hours ago, Simeon said:

If you go from your engine, to a catch can and then on to the carb you can, in theory, get the best of both worlds.

 

I used basically the same setup in two of my cars.  The pastel blue car, I bought a cheap catch can off of eBay, mounted it and shamelessly vented it to the ground.  On the Polaris car, I was more environmentally responsible and figured a way to run the fumes back to the carb.

 

Mark92131

 

 

[kbmb02]

23 hours ago, TobyB said:

What I found with a catch can was that the length and diameter of the hose connecting to the can is relatively important-

when the engine's running, there's a certain amount of pulsation in the breather hose itself.  If the hose volume is greater

than that pulsation and there is no restriction on the vent at the end most of the oil and vapor is just moved back and

forth inside the breather tube.  Upset that balance, and lots more oil makes its way out, even on a healthy engine.

 

Okay, TobyB ... diagnostics, please?

 

This was an oddity; hot compression test and leak-down numbers were lousy, expected this 3+ year old race engine to be down on power. But it wasn't - roughly the same as new/similar builds, 160 HP at the wheels - and with that number in his memory, the driver promptly went out and put down some of the fastest laps at the next event. -KB

 

 

[Dudeland]

I did a catch can to save blow by going down the carb. The butt dyno thought it was a good move. I also found that it ran nice regardless of the weather. For 20$ I would say it was worth it.


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[xr4tic]

Too small of a tube diameter can increase the velocity of the air in the tube which helps carry the oil in the air further.

 

If you're spitting out oil, you definitely don't want to route it back to the air cleaner, unless you route it through a catch can first.  Just make sure the catch can isn't empty, or else stuff some stainless steel pot scrubbers in it to give the oil something to catch onto before coming back out.  Ideally it would have a divider of some sort between the inlet and outlet (the cheapy ebay ones are empty)

 

 

[backeis]

Thanks for the tip with the steel wool. I'll give that a try. My catch can doesn't seem to have an outlet, so I guess there's different/better ones. For when it would have in and out, I assume it wouldn't have a breather/filter and rather be sealed completely.

[thinksound_mike]

try changing your oil first and see if that solves your issue. if the oil is two years old, it's likely absorbed a fair amount of moisture. put in a good filter and fresh 20w50 oil and see if it goes away. i have a similar set-up and no smoke/steam. I change my oil 2x a year and don't drive it all that much. well, i haven't been driving it at all... it's in pieces. 

 

 

[Preyupy]

On ‎6‎/‎21‎/‎2018 at 11:35 AM, kbmb02 said:

 

Okay, TobyB ... diagnostics, please?

 

This was an oddity; hot compression test and leak-down numbers were lousy, expected this 3+ year old race engine to be down on power. But it wasn't - roughly the same as new/similar builds, 160 HP at the wheels - and with that number in his memory, the driver promptly went out and put down some of the fastest laps at the next event. -KB

 

 

nick-dyno-run_480.mov

Ken,

I have seen a number of engines that acted exactly like this,  The only thing we could figure out is the rings were just a bit loose in the grooves and when they are being forced at high speed to seat on one side or the other in the groove they seal better.  You might also have some valve guides that are getting a bit loose and at low speeds the valve does not always seat it's self square when it's closed.  When I slams shut at 7000 rpm it seats better.  I have seen huge differences in Leak down numbers by just putting a screw driver on the rocker or spring retainer and thumping it with your hand with the tester hooked up, the valve just re-seats it's self and the number goes down. 

 

It does not mean they don't need to be freshened up it just means it's not costing you HP.