carb tuning

[ZeroGravity58]

hello all. Since i have put the new intake and all on my truck it has thrown my carb tuning for a whirl. It used to run fine. Now when its cold expecially it likes to pop thru the carb and stall if i do more then ease into the gas. It will do that for the first 10 minutes i drive it. Then after it warms up if you get into it hard it spits and sputters for abit then it takes off. If you are cruising and let off the gas then give it about 1/2 throttle the truck will jerk all around. right now its jetted 74/82 i think. When i floor it i can smell it is runnign rich and my plugs are on the black side so i need to jet it down about on the secondaries. For the backfiring and all should i put in bigger squirters? accelerator pump cam? Im running the 750 double pumper with mechanical secondaries. I need some ideas because im stumped http://www.s10v8.com/images/smilies/icon_confused.gif

[Scotty_S-15]

OK, exact same carb, with no changes to it, and it ran perfectly before, correct?

* What was the old manifold, and what is the new manifold.? The new manifold may not have manifold heat.... or is it an air-gap?

* The most obvious thing is, are you sure the manifold is on OK? No vacuum leaks? is the carb gasket compatible with both carb and new manifold?

* I see you\'re in Maryland, so it\'s not like you\'re dealing with sub-zero conditions, but any chance that there\'s been a 30 deg. drop in temps since you last ran it with the old manifold?

* Does it idle the same as before?

* Any chance you popped the power valve(s) with all that backfiring?

................. Good Luck!

[fastw23s]

Generally, when you experience a pop, or backfire off idle, it is showing a lean condidtion, and can normally be remedied with a bigger pump shot.



Either step up 2 sizes in the squirter, or try installing a bigger pump cam, I would start with the squirter.



Why do you have such a large spread between the primary and secondary jets.



I would try squaring your jets and work from there.

[ZeroGravity58]

the old intake was a performer and the new intake is an air gap. It ran fine before and it ran OK after i got it done but it still hesitated. Now that its gotten colder it has gotten worse. When the motor is cold it pops and stalls. after it warms up it hesitates really bad and still pops but it wont stall. Im almost positive there is no vacuum leaks because it is fine after its rolling, then i can stomp it and burn the tires off. Its just off the line. There has been a big drop in temps since i put the intake on and it does idle diffrent. It idles really low when its cold...Like 500 RPMs, but after it warms up its a 700 RPM\'s. Also it doesnt pop when the choke is on, it pops when the choke is off and the motor hasnt warmed up yet. what should i jet it down to?74/80? thats what i was going to try.

[Scotty_S-15]

Knowing what we know now, I\'d think along the lines of what Fastw23s, playing with the accel. pump shot, etc. Are you sure there is no play in the accel. pump linkage? It should squirt gas at the very first crack of the throttle. Just removing and replacing the bowl can throw that adjustment out of whack. (wiggle the bowl when the screws are nearly tight, and you can see how easily that can happen)

......... But it still sounds lean overall, (again thinking of a vacuum leak). If you\'re sure that\'s OK, then try like 76 & 78 jets. And thinking about the black plugs, are you sure the power valve is OK? Maybe if you\'ve got a 6.5, you might want to try a 5.0, if you\'ve got any \"cam\" at all....

........... If that doesn\'t work, I\'d get a small propane burner, and install it in the air-gap under the carb.... http://www.s10v8.com/images/smilies/icon_eek.gif http://www.s10v8.com/images/smilies/icon_razz.gif Juuust kidding!

[ZeroGravity58]

haha....like the burners they used to heat food up....Im going to rejet it and i am running the 6.5 powervalve. I may have blow it but im not sure...it has been popping alot...expecially when i put the new intake on and i had the timing was to far retarded. I dont have a power valve that is any lower tho. It doesnt have any play in the linkage. Im alsmost positive there isnt a vacuum leak. If i push down on the accel pump lever and squirt fuel into the motor it bogs down. usually if theres a vacuum leak and you do that the motor revs up.

[raceingmichael]

go to www.bob.2000.com/carb. you will fine all you need there. good luck

[Truckman91]

I would also check your timing, make sure your distributor holdown hasent come loose on your new intake.

[fastw23s]

Good advice.



Hell my motor idles at about 900-1000 cold and then goes up to 1100-1200 warm.



Powervalve size is best determined with a vacuum gauge and check it just off idle,is that right, someone may need to correct that, then cut that in half to know what size to buy.



I usually have bigger cams when tuning.

[ZeroGravity58]

well then mine is way to big...i run 10 inches of vacuum in gear. when i open the throttle abit im down low...Maybe thats why ive been getting such terrible gas milage....When i drive it easy i get like 10 MPG. That hold down hasnt come loose i checked. I run about 18 degrees initial timing and around 38 total with my full mechanical distributor.

[fastw23s]

Yep it sounds as tho you are getting there.

[Scotty_S-15]

On 2004-11-30 22:22, ZeroGravity58 wrote:

..................Maybe thats why ive been getting such terrible gas milage......................



ANDblack sparkplugs..... http://www.s10v8.com/images/smilies/icon_wink.gif You see, you might fix two things at once!

[ZeroGravity58]

i found out i had power valve to small. i went up to a 8.5 since i pull 11 inches of vacuum in gear. Holley says to go 2 inches of vacuum below so thats what i did. I rejetted the carb from 72/80 to 71/77. I also changed the primary squirter from 35 to 31....But BOTH squirters are 35\'s...Should i change them both to 31\'s? 35 seems alittle big. It runs alot better when you get into it hard BUT it still pops thru the carb is you get on the pedal enough to open the primarys but not enough to start opening up the secondaries. With that it is really erratic. If i do it onetime it will pop but then the next time it wont. Besides that its running fine. The only other thing i found is that the electrodes on some of my plugs are blistered. Im running the delco CR45TS plugs...I havent changed plugs yet...could that be my problem? also what plud should i go to? they have only been in for about 2 months.

[wild85]

I am using the same plugs as you are.mine are white and slightly tan colored, so I am lean I think.I get a constant bog and cough when I even lightly get on the gas, but who cares now, I am going to tear the motor apart.

[ZeroGravity58]

should i rejet and change the primary jets to something higher?

[wild85]

You must still be rich. I have a similar motor to you, I think a bit more motor and am running the stock 65, 68 jets. I don\'t think the jets will solve your problem If you need to jet them that much from the stock ones.

[ZeroGravity58]

stock for the carb was 70/80

[wild85]

You got rid of the street avenger then. what do you have now double pumper?

[ZeroGravity58]

yea a 750 double pumper, mechanical secondaries. I had to many problems with the vacuum secondaries on the street avenger so i sold it.

[wild85]

well in my opinnion, that carb is too big for your engine, I had a 750 vac sec on mine to begin with and had similar symptoms to yours, would never ever run proper. my plugs were black like yours.

[wild85]

Another example is a guy who has a 67 chevelle 383 and runs 10.50\'s and he only has a 650 dp. So a 750 dp seems like it will only give you problems. Assuming you have a 355 and rev it to 5500rpm you only need 480cfm of air, you are 270cfm over your engines needs.

[ZeroGravity58]

could i jet it leaner to make up for it? i dont feel like buying another carb

[wild85]

You could jet it down forsure, I think the problem with too much carb is that you don\'t get a good signal for fuel. I would try 66,69 if I were you. one size bigger than the 670 avenger has.

[ZeroGravity58]

wow thats jetting it down alot right there....i could give it a try tho...what do i have to lose...

[wild85]

I\'m not pinging with 43 degrees timing and i think i have more motor than you by the sounds of it. Would you agree?

[ZeroGravity58]

im not sure....the big reason im so slow now is because of my rearend...if i had some better gears and a posi id be alot quicker. But i dont know they are probally pretty close. I big problem with me is my 8.5:1 compression. You have the same problem because you run the 882 heads also.

[wild85]

mine is 0 decked though, adds compression

[ZeroGravity58]

yea mines not...mine is stock heads and stock deck block. Im getting some new heads soon. Either a set of world products, edelbrocks, or darts

[wild85]

how bout AFR\'s, they are 1295, darts are 1070, may be worth the extra coin for the AFR\'s

[ZeroGravity58]

yea i thought about AFR\'s but they are money...but i wouldnt have to buy another set for awhile....I thought about going with 58CC chambers what do you think? right now i have 76cc chamber with flattop pistons with 4cc valve reliefs. stock deck height. so im only running around 8.5:1 compression (estimated) I could get away with 10:1 with 93. 58CC chambers should put me around 10:1 shouldnt they?

[wild85]

says your compresson is 8.74:1. With 58cc it is 10.572:1. which is too high. you can run 10:1 on 90 octane with aluminum heads.

[ZeroGravity58]

i will be running 93 octane. i will stick with 64cc

[fastw23s]

Wild, I would be very suspect of this guy claiming to be running a 650!!!

[wild85]

it has been gone through by some carb guru, so it probably flows more than that.

[ZeroGravity58]

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&category=33617&item=7937857 011&rd=1



What do you think? ive also been looking at protopline and world sportsman. they are all good heads.

[ZeroGravity58]

i rejetted the carb and drove the truck awhile today. I decided to change the plug and when i pulled them they were black on oneside and had slight color on the other so im getting closer. Before they used to be all black. I put the new plugs in a drove it hard around the block, cam back a pulled a few and the one side of them is already turning black. Tomorrow im going to get it down again from 71/77 to 69/75. Becsides that a cured alof of the popping problems. Im running the green cam on the primary side and the white one on the secondary side. It seems like the motor like the pump shot fast. Im also going to buy another 31 squirter for the secondaries and take off the 35 squirter