vacuum gauge reading
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vacuum gauge reading
I have about 600 miles on the rebuilt 400 now... bored 40, flat tops, stock heads and normal mods discussed here... still haven't found that exact timing sweet spot and still have some high temp problems... I started just checking out plug condition and decided to check vacuum readings... I plugged in on the full port of my edelbrock carb and found the needle bouncing very rapidly between 1" to 3" in between 15 and 20.... The slightest bit of throttle and it quits... I installed a Comp 255DEH cam.... anybody have any ideas?? my resources say bad valve guides, but the heads were rebuilt??? Could the dual patter cam give that type of reading at idle???
Thanks
Thanks
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From: Franklin, GA
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From: chicago burbs
aftermarket cams will give "late valve timing" or "sticking valve" readings and such. at the top of the page in the bigpond link they suggest pinching the hose to the gauge to smooth out the reading, i wonder if this helps you're reading? my engine also varies about 3" between 4"-7" at idle. i don't suspect any problems in my engine, and from memory if i raise the the idle speed slightly the vacuum comes up and evens out. i would attribute it to the cam, if the needle was still bouncy with a tad of throttle i'd worry. can you take a reading directly off the intake manifold?
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thanks guys... you have set my mind at ease... and to think I actually half guessed that myself... there may be hope yet
now onto the mechanical advance that takes forever to wind out... I was at 3500 RPM or so and it still acted like it wanted advance more... I am kind of skiddish standing in front of the truck, timing light in one hand, pushing on the carb linkage and checking between the timing marks and multimeter.. I'd hate to see her pop into gear at 3500 RPM and me in the way... I think too much about those situations I guess.
now onto the mechanical advance that takes forever to wind out... I was at 3500 RPM or so and it still acted like it wanted advance more... I am kind of skiddish standing in front of the truck, timing light in one hand, pushing on the carb linkage and checking between the timing marks and multimeter.. I'd hate to see her pop into gear at 3500 RPM and me in the way... I think too much about those situations I guess.
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A wavering reading is different than a tick-tick-tick type reading. Check out that website that was posted. A wavering reading just means you have some more carb tuning to do. A tick-tick-tick type reading is a mechanical problem. Usually a bad valve. The intake valve will make a bigger jump than an exhaust. It takes a pretty big cam to make a jumpy vac reading. Your 255DEH should be smooth and steady.
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From: Franklin, GA
All credit goes to Torque1st for the posted site! It came off of his personal list of sites. If it wasn't for Torque1st, Mil1ion and Franklin2, most of us would be lying in the floor kickin' and screaming over these trucks! It would do y'all a lot of good to read the sticky sections at the begining of the sections. These guys have provided a wealth of info right at our fingertips, free for the browzing. We should at the very least, take advantage of the oportunity to use what they have provided.
What's that? Do I hear violins in the background? [tender moment] I think I'll go cry now. Goodnight!
What's that? Do I hear violins in the background? [tender moment] I think I'll go cry now. Goodnight!
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Thanks, now I am crying... -hehe
I have hundreds of sites in my favorites list that I haven't added to that list also. Sites for materials and more info, all kinds of stuff. That list needs some organizing also. I just need to use a good editor on it. I have a huge text file full of snippets of information gleaned from various sources and FTE users (posts) here over the years also. One of these days I will publish it after it is cleaned up and organized a little better.
I have hundreds of sites in my favorites list that I haven't added to that list also. Sites for materials and more info, all kinds of stuff. That list needs some organizing also. I just need to use a good editor on it. I have a huge text file full of snippets of information gleaned from various sources and FTE users (posts) here over the years also. One of these days I will publish it after it is cleaned up and organized a little better.
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I did have that website in my favorites... looks like my resources need organized as well... it is pretty bad when you go to save a site to your favorites, thinking you really found something, and then you get the "overwrite?" message...
Again, thanks for the insight... I think my condition is tuning related as the needle bounce is very rapid and very slight in range... I'll probably fool with the idle mixture and see what results I get...
Again, thanks for the insight... I think my condition is tuning related as the needle bounce is very rapid and very slight in range... I'll probably fool with the idle mixture and see what results I get...
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well I checked my carb idle mixture and I was a little on the lean side... My vac gauge is still twitching but quits with a pinch of the hose... the reading went up 1.5" (17 to 18.5) after the carb adjustment... I don't have any reason to think anything is wrong other than this is my first rebuild and so far I can't believe it is running so well... I thought spending some time tuning might address my running at higher temps and search for that perfect timing spot... After the carb adjustment, it feels like Icould advance a tad more now...
What are your thoughts on the mechanical advance in regards to reaching total advance in a given RPM range... checking total timing it looked like I would have to go around 4000 RPM before it stopped... I read that 2500 RPM is better...
What are your thoughts on the mechanical advance in regards to reaching total advance in a given RPM range... checking total timing it looked like I would have to go around 4000 RPM before it stopped... I read that 2500 RPM is better...
Posting Legend
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Mech advance should reach full advance at 4000 RPM's on the engine. Some sources will list distributor numbers which you must double to get engine numbers.
If I knew what the part number was for a 1971 distributor on a 400 I could post full distributor specs. Anybody have a parts microfiche for a 1971 LTD? Should be a D1AF-??
If I knew what the part number was for a 1971 distributor on a 400 I could post full distributor specs. Anybody have a parts microfiche for a 1971 LTD? Should be a D1AF-??