AFAIK, the timing isn't adjustable in the XFI. I don't have a steady vaccum above 8inHg. It bounces but as low as 5inHg.
Setting the timing was kinda confusing. We set it all according to XFI instructions and recommendations for this engine. No vaccum can on the dizzy. It's a MSD pro billet.
Wouldn't eliminating the vaccum reference to the regulator lean it out rather than richen?
No, the vacuum lowers fuel pressure, so removing it will increase fuel pressure, and make it more rich. The vacuum is applied on the same side of the diaphram as the spring, so the vacuum actually helps compress the spring and reduce the pressure. My engine on the gauge used to makes about 12 inHg of vacuum at idle, but according to the MAP with the EFI now, it makes about 15 inHg and it lowers my fuel pressure from 50 psi down to about 42 or so.
If you only have 5" of vacuum at times, then I would definitely put the included bushing in that MSD distributor to "lock" the timing (eliminate the mechanical advance). What MSD box do you have 6AL, Digital 6+, 6AL2? If it has a start retard (Digital 6+ does, maybe 6AL2 does as well?), enable it. If not, then use a switch (grounding the white wire on the 6AL will do it) to enable you to shut the spark off to crank the engine easier if everything is hot. With that cam and your setup, the only reason you need a timnig retard is to start it up. What stall converter do you have?
With no vacuum advance, and only mechanical, your engine has very low timing at idle, your cam probably doesn't like that. Locking the distributor will give you a lot of that timing back, it will be more responsive and idle smoother, actually giving you a more stable idle, you will probably have to turn the idle down some actually to get back to where you were. It just may crank harder, but you can fix that, may not even be a problem.
Also, with only 5" of vacuum, I hope you have hydroboost brakes?
i no expert. but if vac is bouncing at idle then i bet the fuel pump dont have a clue what to think.
mabye do a storage tank of sorts to filter the vac signal and try that. kind of like a brake booster storage bottle for big cams.
That's a good thought, but you don't want to do that with the regulator. Because the storage tanks usually work with a check valve to store the vacuum when the engine loses it. However, if you hook the regulator up to vacuum, then when you floor the engine, vacuum is supposed to drop, and increase fuel pressure. However, the storage tank would still supply vacuum to the regulator, keeping fuel pressure lower, causing the engine to go lean at WOT, not good. You want the regulator to closely follow the engine load.
You sure you don't mean a fuel pressure regulator?
