Typically to do this, knowing the total mechanical advance capability of the distributor, you simply do the math. 16 deg initial plus 20 mechanical gives you 36 deg. Done. But what if you were trying to determine the best curve for your combination? I am currently running 21 initial advance with vacuum advance disconnected giving me a a total of about 35 deg all in around 2500 rpm. I selected 21 because it gives me the best idle quality and vacuum around 9'hg at about 900 to 1000 rpm. 21 seems higher than most threads I have seen on the subject. I suspect my carb could be part of the problem. Any thoughts or comments? Getting ready to send my distributor back to DUI for a tune up.

Are there distributors out there that have an adjustable mechanical advance? I know some have bushings that can be changed out but I had to fashion my own stop for my D.U.I which was originally set up for 14deg initial advance..(22deg mech).

 

Maybe it's just me, but doesn't 16* initial plus 10* mechanical equal 26*? A distributor with only 10* mechanical advance seems a bit slow, and 9'hg @900 is very low as well (is this your idle?).

Either I don't understand what you are trying to convey or you don't understand basic timing principles.

Somewhat confused.

 

I'm running 24 degrees initial with the lightest springs all in by 2500. An adjustable mechanical would be nice other than bushings, I've been looking at the msd ecurve but I'm not for sure it would fit behind the blower I got in the garage if I ever end up going that route, programable curves with no box would be nice.

 

This one looks promising but still a box hide.
http://www.aldonamethyst.co.uk/

 

Sorry, my public school math has failed me again, that should read 20*. That is just an example. Yes 900 to 1000 rpm is my idle. Oh and my combination is not stock so 9" hg vacuum is not too bad. Need to keep it steady however or the power valves will flutter and the car will idle very rich. I guess my question is, is there a rule of thumb for establishing you initial advance?

 

I had a hard time finding spring combo that would work for me, ended up drilling my weights to slow down the advance. Works well but I would like to do it right and figure out what I need and have to pros set it up correctly.

 

I ran into the whole power valve issue even got into a heated argument or two here on the forum about it. I ended up plugging both valves and squaring up. Idles like a champ and no stumble or stutter.

 

Now that is interesting. I have PVs in both my primary and secondary bowls. Been chasing tune for a couple of years now, the best investment i ever made was a LM-1000 wide band O2 analyzer. When I dyno my car, I log my own O2, RPM and vacuum on my lap top in the car and watch it real time. Its time to get a correct carb for the car. I have been running a 830 CFM marine carb for years. Looking to go to a 950 Holley.

 

OK so I bought a pro systems 780 cfm double pumper that flows 820 cfm according to prosystems which was built off a 950 hp and for what it's worth you'd be as well off going holley ultra hp 950 and adding jet extentions. My vacuum in gear was 7" and when I'd power valve just the primary it was a 3.5 and it would still foul the plugs at idle, i dropped to a 2.5 and it wouldnt foul the plugs but I'd get a lean stumble between tip in and coming off the idle circuit, the transition. My plugs would get sooty pretty fast but when I did plug test with a new plug only wot the plugs were fine. So I borrowed an lm1 got the wot dialed in but idle was off the chart for lean which meant I had a misfire at idle changed plugs, wires, cap, rotor, ect car would only idle with 2.5 turns on the idle air screws so I ran through the gambit on idle air bleeds and in order to get it to idle below 2.5 turns, the air bleeds were decreased drastically which made it idle super rich at idle. Prosystems suggested locking the timing out which I tried and wasn't that fond of so I went back normal on the distributor. So I got in few arguments here on the forum and to there credit the power valves shouldn't see action until tip in, which I agree with except I think that with holes drilled through the throttle plates and given duration of the cam shaft and vacuum idle being 1000 rpm in park and 850 in gear I think they can be effected by idle. I changed intake gaskets 3 times thinking maybe I had a leak but my vacuum was steady and not really indicating a vacuum leak. Another friend of mine suggested plugging the power valve and jetting up. With the secondary power valve being plugged I decided to try it although i wasn't crazy about the thought of that. Now then I didn't have the lm1 to check the idle afr but it idles consistently instead of fluctuating like it did before, which wasn't predictable and was erratic and it don't foul plugs at idle now, which was the only time it fouled plugs. As sure as i post this it will draw criticism but this was my experience. You just have square the Jets and go ten sizes up, I assume that your currently running both power valves so plug them both and jet up 10 sizes. I'm interested in your results. Good luck with your project.
Blue

 

I am having much the same problems. Off idle stumble and sooty plugs or I run to lean of idle....cant win. My number 1 plug gets so bad that I have to periodically clean it especially if I allow the car to idle for and extended period. Really at my wits end with this. I have read many articles about proper timing and how to adjust a Holley using a vacuum gauge to no avail. I think when you depart from stock setups, rules may not change but I think there are more variables to consider. There is a reason why production engines in the early days had low compression, high vacuum and conservative cams. These engines are a lot more forgiving regarding set up. Maybe I will try plugging the PVs and jet up and see where that takes me. I have tried everything else and my only recourse is to try a new carb and make sure my spark delivery is what it is supposed to be, tune up distributor, new wires and plugs. Thanks for sharing your experience.

 

Sounds identical to my experience and I too was at wits end. You can buy the plugs at shucks or orielys or on line cheaper than a power valve. What size camshaft you running?

 

Maybe this is kinda late to try now but did either of u try drilling a hole in the throttle plate to add more air at idle? Thats the oldest trick for a big cam using any carb because of the low vacuum cant pull enough air for all cubic inches thats needed - just not enough mix in the cyl even if correct mix ratio. It just adds more air at very low rpm and u still have to add more fuel/enrichen it to match. I think some Holleys (and a lot of Demon carbs) had 4 corner idle built in for this instead of the drilling holes in throttle plate.

Just curious as this sounds pretty interesting,
cardo0

 

Mine has holes in the throttle plates and four corner idling. The holes came that way from prosystems. My brother talked me into the prosystems. Not a bad carb, but I wouldn't say any better than a Holley ultra hp with jet extentions, might be cheaper though with the pro system. It does have bullet base plate but don't know what that's worth. But the ops issues are identical to what mine were and I'll bet plugging the power valves solves his problems.

 

Hey, see my info below regarding setup. I also had a lot of problems getting enough fuel from my acceleration pump. Had to go with the large size and custom file a cam for it. I would get nasty lean spikes every time I punched it. The LM-1 was a huge help in dialing that in.

Do you run vacuum advance?

 

I didn't have any issues going from idle to wot just casually accelerating. And only when I got the power valves to not foul the plugs but still had power valves.

 

Finishing up my seat installation today and have one plug, I think Advance Auto Parts also sells them, may need to pick up some larger jets. Probably need some in the 85+ range. Would yor recommending matching the sizes for primary and secondaries?

Its funny this thread has turned into a carb thread......just as well keep it hidden, I have learned as I am sure you have there are some topics that always attract the same cast self ordained experts.....

 

Just keep in mind that when this is done when cruising on the main jet (around 2600 rpm) and above range your AFR will be very rich.

Neal

 

You don't need jets that big!

Have you considered an electronic programmable dist'r to dial in the exact curve your engine requires? Anyway, I used my stock HEI springs to set initial idling @ ~700 rpm in neutral, to 18*. I then modified my dist'r by restricting mech'l advance to 14* for a total of 32*. Then I went back and played with the spring combinations to bring in the advance quicker. Trial and error. Get your timing figured out first. Then play with the carb. My 650 Holley Street HP had a hesitation off idle and going to wot. Put in bigger squirters and changed the primary cam to orange and after some trial and error it's gone! It was also running a bit lean at cruise so I changed the primaries from #68 to #70. Seems to be working but still doing some other tweaks. Car runs great just the same.

 

I had a discussion with the forum expert "Lars" over the weekend and we agree on timing settings.

Do you have high VE, modern fast burn heads, good A/F ratio, indexed or multi tip plugs? Or possibly even a car with OD driven at lower rpm creating higher cylinder pressures

Yes, to the above questions would require 3000 3200 all in

 

I like the idea of a programmable distributor provided it doesn't require 10 miles of wire harnesses, who makes them? Also, my statement requiring 85+ was based on plugging the primary PV for experimentation. I am currently running a 78 for my primaries.

What made you select 18* as you initial value? I did exactly what you have done but I came up with 21 initial and 15 mechanical. I based my decision on idle quality and A/F ratios. I experimented with several lower values but the idle was just to rough. Sounded like I had some sort of mega race cam or something.