I have been adjusting and tuning on my motor for about 2 months trying to get it dialed in.

I am running a standard bore 350, RHS Vortex Heads 170 CC intake runner 76 cc combustion chamber
1.94 intake/1.5 exhaust
Comp roller tip rockers 1.5
XE268H .477 intake @.05 and .48 exhaust @.05

MSD Pro Billet Ready to run Dizzy MSD Blaster ss coil, Summit ignition box

Wieland Stealth intake, Holly 750 Vacuum Secondary .035 squirter Jetted 72 pri/75 secondary

TCI torque converter turned to stall @ 2300 RPM

My problem is that at low RPM if I accelerate quickly the car misses, even stalls, When it backfires it is always thru the carb and it is usually white smoke. I disconnected and plugged my Vacuum advance. My experience is that when running a ignition box the vac advance wasn’t necessary. I have my timing set at 18 initial and am all in a 3000 with the total at 38. I am using one light and on medium spring for my mechanical advance. And running Autolite AR133 plugs MSD 8.8 wires which I cut to and crimped I checked the resistance on each wire and they are right at 45 OHMS per foot.

If anyone has any tuning tips I would greatly appreciate it. This is the first motor I have built since High school. I decided to revisit my child hood since my son is so interested in cars especially the 2nd gen camaros I bought him a 73 RS as a graduation present which we have been working on for a couple of years.

I dont know much about vortecs but I thought they were all around 64cc? I really have no Idea and this doesnt help you any just curious.

Can you slowly mash the pedal eventually reaching WOT over a couple seconds problem free? If so sounds like the accelerator pump adjustment or powervalve opening too slow or secondaries opening to fast (sounds like not getting fuel fast enough). If you have this problem no matter how fast you push the accelerator then other problem.

In my experience, a bog or miss upon quick acceleration is a sign of too much gas, not of too little. Of course I would check all the usual suspects, the fuel filter, float level etc first. If the engine has been backfiring through the carb, it would not be a surprise to find that the power valve is blown out. If this is the case, you will have gas dribbling into the carb through the valve so when you punch it the accelerator pump only adds mor gas and overwhelms the engines ability to use it.

Your engine is pretty mild, and imho, a 750 carb, even a vacuum secondary model, is probably too big for your engine. I doubt if this would be the cause of the problems you are having, but you would get better economy and drivability with a 650 or even a 600 cfm carb. Good luck.

I just switched over to a 670 avenger carb same problem. I can slowly accelerate and have no problems. The engine only hesitates when I maash the gas. The Heads are what RHS call old style it is a 12400 part number. I was wondering if I have to much initial advance. It is my understanding that the MSD pro billet dizzys are designed a little hotter. I am gonna play with my timming curve to get my initial down around 14 with total still at 38.
thoughts???

Also my vacuum at idle is only 11.5/12 is the 6.5 power valve to much or should I drop to a 5.5

Pull it into gear and hold the brake while giving it some throttle. Don't do it enough to spin the tires. If you feel the miss during this procedure, then chances are your plugs are fouled....which is easy to do when playing with fuel mixtures while your tuning one. Pull a plug or 2 and see how they read.

well I am stumped I checked the plugs and they look good no sign of fouling they actually almost look perfect when I look at the chart. I have been playng with the springs in the dizzy right now I have a medium and a heavy installed. I even swapped back to the 750 rejetted it to 70 pri and 75 sec and switched to a .031 squirter. test drove and top end is much better with the 750 but when I mash the gas it pops like no tomorrow. ease into the throttle and it runs like a scalded ape. however the 670 avenger dosent pop as much when i punch it basically stalls a bit and picks up. One other bit of info i hooked up the vac advance and it did get better.
I even tried to time the car with a vacuum guage connected to manifold vacuum and rotated the dizzy til I maxed out the vacuum and backed down 2 on the guage. however I felt like I was advancing the timming to much so I set it back to 38 total.

Ok fellas where am I screwing up I am ready to drinking heavily and kick the dog

Also my vacuum at idle is only 11.5/12 is the 6.5 power valve to much or should I drop to a 5.5
Didn't see this earlier. That manifold vacuum reading with the cam you're running is a little low. I'm guessing you have a vacuum leak somewhere. You should be 12-14 with the XE268H cam. Another thing, I tried the AR133 plugs once. They are a fairly mild-cold plug, better for the drag strip. I'd try a set of Autolite #26 with your combo. And anything bigger than that 670 carb is too much for what you have. It's always easier to jet a smaller carb up than it is to jet a bigger carb down.

Thanks I will pick up a set of those plugs in the morning. I also switched back to the 670 tonight. I checked around the intake and dont se any signs where there may be a leak. Is there any tricks to finding it around the intake? Other than that all of the other ports are blocked or connected. PVC, However I will check the modulator on the tranny in the AM when I pull it up on the ramps. Thanks for your help GoldenOne I will post my results when I change out the plugs.

also my vacuum at idle is only 11.5/12 is the 6.5 power valve to much or should I drop to a 5.5

my idle is 10" and I run a 8.5, my darn pcv was a vac leak for some reason and Ive tried a few I ended up deleting it and going with breathers but you may want to pull it out and stick your finger over it just to check, So now that you switched carbs you have the exact same problem?
I think the vac at idle should be much higher as well with that cam.

What kind of fuel filter are you running? It is fresh?

Try setting the timing at 32 degrees total just for kicks. Those heads usually dont need 38 degrees tt. then experiment with a larger accelerator pump shot
A bad coil / cap or rotor, coil wire will make it do that too. You can have a condition where it has enough spark current to fire pretty well under light load but not under high loads when your foot is in it. crazy huge plug gap can do it too.

DJ

What kinda carb base gaskets are you using ?

With all these swaps, you could be stuck with a bad gasket and getting a leak that way.

With it idling, check for a vac leak with a propane tank or some other small gas tank just to be sure.

And with it idling, pull one spark plug wire at a time to see if you just have a bad wire or plug. Then if you have one, swap distributors and leave the elec box off. See how much that helps.

Hey Guys, Thanks for all the great information. I swaped over to 26's and disconnected the box. Motor ran awlful steady miss. I shut it down rechecked the plug wires and tryed again it seemed to run better. So I adjusted the timming set it back to 32 readjusted the carb turned both srews in until it started to die and backed it out until I achieved max Vacuum. Test drove and motor seemed to backfire always around 1500 rpm. as a load is being applied or if I step on it quickly it pops and then takes off. So tonight I decided warm the motor up advance the timming by ear. I shut the motor off, started it up again and backed the base timming back until I had no starter drag. I am not sure what the total is at right now but I went on a test run and the motor ran pretty good.
My question is when I pulled the plugs AR133's to swap to the 26's I noticed that 3 and 5 cylinder plugs where very clean. The rest of the plugs showed signs of a very rich condition very black and sooty.

I am still working on this to really nail down what was causing my problems

thanks

RG

I'm not sure if this would cause it, based on the vacuum comments. But I suggested to him before that he could have a problem with the seal between intake and heads, since he is running an 'old style' stealth intake with vortec heads. The RHS heads he has have dual intake patterns. Don't think that was mentioned.

Test drove and motor seemed to backfire always around 1500 rpm. as a load is being applied or if I step on it quickly it pops and then takes off. RG

that still sounds like total timing is to low or lean at the moment you hit the gas

the 1st thing I would do is take the plugs out and re-gap them.

Deffinately sounds like an ignition issue to me.

just wondering if you have had any luck on this issue. i am having the exact same problem on my 454 im starting to wonder if its in the cam/lifters. its my daily driver and im ready to rip my hair out.

I have a 383 stroker 750cfm @ 12* intial timing with the same problem. My local guy who rebuilds dist. said to first disconnect the TACH he said sometimes they will cause interference. Tried it but still did it. Since I had swapped out the Accel shorty plugs after I bought the car and put a set of autolites in it ran good for a little while then started falling on it's face and backfiring through the carb when I mashed it. I was trying everything to no avail then it dawned on me the only thing I changed was the plugs. I kinda choked @ 5$plus per plug but I got em (576s) set the gap at 35 thou. put em in and whoa pin ur ears back.... runs like a stripped assed ape again. Hope this helps. ;)

over the weekend i fixed this problem that has been killing me for months. i was talking to my uncle and he had the same problem. he told me the problem was an exhaust leak. well i did have a small leak at the headers. so i put a set of copper gaskets on and problem solved. so if anyone is having this problem and they have a small exhaust leak, i would start there.

Hey Guys
Thanks for all your comments, I really struggled trying to get my ride to run without backfiring when I punched it. I switched over to the autolite 26's, went all over the motor looking for a vacuum leak, I even reset my lifter preload (drove the car till it was at operating temp set intake valves till I felt resistance and 1/4 turn preload same for exhaust) this did bring my vacuum to a steady 12. tuned the 670 avenger, exhaust was as clean as I have seen it. Went for a test drive same but was a little better however if I really stepped it bang. Backed the timming to 32 Motor was even worse. So bumped the initial up and backed it down until i had no starter drag. Went for a test ride from a dead stop I tromped it and the only smoke I had was the white smoke filling the rear of the car man I love the smell of burning rubber. I just hope my timming is not advanced to much My temps are steady at 180 but I think the under hood temps from the exhaust are hotter maybe its just me. But the motor really responds now when I tromp it.

Thanks All
R

sounds great what is the total at now?

Total is 40 at 3000 rpm

Total is 40 at 3000 rpm
thats probrably close with those big 76cc chambers

So you think that I should be ok? The motor is running great but I was concerned that my timming is to advanced. I usally have the best luck around 36 to 38. I have never went much higher.

I don't understand what you did to get rid of the misfire/backfire. How can an 8° change in spark timing cure a misfire?

I'm with several of the other guys, it sounds like a weak spark in one or several cylinders.

I'd like to know what you did too. I have a breakdown problem at 4500, and last week I could hit 6000 rpm in 1,2,3, gears. Did I overheat the coil or wear out a wire?

See http://www.nastyz28.com/forum/showthread.php?t=99672

My total was 36* when I checked a few years ago.

Sweet man, what a good feeling. I went from a tired 350 that wouldn't turn over bald tires to a pretty fun mustang killer for little money, and a bunch of wrenching

I don't understand what you did to get rid of the misfire/backfire. How can an 8° change in spark timing cure a misfire?

I'm with several of the other guys, it sounds like a weak spark in one or several cylinders.

Glad the backfire is gone for you but, I agree with Brian here. I don't think you've fixed your problem, only masked it. Another thought - are you sure that TDC is actually 0*? Like a slipped harmonic balancer ring.