Demon Carburetor issue

-
Sorry for the delay, I am approaching my finals for school and just haven't had anytime to really check timing, vacuum, or play with the jetting. Hey Mike! Yeah, I kinda feel like I should just take apart a 4160 series 750 cfm holley I have in my closet and swap them, but I will at least swap out things and see what happens. I jetted it down because I thought at the time that 76 and 83's were too rich considering my 670 cfm street avenger had much smaller jets and ran ok. This is the plan for the upcoming Turkey Day since I dont have school or work:

Check timing and adjust to see what happens
Check Engine Vacuum and change power valve if needed.
Test run with fresh gas.

After that switch from 72/76's to factory setting of 76 and 83 jets
Test run

If problem still is happening I was going to switch to a lighter vacuum secondary spring and try switching out springs in the distributor and play around with the timing.

I am hoping that with changing the jets and possibly the timing will help the issue but it is things like this where you really learn about tuning and your motor so even though my car is not running 100% where I would like it to be I am excited just to forget about school/work and tinker around with the car.

Thanks again to everyone for the information and advice!
 
So I changed to the stock 76/83 jets and it runs like crap, it wont even hold an idle in gear no matter what we do, and a vacuum reading is about 10in of vacuum so my power valve is correct with a 4.5. With the jetting I switched it to before at least it ran great and held a better idle, it just had a miss at freeway speed when I nailed it. On the freeway it still stumbles but pretty much dies unless im on the gas at a stop light or kick it in neutral. So I am going to switch back to the jetting I had before and put in a lighter vacuum secondary spring and see what happens with that and report back.
 
Unless a demon carb is designed totally different than a Holley, which I don't think they are, main jets have no effect on idle unless one or both are plugged up with crap. The idle circuit gets it's fuel from the main jets but it regulates it via the idle mixture adjustment screws and air bleeds. If the idle is messed up it isn't because you changed the main jet sizes. You could conceivably totally remove the jets and it would still idle the same. My guess is when you changed the jets something happened to cause one of the metering block gaskets not to not seal or you got dirt/debris in it that has one of the idle feeds restricted
 
So I changed the gaskets while I was at it when I put the 72/76 jets back in and noticed a light brown tinge of debris in the metering blocks and fuel bowls. So with that discovery I am assuming water got in somehow or it is some crap gas that I had in there. Timing is at 18 degrees at idle and all in at about 2500rpm and its at 36 degrees. With the old jets and some fresh gas it runs a lot better and way I had it before it holds an idle. But am thinking I may have had some crap gas which may be why it wouldn't idle but in the ten years I have owned the car it never ran like that before. I am now going to take out the vacuum spring and see which one I have and play around with that and see if it helps the freeway speed hesitation.

Fishy, didnt know that about jetting and idle but good to know now!
 
69dodgedart360 said:
So I changed the gaskets while I was at it when I put the 72/76 jets back in and noticed a light brown tinge of debris in the metering blocks and fuel bowls. So with that discovery I am assuming water got in somehow or it is some crap gas that I had in there. Timing is at 18 degrees at idle and all in at about 2500rpm and its at 36 degrees. With the old jets and some fresh gas it runs a lot better and way I had it before it holds an idle. But am thinking I may have had some crap gas which may be why it wouldn't idle but in the ten years I have owned the car it never ran like that before. I am now going to take out the vacuum spring and see which one I have and play around with that and see if it helps the freeway speed hesitation.

Fishy, didnt know that about jetting and idle but good to know now!
Click to expand...

I hope you gave it a douche with carb cleaner.
 
Now we've gone from a cruise/drivability issue, to a idle issue as well.

I assume that you believe by jetting back up, that you've gone too rich again? I'd like to know whether this new idle problem is from it loading up with fuel? Are you getting black smoke out the pipes when having to feather the throttle? If so, high floats or too much fuel pressure could still be in play. If not, I still think something else is going on.....like a undiscovered vacuum leak.

As far as the PV being just right, I have a different take on that, but that's not what's causing this new problem.
 
69dodgedart360 said:
So I changed the gaskets while I was at it when I put the 72/76 jets back in and noticed a light brown tinge of debris in the metering blocks and fuel bowls. So with that discovery I am assuming water got in somehow or it is some crap gas that I had in there. Timing is at 18 degrees at idle and all in at about 2500rpm and its at 36 degrees. With the old jets and some fresh gas it runs a lot better and way I had it before it holds an idle. But am thinking I may have had some crap gas which may be why it wouldn't idle but in the ten years I have owned the car it never ran like that before. I am now going to take out the vacuum spring and see which one I have and play around with that and see if it helps the freeway speed hesitation.

Fishy, didnt know that about jetting and idle but good to know now!
Click to expand...

Crappy gas may have been the issue. I got some once and it drove me crazy trying to figure out why the car was sluggish and pinging and not idling good. Once I figured it out and cleaned things out all was well again
 
So I purchased a rebuild kit for the carburetor and am going through it at the moment cleaning up everything the best I can. I am going to switch again from the 72/76 jets to the stock 76/83 jets and I am going to stick with the 4.5 power valve due to vacuum reading in gear at idle and leaving in the green pump cam that I had in that ran well before everything got worse. I also see that I have the standard vacuum secondary spring and am going to leave that in and see if it runs any better at cruise with the bigger jets and then go from there with spring changes. Since I am here I am going to change the manifold gasket with hopes that maybe there is a slight vacuum leak I am missing.

So at the moment the plan of action is:
a new manifold gasket
a cleaned carburetor with bigger jets
a green pump cam and see how it responds.

Parts arrive tonight and my next free day is going to be Sunday so I will get back to everyone on Sunday with how it all goes. Thanks again.
 
Sounds like a good plan. Make sure and spray out all the passages in the carb real good with carb cleaner and if you have an air compressor handy blow them out also. With carbs cleanliness is paramount. Also inspect it real good for any aluminum shards that may cause a problem such as blocking a passage.

Here's a little trick that helps if you have to ever take it back apart, apply a thin coating of chap stick (or any type of wax based lip balm) to all the gaskets and seals. The chap stick won't let things stick so the carb will come right back apart real easy without damaging the seals. Hope you don't have to, but just in case
 
Thanks again for the advice Fishy, so I got it going, sprayed the runners to check for vacuum leaks and no vacuum leaks and looks like my problem was in some crappy gas, not enough pump shot and needing to adjust the nut with the arm that rests on top of the pump cam. The motor really likes the green pump cam so I stayed with it. It now idles actually better and there is no bog anywhere in the power band, I did go back and put the 4.5 pv back in too. It has the stock 76-83 jets in it too. Thanks to everyone for steering me in the right direction for solving my tuning issue!
 
You must log in or register to reply here.
-

Back
Top