Help! Weber 40DCOE throttle problems Up-date

[scubabatdan]

Help! Weber 40DCOE throttle problems Update

I am still plagued with carb problems, specifically the stomp test. My 1275cc still bogs down when you stomp on the gas. If you ease into it gently it will go to red line no problem. The engine is stock and I have replaced all the jets, emulsion tubes, venturis and aux venturis to what other people are running on a stock 1275cc engine from https://www.teglerizer.com/dcoe/dcoepictures.htm

I have changed my 40DCOE carb to the following:
30 = choke size Change to 34
4.5 = auxiliary venturi Change to 3.5
125 = main jet Change to 145
170 = air corrector jet Change to 190
45/F9 = idle jet Change to 50/F9
F11 = emulsion tubes Changed to F2
40 = accelerator pump jet
50 = accelerator pump discharge
Added .95" low profile air horns

This is mounted on a 73 Midget with a 1275cc stock engine.
Timing has been triple checked, vacuum is good no leaks in the diaphragm or hose, and it runs smoother when the vacuum advance is connected. Plugs have been change for stock plugs not platinum’s. When I checked the new plugs after a 10 min rev run they were black indicating it is running rich. The engine will start effortlessly with a bump of the key and idles at 950 rpm.

Needless to say I have dumped a chunk of money into this with little to no results. Any other ideas, anyone?!?
Thanks! Ðan

 

[Trevor Jessie]

Re: Help! Weber 40DCOE throttle problems Update

Ehh... switch to a DCOE 45

 

[Morris]

Re: Help! Weber 40DCOE throttle problems Update

Is there some way to temporarily disable the accelerator pump and do a test?

Have you checked the float level/function?

 

[texas_bugeye]

Re: Help! Weber 40DCOE throttle problems Update

well I have a DGV so its not the same type of carb but I had the same problem.It would bog when going from the idle jet into the main jet turned out to be the e tube was to small. went rich for a second but if you eased into it it would not showup.Try going up a size on the etube and see if that does it. It worked for me. Weber.. love and hate them. does it richen up on a steep uphill when at idle? Then its a dead give away on the float level.

 

[racingenglishcars]

Re: Help! Weber 40DCOE throttle problems Update

I would think the 45 DCOE could have even more tendancy to bogging on sudden acceleration. simply because it would allow more sudden air rush.

The problem of bogging comes from the well known problem of steady state running going into a transient state of running. With a carb, the transient allows for a large in-rush of air while the fuel takes a bit longer to come in. (Air is lighter and easier to accelerate than the fuel is.) This has been such a problem that in the past 40 years large auto manufacturers have spent billions of dollars on trying to solve the problem. (The inverse problem is a closing transient, which though doesn't cause bogging, it causes excessively rich mixture and misfiring. These causing excess HC and CO). Even newer cars have the "drive by wire" throttles which is there ONLY to reduce the effect of transients. Older fuel injected cars, like the Rabbit, with K-Jetronic used a very long inlet runner with the injectors mounted very close to the valve. That way the fuel and air arrived at the cylinder at about the same time.

In your case, the air is arriving at the inlet valve before the fuel does. This gives a lean mixture until enough fuel arrives to offset the air quantity. A further complication is that when fuel is injected by the accelerator pump, it doesn't just go into the cylinder. It hits the manifold wall and sticks there (along with the fuel from the idle and main jets). Only after the fuel film has built up to sufficient thickness that the air flow can pull the top of the fuel film into the cylinder, does the engine finally get something resembling a correct mixture, and runs well.

Yes I did a masters project in exactly this problem.

Bottom line.
1. Inject the accelerator pump fuel directly onto the inlet valve.
2. Really dump acceleration injection in. I mean like excessively overboard. (this will give other problems)
3. Reduce your transients (don't stomp on it)
4. Heat the inlet manifold to reduce the fuel film thickness.
5. Go to fuel injection.
6. There might also be a timing issue. Correct timing at low throttle and idle speed, and at low throttle and high speed are not the same as correct timing at high throttle and idle speed.

Just MHO

 

[Trevor Jessie]

Re: Help! Weber 40DCOE throttle problems Update

I guess my comment about the 45 DCOE should have been explained. I see mostly 45DCOE on 1275's, so I would assume that there would be more knowledge/experience to draw upon.

What type of manifold are you using?

 

[racingenglishcars]

Re: Help! Weber 40DCOE throttle problems Update

That is exactly true.
Most A-series go for the 45

The siamese ports cause pulsing which can draw on a larger carb than the 40. But unfortunately they all are prone to starving the center cylinders. The scatter cam helps this.

For this reason, I'm going with a pair of HS4s until I can get a Megasquirt installed.

 

[scubabatdan]

Re: Help! Weber 40DCOE throttle problems Update

Well I have seen in that the 40 was used in smaller cc engines and the 45 in the 1500+ cc engines. It came with the car and just trying to go with what I have LOL
Thanks
Ðan

 

[scubabatdan]

Re: Help! Weber 40DCOE throttle problems Update

Havent tried disableing the accel pumps,,, hmmmmmm and the float is at 8.5mm touching the float valve and 15mm at empty position.
Thanks
Ðan

 

[scubabatdan]

Re: Help! Weber 40DCOE throttle problems Update

A Masters, wow... ok well I just got off the phone with a weber tech and he said that the jets I switched to was good for a 1275cc engine but the emulsion tubes were wrong. The F2 and F11 are preety much the same and are for high end power and will bog at low end. He said the F16's were for the lower end and if I switch to them it WILL correct my problem. Sure ok 59.90 later and on Wendnesday when they get here we will see. He aslo added if it doesnt he wants to know and gave me his extension so we will see. I have checked the timing at 950 no vacum and at 2000 rpm vacum on. and it apears to be set correctly. Does prrrr at an idle and starts immediatly. We will see what the F11's do.
Thanks
Ðan

PS but I want to stomp on it /bcforum/images/%%GRAEMLIN_URL%%/smile.gif

 

[scubabatdan]

Re: Help! Weber 40DCOE throttle problems Update

Will have to check on the manifold when I get home, cant remember off the top of me head.
Ðan

 

[scubabatdan]

Re: Help! Weber 40DCOE throttle problems Update

Might have to look into the HS4s when I start on car #2 but this is for my wife and just needs to start and go. And be dependable. Have been thinking of electronic ignition for it any recommendations? Pros/Cons?
Thanks
Ðan

 

[texas_bugeye]

Re: Help! Weber 40DCOE throttle problems Update

Make sure you readjust the idle mixture after you change the E tube. Hope it works for you.

 

[DrEntropy]

Re: Help! Weber 40DCOE throttle problems Update

In your case, the air is arriving at the inlet valve before the fuel does. This gives a lean mixture until enough fuel arrives to offset the air quantity. A further complication is that when fuel is injected by the accelerator pump, it doesn't just go into the cylinder. It hits the manifold wall and sticks there (along with the fuel from the idle and main jets). Only after the fuel film has built up to sufficient thickness that the air flow can pull the top of the fuel film into the cylinder, does the engine finally get something resembling a correct mixture, and runs well.

The laminar flow issue...

Two suggestions: inlet runners are NOT supposed to be mirror smooth, that accentuates what Donn describes, and F16 E-tubes have a waay different progression characteristic than the F2/F11 ones.


And 45's aren't an answer. The same problem will still be there.

Factory Mini (Cooper S)1275's raced with two 45's bolted up, the center two throats feeding the manifold, one throat from each unit. The two outer throats were "disabled". Deep wallets!!

 

[texas_bugeye]

Re: Help! Weber 40DCOE throttle problems Update

Sort of like racing sailboats they sand theres bottoms with a really corse grit to forum a thin film of water adhision to make the boat more slippery.

 

[scubabatdan]

Re: Help! Weber 40DCOE throttle problems Update

Ok, the manifolds are Cannon 805 intake manifolds.Help any?
Ðan

 

[scubabatdan]

Re: Help! Weber 40DCOE throttle problems Update

Ok the inlet are far from smooth on the inside, looks like they were sand cast. Ju irritating, you go by what someone else has and it just does the same thing. Hope the F16's pan out /bcforum/images/%%GRAEMLIN_URL%%/smile.gif
Ðan

 

[scubabatdan]

Re: Help! Weber 40DCOE throttle problems Update

Hey I am not sanding anybody's bottom here so you forget that! /bcforum/images/%%GRAEMLIN_URL%%/smile.gif LOL
But I get the analogy /bcforum/images/%%GRAEMLIN_URL%%/smile.gif
Thanks
Ðan

 

[racingenglishcars]

Re: Help! Weber 40DCOE throttle problems Update

Factory Mini (Cooper S)1275's raced with two 45's bolted up, the center two throats feeding the manifold, one throat from each unit. The two outer throats were "disabled". Deep wallets!!

And here's mine

 

[DrEntropy]

Re: Help! Weber 40DCOE throttle problems Update

/bcforum/images/%%GRAEMLIN_URL%%/lol.gif

"Deep Pockets Donn"!!! /bcforum/images/%%GRAEMLIN_URL%%/jester.gif