Oh man, this carburetor is giving me fits! I just got my demon carburetor (625 road demon) and i just can't seem to get it to run right! First of all i've got a stumble....only from a stop, and it can be cured by giving a couple of quick punches to the throttle. Here's my setup: 302, .040 over dual plane high-rise intake ("RPM" intake) 351W heads ('69) flat top pistons (i think i have about 9.5:1....60cc chambers) MSD 6al Road Demon (625 vac. sec.) mild cam-- 204/214, .450 lift I've been playing with jet and shooter sizes. Going up in the jets seems to help a little...and give the car more performance. So far i'm at a 77 jet. Even with a 77 jet the "plug test" yielded a lot of white crap on the plugs. The secondaries are at 85's right now. I have a sneaking suspicion that those are too high, especially with my current elevation of 5,700ft. However, no noticeable black smoke out the exhaust, and again, white plugs. Should i go higher? I have 31 shooters in there right now, and those seemed to help the stumble a little....but it's still there. Any ideas???
sounds like an accelerator pump i dont know the layout of a demon carb. i assume its closely related to a holley, but not sure. anyway, on the carter AFB's there are 3 different accelerator pump positons. using the top position offers the biggest shot of fuel. it sounds like you might have a lean condition. i would see if there is an accelerator pump adjustment on your carburetor and tinker with that.
You need to understand that jet size is not the problem. The carb has to transition from the idle circuit to the intermediate circuit, on a smooth fuel flow basis otherwise you get a 'lean' hole in the fuel curve. The pump shot covers this hole on a (fast action) call for power, otherwise a slow advance of the throttle should transition smoothly to the intermediate with out a pump shot, when its right. The idle needle settings (ordinarly) can be adjusted to cover this transition. If not the carb needs internal passage work to attain this. Many 650 carbs are a pain to setup because they are not tuned to any peticular engine size or setup. Some people get lucky, many don't, with the operation of these carbs. It can be done but is a long learning curve the frst time through.
demon Well with Vac sec. you're probably not even opening them with a 302. You have to drop the spring tension down to get them to open. 85 jets are way to big 72 sounds better. 77 in the front sounds to big too. Drop to 70 and see what happens. It sounds like you have a vacuum leak somewhere. Usesome WD-40 and spray it around the carb while it's idling and listen for a change in the rpms. Remember Ford used 600 CFM carbs on the 390 GTs.
Fixed it! I put a bigger cam (orange cam) and it cured it right away. I also put the stock jets back in, 70's in the front and 78's in the back. I realize i probably don't even need a carb this big (625 isn't THAT big). However, higher compression and lighter cars require bigger carburetors, so i can't be that far off. The reason i was playing with jet sizes to cure the stumble is that one of my books says that before you can even begin to cure a stumble you have to get the right size jets in there. It recommended a plug test, and i think that my MSD makes it impossible to get a good plug test since most of the fuel is completely combusted due to the multiple sparks. P.S.--this carb is great....it uses all Holley parts--accelerator pumps, shooters, cams, jets, secondary springs.... (maybe that's why they got sued by Holley).
about that initial post he said he got his carburetor set. however, i have to agree that a high dollar, 650 cfm carb, on a 302 is overkill. ive had several (bone stock) 302's and my 500 cfm (edelbrock) carb was EASILY the best application. *note- im not talking about engines with hipo cams and other go fast goodies, just a simple 4V bolt on. ive had 600, and 625 cfm holley's, carters, and edelbrocks. (as well as the lil 2V), so i know what im talking about. the secondaries on my holley wouldnt even open. but as for the original poster... he did say he had 351W heads on it. if they were just stock heads, there is nothing special about them, and even though he has flat top pistons, i bet his compression is considerably less than he thinks (9:5-1) i was told that bolting on 351W heads dropped compression ridiculously low (like 7:5-1) he also has a very modest camshaft, and a high rise intake. this is probably why his carburetor wont respond at low rpm. its not a power package id want on my maverick, but to each his own.
The reason i bought the demon is because i got a good deal on it. Read as: cheaper than a holley. I don't know where you heard that bolting 351 heads on dropped the compression, but you read wrong...the 351 heads have smaller chambers than the 302 heads. 302 heads (varying with the years, but the ones i had have 64cc chambers. The early year 351 heads have 60.8 cc chambers, and mine have been decked to further increase compression. I'm sure that the 74+ heads have larger chambers, but i wouldn't know...even then i seriously doubt that they could put chambers large enough to drop compression to 7.5:1. The advantage to the 351 heads is larger valves (on the early heads, again). My intake is 1.84 and my exhaust is 1.56. Not huge, i realize, but a decent gain over the stock 302 heads, especially considering that i paid a little over $200 for the set, rebuilt. I'm sorry but i refuse to put Edelbrock garbage on my motor. Especially their carburetors. I have my reasons....Edelbrock being overpriced crap is one of them. The RPM manifold is what i ended up with, and i plan on a larger cam as time allows.
I've run 'em all (I've even ran a Quadrajet on a small block Ford, worked well)... Holley beats them all hands down. My .02 Dave
CACollo: Sounds from your description like you have a nice street combo of parts to your engine. The heads are a nice touch for low budget power. They are apparently overlooked by enthusiasts more than I would have imagined judging by some of the negative posts I am seeing here. Same with the Holley carbs. People just don't know a good thing. The previous poster commenting on your "low compression" might be surprised to learn that the small chambers, large ports, and large valves were common to all 351W heads from 69-77. (the 75-77s have internal emissions ports and are not as good as the 69-74s which were identical) Dave
Heads:69-72 351-2V have 60 cc chambers and get 9.5 on a 351 motor, put these heads on a flat top piston 302 and the ratio drops several tenths. Put these heads on a 73-76 302 and the cr drops to 7.8. Reference: Tom Monroe on building Ford small blocks, page 38 on cr and cylinder head interchange.
Whew, thanks for the positive response guys! I realize it's not a street machine, but it's mine and i love it. Sorry to get so PO'ed about the negative responses i got, but after all the time and money i've put into this car, anything but a positive response is enough to get me going! My motor is an '80, so i should have about 9.2-9.3:1? One last question, if anyone is still reading this post--my heads casting number is D0OE. I have a chance to get a set of the D7TE heads for about $100 (off of a '94 mustang GT). Which would be better, assuming i wanted to port and polish the hell out of either, and get larger valves (1.94/1.60 or 2.02/1.60 if possible) installed?
hey CAcollo, i never intended to post anything inflamatory/negative about your setup. i only posted facts as i knew them. these are based upon both personal experience, experience of freinds with "in common" setups, things ive read, and 2nd hand information. i did state your setup is not one id want for myself, but if yours works well.... more power to ya. there's certainly more than 1 way to fast. about the heads you mentioned as an alternative... 1. are you going to remove your existing heads? 2. ive never dealt with fuel injected setups, as i think a 94 mustang would be. i wouldnt advise you in anyway on something i know little about, but it may be plumbed differently. im sure the valve-train is different, so do your homework before you do anything rash. 3 finally.. casting numbers. you said your existing heads were D0OE. you also said the 94 heads you are looking into are D7TE. as ford casting numbers go... the 1st digit = the decade the heads belong to. C= 1960's, D=70's, E=80's, etc that 2nd digit is the year of the decade ie: D0= 1970 and D7=1977 so, your existing heads are not 1969 unless ford stamped 1970 in them a year early. (i doubt) as for your alledged 94 heads... unless ford kept an identical casting for 17 years, i dont think they are 94 either. (but i suppose as a bareblock head, its possible) in 1977, hipo heads were few and far between. (if existent) i have no idea what your budget is, but those aftermarket heads really work well. a friend of mine has TFS heads with chevy 2.02 valves and the car is lightening! but it tends to run rich and be slightly spongy in the bottom end. he sorta wishes he'd opted for the 1.94's. good luck, but i think that you can get by with your existing windsor heads. at least they flow somewhat better than a stock 302.
There's no such thing as a carb that's too big. It's a myth that's bought on by people buying 1050 dominators and putting them on stock 302's and such. If you think about it, a given engine might only be pulling 100 cfm at 2000 rpm at WOT, and if you have a 500 cfm carb, it's too big. So where does that put a 625? Actually, the booster size/shape/position will determine where the best response is. This is where dominators suck, because they're designed for high-rpm big inch engines, they dont work as well even on a decent 302. Yes, it's possible to get 20 mpg from a 750 double pumper carb, and yes I've done it. Actually, I got over 20 mpg on a mild 460 with an 880 vacuum secondary carb on my old truck. It worked much better than the 600 edelbrock did, mostly because there's so much more that can be done to the holleys. Now, dont let me confuse anyone enough to go out and buy a 850 double pumper for a daily driver, chances are, its designed for a mid-high rpm race engine, and wont work well unless it's totally reworked for a "milder" engine. For most small-inch street cars, a 600-750 works the best in most cases. This is just from my experience, for what it's worth (not much these days).
Roadrage, don't worry about it. I was just a little pissed when i first read it. The reason i was asking about the heads: My heads may actually be off an early '70 or so, all i know is that they are the "first" casting of the 351w heads. The D7TE did come off a '94 mustang...i know as they came off my girlfriend's '94 to be replaced by the TFS twisted wedge heads (very nice heads, i might add, especially with fuel injection). Anyways, i don't have that kind of money at this point, and i've seen a few flow numbers comparing the ported D7TE heads and some aftermarket heads, and they actually can be made to compete fairly well...i was just wondering if i should start with them, or if anyone knew whether my 351w heads would be as good/better starting point. I already have a dremel, so the cost would be marginal to me..
