Jump to content
  • When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.


brianstj

Recommended Posts

I have searched through the forum to find information on potential causes dieseling, and I am still a little unsure what is really going to solve this.

I am running a 38/38 DGES, Bosch blue coil (no ballast resistor), Pertronix, NGK BPR6ES plugs, distributor with vacuum module, timing was set to 25 degrees advance at 2700 RPM (per underhod sticker) with the vacuum line disconnected and plugged, and 91 octane Chevron gas.

The car used to always diesel when I shut it off, and I attributed this to the carb not having an idle shut off solenoid + "some other issue"

I just did a fresh rebuild, and the only things that have changed are a 292 cam, 9.5:1 pistons, 0.040 overbore. With the timing set statically at the first mark before TDC I did the 20 min. 2000 RPM cam break in run, and upon shutting down I got the old familiar dieseling.

So we have Fuel+Air+Ignition happening when the key is shut off. Fuel and air are coming from the carb, so the question becomes, where is the ignition coming from? Hot plugs? Fuel combusting under compression? I believe I have the right heat range plugs and sufficient fuel octane.

Many of the posts I have read point at timing as the major culprit - but what I don't understand is how this effects things once the ignition is cut off? Can anyone explain?

I found one post pointing at Chevron gas - any validity to this?

I thought about the option of an idle shut off solenoid, but then I am not sure if the 38/38 can run them and if this would just be masking some other issue?

I also got some advise to lower the idle speed, and perhaps even the floats in the carb. I still need to do a once over on everything, set the timing while the car is running, and readjust the idle speed before I take the new engine out for its first drive. I just have a bad feeling that this is still going to be an issue. I could always just keep letting the clutch out after I shut it off to prevent tohe engine from turning, but if there is some issue going on that can be fixed, I would prefer to fix it.

Brian

'72 2002

Brian

'72 2002

'91 325i

Link to comment
Share on other sites

Guest Anonymous

If you look at the 32/36 the idle solenoid is in the primary idle jet location. Is it possible to remove the primary idle jet from the '38 and replace it with a idle solenoid from a '32/36 wired into the ignition? Maybe if you posted a pix of the primary idle jet of the 38/38 someone would have a suggestion. Maybe there is another solenoid that fits. It sounds like you need the fuel cut at the carb to get control of the idle run-off.

Link to comment
Share on other sites

Guest Anonymous

You answered your own question: Dieseling. The fact that your engine will run-on with the ignition off or even run backwards doesn't mean it will do that indefinitely or it will produce enough power to propel the car. Dieseling does not require a separate ignition source (excepting glowplugs).

Fuel, Air, Pressure, Heat are all fundamentals for combustion. The ignition source does not have to be from the battery or coil. If you had an idle shut off that worked, there would not be any fuel to permit any running on. The fact that you have a wilder camshaft suggests there is enough valve overlap to permit vaporized fuel to ignite even between cylinders - for any number of reasons. These including residual carbon deposits, sparking plugs that are too hot (retaining too much heat), too much compression, lower octane fuel (I read where you used higher octane, and maybe it is). The same suggestions apply to related conditions such as preignition and detonation.

It is an unlikely possibility that when you turn the ignition key off, there is still enough residual voltage supplied by the turning alternator that might trickle to the coil and voila - the engine continues to fire. This may be the fault of a number of things including a poorly wired ignition switch and starting system.

If you have a lot of crap in your combustion chambers, that may be a source of ignition (without the key on) you might want to look into one of those products that are advertised for that purpose. Some people in Northern California swear by them. Other people swear at them! lol

Link to comment
Share on other sites

you'll need to get the carb and timing sorted for sure.

either one of those (or combo of both) can cause run-on/dieseling.

recheck your plugs while your under the hood. make sure the gap is correct and look for evidence of "hot-spots" or charred carbon etc. a plug that is just a bit too "hot" can also cause run-on eve tho' there may not be any evidence of this "after-glow" in such a short period. (over time, it will charr and may even break apart).

being a brand-spankin'-new motor i'd say that things are just a bit "warmer" than they would be normally.

you may find that you'll have to run cooler plugs and pull back on the timing until the motor has a few miles on it.

anyway, gotta get that carb set correctly not matter what so....start there...set timing correctly...recheck plugs (try cooler ones)...etc.

Jack be nimble - Jack be quick

If Jack isn't, he's gonna get his ass burned!

Link to comment
Share on other sites

I just installed a new engine similiar to yours (built by Eric Kerman, but w/ stock cam, same pistons as you) and I've always used an idle solenoid on my 38/38 to prevent the dieseling problem, as I live in So.AZ and the summers get pretty hot, which can encourage that problem. Follow the suggestions above and, if that doesn't solve the problem, try installing an idle solenoid. The same solenoid works on both 32/36 and 38/38 Webers.

Bob Napier

Link to comment
Share on other sites

ignition timing - another method and setting from your \present course

float level

idle mixture

idle speed

too hot spark plug heat range

sharp edges in the combustion chamber/head

major cooling system issue

fuel shut-off solenoid last choice.

'86 R65 650cc #6128390 22,000m
'64 R27 250cc #383851 18,000m
'11 FORD Transit #T058971 28,000m "Truckette"
'13 500 ABARTH #DT600282 6,666m "TAZIO"

Link to comment
Share on other sites

  • 10 years later...

This is an ancient thread - but I recently installed an idle cutoff solenoid on my 38/38 Weber to help with dieseling in hot weather. It didn't seem to work at all. I triple checked the pin retracted when it got power off the ignition coil, but when I placed the jet and holder into the carb, it never idled. Replacing the stock idle jet holder allowed it to run. When I examined the thread length of the screw vs. the idle solenoid, the solenoid seemed a good 1/8 shorter. The pin also seemed to block the two side holes on the idle jet. Strange. 

Any thoughts to getting this to work? Perhaps I just got a junk aftermarket idle solenoid?

Link to comment
Share on other sites

On 7/7/2019 at 1:21 PM, silasmoon said:

This is an ancient thread - but I recently installed an idle cutoff solenoid on my 38/38 Weber to help with dieseling in hot weather. It didn't seem to work at all. I triple checked the pin retracted when it got power off the ignition coil, but when I placed the jet and holder into the carb, it never idled. Replacing the stock idle jet holder allowed it to run. When I examined the thread length of the screw vs. the idle solenoid, the solenoid seemed a good 1/8 shorter. The pin also seemed to block the two side holes on the idle jet. Strange. 

Any thoughts to getting this to work? Perhaps I just got a junk aftermarket idle solenoid?

 

Isn’t the solenoid valve specifically designed to be used on the 32/36 or other progressive choke carb? The 38/38 has two idle jets and they would both need to be isolated as they are both in play at idle.  If you only isolate one, the engine will idle on the other. 

rtheriaque wrote:

Carbs: They're necessary and barely controlled fuel leaks that sometimes match the air passing through them.

My build blog:http://www.bmw2002faq.com/blog/163-simeons-blog/

Link to comment
Share on other sites

On 7/6/2019 at 8:21 PM, silasmoon said:

 

when I placed the jet and holder into the carb, it never idled. Replacing the stock idle jet holder allowed it to run. When I examined the thread length of the screw vs. the idle solenoid, the solenoid seemed a good 1/8 shorter. The pin also seemed to block the two side holes on the idle jet.

 

Per your observation, not all idle solenoids are identical and interchangeable, nor are all Weber twin-choke downdraft carburetors identical.  For example, idle solenoids found on some OEM Weber carburetors used on '70s Italian cars may not readily interchange with some aftermarket Webers.  Then there are examples of manufacturing "issues" that could result in any mass produced product.  I recall someone showing me an incompletely machined jet that had to be more fully drilled to make it functional.  In other words, you could have the wrong part or even a defectively produced right part.

 

This is probably old news, but consider re-reading this thread. https://www.bmw2002faq.com/forums/topic/165520-weber-3838-best-lean-idle-procedure/

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • BMW Neue Klasse - a birth of a Sports Sedan

    BMW Neue Klasse - a birth of a Sports Sedan

    Unveiling of the Neue Klasse Unveiled in 1961, BMW 1500 sedan was a revolutionary concept at the outset of the '60s. No tail fins or chrome fountains. Instead, what you got was understated and elegant, in a modern sense, exciting to drive as nearly any sports car, and yet still comfortable for four.   The elegant little sedan was an instant sensation. In the 1500, BMW not only found the long-term solution to its dire business straits but, more importantly, created an entirely new
    History of the BMW 2002 and the 02 Series

    History of the BMW 2002 and the 02 Series

    In 1966, BMW was practically unknown in the US unless you were a touring motorcycle enthusiast or had seen an Isetta given away on a quiz show.  BMW’s sales in the US that year were just 1253 cars.  Then BMW 1600-2 came to America’s shores, tripling US sales to 4564 the following year, boosted by favorable articles in the Buff Books. Car and Driver called it “the best $2500 sedan anywhere.”  Road & Track’s road test was equally enthusiastic.  Then, BMW took a cue from American manufacturers,
    The BMW 2002 Production Run

    The BMW 2002 Production Run

    BMW 02 series are like the original Volkswagen Beetles in one way (besides both being German classic cars)—throughout their long production, they all essentially look alike—at least to the uninitiated:  small, boxy, rear-wheel drive, two-door sedan.  Aficionados know better.   Not only were there three other body styles—none, unfortunately, exported to the US—but there were some significant visual and mechanical changes over their eleven-year production run.   I’ve extracted t
  • Upcoming Events

  • Supporting Vendors

×
×
  • Create New...