Loud backfire when engine turned off

Fuel Supply & Ignition Systems

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Mr Brown
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Loud backfire when engine turned off

Post by Mr Brown » Thu Jun 20, 2013 3:18 pm

Been trying to get a pair of empi hpmx 40 carbs working. Fires up fine and drives ok, idles high some times when i pull to a stop. Worst problem is loud backfire a few seconds after I switch of engine??

miniman82
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Re: Loud backfire when engine turned off

Post by miniman82 » Thu Jun 20, 2013 8:15 pm

Did you buy it from this guy?


Image


Sorry, had to go there. :mrgreen:
Image

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Marc
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Re: Loud backfire when engine turned off

Post by Marc » Thu Jun 20, 2013 8:47 pm

IDF Webers (and their clones) are infamous for all manner of freakish behavior when their idle air correction drillings get plugged -and due to their location in the corners of the carb tops that happens often if the aircleaners aren't functioning at 100%, which is why the "Jet Doctor" kits are made to add a snorkel tube to them so they won't swallow trash.
Another thing that can cause odd symptoms like yours is poor synchronization. Start with mechanical synch'ing to ensure that both throttle plates are at 0% and 100% simultaneously. With some linkage designs (crossbar in particular) that's nearly impossible to achieve but until it's perfect you're pissing in the wind.

If the problem doesn't lie with the above issues, a report out the exhaust after switchoff indicates any to all of three problems. It takes fuel, air, and heat to make an explosion. If too much fuel is being drawn in through the idle circuit due to a mixture issue and/or too-high of an idle speed at shutoff, when it hits the hot exhaust it's going to light off. Sometimes enough oxygen can draft in via the tailpipe if it's large, but more often a leak in the system is found. Lastly, if the exhaust system is just too hot, as could be caused by retarded ignition timing, you can still get a pop even if the other conditions are nominal.

What's it idling at when you switch off? Have you tried putting it in 4th gear and dumping the clutch (brake on) as you turn off the key?

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Re: Loud backfire when engine turned off

Post by Mr Brown » Fri Jun 21, 2013 1:26 am

Miniman wish I had bought from uncle buck at least I could have a laugh about it!
Thanks for clear info Marc, I'm sure the air valves are clean. I've been meticulous in cleaning them. In the workshop I can get the idle running fine. When I run the bus up the road and pull to a stop the engine is on a high idle. It will sometimes stay idling high or will settle after a while. Should I focus on tuning screws now? Just wind them all in and start again?
By the way it's a 1776 cc with 55 idle jets
Thanks.

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Re: Loud backfire when engine turned off

Post by Mr Brown » Fri Jun 21, 2013 1:29 am

Meant to say, had to adjust float settings. 10 mm closed and 42 mm open. Could this be a cause?

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Jadewombat
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Re: Loud backfire when engine turned off

Post by Jadewombat » Fri Jun 21, 2013 1:59 pm

I had carb. that would 'diesel' when I switched the key off. I advanced the timing just a few degrees and it didn't do that anymore.

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Marc
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Re: Loud backfire when engine turned off

Post by Marc » Fri Jun 21, 2013 8:55 pm

A high idle that eventually settles down may simply indicate that the throttle plates aren't closing fully against the stop screws, so check that first. You may need more return spring action than the carbs offer. On Buses, because there's ample room between the engine and lid it's a simple enough matter to add a spring directly in line with the cable - often just a little help in tugging the cable back (and levitating the pedal) is enough to allow the carb springs to do their job.

As the engine warms up it "grows" a little, and with some linkage setups you can lose mechanical synchronization. Sometimes the solution is as simple as adding a light spring to one side.

Also, if the manifolds are a snug fit against the sheetmetal (or the bumps in the castings on some heads) they can be tweaked as things warm up and relative dimensions change - sometimes an induction leak develops between manifold and head that disappears when the engine cools back down. If you've ruled out any mechanical problem with the linkage, bear in mind that an intake leak can give a similar symptom.

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Dale M.
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Re: Loud backfire when engine turned off

Post by Dale M. » Sat Jun 22, 2013 10:57 am

Mr Brown wrote:Meant to say, had to adjust float settings. 10 mm closed and 42 mm open. Could this be a cause?
Think Weber stated 10mm closed and 32.5 mm full open, with top plate held in vertical position.... Hope your 42 was a typo...

http://www.piercemanifolds.com/category_s/317.htm

Weber IDF and HPMX are identical in setups...

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Mr Brown
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Re: Loud backfire when engine turned off

Post by Mr Brown » Sat Jun 22, 2013 12:16 pm

Looking in the weber manual it shows 32.5 mm from the closed position ( 10 mm) the illustration shows it being measured from this point 10 mm + 32.5 mm = 42.5 mm it looks fairly clear from the diagram. Have I read this wrong? Or would you suggest 32.5 mm from the carb top, if its to open could this cause the over fueling and backfire when I switch off engine.
Thanks

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Dale M.
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Re: Loud backfire when engine turned off

Post by Dale M. » Sat Jun 22, 2013 12:49 pm

Mr Brown wrote:Looking in the weber manual it shows 32.5 mm from the closed position ( 10 mm) the illustration shows it being measured from this point 10 mm + 32.5 mm = 42.5 mm it looks fairly clear from the diagram. Have I read this wrong? Or would you suggest 32.5 mm from the carb top, if its to open could this cause the over fueling and backfire when I switch off engine.
Thanks
I believe you are interpreting information wrong or wrong information is given in your source..... Its 10mm from top plate to closed position, and it 32.5 mm from top plate to full open....
Weberfloat.jpg
Actually full open position of float is not really going to make much difference other than float may rest on bottom of fuel bowl...

I have more faith in Pierce Manifold site information as he is a major west coat provider of Weber and HPMX carbs and has been in the business for over 30 years....

Dale
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Re: Loud backfire when engine turned off

Post by Mr Brown » Sun Jun 23, 2013 3:43 am

I've attached an image from the robert tomlinson weber manual. This is the measurement I've been working to.
It looks clear that its 42.5 mm from gasket, what do you think?
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Marc
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Re: Loud backfire when engine turned off

Post by Marc » Sun Jun 23, 2013 4:21 am

Not the problem.

socrace
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Re: Loud backfire when engine turned off

Post by socrace » Sun Sep 08, 2013 12:25 am

It's been a few months for this thread, but thought I'd throw this suggestion out there anyway. When ignition key is turned off, the alternator warning light can sometimes power the ignition enough to backfire. Try removing the warning light and see if backfiring stops. If it does, an isolation diode is needed in series with the warning light.

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Re: Loud backfire when engine turned off

Post by danielzink » Sun Oct 27, 2013 9:48 am

socrace wrote:It's been a few months for this thread, but thought I'd throw this suggestion out there anyway. When ignition key is turned off, the alternator warning light can sometimes power the ignition enough to backfire. Try removing the warning light and see if backfiring stops. If it does, an isolation diode is needed in series with the warning light.

Crazy.....

I've had this odd issue for years but never bothered to look into it....happened upon this thread and may have found a cure !

My issue I run the engine for a bit then turn it off..then key it back up (not start it - just "key on") and I'll get a "chug" like it'll do a 1/16th turn of the motor..

I'll have to try the isolation diode trick. Any suggestions on the diode ? I have plenty hanging around.

Thanks, Dan
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Marc
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Re: Loud backfire when engine turned off

Post by Marc » Sun Oct 27, 2013 12:37 pm

It's not really critical, but this is one that would work: http://www.radioshack.com/product/index ... Id=2062591

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