Thread: Thoughts on jetting.

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I have a Speed Demon 750 w/ vacuum secondaries sitting on my ZZ430 clone. (Thats essentially a ZZ-4 with Fast Burn aluminum heads and the GM "Hot Cam" and 1.6 rockers. It's running really fat. The plugs are sooty black, and so are the tailpipes.

I've very carefully set the butterflies, the floats are right on the money, the fuel pressure is about 6 lbs. Idle mixture screws have been adjusted and re-adjusted, and are about 1 turn out. Power valve is 6.5, and the engine pulls a 12" of vacuum at idle. I have a standard Chevy 350 PCV valve in one cover, and a vent in the other.

Timing is per spec - 10 initial and a max of 32 at about 3,000 RPM. I'm running manifold vacuum to the distributor, but have it limited to 10* max, per Sallee Chevrolet recommendation, using a Crane adjustable vacuum control. This engine does not like a lot of advance.

The jets that came with the carb were 83 secondary and 76 primary. I dropped down two sizes on all four corners 81 & 74. Still looks pretty sooty.

I have a tough time putting in fresh plugs and trying to read them because I live in the middle of a subdivision.

Any thoughts?

get an 02 sensor and a hand held gauge, summit and jegs both sell them. weld a 02 sensor bung in ither the headder collector or close to the headder collector in the pipe. plug the hand held gauge in some how get it into the passanger compartment and take a drive. play w/ the jets until under normal driving conditions it runs close to 14.7, a little leaner wouldn't hurt as long as it didn't ping. make shure that it goes rich and not lean, when you mash the gas.

I put an in dash gauge in my Falcon due to tuning the 2300 holley under boost, will be hard to get right. and a lean run unnoticed could mean a burnt up pistion

I think I need to check my power valve.

That was gonna be my question, did it back pop and take out the power valve? Even if it is bad, think I would still be for leaning it out a step or two, that's a lot of jet for a 350 with 750 CFM of carb....

I like Matt167's advice.

Along those same lines I've found that plugs don't color up well with today's gasoline.

A magnifying glass, some bright light and looking down inside near the bottom of the center electrode's ceramic well - for want of several better words - you'll see the color starting there most times.

I've found too that if the car runs ok you can run a little leaner than you thought since it takes a while to burn a hole in the piston or seize it.

Far as jetting goes, take a look at the Edelbrock carb tuning site and see what percentage change they recommend for your altitude and where you want to go.
The info works great for Edelbrocks and Carters and can be made to work on Holley's and their clones by utilizing a little math.

Add the area of all the Holley jets.
IE: .100 jet's on all four corners gives you .400 total.

If the Edelbrock tuning recommendations say to go 5% lean, fgure what jets you need to lean the Holley down 5%.
IE: 95 jets on all corners.

Seems to work ok on a couple Holley's I've tried it on and it really works great on the Edelbrock/Carters.

Especially since moving to 3300' altitude from 350' altitude.

The roadster still runs good down at the river at 450' altitude.

I'm at 15' above mean sea level.

Start with a jet size equalivant to carbs cfm---700 carb=aprox #70 jets
750 carb==#75 jets

I'd probably start down a couple numbers in primaries and up a couple in the secondary----ie- 750 carb # 73 primary and 77 secondary

Don't forget that powervalve will make a difference so numbers will be skewed further if secondary has one

Jerry

the main jets only come in to play past your idle so look at your power valve like said and your idle air bleeds may need to be tune in if running rich at idle if in gear and say it at 10 then a 5.5 would were i would try for power valve size . you can try to crack the rear barrels this may help if you past your idle slot on the front barrels

Denny,

It's a Speed Demon 1402010VE. The specs are on this page.

http://www.barrygrant.com/demon/defa...=29&specs=29cc

I'll check the vacuum at idle in gear tommorrow . . . er . . . later this morning.

Sallee Chevrolet (now Gilbert Chevy) recommended the Demon 750 for my ZZ430 clone, although the Barry Grant website recommended their 650. In any case, I have the 750, and I should be able to make it work.

I'm also going to check the power valve tomorrow.

Pat,

It seems to idle OK with the butterflies set per the Demon book. Both butterflies are set so all that is showing underneath is a small square, so both are slightly cracked. I don't seem to have much trouble with idle. It idles at about 700 with a slight lope. It doesn't seem to load up at stoplights. I don't need to blip it to clear it out.

I've also got a little hesitation when I hit the throttle hard, but I need to fix this lean problem before I solve that one.

I did get a pop or two when I first started it up, so maybe I trashed the power valve. It feels a bit rich everywhere - not just at idle.

Thanks for responding, guys. I'll let you know about the vacuum later.

i have seen more then one of the carbs like you have run out of the box rich on idle they can be tune in but alot of it i think has to do with your air bleeds they are very adjustable . i can take out a AFB and bolt it right on in less then 10 mintes get the AFB running better then the carb you have on it now .your power valves have the blow out check valves they did this before holley did so they should be ok

alot of it is the power valve .yes he need to give the ilde in gear hot and cut that in 1/2 allways work for me

I did some more fiddling this morning. At 800 RPM idle, it's pulling about 17" Hg - solid, no fluctuating. In gear, I lose a couple hundred RPM, and the vacuum drops to around 13" Hg, but it fluctuates an inch or two. So, assuming that the power valve isn't blown, it should be about right.

After checking that, I drove about 10 miles on new plugs this morning, and they came out black as soot.

While I was trying to set the carb, I had trouble adjusting the idle. It felt like the secondary butterflies were open too far, and uncovering too much of the transfer slot. So, I pulled the carb to give it a quick once-over, and reset the secondary idle position.

While I was re-setting the secondary butterflies, I noticed something odd. When I opend the primaries, the secondaries would open with them for a bit, then they would snap shut. I pulled off the electric choke to get a better look. Check the photo below.

The link rod (A) and the bracket (B) are meant to keep the secondaries from opening further than the primaries will allow. I found that the link rod (A) was bent a little. As the primaries opened, the rod would hang up and pull the secondaires open, every time - even under light throttle. Looks to me like this would cause a very rich condition, and also would cause a bog because of the secondaries opening too early.

Whatcha' think?

Denny,

There was a washer on the outside, but there was also a burr where they bent the rod. I filed that down and smothed it out. I re-bent the rod so that it just barely touched when the primaries were closed.

I just went out and drove about five miles. I didn't change the plugs, and they're still black, but all the sooty look is gone. The electrode tip is clean, and the tip of the insulator is whitish-gray.

On the way back home, I hit it hard through first and second. Holy-bleeping-cow! Night and day compared to yesterday. No bog or hesitation, it just went. It surprised me so much I let off the throttle WAY before I intended. With the big tires and the ladder bars, it doesn't slip very much - it just goes.

I'm going to get a spark plug cleaner from Northern Tool and clean up the three sets I fouled up trying to find this problem. Then, I'll have spark plugs for life.

You know, in almost 40 years of working on cars, I never heard of that! Thanks.

that's what I do w/ my dirtbike plug. except I put it in hot, cause it will start easier cold w/ the preheated plug. it's fouled so many times I have to torch it every time I want to start it. I really need a new plug.