part throttle bog/hessitation/backfire

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younggun2.0

younggun2.0

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hey guys. i had my 383 dart out the other day for a short drive. it has developed a wierd problem. this is all new and all of a sudden. at cruise speeds 2000-2500 rpm it runs great. then at idle its perfect. at WOT runs like a raped ape. i was cruising in high gear. 2700 rpm. give it about 1/3-1/2 throttle and she runs super rough, pops and sputters and loosed power. let off and it goes away. floor it and it goes away. it sat for few months in cold weather in my non heated, non insulated garage. could the **** gas be gumming up the carb? it kinda feels like it could be timing related. i didnt have time to pull the dizzy apart but i wonder if its got moisture inside the cap and getting spark scatter.

specs.

383 stock bottom end
m1 inkate
pro form 750
254 @ .050 .525 lift with 1.5 ratio roller tip rockers
tti 1 7/8 headers
4.10 rear gear 28" tire
4500 stall

where would you guys start?
 
Ignition problems would be much worse under more power. I would guess carb problems.
 
67Dart273 said:
Ignition problems would be much worse under more power. I would guess carb problems.
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i have the msd pro billet dizzy with the mech. advance. i wonder if the inside got corroded and is limiting advance or its hanging up.

i have heard horror stories about the new gas causing problems inside the carb if it sits too long too.

i have a fuel cell with the foam inside. wonder if the foam is breaking down and got some sucked into the carb?
 
Check you primary side power valve and/or the pump arm clearance for the accelerator pump.
 
moper said:
Check you primary side power valve and/or the pump arm clearance for the accelerator pump.
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thank you! how much clearance on the accelerator pump arm? i never touched it outta the box. just set floats. idle mixture by vacume and then curb idle. then i jetted the carb down 2 sizes all way around. it did pop once a little while back up thru the carb when it was cold starting. might have popped the power valve. it did kinda feel like it was loading up a bit now that you mention it.
 
holley call for .015 clearance between the pump lever and the linkage at wide open throttle so that the pump does not bottom out...

But...at idle..there should be no clearance between the pump lever and the linkage...if the linkage moves the pump should squirt.


[ame="http://www.youtube.com/watch?v=k-Nx5HEzvlY"]How To Adjust The Accelerator Pump On Holley Carburetors - YouTube[/ame]
 
70aarcuda said:
holley call for .015 clearance between the pump lever and the linkage at wide open throttle so that the pump does not bottom out...

But...at idle..there should be no clearance between the pump lever and the linkage...if the linkage moves the pump should squirt.


How To Adjust The Accelerator Pump On Holley Carburetors - YouTube
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dude you rock! thanks for the video. if it were wrong and it bottoms out would it tear the diaphram and leak into the carb or on bottom side onto intake?
 
if you leaned the jets down, and drive it in cold weather it will likely pop through .and your right about the fuel,10% alcohol in my area will lean the carb out also, but like Moper said you likely took out the power valve unless it is protected.
 
The thing is..........when you are cruisin' down the road and give it throttle..........the pump could be unhooked and it should still run OK
 
67Dart273 said:
The thing is..........when you are cruisin' down the road and give it throttle..........the pump could be unhooked and it should still run OK
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i guess taking the front bowl off and the front metering plate is a good move on all accounts. can check for sludge and check the power valve. will the power valve be noticeably blown just looking at it? how do i check it? or is it just a good idea to change it if its apart?
 
you can vacuum test the power valve ,and run a small wire through the little jets inside the metering block for the fuel enrichment stage.(I use a cutting tip cleaner set or pin type drill bits)then blow out with air.
 
Check to see if the accel. pump squirts a solid stream without any "dead spots". Sounds like you're not getting a solid squirt when you start to open the throttle.
 
jasperjacko said:
Check to see if the accel. pump squirts a solid stream without any "dead spots". Sounds like you're not getting a solid squirt when you start to open the throttle.
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at cruise speeds if i give it about 1/4 throttle it just stumbles and sputters and pops long as i hold it there. like if i wanted to pass a car on the highway. soon as i stab the gas it cleans right up and takes off. it just started too. i will do the vacuum check on power valve. thats what i think it is or its **** in the carb.

thanks for all the help guys
 
Why did you jet the carb down? the carb is set to run in cold also it must be rich for the heaver cold air, all carbs run rich in the summer unless you lean it ,it never was a perfect system the over fuel in the summer would break down the oil, wash the rings down this is why 100000 mile was the limit on most engines until fuel injection came out and you could keep that perfect 14.2 to 1 ratio and engine life doubled. also remove the catalytic on newer cars and you will get black smoke and burn your eyes at idle. while you got the metering block off put the other jets in you will be happy!
 
teringer said:
Why did you jet the carb down? the carb is set to run in cold also it must be rich for the heaver cold air, all carbs run rich in the summer unless you lean it ,it never was a perfect system the over fuel in the summer would break down the oil, wash the rings down this is why 100000 mile was the limit on most engines until fuel injection came out and you could keep that perfect 14.2 to 1 ratio and engine life doubled. also remove the catalytic on newer cars and you will get black smoke and burn your eyes at idle. while you got the metering block off put the other jets in you will be happy!
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i jetted it down in the summer when i first got it fired up. it was pig rich. would foul the plugs. got the timing curve figured out then idle mixture then did the jets. i got 72 up front and 82 out back. ran great up until the other night. jets have been the same all along.
 
Was the outside temp somewhat cold? W/this intake there is no heat under/through the manifold. So the intake may get colder before it warms up. This will cause the fuel to puddle in the lower portion of the intake. Causing a lean condition especially under "light" throttle. The main metering system with 72 jets is right for warm weather but too lean for cold weather. Now you may have a blown power valve which leads to rich mixtures and real noticable at idle. But this will help "some" in cold weather. Long story short, replace the PV and jet up. That is if you are going to drive the car in cold weather. Questions? Is there a thermostat in the cooling system? Do you have a direct fresh air system hooked to the hood?
Definately fuel related...issues here!
 
Do you have an A/F meter in the car?

If so what's it say?
 
Rob Eppler said:
Was the outside temp somewhat cold? W/this intake there is no heat under/through the manifold. So the intake may get colder before it warms up. This will cause the fuel to puddle in the lower portion of the intake. Causing a lean condition especially under "light" throttle. The main metering system with 72 jets is right for warm weather but too lean for cold weather. Now you may have a blown power valve which leads to rich mixtures and real noticable at idle. But this will help "some" in cold weather. Long story short, replace the PV and jet up. That is if you are going to drive the car in cold weather. Questions? Is there a thermostat in the cooling system? Do you have a direct fresh air system hooked to the hood?
Definately fuel related...issues here!
Click to expand...

i had the hood off the car and yes it was cold. like 40* cold. i do have a thermostat, its a 160* the car was running right at 165* it was warmed up too. it has always idled rich. it was even worse before i got the timing set. i am at 22* initial and 36* total all in by 3000rpm (with medium springs in dizzy and 4500 stall converter)
 
Maybe try taking a heat gun and warming the cr@p out of the bottom of your plenum, then go for a a quick drive.

See if having some heat in the manifold helps, then it may show to be a weather condition.

If the accel pump arm doesn't have some play in it at full depress, it may rupture the diaphram. It leaks outside the carb.
 
165 is pretty cold on a T-stat in any climate where the car sees 40 degrees or lower. You might try a 180 or even a 195. I think you've got fuel falling out.
 
Why, after all the threads we have had on this, would anyone run a 160 stat on a street car?

But what he is talking about is fuel drop out. An intake can be so cold that the fuel doesn't want to stay atomized, hits the runners, and kind of dribbles it's way through the system (or so the theory goes).
 
falling out of the air stream ,my 1985 suburban 350 would Ice the throttle plates on a 40deg or below day with high humidity
with a, heated intake, exhaust heat riser ,the only fix was to put the flex tube back on between the air filter and exhaust manifold and running a 195deg t-stat. I would come off the highway and the engine stalled out when stopping, when I removed the air filter lid the ice filled the bottom of the throttle plates.(just a thought)
 
The worst cases I've seen of this was "some girl" who had a late 60's Chev inline 6 pickup, with headers. Installing headers removes the carb heat completely. Of course the "smog" snorkel heat had been removed, and I finally built a "stove" on the header.

The thing was almost completely undrivable in 30s--40's F weather, especialy when wet.

That's beginning to sound like "what this is." Once saw ice on the bottom of a Corvair dune buggy intake.........aftermarket manifold on a hot San Diego day

It's called "refrigeration effect." When you run air at high speed through the carb venturi, that cools the air.........just like your air tools. Water in the air causes more cooling. Then you invoke gasoline into the mix, and this causes MORE cooling.

Google "carburetor icing" on aircraft. THAT has crashed aircraft.
 
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