I've been going nuts to tune a Speed Demon 650 carb on this mild 355 street motor. It started out really rich so I've been leaning out the main jets step by step. I finally got to the point where it's running lean. It developed a hesitation on acceleration. Ok, so I went to step the jets up a couple of sizes and I took a look at the plugs. This is what I got. The tips are white like you'd expect with a lean condition, but the back of the insulator is sooty like it's running rich. They were new plugs before I rejetted last time and probably had 20-30 miles on them. Is it possible that the idle circuit is rich? The exhaust smells a little rich at idle. I tuned the idle mixture with a vacuum gauge and they are about 1 turn out. Anyone? Ideas? Suggestions?

 

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Powdery black carbon,..that's definetly running rich somewhere in the RPM band,..probably at cruise but that's a guess. The tip looks fine to me,..and see how the carbon starts/stops mid-way on the elctrode? That indicates that your heat range is fine.

Your float levels good?

Make sure your idle mix is spot on.

If so, try smaller primary jets,..though stock jetting on a 650 atop a 355 shouldn't be too rich.

The hesitation you speak is probably due a insuffecient pump shot from the accelerator pump,..try two sizes bigger accel. cam.

Laslty, a blown power valve will cause rich condition,..might check that.

Let us know what you find,..curious.

 

How do you know that the engine started out really rich? What are you attributing a rich condition to?

I had to richen the jets on my demon but lean out the squirters. Need more info... And if you have an idle eze, make sure that it is open all of the way.

 

they look VERY rich but the question is what circuit of the carburetor is rich. Lars did a post on how by reading the exhaust gas with a innovate LM-1 air/fuel meter he found his idle system was too rich and on his speed demon so he had to restrict the idle jet. the mighty demons have replacable idle jets plus replacable idle and main air bleeds but the speed demon does not. you could put changable idle jets in the carb if you wish but i think you should only do this level of jetting with a exhaust gas analyzer or wide band o2 sensor based air/fuel meter and the use of a tool such as them is the best way to find out what circuit of the carburetor is too rich!

 

Thanks for the replies.

I believe it's lean because I dropped the primary jets 8 sizes from stock (72 to 64). With the 72's it just ruined the entire plug in a few miles to the point where it would hardly run. I thought is was strange that half of the plug would be clean and the back part sooty. Plus I can't believe that I'd need to go smaller than 64 on the primary side.

The float level is near the bottom of the sight plug.

I was pretty careful setting the idle mixture for maximum vacuum and I made sure all 4 screws are equal.

I guess I should check the power valve, although it's a new carb.

I don't understand how the idle jets work. As far as I know the idle mixture is controlled by the mixture screws. What would I have to do to change those jets? I guess the bigger question is why would I need to. It seems like this carb SHOULD be a good match for this engine.

One other thing I'm not sure about is the idle eze valve. I messed with it a little when I first put the carb on, but nothing I did with it seemed to have any effect. I finally just closed it all the way. I figured it would only come into play on a more radical engine that had problems idling.

At this point, I think my problem is in the idle circuit. Maybe the plugs are loading up at idle and then partially cleaning up while cruising. Thus the half and half plug. This is my brother's car and he'll bring it over this weekend for some more fun!

 

the idle jet controls the mixture at idle and off idle in the case of a demon about 2500 or so rpm at light loads until it gets into the main circuit (both main jet and power system). there is a book on tuning a demon carb that will show pictures of the idle jet but it is on the back side of the metering block. one of the tricks Bill Jenkins did was to put a wire (about 0.025") thru the idle jet of a holley to lean out the idle and off idle circuits. this is in the Bill Jenkins small block chevy book if you have a copy. good luck and happy tuning

 

Demon carbs are pig rich on the idle circuit. i had to reduce the idle feed restrictor on my 750 race Demon from 0.39" to 0.33" to get the low-end cleaned up. i highly recommend the LM-1 tuner, it has worked wonders for my carb. it's the coolest tool in my box hands down. also, how is your timing set?? Demons are very picky about initial set-up. you have to set the idle transfer slots to "little squares" and then set initial timing to best idle. after that, set your idle mixture screws. i found that i needed to re-curve my distributor to only get 18* of mechanical advance and 36* total. if you want to replace the idle feed restrictors, they are the on the opposite side of the metering block from the main jets. each side of the metering block (left and right) there will be five air bleeds in a neat row and the idle feed restrictors are the lone men out towards the top of the block. Jeg's sells replacement bleeds (#6's) and drill kits. you can also get the drill kit from SEARS for a few bucks cheaper. the bits are up to 0.39" and a pin vice is easier to use than a drill motor. the jets are like $6 of a pack of 10 and you need four for each step change. i love my tuner and exhaust clamp, super easy to use and tune with.

 

how can they be rich when the electrode is bright white? Looks lean and hot to me......

 

looks to hot to me as well, almost like the gas is dumping in the cylinders at shut down or idle, the white is hot....needle and seat might have dirt in it?? is gas seen dripping at idle?? don't know much about Demon...

 

If your primaries are too far open at idle the transition slots will be uncovered and she'll run rich to very rich at idle.

Advancing the initial timing will help as will running manifold vacuum to your vacuum advance (not ported or timed vacuum). These two moves will increase your idle speed,..then you can reduce the idle at the carb which closes the primaries and covers the transition slots.

Or, some folks open the the secondaries a bit at idle which allows you to reduce opening of the primaries. There's an adjustment on the passenger's side of the secondary throttle shaft.

The above and checking for a blown power valve is where I'd start.

Lastly,..dropping the jets eight sizes is no bueno,..you're gonna burn a hole in a piston. Go back to stock and tune her two sizes either way. If it needs more or less, there's something wrong elsewhere.

You're rich in one of the circuits (idle, transistion, main, power),..just need to find out which one and why.

Good luck!

 

If your primaries are too far open at idle the transition slots will be uncovered and she'll run rich to very rich at idle.

Advancing the initial timing will help as will running manifold vacuum to your vacuum advance (not ported or timed vacuum). These two moves will increase your idle speed,..then you can reduce the idle at the carb which closes the primaries and covers the transition slots.

Or, some folks open the the secondaries a bit at idle which allows you to reduce opening of the primaries. There's an adjustment on the passenger's side of the secondary throttle shaft.

The above and checking for a blown power valve is where I'd start.

Lastly,..dropping the jets eight sizes is no bueno,..you're gonna burn a hole in a piston. Go back to stock and tune her two sizes either way. If it needs more or less than two sizes from stock, there's something wrong elsewhere.

You're rich in one of the circuits (idle, transistion, main, power),..just need to find out which one and why.

Good luck!

 

Reading spark plugs is an old black art...a very inaccurate art. Notice how white the tip of the electrode is? You have no idea when its soot fouling the plugs. Get it on a chasis dyno and make some pulls, tune the WOT first, optimize your timing etc. Timing will have a lot to do with plug fouling at cruise and part throttle. You can get a wideband and do it that way but its gonna cost you $400 for the lm1. That would be your best bet for full range tuning. Reading spark plugs has WAY too many variables that corrupt the readings.LOL

 

This is what I'd really like to do. Does anyone know of a shop with a chassis dyno in S. Florida?

 

The idle bleeds look like jets on the top of the carb for carbs that replacable idle bleeds. a wire in the idle circut does the same thing and costs nothing if you have some wire laying with fine strands think, .016 does the trick, bend it in a 'L' shape and insert it into the idle circut and the gasket will hold it there.