Hesitation when taking off

Propane, Butane, LPG, GPL, C3H8, C4H10
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Steptoe
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Re: Hesitation when taking off

Post by Steptoe » Sun Jul 20, 2014 1:37 am

and its still backfiring.
Hesitation, now a back fire or have both?
Backfire ...
a backfire thru exhaust on a petrol engine lifting the foot off the throttle is lean mixture... or lpg when I was messing with very lean mixtures it didnt backfire....so not sure for LPG
A back fire thru the carb.. basic mechanics here....to ignite fuel in the inlet manifold/ carb will require a piont of ign.
This can only be a inlet valve open while it is firing.
To do this it is either
1/ not closing which could be over tight rocker adjustment or a VERY burnt inlet valve
2/Have a couple HT leads to spark plugs around the wrong way
3/Poor quality or abused by pulling on, old or burnt HT leads leaking electrons like water thru a hole causing misfire and/ or crossfire Simple to check on a dark night.
In my books If one cant with full confidence run your hand up and down HT leads, with the other hand on a good ground, while the engine is running then u should be doing something so u do.
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C3H8
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Re: Hesitation when taking off

Post by C3H8 » Sun Jul 20, 2014 3:28 pm

Sorry Steptoe but I disagree. Backfires in the exhaust are due to rich conditions (on almost any fuel). Backfires in the intake are an indication of a lean condition, or the other reasons you suggested. Backfires in the exhaust generally happen because excess fuel mixes with leftover air and is ignited by hot gases or burning fuel as the exhaust valve opens. This condition is most noticeable when decelerating rapidly in gear on standard transmissions. When decelerating in gear the engine remains at a higher rpm dragging air through the mixer or carburetor and pulling excess fuel and air into the cylinder. The engine fails to completely combust the mixture leaving a rich condition in the exhaust pipe.

Other then that critical tests include a leak down test for valve failure, ignition faults and/or timing, timing chain problems and/or camshaft issues. All of which you covered I believe.

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Steptoe
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Re: Hesitation when taking off

Post by Steptoe » Sun Jul 20, 2014 5:30 pm

The lean condition on a petrol engine creates very high exhaust temps which can ignite any unburnt fuels in the exhaust systems... rich just causes a lot of nasty smell...lower temps and mixtures that are difficult to ignite
We are talking street cars here not track cars with very hot open headers , dropping into a cnr, lift power off and spurt open flame out the end of the exhausts... which is generally due to very high track, cam durations and overlaps.

This petrol lean condition is common espec with older classic/vintage vechilces running modern higher SG/ octane unleaded formula fuels with original jettings....
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Helmet
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Re: Hesitation when taking off

Post by Helmet » Mon Jul 21, 2014 11:26 am

Alright, the symptoms as of now are:
There is still a hesitation, its just about 1/10th of what it was before I swapped in the new plugs.
I had NGK's before these plugs, the engine didn't like them. I will be sticking with the platinum plugs I have now,
because the engine has never started as easily as it does now (1 or 2 cranks and it fires up).

Now, my exhaust system is fitted with an electric cutout on each pipe. With the cutouts closed, I don't hear anything odd.
But, because I can still feel a slight hesitation, I wanted to know what was going on, and what better way to that than to open up the cutouts.
Now, before this whole mess, I did get backfires..... but that was getting of the throttle after a serious highspeed WOT pull.
Now I get the occasional backfire just letting of the gas at low speed (as low as 25 mph).
And, when driving at a constant speed with the cutouts open, I can hear the engine misfiring.
Unfortunately, my HEI unit is a pro comp. In other words, chinese junk.
So my plan is to switch to a delco-remy, and see if that cures the misfires.

Now I initianelly suspected the lockoffs because the engine acted like something was going on with the fuel flow.
So after putting on a new vaporizer and checking the mixer, in my mind, the lockoffs where next suspect.

And they are still suspect actually. About 75% of the time when I do a WOT pull, there is an annoying, hissing (almost screeching)
sound coming from the area where the lockoffs are located. So the lockoffs are still not of the hook.
'72 Cutlass Supreme, 455 BBO (out of order)
'85 CUCV, HO454

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Steptoe
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Re: Hesitation when taking off

Post by Steptoe » Mon Jul 21, 2014 5:13 pm

Unfortunately, my HEI unit is a pro comp. In other words, chinese junk.
Im no fan of Chinese engineering, but my chainsaw is surprisingly good, and the duel wheel mig welders tun out far more robust and up there with other brands, as has been the last 2 Chinese HEI units ...both now getting the miles well up on them.
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