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Hose, vent pipe to air cleaner, BJ8

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AUSMHLY

AUSMHLY

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BJ8, What is the purpose of the vent pipe hose to air cleaner?
What problems could it create?

I'm helping a friend get his newly acquired 66 BJ8 sorted out.
He had a British Mechanic go though the car, full tune up.
We took it for a ride yesterday to find it back fired, and chugged somewhat.
On the ride back to my house, we pulled over. I found the vent pipe hose was not connected. I attached it, we finished the ride home. No change in performance.
He leaves my house, 10 minutes later come back, car runs worse with the vent pipe hose attached.
So I suggested we leave the vent hose off and I'll look into it with help from the BCF guys.
Before he drove off, I said let me poke around a little and see if anything else is disconnected out of adjustment.

I found 3 items. Low dashed fluid, vent pipe hose was not attached and one of the choke cables was not adjusted correctly.

I took the rear air cleaner off and found no oil in it.
I took both dash pods off to find both carbs were black with a little oil on the bottom rim where the pod meets the carb. I cleaned that area, wiped down the inside to the dash pods and carbs. Put carbs back together, added dash pot oil.
Adjusted the choke cables and confirmed both were working properly. (Mechanic said the choke was not working and worked on it. Sorry guy, I had to redo your work, cable and spring)

Owner left (without the vent pipe attached) once home said it now runs better then how he got it from the mechanic.
Owner will run the car later with the vent pipe hose on to see if there's any difference.
What is the purpose of that hose and how can it affect running performance?
 
Last edited by a moderator:
Re: HOSE, vent pipe to air cleaner BJ8

Every car, even brand new ones, have pressure in the crankcase ('sump'). This is caused by combustion gasses getting by the rings. If that pressure isn't vented somewhere, the pressure will usually cause problems, mainly oil seeping, or pouring, out of gaskets (valve cover, tappet cover, etc.). Older cars, like Healey 100s, had an open tube pointed towards the road ('road draft tubes'), the 6-cyl cars had the hose of which you speak, and new cars have PCV systems (which should probably be called 'Negative Crankcase Systems' because they use manifold vacuum to vent the sump, but 'positive' probably sounded better than 'negative' to the marketing folks).

I bought some aftermarket air filters, which had a very small, 90deg fitting for the crankcase vent. After installing them, an hour or so of driving, I was getting drops of oil on my windshield from the pressure in the valve cover blowing oil out the gasket (ruined the valve cover trying to stop the leak). Your friend's engine compartment will likely get very oily if he doesn't vent the sump somehow. I put an air/oil separator on my brand new Mustang, and it's collecting oil from the sump; not a lot, but enough to gunk up the intake over time.

It doesn't sound like your friend has a good mechanic, or he has one that one that isn't familiar with British cars, and esp. SU carburettors. The first thing any good mechanic does with an engine he's not familiar with is run a cylinder compression test. Did your friend get the results of his?
 
Hi Roger,

First, I agree with Bob on telling your friend to find another mechanic.

Evacuating internal engine gases/vapors through the rear air cleaner for burning through rear cylinder combustion should create any issues unless the volume of vapor is substantial. Should this be the case, you would have more then combustion contamination and misfiring caused by spark plug fouling to deal with. However, as the volume increases, you could intercede with an oil catch can installation between the valve cover and rear carburetor air cleaner to help condense oily vapor and trap it before entering the air cleaner and carburetor air/fuel stream.


Figure 2.jpg
Ray(64BJ8P1)
 
AUSMHLY said:
Why was the car back firing and chugging?
Click to expand...
No dash pot oil will make the car run rough. My first British car I owned I learned that after taking my car to a mechanic who knew British cars. He filled the dash pots and sent me in my way.
 
Venting the crank case to the air filter was a cheap environmental fix to avoid simply discharging to atmosphere. It’s a ‘bodge’. Far better to install a catch tank with filter and avoid feeing dirty air into the rear carb.
 
DerekJ said:
Venting the crank case to the air filter was a cheap environmental fix to avoid simply discharging to atmosphere. It’s a ‘bodge’. Far better to install a catch tank with filter and avoid feeing dirty air into the rear carb.
Click to expand...

Are there catch tanks (all mounting brackets included) made for the BJ8?
 
Hi Roger,

There are a number of Oil Catch Cans on ebay and some have attached/mounted them to the frame. They are not difficult to make and I installed a support bottom bracket next to the rear carburetor in preparation of installing a PCV. The original tubing was kept in place (without modification) connect to the catch can or could be easily reattached to the air cleaner. However, deciding not to install the PCV at that time, I was able to easily attach the catch can's outlet to the rear air cleaner.

Ray(64BJ8P1)
 
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