Excessive blow by through breather hose

Hi everyone. I need some input. I have a 1974 850 Roadster that I've owned for 36 years. Last year I bored it .040 over, new Emgo/JCC cast pistons with Hastings cast rings. The bore was straight< .001 difference from top to bottom. .005 clearance on the pistons. The rings were within factory spec when fitted out of the box. New Kibblewhite valves, guides, springs, viton seals and push rods. New Beryllium valve seats. New studs in the head and cylinders. Copper head gasket, annealed before installation, head sanded on glass...looked good. New timing chain, oil pump and seals. New carbs too. I was running an old Boyer at the time. I put about 200 miles on it before Winter. Re torqued the head and checked valve clearance a couple times during that period. Ran better than new, pulled like a freight train, no smoke, even had it over 100mph no problem.

I went to start it a few weeks ago. Had some trouble, rechecked valve lash, adjusted carbs, rechecked timing, installed a new Tri Spark with single coil and got it running. I always drain the sump before starting after sitting for a time. Always remove the seat to look for oil level and flow. Fresh gas etc..

While watching the oil flow back to the tank which I have done 100's of times over the years, all of a sudden I start getting excessive blow by back to the oil tank through the breather hose. Never heard anything like it. The bike smoked just a little, you could barely see it. Took it out for a short ride and it still performed well. When I got back, the problem still existed. I also run an oil pressure gauge, and oil pressure was good.

I did a cold/dry compression test and had 170lbs on the Left, 180lbs on the Right. Right plug a little more carboned than the Left, but I think the Left carb was running a little lean. I don't have a leak down tester, but think I will purchase one soon.

Does the conical seal on the oil pump have any effect on blow by? I did pull the valve springs to check the machinists work when he fit the valves to the guides before I installed the head. He did nice work. I did everything myself but bore the cylinders, install new valve seats, and fit the valves to the guides. I don't have a problem tearing the thing apart, head, cylinders etc.., but need to know what I'm fixing.

Just looking for a little direction, before I ride it till it breaks. Thanks for your help.

My understanding of how an 850 breathes is that there are connecting holes between the sump and the timing chest and then there is the breather hose off the back of the timing chest.
Question is where is excess oil coming from?
If the pump is clearing the sump adequately is there too much oil in timing chest? You mentioned the seal on the oil pump and that might be one possibility. Another might be the crank oil seal in the timing cover.
Maybe someone more astute on 850's will chime in.
Yes, you have oil circulating back to the tank, but it's taking a path it wasn't meant to use.
IMHO I wouldn't ride it until I figured out why.

No excessive OIL back to the tank, just AIR/ Blow By through the breather hose to the oil tank. Which actually happened while I was looking for oil return to the tank. I did replace every nut, bolt, bushing, bearing, seal and washer on the whole bike when I rebuilt it. Crank case seal too. Everything but the bottom end. Thanks for your input.

Seems somewhat normal for a nice tight rebuild.

Are you running a reed valve on the breather circuit?

Ah, I misunderstood, I thought you had oil returning. Yes, sounds like healthy engine. All of that good compression works in both directions. I put a Mikes XS 650 reed valve on my 750 and have essentially no leaks.

Pete/htown,
No breather/reed valve, just the stock hose from the back of the timing case to the oil tank. The thing IS it didn't do this (blow by to oil tank) during the first 200 miles, and just SUDDENLY started while I was watching for oil return to the tank. I've never experienced that before, since I've owned the bike. So if you think it's normal, I guess I'll install a breather valve as suggested, and keep an eye on things. And yes, I guess this might be the tightest the engine has ever been since I did New everything in the cylinders and head. Once again, the puzzling thing is "Why did it just SUDDENLY start doing this"? Thanks for your input.

NORTSTER1974 said:

Pete/htown,
and just SUDDENLY started while I was watching for oil return to the tank.

Click to expand...

I might say, thank goodness it did. If it hadn't, it could have been a mess in a half a dozen other areas.
Maybe the line was block and finally opened up under the new found pressure.

Now if you want to get into it, you might want take a compressing/cylinder pressure test to help settle your worry or, god for bid, realize them.

Does the conical seal on the oil pump have any effect on blow by?

Click to expand...

It SEALS the Oil Pressure gallery thru the Timing Cover , which goes to the CRANKSHAFT .

If its not there & the spigots note a firm fit in the Timing Cover , youll be pumping INTO the Timing Case ,

See ' Excess Oil in Timing Case , in the 2nd Post .

Itll Also be starving the big ends , to a degree .

If youve fitted a Triumph Oil Pressure Indicator Button outer to the Pressure Relief Valve , youll see the buttons only half ( if that ) Out ,
with the seal ommited . Second hand used grotty dried out mangled seals are therefore a No No . ' Alternative ' fittments in place of the seal may be dislodged or whatever ,
with a similar result .



Half of these new ones on google pics look a bit dodgie . Should be firm clean precission moulded , from something tecnical and heat ressistant . Not soft soggy recycled tripe .

Had a look at the original post and it said you checked oil pressure with a gauge and you had good pressure. How good and with engine well warmed up or cold? If it really is good, then that kind of rules out the oil pump seal and the crank seal. Failure of those should result in a pressure drop. You also said there was little oil return through the breather line.
Only thing I can think of that could generate pressure in the engine is pistons going up and coming down.

Thanks Htown,
I run an oil pressure gauge off the left exhaust rocker. Oil pressure, hot or cold, is never a problem. Just air/blow by in the oil tank. I'm going to install a Dorman 80190 power brake check valve in the breather hose this weekend an rock on. Thanks for your help.