Recently, my 1986 F150 with 4.9L and duraspark conversion has been pinging with less than moderate throttle on various gasoline brands, so I decided to adjust the timing [retard]. After plugging the disconnected distributor vacuum line and connecting a vacuum gauge to a manifold port, the vacuum was 21" Hg. I tested maximum vacuum and obtained 21.5". I believe our elevation here is in the 400 ft range [north Alabama]. I retarded the distributor and lowered the vacuum reading to 19.5" and then had to adjust the idle speed. The "normal" idle speed, for me, is 750 RPM, but after retarding the timing, the speed dropped to less than 500 RPM [by the way, this truck has a manual transmission]. The test drive revealed no pinging at all. Soon, I will split the difference and adjust to 20 or 20.5" and retest, so I can find the "sweet spot". It sure was nice to be able to accelerate and NOT hear pinging!

By the way, on the carburetor I have, there are three vacuum ports on the passenger side, as mounted. They are lined up in sort of a wide "V". The one closest to the front of the engine is where my vacuum advance is attached. I decided to test each port and this is what I found: The front one, has no vacuum at idle and when one blips the throttle, the gauge barely moves and seems to significantly lag behind throttle opening [it may have shown 2-5" Hg with a quick blip]. The rear most port [at top of "V"], also has zero vacuum at idle, but quickly responds to throttle opening with a very wide swing. The middle port [bottom of "V"] has ~5" Hg at idle and does not seem to vary when the throttle is quickly opened.

As an experiment, I connected the distributor vacuum line to the rear most port and tried to drive. The truck bucked abruptly any time the throttle was applied from low speed. Apparantly, the vacuum is applied too quickly.

Just sharing as food for thought. Have a good day!

 

I appreciate this post, and if anyone can provide detailed steps or links to instructions for vacuum/timing/tuning, I would appreciate it (mine's an '81 4.9). After tracking down a bunch of minor problems, I'm ready to move on to tuning this thing right. I get a little pinging/knocking and I'm sure it's timing related. My idle is also uneven and misses just a little. I plan to replace the voltage regulator by next weekend do to some gauge/light flickering/dimming. One thing I notice right now is that my RPMs are low where I want them to be after I start, but after driving around a while, it idles closer to 1000 (much higher than I want). If I shut the truck off and restart, the idle drops back down closer to what I want. Perfectionist that I am, I want to get this rebuilt motor sounding purring like a kitten.

 

Reading your vacuum gauge: Reading your vacuum gauge

 

Thank you for that! People on this site have the coolest facts, advice, and tips.

 

1986- thanks for posting your findings!

I have found that by setting the straight (disconnected vacuum) timing at 8 I get the best overall tune. Then I tune by max vacuum on the gauge and reconnect everything.

 

Hi!
I apologize because I need help for my '77 Ford Maverick ( I Know it is not a truck) but it was hit by one truck 1 year ago, due to the crash all the plastic hoses and wires in the engine compartment were burned and melt, now after a year of weekend after weekend rebuilding it, the car finally started but if I try to accelerate fast, the engine stop and I am think that the vacuum hoses are not connected where must be or the carburetor (Carter YF) is getting excess of air because if I close the top plate and only let a 1/4" wide gap, the car can run well but with very bad fuel economy, may somebody help me?

thanks in advance
pegasus867

 

you'd be better off starting a new thread-
but yes, it does sound like you may have a vacuum leak.
re-read the first post in this thread- if you have the same carb, all the ports that the original poster is talking about must be connected to something or simply plugged.

 

if it was only pinging at less-than-moderate-throttle, why not just dial back on the vacuum advance, instead of pulling your initial setting down?

 

I always set the timing "by ear"....

 

i hear this one a lot (along with "time by vacuum").
that'll work, as long as you're only concerned with the initial timing...

 

X2 on what rusty said. I use vacuum for initial timing and carb adjustment...as a starting point in the tuning process.