View Full Version : Need help: vacuum low on a 302 HO

John C in PA
09-12-2012, 06:50 PM
I hooked up my vacuum guage today on the Donzi (vacuum fitting in the manifold under the STB side of the carb) to check the mixture screws. I was able to improve the vacuum a little but the highest reading I get is 15". (Good for women but not for engines :embarasse). I checked the timing and the pointer lines right up with the mark on the balancer. Should be 10 degree BTDC per the manual.

I loosened the hold down bolt and rotated the distributer (electronic) to advance the spark and the rpm climbed and the motor sounded better. The vacuum went up to the green zone (19"). Is 15" OK (again for boats, we already discussed women)? Is there something I'm missing? Nothing has been changed this season.

I'm going out Thursday trying to get in as many days before it gets cold so any ideas will be appreciated.

John C

John C in PA
09-13-2012, 09:20 AM
Thanks Fixx. Yep, I have the Holley 500 CFM 2 BBL. Why shouldn't I be using a vacuum guage to set the mixture screws?

John C

John C in PA
09-13-2012, 08:32 PM
Fixx, you're correct. I noticed a slight stumble when I nail the throttle but I thought it may be the 50 cc accelerator pump I recently added. So 3/4 turn out max from full in? OK, will do when I'm out burning up $4.10 a gallon fuel tomorrow. I appreciate the detailed reply!

John C

09-13-2012, 09:41 PM
the fact that this engine is in a boat is irrelevant, the engine doesn't know that...

a vacuum gauge can be your very best friend.... IF you use it wisely, and observe what it's telling you, kinda like a stethoscope

15" at idle isn't unusual, off idle at 1500 rpm it may see 19-20" or so, thats normal

MOST important is that the gauge is rock steady in it's operation, any fluctuation could be a poor sealing intake valve, or vacuum leak, or tight valve/lifter adjustment...

you can also use it to set the timing.... :thumbsup: and the best all-around fuel mixture

I prefer to take the reading in the carb or throttle body base, as close to the throttle blade as possible...

John C in PA
09-13-2012, 10:02 PM
Buiz, I've always used a vacuum guage to set idle mixture but most of those were from the days of street rods and the idle speed was a lot higher than 600 RPM where my boat runs. The port in the manifold just below the carb mounting surface is the only vacuum port I can find.

I have the mixture screws set using the vacuum guage but Fixx mentioned a problem I have (off idle stumble). I have all day to play on the lake tomorrow so I can try all recommendations.

My timing light is at least 40 years old and it doesn't have an advance checking capability. Although I may spring for a new one on the way out tomorrow how do you set the timing with a vacuum guage without blowing up the motor?)?

Good replies all, John C

09-14-2012, 11:24 AM
Advance her until it pings, then retard a 1/4", repeat. You don't need no stinkin light ;) Actually that works on cars, not sure if too far advanced timing would ingest water or not...

John C in PA
09-14-2012, 12:07 PM
biffiefl, that's what we called power timing on the street rods. With a stock mild cam I don't think reversion is likely. I'm not thrilled running the motor up to whatever RPM the advance stops at while on the hose in the driveway, tho.

John C