polo mk2 1300...need some diag help

right weve got a polo mk2 1300 MH engine

over a couple of days it started to have a mild judder that turned into a violent jolt (maybe misfire not sure ) and its so bad its undrivable now

it starts on the button and seems to drive reasonbly at very light throttle...any normal to heavy throttle and it jolts like crazy

i thought it was the hall sender and fitted a new dizzy complete with cap and rotor, plugs, leads and coil are very recent as is the fuel pump and carb base

its got the dreaded pierburg carb that wasnt running to bad apart from cold mornings

any ideas what to do next? could it be the carb causing those faults (id planned to weber it when funds allow) no airleaks that i can tell

not sure what to do now , apart from replace random parts

 

With the engine fully warmed up, is the choke flap wide open?
I'm thinking if you had a fault such that this wasn't opening up as it should, you'll have major overfuelling/under-airing which might cause these symptoms I think.

 

chokes working as it should

just set the mixture to 2% and its no differant

 

Air leak under the carb? I've had a few of these with my mk3 spi system.

 

new carb mount recently fitted

 

is the fuel filter clean? no water in there or anything, can you take the jets out to inspect/clean?

 

I used to have an H reg Mk3 Polo GT and had the same problem. A guy at a VW specialist garage fitted a new ECU and sorted the problem for me ....

<*CAVEAT *this was before I fixed cars myself and really knew anything about them many years ago !>

 

Pigbladder
My suggestion would have been carb icing but after a quick search I see you have previous on this matter. It also doesn't sit well with the on set of the problem although it has been a bit cooler of late. You have a quick check of the warm air feed just to rule it out.

 

As it seems to be rev-related, and you've changed everything else on the ignition side, maybe check the wiring to/from the ignition amplifier/TCI unit, or even try another unit from a scrappie, or borrow one from a friend?

 

its not carb icing ...it will do it straight away when i try to move off

filter is clean ...bad/water fuel wouldnt idle well or run at wot would it?

not sure it has a tci unit? , certainly doesnt have a ecu lol

 

vagcat thinks it does

 

any idea where such a unit would be

 

Not really, possibly under the scuttle cover?

 

it's only listed for LHD versions of the MH engine on ETKA though.
have you run it at night to see if there's any arcing from the leads when the hesitation occurs?

 

i will run it tonight and look...trouble is it will idle nicley and rev sweetly with no load

>>>>off to look under rain tray lol

 

** I'm just popping some humble pie in the microwave... **

 

it has indeed got a tci...under the rain tray

so it sounds like it might be that?

 

it's me that can't read...

I still think if it does it under load it's more likely to be in the HT circuit, probably wrong there too though

 

i guess it could also be a crusty dizzy loom....give same symtoms as hall sender