increase the main jet size by 1 or 2 numbers
383 Surge While Cruising
Hello, I have a 383 with 10.5 compression, comp cam xr288hr, 1.6 ratio rockers, afr 195s, Edelbrock Q-jet performer manifold, rebuild Q-jet that was done by Quadrajetpower, and a 4 speed. I have the timing at 26 at idle, 36 total and running 10” of vacuum advance connected to a manifold port on the carb. I have a MSD pro billet distributor and I’m using the lightest springs with stock weights for the advance curve. I currently use an Autolite 3924 spark plug. The car doesn’t hesitate even when at WOT and never runs on. The idle is set at 750 and it runs great but sometimes when I’m cruisng at let’s say about 2500 it’ll surge almost like I’m letting off the gas but if I give it more gas it’ll pick back up immediately. I was thinking about changing one of the lightweight springs to a medium spring but the timing isn’t advancing at idle I believe because on my other motor I wasn’t able to bring the idle down this low because they were so weak.
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Is it true that since all main rods have the same tip diameter, changing the jet will affect both WOT and cruise mixture. So if I just want to richen the cruise mixture I would need to change the rod and not the jet? Conversely, if I want to richen WOT but not cruise, I would increase the jet and the rod so that the area difference is the same at cruise?increase the main jet size by 1 or 2 numbers
for WOT I would concentrate on the secondary fuel as it's much greater.
if you are getting a lean surge, you are really lean, and probably need to richen the mixture everywhere a little. Get a wideband if you want to get into more precise tuning.
if you are getting a lean surge, you are really lean, and probably need to richen the mixture everywhere a little. Get a wideband if you want to get into more precise tuning.
Id look at the idle system first. If that doesn't improve it enough, richen the main system.
I spoke with quadrajetpower regarding the idle mixture screws not having an affect on the idle and they told me that this is normal with a large cam. I’m used to adjusting the idle mixture screws with a vacuum gauge and seeing the needle move to know where I’m at. At least that’s how I’ve done it with my holley’s. I had a little run on before but I richened the idle mixture a bit to get rid of that but I’ll try to richen the mixture up some more to see what happens.Id look at the idle system first. If that doesn't improve it enough, richen the main system.
So the carb currently has 43B primary metering rods in it after speaking with the carb builder and he recommends I go down a size so the next sizes are 42B and 39B but which pair do you recommend me purchasing? Like I said it doesn’t stumble when at WOT but you would still recommend going down a size or two in the main jets as well? He hasn’t told me the jet size yet but I would like to know so when I take it apart I can put the jets in at the same time.for WOT I would concentrate on the secondary fuel as it's much greater.
if you are getting a lean surge, you are really lean, and probably need to richen the mixture everywhere a little. Get a wideband if you want to get into more precise tuning.
Got a calculator?
Info here how to figure it out.
www.digitalcorvettes.com
Info here how to figure it out.
Quadrajet Info by Lars
With permission by the author. Technical Information Bulletin Rev F 4-5-05 Q-Jet Problems I have Seen by Lars Grimsrud Lafayette, CO Missing Fuel Filter Results in: Lack of a fuel filter will allow dirt and debris to get into the carb. This often unseats the needle, resulting...
www.digitalcorvettes.com
There is no substitute for an oxygen sensor/gauge. No guessing what your fuel delivery is doing.....
May have taken a bit longer , but before the advent of affordable A/F meters , we were able to get the job done .There is no substitute for an oxygen sensor/gauge. No guessing what your fuel delivery is doing.....
And how would you get the job done? Guess, make change, guess again, make change, etc., etc., etc.? Still never sure what A/F ratio you have at idle, light loads, heavy loads, and WOT...We aren't in yesteryear any more....and reading the plugs doesn't tell you anything but "average" over some period of timeMay have taken a bit longer , but before the advent of affordable A/F meters , we were able to get the job done .
A carburetor , no matter how refined , doesn't really benefit from the accuracy possible with an A /F meter . to get much closer than we got them " the Neanderthal way"! That would require constant adjustments ,like the computers do. To suit the sort of driving you do , the altitude you're at , what the weather , is among other things . A carb is at best ,a compromise ! Kinda like locating a tent post with a GPS ! BTW those " guesses" we make/ made , were pretty well educated , based on real world experience..
If your mixture screws do nothing then it sounds like your not idling on the idle circuit. It sounds like your throttle blades are opening the transfer slots. On a Qjet the idle circuit does not quit delivering fuel when you get into the main circuit (like a Holley does). Getting the idle circuit right is critical to having your carb work through all its operating range. Also Qjet carbs have the ability to raise or lower the power piston. You could raise the power piston and that will richen up your cruising mixture. Cliff Ruggles has an excellent book on Qjets, I suggest you start with that.
Oh good Lord, 2old2fast..... Whoever reads what you wrote will be rolling their eyes....your "doesn't really benefit" part is classic.... and by the way "the Neanderthal way" is understandable ... Maybe I'm the stupid one... is it not an ideal condition a 12 to 15 to 1 fuel air ratio the ideal target? Yes or No. Does an A/F sensor/gauge give you just THAT? Perhaps you should educate all of us radical idiots who read your response as to what your "Neanderthal way" is to choose/select the correct jets/rods/adjustments etc. to reach the closest target possible at idle, part throttle, part load, heavy load, and WOT ? Sure, altitude, weather, etc. affect things, but surely you aren't serious about that aspect of getting it right the first time........ I'll wait for your detailed response.
I spent days tuning QJ back in olden times, I still have boxes and boxes or parts and somewhere a Rochester parts manual with every carb number and every part for that carb. QJ are great carbs when sorted out, but the are very complex. GM used different main bodies with differing restrictors for engines of various CID and hp, you had to have a carb that was close to start with to ever end up with a good result.
TBI with feedback is how you make your life easy, and if you drive your car alot it's well worth the few extra hundred dollars you spend over a carb.
TBI with feedback is how you make your life easy, and if you drive your car alot it's well worth the few extra hundred dollars you spend over a carb.
You have your opinion , I have mine , both have been adequately stated. Further discussion is useless .Oh good Lord, 2old2fast..... Whoever reads what you wrote will be rolling their eyes....your "doesn't really benefit" part is classic.... and by the way "the Neanderthal way" is understandable ... Maybe I'm the stupid one... is it not an ideal condition a 12 to 15 to 1 fuel air ratio the ideal target? Yes or No. Does an A/F sensor/gauge give you just THAT? Perhaps you should educate all of us radical idiots who read your response as to what your "Neanderthal way" is to choose/select the correct jets/rods/adjustments etc. to reach the closest target possible at idle, part throttle, part load, heavy load, and WOT ? Sure, altitude, weather, etc. affect things, but surely you aren't serious about that aspect of getting it right the first time........ I'll wait for your detailed response.
Funny you mentioned the quadrajet book by cliff because I just spoke with him and he said the carb sounds like it is close and it’s not worth it to take it off the car and send it to him. So I richened the idle mixture screws up a bit so I’ll see how it does on the highway but I’m still thinking it’s lean because of the hard starts when cold.
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