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Old 05-14-2010, 04:15 PM
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tuning help please

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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Old 05-14-2010, 04:16 PM
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i forgot to mention that the main reason i started messing with it is because it seem to lack the power it had a few weeks ago
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Old 05-14-2010, 04:22 PM
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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
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Old 05-14-2010, 04:33 PM
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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
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Old 05-14-2010, 10:06 PM
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You have a heated wide band?Can you post up some a/f ratios?
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Old 05-14-2010, 11:19 PM
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its not heated it came in a kit its made by AEM its called a UEGO II A/F gauge kit

idle bounces between 15.7- 12.5
WOT about 13.5
cruise 12-14
it is never steady its alway bouncing around
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Old 05-14-2010, 11:35 PM
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if i went any bigger in the squirters i would have to convert the diaphram to a 50cc. right?
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Old 05-15-2010, 12:40 AM
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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.
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Old 05-15-2010, 11:41 AM
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Blower motors want a ton of initial advance at idle for good throttle response and clean idle. Yet limited max advance under boost/wot.
The bigger the cam the more initial needed.

17deg at idle is not enough. 34deg at full throttle is too much on pump gas.

lock the timing out and set it at 28-30deg and reset the carb. go for a ride.

From that point you can determine the best idle timing and max timing required and recurve the distributor to get this.

(Or just leave it locked as it will be very close). The critical factor is avoiding excessive spark advance under boost on pump gas.

As mentioned check your exhaust system for leaks that will fool the AFR meter. Check for vacuum leaks around the carb base, blower to manifold gasket, intake manifold gasket. Do a cylinder balance test to find the weak cylinder.

What cam is in this motor?

note: If you were hammering on it with incorrect timing and or lean AFR under boost and suddenly have a power loss, you may have damaged the motor. ( broken ring land, cracked piston, headgasket etc)
do a simple compression test.

I'm betting for your sake its a carb/blower/manifold gasket vacuum leak.

13.5:1 is way too lean under boost. Get it around 11.5 to 12.5 ratio under boost.
If the exhaust checks out temporarily install a simple narrow band 02 sensor and volt meter and compare readings when the engine is running at stoichiometric ratio 14.7:1 as indicated on the narrow band.
This will verify the wide band is functional.

Last edited by F-BIRD'88; 05-15-2010 at 11:51 AM.
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Old 05-15-2010, 12:34 PM
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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
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Old 05-15-2010, 12:39 PM
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Quote:
Originally Posted by Pupsvette76
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?
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Old 05-15-2010, 12:53 PM
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If the advance mechanism is on top under the rotor its pretty simple. Remove the weights and springs. Work the stripped down mechanism by hand so that it is fully advanced. Now wrap the two zip ties across the mechanism so it stays fully locked. Orient the buckles/knots of the zip ties down under so that the rotor goes back on. Reinstall the rotor assuring it is seated correctly.
Loosen of the distributor hold down clamp bolt and start the motor up. reset the timing for 28deg btdc at idle. I will not advance with rpm.

if the advance mechanism is down deep in the distributor (like an Accel or mallory distributor) you will have to dissassemble it it down and block the advance mechanism stop slot.
Some of these distributor designs with the advance mechanism deep in the distributor body allow you to dissassemble them, rotate the center shaft 180deg and slide the pin into a lock hole and resassemble for locked timing.

Pictures of your distributor without the rotor installed will help.
http://www.malloryperformance.com/pdf/29014.pdf

The pro comp chinesium junk is most likely a copy of one of the name brand distributors. search the manufacturers (Mallory, MSD, Accel, etc) tech pages and download the instructions of the distributor that matches yours.

Last edited by F-BIRD'88; 05-15-2010 at 01:03 PM.
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Old 05-15-2010, 01:06 PM
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http://static.summitracing.com/globa...s/sum-1107.pdf
Are these cam timing specs the same as yours?

If so, you need a minimum of 26deg at idle. SO just lockout the timing.
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Old 05-15-2010, 01:11 PM
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various methods to lock out or limit the mechanical and vacuum advance.
A simple advance stop bushing and advance spring swap is not going to cut it.

http://www.hotrodders.com/forum/atta...chmentid=44636

http://www.hotrodders.com/forum/atta...chmentid=35990

http://www.hotrodders.com/forum/atta...chmentid=34464

The wide band sensor only tells you the average AFR of all the cylinders in that cylinder bank. If one of the cylinders has a vacuum leak or a faulty ignition the AFR will show lean and you would have to jack up the fuel mix to try and compensate, therefor making the the other cylinders too rich.
The seeming rich idle (based on the exhaust smell) is from late spark timing. The fuel is still not finished burning when the exhaust valve opens if the spark is late at idle. (Raw, unburned, partially burned fuel in the exhaust)
This also creates a hot exhaust which eats ex header gaskets and risks cracked exhaust ports and header flanges.
Correcting the idle spark timing and finding the bad/lean cylinder will correct the exhaust smell. It will no longer load up and foul plugs.

The idle feed restriction and or idle air bleeds may need adjustment/tuning to correct the idle and off idle afr for smooth throttle response and avoid eventual spark plug fouling. First get the spark advance corrected and the carb throttle blade/idle transfer slot exposure at idle corrected before you monkey with the carbs internal idle circuit calibration.
If you have a aftermarket timing tab/and or balancer, you need to verify true tdc with a piston stop first as they are always out of wack making it impossible to accurately set the timing.

Last edited by F-BIRD'88; 05-15-2010 at 04:58 PM.
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Old 05-15-2010, 06:36 PM
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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
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