runnin lean,how many bump up on jets?

[kcsteck]

I got the victor on and it ran better then the tunnel with the single 4 barrel. It seemed like it gets to 4900 RPM quicker. It runs lean though.plugs look pretty white. or having the scoop forward play into that as well?i have a speed demon 750 carb with 76 jets in front and 80 in rear. How many should i jump up for starters 3 or 4 jet sizes like 79 front and 83 rear? any help apreciated.
Casey

[poncho-pwr]

How about posting a picture of your plugs if you can. If not explain a little better what they look like on the tip/electrode/first thread ring, and how you ran the motor before pulling the plugs to look at them.

[kcsteck]

they look prety dam white. maybe a sliver of some tan on the porcelen part and thirst thread and electrode. Ran hard run went to shore preety quick and shut it down.
http://i68.photobucket.com/albums/i31/kcsteck/MVC-002F.jpg
casey

[502 JET]

I dont know about the scoop but I would start with 4 sizes larger.Put the secondary jets in the primaries and get a set of 87s for the secondaries.

[bp]

recommended standard jets for those are 78 primarys, 86 secondarys. also, they come with 0.033 idle feed restrictors. if you have throttle response issues, or if it runs a little lean at part throttle, bump the ifr's up to .035s, and see if that helps; may want more. also, it's difficult to get these carbs to work correctly if the idle isn't set up to grant specs.
checking plugs after running at part throttle for any length of time can be misleading. your idle circuit could be lean, and that won't be corrected with main jets. keep bumping up the mains, you'll just lose rpm, but the plugs will still look lean from running at part throttle.

[bakerjet]

hey bp just a question, are you saying going up in jet size is gonna lose rpm because i'm a little lean and want to step up but not if it's gonna hurt my top end?

[kcsteck]

what and where are the (IFR) Idle feed restrictors located on the demon carb>
Casey

[kcsteck]

where is the part number on the demon so I can make sure this thing even is a a 750?
Casey

[Moneypitt]

hey bp just a question, are you saying going up in jet size is gonna lose rpm because i'm a little lean and want to step up but not if it's gonna hurt my top end?
What that means is you will have to go beyond what is "perfect" to find out where the motor starts to lay down on the top end. And then come back down to what is right for your application and weather conditions. Popping out the headers is a good sign that you are delivering more fuel than you can burn, (rich), and popping out the carb is just the opposite, (lean). There is a happy "in between", but it can take awhile to find it. Two things to remember, you never hurt 'em FAT, or COOL, but the other end of that is too LEAN and HOT, which will hurt parts everytime. So, sneak up on adding fuel until it goes over the top so to speak,(safe) and then come back down until the performance picks back up. (risky)........Another loose guideline is if you have to change more than 4 numbers in either direction, you may have the wrong CFM carb for your application.............Unless the Demon is way different than most carbs, the idle mixture has very little, if anything, to do with WOT operation............MP

[DUCKY]

I noticed you say that it "gets to 4900 quicker" as though it was turning the same rpm before the intake change. Have you checked your valve springs lately? I had a motor that the plugs looked like that. It would pull hard to about 4600 and pop lightly out the exhaust. Turns out that the valves were floating a little and it was washing the plugs white. I even went to a set of alcohol jets (110's in the secondaries) just to see a difference and that was none other than it blubbered from 3-4000 rpm then it cleaned up and ran the same as always.
Put .100" shims under the springs, and it ripped to 6100.....
Just a thought...

[kcsteck]

how do I check my valve springs? and you put these so called shims on a OLds and it went to 6100RPMs?
Casey

[MACHINEHEAD]

That plug looks great to me.

[bp]

That plug looks great to me.
looks fine to me too.
as density altitude increases (air temps higher, humidity higher, etc.), there is less oxygen in each cubic foot of air. conversely, when it's cold and at sea level, there's more oxyge in each cubic foot of air. the right air fuel ratio will get you in the ballpark, where the engine will run fairly well through the entire range. unless you're going to change jets all the time, you want it to have enough fuel/o2 to not be too lean when air is good (cool days, sea level), and not be too fat when air is not so good (hot, humid, higher elevation lakes). being "too fat" is not necessarily a good thing, as raw fuel can wash cylinders or contaminate oil. being "too lean" is not good either, as cylinder temps can get way too hot. absent egt for all cylinders, it's not a bad idea to go with standard spec jetting. moving one size down for hot times can help.
generally, demons come from the factory jetted on the fat side, except for the idle circuit. the ifr's are located in the inside face of the metering blocks, about 1/2" down from the top, one for each idle circuit. these restrictors adjust how much fuel runs through the transition slots in the venturis and feed the engine all through the rpm range (this is not a holley!). since jets load the engine all the time, with standard ifr's you can experience a leaning condition at part throttle, like at cruise rpm. if that's occurring, increasing ifr size can help address that. all speed demons and mighty demons have replaceable ifrs, but care must be taken as they are very small and the metering blocks can be messed up if they're not threaded in perfectly.
transition slots are tiny slots in the venturis, right at the butterflys. to set idle correctly, there should only be 0.020 of transition slot showing below each butterfly at idle (to check/set this, the carb needs to be off the engine). once set, idle is adjusted with the 4 corner mixture screws. with one carb, should start with the mixture screws 1 turn open. if you adjust idle by opening the butterflys via the outside set screws, you won't have enough transition slot available for acceleration, and you can experience bogs. correctly setting idle is critical to demon performance.
i don't have the part numbers with me. i'll try and get time this evening to look it up.

[bp]

here, check this link. you can find most all the part numbers that are available, and maybe figure out what you have.
http://www.barrygrant.com/demon/default.aspx?page=29