Dart pro1 200 cc

[BAD67]

I have a set of dart pro 1 200 cc heads on my motor right now,and ever since i put them on now im always real rich at idle no matter what i do to the carb i cannot get it leaned out.iI have tried 2 diff carbs now and get the exact same results out of both.I have checked and rechecked the ignition system for spark,and everything is ok with the msd box and coil also brand new msd 8.5 wires.my motor is a 355 sbc with 10:1 comp. Cam is a lunati .517 .543 lift 239 249 duration @.050.Carbs im using is a 650 mighty demon and i also have a holley 750 dp.Before i swapped cylinder heads i never had this prob,i never fouled a single plug.Plugs that i have in it now are a hot street plug autolite 3924.Did dart sell me the wrong head for my application,because when i called them this is the head they told me to go with,part #11321112. Any help appreciated

[BAD67]

Is anybody from dart gonna reply?:confused:

[PRO 1]

The reason for running rich is not the heads, but the fact the universal carb needs to be tuned to your engine. Carbs are not a bolt on item that are not jsut a plug and play deal. With some tuning, jetting, power valves, squirters, etc, you can and should be able to get that to go away. The head of choice is perfect for that engine and it should make great power when it is finally tuned right.

Thanks

John

[Awesome Bill]

Your engine is not running rich, in fact it is very lean. Now with the wrong cylinder head, the fuel is being lifted out and just falling into the ports. This gives the user, you, the idea I am running lean in some cylinders and rich in others, especially when it stinks and burns your eyes out the back of the car. But, the fuel is entering your combustion chamber in liguid form and will not burn. The only thing you can do to help this is get a screen type spacer, like a door screem to help break the fuel up as it leaves the booster or get a custom very sensitive skirted booster that will allow the fuel to leave a little smaller giving it some chance to tumlble and break up before entering the combustion chamber. The ports with the easiest path of least resistance is going to get solid fuel and the other ones will get very little. Also creating a leam conditon in those cylinder because of not enough fuel to jump over and light the other fuel atoms at once. Spontanious combustion is what your after and not getting. Very poor choice of heads for a 350. All out race engine would maybe use those heads @ 5000 and above rpm . Hate to say it but you can put a little 570 or even smaller carb like a 390 and it will fix your problem. Hurts top end power. You can tune your carb all you want, but the idle and part throttle conditions will be jacked up because of slow air speed in the intake and heads.

[BAD67]

Your engine is not running rich, in fact it is very lean. Now with the wrong cylinder head, the fuel is being lifted out and just falling into the ports. This gives the user, you, the idea I am running lean in some cylinders and rich in others, especially when it stinks and burns your eyes out the back of the car. But, the fuel is entering your combustion chamber in liguid form and will not burn. The only thing you can do to help this is get a screen type spacer, like a door screem to help break the fuel up as it leaves the booster or get a custom very sensitive skirted booster that will allow the fuel to leave a little smaller giving it some chance to tumlble and break up before entering the combustion chamber. The ports with the easiest path of least resistance is going to get solid fuel and the other ones will get very little. Also creating a leam conditon in those cylinder because of not enough fuel to jump over and light the other fuel atoms at once. Spontanious combustion is what your after and not getting. Very poor choice of heads for a 350. All out race engine would maybe use those heads @ 5000 and above rpm . Hate to say it but you can put a little 570 or even smaller carb like a 390 and it will fix your problem. Hurts top end power. You can tune your carb all you want, but the idle and part throttle conditions will be jacked up because of slow air speed in the intake and heads.

THEN WHY DID DART RECCOMEND THIS HEAD AFTER I COMPLETELY TOLD THEM MY EXACT MOTOR COMBINATION WHEN I CALLED THEM TO SEE WHAT HEADS WOULD WORK BEST WITH MY APPLICATION????????? DART TECHS DONT BE AFRAID TO JUMP IN ON THIS POST.

[want-a-be]

Probably thinking wrong here, but I'd like to ask a few questions if I may.

Have you checked your vacuum at idle? If so, what is it please? A common mistake is to drop the engine vacuum below the rated vacuum of the power valve. When this happens the power valve never closes and will constantly dump too much fuel into your engine. Take a vac reading and buy a power valve that is about 4-5 inches of vacuum below your engine's at idle. I might be a bit wrong about the numbers; it's been a while since I've run a prower valve. I don't like them and think of them as a crutch for those who can't properly tune a carb.

Have you checked your float level? On a gas engine you don't need the gas running out of your bowl sight hole. That is more for Alky carbs, as they are higher volume. If the float gets too high the fuel gets siphoned out of the float bowl waaayyyy too easily.

You also never mentioned the lobe seperation you had on your cam. I've built a few 355s that were racing in a vacuum rule class that was a bit more lift and duration than you are running that easily satisfied the rule. Widen the lobes a bit and you can bump the vacuum up.
For the street, you still need enough air speed to keep the fuel atomized.

What head did you have before you put your new heads on? Will probably not be too popular with this opinion, but here is another possibility. It may be that your old heads had to work a lot harder to draw the fuel out of the bowls. Now you have a lot more efficient set of heads and they have to work less to to draw the fuel, possibly causing a rich condition.


I wouldn't give up on this combination just yet. Just have to refine it. Your engine is a package. Anything on it is always going to effect it all. Nothing is going to be bolted on and forgotten about.


I've built several ford and chevy street engines with 200cc heads. Just recently a 347ci stroker 302 with the Dart Roush 200s that made excellent street power.

Of course all this is only my opinion. I'm sure to get some dissagreement from this crowd, as it's been a few years since I've had access to a proper dyno facility to play in.

Hope this helps and doesn't make anyone mad.

Thanks, Don

[want-a-be]

WOW !!!!sorry for being a bit too wordy on that last post.

[BAD67]

My vacumn at idle is between 8-10 with the car in gear im currently using a 3.5 power valve,the float level is also set correctly just below the opening.the carb is a brand new mighty demon 650,so its not that the carb is too big.cam is a solid lunati .517 .543 lift 239 249 duration @.050 106 lobe seperation.

[want-a-be]

A cam with a lob seperation of around 112-114 might help your lack of vacuum. This will more then likely move your torque curve around some but it might help some of your problems. I prefer, and have had good luck with, the LS of something like this for the street. Lunati has some cams that ought to be close to yours. If you can't find one, a customer will be taking one out this year to update to a solid roller, as he no longer has a vacuum rule to adhear to.

Not familiar with the Mighty Demon. Is it a vacuum secondary?

[BAD67]

mechanical secondary's 3000 lb car runs 11.80s on pump gas stock suspension.

[want-a-be]

but your still driving this on the street aren't you? What jet size are you running in this and your other carb?

[want-a-be]

hey I sent you a pm. click on private messages near the top and let me know.

Thanks, Don

[Awesome Bill]

theres your problem, thos carbs are near junk out of the box and need the idle air bleeds closed ½ of what they are. Take your fingers while the engine is idling and close off the outer air bleeds completely and see if it kills the engine or makes it idle great. Bet it makes the idle better!

[BAD67]

do they sell smaller air bleeds for them ?when i spoke to tech at barry grant they told me the complete opposite of what you said ,they said the air bleeds may need to be bigger

[Awesome Bill]

take something like 2+2 plastic spray extensions, cut them off in 1" lengths, place each one over the outside air bleed or something close the that size to cut down on the air @ idle see what happens, you should then go rich, casuing you to back the air idle bleeds out a little. Adjust all 4 air bleeds @ 1 turn out from base. Do not overtighten these, it will damage the seat. Start the car and if you have to add or open the bleeds, then you air bleeds for the idel circuits are too big. let me know.

[BAD67]

i have to be out about 1 1/2 turns on the mixture screws. the idle air bleeds are.070 how small should i go?

[Awesome Bill]

just take your finges while running and close off the 2 front outer air bleeds and see if it stabalizes. out!. I would say .060" just buy a kit from BG if your going to keep that carb. They do work well once you get them figured out. Idle quality and throttle response is what your after if you don't have a dyno.

[lun40119]

I think that this might be a good situation to get the car on a chassis dyno and do some tuning. That is what I like to do when I change something in my combination. We start with just idling, then do some part throttle cruising with the engine and then finish up with some wide open pulls. I thought that it was extremely helpful. In my situation, the engine prefers to idle just a little rich. If I try to set the low speed circuit to the same A/F as main circuit, seems to have a hard time during the transition. I am still working on it too. :D I have alot less vacuum than what you are working with, my cam is ground on a 106 lobe sep. Either way, it is hard to tell without getting the facts. You can guess all day long, but 75-100 bucks will answer all of your questions.