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Discussion Starter · #1 ·
Question on vacuum while carb tuning. 351W 385HP carb FRPP crate engine. I was expecting to see vacuum readings about 15-20 and it seems like about the highest mine wants to go is about 14. Is this unusual or possibly ok? Rookie here... Thanks.
 

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That's about all you're gonna get with that motor. You might be able to squeeze 14 1/2" if you really work at it.
Frank
 

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If you advance the timing a little it will also raise the vacuum but make sure you don't get so high that you have detonation. In other words - follow the mfgrs recommendation on timing and go no more than 2 degrees further advance with premium fuel.
 

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Discussion Starter · #5 ·
Thanks guys, big help. I'll spend a little time seeing if I can get it a bit higher and a little steadier. It does seem to start, idle and run better seeing as how I spent some time on the timing, float levels, and mixture settings this weekend. As always, thank you.
 

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Jeff cam duration play a major part in manifold vacum. Not unual to see 14 with a looby cam. More importand that the vacum is steady as shown by the needle. What your seeing as you watch vacum gauge while adjusting mixture is similar to what you see if watching the tach while doing the mixture adjustment,just a bit more accurate. It's showing you when the engine reach's it's highest steady idle speed. If you watch the vacum gauge you'll notice if the mixture screw is turned far enough to cause engine idle to slow or stumble,vacum drops or needle could flicker. You'll notice same looking at tach.As carb is adjusted closer for correct mixture idle speed picks up. Increase in idle shows as higher vacum reading.Smotther idle makes needle more steady since vacum gauge will show misfires from being too lean or fat. Same happens as timing is adjusted. Advanced equals higher vacum reading. Trouble is if setting timeing by highest vacum reading you'll end up somewhere around 30* initial timing,but vacum reading should be noted as timing is set for your base line refrence for the tune. I like to use both at same time when setting mixture, and both should be steady needle when right. Takes time to get engine in perfect tune. Once carb is close,time to look at initial timeing and total along with rate of advance.Once distributor curve is dailed in back to tuneing jetting and fuel curve in carb again. Great way to kill a weekend and keep from cutting the grass. :D
 
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