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More SU tricks[140-160/any] posted by Topi on
Wednesday, 10 February 1999, at 11:35 a.m.
Good to see there's many SU gurus alive! I've been testing many ways to make these suckers to work on 1/2 and full race engines and here's few tricks: A. By using different viscosity motor oils you can adjust the mixture by dampening the piston travel(thick oil=lean) B. By not filling the damper up full you can enrichen the top end C. By shimming / grinding the plunger piston you can get an acc pump effect D. By grinding the piston ahead of the needle valve you can increase the air flow and change the fuel curve (look at any Japanese performance bike for the shape)E. By stacking 4 bakelite spacers between barbs and the manifold (plus longer studs)fuel won't boil = no vapor locks F. By drilling the needle jets you can feed 200+ HP (if you don't do that needle valves act like main jets!) G. By grinding the jets thinner you can adjust the mixture and mixture curve H. By stacking more cover gaskets you can adjust the float level (there's a limit how much you can bend the valve tab and they brake easily) Well, gurus out there, let me hear your tricks!
--"142s for ever"
TRS-USA
Re: More SU tricks[140-160/any] posted by Glenn Goodspeed on
Wednesday, 10 February 1999, at 12:58 p.m.
Excellent selection of tricks. I guess if I ever have trouble with the SUs, I might try some of them.
Actually, I have tried different viscosity oils in the dashpots, and I think I can add to Topi's synopsis of the effect. I believe using a more viscous oil will cause temporary enrichment of the mixture on acceleration, rather than making it leaner. The dashpot slows the progress of the piston as it opens, leaving a smaller opening for the air to flow through. Less air equals more fuel, given the same vacuum. At a constant engine speed, the dashpot oil will not affect the mixture.
One way to test this is to remove all oil from the dashpots, making the viscosity as thin as air. In this case, the mixture will be so lean on acceleration that the car will bog down, but when the engine reaches a constant speed, it will run fine. (I did this once by forgetting to top up the dashpots for a few weeks.) -Glenn.
Goodspeed's Volvo 1800 Newsletter
Re: More SU tricks[140-160/any] posted by Topi on
Wednesday, 10 February 1999, at 10:18 p.m.
Glenn, I think we both are right! You, if you got a slow reving engine and little overlap (=good vacum signal), and me if we're talking about racing engines (like I was). Don't forget; When you "increase the air flow" (opening the throttle) you are lifting the pistons AND the needle = More fuel. Untill the vacum piston raises there won't be more air even the throttle is full open! On race motor, piston will pop up, give a LOT of fuel BEFORE the air flow catches up = Richer mixture. you can actually see this. this condition will happen on stock motor as well if you open the bottom seat "10 - edges" - about 1.5 turn. Check it out! You hear all the time; "SUs are so simple", ain't so if you start tinkering with them. Weber DCOE is a piece of cake compared to this.
--"142s for ever"
TRS-USA
Re: More SU tricks[140-160/any] posted by Thilo on
Thursday, 11 February 1999, at 10:26 a.m.
Most of the tricks for the SU's are correct for the side draft
Zenith-Strombergs or Pierburgs (old Mercedes) as well. However for any kind of serious tuning SU's are far better as metering needles and parts are available in a broader variety.
Other accessories such as chrome air filters, ram pipes, polished housings, etc are more frequent available for SU's. Don't use HIF (Horitontal Internal Float) SU's; theses have been designed to meet US emission requirements and are not suitable for modified engines.
If you need more flow you should use a pair of 2" HS8 SU's, as used on factory Volvo rallye cars in the past. The inlet madifold needs to be widened however. The carbs can be found on old inline six Jaguars.
BTW:
Anyone got a PC-file containing SU needle charts?
The Saint
Re: More SU tricks[140-160/any] posted by meb on
Thursday, 11 February 1999, at 12:13 p.m.
To increase flow into the SU's I have:
1. Clipped the end of the split pins which holds the butterfly/disks in place.(Use Loctite to keep the screws from backing out.)
2. Tapered the butterfly/disks' leading and trailing edges to about 1/16". (They are normally about 1/8" thick.)
3. Used "stub stacks" in the K&N filters.
I have been running the carbs with the above mods for about 10 years. The next time I have the carbs apart, I plan on tapering the butterfly's spindle shaft and using countersunk screws to hold them in place. I may also remove half of the spindle(lengthwise).
meb