Demon 850 dieseling when shut off

[40k Later]

I'm experiencing dieseling after I shut the car off. It idles at 900RPM, has the factory jets, idle screws are all 1/2 turn out. The B/G website has a troubleshooting chart but not for this issue. Easy fix or is it a miriad of things?

[Cobrabill]

Half a turn is too little.Seems like the idle screw is holding the throttle open to compensate for too little idle circuit fuel.Open them up to a full turnturn& 1/2 out.then back the idle screw back down to 900rpm.If it makes a difference-you're headed in the right direction.

[blykins]

Might be idling too high.....mine did the same thing....until I turned the idle down.

[CJ428CJ]

You might also try retarding the timing a couple of degrees. Last but not least, another trick that might allow you to "live with it" is to put the car in neutral and release the clutch before turning the key off. The extra drag created by gearbox will often result in the car shutting down without dieseling.

Chris

[40k Later]

Barely stays running below 900rpm. I turned my four idle mix screws out to just over 1 full turn and the car stumbles when I nail it. Also the plugs are black. The B/G website recommends raising the float level for the stumble which I did and it helped some but theres way more car waiting underneath those fuel soaked plugs. The dieseling comes and goes. It did it real bad with high temps but after installing dual fans it seldom goes over 180.

Carb gurus speak freely, I need crisp throttle and brown plugs with no dieseling!!!

[Bruce Edwards]

If the plugs are black she is set to rich. Recheck the float level. If it is ok you need to drop your jet size or you have a blown power valve.

[40k Later]

The carb has 650 miles on it so I'm hoping no power valve problems. I've spent alot of time this past weekend researching B/G carburetors and the consensus is they run rich most of the time when at or near idle. I reckon I'll give B/G a call and see what they propose.

[40k Later]

Spoke with the good people at Barry Grant today and before I could get two words out of my mouth I was informed the 850CFM was much too large for my 460.
When I bought the engine from Motorsport I asked which carb they used to obtain the rated horsepower. I was told they used an 850 Holley out of the box with minor adjustments. My thinking, the B/G is an improvement, why not go with that. Same CFM, WRONG. Quote "Our carbs flow significantly more fuel than a Holley. He told me to stop messing with it, because no amount of adjusting will keep it from running rich. He said send it in and they will change the boosters to the larger diameter so theres more air and less fuel. $65.00 plus round trip shipping. There goes another $100.00 to my hobby.

[Bruce Edwards]

One problem. The boosters have NO effect at idle. The idle system is controlled by bleed holes in the carb has is not tied to the boosters. When you "tip" the throttle blades past a certain point the main system / boosters kick in to play.
If a carb flows 850 on a bench it should not matter who makes it. 850 is 850 on a flow bench.
I can not see how a 850 is to large for a 460. We use them. Hell we install 750's on 351's and have no flow problems.
A carb that flows more fuel with the same cfm is just plain set to rich. Don't forget that all of the fuel after idle goes in by way of the jets and the boosters size has more to do with the way that fuel is mixed in the venture. Booster size will play a part in mix at different flow rates but it still can not dump any more fuel than is let into the system by the jets.
Of course if the power valve is blown it dumps fuel in the engine and no amount of adjustment will solve the problem.

[Randy Rosenberg]

This won't fix your Demon problem, however I need to insert here that I just had my 750HP re-calibrated by Power System for my create 460 (535HP), and WOW! What a difference! Starts immediately when cold, idles smooth after just 15 to 20 seconds at high idle (no choke, just my foot), & smooth transitions between circuits. It does, however, run-on a tiny bit about 10% of the time when I shut her down, but I attribute this to the poor winter gas - I just make sure she's in gear and ease the clutch out a bit when I shut her down. I don't have enough miles on the "newly calibrated" carb to see the impact on MPG, however I'm able to get into 5th a lot sooner than before, as she now runs smooth 1300, 1400, 1500 RPM.

I know this doesn't help you with your 850 Demon, but I just wanted to point out that maybe a 750 would be better, as I'm really happy with my 750 on my 460.

YMMV,
Randy R...

[TWM]

....seems as if Bruce Edwards has pinpointed the problem - should be metering fuel and air through the idle circuit. The CFM rating should not be an issue.

When the engine is idling, take a look down the secondary venturi (careful here). If fuel is slobbering (old tech term) into the secondary venturi, it means that the throttle plate isn't closed - when the throttle plate isn't closed, fuel will continue to be pulled through the main metering circuit after shut off and cause the dieseling that you describe. If the problem is intermittent, the throttle may be sticking slightly open, occassionally, for some reason. If the dieseling always occurs, it is time to go to the basic adjustments. As Bruce Edwards mentioned, make sure that the power valve is OK and then get the float level right. Then check for mis-adjusted idle speed screw and idle mixture screws. ...time to hook up the vacuum gauge. Hope that helps.
TWM

[40k Later]

I haven't messed with the secondary plate adjustment at all and no fuel is visible. The slot which is visible is square in dimension. The floats are adjusted to the bottom line of the site glass.
I would love a carb which would allow me to get out of the car and have it stay running after a couple of minutes. I sit there for at least five minutes to keep it running.
The B/G website also confirms what the tech mentioned about having too much carb.
I can make the carb stop dieseling at shut off but with the particular setting but then the plugs are black with soot.
So are we pointing toward the jets now? Assuming I don't need to mail in my carb. What is a good indication of the power valve being defective? Good stuff here, I aprreciate all of it

[Bruce Edwards]

First off I see no way that a 850 could be to large for a built 460 style engine. Get the model number of the carb and check the jet size. I would pull the carb after a short start up and check for fuel running out of the bottom in the area of the power valve bleed hole.
Some cabs are set for different things. If your carb is set up for a blown motor it will be to rich for your set up.

[Curt C.]

Go to the book store and get one of the books on how to tune Holleys or BG's. I believe Cartech makes them both.

You have multiple circuits here that need to be addressed. Hook up a vacuum guage to the carb and set the idle screws with it. You are looking to achieve the highest vacuum while maintaining the desired idle RPM. Make sure the throttle blades are set accordingl to BG specification. Also, you have to set the timing accroding to BG - initial at 16 - 18. Make sure total is where the engine manufacturer suggests. This will help a lot. It should not diesel after doing these items but if it does you can pull a degree or 2 out of the initial or slightly richen the idel circuit.

With the idle mixture set then start looking at the jets by reading the plugs. Better yet, go out and get a wideband 02 sensor from the likes of PLX or Innovate ($350). Install a bung in the exhaust, plug in the 02 and carb tuning becomes much easier. Or, put it on the Dyno and use their wideband.

By the way, I would think an 850 would not be too much carb either.

[SCOBRAC]

A myraid is exactly 10,000. I doubt it is that. Did it run on prior to installing tthe Demon? Was there a carb on the engine before that it didn't run on?

[40k Later]

1st carb on new crate engine, didn't diesel in the beginning but now after 600 miles of driving it did constantly. I have made the following adjustments. 900rpm @ 180 degrees water temp, the four corner idle screws out 1 full turn with the float bowls at the bottom of the site glass. It runs very choppy, stumbles when you nail it, smells like fuel, and sometimes diesels' sometimes not. Plugs are black. Incidentally no black smoke whatsoever and the tips of the side pipe don't appear to be covered in soot.

[Bruce Edwards]

Let me ask a dumb question. How is the spark? A weak spark will cause the same kind of problems.
When the engine was new how did it run? Same as now with the black plugs or is that part as well as the run on new?
I see in your listing you are running a NOS system, is it leaking fuel into the engine?

[Bruce Edwards]

One other question that will help make this a bit clearer at least for me. If you shut off the idle screws will the car run better or shut off? If you turn them in and the engine smooths out the idle system is dumping to much fuel. No other thing will do that short of your NOS system leaking fuel. If the engine has EVER backfired the powervalve could be bad. If the carb sat on a shelf someplace for a long time the valve could be bad. If the engine idles with the screws turn almost all the way in and the float level and bleed holes are clean it MUST be bad.

[40k Later]

If I turn the idle screws in to a half a turn or less the car starts to die, if I turn them out to 1 1/2 the idle picks up. Haven't lit off the nitrous until my timing controller is connected. As far as spark I assume the msd 6al new off the shelf is working just fine. What do you guys think about putting smaller jets in?

[Bruce Edwards]

Install smaller jets. Sounds like from your last post that the idle system is working. If it has a flat spot off idle the main system is to rich and thus the fowled plugs. You did not say if it ran better when you first started driving the car.
Is this a double pump carb?