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7/21/12 12:14pm - Original Message: 'Any one have an idea ? zz4 running issue '
tonytheroofer
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Rochester, NY - USA

Vette(s):
1980 L- 82 fully restored and upgraded.

Joined: 6/12/2012
Posts: 95
Hey I figured I would ask for some ideas , 

1980 Vette ,ZZ4 motor( 2500 miles )  3.55 rear , headers,  2 1/2 dual exhaust . holley 750 vac secondaries . 

It seams like it miss fires when I hit it and the rpm and power climb , I hear it in the exhaust and feel it , it does seam to get past it and builds power and rpm . So it is not a stumble it is more like a miss fire , I am sure there is power loss but it feels slight meaning the car is not hesitating or bucking it powers through it. 

To me it seams to happen when the 4 barrels are all opening , it is a new holley (750 vac ) I set it up with a vac gauge and on initial install i had 12 or 13 inches of vac , after i worked on mixture screws an now  I got a solid 17 inches of vac. 

Car idles perfect  , starts perfect , runs smooth , cool , and for the most part is tip top ! 

So any ideas where I can go searching for this apparent miss firing ? My gut tells me the carb is leaning out or that when the back 2 begin to open I am not rich enough and am fuel starved and I should have bought the 750 double pumper not the single pump vac secondaries .. 

Or not ...lol 




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1980 , ZZ430 , 3.92 rear , 700R4 2400 stall NEW EZ EFI 
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7/21/12 8:23pm - Reply: 'Re: Any one have an idea ? zz4 running issue '
Jaws79 Lifetime Member
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Sykesville, MD - USA

Vette(s):
1979 Corvette Coupe Corvette Light Blue Midnight Blue interior Mirrored Glass T-tops

Joined: 4/27/2008
Posts: 4211
Are the vaccuum and mechanical advances on the diostriuter working? That would possibly be the problem. It also could be the accelerator pump in the carb. Just something to try.
 
Barry
 
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7/21/12 8:50pm - Reply: 'Re: Any one have an idea ? zz4 running issue '
tonytheroofer
Former Member
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Rochester, NY - USA

Vette(s):
1980 L- 82 fully restored and upgraded.

Joined: 6/12/2012
Posts: 95
Yep advances work perfectly I am all in at 2500 rpm with mechanical advance with 34 degrees, 

here is something . . .  If i mash peddle into carpet no issue at all ,, BUT lets say I am in manual 2 and i run motor up evenly with out punching it , , it is there i feel and here it breaking up but the minute i mash gas all good.  

I just pulled 3 random plugs all clean enough to re-box and return to part store , gap was a tiny bit tight so i adjusted it and will do the remaining 5 later . 

Is it possible this vac secondary is not opening when it should and I am leaning out the motor causing this issue cause a mis fire may be to strong a word to use it is more a poping sound in exhaust, 


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1980 , ZZ430 , 3.92 rear , 700R4 2400 stall NEW EZ EFI 
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7/21/12 11:39pm - Reply: 'Re: Any one have an idea ? zz4 running issue '
tonytheroofer
Former Member
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Rochester, NY - USA

Vette(s):
1980 L- 82 fully restored and upgraded.

Joined: 6/12/2012
Posts: 95
Well double posting should not be allowed he he he he 

Here is what i did , When I did the headers I changed plugs because I broke one , I never gaped them figuring they would be pre-set and close enough , I just did 6 of the 8 and all where tight I would say 35 and I set them to 45 as called for , I drove the car and it did not do it. I ran first and second to rpm limit with out opening all 4 barrels and no popping or stuttering, so in the am i am gonna do 7 and 8 plugs to finish it up.

Now why that made a difference I have no idea , every plug I did pull and re-gap was perfect I could have put them back in a box and returned them . So is the gap that critical I never would have thought so and I am hoping that I am fixed and not just lucky that the last ride was a fluke.


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1980 , ZZ430 , 3.92 rear , 700R4 2400 stall NEW EZ EFI 
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7/22/12 8:18am - Reply: 'Re: Any one have an idea ? zz4 running issue '
lukesvette Lifetime Member
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HOWELL, NJ - USA

Vette(s):
1979, Targa Blue (72 Color), Pace Car rear spoiler, L88 hood, Dark blue factory interior, 525HP 406, HD 700R4, 370 gears,Steeroids, composite rear spring, TT IIs wrapped in T/A Radials.

Joined: 5/18/2004
Posts: 6804
I once dropped a plug when doing a plug change and didn't realize it landed right on the electrode and closed the gap completely. The car was backfiring when I decelerated and eventually went through all the plugs and regaled this one and went back to normal.

Not sure though if you were already at a .035 gap, that this would have caused the problem, but I guess anything is possible. Maybe in combination with the heat range of the plug...

My guess would have been a low end fuel issue from your description.

Good luck.

Paul


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7/22/12 11:38am - Reply: 'Re: Any one have an idea ? zz4 running issue '
tonytheroofer
Former Member
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Rochester, NY - USA

Vette(s):
1980 L- 82 fully restored and upgraded.

Joined: 6/12/2012
Posts: 95
I am on the same page with a low end fuel issue , However the only reason I checked any plugs was because it seamed the best first place to look , I know the carb has good fuel preassure , it is adjusted perfectly and both bowls are set to proper fill limits. It still could be a jetting issue but all i read on the ZZ4 motor says 750 is the right carb for the engine. 

So I decided lets check the ignition , now after I re gaped the plugs and it appeared to correct the issue I also started to search Google .  What I think I have learned is a small gap can lead to un-burned fuel and that the bigger 45 gap provides a much hotter and bigger spark ensuring all the fuel is lit off. So if that is right what I herd or called a mis-fire may have been un-burnt fuel going off in the exhaust system making it sound like a misfire.

Let me end by saying that's what I gleaned from reading a dozen or more pages on line.  And if the issue is not resolved I will post back ..... 


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1980 , ZZ430 , 3.92 rear , 700R4 2400 stall NEW EZ EFI 
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