help diagnose a carb flat spot /bog?

[steveo3002]

ive got a weber dmtr kit on my old mk2 polo

runs fairly well and returns good mpg...on niggle is , if I'm around 1500-2000rpm , say turning a junction onto a reasonble hill and put my foot flat down i get a stumble /flat spot

if i gradualy put my foot down instead its not noticable

any ideas how to diagnose whats up...I'm guessing its either weak or rich ...how would i know which?

i know a rolling road would fix it ..its a cheapie and i cant afford such things

[pauldaf44]

Is the engine timing correct? And if it has points are they correctly gapped. don't use feeler gauges check the dwell angle using a multimeter it takes into account wear in the dizzy

[steveo3002]

its the later non points ign

timing is spot according to my fancy timing light

[kapri]

Check your accelerator pump jet is working first.

[steveo3002]

how would i do that

btw , its not a massive bogg ...just slightly annoying

[wilko]

If you take the air filter off with the engine turned off & then open the throttle, you should see petrol squirt into the top of the carb - should be a nice clean jet of fuel. If the diaphram on the accerator pump is shagged or the jet the fuel passes through is blocked or the linkage is faulty, you'll only get a splutter, or none.

If that's OK, you'll have to try to use some educated guesswork to discover what is wrong - I'd hazard a guess that the venturi size is a little on the large side for your engine (standard 1300?), and there isn't enough air pressure acting on the jets at 1500 rpm / full throttle, and not enough air velocity through the carb - the fact it goes OK when you add throttle gradually would tend to support this idea... You might be able to improve the situation by richening up the idle mixture... Is the opening of the secondary throttle controlled by vacum diaphram, or is it a manual linkage?

[steveo3002]

right ive checked whats happening...i get a even needle sized stream of fuel from where you said , just a low pressure stream , no spluttering but hardly what id call a squirt. should it blast out or what ?

think the secondary opens via the linkage ?

yes its a standard engine , and the weber is the proper kit

[wilko]

When you say "proper kit", you mean it's the webcon recommended conversion, or is it a secondhand carb? Are you sure you got one with the right size throttles? The list I have shows 28/32, 32/34 & 34/34 all available & the venturis go up on each accordingly.

Anyhoo - bit of theory: a fixed venturi carb basically has 3 elements effecting mixture which overlap each other, on a fairly standard engine:

Up to 2000rpm - idle jet & idle mixture screw. The screw adjusts how much air gets in to mix with the fuel, the jet controls the amount of fuel

2000-4000rpm - main jets - bigger jet size = more fuel = richer mixture, and vice versa

over 4000rpm - air correction jets - bigger jet size = more air = leaner mixture, and vice versa

I should mention emulsion tubes - but unless they are wildly out, the reasons for changing them are more a case of fine tuning, they effect the way fuel is presented under different conditions.

So, with your problem being at 1500-2000rpm, you can completely ignore the air correction jets, the cure is likely to come from either or both of the idle settings or the main jets, as this is the point where the former starts to stop effecting things and the latter begins to start...

First thing to do is eliminate the idle mixture adjustment - there should be a screw set into the base of the carb somewhere, it often faces away from the engine (towards the bulkhead on your car) to allow room for adjustment. Get the engine thoroughly warmed up and running with no choke, then up the idle speed slightly to about 1000rpm on the idle speed adjustment. Screw the idle mixture screw in (right) about 1/4 or a turn at a time - the engine will eventually start to struggle and could stall. When you get to this point, back the screw off until it's back to normal, then reset the idle speed and check it's still OK - It's important to do all of this with the air filter on the carb.

Road test that and see if it's any better...

Next thing I'd do personally is have a frig about with the timing - older engines seem to be fairly "individual" - I don't bother with a timing light on the cars we banger race, I do them by ear - you'll often find you can "get away" with a little bit more advance than the factory setting, and this can help make them a bit more responsive.

If none of this cures it, then it's going to need jet changes, at which point I'd get main jets that are "10" either side of the current ones and go from there with trial and error...

[steveo3002]

right thanks for the advice

its the proper webcon conversion kit

i have tinkered with the timing before with the hopes of more power..it likes the std setting best