[Fot] Intake manifold vacuum and no high RPM

[chasgee22 at gmail.com]

Make sure the bowl vents are clear and working properly.  I don't know much about SUs, but how about float level?  Is it simply running out of gas the longer you drive it?

I've also seen this happen when a coil goes bad.  You might just switch out the coil first to see if things improve.

Good Luck!

Chuck

> On Nov 28, 2016, at 3:58 PM, Mike Harmuth <ofracer at gmail.com> wrote:
> 
> Thanks for the replies, here are the answers to some of the questions:
> 
> Valve springs, - Dual springs with alloy retainers. The engine builder tested and shimmed them to a uniform pressure but I don't know the number off the top of my head. I'll ask him tomorrow
> 
> Silver springs in the carbs, the lightest they make I believe. I have a set of heaver ones (red I believe) that I can try. The pistons are filled ~ 3/4 of the way,  up with SU branded oil and the dampers are in place.
> 
> The fuel pump, Holley red, all the lines, micron filter before the pump and the regulator were changed last winter because I didn't know how old they were. I had the problem then as well, before and after the replacement. I opened the cell up two weeks ago. The foam is in great shape, the pickups and intake line look to be solid as well. The ATL cell is 6 years old.
> 
> Fuel pressure is measured by a gauge at the regulator 2 feet from the carbs.
> 
> I'm not sure that the problem isn't electrical but the car runs with a alternator (confirmed working). The distributor was built with a profile to match my engine by Jeff at Advanced Distributors. I've changed the points (using his high speed set), rotor, cap, wires and capacitor over the summer, just in case, but with no affect. Timing is steady, double row timing chain.
> 
> thanks for all the info.
> 
> mike h
> 
>> On Mon, Nov 28, 2016 at 5:05 PM, Sam Halkias <atr6racer at hotmail.com> wrote:
>> What are you running for valve springs? Sounds like valve float to me.
>> 
>> Sam
>> 
>> Sent from my Verizon Wireless 4G LTE DROID
>> 
>> 
>> Mike Harmuth <ofracer at 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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