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Thanks everyone.
I=E2=80=99m glad to know that we=E2=80=99re not alone.=20
The carbs were rebushed by Joe Curto last winter. Running red, yellow, blue a=
nd no springs. It happened in every case, with slight differences in final h=
eight depending on spring rate. Checked for vacuum leaks: we run the graph=
ite hi performance manifold gaskets. We did find a slight gouge in one of th=
e rear Manifold gasket faces. Dave did some harmonic calculations, treating t=
he manifold and carb like a recorder and found that there may be a high pres=
sure 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 harmonics t=
hat 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 to check the e=
ffect since, but that supports the idea that it might be due to a vacuum lea=
k.
Cranking compression is all within 10 psi of each other.
Using my last stash of 10W oil in the dampers.
Could not swap carbs due to linkage issues, but we did swap pistons. No chan=
ge: rear still dropped.
We did see an increase of about 0.5 in air/fuel ratio when the piston droppe=
d. That is because a fatter part of the needle closed down the fuel toroid, a=
s it should. That change in air/fuel ratio went away when we manually held t=
he piston open.
I have to agree that it does not seem to impact power much because the peak H=
P on the dyno was about the same whether we held the piston open or not. In=
tuitively that does not make sense, but there it is.....
But to return to the question, it does not seem related to the crank harmoni=
cs, I surmise after seeing all the responses.
For what it=E2=80=99s worth, we are running a Tilton Aluminum flywheel ( I f=
orget the Weight) with a TR6 pressure plate and three puck disc. As a left-o=
ver from =E2=80=9Cback in the day,=E2=80=9D I run a Corvair damper. It=E2=80=
=99s relatively massive with high inertia due to it=E2=80=99s diameter. It s=
eems to work well. Have had no fretting of the nose if the crank that we=E2=80=
=99ve seen with the small dampers available in kits today, or with none at a=
ll.
Sent from my iPhone
> On Aug 25, 2020, at 10:42 AM, Kas Kastner <kaskastner@gmail.com> wrote:
>=20
> =EF=BB=BF
> I encountered this magic move the carburetor piston many many times on the=
engine dyno. Could find no reason for the action and never did. It did no=
t seem to affect the power (you'd think it would) and was not always there. M=
y last thoughts about this wasa the cross over balance tube may have some ef=
fect. I'd stop that particular run, idle the engine a minute and start over a=
nd the event would NOT occur again. It was just plain weird to stand there w=
atching the pistons of the S.U's and all of a sudden one of them would slo=
wly drop down about halfway. My only suspect was harmonics. =20
> Never be beaten by equipment.
>=20
>=20
>=20
>> On Tue, Aug 25, 2020 at 4:50 AM Phil Gott via Fot <fot@autox.team.net> wr=
ote:
>> Can anyone help with this weird phenomenon? The discussion of the second h=
armonic being at 5200 rpm on the TR4 crank got me thinking:
>> On a chassis dyno we could watch the SU (HS 6) pistons during the dyno P=
ulls. We started each pull at 3000 rpm. The pistons went all the way to the t=
op almost immediately after going to full throttle. The front carb piston st=
ayed all the way up for the entire run. The piston in the rear carb DROPPED D=
OWN to about 3/4 open at around 5300 rpm and stayed there for the rest of th=
e pull. This repeated consistently.
>> As these depend on air flow via Bernouli=E2=80=99s law, I examined the va=
lve train thoroughly, fearing a stuck valve, problems with the valve train, a=
bad lifter etc. but everything looked fine. No noises from errant valve ope=
ration either.=20
>> The crank harmonic discussion got me thinking. Could it be something to d=
o 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=99=
s 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 situati=
ons to see what happens.
>> Any thoughts as to why the Piston in the rear carb only should drop at ab=
out 5300 and above?
>> Phil Gott
>>=20
>>=20
>> Sent from my iPhone
>> _______________________________________________
>> fot@autox.team.net
>>=20
>> http://www.fot-racing.com
>>=20
>> Donate: http://www.team.net/donate.html
>> Archive: http://autox.team.net/archive http://www.team.net/pipermail/fot
>> Unsubscribe/Manage: http://autox.team.net/mailman/options/fot/kaskastner@=
gmail.com
>>=20
>>=20
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<html><head><meta http-equiv=3D"content-type" content=3D"text/html; charset=3D=
utf-8"></head><body dir=3D"auto">Thanks everyone.<div>I=E2=80=99m glad to kn=
ow that we=E2=80=99re not alone. </div><div>The carbs were rebushed by J=
oe Curto last winter. Running red, yellow, blue and no springs. It happened i=
n every case, with slight differences in final height depending on spring ra=
te. Checked for vacuum leaks: we run the graphite hi performance mani=
fold gaskets. We did find a slight gouge in one of the rear Manifold gasket f=
aces. Dave did some harmonic calculations, treating the manifold and carb li=
ke a recorder and found that there may be a high pressure area under the two=
air holes of the piston at just about 5300 rpm. That slight gouge could hav=
e been contributing to some high pressure harmonics that would make the pist=
on drop, just like a recorder will squeak if each air hole is not closed com=
pletely. Have not been back on the dyno to check the effect since, but that s=
upports the idea that it might be due to a vacuum leak.</div><div>Cranking c=
ompression is all within 10 psi of each other.</div><div>Using my last stash=
of 10W oil in the dampers.</div><div>Could not swap carbs due to linkage is=
sues, but we did swap pistons. No change: rear still dropped.</div><div>We d=
id see an increase of about 0.5 in air/fuel ratio when the piston dropped. T=
hat is because a fatter part of the needle closed down the fuel toroid, as i=
t should. That change in air/fuel ratio went away when we manually hel=
d the piston open.<br>I have to agree that it does not seem to impact power m=
uch because the peak HP on the dyno was about the same whether we held the p=
iston open or not. Intuitively that does not make sense, but there it i=
s.....</div><div>But to return to the question, it does not seem related to t=
he crank harmonics, I surmise after seeing all the responses.</div><div>For w=
hat it=E2=80=99s worth, we are running a Tilton Aluminum flywheel ( I forget=
the Weight) with a TR6 pressure plate and three puck disc. As a left-over f=
rom =E2=80=9Cback in the day,=E2=80=9D I run a Corvair damper. It=E2=80=99s r=
elatively massive with high inertia due to it=E2=80=99s diameter. It seems t=
o work well. Have had no fretting of the nose if the crank that we=E2=80=99v=
e seen with the small dampers available in kits today, or with none at all.<=
/div><br><div dir=3D"ltr">Sent from my iPhone</div><div dir=3D"ltr"><br><blo=
ckquote type=3D"cite">On Aug 25, 2020, at 10:42 AM, Kas Kastner <kaskastn=
er@gmail.com> wrote:<br><br></blockquote></div><blockquote type=3D"cite">=
<div dir=3D"ltr">=EF=BB=BF<div dir=3D"ltr">I encountered this magic move the=
carburetor piston many many times on the engine dyno. Could=
find no reason for the action and never did. It did not seem to a=
ffect the power (you'd think it would) and was not always there. My last tho=
ughts about this wasa the cross over balance tube may have some effect. I'd s=
top that particular run, idle the engine a minute and start over and th=
e event would NOT occur again. It was just plain weird to stand there watchi=
ng the pistons of the S.U's and all of a sudden one of them woul=
d slowly drop down about halfway. My only suspect was harmonics. =
<br clear=3D"all"><div><div dir=3D"ltr" class=3D"gmail_signature" data-smart=
mail=3D"gmail_signature"><p><strong><font color=3D"#000099">Never be beaten b=
y equipment.</font></strong></p></div></div><br></div><br><div class=3D"gmai=
l_quote"><div dir=3D"ltr" class=3D"gmail_attr">On Tue, Aug 25, 2020 at 4:50 A=
M Phil Gott via Fot <<a href=3D"mailto:fot@autox.team.net";>fot@autox.team=
.net</a>> wrote:<br></div><blockquote class=3D"gmail_quote" style=3D"marg=
in:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex=
">Can anyone help with this weird phenomenon? The discussion of the second h=
armonic being at 5200 rpm on the TR4 crank got me thinking:<br>
On a chassis dyno we could watch the SU (HS 6) pistons during the dyno=
Pulls. We started each pull at 3000 rpm. The pistons went all the way to th=
e top almost immediately after going to full throttle. The front carb piston=
stayed all the way up for the entire run. The piston in the rear carb DROPP=
ED DOWN to about 3/4 open at around 5300 rpm and stayed there for the rest o=
f the pull. This repeated consistently.<br>
As these depend on air flow via Bernouli=E2=80=99s law, I examined the valve=
train thoroughly, fearing a stuck valve, problems with the valve train, a b=
ad lifter etc. but everything looked fine. No noises from errant valve opera=
tion either. <br>
The crank harmonic discussion got me thinking. Could it be something to do w=
ith vibration that would upset the air flow or cause the piston to drop? Not=
e 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 l=
atest video at Thompson. He and I are thinking about putting a go pro camera=
and a light under the hood to watch that carb in real life track situations=
to see what happens.<br>
Any thoughts as to why the Piston in the rear carb only should drop at about=
5300 and above?<br>
Phil Gott<br>
<br>
<br>
Sent from my iPhone<br>
_______________________________________________<br>
<a href=3D"mailto:fot@autox.team.net"; target=3D"_blank">fot@autox.team.net</=
a><br>
<br>
<a href=3D"http://www.fot-racing.com"; rel=3D"noreferrer" target=3D"_blank">h=
ttp://www.fot-racing.com</a><br>
<br>
t=3D"_blank">http://www.team.net/donate.html</a><br>
Archive: <a href=3D"http://autox.team.net/archive"; rel=3D"noreferrer" target=
=3D"_blank">http://autox.team.net/archive</a> <a href=3D"http://www.team.net=
/pipermail/fotUnsubscribe/Manage" rel=3D"noreferrer" target=3D"_blank">http:=
//www.team.net/pipermail/fot<br>
/kaskastner@gmail.com" rel=3D"noreferrer" target=3D"_blank">http://autox.tea=
m.net/mailman/options/fot/kaskastner@gmail.com</a><br>
<br>
<br>
</blockquote></div>
</div></blockquote></body></html>=
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_______________________________________________
fot@autox.team.net
http://www.fot-racing.com
Archive: http://autox.team.net/archive http://www.team.net/pipermail/fot
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