Topic: CB350F Blowby Gas circulator importance

[TomL]

I have a 350F and I was wondering what the importance of this system is

http://hondacb350.com/page2/files/CB350-400_09.pdf

Would this improve economy?  My bike is currently missing the hose from the cylinder head to the air box.  I'm not sure about the rest of the system (the part beneath the air filter) because I never really looked into it.  Does anyone have one of these?

[kajtek]

i thought the system was only present in the 400f? i may of course be wrong...

its there to stop the venting of engine gases to the atmosphere. an environmental thing, which is present in every vehicle nowadays but was only brought in around the time of these bikes. on my 400f it was damaged so i removed it and have a long hose from the head breather that reaches down to where the fuel overflows are. it should really have a filter on it but havent got round to it yet. in terms of performance, it wont make a difference. in theory it actually reduces performance as gases make the engine burn hotter, but apparently in these circumstances the effect is negligible.

ashley

[TwoTired]

The 350F preceded the EPA mandate to reduce unburned hydrocarbons from these machines.
The 400F does indeed have the crankcase gas recirculator.  Which helps keep carcinogens out of lungs.
If you have the opportunity to fit one to your bike, it is worthwhile.

The theory that the crankcase gas recirculation reduces power is bogus, imo.
To me, it seems like a desperate excuse for justification of crankcase breather recirculation removal.
In fact, negative crankcase pressure can help ring seal and increase power, proven in race machines.

That's just how I see it, anyway.

[TomL]

So it seems as though 350s never had them.  That makes perfect sense as to why my bike does not.  Maybe I should run a hose down out the bottom of the frame for anything that decides to exit the breather hole.

[Bodi]

You can see at the top of the page linked it's in the "400F Technical Supplement", showing stuff that's different on the 400 compared to the 350. The engines - and whole bikes - are pretty similar and Honda amalgamated them in one service manual, basically adding the 400F "different stuff" at the end of the 350-four manual.

[camelman]

I haven't seen any schematics showing the routing, but I believe the crank case breather (PCV) tube for the 350F just goes to the intake plenum box.  As long as you aren't blowing a lot of oil through that breather tube, you shouldn't have an issue.  The concept is to catch the poorly combusted gases that blow past the piston and into the crankcase.  By redirecting those gases back into the intake, you give the engine a second shot at fully combusting them into CO2 and H2O.
One benefit from hooking the breather up is that you will help pull a vacuum on the crankcase (or at least decrease the positive pressure in it).  That will show up as better top range power and smoother high rpms.

The comment above about a decrease in power does hold some truth, but the real decrease in power comes from EGR (exhaust gas recirculation).  EGR came about to lower combustion temperatures to decrease the liklihood of NO2 production (which really kicks off above 1800F or so).  EGR actually improves efficiency, but decreases power by adding gases (CO2 and H2O) that are essentially inert for the combustion reaction we have in our engines.  So, when you hear about EGR and PCV, the real culprit for power reduction is EGR, and even it is not very much.

Camelman

[TwoTired]

Be aware that the plenum chamber behind the carbs has a drain tube similar to the CB500/550.  This tube is used as a drain in the event a carb overflows, so that gas does not well up in that chamber.  (The inlet to that tube was very small and had a breathable foam plug in it, to allow gas to drain out but bugs and air inducted were minimal.  Using that tube for the crankcase breather was not what Honda had intended. If the chamber does fill with fuel, and you have it hooked up to the engine breather, it will either not drain off or it will flush the engine with raw gasoline.  Worse, a backfire may cause ignition and fire catastrophe which is exactly what the drain was supposed to prevent.

I do favor a positive crankcase breather, but that particular plenum drain nipple is just not the right connection point, unless some scheme is incorporated that still positively keeps gasoline from welling in the air chamber.

http://www.cmsnl.com/honda-cb350ffour-usa_model435/partslist/F++14.html

[TomL]

Does anyone have a picture of the complete original breather tube routing?  I want to know what is supposed to go where.  Right now I have what looks like the bottom portion of the drain tube.  I have no tubing going to or coming from the air box.  If I can make the air a little bit cleaner while still riding my bike I would be all for it.  Smoother top end is always good too 

[faux fiddy]

I think the hose on my first  350 went out the bottom by the rear stand.

[midnightrider]

Be aware that the plenum chamber behind the carbs has a drain tube similar to the CB500/550.  This tube is used as a drain in the event a carb overflows, so that gas does not well up in that chamber.  (The inlet to that tube was very small and had a breathable foam plug in it, to allow gas to drain out but bugs and air inducted were minimal.  Using that tube for the crankcase breather was not what Honda had intended. If the chamber does fill with fuel, and you have it hooked up to the engine breather, it will either not drain off or it will flush the engine with raw gasoline.  Worse, a backfire may cause ignition and fire catastrophe which is exactly what the drain was supposed to prevent.

I do favor a positive crankcase breather, but that particular plenum drain nipple is just not the right connection point, unless some scheme is incorporated that still positively keeps gasoline from welling in the air chamber.

http://www.cmsnl.com/honda-cb350ffour-usa_model435/partslist/F++14.html

So on the 350F, are the head breather, crankcase breather, and intake plenum drain  all supposed to be routed to the bottom of the frame with three separate tubes?

[TwoTired]

I think the 350f and 500 only have two. The engine top cover breather and the air plenum breather behind the carbs.  The tubes just dump all that noxious stuff down below the frame.

Cheers,

[cookindaddy]

The 77 and 78 750K have a similar system. For what it is worth, the drain line out which exits near the center stand has a removable plastic plug in the end which you are supposed to pull once in a while (at oil changes?) to drain what accumulates. This would prevent the cruddy oil residue from exiting in front of your rear wheel I suppose.

[midnightrider]

I think the 350f and 500 only have two. The engine top cover breather and the air plenum breather behind the carbs.  The tubes just dump all that noxious stuff down below the frame.

Cheers,

That's right...I was confused.  Thanks TT!

[TwoTired]

I think the 350f and 500 only have two. The engine top cover breather and the air plenum breather behind the carbs.  The tubes just dump all that noxious stuff down below the frame.

Cheers,

That's right...I was confused.  Thanks TT!

Just in case to don't know, the plenum tube is a reduced diameter tube.  Beg end on the plenum fitting much smaller down below frame.  Also there is a loosely fitting piece of open cell foam in the end, to reduce the ingestion of bugs and road grime.

The top cover "breather" doesn't have much flow to it.  It's mostly just to equalize pressures inside and outside.  Piston blowby will vent the pressure out there, though.

Cheers,