Hi everyone, I recently installed a Speed Demon 850 Vac secondary on my 455, Idles well and runs okay (no Black smoke), But I have on o2 sensor and gauge installed that reads rich almost across the board. The engine idles well and pulls 17 in vac. at 900 rpm, has 10:1 Comp. Cam is a crower with about .510 lift/236 duration, heads have stage one valves and light bowl work performer intake, headers, 2.25 exhaust I have installed 78 primary jets and 85 secondaries (came with 85/92), the carb stays in the middle of the rich area on the gauge, but will go to the amber "good mixture" area briefly at moderate throttle, and lean very briefly when I accelerate hard, but always returns quickly to the middle of the rich range, even with the throttle still down. The Barry Grant Tech says that the 850 is way to big and is puddleing gas in my intake, that my leaner jets are covering up the problem somewhat, and that o2 sensors do not work on non-injected vehicles. Was a little rude about it in fact. Some of that sounds like bull to me, but the jetting changes seem extreme to still read rich across the board. Any help would be appreciated.. PS. I recently installed Chevy lifters and adjustable pushrods, first time in 5 years, 2 cams, and three sets of lifters that my engine is quite, real quite. Thanks to all on this board for the info.
what brand is your 02 /gauge set up? did you make one? a wide band o2 sensor is needed for reading a carburated card. I read an article in hot rod about this. i'i'l post an article if i can find it.
thanks for the reply Thanks for the reply, I'll look for the article online. The sensors a Bosch narrowband, with an autometer "gauge". Only seems to work with the car driving around though. never stays hot enough at low speed or idle. Installed a heated sensor today, reads leaner, at least at idle Will try to spend some time this weekend tuning. The gauge seems to give a good indication of the tuning though, worked well with tuning my Q-jet. Got everything pretty close, plugs looked good at least.
The only advice you should take from barry grant on tuning this is what they say on the cassette that came with the carb. I have an 850 Mighty demon on mine, and I think it's the right carb, may be a little too much but I think a 750 wouldn't be enough. I've always been told that Buicks like gas, and from what I have tried it appears true. Your idle sounds like it's set a little lean by what you said the sensor is saying and you can adjust this by the four little screws (2 on each side) of the carb. Adjust to where it isn't lean and you have the best vacuum. Off idle, it's all jets, squirters, etc. You may still be jetted a little too rich which is why you run in the rich range. As for the lean on acceleration, you can try messing with the vacuum spring on the secondaries, accelerator pump, the accelerator pump cams and the squirters on the top of the carb. I almost think that you need a lighter secondary spring and to go a few sizes down on your jets and start from there. It'll take time to get it just right....
You do not want 14.7:1 at WOT(center of scale). Reading in the middle of the rich scale is probably close to where you want to be, voltage of 900mV is close to the reading you want for good power production. A narrowband O2 does not read accurately at WOT, but if you're around 900mV you're in the ballpark. Best bet for an accurate reading you can tune with is a wideband. I usually shoot for 12.5:1 or so when I tune on a chassis dyno.
Dennis, I hope you don't take this the wrong way, but why did you change jetting and settle on the jet sizes that you did? Tuning at the track for MPH is the best way to go (opinion). In my limited experience with the turbo Buicks, the narrow-band O2 sensors don't give enough precision to be able to do a whole lot of tuning. But, that being said: readings in the 800mv range should have you in the 12:1 territory that you need to be in at WOT (to add support to what Rob and Artie said). I'd throw the stock jets back in and call it good, unless you have some track testing or a wide-band sensor. Hope this helps a bit, Ramin
Hi, Not at all, Just trying to follow the o2 sensors. they read max rich with stock calibration. I have since installed heated o2 sensors and they seem more stable, and I have richend the mixture up more (82,88). The gauge swings alot buts stays around themiddle during idle and cruise. will watch the plugs to see if I am really in the right range. Thanks for the reply.
Cruise and power... There are a couple of ways to get the cruise and power in the right range. One is a larger spread between the primary and secondary. Go smaller in the primary, larger in the secondary. THis can help get you cruise leaned out and keep full throttle withthe fuel it needs. So you would tune cruise with the primary's and power with teh secondarys. The other way is to drill up the 2 holes behind your powervalve that feed the main circuit and install a smaller primary jet. You can also play with the power valve vaccum rating. I believe they go from 2.5 - 8.5in hg. (vaccum). A 2.5 power valve with open later, closer to full throttle where a 8.5 would open sooner at a lighter throttle. Vaccum secondary carbs are generally designed for a tight or stock torque converter and mild engines. I don't know your exact combo but a 236 cam with a stall converter could be a bit restricted with a vaccum seconday carb. It would feel a little lazy in first and pull like a freight train in second. Just something you should know.......
