su carbs ??

[Boom death]

so i got my su's installed on my l20b today got it running tuned it as best i could... dont have my uni-sync yet had to order it from parts store will be in hopefully next week so its probably not tuned 100% ... but my question is car idles fine and has great accelleration of the start but when it gets to the higher rpms like above 4500-5500 it feels like their is a slight bog or hesitation untill u shift to the next gear and rpms lower then it runs great and wanted to know if their is a knowing if their are both sucking the same amount of air without the uni-sync or will i have to wait

quick pic cuz u guys love them :thumbup:

yeah their rough just waiting till my next day off to polish them up

i also wanted to know what people are running for filters and stacks i currently dont have any and want to see what my options were?

 

[Silver04rs]

There's a write up in the how to section on how to adapt some cheaper filters. There is also a link to a few cool parts in there

 

 

Travis

[wayno]

That is the way mine acts also, the L20b falls on it's face around 4500rpms, I suppose if the proper cam is used, you can move the power band up.

You need to find a heat shield and install it, otherwise you will have heat issues, you don't want the exhaust manifold that close to the SU bowls/bowl fuel tubing unprotected.

[72240z]

It looks like their is a shield on there? 

Are you sure you dont have throttle shaft leaks? You could be leaning out. 

You can sync 2 using a rubber hose pretty easily. 

[Sealik]

Check the float level..????.....also....a short hose with a small funnel jammed in one end works well to balance air flow.

Obviously the funnel end goes over your ear....the other end of hose goes in exactly the same spot per SU.

 

What needles are in the SUs?

[wayno]

This has a heat shield, the return springs connect to it.

 

 

I have issues when it gets hot outside, and I live in Washington where it is cold, he lives where it is hot all the time.

[datzenmike]

The SUs are maxed out at 4-5K and run lean? Stiffer dome springs retards when the piston reaches max height but richens up the whole range.

 

 

Maybe these are jetted for an L18SSS or even an L16SSS motor?

[Stoffregen Motorsports]

Another commonly overlooked detail is full throttle. Have you checked to be sure they are both getting full throttle?

[bonvo]

do as stated above and make sure the are opening all the way but it could be that the l20 is just running out of steam at that high of an rpm range the stock cam teamed with the lower compression means they arent a very high reving motor from the factory team that with the fact that its an old engine and probably tired 

[72240z]

Yes those headers do have a heat shield Wayno but unless I'm mistaken his isnt a bare cast manifold. It has the stamped heat shield over it. I have run errands in 100+ degrees with stock cast manifold and no shield so I venture to say its not the main issue though could be compounding one. No one could argue the less heat on the carbs the better though... 

Mike you dont need and really shouldn't change the spring tension in the domes. The proper way to richen the range (from the manual anyway) is to change the oil in the domes. Different viscosity will yield different a/f ratios.

Just looking at the pics of those su's I feel they need a rebuild or at least a refresh. Those ganky fuel lines could be interfering with the nozzle return, full open throttle could be off, the shafts could be leaking, the domes could not be moving freely through their entire range of movement, the floats could be set too high or low, they could be way out of sync or tune for that matter etc... There are quite a few things that will cause this issue. Thats l20b issues aside too. Get a good guide, I suggest the z therapy one, and go over them top to bottom. When your done they will run great. 

[72240z]

I shouldn't say it so hard line I guess. There are different springs. I should say it shouldn't be necessarily more like. The SU manual and z therapy says change the fluid but I know they do sell and people do use different spring tensions, esp on the track and usually always with a highly modified motor. 

[TFM1066]

You need to find out if your going lean or rich at the said RPM, more then likely rich, but you need to figure that out. To adjust top end rich or lean you need to make the needles longer (leaner) or shorter (richer)  their should be a allen screw on the slide to make this adjustment, @ .015-.020 at a time.

Also one HUGE item your missing is the base for the the air cleaner that has the throttle horn/bells on them, without these bell your giving up @ 20%-30% of clean stright air flow into the carbs, trying to tune without these is almost pointless.

[datzenmike]

IIRC... When you suddenly floor the gas and the damper oil is too thin the piston will rise too fast and lean out. Thickening the damper oil slows the rate that the piston rises when you suddenly floor it. It doesn't limit the piston travel, it just momentarily holds it back causing it to run rich and this will act like an accelerator pump on a regular carb. Damper oil is damper oil and 20W is recommended by Nissan. I don't think people vary the damper oil to set their a/f ratio.

[72240z]

I'm sorry to spam up the thread but thats completely wrong info regarding the needle. Once you index the needle you never touch it again. Never do you tune by adjusting the needle, ever. Too low and you bottom it out in the nozzle damaging it, too high and you'll always be rich. You raise and lower the nozzle to tune by way of the adjustment screw under the carb. 

Its easy to tell if its going lean or rich just drive for over a min in the problematic rpm range immediately pull over and shut the engine off. Once it cools down enough to take the plugs out read them to see what a/f ratio you have at that rpm and adjust accordingly. 

Not running at least velocity stacks is def hurting flow but I doubt thats the problem. Again though just like heat it could compound one.

Mike, its common practice. Track people especially in class spend hours working out the perfect oil to maintain the best ratio under certain conditions.
http://www.jetlink.net/~okayfine/su/damper.html#anchor74937
check out "advanced techniques". Though by no means any official instruction you can see what I am referring to. 

[wayno]

 

Yes those headers do have a heat shield Wayno but unless I'm mistaken his isnt a bare cast manifold. It has the stamped heat shield over it. I have run errands in 100+ degrees with stock cast manifold and no shield so I venture to say its not the main issue though could be compounding one. No one could argue the less heat on the carbs the better though...

 

 

______________________________________________________________________________________________________________________________________________________________________________

 

 

I am not saying that not having a stock datsun SU type heat shield is causing his high rpm issues, I am just saying he does not have one at all, they all came with them from the factory as far as I know, MGs and datsuns alike, but I have never seen a Volvo SU setup, but I would bet $10.00 that they have them also, they are there for a reason.

 

EDIT

Z therapy sells heat shields for a $100.00 without a core, I know it is spendy, and I am also sure that one can be found somewhere else for less, or one can be made from a thin flat piece of sheet metal.

[Boom death]

Look like I still got a lot of work to do to my set up lol it's not like it completely falls on its face at high rpms but u can feel its not consistent all the way thru the rpms buts it's definately better than my stock hitachi for sure ... And yes it does have a heat shield mounted to the bottom of the exhaust manifold or right underneath the bowls but I am looking to see we're I can buy a piece of aluminum wide enough for extra heat protection ... Now for filters or stacks what do u guys use??

[72240z]

I hacked up a stock housing and used a k&n filter. The housing has velocity stacks built in and the filter flows great. I never got a chance to make it look prettier before going to triples.

Home depot and lowes sells sheet steam and alum big enough to fab a simple additional shield. I think steal is a better shield then alum, doesnt alum conduct heat better?

[TFM1066]

Never say "never", so I suppose that the SU carbs came on a L-16 small bore small stroke motor and now there on a L-20b little bit bigger bore and much longer stroke, larger valves, and the fuel demand and curve is the exact same?? I beg to disagree.

[Sealik]

I bought a wide band to help tuning....also just use a garden hose to balance flow.

After the needle is "indexed" in the slide/dashpot...it should stay in that position, shoulder is flush with the base of slide

You can drop or lower the seat/jet which wil change the mixture relative to RPM....but....you're just changing the stages of the needle relative to the jet.

Size/taper of needle is the only way to properly change A/F ratios over the board.

If I was to hazard a guess...those carbs are running lean at WOT????

Heavier oil helps dampen idle dashpot 'float' (?)...ie....high duration cam, also slows dashpot on accleration....<...richer mixture.

Some people change springs....or run without any fluid.......?????.....all depends on app....not really conducive for stock/street. 

 

My jets are raised up to compensate for a rich idle.....yet... WOT is now 2 points off.

Basically................ I need different (profiled) needles.

Horns will increase air flow.............. but........... only 'noticeable' at a higher RPM.

 

I cut some horns out of a 240Z air cleaner

 

 

I have two 'heat shields'...one between IM and carbs and the other on top of the EM/which also shields and mounts the throttle cable.

 

 

 

 

http://www.zcarz.us/TechnicalInformationPageCarburation.htm

[72240z]

Never say "never", so I suppose that the SU carbs came on a L-16 small bore small stroke motor and now there on a L-20b little bit bigger bore and much longer stroke, larger valves, and the fuel demand and curve is the exact same?? I beg to disagree.

 

 

 

NEVER, you NEVER tune by moving the needle. It's flat out retarded for the reasons already mentioned. You index the needle, tighten the key way screw and never touch it again. Every manual every written is going to say the same thing. 

 

For different motors you just change the needle. Thats why there are many to choose from...

[Stoffregen Motorsports]

I cut some horns out of a 240Z air cleaner

 

 

 

 

Look at this pic closely and tell me what is wrong with the backing plates. The rear carb backing plate is upside down, the breather holes are on the bottom. Another common mistake. Be very careful not to plug the vent hole just above and to the side of the carb opening. Blocking this can really cause issues.

 

[Stoffregen Motorsports]

I thought I knew a lot about SU's, but then my father in law wanted me to rebuild his off his 240Z. He bought rebuild kits from Z Therapy and sent all the stuff to me. In the box was a DVD. I almost chucked it, but just for the hell of it I put it in the DVD player out in the shop and Man! That DVD is full of info and I'm glad I watched it.

 

If you've never had SU's apart, and are not one hell of a mechanic, spend the money and send them off to someone who knows what they are doing. There is a lot more going on there than you might tkhink and one wrong tweak or adjustment (like the needles) and your whole baseline is shot.

 

This is one more reason I like DGV Webers (on a 4cyl).

[Sealik]

 

 

Look at this pic closely and tell me what is wrong with the backing plates. The rear carb backing plate is upside down, the breather holse are on the bottom. Another common mistake. Be very careful not to plug the vent hole just above and to the side of the carb opening. Blocking this can really cause issues.

 

 

 

........................................................................................................................................................................

 

Good eye.......You are correct.....didn't notice until after I mounted the horns.

It has been rotated.........all is good

[Stoffregen Motorsports]

I just thought of one more thing. We are debating the tuning and setup of SU's, but after re-reading the original post, there may be something more going on than carburetion problems. Ignition? Cam Timing? Fuel flow? Have all these areas been addressed or inspected?

 

That high RPM hesitation could be a bad set of points or a bad coil.

[Boom death]

I bought a wide band to help tuning....also just use a garden hose to balance flow.

After the needle is "indexed" in the slide/dashpot...it should stay in that position, shoulder is flush with the base of slide

You can drop or lower the seat/jet which wil change the mixture relative to RPM....but....you're just changing the stages of the needle relative to the jet.

Size/taper of needle is the only way to properly change A/F ratios over the board.

If I was to hazard a guess...those carbs are running lean at WOT????

Heavier oil helps dampen idle dashpot 'float' (?)...ie....high duration cam, also slows dashpot on accleration....<...richer mixture.

Some people change springs....or run without any fluid.......?????.....all depends on app....not really conducive for stock/street.

 

My jets are raised up to compensate for a rich idle.....yet... WOT is now 2 points off.

Basically................ I need different (profiled) needles.

Horns will increase air flow.............. but........... only 'noticeable' at a higher RPM.

 

I cut some horns out of a 240Z air cleaner

 

 

I have two 'heat shields'...one between IM and carbs and the other on top of the EM/which also shields and mounts the throttle cable.

 

 

 

 

http://www.zcarz.us/TechnicalInformationPageCarburation.htm

I like that idea their is a 240z at my local junkyard that has an air cleaner gonna head out tomorrow to pick it up and make it fit on mines thanks for the ideas guys