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Sounds like the open carb is the problem, check for sticktion and spring pr=
essure. The way the needles are designed the piston should rise slowly to i=
ts max point and stay there. the incorrect operation looks like the pistons=
opening to their max point instantaneously. Most race SU's function like t=
he latter and there is nothing wrong with that as long as the AFR is approp=
riate at all points, where the problem lies is when you can't get from your=
trailer to the pit late without having to rev the Sxxx out of it, that's t=
he classic example.
On a side note a motor can only use so much air and if your carbs are large=
r than necessary the pistons will find the place they like to live and that=
's what you get. So just because the pistons are not all the way up doesn't=
mean they are not working properly, it just means if you want the pistons =
up all the way you will need more displacement, haha.
Rick
"Doing the right thing is always the right thing, regardless of consequence=
s"
=20
On Tuesday, August 25, 2020, 12:14:18 PM EDT, Phil Gott via Fot <fot@au=
tox.team.net> wrote: =20
=20
Thanks everyone.I=E2=80=99m glad to know that we=E2=80=99re not alone.=C2=
=A0The carbs were rebushed by Joe Curto last winter. Running red, yellow, b=
lue and no springs. It happened in every case, with slight differences in f=
inal height depending on spring rate. =C2=A0 Checked for vacuum leaks: we r=
un the graphite hi performance manifold gaskets. We did find a slight gouge=
in one of the rear Manifold gasket faces. Dave did some harmonic calculati=
ons, treating the manifold and carb like a recorder and found that there ma=
y be a high pressure area under the two air holes of the piston at just abo=
ut 5300 rpm. That slight gouge could have been contributing to some high pr=
essure harmonics that would make the piston drop, just like a recorder will=
squeak if each air hole is not closed completely. Have not been back on th=
e dyno to check the effect since, but that supports the idea that it might =
be due to a vacuum leak.Cranking compression is all within 10 psi of each o=
ther.Using my last stash of 10W oil in the dampers.Could not swap carbs due=
to linkage issues, but we did swap pistons. No change: rear still dropped.=
We did see an increase of about 0.5 in air/fuel ratio when the piston dropp=
ed. That is because a fatter part of the needle closed down the fuel toroid=
, as it should. =C2=A0That change in air/fuel ratio went away when we manua=
lly held the piston open.
I have to agree that it does not seem to impact power much because the peak=
HP on the dyno was about the same whether we held the piston open or not. =
=C2=A0Intuitively that does not make sense, but there it is.....But to retu=
rn to the question, it does not seem related to the crank harmonics, I surm=
ise after seeing all the responses.For what it=E2=80=99s worth, we are runn=
ing a Tilton Aluminum flywheel ( I forget the Weight) with a TR6 pressure p=
late and three puck disc. As a left-over from =E2=80=9Cback in the day,=E2=
=80=9D I run a Corvair damper. It=E2=80=99s relatively massive with high in=
ertia due to it=E2=80=99s diameter. It seems to work well. Have had no fret=
ting of the nose if the crank that we=E2=80=99ve seen with the small damper=
s available in kits today, or with none at all.
Sent from my iPhone
On Aug 25, 2020, at 10:42 AM, Kas Kastner <kaskastner@gmail.com> wrote:
=EF=BB=BFI encountered this magic move the carburetor=C2=A0piston many many=
times=C2=A0on the engine dyno.=C2=A0 Could find no reason=C2=A0for the act=
ion and never did.=C2=A0 It did not seem to affect the power (you'd think i=
t would) and was not always there. My last thoughts about this wasa the cro=
ss over balance tube may have some effect. I'd stop that particular=C2=A0ru=
n, idle the engine a minute and start over and the event would NOT occur ag=
ain. It was just plain weird to stand there watching the pistons of the S.U=
's=C2=A0 =C2=A0and all of a sudden one of them would slowly drop down about=
halfway. My only suspect was harmonics.=C2=A0=C2=A0
Never be beaten by equipment.
On Tue, Aug 25, 2020 at 4:50 AM Phil Gott via Fot <fot@autox.team.net> wrot=
e:
Can anyone help with this weird phenomenon? The discussion of the second ha=
rmonic being at 5200 rpm on the TR4 crank got me thinking:
On a chassis dyno we could watch the SU (HS 6)=C2=A0 pistons during the dyn=
o Pulls. We started each pull at 3000 rpm. The pistons went all the way to =
the top almost immediately after going to full throttle. The front carb pis=
ton stayed all the way up for the entire run. The piston in the rear carb D=
ROPPED DOWN to about 3/4 open at around 5300 rpm and stayed there for the r=
est of the pull. This repeated consistently.
As these depend on air flow via Bernouli=E2=80=99s law, I examined the valv=
e train thoroughly, fearing a stuck valve, problems with the valve train, a=
bad lifter etc. but everything looked fine. No noises from errant valve op=
eration either.=20
The crank harmonic discussion got me thinking. Could it be something to do =
with vibration that would upset the air flow or cause the piston to drop? N=
ote that the piston was steady when it dropped. No apparent fluttering. I=
=E2=80=99m stumped.
The car runs very strong nonetheless. You see the results in Dave=E2=80=99s=
latest video at Thompson. He and I are thinking about putting a go pro cam=
era and a light under the hood to watch that carb in real life track situat=
ions to see what happens.
Any thoughts as to why the Piston in the rear carb only should drop at abou=
t 5300 and above?
Phil Gott
Sent from my iPhone
_______________________________________________
fot@autox.team.net
http://www.fot-racing.com
Archive: http://autox.team.net/archive http://www.team.net/pipermail/fot
ail.com
_______________________________________________
fot@autox.team.net
http://www.fot-racing.com
Archive: http://autox.team.net/archive http://www.team.net/pipermail/fot
tt.net
=20
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<html><head></head><body><div class=3D"ydp1188f210yahoo-style-wrap" style=
=3D"font-family:Helvetica Neue, Helvetica, Arial, sans-serif;font-size:16px=
;"><div><div dir=3D"ltr" data-setdir=3D"false">Sounds like the open carb is=
the problem, check for sticktion and spring pressure. The way the needles =
are designed the piston should rise slowly to its max point and stay there.=
the incorrect operation looks like the pistons opening to their max point =
instantaneously. Most race SU's function like the latter and there is nothi=
ng wrong with that as long as the AFR is appropriate at all points, where t=
he problem lies is when you can't get from your trailer to the pit late wit=
hout having to rev the Sxxx out of it, that's the classic example.</div><di=
v dir=3D"ltr" data-setdir=3D"false"><br></div><div dir=3D"ltr" data-setdir=
=3D"false">On a side note a motor can only use so much air and if your carb=
s are larger than necessary the pistons will find the place they like to li=
ve and that's what you get. So just because the pistons are not all the way=
up doesn't mean they are not working properly, it just means if you want t=
he pistons up all the way you will need more displacement, haha.</div><div =
dir=3D"ltr" data-setdir=3D"false"><br></div><div dir=3D"ltr" data-setdir=3D=
"false">Rick</div><div><br></div><div class=3D"ydp1188f210signature"><div s=
tyle=3D"font-family:Helvetica, Arial, sans-serif;font-size:16px;"><div dir=
=3D"ltr"><br></div><div dir=3D"ltr">"<i>Doing the right thing is always the=
right thing, regardless of consequences</i>"</div><div dir=3D"ltr"><br></d=
iv><div dir=3D"ltr"><br></div></div></div></div>
<div><br></div><div><br></div>
=20
</div><div id=3D"yahoo_quoted_8394273783" class=3D"yahoo_quoted">
<div style=3D"font-family:'Helvetica Neue', Helvetica, Arial, s=
ans-serif;font-size:13px;color:#26282a;">
=20
<div>
On Tuesday, August 25, 2020, 12:14:18 PM EDT, Phil Gott=
via Fot <fot@autox.team.net> wrote:
</div>
<div><br></div>
<div><br></div>
<div><div id=3D"yiv7751268599"><div>Thanks everyone.<div>I=
=E2=80=99m glad to know that we=E2=80=99re not alone. </div><div>The c=
arbs were rebushed by Joe Curto last winter. Running red, yellow, blue and =
no springs. It happened in every case, with slight differences in final hei=
ght depending on spring rate. Checked for vacuum leaks: we run the g=
raphite hi performance manifold gaskets. We did find a slight gouge in one =
of the rear Manifold gasket faces. Dave did some harmonic calculations, tre=
ating the manifold and carb like a recorder and found that there may be a h=
igh pressure area under the two air holes of the piston at just about 5300 =
rpm. That slight gouge could have been contributing to some high pressure h=
armonics that would make the piston drop, just like a recorder will squeak =
if each air hole is not closed completely. Have not been back on the dyno t=
o check the effect since, but that supports the idea that it might be due t=
o a vacuum leak.</div><div>Cranking compression is all within 10 psi of eac=
h other.</div><div>Using my last stash of 10W oil in the dampers.</div><div=
>Could not swap carbs due to linkage issues, but we did swap pistons. No ch=
ange: rear still dropped.</div><div>We did see an increase of about 0.5 in =
air/fuel ratio when the piston dropped. That is because a fatter part of th=
e needle closed down the fuel toroid, as it should. That change in ai=
r/fuel ratio went away when we manually held the piston open.<br>I have to =
agree that it does not seem to impact power much because the peak HP on the=
dyno was about the same whether we held the piston open or not. Intu=
itively that does not make sense, but there it is.....</div><div>But to ret=
urn to the question, it does not seem related to the crank harmonics, I sur=
mise after seeing all the responses.</div><div>For what it=E2=80=99s worth,=
we are running a Tilton Aluminum flywheel ( I forget the Weight) with a TR=
6 pressure plate and three puck disc. As a left-over from =E2=80=9Cback in =
the day,=E2=80=9D I run a Corvair damper. It=E2=80=99s relatively massive w=
ith high inertia due to it=E2=80=99s diameter. It seems to work well. Have =
had no fretting of the nose if the crank that we=E2=80=99ve seen with the s=
mall dampers available in kits today, or with none at all.</div><br><div di=
r=3D"ltr">Sent from my iPhone</div><div dir=3D"ltr"><br><blockquote type=3D=
"cite">On Aug 25, 2020, at 10:42 AM, Kas Kastner <kaskastner@gmail.com&g=
t; wrote:<br><br></blockquote></div><blockquote type=3D"cite"><div dir=3D"l=
tr">=EF=BB=BF<div dir=3D"ltr">I encountered this magic move the carburetor&=
nbsp;piston many many times on the engine dyno. Could find no re=
ason for the action and never did. It did not seem to affect the=
power (you'd think it would) and was not always there. My last thoughts ab=
out this wasa the cross over balance tube may have some effect. I'd stop th=
at particular run, idle the engine a minute and start over and the eve=
nt would NOT occur again. It was just plain weird to stand there watching t=
he pistons of the S.U's and all of a sudden one of them would s=
lowly drop down about halfway. My only suspect was harmonics. <b=
r clear=3D"all"><div><div dir=3D"ltr" class=3D"yiv7751268599gmail_signature=
"><p><strong><font color=3D"#000099">Never be beaten by equipment.</font></=
strong></p></div></div><br></div><br><div class=3D"yiv7751268599gmail_quote=
"><div dir=3D"ltr" class=3D"yiv7751268599gmail_attr">On Tue, Aug 25, 2020 a=
t 4:50 AM Phil Gott via Fot <<a rel=3D"nofollow" ymailto=3D"mailto:fot@a=
utox.team.net" target=3D"_blank" href=3D"mailto:fot@autox.team.net";>fot@aut=
ox.team.net</a>> wrote:<br></div><blockquote class=3D"yiv7751268599gmail=
_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204=
,204);padding-left:1ex;">Can anyone help with this weird phenomenon? The di=
scussion of the second harmonic being at 5200 rpm on the TR4 crank got me t=
hinking:<br>
On a chassis dyno we could watch the SU (HS 6) pistons during the dyn=
o Pulls. We started each pull at 3000 rpm. The pistons went all the way to =
the top almost immediately after going to full throttle. The front carb pis=
ton stayed all the way up for the entire run. The piston in the rear carb D=
ROPPED DOWN to about 3/4 open at around 5300 rpm and stayed there for the r=
est of the pull. This repeated consistently.<br>
As these depend on air flow via Bernouli=E2=80=99s law, I examined the valv=
e train thoroughly, fearing a stuck valve, problems with the valve train, a=
bad lifter etc. but everything looked fine. No noises from errant valve op=
eration either. <br>
The crank harmonic discussion got me thinking. Could it be something to do =
with vibration that would upset the air flow or cause the piston to drop? N=
ote that the piston was steady when it dropped. No apparent fluttering. I=
=E2=80=99m stumped.<br>
The car runs very strong nonetheless. You see the results in Dave=E2=80=99s=
latest video at Thompson. He and I are thinking about putting a go pro cam=
era and a light under the hood to watch that carb in real life track situat=
ions to see what happens.<br>
Any thoughts as to why the Piston in the rear carb only should drop at abou=
t 5300 and above?<br>
Phil Gott<br>
<br>
<br>
Sent from my iPhone<br>
_______________________________________________<br>
<a rel=3D"nofollow" ymailto=3D"mailto:fot@autox.team.net"; target=3D"_blank"=
href=3D"mailto:fot@autox.team.net";>fot@autox.team.net</a><br>
<br>
<a rel=3D"nofollow" target=3D"_blank" href=3D"http://www.fot-racing.com";>ht=
tp://www.fot-racing.com</a><br>
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onate.html">http://www.team.net/donate.html</a><br>
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//autox.team.net/mailman/options/fot/kaskastner@gmail.com">http://autox.tea=
m.net/mailman/options/fot/kaskastner@gmail.com</a><br>
<br>
<br>
</blockquote></div>
</div></blockquote></div></div>____________________________________________=
___<br><a ymailto=3D"mailto:fot@autox.team.net"; href=3D"mailto:fot@autox.te=
am.net">fot@autox.team.net</a><br><br><a href=3D"http://www.fot-racing.com"=
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