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FAT CAM spark tuning

There is a certain rpm at which the timing is descibed as "all in". The hotter the cam, the lower the rpm at which the timing is all added.

Basically total timing is in the mid 30's and it's all in with the k cam around 2600rpm. That's a fine starting point on that end of the curve.

Now I know I have said this a a few times so sorry to belabor it- but set the idle to 1000rpm, and the timing to 5 degrees. Keep increasing the timing until you get the lowest map value (you may need to keep adjusting the idle speed to maintain 1000rpm).

That will give you the baseline from which to start adding timing.

I find it's really helpful to have that timing value as a starting point.
 
Captain Bondo said:
There is a certain rpm at which the timing is described as "all in". The hotter the cam, the lower the rpm at which the timing is all added.

Basically total timing is in the mid 30's and it's all in with the k cam around 2600rpm. That's a fine starting point on that end of the curve.
Click to expand...

I couldn't agree more. Also agreed on getting the timing of the cam to a truly known-for-sure value. Taking a hundred thou off the deck....and not knowing for sure if that head had or had not been shaved priorly, and not knowing what thickness HG is in there....makes guessing where the cam is at advanced one tooth pretty much just that: a guess. [and going 9+ degrees either way at a time is not exactly a 'fine' adjustment]

....a degree wheel might be required to find out where things are at....

the 'fatter' the cam, the less forgiving it is to being mistimed. A mild cam can be off a lot more and still run.

my comment re lash was based on that info not being given, other than the cam spec sheet listing 8 thou and 10 thou, intake and exhaust. If the lash is that tight, then THAT might be exacerbating the low idle vacuum problem.

Starting out at initial base timing of 5BTDC IS a good place to start. Agreed. Depending on how 'fast' the advance is ramped up [and at what rpm that ramp up is started], I suspect that the initial may need to go towards 10 or more. [actually, I suspect closer to 18 initial]

.....but....5 is a damn good place to start. Once the cam's actual timing is known, then playing with initial spark timing to find highest idle vac is the next step.

TF
 
Howdy again. Just wanted to post an update to this. To be honest, this thread probably shouldnt be in the EMS forum, probably more in the maintenance sections.

Problem was... valve shims. To say the least, they were much too tall (no valve clearance). Re-shimmed at .55mm (.022") valve clearance and things are MUCH more peachy. Time to start (re)tuning everything back to normal.

Also got an adjustable cam gear. Round tooth, holla holla. I took measurements from a few places on the front of the head and the height measured out to 143.7mm. A total of 2.4mm (.094") was shaved off.
 
Glad the plugs helped you out, but I bet if you really dug into that carb with a wide band, and nailed the vac advance setup, the beast would be even better. It would make a great tech article that would help out alot of guys, but the carb tuning is quite in depth. An article on vac advance setup would be helpful, as very little has been written on it, and nailing that down really tamed the big cam I'm running.
 
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