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Help with high rpm backfire
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NeedAName
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PostPosted: Mon Mar 04, 2024 6:24 pm    Post subject: Help with high rpm backfire Reply with quote

Hi all, I have a backfire I could use some advice on.

It only happens at higher rpms when you accelerate quickly. The motor is a 1972 dual port with svda distributor with electronic ignition and 34p 3 carb.

I've done some testing and parts swapping, still no luck. I've swapped coil, carb, cap, rotor, plugs &wires, adjusted valves. Fuel pressure is 3psi that drops as its revved up.

I replaced intake manifold, boots, gaskets because the hear risers were plugged up. The timing hasn't been touched in years and always ran great.

If you rev it in neutral it seems to load up with gas and if you rev it quick at that point it will backfire out of the exhaust with a visible flame when it's dark.

Maybe I'm overlooking something? Any advice is appreciated
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jinx758
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PostPosted: Mon Mar 04, 2024 7:42 pm    Post subject: Re: Help with high rpm backfire Reply with quote

Howdy ...

Most backfires happen on deceleration due to an exhaust leak.
Yours is different. HMMM

Maybe your fuel pump rod is sticking in the Bakelite/phenolic base. This could explain the drop in fuel pressure, resulting in a lean condition.
But a non venting gas tank &/or a partially clogged fuel line would do the same thing.
Not sure if these would cause a backfire tho.

Just blindly throwing darts ... stay safe

safe
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VW_Jimbo Premium Member
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PostPosted: Mon Mar 04, 2024 11:36 pm    Post subject: Re: Help with high rpm backfire Reply with quote

Too much fuel or poor spark.

Air leak in exhaust system.

You have both.
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ashman40
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PostPosted: Tue Mar 05, 2024 8:11 am    Post subject: Re: Help with high rpm backfire Reply with quote

NeedAName wrote:
It only happens at higher rpms when you accelerate quickly. ... svda distributor with electronic ignition ... The timing hasn't been touched in years and always ran great.

It would be helpful to know the actual ignition timing? Both at idle and at full mechanical advance @3500rpm (vacuum hose disconnected and plugged). This will give a better idea if the ignition timing is over advancing... or worse, under advancing.

One more thing to check... compression test.


When you say it backfires at higher rpms when you rev it quick... is there any problem if you hold a steady high rpm for a long time (eg. cruise at 60mph?)
Is it only when you blip the throttle? Have you checked your accelerator pump output? Have you tried adjusting the accelerator pump linkage?
From your description it backfires both on the road when you accelerate at high rpms AND if you blip the throttle while sitting in the driveway. Is this correct?

Even an overly rich mixture won't backfire in the exhaust without additional air (oxygen) leaking into the exhaust system to feed the fuel. As suggested, look for a leak in your exhaust. Look for black soot marks.
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NeedAName
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PostPosted: Tue Mar 05, 2024 10:02 am    Post subject: Re: Help with high rpm backfire Reply with quote

Thanks everyone

-You're right about the timing. I assumed if the distributor hasn't moved then the timing can't change?

-You can hold steady rpm fine, but when you gas it, it will backfire. So only sudden acceleration does it.

-accelerator pump gives a nice squirter of fuel, haven't tried adjusting it. Tried a second good carb and acted the same

-It backfires the same in neutral sitting in the driveway as it does while driving

-I did fix a little exhaust leak when the heater box connects to the muffler. Couldn't find any others.
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DesertSasquatchXploration
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PostPosted: Tue Mar 05, 2024 10:46 am    Post subject: Re: Help with high rpm backfire Reply with quote

You need to look at the float level. I would hold a flashlight down the carb and watch closely as you rev it up and it backfires. Look for fuel coming out of the vent tube. Adding a 3 inch section of fuel line on the vent tube is a good idea.
3psi is little high I run mine at 1.5

Your engine displacement is 1972cc?
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gt1953
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PostPosted: Tue Mar 05, 2024 11:13 am    Post subject: Re: Help with high rpm backfire Reply with quote

If you happen to have another complete distributor swap it out. It may be the condensor giving you a fit.
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Dougy Dee
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PostPosted: Tue Mar 05, 2024 12:02 pm    Post subject: Re: Help with high rpm backfire Reply with quote

Electronic ignition doesn't like a low voltage situation. Make sure your charging/battery situation is working correctly.
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NeedAName
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PostPosted: Tue Mar 05, 2024 7:24 pm    Post subject: Re: Help with high rpm backfire Reply with quote

Thanks for the replies.

-I'll have to check the vent tube. I tried two different carburetors and no difference. The motor is a 1600cc, 1972 case.

-The generator is putting out 13.4v, so should be good there.

-Starting to think a valve is sticking. It will pop out of the carburetor also, but very rarely.

Little back story, it sat for 8 months or so when I couldn't drive it (injury). I started it up every week minimum. Ran perfect before that, so it's a little odd. It has fresh gas in it currently.
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VW_Jimbo Premium Member
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PostPosted: Tue Mar 05, 2024 8:06 pm    Post subject: Re: Help with high rpm backfire Reply with quote

It sat! Ah ha!

Change the oil and fuel filter. Take that thing out on the open road and run it for a long stretch without moving the throttle much. A prolonged drive at steady speed. Then letís hear what happens.
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67rustavenger wrote:
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NeedAName
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PostPosted: Sat Mar 09, 2024 3:14 pm    Post subject: Re: Help with high rpm backfire Reply with quote

Thanks for all the replies. No overflow out of carb vent.

I seafoamed thru the carb before I replaced the intake manifold with the rust hole in the heat riser. Was thinking sticky valve.

The seafoam foulded my new #1 spark plug. So even after I fixed the original leak/problem it still wasn't burning all the fuel/still backfiring. Back to running strong now, fingers crossed.
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