625 Demon Carb

[rwantin]

Good morning,

Seeking out someone that knows more about this stuff than myself (probably most of you). I've rebuilt and tuned motorcycle carburetors over the last twenty years or so, so not completely alien territory, but these are a bit different of course.

I bought my Mark II used, and am having a fantastic time with it. It runs really, really strong (only about 4,000 miles on the .030 over rebuild). It's an 89 302 HO, Ford Racing F cam, stock compression, ported, roller rockers, knockoff polished intake, F/F headers, and a older Barry Grant 625 Demon carb that looks all the world like a Holley to these aging eyes. It's the last part I am going to try my hand at tuning today.

It seems to me that it is running VERY rich at idle, and pulling a plug yesterday did little to dispel that notion. I understand with the nature of this engine, a BIT rich is probably fine, maybe even necessary, but I'm thinking, again, too much. Idle with this cam appears to be around 750 rpm, but warm (or hot) it's dropping to 500-600. Before I set warm idle I wonder if that is a function of the over-rich situation.

Altering the mixture seems obvious, and setting the float heights looks pretty easy on these with the lock screws and glass bowl eyes. It is my understanding that the midway point is a good baseline (and this is where I am seeing it), but maybe too much in this application? Not sure.

I also keep running across the apparently new-and-much-improved 625 Street Demon, which appears to have been introduced a few years ago. Anyone have any experience with it? It seems pretty reasonably priced and I'm not afraid of spending some money to make it run better.

I also realize that the carburetor subject may be much like oil - everyone has a different opinion on it. I'd certainly entertain other options as well.

Any insights/guidance beyond greatly appreciated.

-Robert

[Todd Buttrick]

No experience but that looks as though it's based on the Rochester carb. Not many have had good luck with the equivalent Edlebrock carb on these cars. If you're going to invest, take a serious look at the Quick Fuel 650DP. (SS-650-AN)

[skullandbones]

That doesn't look like a Holley to me. It reminds me of the Quadrajets but probably doesn't have the asymmetrical butterflies. If you haven't done this, check the secondary butterfly adjustment. If it has one, it allows you to fine tune that. They can actually be open too much. Next: the fuel float level I'm not sure on this one but I would start at 1/3 from the bottom of the window instead of half way. I like that feature. I would set the idle at around 800 or so as a starting point. BTW: the recommendation for the carb in post 2 is a good one. If you can't get this to work go to that one. Several friends I know have it or at least the same brand. The technical support is excellent from them, too.

Good luck,

WEK.

[rwantin]

Thanks much guys, I'll give that a try. Sorry, I should have said...the pic attached is the new one (the older Demon I have is a ringer for a Holley). The Quick Fuel setup sounds very intriguing as well - will check into that to be sure.

Again, much appreciated.

[CraigS]

You should be able to get the idle mixture just about perfect w/ the idle screws. If not, there is something wrong. Now, for the rest of the engine's range, that is a little harder. Seems to me a lot of these carbs are jetted really rich from the factory. I guess that is safer for them to reduce the number of complaints. If you want to mess w/ your carb I strongly recommend some kind of recording wide band oxygen sensor tool . I have an innovate1 and I understand that the current Innovate2 is better. I love my QF ss650An but will tell you that I have changed nearly every jet in it. It was lean at a steady 55 mph/1600rpm so it bucked and surged. It was extremely rich at full throttle. I enjoy the process but have bought a ton of jets to get it where it is today.

[Todd Buttrick]

You should be able to get the idle mixture just about perfect w/ the idle screws. If not, there is something wrong. Now, for the rest of the engine's range, that is a little harder. Seems to me a lot of these carbs are jetted really rich from the factory. I guess that is safer for them to reduce the number of complaints. If you want to mess w/ your carb I strongly recommend some kind of recording wide band oxygen sensor tool . I have an innovate1 and I understand that the current Innovate2 is better. I love my QF ss650An but will tell you that I have changed nearly every jet in it. It was lean at a steady 55 mph/1600rpm so it bucked and surged. It was extremely rich at full throttle. I enjoy the process but have bought a ton of jets to get it where it is today.

Yes indeed. What Craig says, spend the money on a jet kit (http://www.holley.com/36-181.asp). And some reusable Moroso meter block (http://www.summitracing.com/parts/mor-65222) and float bowl gaskets (http://www.summitracing.com/parts/mor-65224). Start with your IA screws one turn out and make sure you have your primary transition slot within range.

http://www.holley.com/data/products/pictures/large36-181v2.jpg

[rwantin]

Much appreciated (again). I had a look on the Jeg's site - looks very promising. I went through a not completely dissimilar process with the flatslides on an older Ducati. It came from Colorado and was jetted/set accordingly (not to mention some very old gas but that's another story). It's running fantastic now. I'm guessing that this process would be a bit more complicated.