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

[Captain Obvious]

Are you sure you don't have any vacuum leaks anywhere? The big difference in carb settings between front and rear seems odd. I would expect that there might be some smallish difference required between the two, but you've got more than a full turn difference between the two? And the front one is just 3/4 turn down from home stop? That doesn't sound like it should be enough. Makes me wonder if there's not something wrong somewhere.

And are you double dog sure you're damper dashpots are working properly? The lean on pedal transition could be a damper issue. That's exactly why they're there.

[siteunseen]

Do you have the vacuum advance coming off the front carb?

Is the connecting rod in between front and rear carbs set up right, I made that mistake and had a helluva time getting them synchronized until I realized my mistake.

[blodi]

23 hours ago, Captain Obvious said:

Are you sure you don't have any vacuum leaks anywhere? The big difference in carb settings between front and rear seems odd. I would expect that there might be some smallish difference required between the two, but you've got more than a full turn difference between the two? And the front one is just 3/4 turn down from home stop? That doesn't sound like it should be enough. Makes me wonder if there's not something wrong somewhere.

And are you double dog sure you're damper dashpots are working properly? The lean on pedal transition could be a damper issue. That's exactly why they're there.

Once I reset the float levels, the difference between the front and rear carb seems to be much more even. The plugs seems to confirm this as well. 

I've sprayed carb cleaner around the carbs and intake manifold and have seen no evidence of vacuum leaks. 

Damper dash pots seems to be working correctly. They open evenly when bliping the throttle and fully (by hand). Fluid level is good. I'm using ATF...maybe I need something thicker? 

 

[blodi]

On 6/12/2016 at 10:31 AM, siteunseen said:

Do you have the vacuum advance coming off the front carb?

Is the connecting rod in between front and rear carbs set up right, I made that mistake and had a helluva time getting them synchronized until I realized my mistake.

Yes, advance is coming from the front carb. 

The carbs are synced very evenly. 

[siteunseen]

Sounds like you've covered all the basics. Maybe changing to a thicker 20 weight would help the dashpots transition like captain obvious says.

I use 3 in 1 blue label from Lowes hardware,  $3.99 IIRC.

[siteunseen]

54 minutes ago, blodi said:

Yes, advance is coming from the front carb. 

The carbs are synced very evenly. 

When you do the high rpm sync do you use the screw/spring at the rear carb to match air flow. The one on the linkage flat?

[blodi]

3 hours ago, siteunseen said:

Sounds like you've covered all the basics. Maybe changing to a thicker 20 weight would help the dashpots transition like captain obvious says.

I use 3 in 1 blue label from Lowes hardware,  $3.99 IIRC.

I'll have to pick some up and give it a shot. 

3 hours ago, siteunseen said:

When you do the high rpm sync do you use the screw/spring at the rear carb to match air flow. The one on the linkage flat?

Honestly, I've just synced the carbs at idle. What RPM should I check them at? 

[siteunseen]

When you get them even at idle turn that very top screw/ spring until you get 2500 or 3000. I do 2500 but Ztherapy dvd shows 3000. It supposed be your cruising speed. My timing light shows rpms but you can stick your head in the cab and look at the gauge.

Once you have it at high rpms measure air flow on the front. Then turn the screw/ spring to the left of the rear carb until it matches the front carb. Thats how I do mine. If you search "quick and dirty su tuning" Blue has a good write up on the procedure. There's youtube videos also. @Jarvo2 has some good info too. 

[rossiz]

+1 on what site says above. there are two linkage sync points: at idle the linkage is resting on the throttle stop screws at each carb, adjusting them sets the resting point of the butterfly in the throttle body. the second adjustment syncs them when you are actually driving - it's the screw between the two linkage systems. these two adjustments are completely separate, so one can be spot on and the other quite off... same principle for measure/adjust, different screw to turn.

[siteunseen]

This first one opens in a pdf type file, the next two are threads from classiczcars.  Hope it they help. 

SU tuning.rtf

 

 

[blodi]

OK, good links!  I got the carbs balanced last night at 3K rpm. Didn't have a chance to drive it though and observe RPMS. I just had to turn the balance screw in ~1/4 turn to get the rear carb balanced. 

 

[Stanley]

What springs are you running? I was running lean at top end and full throttle (stock 240 displacement, head and exhaust mods), installed SM's after modding the N-27's didn't do it) but it still leaned out at transition to full throttle. Finally fixed it by installing the red springs; they're stiffer. Still not perfect; the british needles are thicker at the base than the Hitachi SU needles, so I had to  set the mix nuts richer to avoid running lean at bottom  end. Now it's a  little too rich at idle, otherwise OK. Also gas mileage went down with the red springs. Don't know what else to do though. If I mod the SM's by turning them down at the base, then the mix would need to be reset which would cancel out the richer top end they provide.

Seems like there's a very narrow range of mix settings with the SM's. 1/8 of a turn off and it's no good.