Fine tuning the 1275, with unknowns . . .

[vagt6]

I have a bit of a challenge on my recently purchased '74 MKIII Midget and I'd appreciate input from you 1275 gurus out there on how to fine tune the engine.

The engine appears to be in very good shape (it's sure is clean!), rebuilt approximately 8K miles/5 years ago. It has roller rockers; upgraded cam; Pertronix; HS2s rebuilt; Datsun starter; head ported/polished; oil cooler; Haynes electric fan, and more. It does not smoke, leak, etc., and runs well.

I have documentation for all of this work, but not the cam, only that it's not stock (I've asked the P.O., he couldn't help as he's not the "mechanical type"). As such, I'm not sure where to set the valve lash. Thankfully, it's in good tune now so I'll probably see what the valve clearances are now and stick with those settings for the time being.

I'd like to assume that if it's a street cam (it idles at about 1050 RPM - below that it's choppy) the lash may be something like .014 on intake and maybe .016ish exhaust (this is inferred from APT and Piper websites for their popular street cams), but who knows? I'm pulling the valve cover this weekend to do a bit of exploring in there.

Reading the plugs, they look pretty good. The old plugs were moderately carbon fouled so I backed off the jet adjusting nuts a couple of flats and the newly installed plugs with about 100 miles look much better. The plug ground straps look a little burned (no color deliniation line on the strap) and the porcelain is snow white - no metal spots).

New car (to me), I don't want to assume anything. What do you think? How would you proceed? The valve lash must be adjusted someday, what to do?

Thanks for all the help.

 

[Hap Waldrop]

Mark, this cars are not that precise, so whether you did it a .014-.016" would not be any big deal, there that much room for human error,in how a couple different people feel the feeler gauges, so don't sweat, .014,.015 or .016 they will all work just fine, just pick one of those numbers and go with, and yes you are correct to assume, almost all A-series cams lash are somewhere between .014-.016". I will give youthis piece of advice when adjusting valves on roller rocker arms, muvoe the feeler gauge from side to saide, not inward and outward like you would with regular rocker arm, in other word from side to side on the roller tip, into the roller tip.

 

[dklawson]

I thought A-series tappet clearances (with a stock cam and stock rocker arms) was typically 0.012"

Most documentation I've seen says that you multiply the standard gap by the altered ratio of the rocker arms if the arms are not stock. For example, if the standard arms have a ratio of 1.22:1 and the roller rockers you have are 1.5:1, the new gap would be 0.012" x (1.5 / 1.22) = 0.0148 or realistically 0.015".

Regardless, I'll bet most of us have wider gaps than that. What's the old saying... better to hear the valves than burn them?

 

[vagt6]

None of this stuff is stock, though, Doug. Both cam and roller rockers are aftermarket and I don't know the specs (and I'm not going to tear the engine down to find out!). Probably the valves, too.

Anyway, with ideas from Hap and others I'll muddle through.

Thanks, guys!

 

[dklawson]

Well, you can always put a dial indicator over a pushrod to measure the lift the cam is giving you and you can carefully use a scale or calipers to measure the relative lengths of the rocker arms to determine their ratio. That information won't fully enlighten you but you'll have a better idea how those two parts compare to stock.

 

[Pythias]

Not that this will help any, but the recommended lash on the cam I've got in my 1098 says .018 HOT. It is an odd one from Oregon Cam Regrinders that has 107 degrees seperation. Not much on the top end, but tons of low end grunt.