Carb & timing worries
07-17-2008, 06:39 AM
#1
Carb & timing worries 
Heres my situation thus far:I recently yanked out the 305 out of my '88 RS and swapped with a low mileage rebuilt '72 350 bored .040 over. It retains the stock heads, with a new lunati voodoo cam (mild 207/213) new timing set and lifters with an edelbrock performer intake and carb, believed to be a 600 cfm. I got it from a friend about year ago and IIRC thats what he said it was. Only other mods are i put an HEI distributor in it (non comp style) and it has SLP shorties with the SLP dual Y-pipe into the Borla cat back. Obviously im very very inexperienced with carb set up cars, but am very interested in learn about them
My concern is this:
When I try to get a reading to set the timing (with the vacuum advance hose plugged)it shows that the timing mark is way advanced (its the original timing tab). If the timing tab had enough marks on it it'd probably read like 26* at idle!?!? What have i done wrong here?
I know the base timing for this engine should be somewheres around 8-10*. I know the centrifugal advance bushings are worn (gotta curve kit on order) and that they aren't sticking. A little over a year ago I replaced them and think i out the medium weight ones in.
Also the tach in the car was always off by like 500-600 RPM. When i did a tune up on (back when it had the 305 in it) I just dissconected the yellow lead going to the cap. Now with this new engine it was hooked up just for sh*ts and giggles to see what'd happen and its reading pegged way out. Like its hitting the oil pressure needle. I was thinking somewhere the ground isn't making contact. Anyone ever had this happen? Any suggestions?
Lastly just out of curiosity, the old style temp sensor for this '72 and the one that the 305 used are not the same size. The '72's is much bigger and I have tried using the '72 style but am getting no reading. The temp sensor worked fine with the 305. Does any one know if the Ohms are different in the 2 styles and maybe thats why im not getting a reading?
07-17-2008, 11:14 AM
#2
You should swap out the sensors. The timing should be way lower, have you tried adjusting? The carb can be tuned easily, just have somone shw you what you need to do, much easier in person than being told over a forum. The tach in our camaros are pieces of ****, they act like crap and are worthless. I installed a 5 inch aftermarket tach due to the fact i need one and it looks better as well as right on the spot.
07-17-2008, 05:11 PM
#3
Zones,
Thanks for the reply, really! Okay I'll deff swap sensors and find some kind of fitting to adapt to the large sized bore in the '72 heads. Yes I've been playing with the timing for past 3 days. The only place it'll run is at least 24 or 26* advanced. Otherwise it'l die. I've been told 3rd gen tach's are garbage.
I've had 4 3rd gen cars. 3 have had tachs and this one is the first that i've ever gotten wacky readings, however this is also the highest mileage/worn third gen i've had as well. But the timing thing really stumps me. At the suggestion of 86IROC (brian) i pulled the distributor, brought the balancer to 0* on the timing tab and THEN put the distribtor back in at No.1 cyl. (o about 5:30) any more advice is GREATLY appreciated I hate not being able to drive my '88!!
IF when i pulled the distributor i pulled it out when No. 1 was @ BDC would this exlain my situation?
Thanks for the reply, really! Okay I'll deff swap sensors and find some kind of fitting to adapt to the large sized bore in the '72 heads. Yes I've been playing with the timing for past 3 days. The only place it'll run is at least 24 or 26* advanced. Otherwise it'l die. I've been told 3rd gen tach's are garbage.
I've had 4 3rd gen cars. 3 have had tachs and this one is the first that i've ever gotten wacky readings, however this is also the highest mileage/worn third gen i've had as well. But the timing thing really stumps me. At the suggestion of 86IROC (brian) i pulled the distributor, brought the balancer to 0* on the timing tab and THEN put the distribtor back in at No.1 cyl. (o about 5:30) any more advice is GREATLY appreciated I hate not being able to drive my '88!!
IF when i pulled the distributor i pulled it out when No. 1 was @ BDC would this exlain my situation?
07-17-2008, 08:03 PM
#4
Well, there is the possibility of the ballancer inertia ring slipping, giving a false reading. Then there is the different timing marks for different timing covers. If it runs fine with the timing like that, i guess it is fine, my stock L98 with a carb conversion runs with 10* initial.
07-18-2008, 10:18 AM
#6
http://www.corvette-restoration.com/.../Timing101.pdf
http://en.wikipedia.org/wiki/Ignition_timing
the vacuum advance is a complement to the mechanical advance in the distributor for better fuel economy by advancing ignition timing when there is no load (high vacuum) on the engine. and this basically requires the carb or fuel injection system to be tuned to run on the lean side under a high vacuum low load situation... because lean = fuel economy and lean requires more ignition timing. i'm not that knowledable about hei distributors and ignitions but it seems logical that for any given distributor with some set mechanical timing curve from the factory and whatever spring in the vacuum advance was done so for a specific engine setup (with factory heads, cam, etc.) If you go using such a distributor on a completely different engine, it's not going to apply and provide an optimum timing curve and vacuum advance, or at least don't be surprised if it doesn't and you need to change springs in the distributor to modify the mechanical advance curve or the vacuum advance curve.
Base timing is usually set at 8-10 btdc.
the mechanical advance (not vacuum adv) typically provides 24 degrees of advance at most, which yields the maximum of 34 dbtc total advance which is soley rpm dependent.
Now add in the vaccum advance, which adds degrees of advance to ignition timing when there is high intake vacuum (which means low load) and expects the a/f ratio to be on the lean side. At full throttle or high load conditions, there's little or no intake vacuum so the vacuum advance provides no additional advance and you max ignition timing (under load) is 34 degrees.
coincidently, i have a scangauge hooked up to my car (camaro ls1) and at idle with clutch in at 800 rpm the timing is 20 degrees +/- a little, because the computer is set up to fuel on the lean side for economy and emissions. For a carb setup, where the carb maintains a 12:1 a/f ratio all the time including idle no load, then you would want ignition timing around the base setting of 10 btdc. hope that makes sense and helps
http://en.wikipedia.org/wiki/Ignition_timing
the vacuum advance is a complement to the mechanical advance in the distributor for better fuel economy by advancing ignition timing when there is no load (high vacuum) on the engine. and this basically requires the carb or fuel injection system to be tuned to run on the lean side under a high vacuum low load situation... because lean = fuel economy and lean requires more ignition timing. i'm not that knowledable about hei distributors and ignitions but it seems logical that for any given distributor with some set mechanical timing curve from the factory and whatever spring in the vacuum advance was done so for a specific engine setup (with factory heads, cam, etc.) If you go using such a distributor on a completely different engine, it's not going to apply and provide an optimum timing curve and vacuum advance, or at least don't be surprised if it doesn't and you need to change springs in the distributor to modify the mechanical advance curve or the vacuum advance curve.
Base timing is usually set at 8-10 btdc.
the mechanical advance (not vacuum adv) typically provides 24 degrees of advance at most, which yields the maximum of 34 dbtc total advance which is soley rpm dependent.
Now add in the vaccum advance, which adds degrees of advance to ignition timing when there is high intake vacuum (which means low load) and expects the a/f ratio to be on the lean side. At full throttle or high load conditions, there's little or no intake vacuum so the vacuum advance provides no additional advance and you max ignition timing (under load) is 34 degrees.
coincidently, i have a scangauge hooked up to my car (camaro ls1) and at idle with clutch in at 800 rpm the timing is 20 degrees +/- a little, because the computer is set up to fuel on the lean side for economy and emissions. For a carb setup, where the carb maintains a 12:1 a/f ratio all the time including idle no load, then you would want ignition timing around the base setting of 10 btdc. hope that makes sense and helps