Finally got the floats set around 19mm down from top of bowl, new N-27's and nozzles. 3 1/2 turns down still too lean when book (FSM) says 2.5 for altitude and temp. Well don't know turns down exactly since I made a couple stop light adjustments yesterday to fix stalling problem, maybe 4.

On the freeway, about 70 mph on my (optimistic due to smaller tires) speedometer at 3000 rpm or so, when I hit the gas it bogs down (lose a few rpm) before it picks up. No problem on acceleration if lighter on gas pedal.

I'm guessing it's leaning out, maybe needs thicker oil in dashpot or heavier springs. I've read some stuff like this: SU Variable Choke Carburettors Explained but zero real-world experience with springs, race tuning, etc.

Have some modded needles but want to follow the one thing at a time rule so I know what's happening.

73 Z running stock 240 bottom end with stock Y-70 head and cam, K&N's with velocity stacks, and 2 1/4" exhaust, '72 round-tops, otherwise stock. The Y-70 seems to have a bad-arse midrange and no top end (and the correct headers with smaller diameter primaries don't seem to be available) and feels totally different from the stock head it had before. Y-70 seems to need more gas at mid-range.

The link above seems to say straight 20W-50 is too thick and gets sucked up in the engine too fast. I'm using 20 wt three in one oil with electric motor picture on container.

A way to tell if it's the oil, springs, or both? US supplier for springs? Which springs would be one step heavier than stock? Over my head here, no pun intended.

Edited July 6, 201410 yr by Stanley

Oil only dampens piston vibration and controls movement of piston during acceleration and deceleration. Thicker oil enriches acceleration and leans deceleration. Thinner oil leans acceleration and enriches deceleration.

Spring and weight (added to piston) affect height of piston at each RPM. More weight or stronger springs keep piston lower than norm at an rpm. This restricts air flow but causes more depression and draws more fuel however the annular puddle of fuel around the needle is also reduced with lower piston height so the extra fuel added by the depression is not linear. Simply lowering the jet or shaping the needle may accomplish adding more fuel w/o the restriction of doing it with a heavier spring or heavier piston.

One wants the piston to rise to maximum height to minimize its airflow restriction but I'm not sure if it should be at max torque or max hp. (I would guess at the max torque point). Once the piston maxs out, all rpms above will draw more fuel as the depression increases in magnitude (more suction on the annular fuel puddle). I'd like to map this stuff someday. Cptn. Obvious and I had planned to do this but I moved from his neighbourhood too early. A few months with an engine dyno would allow for making math models for these adjustable parameters.

Edited July 7, 201410 yr by Blue

OK, that helps, maybe I'm starting to get it. Richer acceleration and leaner deceleration sound like the right direction.

Was re-reading Chapter 1 of the Des Hammill book today, if I understand, he says the heavier springs are for fuel economy. Also, in other sections, (I think) he's saying something to the effect that the bogging I described is normal for stock SU's, but can be tuned out, with the right combination of testing and voodoo (knowledge).

Maybe ditch the 20 weight and try 10W-30, first. So easy it might work.

Yeah. What he said.

(Only additional independent thought I can provide is that the piston max rise is NOT simply max RPM. You can easily run the engine up to max RPM by grabbing the throttle linkage while sitting still, and the pistons will not rise all the way to achieve that feat. They will reach max altitude when the engine sucks the most air. Might be at max torque, might be WOT at max RPM.)

Maybe ditch the 20 weight and try 10W-30, first.

If your leaning out at throttle punch, then you're going the wrong way with the oil.

Ditch the 20 weight and replace with 30 weight.

Like Castrol HD-30? Getting hard to find.

Yup. Like that.

You're having trouble finding HD-30? I've not had that problem. I've found that HD-30 is easy to find. What's hard to find is non-detergent 30W. At least in my area.

I'm not certain that the 3-in1 oil is a true 20. It always seemed thin to me. I used 20 wt. motorcycle fork oil in mine.

John Deere Hydrostat (hydraulic) fluid, straight 20 weight, no additives, farm supply stores will have it in quarts. Each carb will take about 2cc of oil. I use a syringe, without needle, sourced at a local drug store, to fill them without a mess, and have a piece of tubing that goes on the end to remove fluids, if needed. The syringe comes in handy if you need any liquid in a small confined space.

Bonzi Lon

Ahhhh! 30 weight. Oh yeah, it's available.

Yup. Like that.

You're having trouble finding HD-30? I've not had that problem. I've found that HD-30 is easy to find. What's hard to find is non-detergent 30W. At least in my area.

I found some non-detergent 30W in Wal-mart a few years back.