CDaniel525

I just installed my speed demon 750 mech. secondaries the other day and everything went on great...I took an alternative to buying a fuel line or fuel log as I am running a fuel pressure regulator on the fenderwell b/c of my nitrous setup....I used 2 of the out ports, one to each fuel bowl, where I had AN adapters and AN hose ends..just the cheap Earl;s Superstock ends....just slide on to the adapter....nothing hard or expensive.

Unfortunately, I didnt read Lars basic tuning artice on the carb before I installed it....Sometime this week, Im going to pull it off and start from scratch with his suggestions. but I am curious as to how I have it tuned now....
When started up, the car seems to idle decently, but took some time doing so. And at RPMs, it seems to run pretty well...however I havent driven the car yet... However, there is a pretty bad stutter when I give it gas...One time it even back fired or popped with smoke coming out of the carb, but it kept running. My thinking, is that the carb is too lean? and thats why it backfired and stutters when I give it gas?? But, im not expert and could be completely wrong. The primary jets dont even make a difference for the car when idleing, correct? but could that be the issue why it stutters when I give it gas? Doesnt make sense for a new carb to be lean on the primary jet since its brand new.
FYI, I have a zz4, hotcam (218/228 @.050 and 525/525 lift) and wasnt sure about the vacuum ports setup, so I hooked up the brake booster and valvecover pcv valve into the large port in the rear of the carb. The vacuum advance to the dist. is in the right side little port when looking at the carb from the passenger side. The headlights and everything else is in the left port, again looking from passenger side. The weird thing was, that the left vacuum port on the demon sucked all kinds of vacuum, but the right side seemed to suck barely any. Is this supposed to be this way? I checked the timing and have my normal 12 btdc @ idle and it advances to whatever the mark is that I put on there (i think 32btdc) almost right after I give it throotle.

Any help is greatly appreciated so I could have an understanding to tuning it properly when I do pull it off and start with lars' suggestions.

Thanks a lot!!
Chris

LAvetteman

Yes, do exactly what Lars says in his paper, it will make a difference.

You are not jetted to lean, infact you may find just the opposite.

Demon makes a special carb for the ZZ4, it is a 650. A 750 with stock jets will more than likely be too rich.

I had the same problem on off idle hesitation. I found the idle screws not set like Lars mentions in his paper, and both idle speed screws not adjusted the same amount, and the sccelerator pump springs not set correctly to be the problem.

Vac ports, yes, one of the pass side ports will have full vac the other won't.

Enjoy your demon, they are great carbs. :thumbs:

CDaniel525

Thanks for the thoughts LAvetteman,

im going to follow lars paper and see most of your issues were what he tackled in the paper, but i dont remember seeing anything about the acc. pump springs in there...is there somewhere i can read about that or can you articulate the problem is caused and explain how to adjust properly.

Thanks a lot
chris

LAvetteman

He mentions it in his paper. Its the spring inbetween the accelrator pump linkage. It adjust how quickly the pump shoots fuel through the squirters. If the squirt is late your off idle mixture is momentarily too lean causing the hesitation and the backfire through carb.(sign of leaness)

But I would make sure everting is right with the transfer slot coverage(idle speed adjustment) on both pri and sec and idle mix screws first. Most important.


[Modified by LAvetteman, 12:35 AM 9/22/2003]

CDaniel525

YES!! I just finished doing it now...everything was all messed up...i bought the carb off of ebay, but it was brand new but didnt include instructions or anything....the idle speed screws were different fomr the prim to the secondary.....and all the idle mixture screws were also different....after reading his paper and adjusting it off the car and then installing it, it barely ran at all....i kept turning the idle speed screw in the same on each side 1/4 turn each time and it would idle for a few secs at a high rpm and then almost stall.....so i figured out that the 3/4 turn out on the idle mixtures was too lean to keep it running....so turned out 1/4 more and idled smooth but at 1300 rpms...turned out on the idle speed thje same on each side and run perfect @ 800 rpms....still a lil hesitation when getting on throotle, but anouther 1/4 turn out on the idle mixture and turn out on the idle screws and it idles at 850 and no hesitation :cool: All in all, the idle mixture screws were out 1 1/4 turn and idle speed were identitical to one another... :thumbs:
I gotta get it outta my driveway and run it hard now to see about jet changes :D

Bob Turner

74Vette,

As far as your problem at hand is concerned, I would definitely set up the carb as Lars has suggested (and the manual). Your accelerator pump might not be adjusted correctly...I did set the idle transfer slots at .020" yesterday- easy. I would also check the float levels as my float level was set WAY TOO HIGH and fuel was spilling in from the top of the carb above the boosters. Check yours....

I am ALSO in the process of tuning my 850 Demon (vac sec) on my 454 and learning how it works.
The amazing thing is that this carb is jetted at:

Primaries- 85
Secondaries- 93

The DEMON 750 is jetted at:

Primaries- 76
Secondaries- 83
http://www.bgfuel.com/demon/default....e=29&specs=29b

My motor is running way too rich and I am NOT getting the power out of it that I should. The plugs were fouled beyond belief.

As you know, I could run EITHER the 750 or the 850 and the motor would run fine with some adjustments. Based on this, I have decided to change out the primary jets to 78 and change the secondaries to 85.... This should help quite a bit thanks to 467RATMAN and LAvette for their input here:

http://forums.corvetteforum.com/zerothread?id=653633

I ordered a jet kit and a vac sec spring kit today...

I would strongly suggest getting a hold of a jet kit as I would tend to believe that your carb is jetted over rich since you have a 350 (based on my experience here). Like I said before, I am learning!!

Let us know what adjustments you make...

Bob :cheers:

Ryan77

I have the exact same carb as 74vette and I'll be installing it on my 383 soon. Where can I find Lars' tuning paper?

Bob Turner

https://www.corvetteforum.com/techti....php?TopicID=3

Ryan77

Thanks :cheers:

dman535

I am running a 750 VS on my LT-1 cammed 355. I found that the factory jetting was to lean. I ended up running 81/84 jetting with a 7.5 power valve. The car runs much better and the fuel economy has actually increased.

LFZ

i had the same carb on my 74...and i couldn't get it tuned right if my life depended on it...i used Lars papers and had lars help me numerous times with advice via e-mail...couldn't get it to idle right, stop smoking and hesitating like crazy. i may have just had a bad carb, but i got completely fed up and destroyed it into a million pieces. :mad

ScubaJKD

Could someone point me to the lars article. Thanks

Bob Turner

https://www.corvetteforum.com/techti....php?TopicID=3

sharkthehunter77

How many ponies is a demon 750 worth on your ZZ4? I ask becuase i just got a ZZ4 with ported heads and intake from Sallee chevrolet in oregon. I am using the rebuilt Q-jet that came with the car when i bought it, but i am curious as to how much horsepower a demon would add!

Graemeinvette

I am going through the same problem with my 650Speed Demon on my 350 :mad Originally jetted at 70 primaries and 78 secondaries and it was popping and banging under acceleration :bs Came down 2 sizes and its still running rich, so I have ordered some more jets to bring me down 2 sizes at a time. I cant believe how rich they set it originally :eek:
:cheers: