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Knocking on hard acceleration and dieseling at shut off
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EODTECH*
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PostPosted: Tue Jun 01, 2021 6:48 pm    Post subject: Knocking on hard acceleration and dieseling at shut off Reply with quote

New 1600 DP. Knocks on hard acceleration. At first it seemed like it was happening only in 4th when climbing a grade. Now in 2nd, 3rd and 4th...under hard acceleration (hill or not) it knocks.

When it was first installed about a month ago...my tech told me..."I didn't like the idle. So I left it a little high." Hmmmm.

When I first drove it out of the shop about a month and a half ago...no issues...then...it started dieseling...just a bit. About 3 weeks later...it started knocking in 4th...going up hill/hard acceleration. Today I noticed it knocking in 2, 3 and 4...and again, on hard acceleration. If I ease up on the accelerator....it disappears.

And now the dieseling is almost every time I shut it off. And it's getting worse. Sometimes in the morning just as I turn the key to fire it up, the last bit of knocking occurs just before it starts. As if the dieseling was on "pause" just as it finished shutting down the night before.

34 PICT carb, Petronix distributor. No sediment in the fuel filter. I use the same fuel in the Bug I use in my GTI. And it doesn't knock.

I've read thru this Forum...a lot of similar probs...but mostly on rebuilt older engines. This is a new case, w/new heads, valves...all new everything from fuel pump to clutch plate etc.

In all other respects, it starts beautifully every time, responds quick to the pedal, idles nicely w/no knocking.

Some of the gurus here have said check idle...fuel cut-off and a lot of other things.

My tech has since retired and moved away. There's no one here now who knows air cooled. And unless I have a pic where to make adjustments to the carb...I'm lost

.(The next reliable tech is 85 miles away.)

Am I doing more and more damage as I drive it? Would it even make it 85 miles w/o throwing a rod or a bearing or some other disastrous lethal thing?

Any help what to do?


TIA
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PostPosted: Tue Jun 01, 2021 7:13 pm    Post subject: Re: Knocking on hard acceleration and dieseling at shut off Reply with quote

Tow it to a known good drivability tech. Anyone who knows cars. They could take a look and let you know what may be happening.

What kind of gas are you using?

Have you tried a higher grade. Any idea how to set timing? Are you tooled up to do that?
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PostPosted: Tue Jun 01, 2021 7:46 pm    Post subject: Re: Knocking on hard acceleration and dieseling at shut off Reply with quote

You need to recheck your timing. Sounds like it's too far advanced. Also this will make the engine run hot, and diesel.

You really need to figure it out quick, or you might melt a piston.
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EODTECH*
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PostPosted: Tue Jun 01, 2021 7:50 pm    Post subject: Re: Knocking on hard acceleration and dieseling at shut off Reply with quote

Thanks. I'll start making calls tomorrow.
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PostPosted: Tue Jun 01, 2021 8:17 pm    Post subject: Re: Knocking on hard acceleration and dieseling at shut off Reply with quote

IMO, one or more of the cylinders have too high of a compression ratio for the fuel and load involved.

It's running on and getting worse because you're beating the shit out of it.
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EODTECH*
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PostPosted: Tue Jun 01, 2021 10:15 pm    Post subject: Re: Knocking on hard acceleration and dieseling at shut off Reply with quote

goober wrote:
IMO, one or more of the cylinders have too high of a compression ratio for the fuel and load involved.

It's running on and getting worse because you're beating the shit out of it.


Beatin the shit out of it? I'm driving it like normal.
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PostPosted: Tue Jun 01, 2021 10:43 pm    Post subject: Re: Knocking on hard acceleration and dieseling at shut off Reply with quote

Pruneman99 wrote:
You need to recheck your timing. Sounds like it's too far advanced. Also this will make the engine run hot, and diesel.

You really need to figure it out quick, or you might melt a piston.


This is where I’d start trouble shooting.
Where is the timing set? How was the timing set?
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EODTECH*
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PostPosted: Tue Jun 01, 2021 10:57 pm    Post subject: Re: Knocking on hard acceleration and dieseling at shut off Reply with quote

Xevin wrote:
Pruneman99 wrote:
You need to recheck your timing. Sounds like it's too far advanced. Also this will make the engine run hot, and diesel.

You really need to figure it out quick, or you might melt a piston.


This is where I’d start trouble shooting.
Where is the timing set? How was the timing set?


I can't answer that. I left it all in the hands of my tech. He said he would make any adjustments as time went by....but he retired and moved on.
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PostPosted: Tue Jun 01, 2021 11:23 pm    Post subject: Re: Knocking on hard acceleration and dieseling at shut off Reply with quote

How many miles on engine since you rolled it from your “Tech”. Did the tech just do a 20 minute break-in technique on an engine stand and stab it in your car? Expecting you to drive it under load in different load conditions? Do you have the ability to adjust (check ) valve lash? Lots of oil changes and tune up procedures in a new engine within the first 1000 miles in my opinion.
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EODTECH*
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PostPosted: Wed Jun 02, 2021 8:02 am    Post subject: Re: Knocking on hard acceleration and dieseling at shut off Reply with quote

Less than 300 miles driven since the swap from the 1200 to the 1600. He/we followed the engine rebuilders directions to the T. 3 oil changes. First for initial firing up, then a flush, another after 300 miles and just recently the 3rd oil change.
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PostPosted: Wed Jun 02, 2021 8:03 am    Post subject: Re: Knocking on hard acceleration and dieseling at shut off Reply with quote

I'd agree it sounds like the engine may have the timing too advanced or you may need to go up in octane rating on the fuel you're using, but there may be other issues at hand.

Pull the spark plugs and post a clear well lit close up picture of the electrode end of of them. You may have carbon fouling which can lead to hot spots and the resulting pinging and run on -- if this is the case, adjustments to the carburetor may be necessary, or possibly a change of plug heat range may be required.

Run through a standard tune up procedure of valve adjustment, spark plug gap, verify choke adjustment, adjust dwell (if running points), timing set, and warm idle speed and mixture set. If you aren't familiar with how to perform these tasks, I would highly advise you learn. If you don't have at least one repair manual then you really need to get one, but in the mean time you can read through some of the materials available in the Technical Instructions Manuals section located here: https://www.thesamba.com/vw/archives/info/. There's no wizardry involved in basic maintenance of these cars -- it's so easy, even a crew chief can do it Wink .
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EODTECH*
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PostPosted: Wed Jun 02, 2021 5:41 pm    Post subject: Re: Knocking on hard acceleration and dieseling at shut off Reply with quote

Understood. I find Muirs' book a total let down. It's got language in it from the '70s I don't understand. And for a lot of things, he says...."better off just taking it to the garage." I know there are way better manuals out there. I just haven't bought them yet.

However, it seems what you suggested might be wrong in the first sentence...IS what was wrong. I found a new tech (40 years) working on all air cooled's....and his first question was "What octane are you running?" I told him, "of course" I'm running the cheap stuff...87. He said he'd bet a months wages that that's the problem right there. Add octane boost immediately. I did that...cannot re-create the knocking and definitely does not diesel anymore.

He also adjusted timing/idle. It is now @ 8 deg BTDC. Seems to fire up, accelerate and idle like factory. (According to him.) And it sounds good to me...so far. He told me tho...the keep an ear on it. There might be other issues.
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PostPosted: Wed Jun 02, 2021 6:32 pm    Post subject: Re: Knocking on hard acceleration and dieseling at shut off Reply with quote

The timing would be the fix, not the octane boost.

Unless your engine was built by a PO with high compression, it'll run just fine on regular 87 (in the USA) gas.

What you often get is some knucklehead mechanic who tries to time the thing by ear rather than by using a light and that winds up with it being too advanced and then you're on the road to meltdown.
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EODTECH*
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PostPosted: Sun Jun 13, 2021 1:50 am    Post subject: Re: Knocking on hard acceleration and dieseling at shut off Reply with quote

See below.

Last edited by EODTECH* on Sun Jun 13, 2021 2:04 am; edited 1 time in total
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PostPosted: Sun Jun 13, 2021 2:01 am    Post subject: Re: Knocking on hard acceleration and dieseling at shut off Reply with quote

So...he did properly adjust the timing as I said to 8 deg BTDC. Dieseling completely gone.


Here's what the engine builder sent me.

"The piston size is 87mm which makes the displacement 1641cc.

The cam shaft is an Engle W90 which is the closest spec cam we can find compared to the original VW one.

The distributor is the Pertronix with vacuum advance, not mechanical like the 009. "

There is still a 'slight" knocking issue whenever I'm accelerating uphill in 3rd and 4th. If I ease up just a bit on the accelerator...it doesn't knock. I told my tech this...and he says it should never knock under any circumstance if the carb, timing etc...is all set properly. He's now convinced that it's a combination of issues w/the carb. Not jetted properly, choke issues and he's not impressed w/the aftermarket unit. He's going to rebuild a Solex 34 PICT and replace the current carb.
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PostPosted: Sun Jun 13, 2021 2:17 am    Post subject: Re: Knocking on hard acceleration and dieseling at shut off Reply with quote

In the meantime...............BUY AND USE THE HIGHEST OCTANE GASOLINE AVAILABLE IN YOUR AREA, or stop driving the car untill ALL the issues with your motor are resolved. Unless you wish to destroy your engine. Knocking is a very bad thing anytime, to any degree!
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PostPosted: Sun Jun 13, 2021 8:34 am    Post subject: Re: Knocking on hard acceleration and dieseling at shut off Reply with quote

What compression ratio did the builder use? How much TOTAL advance is that distributor making?

The human ear cannot hear knocking at low levels, so if you are still hearing it on occasion, it is a lot worse than what you are hearing. Modern knock sensors can detect it way before a human can hear it.
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PostPosted: Sun Jun 13, 2021 9:12 am    Post subject: Re: Knocking on hard acceleration and dieseling at shut off Reply with quote

removed my post
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EODTECH*
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PostPosted: Mon Jun 14, 2021 8:24 pm    Post subject: Re: Knocking on hard acceleration and dieseling at shut off Reply with quote

Q-Dog wrote:
What compression ratio did the builder use? How much TOTAL advance is that distributor making?

The human ear cannot hear knocking at low levels, so if you are still hearing it on occasion, it is a lot worse than what you are hearing. Modern knock sensors can detect it way before a human can hear it.


Hmmmm...ok thanks. Here's an answer from the builder.

"On the timing for a stock engine typical start point is around 6 degrees advanced with a maximum of 28 to 30 degrees."

As I said in my last post...my tech is rebuilding a Solex 34PICT 3 to replace the aftermarket unit the rebuilder used. He says the knocking issue is something to do w/that carb. Again, He already reset the timing to what he's being doing for 1641 DP for decades. This massively reduced the knocking and ended the dieseling immediately. And since, I've been running only Chevron 94 octane.
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PostPosted: Mon Jun 14, 2021 8:30 pm    Post subject: Re: Knocking on hard acceleration and dieseling at shut off Reply with quote

EODTECH* wrote:
Q-Dog wrote:
What compression ratio did the builder use? How much TOTAL advance is that distributor making?

The human ear cannot hear knocking at low levels, so if you are still hearing it on occasion, it is a lot worse than what you are hearing. Modern knock sensors can detect it way before a human can hear it.


Hmmmm...ok thanks. Here's an answer from the builder.

"On the timing for a stock engine typical start point is around 6 degrees advanced with a maximum of 28 to 30 degrees."

As I said in my last post...my tech is rebuilding a Solex 34PICT 3 to replace the aftermarket unit the rebuilder used. He says the knocking issue is something to do w/that carb. Again, He already reset the timing to what he's being doing for 1641 DP for decades. This massively reduced the knocking and ended the dieseling immediately. And since, I've been running only Chevron 94 octane.
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