[base1969]

engine idles great. it sputters and dies when i put it in gear. it's a 383, rpm heads, no idea on the cam. carb is a speed demon 670 m/s. the highest vac reading i get is 12 at idle. floats are good right in the middle on the sight-glasses. ignition is msd with brand new blaster 2 coil. i replaced it because i pulled the coil wire and it was full of oil. initial is 18, and total is 36. vac adv is 10º i've tried initials of 14 through 20, with 18 being the best. i found a bad plug wire and replaced it. i adjusted the carb with a best vac reading of 12 at idle and 5 in gear (right before it dies.) i read here that vac diaphrams are rated to activate around 10-12 inches? i was gonna pull the plugs to inspect but it got dark and my boys showed up with beer. any help/ideas are appreciated.

 

[sevenzeronova]

What rpm in park and then in gear?

 

[base1969]

i'd have to guess, as i don't have a tach. i would say around 1200/500. i also tried raising the idle to about 1800/1100 in drive. this same car has run well in the past with improper tuning and pure dumb luck.

 

[mdw66nova]

A little history might help. Recent rebuild? Does the Demon have the adjustment screw under the breather mounting bolt? If so, this will help. Read the instructions for the Demon. What is the throttle response when in park?

 

[base1969]

i adjusted the 4 idle mixture screws to best vac of 12. the vac diaphram does not seem to be functioning due to the low vac of 5 under load/in gear. i know it works at idle because my balancer shows the add'l 10º
as far as the history, it was built at a good local speed shop with high reviews. my brother knows even less than i do and didn't bother to save the build sheet, or even the parts list. he ended up selling it to my other brother with no info and i'm trying to figure it out.
it seems to me that it's an ignition problem. i hope the plugs show something obvious.

 

[base1969]

the throttle response in gear is sputtery and missy. it's awesome in park; no issues. it runs great in park and sounds excellent when i rev it up for cheap thrills.

 

[mdw66nova]

Is it back firing through the carb?

 

[base1969]

no backfires at all. it just fails under load. i initially thought it was the coil but i replaced it and same result.

 

[mdw66nova]

You said this motor has run well before. Have you adjusted the valves lately. Not enough valve lash can cause it to die when put in gear. My engine had similar issues but I worked it out with hotter plugs, fuel mixture etc. I heard Demons without that added adjustment screw are hard to tune. When does the accelorator pump come in. Delay it a little. Does it have a choke?

 

[base1969]

it's a hydraulic roller cam. i didn't assemble the eng and i have never adjusted the valves or any valves for that matter. they're not ticking but now that you brought it up i do hear some noise coming up through the intake and carb. hard to describe but kinda sounds like an exhaust leak. i did tighten the headers and collectors so there aren't any exterior leaks. i don't know enough about carbs to check the pump.
i'm pretty much at the level of diagnosing i feel comfortable with, if my plugs don't show something obvious i'll have to take it to a shop. the problem in the past has been finding a good one. most shops say they can do everything...not the case.

 

[Esteban mejia]

i just finished tuning my 383 stroker about 1 month ago, its got brodix aluminum heads, kb pistons, and a 650 vacuum secondary speed demon, i too have it at 18 initial timing, with 16 degree stops on the distributor,all 4 my mixture screws are all backed off 1 full turn, it idles at 1k in park and in neutral, and in gear it drops to 850 ish, did you try raising the float level? i had to do my valve when trying to tune the engine it was running really rough and wouldnt idle right in gear, it was dieseling and spitting out the venturies and want to die. , try to adjust the valves, the difference between mine and yours is with my engine is im running a hydraulic flat tappet cam. if you go to the demon website, they actually have a troubleshoot section for their carbs with 2 sections,(symptoms) and (Remedies) but yeah id raise the float levels a little more, check the valves and open up the mixture screws out more.

 

[linthicum]

Not sure about the Demon's but if your vac is dropping to 5hg in drive your power valve may be opening up and flooding you out. I am a Holley guy and I do not even know if Demon's have power valves. So for conversation sake let's say they do. Your power valve is held in by vacuum and opens up when you floor it and vacuum drops off. So if your vacuum is 5hg then you are going to need a 4 or 4.5 power valve.
Also I would try to get your idle down to 800 when you are setting your timing. You can turn it back up after you have set it. I did not see what kind of distributor you are running or what you did to it. But if your rpm is up you may be into your advance when you set your timing.

 

[base1969]

thanks guys. the dizzy is an msd out of the box. i'll look at the demon web site. i'm also gonna pull the plugs and check'em. while they're off, i'll do a compression test. that should give me an idea on the valves.

 

[73Ventura]

Heres something to try just for the hell of it. Disconnect the vacuum advance at the carb and plug the input on the carb. Bring your idle back up and see how it runs.

 

[87monteSS]

have you verified fuel pressure? did you look down the carb with the car idleing and make sure its not sucking fuel in through the boosters in idle and when put in gear? hook a tach up to the car, it could be idleing way too low and you think its higher. somethings weird being that it drops 7in of vacuum when put in gear, does it sputter and die instantly when put in gear, or does it only sputter and die when reved up in gear? you dont have the curb idle screw turned in very far do you? it almost sounds like its starving for fuel, try turning the idle mixture screws out some more and see if that helps

 

[base1969]

have you verified fuel pressure? did you look down the carb with the car idleing and make sure its not sucking fuel in through the boosters in idle and when put in gear? hook a tach up to the car, it could be idleing way too low and you think its higher. somethings weird being that it drops 7in of vacuum when put in gear, does it sputter and die instantly when put in gear, or does it only sputter and die when reved up in gear? you dont have the curb idle screw turned in very far do you? it almost sounds like its starving for fuel, try turning the idle mixture screws out some more and see if that helps

it sputters and dies when shifted into drive. i can give it fuel and it will sputter longer but eventually die. the curb idle screw and a/f mixture screws are at the highest vac of 12. i've backed them out further but i lost vac and it ran worse. the fuel pressure seems good. the sight glasses on the floats show the fuel level right at the middle. what gets me is that it runs great in park, but will only fail under load.

 

[ncfireman1918]

Has this combo run well in the past? Maybe take a look at torque converter. Too low a stall speed will give the exact result you are seeing. The converter and the cam need to be pretty well matched, especially if the cam is fairly aggressive. I just went through this on a new engine, ended up swapping cams for something a little less radical.

 

[base1969]

this combo has run excellent in the past. you bring up a good point about the converter. the machine shop that built the engine matched the converter. my brother (his car) just had the trans rebuilt. i'll get underneath and see if it's the same one. it was a b&m 2800 stall. thank you for the advice, i didn't even think to check if the converter was replaced.

here's a video of the car when it ran well.