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Sticky Twin Carbs

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#1 Tonylamb

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Posted 18 October 2021 - 08:06 PM

i've fitted twin HS4's to replace my HIF44. they will need a bit of needle tuning at some point. But for the moment I have an issue. The car starts and idles at around 800rpm but when I drive it the throttle sticks open at around 1800/2000 rpm. Their are three springs pulling on the throttles to shut them. sometimes blipping the throttle releases the sticking sometimes it doesnt. I'm wondering if the throttle spindle might be bent so it doesn't allow them throttles to shut. But its not every time. any ideas of what to look for. I don't think its the throttle cable itself but i suppose i could change it and find out.

#2 T.Harper

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Posted 19 October 2021 - 05:23 AM

This can happen if the nut on the throttle shaft is overtightened.  I have found that backing it off 1/2 a turn or so helps



#3 Tonylamb

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Posted 19 October 2021 - 07:20 AM

Ok thanks that makes sense can you point me to that nut? Presume its on the outside of the carbs with the linkage between the two? Can't seem to find a relevent diagram on the net?

Also and apologies for this question. When you are looking down on the top of the carbs which way do you turn the jet nuts to weaken the mixture. Just want to make sure I get a base setting before thinking about needles/springs/oil etc.

If anyone wants a Vizarded HIF 44 let me know.

#4 T.Harper

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Posted 19 October 2021 - 09:41 AM

Weakening mixture is anticlockwise from the top.

 

Throttle shaft nut is a long, hexagonal nut on the inside of the carbs, that the linkage slots into.  It should have a retaining washer with two spikes folded over the flats (this will obviously have to be bent up to release the nut).



#5 Tonylamb

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Posted 19 October 2021 - 11:49 AM

Thanks I will start by letting that off by half a turn and see if it free's things up.

#6 sonscar

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Posted 19 October 2021 - 05:58 PM

While you are in that area check for wear in the spindle and bushes.Wear here is all but impossible to tune out.Steve..

#7 Tonylamb

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Posted 03 November 2021 - 08:32 AM

So im still having some problems. These are brand new HS4's so there shouldn't be any wear issues. I've changed the throttle cable re set the jets to to 1.5 turns below the top. disconnected the two carbs from each other. using the flow meter and the individual throttle screws I've matched the flows. Reconnected everything and no sticking which is good. Using colour tune I set mixtures with an idle of about 1000rpm. it has a fast road cam 1360 motor. also has a lightened flywheel

problem is engine stalls everytime you stop. I'm wondering if it just needs a higher tickover. perhaps 1200 because of the flywheel and cam. seems a bit fuel wasteful but I'm not over bothered by that. Have i done something wrong? Any thoughts.

Perhaps go back to the HIF 44 LOL it doesn't look or sound as good but the performance is negligable.

#8 Tonylamb

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Posted 03 November 2021 - 08:33 AM

by the way these carbs don't seem to have a throttle spindle nut on the outside of the carbs they are on the inside on the spindles themselves. without the throttle cable the throttles snap back shut on the stops nicely.

#9 cal844

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Posted 03 November 2021 - 09:34 AM

I'd up the idle slightly, a little at a time

#10 sonscar

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Posted 03 November 2021 - 11:37 AM

I would think it is too weak,what the law says and what the engine needs are not always the same.Steve..

#11 Tonylamb

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Posted 03 November 2021 - 09:28 PM

Interesting because when I lifted the pistons the engine nite doesnt rise like it should. I made the mixture richer but maybe i need to go up a bit more

#12 T.Harper

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Posted 04 November 2021 - 11:06 AM

Look for vacuum leaks on the manifold gasket or carb to manifold joints.  Cutting out after blipping throttle can be a sign of air leaking in during high vacuum and overly weakening mixture.



#13 Spider

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Posted 04 November 2021 - 06:35 PM

The throttle linkage shaft between the carbs, at idle, must have some end float, ie, movement between the carbs. If it's at all tight, it will cause the butterfly to jam against the carb bodies as the buttery comes to the idle position (holding them open that little bit),  where they need to be dead centre. Say 0.020" (0.5 mm) end float is well enough here.



#14 Tonylamb

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Posted 05 November 2021 - 09:32 PM

Presume the best way to find a leak is to spray a bit of WD40 along the gasket.

#15 Tonylamb

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Posted 05 November 2021 - 09:35 PM

Regarding the end float do you mean some sideways movement of the linkage between the carbs. Do I get that by loosening the nuts either end of the linkage.





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