flat spot between 3-4,000 RPM 1987 1.8 GTI CAB

hi guys, new to using this forum as i normally use the fb site but thought id give this ago so apologies if ive done anything wrong. so i have a 1987 mk1 cab 1.8 gti which is running pretty well but i have an an annoying flat spot where it really bogs down if im accelerating throttle fully open between 3-4,000 rpm,it will do it more when hot and not so bad when cold,its almost like fuel starvation?! all the usual things have been changed, coil plugs leads,rotor,new fule pump and pick up pump, could it be a sensor of some sort??? ive checked timing when replacing the cambelt and thats all good too. you can back off the throttle and its fine its just when you try and open it up it bogs down (REALLY ANNOYING) but once over 4,000 rpm it flys. ANY HELP would be fantastic. thanks scott 

What is the CO reading from the last MOT?

Have a read up/search on the WUR, (warm up regulator) as this controls the fuel pressure.

 

mark1gls said

What is the CO reading from the last MOT?

Have a read up/search on the WUR, (warm up regulator) as this controls the fuel pressure.

 

adjusted the CO to what it should be but no i have not read about WUR, where is this located please?

The WUR changes the ratio between the plunger and the air flow to compensate for a cold engine. Id imagine if it were WUR related it'd be more likely to be through out the rev range - not just at a specific rpm.

Yomp said

The WUR changes the ratio between the plunger and the air flow to compensate for a cold engine. Id imagine if it were WUR related it'd be throught the rev range not just at a specific rpm.

  any ideas in that case then please? It's got to be fuel related to me…if you back off when it bogs down it clears and you get full throttle again and revs right round, I've ended up learning how to drive it so it doesn't do it but it would be nice to get it right?! Thanks again scott 

Is it a new problem or since you had it?

Sticky plunger or air flap in the metering head maybe? - If you think its fuel related.

Does it stutter in the flat spot or just die or feel as if its
 running evenly but just flat on power?

Fuel starvation - like a blocked pipe tends to occur under load but if your managing to back off and then rev right through its not a system starvation so to speak.

I would also check the advance / retard mechanism in the dizzy. If thats playing up you could have flat spots but maybe not so savage as you describe so I'm not convinced with that suggestion! More thinking out loud!

BTW the WUR is located to the left of the oil filter housing. Has two fuel pipes going to it from the metering head.

Yomp said

I would also check the advance / retard mechanism in the dizzy. If thats playing up you could have flat spots but maybe not so savage as you describe so I'm not convinced with that suggestion! More thinking out loud!

The braided rubber hose connecting the vacuum hoses is the common problem as the rubber goes hard over time and leaks air…

 

mark1gls said

Yomp said

I would also check the advance / retard mechanism in the dizzy. If thats playing up you could have flat spots but maybe not so savage as you describe so I'm not convinced with that suggestion! More thinking out loud!

The braided rubber hose connecting the vacuum hoses is the common problem as the rubber goes hard over time and leaks air…

 

  so tried it this morning from cold driving it hard (ish) which I don't like doing but had to to double check and it's totally fine when cold pulls like a train right through the Rev range under load…soon as it got up to temperature the hesitation and bogging down happens under load. It's done it since I've purchased it, previous owner said he'd never noticed it (don't believe him) but loved the car so took a chance on it! Any thoughts now we know it's fine when cold? Thanks again for your help all 

Yomp said

Does it stutter in the flat spot or just die or feel as if its
 running evenly but just flat on power?

 it stutters pretty badly between 2500-3500rmp but only when up to temperature if that helps?! If you drive it half throttle like Mrs daisy you'd never know it had an issue but I don't see the fun it that! 

What fuel are you using, normal unleaded or super unleaded?

Does the fuel pump get noisier at all?

Fuel mixture at 2% CO is where you want to be.

 

mine ws doing this and advancing the dizzy a little resolved it

mark1gls said

What fuel are you using, normal unleaded or super unleaded?

Does the fuel pump get noisier at all?

Fuel mixture at 2% CO is where you want to be.

 

fuel pump stays the same now its been replaced. will check mixture tomorrow lunch thanks and im just using normal unleaded, should i use super unleaded?

Alps said

mine ws doing this and advancing the dizzy a little resolved it

great thanks will try and report thanks

I tend to ignore factory settings on timing and go by my ear. Head skins, rebores changing comp ratios, differences in modern fuels etc  make  original settings in accurate. I keep advancing until I hear any form of pinking and then back iit off until the pinking stops completely.  Then take a reading with the timing light to see where it is for future reference. May well be worth swapping another WUR to see if it makes a difference as you say it's different between hot and cold running. All our guesses are valid but there are so many things on a k jet that give the same symptoms that without a pressure test it's difficult to say with confidence. It may not even be fuel related at this stage. 

Any other things to check guys?? Cars now off the road for winter so I've got time to try things now…I guess WUR is next to try? Just to confirm it bogs down when accelerator is completely open at 2000RPM , if you back off the throttle slightly it's fine it just can't seen to take fully throttle when hot. Thanks in advance