SU's- lean at partial throttle and high RPM.

[blodi]

Question is about carb tuning...

Running stock round top SU's. Z therapy rebuild kits, ATF in the dashpots,  SM needles. 260Z intake manifold. Electronic fuel pump.  Again...L28, high compression, E31 head, 500/500 cam. Ported, 6-1 header...etc. 

I just got an Innovate LM-2 the other day to see how things were looking in the tune. Overall...idle...for some reason...it's reading in the 30-40's...which makes no sense...but regardless...

At cruise...things look pretty good...between 15-16. Rolling into throttle at 3.5K at about 30-40% throttle...things go lean...like 17's and there is a slight stutter. But if I push throttle beyond that (50-60%) it richens back up nicely and pulls well.  

WOT...things progressively get richer down to 12.5-13.0  and hold steady...until I get to maybe 5K+ and then it just starts progressively leaning back out to 14's by 6K...and that's all the revs I'm putting into the engine so far.  

What are my options for these two issues? Is this engine asking too much of "stock" SU's? to deliver at high RPM? And suggestions for the partial throttle lean out?

I think I may eventually want to go to Tripple Webers (40's or 45's?) and fortunately when I bought this engine it came a label for what jets to use! So that will take a lot of guess work out of them down the road hopefully. 

But I want to get the SU's working well for the time being. 

[Jarvo2]

Sounds like a fuel mapping (needle profile) issue based on your configuration.  I've never heard of anyone running SM's say that it runs too lean, always too rich.  Might want to look at some other needle profiles or to grind down your SMs even more.  I know several on the board here have done it.  Attached is an SU book that I've acquired in my travels.  You may also want to monitor your fuel pressure, should be low for carb'ed engines, but you'll want to make sure your electric pump is providing enough fuel and the bowls aren't drying out at high RPMs.

 

 

Fuel_SU-Carb_bmc_needle_chart_1965.pdf

[blodi]

Thanks for that. The pump is the K&N unit the MSA sells...low pressure...should be plenty of flow. 

Found this on seating the needles...might be something i need to try first. The rear carb is only maybe 3/4 of a turn out and the front is closer to 2-2 1/4. 

At the bottom...

http://www.sonic.net/~kyle/su.html

 

[Jarvo2]

Agree that you should look how your needles are seated, and you may also want to check your float bowl levels.  Here's some more I was able to dig up...

SU_Needles.xls

Needle charts pp 47 to 49.pdf

[blodi]

Yeah, I'm going to start with the needle seating...as my rear carb needs to be leaned out a couple turns from 2.5 to get it "correct" with my colortune. I'll also double check the float levels too. 

 

[HuD 91gt]

I recently did a comparison with my SM needles and stock '71 needles on the stock L24.  I had been running the SM needles for the past year or so on some freshly rebuild SU's using the ZT kit.

My SU reading's are similar to yours, besides at idle where I have to run them at 10.5:1-11-1 at idle to get a good mixture to redline.  When I did this with the stock needles, everything was quite a bit more lean.  The SM needles are definitely going in the right direction, but still need to keep a similar profile, except at the base where i'd like to see them thicker for a leaner idle (Or thin out everywhere else).

You have a larger modified motor where i'm sure they are much too lean.   Time to start searching on needle modification, or try adjusting where the needles are seated (I used the ZT method of holding the choke tube up into the carb, while dropping the piston and needle down.  Pull the piston out and tighten the needle where it sits).  I've heard of some people fiddling with the needle height to get different results.

Edited June 3, 2016 by HuD 91gt

[HuD 91gt]

As for your rear carb requiring more turns, that could quite easily be float level causing that difference.

[rossiz]

Set float levels first or you'll be chasing your tail with anything else you do to compensate for them. Carbs want to be tuned in a specific order from gross adjustment to fine. 

[siteunseen]

Plus one on what Rossi says above. And glue the cork gasket onto the lids of the float bowl RIGHT BEFORE TIGHTENING IT DOWN (sorry for the caps, I'm not too good with my phone). I put RTV on mine right off the bat, sat there maybe 10 minutes and had to reapply after removing the dried up RTV. 8^(

[psdenno]

11 minutes ago, siteunseen said:

And glue the cork gasket onto the lids of the float bowl RIGHT BEFORE TIGHTENING IT DOWN 

Glue the float bowl gaskets?  Curious why.  I've never done that when opening the bowls or putting new gaskets on the bowls.  Have i been doing it wrong or just the hard way?  

Dennis

[siteunseen]

Sticking them to the lids made getting the lids on and off easier for me plus they won't tear as quick.  They last a little longer too, unlike a wine bottle's cork.

[esmit208]

Since it sounds  like you will be in the float bowls make sure there is nothing obstructing the operation of the "GROSE JETS" that handle the function of emulsion tubes on triples. Under WOT conditions the level in the bowl decreases and if you have an obstruction it could create a lean condition. It takes very little obstruction before you are popping and spitting. I had about a 20% obstruction on the way to Road Atlanta years ago and was forced to stop in Tuscaloosa Alabama to disassemble the rear carb. Just probably some trash that finally made its way up from the gas tank that eventually block the needle seat.