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Author Topic: car backfires & transmission problem  (Read 1389 times)
jk79ta
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« on: September 16, 2011, 08:43:46 AM »

It's probably been touched on here before, but when I let off the gas while cruising, I hear popping. I think it's backfiring out of the exhaust, but I can't tell for sure. I have a 1975 pontiac 400 with a 1979 301 quadrajet. Also have new true dual exhaust but no X or H pipe. Wasn't available locally. Has anyone ever had this problem? Another question, I have a re-built turbo 350 transmission in the car. I know it's overfilled with fluid but haven't had a chance to remove some. Will this cause the transmission to shift at high rpm's? It shifts manually ok, but it really needs to reach high rpm's to shift automatically. Modulator is new and hooked to manifold vacuum.
Thanks.
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formula403
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« Reply #1 on: September 16, 2011, 10:29:31 AM »

when i got my new exhaust installed i got popping and crackling sounds like that, it went away eventually after the breaking in period.
ive been told never to overfill any fluids, motor or tranny! they should be filled exactually to the mark on your dipstick.
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1978 Formula Firebrd 403
usafaux102
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« Reply #2 on: September 16, 2011, 05:40:12 PM »

Actualy, it does not hurt the trans or engine if you are over full by a minimal amount up to a quart. In fact, many times, guys will do this intentionaly to avoid the trans pump or eng/oil pump and hence bearings from momentarily running dry under heavy acceleration or hard cornering. The best option is to use a deep sump pan for a higher overfill need. The pumps will suck in only as much as they can any way so it doesn't matter if you are sucking from a pail or an ocean. The pump s will only move what they will move. The only thing I can think of in the case of an engine would be that with a high overfill/ small volume sump, the windage or actual churning from the counter weights on the crankshaft could cause millions of little bubbles in the oil which could cause poor lubrication of internal parts. As far as the back fire in the exhaust, timming, both ignition and valve could contribute to that. A long valve overlap or certain cam profiles could do this as well. I assume you have no burnt valves or old/weak or broken valve springs.
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usafaux102
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« Reply #3 on: September 16, 2011, 05:41:43 PM »

As for the trans shifting, try backing off on the modulator valve setting to see if your rpm drops when shifting under normal acceleration.
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AngryBird1
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« Reply #4 on: September 16, 2011, 07:01:41 PM »

It could also be shifting late due to the TV cable on the carb being set too tight.  If the kickdown cable adjustment is okay, it could be the governer sticking or possibly someone has swapped out the governer weights/springs.  If the governor wasn't touched, my money is on the cable being set incorrectly.
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jk79ta
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« Reply #5 on: September 19, 2011, 08:23:25 AM »

Thanks everyone. Forgot to mention, the engine was re-built also. Not sure about the cam, I think it's stock, but it was new when I got the engine and wasn't told. We messed around a little turning the screw on the modulator, didn't do much good. I don't know how far you can turn that, we were a little cautious about it. I don't know that we have the kickdown cable adjusted properly, but it won't upshift from 1st to 2nd to 3rd until it hits higher rpms. Not just 2nd to 3rd. Maybe I'll disconnect it and give it a drive. The backfire is driving me nuts. Car runs well, just backfires when I let off the gas. Do you think it's the 301 carb causing that? Like I said the engine is re-built, so I would hope there's no problems inside it. Look forward to everyone's comments.
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Black Sheep
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« Reply #6 on: September 19, 2011, 10:26:44 AM »

On a TH350 the kickdown cable does nothing but provide for the kickdown.  Mine has never been hooked up.  Make sure the vacuum line to the modulator is hooked up and modulator is working.

On 700-r4 and 2004-r, the cable must be attached and adjusted properly.
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usafaux102
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« Reply #7 on: September 19, 2011, 03:23:31 PM »

On the back fire issue, try making some mixture adjustments. Go opposite of where it is at right now first, then try the other way if no positive results are gained. My thinking is that if the fuel isn't all being consumed during combustion( which it usually will not be) and is too rich, it could be pulling a rich enough mixture into the exhaust under off throttle /high vacuum conditions to be causing the fuel to ignite in the exhaust system.
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Aldo
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« Reply #8 on: September 19, 2011, 06:19:11 PM »

Poping in exhaust due to letting the gas off can be caused buy many things.

1 leaking headders  (fuel is combusting in them because of air leak)
2 pour engine break in ,(messed up the lobes on the cam)
3 depending on engine specs and heads rockers are out of adjustment (if stock tourqe to specs and go)
4 timing.and correct gap in plugs
5 to much fuel running through carb.



In my case i had the very same thing,turned out to be a bad cam ,the break in was not done correct and messed up the cam.
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LilSki
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« Reply #9 on: September 20, 2011, 07:58:44 AM »

Actually too lean will cause popping in the exhaust not too rich.
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Kevin
79 Trans Am: (77 nose), WS6, M-21, 461, Hotchkis TVS, PTFB Contol arms. 
85 Trans Am: Vortec 350 w/LT1 cam/T56, TPI conversion w/Megasquirt II.
jk79ta
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« Reply #10 on: September 20, 2011, 08:18:50 AM »

Thanks for all the ideas. So, maybe my carb, used on a 301, isn't supplying enough fuel for the 400. Guess that makes sense. Anybody every remove the covers over the screws on a 1979 quadrajet? I read in Cliff Ruggles book that they have metal covers.
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Grand73Am
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« Reply #11 on: September 20, 2011, 10:22:53 AM »

Yes, there may be metal caps over the idle mixture screws. There's a part of the base around each screw that you would cut away to gain access to the caps, so you can use a small sharp chisel to drive the caps out. Sorry, don't have time to show pics right now. I don't know if Cliff put a picture of where to cut in the book or not. Maybe someone else will.
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Steve F.
LilSki
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« Reply #12 on: September 20, 2011, 11:28:42 AM »

What I have done to remove the plugs is gently drill a small hole in the cap. I then take a small self tapping screw and screw it into the cap. as it starts to bite the cap should start to spin and you can pull the cap off.
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Kevin
79 Trans Am: (77 nose), WS6, M-21, 461, Hotchkis TVS, PTFB Contol arms. 
85 Trans Am: Vortec 350 w/LT1 cam/T56, TPI conversion w/Megasquirt II.
Aldo
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« Reply #13 on: September 20, 2011, 06:28:22 PM »

That cant be,im running a 301 carb on a lightly moded 400 with no issues.....something else might be wrong.my carb has the mixture screws in plane site,no caps,maybe its not original to the 301 i had before but im almost shure its correct.
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DriveIt
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« Reply #14 on: September 20, 2011, 08:58:34 PM »

Only thing that causes backfire is unburnt fuel building up in the exhaust and then igniting, causing the explosion of gasses that you hear as a bang.  For some reason your not getting full combustion.  A friends Falcon had the same issue - runs fine, but when coasting in gear it would backfire so bad that it sounded like a 12ga going off, even balloned one of his mufflers - so we changed the points in his distributor and problem solved.  You could have a weak coil or bad plugs/wires. 

What we need to know is: how long has this been going on?  Diid it start on its own or after a certain modification?  If it was after a mod, which one?

If nothing has been changed and the problem started on its own, then there isn't a problem with your mixture screws. 

Are you positive it's backfire and not just the burbling sound of a free flowing dual exhaust?
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Kevin
'78 Firebird Esprit, Oldsmobile 425 on Ethanol

Here's my build.   http://transamcountry.com/community/index.php?topic=44268.msg393763#msg393763
jk79ta
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Posts: 148


« Reply #15 on: September 21, 2011, 08:19:32 AM »

I believe that starting in 79, the carbs had metal caps over the screws, so yours have already been removed by the previous owner. I've got less than 100 miles on the re-built engine. The carb came off the original to the car 1979 301. I've been reluctant to drive the car because of the transmission issue. So I guess it's been backfiring since I put it in the car this summer. I'm pretty sure it's a backfire, never had a car with true duals before, I hear it when I'm in the car and let off the gas, but I haven't heard it from outside. I'm going to try to get the tranny right this weekend and also try to get the caps off the carb and do some adjusting. Any suggested starting points for the screws?
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