chevyart

put new carb on my 355 sbc(car normally runs 11.00(aprox) with old carb(aprox 700 cfms)(new carb is 750 with proform center section). first pass and the car seemed to respond and rev up better than ever before and i figured a 11.80 to 10.90 pass in the 1/4. got my slip and couldnt believe it (11.34) car dideverything right and the 60foottime was poor, but not a factor in the ET. mph was right for the Et, but w ETwas just not low enough. next pass i had the same 60 ft. and out a little in low gear(its a glide) i felt a little something like a miss or rumble or something and comlpeted the pass and ran a slower 11.47. now i am reall confused. car didnt seem to do anything wrong, but just didnt feel as good as the first run. Now here is the real problem i am looking into. 3rd pass i got the same 60 foot as the oither runs and this time when i shifted(seemed to take a little longere to hit the shift point(7500 rpms) it hit hard into the shift, but them seemed to pause for a littlle while at around 6000 rpms and started to go up (very slowly ) to the 7600 rpms that i go through the traps at.. there was no misses in the engine and it did reach the high rpms going through the traps. car has never reacted like this before. it is a good 8 inch dynamic converter(5500 stall speen) but it has been in the car for 15 years and has only about 70 passes on it. could this be the converter going BAD.. CAR DOES NOT MISS OR POP, SO I DONT THINK IT IS AN ENGINE PART. only other thing i was thinking about (and i dont think it could happen) is that in the new carb i rebuilt i put in a 3.5 power valve in front so the car would not load up in pits. is it possible that this valv is not opening at higher prms and shift points. reason i think not is that it ran so good on the first pass and decent on the second pass. i am not knowledgeable about converters so i really need some good advice and opinions as to what my problem might be thanks art

BEAST477

Do you still have your old carb? If so I would put it back on and see if the problem is the new carb. If that is the only thing you changed I would guess it's the carb.

chevyart

hey beast 477 i had the same idea before i started to think about the converter bit. the only thing i changed was the carb, so i am going to rebuild it and try it again. first i am going to take the power valve out ouf the new one on car and up the jets another 8 sizes and try that, and then if still no good, i will put the original carb on the car and see what happens. if still not right, then i will pursue the converter as being the culprit. any idea why a carb would do what i described in my original post thanks art

kwkracing

throw the pro form part out and start over

cepx111

Chevy art, your post is confusing to say the least, first you state that the carb is new then you start talking about rebuilding it later on, then you say you expected a 11:80 to 10:90 pass, but you couldnt believe a 11:34 ET, that ET by your own admission is in the spectrum of your expectations.

Then the part about your sixty foot times not effecting your ET's - absolutely untrue.
A 1 tenth drop in your sixty will have a profound effect on your ET, usually at least 2 tenths maybe more depending on the car and how fast it is.
Then you say on the 11:47 pass you felt like it had a miss and or rumble and it just didnt feel right, then later on when your blaming the converter your atiment about it NOT having a miss or a rumble, so that is also confusing.

Now on to the converter, what makes you think the converter is bad if you changed carbs and it slowed down? Maybe it's just me but it sounds like that carb is way off for your combo and just needs some tuning, most do.

Since your post is all over the place and confusing your not gonna get many people to chime in, perhaps you should re-read your own post and correct any misleading info and slow down and try again, but since I'm already here - I'll take a stab.

Pulling the power valves out is a good idea but I dont go up 8 jet sizes, I go more like 5.

Sqauring the jetting is also a good idea if its a all out racecar, I'd say for your combo, jetting it around 73-76 sqaure would be a safe starting point for a good running 350, but not knowing all your engine specs it's just a ball park guesstimate.

Also I'd put the old carb back on and run it one time and see if it runs back to where it was running prior to the carb swap, that would be another indication that your carb is the culprit and not the converter going south.
Sometimes you gotta back up before you can move forward> food for thought.

I hope you get it sorted out>Cp

TheRabbit

I didn't read your entire post, but if the motor felt like it just stopped for a split second it could be a fuel problem. I had this same problem first of the year. My motor at about 180' would act like somebody cut it off then right back on. My fuel pickup lines were in the front of the tank and It was getting air in the fuel lines when I launched.
Make sure you're not loosing fuel pressure when going down the track before you make any changes.

bixblk

does the new carb have jet extensions,how about the old carb ???
I was also confused a bit ???

traysvega

Same thin I was wondering and aslo if the float levels where high enough.

Scooterz

good idea... floats/jet extensions. Had that happen too & it causes a flat spot in RPMS...

chevyart

hey cpx only thing i didnt explain right is in the second line i meant to say that i thought it would be a 10.80 to 10.90 pass and i said 11.90 for first number instead of 10.90. all the rest i would not change.. i said my 60 foot time didnt make a difference in that it was poor (high 160s) and they were all the same . but the ETs dropped progressively from 1135 up to 1145 up to 1155. first run felt great as i said but et was poor for how it felt and the next two runs felt a little funny and got slower each additional run. and stranger still is that the MPHs were all about the same(119.5 mph) i would think that if it was a carb problem all the runs would all have been all the same(bad) but i had the good first run. i kmow this car well and something is wrong. carb i put on is a 750 and has the proform center, jet extensions and all the other goods stuff. i know the jetting is riught and the floats are adjusted properly. only thin on carb that i question is the front power valve, which is the 3.5 and just ma its is too low. most carbs have the 6.5 power valve. is is possible the it isnt opening when it is supposed to due toi the low number(3.5) i am putting in a 6.5 in front and going back out and try the car out again.. i know nothing else is wrong with that carb that i did on the rebuild with new center section and the jets are right for the setup, as prescribed by proform for that center section. i will also pay close attention to fuel pressure going through the traps. i will also rebuild the old carb and try it if the car still has the problem. it is the logicasl thing to do.. old carb was almost stock. only the choke housing was milled and the jets were stock size for the 700 cfm carb and there werent even jet extensions, and no power valve in rear and the car ran like a clock around 10.95 to 11.00.. i will chime back in after i run the car. any comments on what i just said thanks art