help me tune my Demon

[Chevy305]

ok so I have been tinkering with my new 355 trying to get it tuned propperly. It has a balanced Eagle rotating assembly, Patriot Perfromance aluminum heads, Lunati Voodoo cam 262* intake 268* exhaust. On top of that it has a Weiand Action+ intake and a 725cfm Road Demon Jr. with Headman Elite headers to true dual Pypes Violator mufflers.

What I am having problems with is that with the new mufflers that I just put on is that once I hit 2-2500 rpm and up under medium to heavy acceleration the exhaust sound really crackily and is actually pretty deffening. The Pypes Violator mufflers are a straight though design with louvers like a glass pack. I also have a big a flat spot in acceleration occuring at about the same time through about 4000 rpm. I have already changed out my accelerator pump cam to the blue one in the #2 hole which has eliminated my off idle stumble. And I swapped in the lighter short yellow vac secondary spring and noticed an improvement but the flat spot is still there. I have not changed jets yet, I have a set on order now. Also according to the Demon carb instruction manual for my setup requires a 8.5" power valve over the 6.5" factory one. What do you guys think?

As far as timing goes because I know someone will ask, its only adjusted by ear because my accessory setup doesn't allow me to see the timing tab. Also I believe the reason the engine just falls flat on its face after 5000rpm is due to a crappy factory distributor even with a pertronix flame thrower coil and module. I have an MSD system on order now too.

 

[ChrisPerry]

I'm not a tuning expert, but if the timing isn't set correctly I think trying to tune the carb is a waist of effort. That "crackly" exhaust note might be engine damaging. Don't risk that new motor on "set by ear" timing.


Edit: If you can, get the motor to top dead center. Then make a mark on the balancer or crank pulley that you can see with it running (even if it is on the bottom) make your own timing pointer, and there you go.

 

[Chevy305]

I'm not a tuning expert, but if the timing isn't set correctly I think trying to tune the carb is a waist of effort. That "crackly" exhaust note might be engine damaging. Don't risk that new motor on "set by ear" timing.


Edit: If you can, get the motor to top dead center. Then make a mark on the balancer or crank pulley that you can see with it running (even if it is on the bottom) make your own timing pointer, and there you go.

I am thinking this might be due to not enough total timing and or to the jets being too lean.

I will make a new timing tab location and see how it goes.

Anyone think I should switch to 89 or even 87 octane? I am running 93 now and have a compression of 9.45 with my aluminum heads.

 

[zombie1969]

I wouldnt mess with the jets on a new carb just yet with out setting everthing else up properly because the companies that make these carbs know what there doing.The power valve sure if you have the wrong one because that will cause it to be lean or rich depending on which way its off.The timming is the thing to get right first then check for proper secondary opening at WOT under a load.As far as the exaust goes you dug your hole now you need to lie in it or get differant mufflers.

 

[Chevy305]

I wouldnt mess with the jets on a new carb just yet with out setting everthing else up properly because the companies that make these carbs know what there doing.The power valve sure if you have the wrong one because that will cause it to be lean or rich depending on which way its off.The timming is the thing to get right first then check for proper secondary opening at WOT under a load.As far as the exaust goes you dug your hole now you need to lie in it or get differant mufflers.

Ok back at the tuning today. I just bought an MSD distributor, wires, timing tape, the new power valve, and a set of jets. However I forgot to order a cross over tube for my exhaust because i'm pretty sure that will clear some crackling up.

 

[folkenheath]

A few tips...

http://www.barrygrant.com/demon/default.aspx?page=28

 

[Chevy305]

Ok been at tuning all day today. I got a piece of timing tape and set the timing to 13* initial and 37* total. However the engine would still give me the crackling under acceleration and popped through the exhaust just sitting reving above 2500rpm in the driveway. Going off of the advice of the summit racing tech hot line I checked the condition of my spark plugs. They turned out to sooted up with carbon pretty well so I started jetting down. Now after my previous post, I have actually upped the jet size from the factory 70 to 73. Since I am experiencing an extreme rich condition I decided to jet down to 68 which helped but then at 65 it eliminated the exhaust pop. Now I readjusted the idle mixture screws however my idle vacuum is only 13-14". The engine sounds smooth but it completely falls on its face now when trying to accelerate hard. I guess I need to up the squirter size since the massive over jetting was covering up the idle to main circuits problems. However, I still have the same bad crackling under acceleration even with the smaller jets. Should I keep jetting down and get larger squirters? Could the carb too big? Any advice on this would be very helpful

 

[folkenheath]

Are you sure you don't have a mechanical problem? Popping through the exhaust sitting lightly revving in the driveway doesn't sound good.

Did you try changing your spark plugs. If the problem goes away when you change your plugs then you may need a better ignition system if the plugs foul again and you aren't pig rich.

Sometimes when your cam has a lot of overlap it can foul plugs easier at idle, and a better ignition can help that. It all depends on your combo.

 

[496truck]

The crackling sound is most likely just the sound of the mufflers. I built a 79 Z28 with a nice 355 and a set of Flowtech Terminators and they rumbled during idle/normal driving but sounded like old school glass packs when I opened it up.

A thought on falling flat on it's face, too soft springs in the dist bringing total timing in too soon?

 

[Chevy305]

Are you sure you don't have a mechanical problem? Popping through the exhaust sitting lightly revving in the driveway doesn't sound good.

Did you try changing your spark plugs. If the problem goes away when you change your plugs then you may need a better ignition system if the plugs foul again and you aren't pig rich.

Sometimes when your cam has a lot of overlap it can foul plugs easier at idle, and a better ignition can help that. It all depends on your combo.

Everything in my ignition is brand new. I have a MSD street fire HEI, super conductor wires, and AC Delco plugs. In the future I do plan to add an ignition box.

Today I will clean all the plugs up and see if they continue to foul.

The crackling sound is most likely just the sound of the mufflers. I built a 79 Z28 with a nice 355 and a set of Flowtech Terminators and they rumbled during idle/normal driving but sounded like old school glass packs when I opened it up.

A thought on falling flat on it's face, too soft springs in the dist bringing total timing in too soon?

I am thinking that the crackling is mostly the mufflers. I have an H-Pipe on order so I hope balancing the exhaust pulses will help.

As far as the dist springs, they are factory MSD and I see the full advance at about 2500rpm.

 

[folkenheath]

Everything in my ignition is brand new. I have a MSD street fire HEI, super conductor wires, and AC Delco plugs. In the future I do plan to add an ignition box.

Today I will clean all the plugs up and see if they continue to foul.



I am thinking that the crackling is mostly the mufflers. I have an H-Pipe on order so I hope balancing the exhaust pulses will help.

As far as the dist springs, they are factory MSD and I see the full advance at about 2500rpm.

That sounds like a perfect start for the ignition, and a 6A or 6AL would finish it off excellent, those springs are good too, if it was too fast you would get detonation under load. Also, is this popping sound a missing and popping/banging in the exhaust? Or just the obnoxious popping of the mufflers when you rap the throttle and let off again? Does it do it at steady RPM?

Did you ever get a timing light to set the timing correctly? Disconnect the vacuum advance, rev to 3000 RPM, set it at 34 - 36, and then reconnect the advance.

 

[Chevy305]

That sounds like a perfect start for the ignition, and a 6A or 6AL would finish it off excellent, those springs are good too, if it was too fast you would get detonation under load. Also, is this popping sound a missing and popping/banging in the exhaust? Or just the obnoxious popping of the mufflers when you rap the throttle and let off again? Does it do it at steady RPM?

Did you ever get a timing light to set the timing correctly? Disconnect the vacuum advance, rev to 3000 RPM, set it at 34 - 36, and then reconnect the advance.

It did it at a steady sustained rpm around 2500 and up. From the engine side everything sounds, looks, and feels smooth until you walk behind the truck to hear it popping a little like a miss through the exhaust. Now that I have jetted down to 65 the miss is gone. As I was explained to me by the summit tech guy, he said that the carb is dumping too much fuel into the engine and not completely burning until it hits the hot exhaust and igniting in the exhaust.

Yes I did hook up the timing light and now I know that the timing is 13 initial and 37 total. That is exactly how I set the timing too.

 

[folkenheath]

It did it at a steady sustained rpm around 2500 and up. From the engine side everything sounds, looks, and feels smooth until you walk behind the truck to hear it popping a little like a miss through the exhaust. Now that I have jetted down to 65 the miss is gone. As I was explained to me by the summit tech guy, he said that the carb is dumping too much fuel into the engine and not completely burning until it hits the hot exhaust and igniting in the exhaust.

Yes I did hook up the timing light and now I know that the timing is 13 initial and 37 total. That is exactly how I set the timing too.

37 total without the vacuum advance I hope? Otherwise at WOT your timing would only be about 20 degrees.

Also, if that is the sound, popping and banging in the exhaust from unbured fuel, it can come from fouled plugs too, even if the engine is tuned right at that time. Or your jets could be correct, but your idle is way too rich and fouling the plugs.

be careful, you don't want to make it lean if the plugs are bad and you change them after the fact.

 

[Chevy305]

I think I figured out my problem, I think my secondary springs are way too loose. The engine bogs until 4k rpm then takes off like it should. I think this may be the cause of my mid range rpm crackling when accelerating hard since the secondaries are open at that time.

Also I cleaned the spark plugs today and was then experiencing some lean back firing with the #65 jets so I upped it to #68 and I will check the plugs again tomorrow.

 

[phreddei166]

I had similiar driveability problems. Switched from ported vacumn to direct manifold vacumn on dizzy. Much improved. Just try it !

 

[76zimmer]

try changing plugs too. Does Patiot have a recommended plug for those cylinder heads?

 

[Chevy305]

I had similiar driveability problems. Switched from ported vacumn to direct manifold vacumn on dizzy. Much improved. Just try it !

I drove without the vac advance hooked up and noticed no difference in performance.

try changing plugs too. Does Patiot have a recommended plug for those cylinder heads?

Yes and they are the plugs that I am running. I cant remember the AC Delco equivelent which I am running but they are Champion RC12YC.

 

[folkenheath]

I drove without the vac advance hooked up and noticed no difference in performance.

You shouldn't notice any difference at WOT, but if you hook it up to FULL manifold vacuum you will notice a part throttle difference and a better idle. In fact you will probably have to turn the idle down a little to compensate for how much the engine will like the advance at idle. If not then something is not right, there is a vacuum leak somewhere or some related problem.

Now if you have to pass an emmisions test switch it back to ported vacuum.

 

[Chevy305]

Ok so I did a little more tuning.

I looked at the plugs and they are all gray indicating a lean condition at the #68 jets. So I jetted back to the factory #70 but I still have a bad hesitation. I tried the #37 squirter but that gave me a slight black puff through the exhaust so I am sticking with the #35 nozzle. The pump cams I have tried are the blue in the #1 and #2 hole, now I am running the black at the #1 hole which is supposed to be the biggest combination without going to the 50cc pump. Then why do I still have hesitation? If I dry to dump the clutch the engine all but nearly dies and I even get a backfire.

Also I changed the secondary spring to a stiffer spring and this has removed my mid range flat spot. so once I am on the main circuits the acceleration and power is constant but the crackling is still there.

 

[Chevy305]

EDIT: ok so the black pump cam is not the biggest, apparently the blue one is: