SU adjustments for L28

[Stanley]

We've been doing some other stuff for the car so took my first test drive yesterday. New stage II engine. Carbs same as before, N-27 needles modified for richer mid-range and top end, and red SU springs (slightly stiffer than stock), as was required to get the 240 running right with  Y70 head, 2 1/4 inch pipe and K&N's.

Idle set at 1500 rpm in Park, drops to 700 in gear (to low for cam). AF ratio is abut 13.5 at 2000-2500 rpm which is per engine builder's spec, but drops to 11.5 (smoky) at idle.  The engine's not broken in enough to try top end yet. I'm going to try FSM tune procedure today but doubt if adjusting the mix nuts will do the job. Worth a try before modifying anything though.  Easy enough to take a little off the needles at the midrange stations. As per SU documentation posted in an earlier thread, if you reduce the section at idle, stations 1 and 2, it will richen the whole needle since the taper needs to be even through the length of the needle. My thinking is to take a little more off midrange and top end, and raise the mix nuts to get a cleaner burn at idle.

Also thinking about changing to the yellow springs, one step stiffer than red springs. Maybe they'll keep the pistons down a bit more at idle, keeping it leaner. The bigger engine should make plenty of vacuum to get the pistons where they need to be at high rpm. Obviously I'm guessing at some things here. So, any ideas ?

Not used to all the low-end torque. It's like a wild horse that wants to run.  Gotta be extra careful with the right foot.

Thanks Chickenman for the Zeitronics recommendation.

[JSM]

What oil are you running?  I have a stumble and the recommendation was to increase SU oil viscosity with a relatively stock L28 engine now.  I haven't tried it yet and I see you've increased the spring tension already. Wonder if some experimentation may not be in order.

I had an L28 with a 530 lift, 306 duration Isky cam, bored, ported, etc., with stock SU's and it ran wonderful.  I swapped over from OER 45's as I didn't want the hassle at the time.  Wish I kept that car.

[Captain Obvious]

Maybe it's just me, but I would be interested in seeing how a pair of good condition, well set-up, clean, straight up stock, not modified pair of SU's would work for you.

I wouldn't be surprised to hear they worked great. Maybe a fraction of a turn down further down to account for the small change in displacement. Maybe not even that.

[ajmcforester]

I got a friend running stock SU on a 280 setup

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[Mark Maras]

1 hour ago, Captain Obvious said:

Maybe it's just me, but I would be interested in seeing how a pair of good condition, well set-up, clean, straight up stock, not modified pair of SU's would work for you.

I wouldn't be surprised to hear they worked great. Maybe a fraction of a turn down further down to account for the small change in displacement. Maybe not even that.

 Not just you. I'm sure a clean original set would run great. It's been done countless times.

[Stanley]

Maybe, but according to the official SU publications, just putting K&N's instead of stock air cleaner, or a bigger exhaust pipe will require modified or different needles. 280 is 20% more displacement. We've got the mix nuts set 4 turns down already just to keep from going lean at 2500 rpm, and it's making it too rich at idle. I assume the engine builder knows what he's doing, so I'm following his recommendations for A/F ratio etc. to the letter. I think in theory you are correct when the engine is at steady state. But without accelerating you won't go anywhere. If you drive like a kid who sees a cop in the rear view then maybe.

I wish 0.095 Hitachi SU needles were available in various richer or leaner configurations for modified engines like the 0.099 European SU needles are. But then we wouldn't need to be as creative. Look at the needles they have to use for 300 HP L-series engines. Mine isn't stage 4 (and I'm glad) but it's got a big cam, headers etc.

I did find a defect today. The front carb piston was harder to push up than the rear. Also, the front carb had to be set richer than the rear to run "right". I swapped the plungers which made the rear harder. We tested it with another plunger from a freshly rebuilt set of SU's, that one was stiff too, so I'm assuming that hard is correct and there's something wrong with the "easy" one. Fortunately I have an extra set of NOS plungers so I'm going to swap them in tomorrow.

[Captain Obvious]

The pistons are supposed to be hard to push up. I agree with the assessment that there is something wrong with the "easy" one.

Sounds like the check valve jiggly bits on the stalk are not sealing correctly.

[Mark Maras]

 If different jiggly bits don't cure it, it could be a clearance issue between the pistons and chambers. You might try swapping the pistons from one carb to the other and see if things improve. The pistons and chambers aren't the same and they do get switched accidentally. I remember a test in ZTherapy's "Just SU's" that calls for removing the chambers and pistons. Tape up (seal) the holes in the bottom of the pistons. Invert the chambers and pistons, then holding the needles, release the pistons at the same time and compare the drop times.

[Stanley]

On 2/1/2018 at 11:01 AM, JSM said:

What oil are you running?  I have a stumble and the recommendation was to increase SU oil viscosity with a relatively stock L28 engine now.  I haven't tried it yet and I see you've increased the spring tension already. Wonder if some experimentation may not be in order.

I use 30 wt. but FSM says use 10W-30.  There was a discussion about where the oil goes and why.  It eventually goes so main thing is fill it to the upper ring before it gets to the lower ring. I tried an experiment that seemed to show that the level of oil affected the pressure need to push up the piston. If it' not done carefully checking the level will make it go down.

IIRC  the SU tuning manual says if the carb is big for the motor use lighter springs; if carb is too small for the motor use heavier springs. 

Is it true that some racers run without springs ?  Hope not, cause that might confuse the issue.