Well I am running my stock L98 longblock. I switch from tpi to carb.

I added a holley 750vs #3310s and a team-g intake #7525 with a rpm range of 2000-6500.

I am currently have a problem. The motor has a dead mis. I installed a non computer controled Hei dist. Everything is new in it except for the vaccum advance and the module, which I had checked. It has a accel super coil, cap and rotor also.

The plugs are new and the wires are in good shape. My timing is a 4*btdc and I cant find any vaccum leaks. There are no cross wires and the dist is in right.

The car backfires through the carb if I hit the gas hard from idle. I have tried a few different things with the jets.

Things I have done to the carb: I added a rear metering block, jet extensions & notch float. I installed a powervalve block for track reasons (may need to change). I installed a quick fuels adjustable vaccum secondaries diaphram. I am currently running the powervalve block with 76 jets in the front and 78 in the back with a 31 squirter.

The car run some better and does not backfire as often. I still have a dead mis. It will run smooth if I give it steady pedal, but if I romp on it, it will hesitate/cutout maybe even backfire. I will let out of it and it will take on off an run with steady pressure, but as soon as Im back on the throttle, it starts sputtering.

Is the carb just too big for the stock L98? Can I get the carb adjusted right for it, or is my problem elsewere?

I have a 4.5 and a 6.5 powervalve at home that I have tried with different jets. At first the plugs would soot up even after a few passes. Now they clean themself off and have just a hint of tan on them after a couple of passes.

I need help, this is driving me crazy.

thanks.

 

No, lots of people use the 750 vacuum secondary 3310 and a dual plane aluminum intake like that on even milder motors.

So the good news is that you do NOT have a basic mismatch of parts. You're OK there.

The bad news is that you still have the problem. I agree that this does not sound ignition-related.

Is this a situation where it stumbles for a second and then takes off like a bat outta he11 OR it CONTINUOUSLY stumbles and misses for as long as you hold it wide open?

 

i thought all team g's were single plane not dual. i know mine is a single.

 

As of right now, it stumbles and misses as long as you are in the throttle hard. As soon as I let up and keep slight pressure to hold the speed, it does ok. It still misses while going down the road.

And Spartyon, yes it is a single plane intake. It is considered a street ram I think. Check holleys site for weiand #7525. It has egr and choke provisions, but I have blocking plates on them.

 

update

I replace the powervalve block with a 6.5 and put 74 jets in front and 78 in back.

I also put a timing light on each plug wire and found one that wasnt burnt into, but was not firing every time. I replaced it and now my mis is slight if now gone.

It still stumbles when I hit the throttle hard, then smooths out and then starts stumbling again at higher rpms.

I did bump the timing to about 9*btdc. the car seems to start fine.

What should I do next?

Oh yeah, the backfire is gone now. Must have been in the timing.

 

I tried a vaccum gauge on the intake. It pulled 17" with the car warmed up and in gear.

What powervalve should I use?

Oh yeah, it held steady at 17" and did not drop. So I guess it rules out the a vaccum leak. Is that correct?

 

a steady vacuum doesn't rule out a vacuum leak, it just means your valves are operating and seating correctly. I wouldn't waste much more time looking for a vacuum leak as it's very unlikely to be causing you a problem.

A 750cfm is a little big for a single plane on a 350. Just fine for a dual plane intake though.
To cure the stumble you need to work more with the accelerator pump than the power valve.
Your vacuum is close enough to stock that you could use a stock application power valve.
If you insist on keeping the single plane intake and swapping accelerator pump cams doesn't completely cure the stumble, the you'll have to upgrade to a better torque converter to get it smooth.

 

4 btdc seems like not quite enough for your timing. I thought you were suppose to be closer to 8-12btdc.

 

Yeah, I bumped the timing to 8*. It didnt make much difference.

 

Are you sure your guage is right on the block. If that is off it could show you at 8* and you really not be. You said your distributer is new right. Kinda sounds like your mechanical advance could be gummed up. Or maybe you need to look at your spring choice on the advance. Just a thought.

 

well, when I said it didnt make much of a difference, I ment in the sputtering of the carb.

I am running new weights with the nautral springs. I also checked the vaccum advance for leaks, didnt find any. I still plan on replacing it.

 

update.

I replaced the plug wires, plugs, went from a 31 squirter to a 35, and went to a 8.5 powervalve.

Everything seems 100% better, except for it sputtering up around 4000 rpm. I think I arm running out of timing.

I replace the stock weights in the dist, but I dont think they are working right. With a timing light on #1 plug with the vaccum advance plugged I get about a 8* reading. When I rev the motor to say 2500-3000 rpm it only goes to about 12-14*.

I think I will check the weights and springs to see what is up.

 

Lack of sufficient advance will definitely hurt you up top- including some sputtering, depending how bad it is. 12-14* all revved up is not nearly enough. What distributor are you using (old style large cap HEI)? What weights were in it and what weights are in it now?

BTW- a LOT of the gyrations you are going through with the carb tuning are probably related to your lack of centrifugal advance. I wouldn't be at all surprised if a lot of your carb tuning issues "magically" go away when you get the centrifugal advance working right. If you don't have at least 22-24* of total timing over 3500-4000 things get "funky"- weird stuff that just can't be tuned around by fiddling with the carb. Not to mention you are leaving a LOT of power on the table. You should shoot for about 32-36* total timing for a typical street N/A performance small block Chevy (excluding vacuum advance) all-in by about 3000 RPMs.

 

Well, I had the spectra advance kit on it. I took them off last night and cleaned up the originals and put back on. And yes it is the old style large cap HEI. This fixed my advancing problem. I put it to 12* base. I dont have a timing tape on it, but the mark would move to straight up or even more tword the passenger side. So I should be getting good advance now.


I did find that If I take off at wot to make a pass im fine. But as soon as I ease up on the throttle (not letting completely off)between 2500-5500 rpm that it starts stumbling ( shuttering). But it goes away as soon as I apply more throttle.

What is going on? I cant figure it out?

 

You should be pulling way more vacuum than 17 with a l98 cam. I pull 23psi vacuum with a lt4 camshaft...

I think your running rich because you have to much carb. I have smaller jets in my carb than you. And its a 670cfm carb.

 

I agree with 88Camaro350 on the running rich. I just helped a buddy get his newly rebuilt 355 running and he had similar problems. Turns out his was running rich and fouled out his new plugs. Spark plugs can only take being ran rich a brief time. The soot is a good indication of running rich, have you checked your tailpipes for black soot? If so jet it down.
I don't know if it is rich at idle but my guess is yes. Hook up a vacuum gauge and set your idle mixture screws till you get the most vacuum, then adjust the idle speed screw back down to prefered idle speed.

Hope that helps.

 

I called holley tech line and they said that the factory jets were 72 primary and the back plate was a -21 which is the same as 75 jets.

I will try this and make a few passes and check the plugs. I hope to get this fixed.

And no I have not adjusted my idle mixture screws with the vaccum gauge yet. I will give it a try. I think I am about 1 1/2 turns out right now.