I have a 1990 RS that has nothing left stock about it except for the body. The engine is a .030 flat top 355 9.6:1 compression with a comp cams Magnum 305. Thats .525 lift 253 degrees duration at .050. pro comp heads 210 cc intake runners, 64 cc chambers 1.6:1 rockers. high rise intake and quickfuel 750 cfm carb. My problem is that it wont rev up. you have to very slowly give it gas to get any rpm and it acts then like it doesnt want it. now if i roll into the throttle going down the road the car is fast as hell but i cannot crack the throttle, it dies if you do get it to rev up and let it fall, and very obviously there is something wrong. It has plenty of timing. I am running locked out timing with a msd style distributor and gm performance parts CD boxes. The new engine only made 9 lbs of manifold vacuum with the bigger intake and I accounted for that and replaced the power valve with the correct one. The reson i think its the carb is that it did this one the previous engine package just not as severe. I could slowly rev it, but not crack the throttle. there has to be a solution. I did notice that when i do this the manifold vacuum instantly dropped to zero and quickly jumped back up as it continued to idle. I simply just cant figure this one out, any advice would be great. Thanks!

 

Is the accelerator pump working?

 

Yes the pump is working but the more ive read and the more i think about it im thinking that the pump is the direction im going to go in. I remember noticing now that the problem started right after having the carburetor rebuilt by the buddy of my mechanic that built the engine and put 305 head gaskets in a 355 (why i had to rebuild the thing this go around). Long story short i believe my quickfuel carb did come with 50cc pumps, or bigger nozzles than are on it now. I distinctly remember a much thicker and longer fuel charge from the accelerator pump. I just never imagined that this new top end would change that much. Question is now should i put 50 cc diaphrams in it or try to get bigger shooters?

 

That is a lot of cam for a 355 with low compression. Locked out timing is a good idea...but you should probably still have vacuum advance to add another 15* of advance at cruise. That might help some.

Check to make sure the pumps are adding fuel with even the slightest movement of the throttle. They will cover up a lean stumble, but not fix your slow revving situation (the pump shot only helps for a split second).

What transmission are you using? If it is a stick, and you have it in a higher gear and are punching it at 2000RPM cruise, you may not be able to tune it out. A high stall converter in an auto will mask this problem. Again, lots of ignition advance will be your friend.

 

Punching it in gear isnt a problem. get about 2500 rpm in it and it and its screaming to 7000 before you know it. my problem is instantly right off idle. if i slowly open the blades it can climb but if i crack the primary blades all the way back until the secondaries start to open it just hiccups. no backfire nothing else. its got all the timing it can handle without dragging the starter, more than i really want in it. It is a 5 speed with a 4.10 gear. really has no problem pulling the trannys geared for a 3.08 i think i need a little more tuning of the idle air screws to maintain an idle and im thinking some work with the accelerator pumps may help. I am worried however that i dont have enough compression for the cam but i feel confident ill be ok. otherwise it runs incredible. completely blew my mind on the test drive ( when i got up to speed). I scared myself, and havent done that in a while

 

Your 750 should have 30cc diaphrams front and rear out of the box. This is what you need, if you have a 50cc front or rear trash it! A 50cc comes on the rear of an 850 usually and is too much for your combo. What size squirters do you have? A stock 750 has 28 in front 31 in rear and some are 31 f&r. I run anywhere from a 28-34 in the front and leave the 31 in the rear, just depends on what the engine wants. Is your accelorator pump linkage adjusted correctly in the front and rear and do you know how to do this, if so check it. Your idle mixture screws have a direct affect on part throttle so get them set right before you tune the pump shot. Idle vacuum is critical for idle quality so tune your idle mixture screws for the highest vacuum and you may consider getting a vacuum reserve tank, it will help your brakes as well if they are power.

 

the motor has an insane cam in it. it only makes 9 in. of vaccum at idle. the thing thats bugging me however is its a quickfuel 750 carb and I once had an issue with it and couldnt figure it out so i had my mechanic put a kit in it, and when i got it back it idled great but then i couldnt crack the throttle. It noticable has a much smaller pump shot than before i could have sworn it had 50 cc pumps. had a much much thicker shot and also lasted much longer. the pump arm is adjusted correctly ive checked it twice. My mechanic is the same guy that put 305 head gaskets in my 350 which is why i had to rebuild this motor recently, so id be willing to bet he just found a couple 30 cc pumps from the shelf and stuck in there. I guess i just need to find out what quickfuel carbs come with. This thing really needs some help at idle though, thats why im thinking 50 cc or bigger squirters

 

just noticed you have the 262H magnum cam malibu2envy. I have the 305H

 

Yes you have far too much duration for your compression which is one reason you have no vacuum. Ok your squirters have nothing to do with your accelorator pump diaphram volume they are two different things absolutely. Your squirters are on your main body atop the primary and secondary venturis "throat, holes," and spray your pump shot into the venturi. Your 30cc or 50cc accelorator pump diaphrams are on your front and rear fuel bowls at the bottom and "store" fuel for the pump shot and it's only the amount of volume 30 or 50 and 30 is what you need. Your squirters have a number on them to say what size they are. Your diaphrams you just look at how thick they are to determine if they are 30 or 50 and the diaphram housing that bolts to your fuel bowls is NOT interchangeable between the two sizes so unless your "mechanic" had some different housings nothing has changed there. Understand what I am talking about? How did you set your Pump shot linkage with the bolt and nut on the arm? At wide open throttle you need 30 thousandths clearance before the diaphram "bottoms out" which damages it. Another thing to consider is getting a wideband to tune it with, they are cheap $200 thru Innovate and will help you year round. What is your timing set at? Also if this "mechanic" put 305 head gaskets on a 350 loose this retards number and don't let him touch your car!!!!!!!!!!

 

the reason I built the motor is because i did loose his number. he will never touch anything of mine again trust me. And i know what all of these parts are. I know the nozzles the numbers the sizes etc. i just know that the pump shot is very very different than it once was when the carb was in its original condition. could he have put the secondary shooter on the primary side and primary shooter on the secondary side? and i have 15 thousandths clearance as thats what ive seen most common. I wish i could walk outside and check the squirter numbers but im at college and the car is an hour away. but i will be going home to work on it to night. So i gues the first thing to check is gonna be the squirters and then maybe pump cam? I realize the cam is too big i went from 52 ccs to 64 cc heads and lost all my compression. I want to try very hard to make this cam work and if i just cant ill have to change it. Thanks for all of yours and everyones help. I think this will drastically decrease the time i spend spinning my wheels on this car. I also have a cam to put in my buddies 99 t/a this weekend.

 

Ok so is he above stated compression actual compression with the current 64cc heads and did you calculate exact compression or is it just what the pistons are? Ya I guess that mechanic is pretty stupid, check and make sure he don't have the metering blocks switched from front to back, the front should have the power valve and the rear should not. I take it your carb has 4 corner idle screws since it's a Quick Fuel? The pump cam is more fine tuning and to be honest usually the stock ones work pretty good in most cases. Make ONE change at a time is the best thing you can do so you find the problem.

 

yes i recalculated it with 64 cc heads .30 over bore deck height etc. i dont remember exact specs but i did the math. it is a four corner idle. that mechanic said the front two screws about 2 turns out and the back about a half out but if i understand the holley tuning guide correctly they should all be the same. is that correct? i want to tune the idle circuit first and try to get a little more vacuum out of it then im goin to check the shooters, and yes the metering blocks are correct

 

If that's what your idle mixture screws are at they are not even close! Yes they all need to be even, start about 1 1/4 turns out and see what happens. After that never go more than 1 3/4 MAX, 1 1/2 is average. There is so "set" turn number in fact this greatly affects "tip in" throttle and is very sensative, once you get close 1/16 of a turn is alot. *TIP* DO NOT crank down on those mixture screws it will make a ridge on the tip of the screw then they are trash! For squirters start with 28 in front and 31 rear, then if you want to make a change go to 31 in the front also.

 

ok so heres the latest i went over and messed with the car. sadly i did not get your last message before hand so i went and set all of the mixture screws even off of my gut (why would they invent a four corner idle if you could just run the fronts? duh) anyway theyre more then 1 3/4 however 2.5 is where i got it to run the best however it didnt solve the problem it definitely helped. I have 31 size shooters front and rear but ive somewhat narrowed down whats happening. if i open the throttle fairly slowly i can get it to rev but if i flick it or bring it back all the way until the secondary kicks in thats when the problem occurs. I got it out and did drive it up the road however and once its wound up it runs great pulls great etc. but it just will not come off idle smooth and when i hit the secondarys it cuts out completely. at this point i am wondering if my distributor is advancing but i dont know why it would cut out right at the secondary is its ignition though. other than this the car runs great and its very annoying since i really want to be able to drive it. I just cant tell if its getting too much air or too much fuel but it has to be one or the other. Im posting a youtube video of me trying to rev it. I know its difficult to tune a carb over the internet but ive made great progress with your help and really appreciate it. http://www.youtube.com/watch?v=bNwVHxAd6yw(at :46 is where i first cracked it hard enough to make it cut out. adjusting the idle screws greatly helped that initial off idle but i cant give it much there maybe bring it from 1000 rpm to 2000 max. any more than that or hitting the secondaries makes it cut out. It just acts like a hiccup or burp. vacuum drops to zero and if you hold the throttle when it hiccups itll eventually catch itself. like i said it never did this with the same cam just different heads and intake and a holley brand 750 right out of the box 0-4779c mech secondary double pumper was the original. i know its alot of cam but i just know this is carb or ignition if nothing else my gut says it. thanks for all the help

 

Just as a test...up your jets 4 sizes front and back and see if it cures your problem. You wouldn't want to keep it this way, it is just a test. I have a similar stumble with my mighty demon. At certain speeds (ie. throttle position), a quick tip of the throttle will not give enough pump shot to cover up the hole. You are going lean. If the jets cure it, you need to get more fuel in through the mains without screwing up the whole fuel curve. HSAB's, PVCR's, and float setting will be your friend in this case. So will lots of vacuum advance in the distributor.

 

well the distributor is locked out. ill see if i can do that tomorrow. i dont have jets on hand and maybe oreilly will have some. i almost want to try bigger pump nozzles just to see what it does. this is all thats holding me back. im very curious too if the mixture screws are wrong. it definately got better when i adjusted them and i apparently still have them too far out.

 

btw fill me in on what pvcr and hsabs are. sry im old school

 

Looks like a nice car! Carbs can be fun sometimes huh lol! Don't worry about your vacuum going away when you open the throttle that is how it should be, when the throttle blades are open it can't have vacuum. Wish you had got that last message, set the idle mixture screews like I said before and give that a go. I have never personally run the timing locked out on anything so I don't have much input on that sadly. Dumb question, what's your fuel pressure?
Also I just thought of something! I once had this problem on a used carb I bought and you would never believe the problem! Ok now your linkage from the primary to secondary throttle blades on the baseplate are they adjustable? Some are. If they are not often everybody including myself checks that the secondary blades are opening a full 90 degrees at wide open throttle looking with the carb off using a simple square, if they are not at full 90 degrees you can bend the linkage that connects the primary and secondary throttle blade shafts. A used one I had was bent too much and the secondary was more open at idle then when you would rev it it would actually close some first then open back up. This can also be the case if you have the secondarys opened too much at idle. To check this do it with th engine and fuel pump off or with the carb off. Sounds crazy I know but I have seen it and it took me awhile to find it on mine! Keep me posted on your progress!

 

ill be honest im not very sure what you mean by the linkage but ill try to see if i see anything. it is a mechanical secondary. should the secondary be completely closed? and when i check should it be at 90 degrees? and if it is incorrect it would not make a 90 degree angle? thanks for all you guys input. as for fuel pressure ironically i ran it out today i think but like i said its been doing this for months i know its not fuel pressure. could the float level do this too. i definitely think its something wrong with the secondaries though. thanks guys

 

Ok on the opposite side of the throttle linkage there is other linkage on the passenger side, all it is is a little piece of "thick wire" that connects the primary and secondary throttle blades together. You check that it opens 90 degrees with the carb off. While you have the carb off look under the throttle blades on the front and rear there will be "transition slots" VERY small vertical slots in the baseplate itself and for both front and back NO more than 30 thousands can be exposed under the throttle blade in the closed throttle position or it will be trying to idle on your main jet circuit not the idle circuit. These are things to just rule out, the main thing is to look that you are not having the strange problem I once had where when you open the secondarys that they close briefly then open back up, it should just only open not close first (this can only happen if that linkage is out of adjustment or the secondary is open to much in the closed idle position). Food for thought. Wish I was there I could fix it for ya!

 

i will be checking it tomorrow. one of these things has to fix it. i really cant thank everyone enough for all the help cant wait to post a fixed video. im sure tomorrow i can make it do something. thanks guys.

 

have you checked the diaphram in the acc pump yet? i once by mistake put the acc pump spring in upside down. i cut the the little pink diaphram inside there. it acted exactly as you describe. pull the front bowl, flip it over and remove the 4 little screws. something easy to rule out.

 

did it still give a good pump shot when the diaphragm was cut?

 

Here is todays videohttp://www.youtube.com/watch?v=HDdokTpUn00, at :37 was the backfire it has a very distinct pop. Heres the latest. It blew a fireball back in my face. SO first thing i did was take the carb off and start checking things. the acc. pump diaphragm is good. The transition slots were way off primarys about .100 and the secondarys 0 i set them both to .030 and eventually had to give it just a little more to idle so primarys call it .050 and .030 on the secondary. I set the mixture screws at 1 3/4 and actually set them another 1/16 of a turn in, (theyre perfect great call malibu2envy). I noticed it has 72 jets in the fron and im assuming 78 jets in the rear. #31 acc. pump nozzles front and rear. next thing im going to do is reset the timing since i simply cannot find anything wrong with the carb

 

I was always told that most carb problems are fixed with the ignition curve! Having been through it a few times, I believe that now.

Popping and shooting fireballs is USUALLY retarded timing. Could be other things, but I would start with the timing.

 

it sounds like your lean popping. go up on your squirter.

if it pops at 37 go to 40.. keep going up until the pop and or hesitation goes away.

you really dont need a a50cc acc pump until you need a 37 or highr nozzle.

you have yours already, so consider that a plus. when you go to a 40 or higher make sure to use the hollow squirter srcew.

after you get rid of the lean pop then you can play with the pump cams.

 

well guys i still have nothing. The car runs ok. Not great but ok. Still has that flat spot right off idle. Still very obviously is not correct. Latest thing is when i dog it for three gears. Get up to about 120 or so and then slow down and pull into the driveway it goes from about 1100 rpm idle down to a very labored 700 or 600 and i have to tap the gas to keep it running. Im seriously considering going up to work and buying one there.(RCR) think 830 cfm is too much for a flat top 355 with wicked heads and a massive intake on it? its only 70 more cfm than i have?

 

A bigger carb wont fix too much cam in a low compression 355 with not enough ignition advance. If you want to experiment...skip the 830cfm carb and try a GM performance parts HEI (about $190) with 20* initial advance and the vacuum advance hooked to full manifold vacuum.

The 830 could be a real good carb if it has the annular boosters. If it has the regular dogleg boosters, it will be too much CFM (too little signal).

 

i hear what youre saying but I cant get the timing any more perfect. Id rather put the 190 in that carb and completely rule that out. if i still have a problem ill look futher into ignition. Ive just got that gut feeling on this one. I cant figure out why my idle would be all over the place with timing and if you can tell me, why annular vs. dogleg, could that be my problem now? Thanks for all the help I wish i could let you guys drive the thing and feel what its doing

 

i currently do have dogleg/downleg boosters

 

wow, ive been reading up on downleg vs. annular. I have the completely wrong cam or completely wrong carb. Ill have to hop on the annular train asap. Might still look into a distributor too. Thanks for the idea!