Z28HO

I have a decently modded 1984 Z28 with the L69 305. I have too many mods too list I will post a link to a page with all of my mods http://www.cfcc-dfw.org/members/140.html
The problems started when I got the bright Idea to put nitrous on the car. I wanted to do it the right way and I did not trust my comp. controlled q-jet for the task. Even though I had a stage to rebuild done on it by the carb shop. The car hauled *** before and made many high 13 second passes. But I needed a little more for those LS-1's So I ordered myself an MSD Digital 6 ignition, MSD Blaster 3 coil, with HEI cap adapter and 10.4 taylor wires. I needed a new distributor so I bought a brand new vaccum advance distributor and pulled the module and put in the MSD triggering harness. I also needed to upgrade the fuel system I went with a Paxton 150gph electric pump with a K*N filter and mallory 2 port regulator and all AN fittings and line -6 or 3/8. I wired up a fuel pressure safety switch and t it into my factory oil switch and removed my factory mechanical pump. I am running Rapidfire 2 plugs for now since I was only going with a 100 shot for start. I have also added 4 auto meter guages mechanical fuel pressure, I reused my O2 sensor and got a air/fuel guage, I also ran nitrous pressure, and a vaccum guage inside. So from what I can gather I bought all the right ****. I forgot to mention the carb. I went with another Q-Jet from carb shop built to my spec as was the previous one only difference this one is non comp controled. I was told by all my friends toss the Q-jet get a holley well I guess I am just a die hard q-jet fan and like their reliability so I got another. Now hear is my problem I get everything hooked up and the car started and it runs like ****. I am getting a real rough idle and smoking bad out the tail pipe thick black. I guessed it was to much fuel and lowered the pressure to 5 from 6. Hoping this would do something but nothing I proceed to think it is an igniton problem. I have never had a vaccum advance setup before so I don't know for sure. The car was a complete pig until it hits about 2500 then it start spinning the tires and screams all the way to 6200. So i guessed it was the distributor and it is not advancing soon enough I mess with that for a week and get no where. I finally decide that it must be the carb. I never would have figured it since it was built to my specs and run on a test engine before shipping. So I call carb shop and they tell me I must have trash in my needle and seat/power piston so I decide to take the carb apart my self and check it out and clean it. I found nothing and put it back on the car and tried it just for grins and it runs the same. Which is barely I can run the car 2 minutes and the plugs are completly fouled and I will sometimes have to wait for the fuel to dry before I can even get the car to restart. Am I going crazy I have already decided I should have left the computer and harness in the car but it is a little late for that now. I have already sent the carb back for them to look at. Am I doing the right things here has any one heard anything like this. Is this a fuel problem with my carb I hope I also have a slight vaccum leak but in this case I would think that might help by leaning the car out. Any feedback would be appreciated.
Thanks Mark

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Mark B.
1984 Z28 5.0. Liter H.0. with 17" Fikse FM/5's
Winner Best Exterior Third Gen. 1999 National Gathering
2000 Dakota R/T Intense Blue Regular Cab
with 18" Budnik Prism 6's

MRZ28HO

You mentioned you bought a "brand new vacuum dist", what brand and is it an OEM replacement? If OEM replacement, check (with an advancing timing light) how much advance timing and where it comes in. I had to modify a vacuum dist for temp use on my 305 and had exactly the same thing happening, but I used a dist advance kit (mr. gasket $5.00) and added the lightest tension spring (also check Damon's tech article on Thirdgen's homepage about vacuum HEI). Guess what happened ... nothing but tire spin from idle all through mid second. Try that, btw, what's you initial timing at?

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George P. Lara
1994 Z28 LT1 T56
1984 Z28 High Output
Member: SCCA, SCFB, SC3GFB

Z28HO

George I tried that first I had an accel performance distributor completely new unit adjustable vaccum advance and springs and weights came with it. I tried every combo i could think I finally decided it was a bad distributor so I went to the parts store and bought an early model rebuilt unit with vaccum advance non adjustable and i changed the springs and weights with the mr gasket kit no change. It was so bad witha fresh set of plugs I would install and just idle the car and not even dive it for around 1-2 minutes and the plugs would be fouled just dripping in black oily gas smelling mess.
Oh by the way the strut tower brace is working out great. I don't know any more I am about fed up with the car thought I am hoping by some miracle that when my carb comes back again I will be able to bolt it on and the car will run fine.

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Mark B.
1984 Z28 5.0. Liter H.0. with 17" Fikse FM/5's
Winner Best Exterior Third Gen. 1999 National Gathering
2000 Dakota R/T Intense Blue Regular Cab
with 18" Budnik Prism 6's

Damon

In this case it MIGHT be the carb. If the plugs are fouling up (black carbon deposits- overly rich) and the thing idles like its got a lumpy cam in it then it just MIGHT be the carb.

I had a WEIRD problem with my Carb Shop QJet once- the secondary throttle plte screws cam loose and the secondary throttles wouldn't go all the way closed sometimes (usually after the first time I got into the throttle). Same problem- crappy idle, fouled up plugs, smoke out the tailpipes, etc. It acted for all the world like a fouled up inlet valve but the thing was NOT flooding out at idle- with fuel spewing out the vents and such like you would expect from that kind of problem.

Its possible you have the same problem or a SIMILAR one where the secondary throttle plates aren't closing tightly in their bores when they come to rest. They SHOULD seat tightly in their bores- almost no light should show around the outside edges of the secondary throttle plates when held up to the light.

I would ordinarily point to ignition first but at idle if the thing is making sparks and the timing is in the ballpark then that's not the problem. And you have replaced enough ignition stuff already without improving the problem.



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94 Firebird Formula M6- No options but Z rated tires. No mods over $10. 13.5@105.
79 Malibu "beater" w/junkyard 400 SBC, tweaked QJet 4bbl, finally a decent set of heads, a few other tricks. A maddening 13.000001 @ 108 on the motor with little traction. No nitrous runs with the new heads yet.
Grandma's old 78 Malibu (33K miles!) soon to have the powertrain from the 79 put in it.

"One of the last remaining QJet tuners on Planet Earth!"

MRZ28HO

What kind of primaries were installed on the Q-jet? I would think it is a bit too fat on fuel. So then the bog at low speeds is related to being to rich, and therefore "putting out the fire" (if you know what I mean ). Have you gotten the carb back yet?

Glad you (and many others) like the STB, unfortunately the carb'ed STB market is very thin. I think that is why aftermarket companies don't mass produced them (less Edelbrock). I currently am not producing them due to lack of mass customer interest, and especially my full-time job sometimes has me working late and through the weekends (man you people buy alot of cell phones ). I am planning on starting back up again and maybe more products (shhh, keep that part quite ). Mind you all as a side-line for extra cash and to help fellow carb'ed 3rd gens.

Z28HO

I am very thankful for your input guys I am still waiting on the carb to return from the carb shop. I am very confident that if it is a carb problem that they can fix it. I have had nothing but good luck with them in the past but I guess there is always that freak problem that seems to always happen to me.
I am not an expert on the carbs buy any means so Damon i was wonder when you mentioned "its possible you have the same problem or a SIMILAR one where the secondary throttle plates aren't closing tightly in their bores when they come to rest. They SHOULD seat tightly in their bores- almost no light should show around the outside edges of the secondary throttle plates when held up to the light."
Am I to be looking at the base of the carb correct because I do believe I could see some light through the carb I can be for sure and can't check it until it returns but hopefully if that was a problem they will have caught it. What I don't understand is it ran perfect on there test engine so they say and it never ran worth a damn on mine.
I need to check my paper work I don't remeber right off what primaries were installed but I will check and let you know


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Mark B.
1984 Z28 5.0. Liter H.0. with 17" Fikse FM/5's
Winner Best Exterior Third Gen. 1999 National Gathering
2000 Dakota R/T Intense Blue Regular Cab
with 18" Budnik Prism 6's

Z28HO

I just found my paperwork from carb shop with my specs. They show primary jet #73 and primary rods 50m and the secondary rods are DP. Let me know what these mean if they are ok or too big.
Thanks

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Mark B.
1984 Z28 5.0. Liter H.0. with 17" Fikse FM/5's
Winner Best Exterior Third Gen. 1999 National Gathering
2000 Dakota R/T Intense Blue Regular Cab
with 18" Budnik Prism 6's

Damon

Tuning is in the ballpark. I don't like the lean primary rods at all, though. 50s are WAY WAY WAY lean primaries. My QJet runs a 76 jet with 44 rods on a 400ci motor. That's a little rich and would probably be even richer on a little 305 but I'd knock that primary rod size down to 44-46, using the same 73 jet. Just make sure you get the RIGHT primary rods- carbs starting with part numbers 170xxxxx use different primary rods than carbs starting with 70xxxxx.

The secondary rods could also use a little fattening up, probably. DPs are a little on the lean side. Stock 305 HO motors use DR rods which are about perfect for 90% of ALL QJet applications.

Neither of these minor tuning isses is causing your immediate problem, however.