Well in the last few days i've been working on the carburetor a lot and i've solved all but one of my problems. It is a 625 road demon. The problem i am having is that the carburetor leans out on part throttle. I have the float level perfect, the jetting perfect, and the idle pretty good (runs a lil rich to compensate for no choke). However, when i am moving along at a steady pace at a lower rpm (about 1500-2000) i can watch the carb lean itself out. I have a heated O2 sensor and a/f gauge to monitor these things, and it also starts to "buck" when it does this. Demon's prescribed method of treatment for this was to put air bleed restrictors in the outside little holes. I have done this and it doesn't seem to have helped. Can anyone offer me any other suggestions?
try restricting the idle air bleeds. They are the inside holes. I think this should help more than restricting the high-speed bleeds
Demon is a carburetor company that makes carbs that are similar to Holley's, only they are made from billet and more adjustable. The Road Demon is a model, they also have Race Demon, Street Demon, etc. The 625 is the size in cfm. All this info is readily available through and internet search.
Just for discussion only, I have the air fuel ratio gauge looking at the right hand sensor on the Lincoln. With the blower on , it leaned the fuel to much in the same range so the answer was to move the AIR CHARGE sensor out of the intake to a cooler location and bump the fuel pressure up on the fuel injection until it just goes away and the improvement shows on the A/F gauge. Your intermediate circuit is to lean. I would put larger main jets in a few sizes up to see if it improves the condition, then decide how to handle the final fix, if any.
I think it's fair to call the Demon's a direct copy with improvements over a Holley. I will get in touch with Demon about the idle air bleeds. My demon does not have replaceable high-speed bleeds (since it is the bottom-of-the-line), my only option was to put smaller high-speed bleeds in which is what demon recommends. To be honest it doesn't make a whole lot of sense to me, but then again i didn't design the thing! As for jetting the mains up a little higher, i went from a size 69 jet to a size 68 trying to get my cruise a/f a little closer to stoich and that seemed to worsen my lean condition. Putting bigger jets in would be no problem, but won't that throw my WOT a/f off too much on the rich side?
Going from 69 to 68 is the wrong way. You went leaner. You must change by at least two sizes or more, to be sure you see any difference. The main jets do have an effect on total WOT a/f ratio but are the main controller of the cruise ratio. The main jets in combination with the power valve determines the WOT a/f ratio. Going smaller on the bleeds richens up the mix. This is do to the tendency of any carb to flow more fuel in relation to the air flow as it rises, because it is not a linear relationship and is why bleeds are used to control this tendency as well as to allow some air to mix with the fuel for better atomization during cruise.
. YEA .............what HE said !! It doesnt make sense about the airbleeds but trust Mr Kerring . The smaller the highspeed airbleed the more fuel is added to a given signal from the engine . The same thing goes for the idle bleed curcuits . If you look closely you will notice that the bleeds for the idle curcuit are considerably larger than the high speed or main carb circuit (spell check ... curcuit , circuit ?). This makes sense if you think about it as the idle curcuit delivers a lot less fuel than the main system
I realized that going from a 69 to a 68 is a step in the wrong direction for the cruise a/f, but i was tuning for WOT (3,000 rpms on a flat area with no acceleration or deceleration) and was basing my jet size strictly on that. I see now how it can affect other rpm ranges, and will jet up a couple of sizes. Thanks everyone!
The power valve is a vacuum operated, spring loaded device and operates at a prescribed level of vacuum for supplying extra fuel at WOT. Most of the time the opening point is set to open at a vacuum level about 2" below cruise vacuum. These valves are offered in opening points from quite low to over 10" of vacuum and in at least two flow capacities. The opening point is the number found on the part. To properly set up a carb, the power valve should have a plug temporarly installed so it does not get into the act of jet testing. Select the main jets to get the cruise A/F desired, note the vacuum level then select a power valve to open about 2" lower in vacuum to replace the plug and go from there if a change is needed to finish up or solve any response problem.
Forgot to comment that these items are why one cannot just take a carb out of the box, bolt it on and go without problems much of the time because the engine setup dictates the jetting and power valve size. It also gets worse when a larger carb than is prudent, is used. Then there are times when bolting on just happens to be close enough by the seat of the pants..
Make sure that your vacuum advance on the distributor is working properly...It can cause a part-throttle lean condition...
