Weber DCNF problem/strange symptoms when starting warm engine

[Pedalpusher]

Hi,

Been frustrated to run into this symptom every time I start warm engine. And I've done and checked a lot of things trying to cure it, no dice! Help appreciated!

1955cc
C55 cam cr10.6:1
DCNF 42's 34 vents

Starts normal when engine is cold. minute or so to keep it running with some helping to run with gas foot. AFR normal all the way.
Then when engine is warm and I stop to crosery store for example and it sits 15-20 mins outside and I start it again. It runs like crap, AFR shows very very lean, like 19. And it doesn't run without keep it running with gas foot. It stays like that 5-10mins and then back to normal.
If I start it right away after shutting it down when warm, no problemo! Only when it sits some time.

I've checked and done float level as per weber's instructions. Fuel pressure 0,2 bar or 3psi. The carbs have been cleaned thoroughly. I've rebuilt the engine year ago in winter time, changing all the bearings, pistons, cylinders, cam and some mods to heads. Didn't notice any mechanical problems which could be the answer. And same symptom continues after the rebuilt.

I really like the smooth behaviour of the DCNF's but it's getting so annoying that I'm this close to change to other carbs during this winter! I wouldn't want but I'm frustrated.

-Mikko-

[alex d]

Sounds like you are boiling the gas, do you run phenolic spacers? make sure the fuel lines are not touching anything hot (exhaust, oil lines, etc)

[Pedalpusher]

That is one possibility!

I've checked the temperature of the fuel bowl sides from the carbs with infra red gauge after shutting down and letting it sit for a while like it would in the parking lot when running like crap. Nothing too serious there...

I have phenolic spacers between carbs and intakes. I need to change the fuel line routing still and hope it's doing some kind of vapor lock in the fuel line before carbs and that is now stopping fuel to run freely to the carbs.

[Martin S.]

It was kind of the opposite problem for me when shutting down a hot engine and re-starting after gassing up or similar.
My engine would act flooded and needed lots of cranking with the pedal all the way down and eventually it would start.
I figured it is because I was using the regular cheapo dome-top fuel pump. The pumps without the shut-off valve in them tend to keep residual pressure in the line to the carb and end up dripping gas into the carb while it sits there after driving. If you wait long enough the gas evaporates.
Originally, VW had installed an external shut off valve that went inline with the supply line and the delivery line to the pump, but these valves are long gone and N/A.
I'm not saying that this is your problem, but your problem made me think of this one.
Here's a new pump that I believe incorporates the shut off valve as the original generator style ones had.

https://www.cip1.ca/ProductDetails.asp?ProductCode=VWC%2D113%2D127%2D025%2DDOE

[Jim Ratto]

Hi,

Been frustrated to run into this symptom every time I start warm engine. And I've done and checked a lot of things trying to cure it, no dice! Help appreciated!

1955cc
C55 cam cr10.6:1
DCNF 42's 34 vents

Starts normal when engine is cold. minute or so to keep it running with some helping to run with gas foot. AFR normal all the way.
Then when engine is warm and I stop to crosery store for example and it sits 15-20 mins outside and I start it again. It runs like crap, AFR shows very very lean, like 19. And it doesn't run without keep it running with gas foot. It stays like that 5-10mins and then back to normal.
If I start it right away after shutting it down when warm, no problemo! Only when it sits some time.

I've checked and done float level as per weber's instructions. Fuel pressure 0,2 bar or 3psi. The carbs have been cleaned thoroughly. I've rebuilt the engine year ago in winter time, changing all the bearings, pistons, cylinders, cam and some mods to heads. Didn't notice any mechanical problems which could be the answer. And same symptom continues after the rebuilt.

I really like the smooth behaviour of the DCNF's but it's getting so annoying that I'm this close to change to other carbs during this winter! I wouldn't want but I'm frustrated.

-Mikko-

I've been fighting this with my 48 IDA Webers and California pump gas for about 15 years, same exact scenario and symptoms. Like you, I've tried everything- except I haven't gotten around to fitting a phenolic spacer set between carbs and intakes.
My friend experienced this same issue with his 40DCNF (and his BSA motorcycle too!) and installed phenolic spacers, and it helped (delay) but not cure the problem.
As the carbs sit over the warm engine after shutoff, they absorb the heat rising off heads. The volatility of CA pump gas (not sure what you run) is super susceptible to "boiling off", especially when we're saddled with the "winter blend" fuel (higher level of butane in it) and hot, low humidity Southern CA days in the winter (yes it happens all the time). The actual fuel in my carburetors doesn't evaporate, (I've parked it in my garage, hot, and waited and then: a) stuck Q tip sticks down jet stack holes to measure "fuel line" from top of carb (it was where it should be) and b) removed carb tops.
What I think is happening, is the fuel is boiling off the most volatile components of the fuel. Leaving stuff that doesn't want to burn when metered by jetting we've found to run best under "normal operating conditions"- Why do I think this? Because upon warm/hot fire up, I can force the motor to behave by opening the mixture screws up an additional 1/4-1/2 turn, which runs fine, until the carburetors return to a normal temp. Then it runs pig rich.
I've tried the urethane/alloy spacers that are out there.... the urethane gaskets don't like fuel. They swell up big time and end up impeding the idle orifice adjacent to butterfly on 48. Car will run horribly once that happens.
Good luck. If you come up with a solid, repeatable cure, let me know.
Before 2002-3, this was NEVER an issue for my Webers. (I've talked to other car enthusiasts, all kinds, running both down-draft and side draft Webers, in IR set up and a lot of guys have been battling same issue).

[Martin S.]

That makes sense in theory, since fuel injection wouldn't have that problem.
Did you try testing the idea by using different gasoline?

[Jim Ratto]

Customers of mine with CIS injected cars are having the same issues.

[Jim Ratto]

Hi,

Been frustrated to run into this symptom every time I start warm engine. And I've done and checked a lot of things trying to cure it, no dice! Help appreciated!

1955cc
C55 cam cr10.6:1
DCNF 42's 34 vents

Starts normal when engine is cold. minute or so to keep it running with some helping to run with gas foot. AFR normal all the way.
Then when engine is warm and I stop to crosery store for example and it sits 15-20 mins outside and I start it again. It runs like crap, AFR shows very very lean, like 19. And it doesn't run without keep it running with gas foot. It stays like that 5-10mins and then back to normal.
If I start it right away after shutting it down when warm, no problemo! Only when it sits some time.

I've checked and done float level as per weber's instructions. Fuel pressure 0,2 bar or 3psi. The carbs have been cleaned thoroughly. I've rebuilt the engine year ago in winter time, changing all the bearings, pistons, cylinders, cam and some mods to heads. Didn't notice any mechanical problems which could be the answer. And same symptom continues after the rebuilt.

I really like the smooth behaviour of the DCNF's but it's getting so annoying that I'm this close to change to other carbs during this winter! I wouldn't want but I'm frustrated.

-Mikko-

By the way, the Cobra/GT40/Ford guys running Webers are dealing with this too. It's funny, very few VW guys running 48's have talked about this problem. I know a few guys, but I don't hear about his as often as I thought I would. I've talked to a few 427FE guys running 48's and they have all battled this. Some have resorted to going to a non-deadhead fuel system (with return) in addition to phenolic blocks under carbs- some have cured it, some have merely stalled it. Some guys have fabbed copies of the old GT40 style cold air cookie sheet in an effort to overcome this.
I've talked to guys running Lamborghini 400GT-style V12 (the version that had 40DCOE's feeding intake in between cam boxes on Espada, Jarama or 45DCOE on Countach), they all are dealing with it. The Miura guys too, in addition to the usual carb-fire nightmare they dread on that car.
Some of my 911 customers are reporting big issues after they track day @ Buttonwillow etc, they come in, let the car sit, then it limps along, barely idling when restarted hot.
I noticed this first while visiting Southern CA in my car in early 2000's. Went back to Bay Area (Northern CA) and it went away, for a few months. Then it came back. Then I moved to Southern CA in 2005, my car in early 2006. Have dealt with it ever since.
Again, good luck and keep me posted if you find a way to beat it.

[Martin S.]

I don't know about in your area, but there have been stories about unscrupulous gas station owners deliberately running a water hose into their gas tanks to make more money. I was told to run premium when someone thought this might have been a problem for me, with the thought that premium comes from a different tank and less likely to be tampered with.
I know this isn't your problem since it has gone on so long but made me think you might try a completely different gas.

[Pedalpusher]

Yes, real gathered and experienced data. Thank you guys big time!

I forgot to mention, I also took off the air cleaners and watched down the barrels after shutting down. Many times during 15 mins. And it didn't drip any gas at all. So that's checked off.

I'm really leaning into Jim's theory about the gas losting some of it's components. I can't really think of any other explanation cause I've checked everything (I think).
And after the bad fuel in the bowls has been replaced with fresh one, gradually, it starts to run good again.

I'm going to re-route the lines this winter to be sure about that also. After that I have to think what I can do about the fuel. Maybe nothing...

I have thought about the return line. On a 61 bug that I'm building on the side of my current running 64, I have planned to do this cause it's so easy in this stage when everything is apart and chance of running efi sometime.

I run the best fuel I can get in here from the pumps. It's Shell 98+. But it doesn't matter, i Have tried others.

I'll definetly keep you posted IF I ever find a cure on this.

[Jim Ratto]

I forgot to mention, I also took off the air cleaners and watched down the barrels after shutting down. Many times during 15 mins. And it didn't drip any gas at all. So that's checked off.

And after the bad fuel in the bowls has been replaced with fresh one, gradually, it starts to run good again.

I'm going to re-route the lines this winter to be sure about that also. After that I have to think what I can do about the fuel. Maybe nothing...

Mine doesn't drip either. And exact same as you- if I run the car after hot startup and drive for 10-15 minutes, eventually it goes away. Thing is, I think I can empty and refresh bowls in less than that time- so that makes me wonder if the hot surface temp is cooking off fresh fuel as it streams in from needle valve (?).
In my head, I always get mad enough at this to almost visualize the fuel level somehow being too low in bowl to allow proper metering through idle jets.
But there are so many variables as to what is really causing it. And I've gone up and down about 0.5-0.75mm with float settings, with no change.
Maybe the intake runners are the culprit, maybe the surface of the intake runners and temp in the runner is hot enough to flash off the fuel before it gets to combustion chamber, etc?
As you can see, lots of frustration and time spent trying to fix this.

[Martin S.]

It's hard to believe that gas sitting for a few minutes loses so much volatility that it can't start and run your motor.
Spray some quick start spray down there and see what happens 

[Jim Ratto]

It's hard to believe that gas sitting for a few minutes loses so much volatility that it can't start and run your motor.
Spray some quick start spray down there and see what happens 

In my case, it's 20min + and the motor will start and run. It does not run as well as I prefer. 
If I shut it off and restart it in "a few minutes", there are no ill effects.

[Martin S.]

It sounds loaded up, but you are convinced it is lean due to a lack of fuel or good cool fuel that's the problem?
What a mystery!
Mikko says his AFR gauge reads super lean at that point, like pinned. I would try installing an AFR gauge and see if yours reads the same way.

[Jim Ratto]

It sounds loaded up, but you are convinced it is lean due to a lack of fuel or good cool fuel that's the problem?
What a mystery!
Mikko says his AFR gauge reads super lean at that point, like pinned. I would try installing an AFR gauge and see if yours reads the same way.

It's lean. Yes have metered it with an AFR tool. The mixture screws will tell me that without having to string up the AFR tool.

[Martin S.]

I get it. That's odd that an engine needs a choke when it's hot. 
With my kads in the winter I used the poor man's choke which is pumping the gas pedal before cranking.

[Martin S.]

Mikko, does it run with the same lameness whether you pump some gas as you start it? How do you start it warm, no gas, opened up or

[Torben Alstrup]

Flooding due to a set of leaky inlet valves could be the issue.
It can also be a coil that is not up to its best. Try and replace the coil with more discharge capacity. Also check that the coil get enough voltage from the key. Sometimes they don´t. Last, check that the electroni´c hall unit is properly secured in the distributor.

T

[Pedalpusher]

Mine doesn't drip either. And exact same as you- if I run the car after hot startup and drive for 10-15 minutes, eventually it goes away. Thing is, I think I can empty and refresh bowls in less than that time- so that makes me wonder if the hot surface temp is cooking off fresh fuel as it streams in from needle valve (?).
In my head, I always get mad enough at this to almost visualize the fuel level somehow being too low in bowl to allow proper metering through idle jets.
But there are so many variables as to what is really causing it. And I've gone up and down about 0.5-0.75mm with float settings, with no change.
Maybe the intake runners are the culprit, maybe the surface of the intake runners and temp in the runner is hot enough to flash off the fuel before it gets to combustion chamber, etc?
As you can see, lots of frustration and time spent trying to fix this.

Yes it could be also the hot surfaces that keep cooking off the fuel until the temps settle down when air starts moving throuh the intakes. Intakes get pretty damn hot after shutting off. When it gets to you, it's hard to just let it go in your head until you've figured it out, I hear you Jim!

[Pedalpusher]

Mikko, does it run with the same lameness whether you pump some gas as you start it? How do you start it warm, no gas, opened up or

I start it with butterflys half opened when hot. Then keep flickering the gas to keep it running on idle. It runs on idle if I don't flicker the gas pedal but barely, sounds like an old harley. Just keeping on.

When cold, I start it with like 3 pumps on gas.

It's not the starting up that's bothering me. It always starts. But the running after that is the problem.

[Pedalpusher]

Flooding due to a set of leaky inlet valves could be the issue.
It can also be a coil that is not up to its best. Try and replace the coil with more discharge capacity. Also check that the coil get enough voltage from the key. Sometimes they don´t. Last, check that the electroni´c hall unit is properly secured in the distributor.

T

Thanks for chipping in. More options I haven't thought about.

Wouldn't leaky inlet valves cause the fuel to run in the throats and intakes? It's not dripping fuel after shutting off so that's maybe not the case.

Coil would be easy to try another one. And check the hall unit.

[modnrod]

I have similar problems here with our local RON98. It gets hot enough here so from lunchtime onwards in summer the pumps at the servo get vapour lock and won't pump fuel until they cool down in the late afternoon.
I've had the same problem with my bikes as well.

Hence why my new engine is set up to run on RON91 (much lower volatility).

Problem fixed.

[nicolas]

i know it has been said, but the plates between the carbs and manifolds do help a bit. the fuel boils out… i have it with my type3 as well. every time i run it and let it warm up and then want to restart it after a few minutes it has no or not enough fuel.
i have type3 manifolds which make it even worse.

so here is what i did/do

*use the spacer plates (got them from an english online store that also sells jets if memory serves me right)
*use an electic pump. it helps to prime the system again. i drained quite a few batteries when i had the mechanical pump. i don't like it as a stock one works just fine, but the electric one is just what it takes to start the car in a few seconds.
*pump your gaspedal just a few times (i do it 7-8 times) and then start.
*once you start it, play the pedal a bit to get the roughspots out.
*then hold the trottle in a bit to let the engine get to breath and get the rpms up by itself.
* if it is at this point it will idle at 750rpms and not stop or stotter anymore

this seems to be a long list, but now it just takes me no more than half a minute to start it and another half to have it rev correctly. just enough to give the others the headstart they need

happy driving 

[Neil Davies]

*use the spacer plates (got them from an english online store that also sells jets if memory serves me right)

Eurocarb?

[Pedalpusher]

Yes I think nicolas is talking about the Eurocarb spacers. I have exactly those too. And run electric pump.
It's allready season over here in Finland, winter time and snow. So further experimenting is gonna happen in spring time.

But my starting procedure is allready like you are doing it nicolas! Minus pumping the gaspedal when hot, I'll have to try that.

[nicolas]

*use the spacer plates (got them from an english online store that also sells jets if memory serves me right)

Eurocarb?

yes! correct. that is where i got mine.

Yes I think nicolas is talking about the Eurocarb spacers. I have exactly those too. And run electric pump.
It's allready season over here in Finland, winter time and snow. So further experimenting is gonna happen in spring time.

But my starting procedure is allready like you are doing it nicolas! Minus pumping the gaspedal when hot, I'll have to try that.

isn't winter time when you scandinavians always gather around a big fire to cook up crazy ideas about engines and build it? 

[Pedalpusher]

isn't winter time when you scandinavians always gather around a big fire to cook up crazy ideas about engines and build it? 

This pretty much sums it up! 

Actually i'm planning on changing my 010 bosch to an ignition through ECU. Staying with carbs still. Maybe get it to behave a little better and I've wanted to try that some time. And why not?
Also been starting on my 2276 FK89 build little by little and on a homestrech with metal finishing my '61 this winter after years spent with it.

[Martin S.]

If you could rig up a manual choke that would be one way to try richer jetting 

[Pedalpusher]

Yes that could work 

[JeeWee]

Hi there, you might want to try this: http://www.123ignition.nl/products.phtml instead of ECU
next to that, how much volt is on your coil? I had an issue with a weak spark, due to 11,9 volt on my coil. But should not really matter when cold or hot... and what spark plugs are you running?

I like the fact you are running DCNF's, got one of those in my collection but currently want to run my Zenith NDIX carbs.