Need help solving backfire problem
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Need help solving backfire problem
The car is an 1970 karmann ghia with a 71 1600cc, duel port stock, sva distributor, h30/31 solex brosol carb.
This car has been sitting in my driveway for 5 years and I would love to be driving it before summer. My problem is
I have a light to heavy backfire when I let up on the gas pedal. It idles fine, accelerates fine but quick release of accelerator causes pops.
Here's what I've done so far to troubleshoot:
1) rebuilt carb
2) new fuel pump
3) tune up( plugs, wires, points,fuel lines and filter, timing 7.5 deg before top dead center)
4) new exhaust
5) new rubber boots and gaskets on manifold
6) compression check range 100-106 psi
I've run out of ideas can anyone out there please help? Because I'm stumped.
This car has been sitting in my driveway for 5 years and I would love to be driving it before summer. My problem is
I have a light to heavy backfire when I let up on the gas pedal. It idles fine, accelerates fine but quick release of accelerator causes pops.
Here's what I've done so far to troubleshoot:
1) rebuilt carb
2) new fuel pump
3) tune up( plugs, wires, points,fuel lines and filter, timing 7.5 deg before top dead center)
4) new exhaust
5) new rubber boots and gaskets on manifold
6) compression check range 100-106 psi
I've run out of ideas can anyone out there please help? Because I'm stumped.
- Piledriver
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Re: Need help solving backfire problem
Welcome to the STF!
Those symptoms typically indicate an exhaust leak.
("typically" is probably an understatement, but I hesitate to say "always", although nothing else even comes to mind)
Most common place to leak: Heater box seals, the HX tend to get all gnarly at the HX>muffler slip joint, resulting in a poor seal at the compression donut. There are various designs for that sealing donut kit, some are the old style donut of krunchy stuff, some look like woven/formed SS compressed wire donut with embedded sealant: Go with the latter, I forget where I got those awhile back, perhaps Marc has a source, I recall that they worked very well, German, about same price)
Beyond a certain point, the ends of the HX need replaced (they sell repair bits)
(Those) Carbs don't have a fuel cut so on decel you will be almost certainly be sucking some unburned fuel into the exhaust.
Add fresh air from a small leak, add heat, mix well... BANG.
If your idle mix is quite rich it can aggravate it.
Those symptoms typically indicate an exhaust leak.
("typically" is probably an understatement, but I hesitate to say "always", although nothing else even comes to mind)
Most common place to leak: Heater box seals, the HX tend to get all gnarly at the HX>muffler slip joint, resulting in a poor seal at the compression donut. There are various designs for that sealing donut kit, some are the old style donut of krunchy stuff, some look like woven/formed SS compressed wire donut with embedded sealant: Go with the latter, I forget where I got those awhile back, perhaps Marc has a source, I recall that they worked very well, German, about same price)
Beyond a certain point, the ends of the HX need replaced (they sell repair bits)
(Those) Carbs don't have a fuel cut so on decel you will be almost certainly be sucking some unburned fuel into the exhaust.
Add fresh air from a small leak, add heat, mix well... BANG.
If your idle mix is quite rich it can aggravate it.
Addendum to Newtons first law:
zero vehicles on jackstands, square gets a fresh 090 and 1911, cabby gets a blower.
EZ3.6 Vanagon after that.(mounted, needs everything finished) then Creamsicle.
zero vehicles on jackstands, square gets a fresh 090 and 1911, cabby gets a blower.
EZ3.6 Vanagon after that.(mounted, needs everything finished) then Creamsicle.
- Marc
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- Joined: Thu May 23, 2002 12:01 am
Re: Need help solving backfire problem
Is there any unusual popping sound when you accelerate rapidly or place a load on the engine when it's cold? A `71 case ID should start with "AE" - does yours?
By "sva" do you mean the original `70 distributor? (number please) ...correct timing for that type of distributor is 0° at idle with the vacuum hose disconnected (or it may be set statically to TDC).
The H30/31PICT carb has the smaller singleport-sized flange and an adapter (or odd-ball DP manifold center section) is needed to graft it onto a dualport engine. Even though the vacuum advance signal from this carb is weak compared to a stock DP distributor, it'll work - you just don't get the full performance of a proper 34PICT carb. One issue with the H30/31PICT is that they often are supplied with an undersized main jet, perhaps appropriate for a 1300 but too lean for a 1600...I'm talkin' a 115 when a 125 or 127.5 is needed.
So, we need to know what main jet is in it as well as the distributor number (preferably the 10-digit Bosch number).
By "sva" do you mean the original `70 distributor? (number please) ...correct timing for that type of distributor is 0° at idle with the vacuum hose disconnected (or it may be set statically to TDC).
The H30/31PICT carb has the smaller singleport-sized flange and an adapter (or odd-ball DP manifold center section) is needed to graft it onto a dualport engine. Even though the vacuum advance signal from this carb is weak compared to a stock DP distributor, it'll work - you just don't get the full performance of a proper 34PICT carb. One issue with the H30/31PICT is that they often are supplied with an undersized main jet, perhaps appropriate for a 1300 but too lean for a 1600...I'm talkin' a 115 when a 125 or 127.5 is needed.
So, we need to know what main jet is in it as well as the distributor number (preferably the 10-digit Bosch number).
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Re: Need help solving backfire problem
Thanks for the quick replies
Piledriver the exhaust that I just added does have the woven embedded type and I'm pretty certain that I did a good job putting it on(knowing that I had finally isolated the source of my frustration. But noooooo!). Also, my carb does have the fuel shutoff ( the little cylinder on the side with a wire to the coil, right?).
Marc the distributor isn't stock, I replaced an 009 that had a dead spot. I will find a ser. # when the sun rises.
On a side note I tuned the car using Rob and Dave's aircooled volkswagen pages as my guide and just noticed that I mistook the 10deg. btc believing that it was the 7.5deg btc mark on my 3 notch pulley. Also they say in adjusting the carb to start the carb adjustment by turning the volume adjustment screw 2 1/2 turns out. initially I interpreted a turn to mean a 360 deg. rotation but yesterday I tried 180 deg. rotations and it seemed to idle better. It is still backfiring though.
Piledriver the exhaust that I just added does have the woven embedded type and I'm pretty certain that I did a good job putting it on(knowing that I had finally isolated the source of my frustration. But noooooo!). Also, my carb does have the fuel shutoff ( the little cylinder on the side with a wire to the coil, right?).
Marc the distributor isn't stock, I replaced an 009 that had a dead spot. I will find a ser. # when the sun rises.
On a side note I tuned the car using Rob and Dave's aircooled volkswagen pages as my guide and just noticed that I mistook the 10deg. btc believing that it was the 7.5deg btc mark on my 3 notch pulley. Also they say in adjusting the carb to start the carb adjustment by turning the volume adjustment screw 2 1/2 turns out. initially I interpreted a turn to mean a 360 deg. rotation but yesterday I tried 180 deg. rotations and it seemed to idle better. It is still backfiring though.
- Marc
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Re: Need help solving backfire problem
I edited my post this morning after I got a cup of coffee in me. Also, you say it sat for 5 years...how old is the gas?
- Piledriver
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Re: Need help solving backfire problem
The idle fuel shutoff turns on/off with the ignition, although turning it on/off via logic is an interesting idea.
Addendum to Newtons first law:
zero vehicles on jackstands, square gets a fresh 090 and 1911, cabby gets a blower.
EZ3.6 Vanagon after that.(mounted, needs everything finished) then Creamsicle.
zero vehicles on jackstands, square gets a fresh 090 and 1911, cabby gets a blower.
EZ3.6 Vanagon after that.(mounted, needs everything finished) then Creamsicle.
- Marc
- Moderator
- Posts: 23741
- Joined: Thu May 23, 2002 12:01 am
Re: Need help solving backfire problem
Shouldn't need anything that sophisticated - an adjustable vacuum-operated switch, set to open at an inch or two over normal idle vacuum should do the trick. The stock diaphragm throttle positioner fitted in the late `60s/early `70s could be adjusted to delay return-to-idle enough to reduce decel richness some, but at best that makes it seem as though you don't know how to drive and at worst could be a safety problem - and most of those were tossed out decades ago.
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Re: Need help solving backfire problem
marc-
1) the # on the distributer is 113 905 205 AI
2) the fuel was bad so I drained it, cleaned the tank filter, replaced all the rubber fuel hoses, replaced the pre fuel pump filter, replaced the fuel pump and rebuilt the carb, and replaced the air filter.
3) the case does start with AE.
4) today I tried setting the timing at 0 deg and the engine didn't run very well. The VW service manuel says 1971's with sva dist. should be set at 7.5 btc so I tried that and with my new knowledge that the second pulley notch is 7.5btc and not the 3rd notch, and now it runs like a champ, except for the annoying cough when I release the accelerator. It's better than before no backfiring but its definately not purring like it should.
piledriver- I'm not sure I understand about the idle fuel shutoff. Ithought that just kills the fuel supply with the ignition to prevent
run on. That isn't a problem that I have.
1) the # on the distributer is 113 905 205 AI
2) the fuel was bad so I drained it, cleaned the tank filter, replaced all the rubber fuel hoses, replaced the pre fuel pump filter, replaced the fuel pump and rebuilt the carb, and replaced the air filter.
3) the case does start with AE.
4) today I tried setting the timing at 0 deg and the engine didn't run very well. The VW service manuel says 1971's with sva dist. should be set at 7.5 btc so I tried that and with my new knowledge that the second pulley notch is 7.5btc and not the 3rd notch, and now it runs like a champ, except for the annoying cough when I release the accelerator. It's better than before no backfiring but its definately not purring like it should.
piledriver- I'm not sure I understand about the idle fuel shutoff. Ithought that just kills the fuel supply with the ignition to prevent
run on. That isn't a problem that I have.
- Marc
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Re: Need help solving backfire problem
No 10-digit Bosch number on that distributor, eh? The number you gave is the VW P/N, and I've never heard of an "AI". most `71s had an "AJ" (4-speed) or "AH" (AutoStick) so AI sounds contemporary, but the AH and AJ were dual-vacuum, dual advance "DVDA" models for which initial timing was 5°ATDC. "SVDA" denotes single-vacuum, dual-advance with a single vacuum canister connection and a centrifugal-advance mechanism built into the shaft. To me "SVA" would mean a vacuum-only distributor with no centrifugal mechanism as were used on SP engines; most of those are set at 0°.
I know of no 1971 application of either an SVDA or an SVA, to the best of my knowledge all were DVDA - perhaps your book references a non-US-market engine?
Generally when someone refers to an SVDA it's specifically the 1974 4-speed federal model 0 231 170 034 (VW P/N 043 905 205) for which initial timing is 7.5° BTDC but there are some others which meet the definition. As I said above, the H30/31PICT doesn't provide a strong enough vacuum signal to reap the full advantages of the 0 231 170 034 but they will work together.
To find out what size your main jet is you should take the top off of the carb - it's less tricky when you can see what you're doing, and odds are that your carb is filthy inside with crud and deposits of what was once gasoline anyway so this'll give you a chance to clean that out before it finds its way into one of the jets or internal passages. Most of the time the carb top gasket will survive and can be reused, but you should at least know where in town you can get your hands on one before opening up the carb.
I know of no 1971 application of either an SVDA or an SVA, to the best of my knowledge all were DVDA - perhaps your book references a non-US-market engine?
Generally when someone refers to an SVDA it's specifically the 1974 4-speed federal model 0 231 170 034 (VW P/N 043 905 205) for which initial timing is 7.5° BTDC but there are some others which meet the definition. As I said above, the H30/31PICT doesn't provide a strong enough vacuum signal to reap the full advantages of the 0 231 170 034 but they will work together.
To find out what size your main jet is you should take the top off of the carb - it's less tricky when you can see what you're doing, and odds are that your carb is filthy inside with crud and deposits of what was once gasoline anyway so this'll give you a chance to clean that out before it finds its way into one of the jets or internal passages. Most of the time the carb top gasket will survive and can be reused, but you should at least know where in town you can get your hands on one before opening up the carb.
- Piledriver
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Re: Need help solving backfire problem
The general method to set timing these days is:
disconnect vacuum line from can
Rev to ~3500 RPM, set timing to ~32BTDC (on a T1, ~27-28 BTDC for T4)
Lock down distributor, reconnect vacuum can, and deal with the idle adjustments again.
(allow idle timing to fall where it may, but note what it is)
Note: helps greatly to be CERTAIN where TDC really is.
You may find that the idle timing is still in spec, but the distributors that are currently installed in engines are all over the place spec and wear wise, so it's best to set the top end where too advanced or too retarded really hurts.
It would be awesome if someone offered a service rebuilding these old distributors to better than new condition, and recurved them to suit.
(Shameless plug for Marc as he won't blow his own horn, but I can )
disconnect vacuum line from can
Rev to ~3500 RPM, set timing to ~32BTDC (on a T1, ~27-28 BTDC for T4)
Lock down distributor, reconnect vacuum can, and deal with the idle adjustments again.
(allow idle timing to fall where it may, but note what it is)
Note: helps greatly to be CERTAIN where TDC really is.
You may find that the idle timing is still in spec, but the distributors that are currently installed in engines are all over the place spec and wear wise, so it's best to set the top end where too advanced or too retarded really hurts.
It would be awesome if someone offered a service rebuilding these old distributors to better than new condition, and recurved them to suit.
(Shameless plug for Marc as he won't blow his own horn, but I can )
Addendum to Newtons first law:
zero vehicles on jackstands, square gets a fresh 090 and 1911, cabby gets a blower.
EZ3.6 Vanagon after that.(mounted, needs everything finished) then Creamsicle.
zero vehicles on jackstands, square gets a fresh 090 and 1911, cabby gets a blower.
EZ3.6 Vanagon after that.(mounted, needs everything finished) then Creamsicle.
- Marc
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- Joined: Thu May 23, 2002 12:01 am
Re: Need help solving backfire problem
I would not advise using any such "general method" without knowing the specifics of the distributor you're dealing with. 32° BTDC total (at an RPM high enough to bring about maximum centrifugal advance) would be at the upper end of the 28-32° range normally recommended for a centrifugal-only distributor like a 0 231 178 009, for which the advance is "all in" by 3000RPM or so...but would not necessarily be correct for others - it depends upon their particular curve. For example, the `71 4-speed's 0 231 167 049 DVDA gets its full centrifugal advance of 22-25° by 3800RPM...add the 7.5° initial setting to that and you would be looking for ~31° total at 3800RPM (not 3500). Similarly, the 0 231 170 034 SVDA would be set at ~30° total at 3800RPM.
And of course, if you were to use that procedure on a vacuum-only distributor like a `70 0 231 137 036, the total advance attainable once the vacuum hose was reconnected would be a destructive ~75°.
Don't guess. Identify the distributor and set the timing accordingly.
And of course, if you were to use that procedure on a vacuum-only distributor like a `70 0 231 137 036, the total advance attainable once the vacuum hose was reconnected would be a destructive ~75°.
Don't guess. Identify the distributor and set the timing accordingly.
- Piledriver
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Re: Need help solving backfire problem
I run >70 BTDC at low map/higher RPM and low CHT keeps the CHT and EGT down.
(I also cruise at 19-22:1, YMMV, admittedly via a programmable crank trigger ignition/EMS)
I admit the vacuum only setups exist, I haven't seen one actually in use for 20+ years.
IIRC the latest carb they worked with was a 28pict.
(I also cruise at 19-22:1, YMMV, admittedly via a programmable crank trigger ignition/EMS)
I admit the vacuum only setups exist, I haven't seen one actually in use for 20+ years.
IIRC the latest carb they worked with was a 28pict.
Addendum to Newtons first law:
zero vehicles on jackstands, square gets a fresh 090 and 1911, cabby gets a blower.
EZ3.6 Vanagon after that.(mounted, needs everything finished) then Creamsicle.
zero vehicles on jackstands, square gets a fresh 090 and 1911, cabby gets a blower.
EZ3.6 Vanagon after that.(mounted, needs everything finished) then Creamsicle.
- Marc
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- Posts: 23741
- Joined: Thu May 23, 2002 12:01 am
Re: Need help solving backfire problem
Vacuum-only were used through `70 (US market) and presumably beyond that in other countries where the 1200 and 13/15/1600 SP engines were available after `70...so add 28PICT-1, 28PICT-2, 30PICT, 30PICT-1, 30PICT-2, and 30PICT-3 to that list.
- Piledriver
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Re: Need help solving backfire problem
I stand happily corrected, but this guy still has a backfire.
I have seen rusted through heat risers... Could also be a gimpy gasket, anywhere.
(including the heat riser<>muffler connections, and perhaps even at the HX outlets)
You can find gross leaks on intakes with (low pressure ~5 psi) compressed air, at least on TDis as they have ~no overlap.
(pull the rockers on a T1/T4)
Haven't personally tried it on a stock T1 exhaust system, but it seems ~reasonable esp if new.
Listen for the hiss or use soapy water in a spray bottle, look for bubbles.
Unsophisticated. Cheap. Very effective.
I have seen rusted through heat risers... Could also be a gimpy gasket, anywhere.
(including the heat riser<>muffler connections, and perhaps even at the HX outlets)
You can find gross leaks on intakes with (low pressure ~5 psi) compressed air, at least on TDis as they have ~no overlap.
(pull the rockers on a T1/T4)
Haven't personally tried it on a stock T1 exhaust system, but it seems ~reasonable esp if new.
Listen for the hiss or use soapy water in a spray bottle, look for bubbles.
Unsophisticated. Cheap. Very effective.
Addendum to Newtons first law:
zero vehicles on jackstands, square gets a fresh 090 and 1911, cabby gets a blower.
EZ3.6 Vanagon after that.(mounted, needs everything finished) then Creamsicle.
zero vehicles on jackstands, square gets a fresh 090 and 1911, cabby gets a blower.
EZ3.6 Vanagon after that.(mounted, needs everything finished) then Creamsicle.
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- Joined: Sat May 19, 2012 10:35 pm
Re: Need help solving backfire problem
piledriver- you were correct to suspect a leak on the heat riser. Turns out I didn't tighten one down enough and lost a nut, causing all the problems. Yesterday it ran like a champ so I took my wife for a ride with the top down enjoying the warm California sun.
And then when I got it home I had a stone dead battery(brand new battery) and a severe oil leak that looks to be originating from the pulley, fuel pump, or oil sending unit. Oh well, for about an hour I was all smiles driving down the road in my baby. This is the first car I have bought myself and have owned her for 31 years and she WILL ride again!! Thanks for the assistance. I think I should start a new post regarding the electrical issue and the leak.
P.S. how do I go about posting photo's?
And then when I got it home I had a stone dead battery(brand new battery) and a severe oil leak that looks to be originating from the pulley, fuel pump, or oil sending unit. Oh well, for about an hour I was all smiles driving down the road in my baby. This is the first car I have bought myself and have owned her for 31 years and she WILL ride again!! Thanks for the assistance. I think I should start a new post regarding the electrical issue and the leak.
P.S. how do I go about posting photo's?