Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
1903 Won't Respond to Tip-In Procedure
Cliff Ruggles:
If I supplied the advance can you woln't need another one. You do NOT need a really early starting point unless you plan on running manifold vacuum to it and your engine isn't making much vacuum at idle speed. I consider that move more of a "crutch" anyhow, vs correctly tuning the carbs idle system and ideal initial timing in it right to start with.
You can "adjust" the amount added by changing the distance between the rod and the stop, or making the window it rides in longer or shorter.
Stock it's around 14-15 degrees, or should be close to that. Use a timing light to determine exactly how much it adds, hook up and unhook manifold vacuum to it or use a vacuum pump and watch the timing with a timing light.
Use drill bits as measuring tools when making changes.
The cold intake is part of your issues. They require complete "heat soak" before things will work well and why you saw minor issues on cold mornings.
A wet flow system needs a hot intake and why dating clear back to as far as I can remember EVERYTHING I've ever worked on, old cars, tractors, etc used exhaust heat to warm the intake for improved thermal efficiency and able to run leaner mixtures, etc.
Not really a deal breaker, just be aware that ALL tuning needs to be done with the engine fully warmed up and intake fully heat soaked. Things will also get better in hot weather.
You may read some conflicting information on that subject as there as still folks out there on various Forums recommending to block heat crossovers and run cold intakes, etc. Heat is your friend with these things, and at most you might see a very slight improvement if drag racing and running a dead cold intake but you will need to "flood" the engine with fuel and pour the timing to it to see any improvement. Even then it is difficult if not near IMPOSSIBLE to continue to run (race) that way IF you win and get into later rounds as the staff at the track will be trying to "hot-lap" you back to staging so they can get finished up and home at a decent hour. I've lost count of how many times I've beat up on much faster and really well prepared "race" cars (I race my car fully warmed up and heat soaked every round) with my daily driver in final rounds even though they had all their friends and beer drinking buddies pushing the car in staging, dry ice and even bug sprayers out trying to cool the engine back down to where it was on their last pass.....hint, hint.
continued....
Cliff Ruggles:
Anyhow, for street driven vehicles a working heat crossover is your friend, unless you live on the equator and it seldom drops much below 90 degrees outside....FWIW.
For timing I have found over the years that about 8-12 initial, 10-12 from the mechanical advance (20-24 at the crank) and about 10-15 from the vacuum unit are good numbers to work with.
Of course IF you have made poor choices for CID vs COMPRESSION vs CAM SPECS you may need to add MORE timing in a lot of places to make things happy. For well thought out street and street/strip engine builds the numbers above will typically get you there.
I do NOT like, use or advocate super-quick mechanical advance curves and a good engine build woln't need them anyhow. The advance curve (mechanical) should start right off idle, or about 900-1000rpm's and steadily increase to full advance around 2800-3200 rpms or so. No need to bring it in any sooner and ALWAYS use the vacuum advance. It is a load sensing device and required to improve engine efficiency, burn leaner mixtures, improve throttle response, fuel economy etc in the "normal" driving range.
Anyone who tells you to ditch the VA and just install a POS spring weight kit to get all the timing in right off idle needs to keep their day jobs.....IMHO.......Cliff
bry593:
Rigged a vacuum gauge into the cab before I went on my trip this morning. Cruising down the turnpike at 75mph, here are my vac readings in in Hg:
22 - This is higher than idle (19.5), but only happens coasting or going downhill....
18-22 - It has to be pretty flat and not much wind. Very little time is actually spent in this range.
14-18 - It stays about 16 unless the converter clutch drops out. This is where the majority of cruise happens. Need the vac advance to be in at this range.
10-14 - Converter clutch dropout, hills, headwinds and slight accelerations . This is probably a 1/3 of cruise time.
6-10 - Leaving stop lights, medium throttle
Based on the above, a vac advnace that starts around 8-10 and is all in by 14-16 is probably about right.
From my previous two selections:
VC1831 / AR7 6-8, 12@14-16
VC1828 / AR4 6-9, 10@12-14
Both look pretty good for this application. But first, I'll check the stamping on Cliff's advance and see if it is close.
Edit: I went ahead and ordered a VC1831 / VC217.
Cliff Ruggles:
I didn't get a chance to check one mine today in the shop but it's going to start around 8-9" and all in around 11-12 if memory serves me correctly.
If you didn't notice the vast majority of the time your engine is making enough vacuum to both apply the vacuum advance and the power piston would be DOWN no matter what power piston spring you'd have in it.
That's the way it's supposed to work, and what I've seen with all the street engines I've had in my cars over the years.
The "moderate" accl from a stoplight will vary the vacuum readings a LOT depending on how heavy right foot is. For "normal" driving those vacuum numbers should be pretty high......
bry593:
I put the pedal to the metal at a rolling 30mph and had ping in the primaries. It did not do this when the float was flooding and excessively rich. Probably needs more fuel and current calibration is 74 jets with 50C rods. Which way should I go, 75 jet or 49 rod? Probably easiest to find a 75 jet.
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