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  #1 (permalink)  
Old 07-07-2003, 01:00 PM
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Top End Miss

I have just bought a 71 chevelle with the following: turbo 350, 3500 stall, 410 rear gears, Msd HEI kit, (6AL, Blaster coil) 355 sbc .30 over Pro Top line heads, Weiand Stealth intake (port matched) Holley dp 750 and a solid roller cam with 1 5/8 headers (3" ehaust back to the rearend then 2 1/2 cat back) the cam specs are as follows: 285/285 242/242 .567/.567 110c and I have added 1.6 roller rockers.... The problem I am having is that @ 4000 up, I am experiancing a top end miss, I have checked the wires and plugs and they seem fine, checked for coil bind (ok) and I really cant seem to run down the problem...I was told that the distributor shaft or gear could be worn because when I lift off the gas it backfires....the timing is right on with 36 degrees of total timing and the timing chain is new...could my problem be distributor related? it is a stock HEI distributor with the Msd Kit...any info would be very helpful...Than you for your time....Tim

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Old 07-07-2003, 02:03 PM
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I would guess trash in the high speed air bleeds. I would also say that where the backfire comes from. I assume it backfires through the exhaust right? If not then start looking for vacumm leaks and exhaust leakes aroung the header flanges.
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Old 07-08-2003, 01:48 PM
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yes, it backfires through the exhaust, Ill try another carb on it later on today, but the carb thats on it is a brand new 670 holley avenger(vac secondarys) Ill clean and re gasket the carb just to be sure, I looked the car over real good today and found that it had a el cheapo electric fuel pump, (proboly 3-4 psi) I hadnt noticed this because it still has a stock pump mounted and working....I think this could be part of the problem but that still doesnt quite explain the backfiring..Im going to re-check the timing, plugs, wires, distrbutor etc..try a new carb and if that doesnt do it, Ill definatly need some new ideas on this one...
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Old 07-08-2003, 06:22 PM
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I recently corrected a problem just like this one.
Missing as out of gas.
The Holly Mechanical 7-9 PSI pump was sucking the lower 3-4 pressure and flow pump dry and leaning it out.
I took off the electric pump, and all was well and smoking.
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Old 07-08-2003, 08:29 PM
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Sounds as if the pump can't supply the engine at or near top speed. the backfiring comes from the lean mixture when the carb bowls are empty.
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Old 07-08-2003, 08:56 PM
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Top end miss sounds like spark plugs to me. back the timeing of about 2*, my help the backfireing. Good luck.

Troy,
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Old 07-08-2003, 09:16 PM
 
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It could be lean from lack of fuel pressure,leak at intake or carb gasket,and in my case,the grommet from the power brake booster had broken in two, check all vacuum lines,even on the modulator on the back of the transmission,I've had mine come off a couple of times til I put a small hose clamp on it. I also took my pcv valve off and installed K&N filters in both valve covers,but I don't drive on the street much anymore.
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Old 07-09-2003, 06:13 AM
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Quote:
Originally posted by troy-curt
Top end miss sounds like spark plugs to me. back the timeing of about 2*, my help the backfireing. Good luck.

Troy,
I disconnected the cheap electric pump made some plug changes and swaped carbs, I borrowed a new 750DP, It definatly helped but to me it still feels a little weak from 4000-6500, this little mouse should pull till the cows come home but Im still experiancing some surging at full throttle..and since it still a stock mech fuel pump I may still yet be running out of go fast stuff...hmmm...I have the holley electric "blue" pump and a regulator laying around, would it be up to par? Im not sure of the horse power potential of the motor because I did not build it...I think the Holley pump is rated at 110gph, just how much horse power would it support @ 7psi? Im quessing the 355 is making around 375-390 hp from "seat of the pants" this mouse should make at least 450 hp, I do know the motor is safe up to and proboly past 7000, would a cam and intake change be wise? or can I just get bye with changing the dual plane intake to a single plane to find this mouses potential?
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Old 07-09-2003, 08:10 AM
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You should be developing max at low RPM now.

Might try backing the rockers of a few thousands at a time.

Is this a street car or drag car? With 410 gears it can`t be a cruser. 65 mph =5000 RPM + hth

Troy
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Old 07-09-2003, 07:01 PM
 
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A GOOD mechanical pump will more than support the HP that you are making, you don't need an electric pump and surging is from a lean condition. Loosen the valve lash for more low end, which is very hard on parts,tighten the valve for top end.
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Old 07-12-2003, 05:29 PM
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Quote:
Originally posted by troy-curt
You should be developing max at low RPM now.

Might try backing the rockers of a few thousands at a time.

Is this a street car or drag car? With 410 gears it can`t be a cruser. 65 mph =5000 RPM + hth

Troy

well, Im glad to say I have cured the fuel starvation problem and found some other problems that I have fixed, for one, it had stock rocker arm studs (some craked 2 broke) I replaced those with ARP studs. I also went ahead and replaced the distributor with an MSD billet and I tried a 2" spacer and a 800 dp holley..(really woke up this mouse) and yes I tried adjusting the lash, the recomended lash is, .016 int .018 exh.. I tried .018 int .020 exh..It definatly has more torque but at Idle it sounds like a stocker..lol ..did I maybe go to far? and to answer another question, yes this is a street-strip car in every sense of the word....My goal is to eventualy run mid 10's in this ride....<<<<long way to go.....
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  #12 (permalink)  
Old 07-12-2003, 05:40 PM
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Sounds to me like your on the right track, just keep playing with the adjustments. no two eng. are the same. If it runs like you want it to who cares what it sounds like(sleeper).

10s is a good goal, i`ts very hard to drive a 10 sec car on the street, and keep it in the 10s. Good Luck,

Troy.
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Old 07-12-2003, 05:46 PM
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Quote:
Originally posted by troy-curt
Sounds to me like your on the right track, just keep playing with the adjustments. no two eng. are the same. If it runs like you want it to who cares what it sounds like(sleeper).

10s is a good goal, i`ts very hard to drive a 10 sec car on the street, and keep it in the 10s. Good Luck,

Troy.

yeah, it will definatly be a bottle baby...
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