Qwk & Dirty Accl Pump Adj.

[geodoc]

Got a pair of used 36 dellortos for the loop project (w/ Convert eng., Megacycle X10, 949cc). They look like they might have come from a Ducati or somesuch. Accelerator pump adj. screws have obviously been moved.

Seems I saw somewhere a "pump 'X' times and measure output qty." procedure. Short of that, any quick and dirty adjustment method known?

Also BTW, anybody want to trade a pair of 60/3 slides for my extra pair of 50/3's?

George

 

[john zibell]

You might find something on Greg's pages http://thisoldtractor.com/gtbender/dellorto.htm

I've always started with minimal adjustment to the accelerator pumps. If they are even, start by making 1/4 turn adjustments and see how it goes. Turning the screw in results in less fuel I believe, but don't hold me to that as it has been a few years since I've made any pump adjustments.

 

[geodoc]

Heh John,

Didn't see anything on Greg's site for the accelerator pump set-up. I've Googled around with no luck.

It's correct that screwing the pump adjuster reduces the length of the "squirt", and screwing it out increases it.

You might find something on Greg's pages http://thisoldtractor.com/gtbender/dellorto.htm

I've always started with minimal adjustment to the accelerator pumps. If they are even, start by making 1/4 turn adjustments and see how it goes. Turning the screw in results in less fuel I believe, but don't hold me to that as it has been a few years since I've made any pump adjustments.

 

[contractor20]

Geodoc have another look at Gregs site under Guzzitech.dk archive
I Quote
As shown in figure 27, on opening the throttle slide (9), lever (8) controlled by a special cam (7) cast into the front of of the throttle slide, acts directly on the pump diaphragm ( 1), I held out by the spring (2).

This diaphragm, through the delivery valve (4) and pump jet (5), pumps fuel into tne main barrel (10).

On closing the throttle, the diaphragm returns to its original position, pushed by the spring and drawing fuel up from float chamber through the inlet valve (6).

The pump injection amount can be changed by adjusting the screw (3) which controls the travel of the diaphragm and consequently the volume of fuel pumped out.

The start of pump operation is determined by the particular configuration of the cam (7) cast in the front of the slide (9).

3.7.2 Selection of correct pump jet and slide pump cam

fig. 28 (left) fig. 29 (right)

The profile of the cam in the throttle slide controls the action of the accelerator pump.

For example, cams having the operating ramp high up in the throttle valve (see figure 28) make the pump start to work immediately the throttle opens.

Operating ramps lower down in the slide delay the spraying action of the pump.

Having selected the cam type, to produce immediate or delayed pickup from engine idle, the pump jet size can then be chosen.

The size of pump jet selected determines the duration of fuel delivery, so the larger the pump jet used the shorter the pump spraying interval and vice versa. The quantity of fuel sprayed out has already been fixed.

Pump jet selection must be effected with the engine running with rapid full-throttle acceleration; under these circumstances the optimum jet size should allow the engine to pick up regularly and promptly, rapidly increasing engine speed in every acceleration-speed range.

It was a bit hidden in jens stuff under TECHNICAL ,then Dellorto manual, then Acceleration :whistle: :whistle:

 

[geodoc]

What I was looking for was more in he way of a rule-of thumb or close-enough method to adjust the fuel delivered by the accelerator pump. Sort of like the difference between adjusting the idle air screw not by a CO spec, but by "turn the idle screw out to just past the fastest RPM and then back in an RCH"

I had already read the manual (available on-line at other places besides Greg's site), and from this you can see that: 1) the slide ramp determines the timing of flow, 2) the nozzle size determines the volume and 3) the adjustor screw determines pump stroke.

The only thing you can tweak without spending some (jets) or a lot (slides) is the adjustor screw. In keeping with my method of testing remedies for any mechanical problem by starting with the easiest / cheapest thing first, I concentrate on the adjuster screw.

I ended up adjusting it out (lengthening the stroke / more fuel delivery) to a point when I opened the throttle "briskly", I could feel a bit of hesitation from too much fuel. I then turned in the screw about 1 turn to get a nice pick-up with said abrupt throttle opening.

Admittedly, this is a less than scientific method, but has the advantage of being easy and cheap if not precise.

 

[john zibell]

I ended up adjusting it out (lengthening the stroke / more fuel delivery) to a point when I opened the throttle "briskly", I could feel a bit of hesitation from too much fuel. I then turned in the screw about 1 turn to get a nice pick-up with said abrupt throttle opening.

Admittedly, this is a less than scientific method, but has the advantage of being easy and cheap if not precise.

That is about what any carb tuner would do. If you get it to where you are satisfied with the performance that is all you want. Also very similar to what I suggested in my first post. Just adjust the screw until you are satisfied with the result.

BTW, for idle mix adjustment, you technique is the opposite of mine. I make it leaner until the engine just starts to stumble, then richer by half a turn.