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tuning help please

2K views 20 replies 5 participants last post by  Pupsvette76 
#1 ·
ok heres the lowdown
i have a sbc 350 with a weiand 177 supercharger on it
it was running really well about 3 weeks ago i dont drive it everyday
but it started to feel like it had a little miss in it so i changed out the plugs and tried a different brand plug and gapped it down a little from .035 to .030
the little miss is still there i know it sounds stupid but i changed the plugs again to another brand gapped at .035 again just to see if it was the gapped that make me keep fouling plugs
ok the car has a A/F gauge with an oxygen sensor on bank 2 so i kinda use it to tell me where to set my idle mixture screws but i tried leaning idle out a little to fix the problem but it still smells rich
here is where i am really confused i used the proper method of setting idle mixture screw... with a vacuum gauge but when i do this the best vacuum reading seems to be on the lean side and when i punch it with the carb set this way i get a backfire so i put a little more timing in it my initial is 17 but i have to keep my total under 34 p.s. i do not have vacuum adv ... same thing happened i could make it rich again but that would lower my vac readings foul my plugs and make the car smell really bad
i already smell like gas if i take the thing out for any amount of time please help
 
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#3 ·
Did you try leaning out the idle mixture screws combined with larger pump shooters to cover the leaner mixture with a bigger shot of fuel?? This will provide a leaner idle and prevent the backfire from happening, the backfire is a lean hole in the fuel curve. You need to prevent it with the correct shooter, not by overrichening the idle.

If a miss just shows up and the related parts of the spark system are ok(plugs, wires, cap/rotor), I might be taking a look at the valvetrain for problems
 
#4 ·
i have had the valve covers off everything is tight and the valve are adjusted so i

i also checked combustion temps with an infared thermometer it seems like cyl 4 is a little cold so if it not the plugs maybe i'll pick up an extra wire and try that
i will do what you said though... leaning out the idle and putting bigger squirters in it i have .035 now should i jump to a .037 or a .039 what do you suggest i think i may put a smaller jet in the rear bowl because i have to nearly close the idle screw all the way to get a good vac reading
 
#8 ·
When you tune with an AFR gage you must have a leak free exhaust in front of the AFR or you will get erratic and generally very lean readings.

I'd check your exhaust system. I use solid copper gaskets both at the heads and collectors. Make sure the gasket surfaces are flat and clean. Use a good file and a flat plate to check flatness.

You may be knocking on the door of a flat cam. When you adjust valves be sure you check any that require a major adjustment.

Also a blown head gasket can cause a miss especially at idle.
 
#9 ·
i dont think its a flat cam because i do have steady vacuum at all operating ranges and it does have a blower cam .488 lift with a 292 advert duration
that funny you said that about locking out the dist i have put up two post already trying to figure out how to do it but i just cant see how i would do it with my dist i got one off of ebay it is a pro comp mechanical adv dist i would like to put more timing in it intially but i know i have to keep the total down and i would like to change the advance stop so that i can limit the advancing mechanism and run a little more intial but again i just do wanna weld it just in case i dont like it and i cant see any place to zip tie it and lock the post
all in all i do believe it is just a hole in the fuel curve and needs proper tuning as someone said above
then again thats why i ask the question cuz u guys know best!!
anyway i have checked to see if any valves need dramatic adjusting and it was ok
 
#10 ·
Pupsvette76 said:
i dont think its a flat cam because i do have steady vacuum at all operating ranges and it does have a blower cam .488 lift with a 292 advert duration
that funny you said that about locking out the dist i have put up two post already trying to figure out how to do it but i just cant see how i would do it with my dist i got one off of ebay it is a pro comp mechanical adv dist i would like to put more timing in it intially but i know i have to keep the total down and i would like to change the advance stop so that i can limit the advancing mechanism and run a little more intial but again i just do wanna weld it just in case i dont like it and i cant see any place to zip tie it and lock the post
all in all i do believe it is just a hole in the fuel curve and needs proper tuning as someone said above
then again thats why i ask the question cuz u guys know best!!
anyway i have checked to see if any valves need dramatic adjusting and it was ok
Got a photo of the dist. w/the cap and rotor off?
 
#11 ·
great point.
my dist is a mimic of an MSD
i have had problems with two on three of these pro comp dist the one on this car is the only one i have not had to return
i am going down to the garage to take the cap off now and look at it
yes that is my cam the one you posted up... the summit cam
locking out my dist will not effect power output.. right? it maybe hard to start with hot engine temps though or is that a myth
thanks for the advice
 
#13 · (Edited)
Pupsvette76 said:
are there any negatives to locking out the advance mechanism
it sounds like its my only choice but i just wanna know what im in for otherwise
I could rattle on for hours regarding locked out vs a spark curve with a roots blown engine.
The best I can say..if she,s a track only deal..lock her out.If you see a lot of street miles..run a spark curve.
With a non vac type distributor,you need to invest in a cd box that pulls out timing under boost with a 2 bar map sensor.
This will allow you to run the initial you require...more total at cruise/non load conditions and then pull x amount of total out based on manifold pressure.(boost).
My current setup with a 18* stop bushing..has the initial at 22* and pushing 40* at 2800 rpm with light springs.
I have my in car boost retard knob set to pull out a degree for every lb of boost.
So,here I have decent initial..plenty of advance for cruising(mpg),and no more than 32* total with 8 lbs of boost.
You can have your cake and eat it also..you just have to take advantage of modern day electronics and sensors.
I run aluminum heads and keep my coolant temp under 190*f with a 13.5/1 cruise a/f ratio and 11.8/1 wot a/f ratio.
No detonation issues on bp 93 on the street with my 11 under 8-71 blown 461 cid bbc in a 68 camaro and pulls down an honest 700+ hp and 13 mpg to boot with dual 850 double pumps.
 
#14 ·
this is a street car and i really dont have the money for the boost retard box($329) im not so concerned with gas mileage seeing as how its a toy not a daily driver i want it to run properly and i want to tune it the best i can i am 20 years old and i go to school for automotive engineering im a motorhead this is what i will be doing with my life so i wanna learn everything i can
 
#17 ·
Pupsvette76 said:
this is a street car and i really dont have the money for the boost retard box($329) im not so concerned with gas mileage seeing as how its a toy not a daily driver i want it to run properly and i want to tune it the best i can i am 20 years old and i go to school for automotive engineering im a motorhead this is what i will be doing with my life so i wanna learn everything i can
Getting good mpg at cruise rpm is an indicator of a good tune up.Guys in the know with blown units achieve great driveability with manifold vacuum and then set the carb ckts and total timing under boost on KILL.
Easy to do with smaller main jetting..boost referancing the primary power valve and opening up the pvcr and using a manifold absolute pressure sensor in conjunction with a retard box to lay back total timing per lbs of boost.
Imho..you will recoup the cost of the box in a short time with fuel savings alone..a cleaner and safer tune that your ring package will thank you for..and no need to buy stock in spark plugs.
Fwiw..your a 20 yr old and I think you have a bright future in automotive engineering with your desire to learn. :thumbup:
 
#18 ·
i will probably pick one up when i can get the money... im kinda attached to the one i have in there cuz i have made so many adjustments on it to make it work right really weird lol
i have an extra GM HEI dist but i really dont wanna wire it into the msd box plus there will probably be a firewall clearance issue with it
 
#19 ·
it took me a while to update but heres what i did and what happened
i locked the timing out at 29 deg
the car idled much better and the smell of rich exhaust went away but the car was still not the car it was three weeks ago (slow now)
checked valvetrain no loose rockers
then i tried a bigger shooter to alleviate the backfire coming off the idle circuit not only did it still backfire when i floored it going from 35 primary 35 sec to 37 primary and 40 sec but now it just ran like a dog just flat out sluggish i am so frustrated with this car it ran great a few weeks ago now its not runnning the same i have been working on this car for 2 years and i cant get this motor right and reliable the summer is here and i finally wanna get some use out of this car
no matter what i do i feel like it still lacks that power it had
it really feels like there is a plug wire off . like im running on 7 cylinders
 
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