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what is causing my demon to idle +200 rpm after a drive?

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MY FIRST 69

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Discussion starter · #1 ·
** i posted this on pro-touring as well **

im running a 750 mechanical secondary mighty demon on my carbed ls1. when i first start the car it idles at around 900-950 but after a quick drive around the block it will idle at 1100 rpm. what is the major cause for this?

the carb is out of the box stock. i literally took the carb off my old 355 and threw it atop my ls1 and ran it as is. i checked the gap between my accelerator pump linkage with a .0012 feeler gauge, that checked out. i have two large springs pulling the linkage shut (sometimes wonder if the springs have too much tension) and im running a home made heim joint throttle linkage with aluminum holley bushings like these at the carb linkage and at the throttle pedal arm.

Image


i noticed my flots are a tad low, about 1/4 of the way in both the front and back. i didnt recheck the transfer slots before installing the carb, but it ran great on my old small block. the idle screws are about 1/2 turn out if my memory serves me. the idle is a tad rich but the car has no stumble. i have the idle ease screw but it never changes anything when i mess with it.

my cam is rather large and only pulls 10" of vacuum when the gauge is hooked up to the carb.

Texas Speed Magic Stick 3 cam - 237/242 .603/.609 113LSA

i programmed my ignition to run 17* timing from 0-1000 rpm and slowly ramp it up to 29* total at 3000 rpm.

also my fuel pressure is at 6-6.5 psi constantly.

i can press on the linkage with my hand and the idle will lower as long as i put some force on it.

and lastly. here is a pic of my home made linkage. the throttle arm sits way low compared to the height of the ls1 intake. so i bent up some threaded rod. the Z shape needs to be revised. its actually too long and i cant achieve full throttle at the moment. i was thinking of making my new linkage a lazy L shape.

Image


Image


should i run this return spring bracket instead? (my old linkage is pictured. too bad it was way too short to make it to the throttle arm).


Image
 
Discussion starter · #4 ·
its been very reliable since i built it. starts up every time. i am yet to experience all of its potential because i have a busted t-56 (bad 3rd) behind it and my throttle linkage is off so the secondaries dont open all the way. other than that, its very easy to set the timing on.

Because the manifold is warming up. You need to set the idle with everything at opp. temp.
Click to expand...
so the linkage is warming up and getting relaxed? if i remove the return springs the idle will rise on its own also.
 
Discussion starter · #7 ·
no not yet.

is something too tight? on my old 355 i only used one return spring and never had the floating idle problem. the ls1 firing order sounds weird and my cam isnt as lumpy in the 1000 prm range. it gets pretty lumpy at 850 or so.
 
Discussion starter · #8 ·
found out what the problem was today, but im yet to fix it.

i drove the car and got it hot before i took some idle out if it. drove it around the block again and still had the 1100rpm idle. i then removed the carb from the engine and flipped it over to take a look at the transfer slots. i noticed the secondaries were showing a lot more than the primaries. i slowly opened the throttle all the way and closed it slowly. thats when i noticed that the secondary butterflys were sticking open. im not sure if they are binding on the bores of the carb or if the linkage is bent.

im thinking about sending it off to a carb tuner and having it taken apart and inspected.
 
Discussion starter · #9 ·
well i ended up taking the carb to a pro. i had the carb shop take it all apart and inspect it. $350 later i have a remachined and reworked demon.

my dad picked it up for me today. the tuner said the carb was packed full of metal shavings and the metering blocks were all screwed up. he ended up replacing the metering blocks, changing the jet sizes, cleaning everything, and remachining the throttle bores.

it really sucks to know that a huge company such as barry grant sells such a poor product. i guess the myth about demons poor quality was true.
 
#10 ·
MY FIRST 69 said:
well i ended up taking the carb to a pro. i had the carb shop take it all apart and inspect it. $350 later i have a remachined and reworked demon.

my dad picked it up for me today. the tuner said the carb was packed full of metal shavings and the metering blocks were all screwed up. he ended up replacing the metering blocks, changing the jet sizes, cleaning everything, and remachining the throttle bores.

it really sucks to know that a huge company such as barry grant sells such a poor product. i guess the myth about demons poor quality was true.
Click to expand...
Summit racing quit selling the Demon line because of all the returns. I had one of them that had the jets beat in with a hammer!:eek: I have a proform race 750 now and have 0 problems.
 
#11 ·
They had a period a couple years ago where they had a LOT of carbs with metal shavings in them. Sounds like you got one of them. I did too -- except mine did all kinds of weird stuff, was too lean, then too rich . . . idled higher, then low and stalled, weird.
 
Discussion starter · #12 ·
the car runs great with the new upgrades. the tech sheet didnt give too much info as to what was done to the carb. it just states:

new jets 72/82
dual 3.5 power valves
stage II 4150 upgrades

hmmmm, all i can say is this thing dumps some serious fuel now. i warmed my engine up and opened the blades all the way, i was amazed at how huge the pump shot was.

the idle still climbs some when hot. its just one of those things i have to deal with i guess. i would have a super low cugging idle while stone cold and a better hot idle, or a good cold idle with a slight increase (900-1000) rpm.
 
#14 ·
MY FIRST 69 said:
** i posted this on pro-touring as well **

, but it ran great on my old small block. ]
Click to expand...
I have read your entire posts through a couple of times as I thought I was missing something but you did state that carb ran fine on the last application and I believe I read elsewhere where you also stated that the linkage was not binding on the other car as well. Now that it is on this application it had problems so you sent it to a former competitor of ours who charged you 300+ to do what we would have done for 225.00 and still didnt correct the original problem you posted about .If the throttle sticking had been determined to be a defect we would have corrected that issue for no charge. With all that already said and done however please make sure that your linkage [from the gas pedal to the carb] does have a positive throttle stop in place once you get it where you are happy with it .In regards to the idle moving around as one of the other member posted make sure when adjusting the carburetor that the engine is at operating temperature when making the adjustments and also ck for vacuum leaks both hot and cold . One key area to check is at the passenger front corner where the 2 little vacuum tubes are. Make sure that the intake manifold comes out far enough to cover the bottom of the baseplate and tightly captures the gasket. If it does not then a sealing plate will be required as Edelbrock offers one for just this kind of issue. Also you may want to open the secondary butterfly slightly [keep the transfer slots under .020] and close the primary ones down if needed to achieve. Keep in mind that if you close either one of these off too far they will stick in the bore .Given the opportunity we are here to help if possible.
 
#15 ·
MY FIRST 69 said:
** i posted this on pro-touring as well **

im running a 750 mechanical secondary mighty demon on my carbed ls1. when i first start the car it idles at around 900-950 but after a quick drive around the block it will idle at 1100 rpm. what is the major cause for this?

the carb is out of the box stock. i literally took the carb off my old 355 and threw it atop my ls1 and ran it as is. i checked the gap between my accelerator pump linkage with a .0012 feeler gauge, that checked out. i have two large springs pulling the linkage shut (sometimes wonder if the springs have too much tension) and im running a home made heim joint throttle linkage with aluminum holley bushings like these at the carb linkage and at the throttle pedal arm.

Image


i noticed my flots are a tad low, about 1/4 of the way in both the front and back. i didnt recheck the transfer slots before installing the carb, but it ran great on my old small block. the idle screws are about 1/2 turn out if my memory serves me. the idle is a tad rich but the car has no stumble. i have the idle ease screw but it never changes anything when i mess with it.

my cam is rather large and only pulls 10" of vacuum when the gauge is hooked up to the carb.

Texas Speed Magic Stick 3 cam - 237/242 .603/.609 113LSA

i programmed my ignition to run 17* timing from 0-1000 rpm and slowly ramp it up to 29* total at 3000 rpm.

also my fuel pressure is at 6-6.5 psi constantly.

i can press on the linkage with my hand and the idle will lower as long as i put some force on it.

and lastly. here is a pic of my home made linkage. the throttle arm sits way low compared to the height of the ls1 intake. so i bent up some threaded rod. the Z shape needs to be revised. its actually too long and i cant achieve full throttle at the moment. i was thinking of making my new linkage a lazy L shape.

Image


Image


should i run this return spring bracket instead? (my old linkage is pictured. too bad it was way too short to make it to the throttle arm).


Image
Click to expand...
Dougs67camaro said:
Summit racing quit selling the Demon line because of all the returns. I had one of them that had the jets beat in with a hammer!:eek: I have a proform race 750 now and have 0 problems.
Click to expand...
Actually that would be incorrect as Barry chose to close Summits account . Their returns were 10 times that of the average of all of our other 400+ dealers combined and we even went as far as to bolt some of them on engines and they ran fine. Had we continued to do business in that manner we would have had to raise the prices to all of our other dealers .
 
#16 ·
Eric68 said:
They had a period a couple years ago where they had a LOT of carbs with metal shavings in them. Sounds like you got one of them. I did too -- except mine did all kinds of weird stuff, was too lean, then too rich . . . idled higher, then low and stalled, weird.
Click to expand...
That would be correct and we have admitted to and addressed that subject on many of the forums including this one. We found shavings were getting onto the gaskets and parts during the assembly process and there were numerous procedures put into place which has greatly reduced the warranty claims we now see for them. Gaskets and parts are stored and transported differently and some of the assembly tables have vacuum systems under them as well as adding a timed sonic wash for the metering blocks. Do you still have yours? Was it corrected?
 
#17 ·
Tech @ BG said:
That would be correct and we have admitted to and addressed that subject on many of the forums including this one. We found shavings were getting onto the gaskets and parts during the assembly process and there were numerous procedures put into place which has greatly reduced the warranty claims we now see for them. Gaskets and parts are stored and transported differently and some of the assembly tables have vacuum systems under them as well as adding a timed sonic wash for the metering blocks. Do you still have yours? Was it corrected?
Click to expand...
Yup, mine just got a good cleaning and eventually went on another engine. It was an 800cfm Mighty Demon with annular boosters and for some reason, even with a bunch of tuning (using a combination of LM1 wideband and ET slips) my old 750 Speed Demon ran faster on my 410.

To this day, that Speed Demon 750 is still the fastest gasoline carb I've run on that engine. Go figure. The 800AD Mighty Demon I messed with went on a friend's 468 and it ripped -- he wound up selling the car so I don't know what became of it.

Have since converted my engine to E85 and the Speed Demon 750 is still backup for when I switch back to dinosaur fuel for early spring and late fall activities. Presently using a highly modified (ahem, by me) AED 850 Holley based E85 carb.
 
#18 ·
Demon, the carb should have left in the correct running condition in the first place. I bought a Demon 2 years ago and the carb had to be taken apart and something drilled out for it to idle, the holes were never done properly. I did not get charged, but come on, send out the correct stuff from the beginning
 
#19 ·
oger said:
Because the manifold is warming up. You need to set the idle with everything at opp. temp.
Click to expand...
That's what I was thinking. You have no choke, so you must be richening the mixture to get it to run cold.

Also, be sure the mechanical advance isn't kicking in too low. If you're on the boarder line, and increase in idle speed will be magnified if the mechanical advance is kicking in.

Correction: I just reread your post and see that you have an electronic ignition. However you state that you roll on the advance from 1000RPM and the engine is idling hot at 1100RPM...so the same theory applies. Ignition advance will bump your idle speed.
 
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#20 ·
Phatbudde said:
Demon, the carb should have left in the correct running condition in the first place. I bought a Demon 2 years ago and the carb had to be taken apart and something drilled out for it to idle, the holes were never done properly. I did not get charged, but come on, send out the correct stuff from the beginning
Click to expand...
If you re-read his post he stated it ran fine on the first engine it was on. The problems did not occur until it was put on this application and he is still having the original problem even after paying somebody else to work on it. We strive to have every carburetor go out right the first time but I am not aware of anyone in the business that has a perfect record in that area and that is why we all have warranties and a warranty department as well as always looking for ways to improve our QC.The forums are also helpful in that area as we get to see issues sometimes that normally we would not be made aware of. We are sorry you had an issue with one of our products but looks like by your own post that we took care of it.
 
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