engine seems to stop reving for a moment

[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]

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
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]

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

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

[cepx111]

Well chevyart, thanks for clearing that up, it makes more sense now.

I've also been thru the proform center section nightmare.

I installed one my son's 750DP, lets just say I caught hell getting that sucker dialed in, but I will say that once I did get it lined out, that car flys.

Their jet recommendation was WAY off for our combo, now I'm not saying that this is the case with yours but playing with the jetting wouldnt be a bad idea, it can be a cheap fix.

But on the power valve, why even bother?
If this is a race only car a power valve is useless, in a drag car your @ WOT 99.9 % of the time, the power valve circuit only comes into play at part throttle, it's just a transition circuit for driveability, not a tuning aide in a dragcar, plug that hole, jet the carb up and "square" and be done with it.

Goodluck>Cp

[chevyart]

hey cepx11 thanks for quick answer. stock 750 holley comes with 75 square with 2 power valves, so if i remove the front and rear power valves and step up 8 jet sizes (as recommended by holley and quick fuel), then i should start jetting at 83 front and rear. i got the carb from my buddy already done and i only made a few minor changes. he had it jetted 82 in the front and 84 in the rear.i changed back to a front power valve(3.5) and lowered front jets to 74s(down 8 sizes). i really thought that this would help prevent loading up whild driving in the pits. i asked on the forum about the power valve vs no power valve for drag only car and the views were about 50-50 for and against, so i felt the safest way was WITH the power valve in the front. does jetting sound right to you and i guess your thought is to NOT run a power valve. i really appreciate your input. i still have time to go NO POWER VALVE in front. im going racing on sunday thanks art

[cepx111]

Sorry for the delayed response, I work nights. :x

On your jetting, it's hard to say, every combo likes something different. But if I had to make a stab no knowing all your variables, I'd say 80 sqaure would not be a bad starting point.
It also depends on what your trying to accomplish, if speed and ET is all your after, being a tad fat is usually what your looking for, just jet it up till your MPH drops off and then go down 1-2 jets and lock it down.
But if your a bracket racer like me, lean is mean, I think my sons carb is jetted 72-73, no power valves, 50cc secondary accelerator pump.
It's a a failry mild 351W that runs 750's 1/8 low 11's 1/4 and yes it's a frigging ford.

Jetting can be a huge enigma to some folks, but really it's just something that needs to be played with in order to find out EXACTLY what the motor is asking for and what your trying to accomplish.

Gooluck and let us know how it pans out>Cp

[chevyart]

you are a good man cepx. im going to run car tomorrow. gonna start out with the same setup that i experienced my problem with and first watch the fuel pressure guage and then work from there.. this track is up over 1000 ft above sea level and last rrack was about around 300 ft abpve sea level, so i should go a tenth slower for starters over last track. ill let you know the results. thanks again art

[cepx111]

you are a good man cepx. im going to run car tomorrow. gonna start out with the same setup that i experienced my problem with and first watch the fuel pressure guage and then work from there.. this track is up over 1000 ft above sea level and last rrack was about around 300 ft abpve sea level, so i should go a tenth slower for starters over last track. ill let you know the results. thanks again art Hey man, no problem, that's what we're here for, help.

I hope all goes well and you get that car dialed in and you go some rounds>goodluck<:wink:

Me and the boy are getting ready to head out right now, so wish us luck!



Cp

[chevyart]

hey cepx sounds like you got a nice family race team i like that. a family that races together stays together. good luck out there ill talk soon art

[mopar1968]

i agree get rid of the power valves and step up the jet sizes :!: :!:

[cepx111]

hey cepx sounds like you got a nice family race team i like that. a family that races together stays together. good luck out there ill talk soon art Well it definetly has its joys and woes.

I work on that damn ford more than I do my own car :x

I get to pull the trans again tomorrow, converter we put in it last week aint right, lucky me.......

How did it run today?

Cp

[chevyart]

hey cpx went to lebasnon valley(ny) yesterday. first pass dont even count. spun tires so bad i was embarresed. forgot to check air preszsure). 2nd pass was race time. car launched good and ran good but et was only 11.47 and mph was off by about 1-2 mph(lost race by 11 thousandths of a second) i let off just before end because i couldnt see the guy(didnt use my mirror and just looked to side.he caught me and i lost by maybe inches. they let me do a time trial and the car spun alot again(didnt care about that) and when i shifted around 7500 the car had like a bad miss(stutter) or something and it lasted for only a second. i checked the fuel pressure guage(in car) and it read 7 lbs(thats ok). car picked right back up but ran a loust 11.7 (60 ft tome was partially to blame for that. problem is that come and go miss ive been getting since switching carb . i really think it is the 3.5 power valve in there(too low a number), i am putting the vacum guage on there today.still havent seen the power valve that was in old carb, but it definitel the right number power vaLve.. and will try the power valve (the right size if the wrong one has been in there. if it was the right one in there then i will go to straight jets on front anbd rear and step the size up to 84 square.i worry about loading up in the pits, but quick fuel told me i could adjust the idle circuit air abeeds to lean out the idle circuit to help prevent loading in the pits. in my old cargb the power valve in there was definitely the right size. still trying to get the metering blocks off old carb. they are like welded onto the main body. ill keep you posted. any more suggestions ,i am listening thanks art

[cepx111]

hey cpx went to lebasnon valley(ny) yesterday. first pass dont even count. spun tires so bad i was embarresed. forgot to check air preszsure). 2nd pass was race time. car launched good and ran good but et was only 11.47 and mph was off by about 1-2 mph(lost race by 11 thousandths of a second) i let off just before end because i couldnt see the guy(didnt use my mirror and just looked to side.he caught me and i lost by maybe inches. they let me do a time trial and the car spun alot again(didnt care about that) and when i shifted around 7500 the car had like a bad miss(stutter) or something and it lasted for only a second. i checked the fuel pressure guage(in car) and it read 7 lbs(thats ok). car picked right back up but ran a loust 11.7 (60 ft tome was partially to blame for that. problem is that come and go miss ive been getting since switching carb . i really think it is the 3.5 power valve in there(too low a number), i am putting the vacum guage on there today.still havent seen the power valve that was in old carb, but it definitel the right number power vaLve.. and will try the power valve (the right size if the wrong one has been in there. if it was the right one in there then i will go to straight jets on front anbd rear and step the size up to 84 square.i worry about loading up in the pits, but quick fuel told me i could adjust the idle circuit air abeeds to lean out the idle circuit to help prevent loading in the pits. in my old cargb the power valve in there was definitely the right size. still trying to get the metering blocks off old carb. they are like welded onto the main body. ill keep you posted. any more suggestions ,i am listening thanks art As far as the air pressure whoops, I aint never done that :roll: ...lol I think we all have, no biggie.
Damn ya'll only get one time trial? Whats up with that?
We usually get 3 for sure, sometimes 4 but I race at a laid back outlaw track in north mississippi, that would be in god's country if your wondering :wink:

Man I see your still stuck on them PV's, I'd loose them sum bitches before I did another thing to the carb, but 84 sqaure is wicked fat IMO for a 750cfm DP on a 350 anything. I'd go 80 sqaure and work my way up or down till it's dead nuts on.

Note:
As a comparrison I'm jetted 88 sqaure on my 1050 dominator which sits atop a 500CI bigblock.

Also, plugging them and squaring or not squaring up the jetting will NOT make it load up in the pits, that would be the idle circuit causing that problem, just lean that sucker out with a couple turns of the mixture screws (IN), if that dont do it, you got other problems, a BLOWN power valve comes to mind, another reason to loose them suckers.
In certain circumstances you have to add additional air to the idle circuit, a couple small 1/8 inch holes drilled in the primaries will take care of that with a quickness.

Ok, let me know what you decide to do and how it pans out.

Goodluck>Cp

[chevyart]

hey cepx put vacum guage on engine and it reads between 5-8 inches at idle (in low gear) gave it a little gas and it just stays around the 5-8 area.. im gonna try one more time with a 4.5 power valve. the other carb had a 6.5 in there for all these years and the car ran like a clock. im rebuilding that carb this week and its going back on. in the meantime i want to see what the problem is with this carb(if the carb is the problem,) as it will go on my 377 engine next season and i would like it to be right. the 84 square is just about correct for the carb(without power valves in front and rear. some of the stock 750s out of the factory are 75-75 square with power valves in the front and rear. some have smaller than 75 in front and 75 in rear(with power valves in front and rear, so by removing the valves in front and rear and going up 8 sizes it comes in at 84-84. when imgot the carb from my friend it was done without the valves and was 83 front and 84 rear. thats probably what i will go back to when i take out the front power valve, like you suggested. my buddy, who is a super stocked and stocker racer in nhra told me today to get that power valve out, just as you said i should do. i can put bigger air bleeds in the idle circuits if it loads up in the pits. quick fuel told me to do this. so did you,, so i respect all that you are telling me.. holley tells me its definitely not the carb, but the fuel delivery system( elec fuel pump or filter or pressure regulator) but i dont believe that. what do you think thanks art

[cepx111]

Well chevyart, sounds like installing a 4.5 PV would be a complete & total waste of time if a 3.5 didnt work.
Now dont get me wrong, I'm all for baby steps when it comes to tuning but sometimes you can get hung up on too many small changes and get frustrated (you sound frustrated) .
Bite the bullet, make a profound change, you can always go back, if that 6.5 was working in the other carb - I'm gonna go out on a limb and say it will work in the new one.
So stick that 6.5 in there or better yet plug the hole(s) jet up accordingly (not 8 either, more like 5-6) and be done with it.

Cp