Quick and dirty SU Tuning

[madkaw]

Mark,
The carb overflowed after trying to richen FLOAT LEVEL. .
Just did the 10 turns down and seeing fuel method to check float level.
Went out for another spin after putting piston needle shoulder flush with the groove. Engine pulled harder but have run out of sunlight until tomorrow. I know I have run the needles like this on my car before with good results (SM)


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[Mark Maras]

 Was the float adjustment a major tweak or a little one? Yet another thought, float pivot needles can get crusty. To operate properly they should slide and rotate easily in the float base and the needle bosses under the float cover.  Anyway, It sounds like you're on the right track. One more question, Is it backfiring thru the carbs at any particular throttle position or RPM and what wt. oil are you using? Oops that was two. 

[madkaw]

I guess it was too big of a tweak- .
Backfiring happens during tip in under load at lower rpm.
Might need to play with timing a little. Not running a vacuum advance at present since this dizzy is a d6-12 with 24!degrees mechanical advance . At present running 12 static .
I'll be back at it today


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[madkaw]

And when I say over flow I mean shooting out the air vent


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[Captain Obvious]

The manual says the needle shoulder should be flush with the bottom face of the piston, not pushed up so that the shoulder is even with the bottom of the groove.

I'm no carb expert, but here's some other stuff to consider:

The round top nozzle adjusting nuts are M10 x 1.0. This results in .0394 inches drop for each turn.

2.5 turns down is .098 down the needle and at that location along the needle you're running between station 1 and station 2. This means that at 2.5 turns down, the SM needles will idle and no load leaner than stock. It's not until about six turns down that the SM's turn the corner and start running richer than stock.

So if you're getting a transition lean pop from no pedal to much pedal, it might be because you're running lean at no load when you're high on the needle. Pushing the needle up so the shoulder is even with the bottom of the groove is about the same thing as turning the nozzle another turn down so that would help. I don't know where you would be on the needle at light cruise, but you might be leaner than stock there as well.

Here's a chart I whipped up that illustrates the difference between the N-27 and the SM needles:

[Stanley]

With N-27's (stock 240 displacement but some mods) had high end lean-out that I couldn't tune out. SM's and red springs fixed that for me but still too lean at low end tip-in, had to lower mix nuts to drive and idle without the effect Steve described, which seemed to cause excessive rich condition (judging from plug appearance), at mid-range I'm guessing. I think C.O. is correct in post 17. My car seems to be quicker on cooler days, which leads me to suspect a leaner mix is needed for performance on hot days. But any leaner and it craps out as stated above.

I recently set the mix about a 1/16 turn leaner, after looking at the plugs, and I think it's got more power at mid-range, but I can definitely feel it's weaker (leaning out a bit) at low end, which hurts performance off the line unless I rev it up with my foot on the brake (AT) like at the drag strip. Not what I want to do for everyday driving, even if I want to keep up with a leadfoot in a Lexus.

Maybe the solution is to modify the SM's by taking some material off at stations 1 & 2. But then the nice richness at top end would be reduced since the mix nuts would need to be raised (I think) to get it right at idle. Dunno what to do, except experiment.

It's easy to make a float bowl overflow while setting the floats, especially the rear one. Maybe it just means they're set almost right, but not quite.

[madkaw]

Another run with better results. I set the needles flush with piston , not groove. I'm 2.75 turns out now to avoid lean pop, but there's more power there to be had. Going to drive it around like this for a while. Also not running air filter so that leans things out. I've seem to have lost the captive nuts for the backing plate - if anyone wants to sell some.



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[dat260]

I had an overflow issue with the back carb (closer to firewall) I change the gasket and adjusted the overflow lever. Overflowing problem solved but runs bad. Did the same to the front one and adjusted the overflow, both overflow were off by a lot. I followed the FSM to adjust and sync the carbs, went for a drive and it runs terrible. Sync becomes off after a drive. I noticed when I plug the overflow tube on the front carb nothing happens but when I plug the overflow tube on the back one, it stalls. I also hear a hissing sound coming from the back carb. Iam getting discouraged, I spend two days playing with them. I would appreciate some input.

Thanks in advance.

[siteunseen]

Which carbs does your 260 have?  Are they dome shaped or flatter at the top?

[dat260]

Sorry, I should of mentioned it, dome.

thank for the reply

btw, nice car!

 

Edited June 13, 2017 by dat260

[240260280]

@dat260,  I may be in Montreal later this summer.  I work remotely in N.S.  for a company with an office in St. Laurent.  I should be able to come by and fully tune your Z.  I'll PM you when the trip is firm.

 

For your problem, have you looked at, and compared,  plug #2 (which represents the front carb) and plug #5 (which represents the back carb)?

Also make sure all of the bolts holding the air intake manifold to the head are snug.  The manifold  seal should not allow air  to get into the head.

Edited June 13, 2017 by 240260280

[dat260]

That's very kind of you, thank you, let me know.

Everything is like it was before the rear carb started to overflow. The only problem is that I played with the adjustment screws instead of just replacing the gaskets bowls, my fault.

I will check the plugs and I also notices the header tube coming from cyl 1 is reddish in colour compare to the others. Do the bowl pivot a little? While I was changing the gaskets I notices the bowls were slightly turning. I need to check how they are attached to the carbs, maybe thats were the hissing sound comes from.

Anyways, I going to play with them again today and see what gives.