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  #1  
Old May 13th, 11, 03:24 PM
derek derek is offline
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Question Car dies when I floor it. Shop can't solve it.

Hi All,

A while back I had my 71's original Rochester 2v and intake replaced with an Edelbrock 1406 Performer carb and an Edelbrock 2101 Performer intake manifold. A local shop did all the work and afterwards the car idled strong and stable and ran fine. That is, until I noticed that it would basically die if I suddenly floored it from a dead stop or slow roll, either going straight or on a turn out of a parking lot or side street.

So, I took the car back to the shop and they've had it for over a month and cannot figure it out. They haven't taken the carb apart yet though. That's the last resort since they'll start charging me at that point.

So how complicated can this problem possibly be to diagnose? I'm not an expert, but it seems that after 4-6 weeks they could at least tell me what the cause and solution is. Right?

Any thoughts?
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  #2  
Old May 13th, 11, 03:50 PM
Dave's 81 Dave's 81 is offline
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Default Re: Car dies when I floor it. Shop can't solve it

Engine ever been rebuilt? Does it use oil? Electric fuel pump vs mechanical pump? Do you have a gauge on the fuel line for pressure? Too much fuel vs too little fuel? Any spark? Checked the plugs? Are they black or white or completely melted off? Foam in the fuel tank with aftermarket pump? Dirty fuel tank with rust? Any filter in the fuel system? Check the fuel filter? Vacuum leak? Oil smell like gas? Who adjusted the carb? What size is the carb (600, 650, 750 etc.)?

My opinion is that your car has been at the shop too long without a diagnosis. This kind of stuff should have been worked out in a few minutes. If these are buddies, that is a different story. If they are not buddies, get the car to someone else.

I can't diagnose your car over the internet. You can check some of the things above to get started yourself. A filter or fuel pump or spark problem is easy and is part of a basic tuneup.
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  #3  
Old May 13th, 11, 03:59 PM
Blown73 Blown73 is online now
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Default Re: Car dies when I floor it. Shop can't solve it

Its not like its an efi system which can have multiple issues all at once.

You need fuel, air, spark, and compression for the car to run. If all four are there, then you have a timing issue and/or an air/fuel ratio mix up.

If a legitimate shop has had it for 6 weeks and can't figure it out, they probably shouldn't be in business. It would take maybe 2 hours to figure out the problem and probably another 2 to fix it. Go to a different shop on their expense as they at least owe you a running car.
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  #4  
Old May 13th, 11, 04:18 PM
Cousin'76 Cousin'76 is offline
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Default Re: Car dies when I floor it. Shop can't solve it

Gotta agree with Dave: too many possibilities to diagnose remotely.

The Edelbrock's use a "flapper" air gate in the secondaries which should eliminate any over-carburetion potential.

Dave's got it with the fuel pressure possibly being too low or too high. It could also be a symptom of the float level in the carb being too high or too low, which still could be caused by the fuel pressure.

I run a Holley fuel pressure regulator on my 7150 Holley, with it set to 4 p.s.i. The regulator is adjacent the carb, so inertial problems of the fuel don't exist. It's flow more than pressure that you want, but you need enough pressure to overcome acceleration and sticky needle and seats in the carb.

As long as your fuel pump is good, this could be a $25 fix. If you have a timing light, check your initial timing with the vacuum advance disconnected.
Check your vacuum. It should read 18 to 21 inches with a stock or mild cam and can be as low as 12 inches with a more aggressive street cam. A vacuum leak (especially between the carb and intake) will greatly affect your carbs ability to meter correctly.

Too much or too little timing can cause your car to sag at initial WOT. Advancing the timing will also make a bad idle mixture sound better. Sometimes done to cover-up any of the aforementioned problems.

I know that a lot of info, but knowing / correcting your fuel pressure and timing is a great starting point.

All of this could just be a bad accelerator pump in the carb.

Any shop that doesn't appreciate your desire to drive your car isn't the right shop....

Bill
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Old May 13th, 11, 04:26 PM
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Default Re: Car dies when I floor it. Shop can't solve it

I suspect it is running lean and bogging but you didn't mention a characteristic occasional back fire. Having a bigger carb is great until you stop on the gas and suddenly you are ingesting more air than fuel. It runs lean because air won't burn and the air enters the motor faster than the gas can flow.

If timing was off that much it would be a dog under all circumstances and not just when attempting to accelerate quickly. You pump up timing to burn the gas you dump into the motor faster with a bigger carb (get a head start in lighting off the air gas charge because gasoline actuall burns and it takes a measurable amount of time to cross the combustion cylinder from the spark plug).

Big Dave
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Old May 13th, 11, 07:59 PM
Philip Philip is offline
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Default Re: Car dies when I floor it. Shop can't solve it

Derek does it die if you roll into the throttle slowly until it is at wide open?
There are lots of things we can have you check, but need a little more information about the car, your skill level and tools you own, like a timing light and vacuum gauge.
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Old May 14th, 11, 12:13 AM
JAYARTER JAYARTER is offline
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Default Re: Car dies when I floor it. Shop can't solve it

More info on car setup would help

is it new parts or someone else's used parts...?

Is it stock setup or hipo setup.

there are some very good car members that should point you in the right direction!

Jay
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Old May 14th, 11, 12:45 AM
derek derek is offline
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Default Re: Car dies when I floor it. Shop can't solve it

Thanks for all the replies! I'll try to provide as much as I know at the moment:

The car is a 71 with original 350 that was rebuilt about 6 years ago. It ran fine when I originally got, although sluggish. It wouldn't die or stall under hard acceleration, it just didn't accelerate all that fast. It's last tune-up was about 2 years ago.

I had a the shop swap the old rochester 2v and intake with the Edelbrock parts which were purchased new from Summit. The carb is 600cfm. It seemed to run pretty strong, and driveable, except under hard acceleration. So back to the shop I took it.

The last time I talked to them, which was about 2wks ago, they said that sometimes it would accelerate just fine for them, but other times it would bog down and almost stall. However, when I was driving it I felt like it would do it every time. Also, if you stomp on the pedal and it starts to stall, immediately letting off the pedal will bring it back to normal idle. If I accelerate gradually there's no problem and it seems to pull pretty strong at higher rpms, just as long as the accleration isn't "pedal to the metal" in a split second. There's no backfiring at all either.

The fuel pump is mechanical and the shop said they tested the pressure and it's normal.

I believe they said they were going to check a few other things and get back to me, but again, that was two weeks ago. I think I'm done waiting around at this point. If there are no solid answers on Monday I think I'll pick the car up and find another shop.

I hate to take it elsewhere. The folks at the current shop are really nice, but if I can't enjoy my car....

I'll ask about some of the things you kind folks have suggested and post back with results.
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Old May 14th, 11, 01:10 AM
Philip Philip is offline
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Default Re: Car dies when I floor it. Shop can't solve it

Derek Assuming the timing was set properly and a vacuum leak was ruled out the sluggish condition before the upgrade is probably going to point towards the root cause. The addition of the intake and carb would have compounded the problem. You state the engine build was 6 years ago, did you own the car then? If not any proof of what was done? If not we will assume it may have been less than perfect.
I suggest your mechanic looks into the timing chain. Check first to see if there is excessive slack, this can be done without disassembling the engine. If still within specs I would suggest the timing gears be inspected to see if the marks are lined up properly. If the gears are even 1 tooth off it will cause sluggish response to the throttle. If the chain and gears have never been changed it would be a nylon clad pot metal cam gear and could be coming apart changing cam to crankshaft timing greatly. These conditions would cause the problem with lack of response but would be consistent. Your mechanic states it is intermittent, really need to know which.
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Old May 16th, 11, 11:09 AM
Cousin'76 Cousin'76 is offline
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Default Re: Car dies when I floor it. Shop can't solve it

Derek,

Does your car have power brakes?
The vacuum accumulator can leak enough to cause poor vacuum with the brakes on that might affect your off-idle acceleration.
Cheap, quick check that could explain the intermittent stalling that the garage has seen....

Bill
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  #11  
Old Jun 7th, 11, 11:37 PM
derek derek is offline
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Default An update

Thanks everyone for the replies and helpful suggestions. Here's an update:

Finally got the car back from the shop last night. After two months they came to the conclusion that it was the fuel pump. They tested it early on and said it was working fine, then checked other stuff, then switched it out with an electric pump which solved the bogging down issue, but then suffered vapor lock, so they swapped on a new mechanical pump which seems to have fixed the problem. I'm not thrilled about the time it took and the final bill, but happy to get my car back. But....

....after driving it today for about 50 miles I noticed a new problem that wasn't there before. So the saga continues in a new thread about the gas pedal not depressing all the way.
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