Hopefully, you or someone else can give this answer, but I am having the same trouble with my demon as you were having earlier. I just wondered if you found a solution. I have the speed demon and it is rich as can be and falls on its face until I am up to speed. Different jets for this altitude? (remember I am in Rock Springs) just can't lean it out. Thanks and I appreciate your time.

 

I recently finished a very troublesome install of a 625 Road Demon on my 390. Runs great now. Please provide some more details about the problems you are having and I may be able to help you out.

 

Definately give us more details on your build. Cam, compression, timing, jets, etc...I think we can get you there. You will absolutely need to rejet from what came in it. When I called BG they said 1 jet size per 1000 feet. I'm sitting at 7000' here at my house, so I went down 7 numbers front and back.

I am getting mine closer. I'm going to make some more runs this weekend to see if I can get it good enough to call it done. The richness is going away, little by little. My plugs are dark tan after idling now, so I don't have far to go.

-Scouder

 

390- .40 over, 9.5 compression. using comp cams 270h. rpm intake. edel stock heads. I think that covers it. have all msd ignition, also using autolite 3924's gapped to .45 as adivised on this board. my machinist also races using BG and he gave me jets that dropped me 2 steps, and my power valve from 65 to 55 (I beleive that is what the numbers were). may be wrong on pv. jets in front were 70, and in the rear 76. had more trouble with these settings actually, then at stock. thanks for replies

 

Which model of carb?

-Scouder

 

Sorry, duh it is a speed demon 650 mechanical secondaries.

 

If you are reading your plugs after idling, all you are doing is reading the idle mixture. The jet changes have no impact on the idle mixture. That is controlled entirely by the idle circuit. You are not reading the actual running mixture. To do that you need to hold the throttle open at say, 2K to 3K rpm for a couple of minutes, and then shut the engine off WITHOUT letting it go back to idle. Then you can read your plugs and determine what your jet changes are doing.

 

You need to be more specific about what kind of problems you are having. A 650 Speed Deom with mechanical secondaries is a double pumper. That means you have an accelerator pump for both the primary and secondary venturies. I doubt that the power valve is your problem. Most likely you need to change the accelerator pump cams to prevent too much fuel from being dumped into the carb when you punch the throttle. Double pumpers are a pain to tune and get lousy gas mileage. If you are running this carb in a heavy truck (you didn't say), you would probably be better off with a vacuum secondary 650 Speed Demon.

 

Absolutely correct. The richness problem I am solving is at idle. I am already jetted pretty close up on the high end.

-Scouder

 

Coincidentally, I was in Rock Springs today for a while, looking for office space. We are going to expand into your neck of the woods.

OK. You need to refresh my memory on the speed demon. Does it have 4 corner idle? Or is it like the Holley Double pumper with just two idle screws?

Also, where do you have your timing set?

Describe you problem as precisely as you can. Are you rich at idle? Does it stumble off idle under light acceleration, or just when you really stab it? At what RPM does it pick itself up? Have you read your plugs after a good 3000rpm run, without idling, as posted above?

-Scouder

 

I'm looking at buying another carb and was leaning toward the BG. Now I wonder. I may just stick with Edelbrock.

 

I do have the 4 corner idle, and the timing is at 12 right now. before i stepped down the factory jets, it spit and fart until warm, but still run very rich at idle based on plugs, it did not bog with light acceleration, but like you said if you step on it, it would fall on its face picking up around 2500 to 3000. can barely get it to idle with current jets.

maybe debestuss is right!!

 

The jets have absolutely nothing to do with the idle. As I stated before, the idle is controlled solely by the idle circuit which the jets are not part of. I found that in order to get the idle mixture correct, you have to follow BG's instructions to the letter.

First of all, with a 650 Speed Demon, your initial timing should be 14 to 16 degrees BTDC. You must make sure that the idle transfer slot is adjusted properly. Then you need to set your four idle mixture screws about 1.5 turns from full in. Prime the carb and start the engine. Make sure the float levels are set properly. Let the engine fast idle until the normal operating temp is reached. Follow the Demon instructions exactly as stated to adjust the idle mixture screws. Use either an external tachometer or vacuum gauge (I used both) and tune for highest idle speed/vacuum reading. Ideally the four idle mixture screws should end up being between 1 and 2.5 turns from full in. If they vary a little from that, don't worry unless you end up with an off-idle stumble during the transition from the idle circuit to the normal operating circuits (mine ended up at 5/8 turns from full in and the carb runs great). If you have a newer Speed Demon, use the idle bypass screw (underneath the air cleaner hold down stud) to set your final idle speed. Do NOT change the throttle plate settings. Once you get the idle mixute right, than you can play with the jets, accelerator pump cams, etc to tune the carb across the entire operating range.

If you don't understand what I have written, then reread the carb instructions or pick up the book "How to Tune and Win with Demon Carburetors." Its a great book and I found the instructions made much more sense after reading the book.

Good luck.

 

What he said.

It doesn't look like your build is that radical. You should be able to set you idle without doing anything crazy. Since my Mighty Demon was built prior to the EZ-Idle system, I had to drill two 1/16 holes in the primary butterflies to get it to idle without changing the butterfly position from what BG recommends. Like I said, you should NOT have to do anything quite so radical. When you say it won't idle, I am assuming you mean it idles too low, and dies. Start from scratch following the instructions provided exactly, so you have a good baseline, then tune from there. I discovered that timing is very important in getting the engine to idle correctly, mine likes alot of advance.

If you can't get it just right, maybe you should make a trip over here so we can gang up on it one of these days.

Keep us posted on what you do.

-Scouder

 

I didn't say it directly, but hopefully you read between the lines, but as Scouder says, you should return everything, including any jet changes that you made, to what the carb was like as it came out of the box. Check the settings as Demon advises, and adjust only what needs adjusting (Idle transfer slot for example) if something is out of spec. Get the idle circuit tuned right first, then make whatever changes you feel are necessary from that baseline. I started over at least twice with my carb before I was satisfied and I only had to adjust the idle circuit and change the vacuum secondary spring. Didn't have to mess with the jets. The carb runs just like its name ---DEMON!