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Demon Carb tuning tips? 625 road demon - fat, fat, fat, fat, fat

6

67resto-coupe

Member
ok, so trans issues and coolant issues aside, i've run into another issue. a really, really rich carb.

specs: crate 302, e-cam, gt40-P iron heads, 625 road demon, 1.7 rockers, air gap rpm intake, msd ready to run dist.

carb was bolted on right out of the box. i've now fouled a 2nd set of plugs......black - just black - all 8 of 'em, not a tan one in the bunch. exhaust is really, really strong and i haven't had a really smooth run yet. only smooth run was when i put the new plugs in there and it ran really smooth for a short time only, then sputtered.

4 idle mixture screws...sac69 and i turned them all in, then back out 1 1/2 turns. tomorrow, i'll turn them all the way in, then start 1/4 turn out and work my way out.

any other suggestions? from other blogs, some have said it's not a jetting issue. i'll re-read the material that came with the carb and see if reducing the jets may help too.

thanks.
 
Double check the float levels, and make sure you're not spilling over into the venturis. I like the idea of leaning out all four corners on the mixture screws. You really don't need as much fuel on a 4 corner circuit as you do a 2 corner circuit.

You may have to jet it down, though.

What coil are you running? Are you using a capacitive discharge unit on the ignition? What plugs are you using and what are they gapped to?
 
In addition to what John mentioned... Where do you have the idle set? If the idle is set so the butterfly is too far open, it will uncover too much of the idle transfer slot. If there's more than 0.040" showing, the butterflies are too far open, and you may need to drill a very small hole in the primary butterflies.

Does the car run with the idle screws turned all the way in?
 
-using autolite 104 or 764 (equivalent)...gapped at .054. since they're gt40p heads, i get the plugs designated for a '98 explorer 5.0
-using the msd blaster 2 coil
-the car wouldn't start with the 4 idle screws turned 1/4 out.....had to back them out 1 full turn and now it starts
-front and rear floats are running even
-i don't have a tuning tach and the car didn't come with one, so i really don't know where the idle is set....just a guess...just a bit higher before it dies?
-plugs are still black, too

i can't really see where the butterfly is set...i'm on my tip toes trying to look down into the carb and it's really hard to tell....
what else could i do differently? i'm tempted to get this towed to a tuning/dyno shop and have it done...i'm really new at this deal and can't seem to get it right.......

thanks everyone
 
so the Barry Grant tech guy said that the 625 road demon is too big of a carb for this 302. says a 525 road demon....even a 575 speed demon would have been a more suitable fit. recommended a new set of plugs and 16 degrees of initial timing to start with and then go from there. man, i don't want to have to buy a new carb. back in the day when i bought the carb, i swear this was the correct one for my app. i'll try that tonight.
 
The carb you have is just fine, IMHO. Sounds like it needs to be jetted down quite a bit, but Demons and Holleys seem to be pig rich from the factory most of the time. I've heard several people say that the BG tech guys recommended a smaller carb...if you ask me, they are almost TOO conservative with their recommendations.
 
"67resto-coupe" said:
recommended a new set of plugs and 16 degrees of initial timing to start with and then go from there.
Click to expand...

16 degrees of initial advance is nice if you can get away with it. Make sure, you have the other side limited, so you're not exceeding 36 total when the advance is all in.

You may experience some hard starts when warm with that much advance. You'll probably get away with it for a while, but it may expire your starter pre-maturely, if it's a stock one. Not all is lost in this scenario, though ... it gives you a great excuse for a hi-torque mini starter :nk

I run 16 degrees initial on my 408 stroker and jut put the mini starter in. I must say, it's pretty sweet! :10
 
well, the mini starter is on it's way from db electric, i should get it tomorrow. confirmed that it shipped on monday. i haven't had much luck finding any online stuff about how to rebuild a demon. should i follow a guide on rebuilding a holley as a substitute? i found a nice write up on vmf from '03 that listed jetting down to 67 primary, 72 secondary and checking a power valve (?)...much more research to do tonight. maybe i'll start by cleaning my workbench in preparation for this weekend's wrenching.
 
*** UPDATE *** installed the hi-tq mini-starter...sounds great and works great! installed a new tank and cleaned out the inline filter (to the carb) 2x....still filling up with residual debris left in the line. i think that before i start it again, i'll disconnect the fuel line to the carb, route it to a bottle and just bump the starter a few times to blow out whatever crap is left in the line.
also, my in-tank sender filter is junk, so i'll pick up a new one this week. i've taken the carb off and after pouring some of the fuel out of it, i can see much of the same sediment-like crap that was in the tank and filter! suffice it to say, i'm certain some of my carb issues are partially related to dirty internals, but also some fine tuning. i'm going to a parts store for a new gasket and a demon carb tuning book. i'm too inexperienced with this stuff to go without some type of manual. if i can't figure this part out, i'm going to farm it out and pay the pros. group prayer, please. thanks.
 
I have about the same set-up(engine wise) as you, except I have GT-40 cast heads. I am running a 600 holley, and mine runs alittle rich but doen't fowl the plugs. I would go no more than 11-12 initial timing on this mild engine. I would check your accellerator pump lever adjustment for the rich condition. You want about .015 gap between the lever and the adjuster.(it has the spring on the bolt). Just something else for you to check out. :lol I'm thinking that your pump lever is laying right against the adjuster causing it to pump too much fuel. Oh, I have mine jetted at 67 and 72.
 
so, the carb is off and i'm trying to adjust the butterflies to be about .020"....but can't seem to get it that tight. when i back the idle screw out, it reaches a point to where the butterfly is still showing about 1/8 or .125" of the transfer slot. there's no further adjustment available! i'm missing something, right? i'll look into the pump lever now, as i think it is soo close and needs some space. other than that, i'm going to remove the bowls to clean 'em out and will likely look into the jetting adjustment. i've already had the car running with the idle down as low as can be and with all 4 corners at 1 turn out....even at 3/4 turn out. i think the only major options are to lower the front and rear bowl levels and rejet. any more thoughts? i'm at the point where i'm seriously starting to look at the retrotek f/i unit.
 
Too bad I didn't see this post before last night, because would have brought you the feeler gauges. Just go get some. They're cheap and you're going to need them over and over and over for years.
 
so, any tips on how to figure out this .020 and .015 measurements?
Click to expand...


My set of feeler gauges is over 30 years old. It's probably the oldest tool in my inventory. Get yourself some.

FYI, a standard paper clip is approx. 0.035"
 
yeah! so i think i see the light at the end of the tunnel! i put the carb on then left for an 8 day vacation. got back into the car tonight: all 4 corner screws are 1.5 turns out. i readjusted the bowls to the lowest of the 3 hash marks on the sight window. i rejetted to a 67 primary and 72 secondary. adjusted the accelerater arm thingy a bit - loosened it. bumped the motor to get some new fuel into the carb...new tank helped greatly, as the fuel now looks like a really light tea, not honey. checked the timing at 14-16 deg. took it for a spin around the block a few times and it actually ran really smoothly. any other tips on trying to get the proper tune? i think i'll take it for a longer spin in the 'hood tomorrow night while it's still light and put a few more miles on it.
 
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