Weber 40 DCOE`s Stumbling/bogging down

[Martym]

Trying to gain info on what is main cause of bogging down ? my car runs ok & will rev fine on smooth moderate peddle pressing. If at lowish revs I press throttle down quickly, the engine bogs/dies.

My initial thoughts are lack of fuel but have read poss over fueling

[21jigsaw]

Have you looked at the pump jets - for deposits / blockage, springs and plungers for cleanliness / damage ?

[Nigel Riches]

the pump jets have a diaphragm which pushes the fuel through them, it has been known for these to rupture, if I recall they are at the bottom of the float chamber, one for both jets. worth a look.

[Martym]

or i would think to open throttle and look into each choke and see if it get a squirt of fuel.. ?

[Duckpit]

Silly question, but have you balanced and tuned the carbs recently ?

[Jonathan Kay]

What's the recommend general approach to diagnosis with these: jump to possible specific problems from faults and some tests, or strip and rebuild regardless?

Thanks

Jonathan

[Duckpit]

I would eliminate obvious first ie

clean air & fuel filters on each

take all jets out and clean and blow thro

check for induction leak

Balance carbs

Tune carbs

[Martym]

Filters are new, jets are clean, carbs are balanced & idle mixture adjusted, When I got car it had factory Caterham fitted jets which I have read run it rich & was the case. Have leaned idle jets & put a more appropriate ET`s in. I am sure it still bogged down with orig set up,as I think it did it when entering where I live,

 

[Duckpit]

What engine ?

Timing?

[BlackSeven]

You should be able to get the Jetting something like close by following recommendations from the likes of the Dave Andrews Jetting calculator but to a large extent you are shooting in the dark until you get it on a rolling road. 

[Martym]

1.6 vx 8v.. Ignition timing now thats another ? as I dont know how timing is ajusted on this as dizzy is fixed ???

[Martym]

The metering hole in the accelerator pump jet has to be large enough to remove any hesitation or stumble caused by the lean condition created by suddenly opening the throttle plates (butterfly valves) at low RPM. Too large a jet will cause a "bogging down" of the engine from too much raw fuel.

The jets are numbered for their hole size in hundredths of a millimetre and are in five hundredths of a millimetre steps (i.e. 35, 40, 45, 50).

Think I will try 35 pump jets current ones are 45 & have read in here a few saying they were told to use 35`s....

[Roger King]

I can't be sure that you don't have other issues, but 35 jets are far more suited than 45 jets. I have never managed to understand why so many engines are fitted with grossly oversized pump jets; it's probably the most common issue I came across and all it gives you is a fluffy and hesitant throttle response with no discernable benefit anywhere else.

[Martym]

Thanks Roger, it I am right you are an engine guy... sure have read your name regards xflows.. i have a little vid but not sure how to post it on here

 

[Martym]

https://www.youtube.com/watch?v=-x-WhaMjRPc&feature=youtu.be

[Roger King]

It's difficult to tell for certain from the video, but it's possible that your carbs would benefit from balancing, adjustment of idle mixture etc.

I have an alternative jetting chart that I seem to recall worked better than std.

Choke 40mm
Main 115
Air  135
Emulsion tube  F7
Slow run  50F9
Pump  35

You may have some of this fitted already, but in particular the standard slow run, pump and air jets were way off target.

[Martym]

Thanks Rodger appreciate your advice... I have ordered some 35 pumps.

Do you mean 30mm Chokes ??? (40mm)

I got 50F8 idles in (had 50F6 in). 115 mains, F9 ET 160 air. in at moment.

have got F7 ET in a bag..

 

[Duckpit]

if it helps my 1.8 silvertop has

33mm chokes, mains 135, 50F9 idle, air 1.6  - runs a dream

My Xflow was exactly the same except 45F9 idle, run well. Initially the xflow bogged down but after a few years sorted it with tuning of carbs and aldon mapped dizzy.

[Roger King]

Whoops! Obviously hit the wrong key and didn't notice.

Yes, I meant 30mm chokes.