BJ8 cam idle vacuum

[57_BN4]

I had my BN4 cam reground to what was supposed to be a BJ8 profile but it looks like the grinder has possibly done something else to it. The idle vac is only 8-9in.hg and it should be in the 16-20 range. The idle is lumpy and it stalls when the clutch is pressed fully down to the carpet if I don't pull the choke a little to raise the idle first. There is also a definite boost in power around 4000-5000rpm on the road but it is a bog standard engine downstairs and supposedly redlines at 4800 so not ideal.

BJ8 cam data I sent to the cam shop:
Intake open - 16 deg BTDC
Intake close - 56 deg ABDC
Duration - 252 deg
Lobe centre - 110 deg ATDC

Exhaust open - 51 deg BBDC
Exhaust close - 21 deg ATDC
Duration - 252 deg
Lobe centre - 75 deg ABDC
valve lift 0.368


Data supplied back with the cam- each value has changed by about 1 degree from the BJ8 profile
Intake open - 17.65 deg BTDC
Intake close - 55.43 deg ABDC
Duration - 253.08 deg

Exhaust open - 52.26 deg BBDC
Exhaust close - 21.85 deg ATDC
Duration - 254.11 deg

valve lift 0.35252

Questions: what vacuum should a BJ8 cam produce in a fairly standard engine?
Is a change of approx 1 degree on the opening and closing values enough to make it sound like this?

I'm going to set the tappets at about 20 thou to see if it de-cams it enough to get the idle vac up, then I'll have to try and degree it to see if what is in there matches the spec sheet. Kind of thinking at the moment that the profile of the cam in the engine isn't what is on the spec sheet.

Andy.

 

[Bob_Spidell]

We had my BJ8's cam ground by Crower to the mildest of five grinds. The idle vacuum is very low--similar to yours IIRC--but I don't have any stalling problems (unless I totally pooch a standing start).

 

[57_BN4]

Thanks Bob. Do you have any duration specs for your cam?

The stalling only occurs when the engine is fully hot and perhaps the carbs are boiling a little, then it just kind of goes to sleep gradually due to the extra friction of the release bearing. Other than that it is very drivable.

Andy.

 

[Bob_Spidell]

ADVERTISED DURATION:

Intake: 250
Exhaust: 250

Intake Lift: 0.432
Exhaust Lift: 0.432


Degreeing info ('correct only at 0.50" tappet lift'):

INTAKE:

Opens 8deg BTDC
Closes 34deg ATDC

EXHAUST:

Opens 42deg BBDC
Closes 0deg TDC

DURATION AT 0.50":

Intake: 222
Exhaust: 222

Lobe separation: 107
Intake lobe lift: .288
Exhaust lobe lift: .288

Clearance intake hot: 0.012"
Clearance exhaust hot: 0.014"


Some fairly big differences, may be due to settings measured at 0.050" of lift at the tappets. My engine has a slightly lopey idle, tons of low-end torque but doesn't want to go much above 4,500rpm without some effort.

 

[steveg]

Andy,
What happens if you spray the carbs with water a little to cool them down when it's stalling?
What happens when you lift the carb pins? Does the carb run faster or slower or speed up then even out?
Have you tried richening or leaning the carbs a little?
Have you checked the timing?
When it stalls, did you try removing the float bowl covers to see if the fuel level is low?
Just some random ideas.

 

[steveg]

The other question maybe the racers on the forum could answer: what happens when you put a relatively hot cam into a car with a relatively constricted intake and exhaust setup.

 

[57_BN4]

Well after another day fiddling about with it I managed to get some actual opening and closing numbers off the cam.

Results:
IO Measured 18.5, claimed 18.4, requested 16 BTDC
IC Measured 75.5, claimed 58.3, requested 56 ABDC

EO Measured 56.5, claimed 53.5, requested 51 BBDC
EC Measured 32.0, claimed 21.7, requested 21 ATDC

So that explains what is going on- and extra ten degrees here, twenty degrees there and suddenly the cam is nothing like what I asked for. @ 16 thou the duration is approximately 273 and 269 degrees...

I set the tappets of #1 with a 17 thou plus 5 thou feeler blades together, then removed the 17 thou one and rotated the engine until the 5 thou blade just nipped/released, then noted the degrees of the wheel I stuck on the front pulley. The claimed figures above are supposedly at 17 thou, BMC uses 16 thou for their figures.

I discussed this with the camshaft shop owner and he claimed that it is not possible to measure a camshaft while it is in the engine and I must return it to him to check with his computer. I didn't try and push the issue of camshafts existing long before computers were invented. After a bit more fluff he agreed to refund my money. So now I'm stuck with a lumpy idle until the engine comes out again for full stripdown to sandblast the chassis.

Steve- I'm pretty sure the idle stalling is mainly caused by low idle torque due to the lack of vacuum. Hot carbs are quite normal, just in this case they have far more of an influence on the idle because of the cam and also it is only firing on every other stroke at idle. I can fairly well say how a standard engine performs with a lumpy cam- still useable but would be better with less of a cam.

Thanks for your thoughts

Andy.