850 Demon carb help PLZ.....

[Jim W]

I just put a very slightly used 850 Demon on a mild 396 BBC.
Basic engine run down is...
Edelbrock RPM air gap, HEI dist., closed chamber oval port heads no porting or polishing, hyd cam with a 900 RPM idle speed, forged TRW pistons with small pop ups, Bassett headers. FYI, fuel level is 1/2 way up the sight glass on primarys and secondarys. I set total mechanical advance at 34 degrees on 91 octane pump gas. 4 corner idle screws are set 1 1/2 turns out.
The problem is, when reving up the engine in neutral (driveway) the accelerator pump shot is strong for the first 1/2 second and then peters out.
I think it needs a longer duration shot.
When I first blip (heavy into the primarys but not secondaries) the throttle, for that first 1/2 second, it's rich.
Should I reduce the shooter size to get a longer shot or play with the acclerator pump cam???
Might also be the power valve not opening? If I crack open the throttle and hold it there, the engine will rev for a split second and then actually die and have to restart the engine.
Never messed with a Demon carb before. Can someone give me a little guidance with this????
Be good, Jim

[junkyardhunter]

A 850 cfm is kinda big for a mild 396ci.You could start with the factory setup that should give you a good starting point.
.http://www.barrygrant.com/demon/default.aspx?page=5

[Jim W]

Thanks junkyard for the link.
The carb is untouched inside, just like it came from the factory.
Not a whole lot of info at the Demon site though. Under troubleshooting they say to richen up idle screws and bigger nozzle but this thing acts like it needs a longer squirt time not just a bigger shot.
Yeah, the 850 is a little big but it's like brand new and the price was right.
Let's see if a Demon expert comes in with some help. Since it's Saturday night, I am not holding my breath...LOL
Be good, Jim

[junkyardhunter]

I ran a speed demon 850 Dp on my 468 chevy (575-600hp) the only change I made to the carb was a jet change (went down 3 sizes from stock).Try changing the the pump cam.

[SmokinLowriderSS]

You'd be better off on that 396 with a 650 or 700 CFM Holley.

[jbone]

You'd be better off on that 396 with a 650 or 700 CFM Holley.
I agree.
I ran a 750 on stck 502 and it ran great.
I'm currently using a Speed Demon on my 500 hp 502 and it is fine.
If you call the tech line at Barry Grant I think they will give you some advise. I've used the help line and was pleased with there customer service.
J

[cfm]

Acc pump is to supply fuel during momentary lag of metering system during a load change caused by increased throttle.
Maybe I'm missing something, but how do you figure out accelerator pump settings when in neutral and no load / change of throttle position ?
I agree that you should consult BG with a baseline setting (jets, PV, and acc pump) for your combo so you'll be closer and require less on the water tuning.

[Jim W]

OK,
I found a web site that has a guy posting up some really info on Demon carbs. Honestly, I have had some pretty poor advise from tech lines and would rather speak to someone who has real world tuning experiance.
Took the carb apart and gave it a good cleaning, re-set the throttle plate openings, have the mixture secrew set at 1 1/2 out and have it back on the engine.
Will see how things go after doing what this guy suggested.
"Maybe I'm missing something, but how do you figure out accelerator pump settings when in neutral and no load / change of throttle position ?"
Agreed CFM, this is not in the water tuning but the engine wont even rev up in neutral much less in the water with the prop turning.
Will post up what happened later today.
Be good, Jim

[jbone]

This might be a silly question, but I know you were in the market for a few things, including a dist. Are you sure you have the firing order right?
No offense intended.
J

[Jim W]

None taken JBONE.
184365729 right????? LOL
Actually, I spoke on the phone with the guy who knows these Demons very well and he thinks it might be a distributor spark problem.
I bought a new HEI for $50.00 and the thing looks sweet but apparently is pretty poor, spark energy wise. Just ordered a MSD HEI from Summit.
It's interesting that no where on the internet is there information about Demon accelerator pump tuning and the guy I spoke to says he never, ever has to mess with the pump shot. Kind of weird since the same engine in a light car could use a smaller shot compared to a heavy car. But, he sells and sets up 4-500 Demon carbs a year so I wont argue with him. Also found out from him, Demon carbs are rated at wet flow and Holley is dry flow. So now my 850 Demon is way too big compared to a 850 Holley.
Yesterday I got the engine fired back up with the carb all reset and it's 50% better but still has a hesitation. I will pop the MSD in when it shows up and see what happens.
Be good, Jim

[jbone]

Did you go with the MSD Billet Ready to Run??
I have that and am very happy.
J

[SmokinLowriderSS]

A co-worker of mine who just recently quit dirt-track racing cars burned up 3 MSD HEI coils in 3 years. Each lasted just about 1 season. Finally switched, and lost the problem. I do not recall what he switched to.
I do hope you don't have this kind of luck, but I'm not exactly hopeful of your prospects after a friend of mine had this kind of experience.
I am really happy with my MSD Pro Billet Ready to Run and high-vibration coil. (marine version)

[kcsteck]

I also have Demon.(Speed Demon 750) on my Olds. The Demon is a whole diff monster then the Holley. Really finicky! I would set the fuel level just below half not at half.When my carb was at half or just pass half ran like crap. Changed the level and it helped alot. Also I changed the Power valve as well. Dont Quite remeber what Number I changed to. Go to Web site and download the pamhlet about your carb. some helpful stuff in there.helped me.
Casey

[Jim W]

jbone and others....
bought the MSD Street Fire HEI. Summit P/N MSD-8362. Should arrive this Thurs.
The boat is pretty much sold but I have to get it to run properly.
Will post back on Thurs after putting in the dist.
Be good, Jim

[Cole Man II]

Jim W,
Go to the top of the page for Gear Heads, there you will see a thread called tech links. The second to last post on this thread has a link to guy talking about how to read spark plugs. This guy sells a book for $10.00 that you can down load on how to tune Demon Carbs.
Good luck.

[Jim W]

Cole Man,
I have the tuning guide already and he's the one that I spoke to on the phone.
Like I said, I bought this cheap azz dist. and am replacing it with the MSD unit after talking with him. Not completly convinced it's the dist. but after talking with him, a weak spark -may- be the problem. One thing he did say is that the dist. drive gears fail quicky.
Since this has already happened once with the old Accell dual point, (someone put in a unneeded, high pressure oil pump in which puts undue strain on the drive gears) I decided to swap it out no matter what. FYI, I now run a much lighter weight oil to keep the pressure down closer to the usual range.
The real fix would be to pull the engine and replace the oil pump but I for one am not going to do that.The next owner can.
Be good, Jim

[Cole Man II]

Sounds good Jim, let us know how it works out.