Flat spot at 2500rpm

[kevtherev]

Never really solved this, but I think I understand it better.
This occurs under full throttle acceleration.
3rd gear from 2000 rpm and up
As I reach 2500 the engine falters like a slight misfire, as the revs rise to 2800 it clears and the power comes in buckets.
It's my belief that this is the point at which the vacuum controlled throttle starts to open.
Does any one habe a description as to the pierburgs vacuum throttle operation?
Or what causes flat spots

(N suffix distributor 120 main jet.)

[937carrera]

It's my belief that this is the point at which the vacuum controlled throttle starts to open.
Does any one habe a description as to the pierburgs vacuum throttle operation?
Or what causes flat spots

I'm not sure what you mean by "vacuum controlled throttle", but that may be my lack of Pierburg knowledge. I know you know about the Wiki, so next port of call would be James Akers Sir Adventure website and videos

Ignition timing is affected by vacuum, which is the result of throttle position, so if vacuum advance is not operating correctly that could cause a flat spot

The gear you are in should not affect how the engine behaves, save for any defect will be amplified in a higher gear due to gearing effects and aerodynamic drag

Has this happened suddenly, been a longstanding problem, or something which used to be minor and has now got worse ?

[itchyfeet]

Had a similar flat spot on acceleration and pinched the 2nd throttle interlock tabs together ( seen on RHS either side of plastic interlock cam) and it largely fixed it, if the interlock tabs are loose then the throttle plate can open slightly and I think it then delays the point at which the 2nd throttle opens because the vacuum is reduced by the throttle leaking.

Interlock stops the throttle opening at lower primary throttle positions

Vacuum comes from two places deep in the carb fed by drillings as marked so make sure they blow through also.

vacuum unit could be faulty also, if you suck on it it should release slowly from memory

plastic cam can be worn or broken too, CJH made some of these, he's not been around recently but maybe soon.
http://forum.club8090.co.uk/viewtopic.p ... 0#p8197287" onclick="window.open(this.href);return false;

[diabolov]

Hi Kev,

I was going to say possible stage 2 diaphragm weak or needs a slight adjustment, but Itchy seems
to have covered that.

I'll add this as my contribution.

Have a read here http://www.cjh.me.uk/MyPhotobucket/albu ... Repair.pdf

https://www.dropbox.com/s/ig2hs3e4bdxlh ... ctions.pdf

Mark

[937carrera]

That's enhanced my understanding of Pierburgs then. I understand the principle of how it works from itchys photos etc, just trying to figure out why VW went for that setup rather than the simpler mechanical linkage / cam type approach

[kevtherev]

Ok.
Firstly thanks.
I have adjusted the tabs itchy, the cam shows wear but I wouldn't say its excessive, maybe a thou or two.
However perfection will be persued.
Great links Diabolov.
That fault chart has probably pointed at the reason(s).
The vacuum throttle unit, and the jetting, so the fact that I have altered the main jet to 120. (Which makes it FLY! ) could have upset it
So
The vac unit doesn't leak but I will change it, I have a few used ones knocking about.
And if necessary, the main jet.

Two questions.
What is the main jet supposed to be (is it 110?)
What action am I looking for in a vac unit, is it constant like the pull down?

[kevtherev]

Update
I have made up a really good, tight secondary throttle assembly from an Ikov Czechoslovakian pierburg rip off that looks almost new.
Mine is sloppy compared to this
Fitting tomorrow for sea trials

[diabolov]

Personally I can't answer the questions

I have no idea what the main jet should be and searching the WWW didn't come up with anything obvious.

Itchy said from memory it was a slow release, I don't remember, just that it pulled when I sucked on it.....I know, I know.
I would have thought that it would hold open all the time the vac was applied, but I don't know..

Just seen your post. Good luck at sea
M

[kevtherev]

Yes me too.
It seems that the opening will follow a cam under constant vacuum.
But during the transfer I suspect the butterfly is opening too early on mine and enriching the mixture.
Reason being the cam follower is worn, (more than I thought) and the follower is in contact with the cam opening the butterfly during low demand

[diabolov]

Hope the rebuild cures that. From what you have said it sound like it should.

M

[kevtherev]

Carburettors are becoming a dark art.
The pierburg is simple, but understanding its overlapping mechanics is perplexing

[itchyfeet]

Two questions.
What is the main jet supposed to be (is it 110?)

It's a god job somebody is keeping a list of useful threads itchylinks

http://forum.club8090.co.uk/viewtopic.p ... n#p8202199" onclick="window.open(this.href);return false;

[kevtherev]

Two questions.
What is the main jet supposed to be (is it 110?)

It's a god job somebody is keeping a list of useful threads itchylinks

http://forum.club8090.co.uk/viewtopic.p ... n#p8202199" onclick="window.open(this.href);return false;

Oh my gosh what an awesome post!

Thank you

[itchyfeet]

What action am I looking for in a vac unit, is it constant like the pull down?

it pulls up quickly then takes about 2 seconds to fall when connected to the carb, tagged you with a vid on fB as it's just easier than getting a vid on here

[itchyfeet]

just trying to figure out why VW went for that setup rather than the simpler mechanical linkage / cam type approach

They didn't becuse VW never made carbs Pierburg did

Carbs like any technology just got more and more complicated probably for minor performance, economy, reliability and emissions reasons that are long forgotten.