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Sputter and backfire at low RPM
Posted by: FlatEarth ()
Date: July 28, 2020 06:42PM

Hi all,

I have a 63' convertible 4spd with a 63 block, 65 140 heads and 4 carb set up. At lower rpm (500-1500 ish) the car wants to buck and sputter and will backfire through the passenger primary carb. once at a higher rpm and wide open throttle it takes off and runs smooth. Here's what I've done so far:

Adjusted valves on both sides (twice)
New distributer (with cap, rotor condenser ect. all parts attatched to distributer)(replaced due to intermittent spark caused by an electronic ignition)
Timing set to 14 degrees with vacuum advance disconnected
New ballest resistor
New plugs (correctly gapped)
New plug wires
New fuel pump
Smoke tested for vacuum leaks. There's none
Carbs balenced using vacuum guage and air/ fuel screws adjusted for peak vacuum


Things done with in the last year (car ran good until about a week ago)
All 4 carbs rebuilt
new piston rings
barrels honned
valves lapped
some valve guides replaced that were out of spec
new valve stem seals
of couse all accompaning gaskets needed to put engine back together
Car ran good after all this was done

So what is causing it to sputter and backfire through the carb at lower rpms and mid throttle but runs good at higher rpm and high throttle?
floats?
Carb accelerator pump?
sticking valve?
Any ideas would be greatly appreciated, I'm running out of things to try

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Re: Sputter and backfire at low RPM
Posted by: gwrench ()
Date: July 28, 2020 06:47PM

A weak accelerator pump will cause backfiring at the carburetor. I'd check that first.

Dale
Loveland, Colorado

1965 Monza 140 barn find

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Re: Sputter and backfire at low RPM
Posted by: American Mel ()
Date: July 28, 2020 06:50PM

WELCOME TO CCF ! ! !

Sounds like you have tried a lot of stuff to get it running.
Too bad you didn't find CCF sooner, these guys around here could have had you running in tip-top shape quite a bit sooner.
It sounds like you passenger-side carb is the issue to me.
I know that you said it was rebuilt, but the signs point to it as the culprit.
Check the accelerator pump, as Dale suggests.
Does it have problems at idle?
Or just from tip-in and up?

-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
WA. state, 1 mile south of the Canadian border,
I am not at the end of the world, but you can see it from here.
Have; '66 Monza Coupe - 140, 4-spd. Daily driver beater
'67 Monza Vert. - 140, 4-spd. Daily driver beater
'67 A/C Moredoor Monza
Have had; '61 Monza coupe, 80hp, PG
'62 Monza Wagon, 4spd.
'63 Spyder,
'65 Corsa
.
non-vair
'04 Dodge Cummins Quad Dually, approaching 400K
'03 Honda Del Sol
17'Terry



Edited 1 time(s). Last edit at 07/28/2020 06:52PM by American Mel.

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Re: Sputter and backfire at low RPM
Posted by: Frank DuVal ()
Date: July 28, 2020 07:26PM

Do you know if the passenger carburetor is a 66 style? Yes, you need Bob Helt's book to know the difference. 66 has an idle circuit that plugs and causes off idle issues. Very very difficult to clean.

Sounds lean. Both accelerator pumps (Dale said) working well? Or, all 4 accelerator pumps working well?

Frank DuVal

Fredericksburg, VA

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Re: Sputter and backfire at low RPM
Posted by: RexJohnson ()
Date: July 29, 2020 08:59AM

I don't have my accelerator pumps hooked up on my car and it doesn't backfire and pop out the carb. So I would take that carb apart and look at anything that could make it lean like float level, make sure that the cluster is flat and blow out all of the passages again.

RJ tools
Salem,Oregon

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Re: Sputter and backfire at low RPM
Posted by: Frank DuVal ()
Date: July 29, 2020 09:35AM

Quote
Rex
I don't have my accelerator pumps hooked up on my car and it doesn't backfire and pop out the carb.

OK, this needs explaining.

Rochester designed the accelerator pumps because they were a necessary item to make the engine perform in all conditions the customer was expected to drive. If they could have been left out, money was to be saved! I know from years of working on cars that weak accelerator pumps cause bog and popping back through carburetor when the throttle is mashed, not just a Corvair HV issue.

How rich did you jet the carbs to eliminate the need for pumps entirely?confused smiley

Frank DuVal

Fredericksburg, VA

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Re: Sputter and backfire at low RPM
Posted by: American Mel ()
Date: July 29, 2020 10:57AM

Frank, I know from experience that it is possible to drive a car without MASHING the pedal down.
One of the family cars I had to drive as a kid had a non-functioning accelerator pump.
You can drive without one, you just can't drive it in a ahem, "Spirited" manner.
Rex does NOT mash his accelerator pedal!
Rex and a few others are virtuosos of the Econo-run.
Accelerator pumps are Fuel Wasters!
Not a good thing when trying to conserve every single last drop of fuel.

-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
WA. state, 1 mile south of the Canadian border,
I am not at the end of the world, but you can see it from here.
Have; '66 Monza Coupe - 140, 4-spd. Daily driver beater
'67 Monza Vert. - 140, 4-spd. Daily driver beater
'67 A/C Moredoor Monza
Have had; '61 Monza coupe, 80hp, PG
'62 Monza Wagon, 4spd.
'63 Spyder,
'65 Corsa
.
non-vair
'04 Dodge Cummins Quad Dually, approaching 400K
'03 Honda Del Sol
17'Terry

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Re: Sputter and backfire at low RPM
Posted by: RexJohnson ()
Date: July 29, 2020 12:14PM

I think that my jets are #52 with the full smog system. Try unhooking them sometime and you will be surprised how well it runs. I have seen other cars at the conventions with the pumps unhooked also. As Mel said I don't mash it down very often but when I need to get going it goes. The chocks have to work and it is alittle cold-blooded when it is cold. Since the pumps are so prone to failing there are a lot of Corvairs driving around without working pumps.

RJ tools
Salem,Oregon

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Re: Sputter and backfire at low RPM
Posted by: joelsplace ()
Date: July 29, 2020 01:40PM

I've had Corvairs with bad accelerator pumps that the only symptom was no hot start. Ran fine otherwise and would start fine with the chokes on. I've never had one pop because of no or weak pump. I've never tried running without them.

Joel
Northlake, TX
5 Ultravans, 114 Corvairs and counting...

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Re: Sputter and backfire at low RPM
Posted by: playerpage ()
Date: July 29, 2020 04:09PM

Does any of this accelerator talk apply to a YH carb?

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Re: Sputter and backfire at low RPM
Posted by: joelsplace ()
Date: July 30, 2020 08:08PM

No. Failed YH accelerator pumps flood the engine or make it really rich.

Joel
Northlake, TX
5 Ultravans, 114 Corvairs and counting...

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Re: Sputter and backfire at low RPM
Posted by: Frank DuVal ()
Date: July 30, 2020 08:56PM

Quote
AM Mel
Frank, I know from experience that it is possible to drive a car without MASHING the pedal down.

Not around here! I have to pull out into 55 MPH heavy traffic road (US 17) to leave my street!grinning smiley

I can get my 95 HP to pop when the pump cups are bad. winking smiley

I too do well in econo runs. Just impractical in this area in everyday driving.

Frank DuVal

Fredericksburg, VA

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Re: Sputter and backfire at low RPM
Posted by: FlatEarth ()
Date: August 02, 2020 03:01PM

Orginal poster here:

Swapped "in question" carb with a known good carb off of my dad's corvair. Car runs good. I have not had a chance to tear into the bad one yet to see if it is a float level issue or accelerator pump. Thanks for the warm welcome and good ideas!

Drove the car around for two days with the borrowed carb (tuned on my car) ran good, no problems. Tried to start the car after sitting in a store parking lot for 10 minutes, engine turns over a few times and then stops. Turn the key, nothing happens. I happened to have a wrench with me, put it on the crank pulley, engine won't budge forward. It'll turn backwards and then forwards but hits a dead stop. I have it towed home, pull all the plugs and start looking with the bore scope, and well... I think the attatched picture shows pretty well what happened.

Attachments:
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Re: Sputter and backfire at low RPM
Posted by: Lane66m ()
Date: August 02, 2020 03:06PM

Dropped valve seat with valve jamming engine?

Al Lane
Ellabell, GA 31308

1966 Monza Coupe, 110 hp, 4 Spd
1966 Monza More Door 110 hp, PG
1968 Camaro SS Coupe 350 CI 295+ hp PG
1964 Greenbrier Deluxe, 6 dr, 80 hp car engine, PG
1947 Farmall A tractor 15 hp


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Re: Sputter and backfire at low RPM
Posted by: steve c goodman ()
Date: August 02, 2020 03:09PM

You are lucky because if engine had been running the piston would have broken seat ring into small pieces damaging both quench area of head as well as piston/ring lands crushed too.

best wishes, Steve
Rear Engine Spec. Inc. Golden, Colo.

1962 spyder 3.0L turbo---1965 Crown V8
1967 monza 110/4---1968 monza 110/4
1971 amante gt 110/4
CORSA/RMC/PPCC/V8 Registry

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Re: Sputter and backfire at low RPM
Posted by: FlatEarth ()
Date: August 02, 2020 04:11PM

Lane66m is right on the money. Definitely lucky it didn’t drop the seat while running. Not real thrilled about the situation, but it could be a lot worse.

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Re: Sputter and backfire at low RPM
Posted by: American Mel ()
Date: August 02, 2020 04:26PM

It looks to me like the Seat HAS broken and there ARE a few pieces stuck into the Quench area. sad smiley

-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
WA. state, 1 mile south of the Canadian border,
I am not at the end of the world, but you can see it from here.
Have; '66 Monza Coupe - 140, 4-spd. Daily driver beater
'67 Monza Vert. - 140, 4-spd. Daily driver beater
'67 A/C Moredoor Monza
Have had; '61 Monza coupe, 80hp, PG
'62 Monza Wagon, 4spd.
'63 Spyder,
'65 Corsa
.
non-vair
'04 Dodge Cummins Quad Dually, approaching 400K
'03 Honda Del Sol
17'Terry

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Re: Sputter and backfire at low RPM
Posted by: Lane66m ()
Date: August 02, 2020 04:43PM

Mel, you are correct, the seat is broken. That piston and cylinder are toast.

Al Lane
Ellabell, GA 31308

1966 Monza Coupe, 110 hp, 4 Spd
1966 Monza More Door 110 hp, PG
1968 Camaro SS Coupe 350 CI 295+ hp PG
1964 Greenbrier Deluxe, 6 dr, 80 hp car engine, PG
1947 Farmall A tractor 15 hp


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Re: Sputter and backfire at low RPM
Posted by: steve c goodman ()
Date: August 02, 2020 04:57PM

Yep those few revolutions of cranking may have done some damage.

best wishes, Steve
Rear Engine Spec. Inc. Golden, Colo.

1962 spyder 3.0L turbo---1965 Crown V8
1967 monza 110/4---1968 monza 110/4
1971 amante gt 110/4
CORSA/RMC/PPCC/V8 Registry

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Re: Sputter and backfire at low RPM
Posted by: American Mel ()
Date: August 02, 2020 05:19PM

Lane66m Wrote:
-------------------------------------------------------
> Mel, you are correct, the seat is broken. That piston and cylinder are toast.


I wouldn't go so far as to say that, without looking at it first.
It is quite probable, but not a certainty.
I have opened running engines, that had past scars on the pistons and heads from broken Valve Seats.
The PO had replaced the dropped seat, reassembled it, and it ran fine.
It might not be a "Perfect Solution", but it works when the damage isn't too great.

-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
WA. state, 1 mile south of the Canadian border,
I am not at the end of the world, but you can see it from here.
Have; '66 Monza Coupe - 140, 4-spd. Daily driver beater
'67 Monza Vert. - 140, 4-spd. Daily driver beater
'67 A/C Moredoor Monza
Have had; '61 Monza coupe, 80hp, PG
'62 Monza Wagon, 4spd.
'63 Spyder,
'65 Corsa
.
non-vair
'04 Dodge Cummins Quad Dually, approaching 400K
'03 Honda Del Sol
17'Terry

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