Detonation under load, Whatup??

[ejoedirte]

Its a '70 340 wearing a Victor intake, big bumpstick, Hookers, a new MP ignition, and a new/remanufactured Eddy 750. I don't have the cam specs but someone built this thing to the edge! Roller rockers/lifters, etc. Initial timing set to 15 ( just won't idle under that), carb mixture on the rich side, and I can't get rid of the detonation issue. It'll idle all day long ( OK a ping or two here and there) but under load with the Vacuum tube plugged it starts to backfire again. Tuning this thing is getting a bit complex for a shadetree guy like me and I may be a bit over my head with this car (shes a mean one) but in this instance it seems to me there is a simple something I'm missing. Are the eddies really pieces of shit, would different metering rod help? Looking for advice from you gurus! Thanks in advance!

[dave571]

Perhaps you have more compression than you know.

I had a similar problem with a car I had bought.

It was a 383. Couldn't tune the ping out, so we threw in a 440.

About a year later we tore the engine down. It had closed heads, and a domed piston. I can't imagine it being less than 12 to one.

[amxauto-x]

Buy a dial indicator and a magnetic base with the long arm setup. Take out all sparkplugs and take off one valve cover. Setup the indicator on the top of the valve retainer. 1st the intake and later one exhuast valve. Turn the engine over and watch the indicator to see what the valve lift is. Then you can get the distributor recurved to match the cam.

[ejoedirte]

Yeah its up there for sure. Whats to be done short of slapping on a Demon?

[ejoedirte]

That would be a helpful bit of info, AMX, however I haven't even gotten to the Vacuum advance yet. I've got too much advance or not enough fuel at this point, I assume. What other variables are there that can be easily adjusted to get this thing to at least maneuver into takeoff position?

[dave571]

Dude, what's "..up there"?

Your compression, or your cam specs?

If your compression is up at 12 to one, you can't tune it out. Impossible, without use of race fuel.

If the cam specs are big, then a distributer curve may make you drivable.

Vaacuum advance shouldn't be a factor under load. The vacuum on the dist vac port drops off under load. Driving with the hose off is duplicating that already.

[mtrv8n]

You need to set timing at 2500rpm, in order to know where to begin. Try setting it to 34 degrees to start. If it won't idle, then it's the carb that needs major tuning.
A vacuum guage would help here, because if you're pulling less than 6 inches at idle, you may need to drill a small hole in the primary butterfly to be able to tune idle.

Part of what happens is that if your idle is set too high, or the vacuum is too low, the carb thinks it's under load and pulls the metering rods up. This richens the idle, but removes the influence of the idle air screws. Now you can't tune idle, and need tons of advance to burn the mix.

Try to get it to idle, even roughly, at 900 rpm. Then, check the vacuum, and adjust the idle screws until the vacuum is greatest, reset the idle back down, and so on, until you've got it set as lean (screws clockwise) as possible.

The Edelbrock web site has an online tuning guide, and it does mention the possibility of the need to drill a small hole to smooth out idle when you have a real big cam. I'm not sure what size, though...

Next you need to adjust the rest of the carb, and most Edelbrocks come tuned either for economy or performance, depending on the model. If yours has a factory electric choke, then it's tuned fairly lean, which will cause detonation under load.
The online guide has a chart which tells what rods n jets are stock for which versions. Starting off with the set for the manual choke version is a good bet.

The other carb consideration is the insuitability of an Edelbrock for a high compression , high output motor. Double pumpers are usually recommended here because they have that second pump which squirts when the secondaries open, improving throttle resonse and reducing detonation under load. The DP's also are tunable on the secondary side, not available to the vacuum secondary carbs like Eddy's.

Your other problem may be the ignition. MP ignitions are sensitive to correct installation, and the ballast must be correctly set up. Even so, they are prone to failure. The distributor may not have the correct advance either.

Also, connecting the vacuum advance may help idle, as it will give a little advance there. You must set the advance with it blocked, but make sure to connect it to the timed (higher) port.

Now, for the last which may well have been put first..
Have you checked for vacuum leaks?? Detonation and poor idle, backfiring, all can be caused by a poor carb to intake seal, or intake to head seal. the way to check is to take a spray bottle with soapy water and spray it around suspect areas. the problem will go away when you've covered the leak.
Good luck!

[amxauto-x]

You won't know how much fuel you need until you know what the cam needs.

Don't worry about the vacumn advance untill when you get the distributor recurved. Which you can't do until you get the specs on the cam--with a dial indicator.

You need to do this in steps;

1] cam

2] dist. & vacumn advance setting

3] carb size, then brand and model.


What is the cylinder pressure? What is the vacumn at idle, at cruise??

Then talk to cuda66273 for his advice on a Demon. But he can't help without the cam specs, no one can.

You say it is running on the rich side, then you say it needs more fuel. That is just opposite to each other. So talk to cuda on the carb info.

[6 packin]

You need to know total advance timing, check it at 3500 so you no its all in. After that try running it at 4-6 intial, I know it wont like it and raw gas will be ozzing from the exhaust, but see if it helps. If it works pull the distributor apart and weld up the slots for the mechanical advance about 3/4 way to the end. orget the vacum advance all togather. File the slots out to make more advance. trick is with that combo, is to run 18-20 deg intial, this means trial and error, for trouble shooting. If you cant run at leats 30 deg total, the compression is costing you power, and lower compression will make more. You can buy thick spacer head gaskets to lower the compression down. Find out what it is. A compression gage wont be much help telling ya. With that big a cam you could have major pressure bleed through the exhaust. If ya through a gage on it and you have 180 psi with that cam, you know what it is.

[ejoedirte]

Thanks fellas!

[pishta]

I pulled apart my MP distributor and found I had a 15.5 plate in it. That means the mechanical advance was giving me 31 degrees at the crank. That is way too much! And I had a 10 degree vacuum can on top of that. That is 51 degrees total timing! If you need 15-20 at idle and 34 at 3000, then you only need like 15 degrees of mechanical advance. try not using the vacuum and limiting your mechanical advance to 15 total. I did this by opening the distributor and drilling a tiny hole in the weight next to the cam that slides in the long slot. You could weld up the slot (make it shorter) or stop the cam from traveling the length of the slot by putting a pin (needle bearing from a U joint cap) in the hole you drilled (making it look like a shorter slot) Now you have 15 total advance with no vacuum advance. Try diffferent springs from a Chevy kit to tailer the curve. If you drill it in the wrong position, you could drill the other weight and move the pin over (you just need one) On paper this should help. I have yet to try mine, but I had nothing to lose. I could always pull the pin and be back where I started!
Feel free to correct any of this, I thought it through in about 15 seconds and it sounded good before I actually drilled my weights.

[23T]

What octane gas do you run?

[ejoedirte]

Yeah thats my other problem, all I can get locally is 93. It will be worth the trip to the Rock, I suspect, to get some of the good stuff.

[cuda66273]

Quote:
"Are the eddies really pieces of shit"

You said it not me.....

You'll never get the Eddy tuned...the 750's have chronic main circuit problems that not even Eddelbrock can fix...we pull everyone of them off and start over, they are not worth the time it takes to get them even close and will not work on a hot cammed motor...drill all the holes you want the best one would be a 1/2" hole right through the side of it.

What you've got going on is a totally wrong curve in the distributor...you can dial it at 2500 3500 or 10,000 and unless you know what springs you've got and where you need to be for your combo you'll never be right.

By the sounds of it your going to need about 20-25* of initial and total of 34* at about 2800, depending on all you other specs, a decent carb that can be adjusted and if you have too much compression for the fuel level you'll need to install a set of Cometic MLS gaskets to bring the compression down to a reasonable level.

I'd be happy to send you our spec sheet and make some recommendations on how you should set it up and what carb size and type would balance the whole package...or you can keep playing around until you detonate a piston out of it...the info is free, email me direct to be sure I get your request.

Don

[mtrv8n]

You should be able to get it going on 93 octane, unless the motor is really wild. Did you look for air leaks yet????

[moparmotorman]

i don't have much to add as most of it has been said already. the 2 things i have to say are 1: victor intake is a race unit with large plenum volume and i don't think any eddy carb will be matched to it w/ a "big" cam. 2: with an engine this "wild" i don't know what a vacum advance dist is doing on it. i would use mopar tach drive dist or msd dist. that's my 2 cents. good luck.

[cuda66273]

Yes both those distributors will work for about $300+ dollars...or we can easily build you a Stage 2 which will work just fine for 1/2 the money.