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Mike,
Sure sounds very similar to tuning issues with my dad's small journal.
Make your adjustments so that the mixture is optimum at racing RPM (5k plus
rpm). 13.5 - 14.5 is still too lean for power, that's a good economy
cruising mixture on the street.
Pay no attention to mixture at lower RPM - doesn't matter in a race car.
You might have to change needle profiles also.
You don't want much vacuum in the intake manifold at wide open throttle.
It's the velocity of the airflow thru the bridge in the carb that creates
the vacuum to lift the dampers, not the vacuum in the manifold runners.
Jason
On Mon, Nov 28, 2016 at 4:23 PM, Mike Harmuth <ofracer@gmail.com> wrote:
> I'm tracking down a problem with the Spit race car (1296 Small Journal,
> 12;1 compression, APT TR74 cam 295 duration and 0.324 lob lift, head ported
> as per Kas's book). Rebuilt European SU HS4 carb setup with AAA needles
> (manifold and carbs checked for vacuum leaks), Advanced Distributor for
> spark (34 degrees at full, mechanical only, advance @ 3.5K RPM)
>
> The problem I have has followed me on two different short blocks, I can
> easily pull 7500RPM in 1st-3rd ( Close ratio TR7 gearing in a Spitfire
> case, 4th gear is 1;1, 4:11 locked diff ) gears. In 4th I'm struggling to
> get over 6K, no matter how long the straight. Just before the last race of
> the season, I installed an air fuel ratio gauge on the header (stahl) and
> found out I'm running (progressively) very lean (19:1) in 4th over 5K. At
> 2K I adjusted it rich, to 12;1 and it keeps in the 13.5- 14.5 range until I
> get over 5K when it really leans out. If I pull the choke to 1/2 full over
> 5K, I get a good ratio again, 13.5- 14%.
>
> To me that seemed like the carb pistons aren't pulling all the way up so I
> drilled and tapped the manifold to get a vacuum reading. It was steady but
> low at 6K, ~20 Inches but would jump to ~28 briefly as the revs dropped if
> I blipped the throttle. Under 6K, it was in the 15 inch range. Normal air
> filters are K&N with stub stacks but I get the same results with open carbs.
>
> Am I correct in assuming that the vacuum is too low or is it that due to
> overlap with a performance cam? Any ideas on where to look to get the ratio
> "normal" at higher revs? Fuel pump is Holley Red, adjusted to 3 PSI at the
> engine compartment and steady through all revs.
>
> I have dyno time scheduled in a few weeks but I'd like to use that time to
> tune not debug things.
>
> thanks
> mike h
> 63 spitfire
>
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>
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<div dir=3D"ltr">Mike,<div>=C2=A0 =C2=A0 Sure sounds very similar to tuning=
issues with my dad's small journal.=C2=A0 Make your adjustments so tha=
t the mixture is optimum at racing RPM (5k plus rpm). =C2=A013.5 - 14.5 is =
still too lean for power, that's a good economy cruising mixture on the=
street.</div><div>Pay no attention to mixture at lower RPM - doesn't m=
atter in a race car.=C2=A0 You might have to change needle profiles also.</=
div><div>You don't want much vacuum in the intake manifold at wide open=
throttle.=C2=A0 It's the velocity of the airflow thru the bridge in th=
e carb that creates the vacuum to lift the dampers, not the vacuum in the m=
anifold runners.</div><div><br></div><div>Jason</div></div><div class=3D"gm=
ail_extra"><br><div class=3D"gmail_quote">On Mon, Nov 28, 2016 at 4:23 PM, =
Mike Harmuth <span dir=3D"ltr"><<a href=3D"mailto:ofracer@gmail.com"; tar=
get=3D"_blank">ofracer@gmail.com</a>></span> wrote:<br><blockquote class=
=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padd=
ing-left:1ex"><div dir=3D"ltr"><div><div><div><div><div><div>I'm tracki=
ng down a problem with the Spit race car (1296 Small Journal, 12;1 compress=
ion, APT TR74 cam 295 duration and 0.324 lob lift, head ported as per Kas&#=
39;s book). Rebuilt European SU HS4 carb setup with AAA needles (manifold a=
nd carbs checked for vacuum leaks), Advanced Distributor for spark (34 degr=
ees at full, mechanical only, advance @ 3.5K RPM)<br><br></div>The problem =
I have has followed me on two different short blocks, I can easily pull 750=
0RPM in 1st-3rd ( Close ratio TR7 gearing in a Spitfire case, 4th gear is 1=
;1,=C2=A0 4:11 locked diff ) gears. In 4th I'm struggling to get over 6=
K, no matter how long the straight. Just before the last race of the season=
, I installed an air fuel ratio gauge on the header (stahl) and found out I=
'm running (progressively)=C2=A0 very lean (19:1) in 4th over 5K. At 2K=
I adjusted it rich, to 12;1 and it keeps in the 13.5- 14.5 range until I g=
et over 5K when it really leans out. If I pull the choke to 1/2=C2=A0 full =
over 5K, I get a good ratio again, 13.5- 14%. <br><br>To me that seemed lik=
e the carb pistons aren't pulling all the way up so I drilled and tappe=
d the manifold to get a vacuum reading. It was steady but low at 6K, ~20 In=
ches but would jump to ~28 briefly as the revs dropped if I blipped the thr=
ottle. Under 6K, it was in the 15 inch range. Normal air filters are K&=
N with stub stacks but I get the same results with open carbs.<br><br></div=
>Am I correct in assuming that the vacuum is too low or is it that due to o=
verlap with a performance cam? Any ideas on where to look to get the ratio =
"normal" at higher revs? Fuel pump is Holley Red, adjusted to 3 P=
SI at the engine compartment and steady through all revs.<br><br></div>I ha=
ve dyno time scheduled in a few weeks but I'd like to use that time to =
tune not debug things.<br><br></div>thanks<br></div>mike h<br></div>63 spit=
fire<br></div>
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<br></blockquote></div><br></div>
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