Club 80-90 Forums

Ooh - here's a thought. There's always a bit of a vacuum in the stage 2 venturi, because the valve is allowed to crack open a fraction, and there may even be a hole in the flap. But that vacuum shouldn't be enough to draw fuel from the main diffuser until the valve is open a lot more.

But... suppose the air corrector jet was blocked (the right hand blue arrow* in the second cutaway diagram) - would a small vacuum then be enough to draw fuel up?

* Sorry, there are four blue arrows - I meant the right hand one of the two larger blue arrows.

Thanks CJH, i'll give the jets a blast with carb cleaner again tonight to see if that does anything.

The plastic interlock part looked OK when I had the carb off:





Would twisting the black shoulder bit of the vacuum diaphragm make any difference? I may have moved it when fiddling.

When I sucked on a tube to test for blockages where the vacuum unit connects it wasn't blocked but it drew fuel out, should this be happening?



Here is a video of the choke coming off the last step: https://youtu.be/rzslTQ0rd5E" onclick="window.open(this.href);return false;

No, twisting the diaphragm won't make any difference. There's some length adjustment possible by turning the plastic ball socket on the end, but it can tolerate quite a bit of misadjustment. That vacuum unit is what opens the stage 2 throttle valve, and the gap either side of the 'tail' of the interlock part is what allows that valve to crack open a fraction.

I'm not sure where in the venturi that vacuum pipe comes from - I'll check this evening - but a bit of petrol isn't surprising since the venturi is where the petrol goes down. In operation the vacuum pulls in the other direction, so rather than petrol coming out it would be air going in.

jamesakers wrote:Thanks CJH, i'll give the jets a blast with carb cleaner again tonight to see if that does anything.

Try a filament of wire as well as the carb cleaner, just to make sure.

I've labeled this image to visualise what might be happening:



I do remember noticing the top of the jet was discoloured compared to the others.

Yep, that's the theory - fingers crossed.

It would fit the symptoms - at low rpm the vacuum isn't sufficient to pull fuel through the diffuser, but when you use the throttle to increase the rpm, the vacuum increases, fuel comes through the diffuser, which then keeps the revs up and maintains the vacuum. When the engine stops the vacuum dissipates and the fuel stops being pulled through.

In fact, like a proper theory, it's testable - if you can drop the revs, say against the clutch and the handbrake, for long enough for the diffuser fuel to stop flowing and burn off, then the revs might remain low after that.

Has the second throttle stop been set properly?

...and did you check the operation of the float valve? If it's not cutting off the supply, could the pump pressure actually force fuel out of the diffuser? Not sure that would fit the observations quite so well, unless the increased pump rate at higher revs is what's doing it.

jamesakers wrote: The plastic interlock part looked OK when I had the carb off:

The U channel holding the interlock. It should have a 0.4mm gap
Might aswell adjust it while it's off

CJH wrote:

jamesakers wrote:Here are some videos I've just filmed of the problem:

Engine stuck on high revs: https://youtu.be/6O6foXx3_Fk (cam looks fine)

Looking down the barrels when stuck: https://youtu.be/NBipxaTe45k

Engine restart resets it: https://youtu.be/_yQqtYNwGuw

I sucked on a pipe connected to the carb where the stage 2 goes in and it's not blocked.

Looking at the second video, there seems to be a fair amount of fuel dribbling down the stage 2 venturi, even though the stage 2 valve appears to be closed.

I took some screen grabs and I think primary is closed and 2nd barel is partly open and that's why fuel is pouring in, it should not be open at all with the primary closed, the plastic interlock is not working.

It's allowed to open a fraction under vacuum - that's what the small gap between the interlock tail and the U-bracket does. Did you spot it open more than that permissible gap?

CJH wrote:Yep, that's the theory - fingers crossed.

It would fit the symptoms - at low rpm the vacuum isn't sufficient to pull fuel through the diffuser, but when you use the throttle to increase the rpm, the vacuum increases, fuel comes through the diffuser, which then keeps the revs up and maintains the vacuum. When the engine stops the vacuum dissipates and the fuel stops being pulled through.

.

Vacuum is high at idle because primary is closed ( the reason why you do your timing with vac pipe disconnected) so if that ^^^ were the case why does it not do it from startup?, as you open the throttle vacuum reduces, only on the open road with your foot down when the primary can no longer supply enough air does the vacuum increase and secondary open,

in my pinion this is happening but when you take your foot off the primary closes but the secondary stays open slightly.

secondary

primary

itchyfeet wrote:
Vacuum is high at idle because primary is closed ( the reason why you do your timing with vac pipe disconnected) so if that ^^^ were the case why does it not do it from startup?, as you open the throttle vacuum reduces, only on the open road with your foot down when the primary can no longer supply enough air does the vacuum increase and secondary open,

in my pinion this is happening but when you take your foot off the primary closes but the secondary stays open slightly.

The vacuum pipe outlet is below the valve, so that's true - vacuum in the pipes is high because the engine is sucking - but the 'suck' from the engine is restricted to the area below the valve. But *inside* the second venturi (above the valve), the vacuum should only able to reach the diffuser to pull fuel through if the secondary valve is open (excluding the small suck that will get through the valve plate hole and the small opening allowed by the interlock).

maybe I-m talking rubbish, vac for 2nd barrel comes from above venturis so would be low at idle.

time for a rethink in vacuum

2nd deffo looks open though