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HS6 slipping?

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Old Jan 29th, 2015, 04:29   #1
volvogv
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Default HS6 slipping?

There's a problem that's been annoying me for some time. It goes like this:
I tune the HS6 carbs to have equal airflow and a nice little RPM bump when the piston is lifted. The engine runs like a tiger for about a week but soon feels lackluster.

I am starting to think that the jets are slowly backing themselves out, thus enriching the mixture over time. It's tempting to put some inspector's lacquer on the flats to see if this is true.

Has anybody experienced this?

Mike
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Old Jan 29th, 2015, 09:40   #2
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Check your spindles for wear first and make sure your dashpot oil is not leaking out.
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Old Jan 29th, 2015, 12:06   #3
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Mike;

It sounds like you have gone through this cycle a few times, so it would seem to me the question which must be asked is: After it runs poorly, what needs to be adjusted for it to run well again?...when you answer that, it would also seem to me, you will have answered the question of where you must concentrate your repair efforts.



...and why would you say that you suspect "jets are slowly backing themselves out"...is jet height adjustment nut (30) what you must adjust to get back to good operation? I suppose nothing is impossible (or should be ruled out when troubleshooting), but I'd be surprised if you are finding that jet heights are changing...I wouldn't expect the adjustment nut to have moved...this is unlikely given that these typically need a wrench to move them (they are preloaded and position is locked in place quite definitely by spring 29) if mixture is changing, something else is changing...perhaps securing setscrew in dashpot (13) is allowing Metering Needle to move...I'd check this ASAP before MN falls out and is consumed...

General Tips: Assure external linkages are lubed and move smoothly through their entire range, and return to idle position with good repeatability. Assure dashpots also move smoothly (and dampened) up, and drop (undampened) down to bridge.

It sounds like you might benefit from attending my "Care and Feeding of your Volvo's SU Carbs" seminar coming up (date to be set, place will be Nardelli's Restaurant, Waterbury CT)...unfortunate for you, it will be held on this side of the country...perhaps you need to schedule and attend a coincedent business meeting in NYC (with a sidetrip)!

Good Hunting from Connecticut!
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Old Jan 29th, 2015, 13:02   #4
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I certainly think it's unlikely that the adjuster nuts are backing out. Even more unlikely that they are backing out equally. An extra flat or two not likely to be noticed beyond a small decrease in tick-over revs and a small drop in mpg. Dropping the jets a couple of flats in the winter often suggested anyway.
You say you have a nice little rpm bump when you raise the pin but you don't say if the revs drop again. If the revs stay up it's running too rich, so maybe a partial answer to the problem is there. George mentions that you might be losing damper oil and that certainly does seem to happen with some carbs. The small brass damper plunger and its D clip is rarely mentioned but they do wear and this affects the damping. Correct oil is 20 weight but some prefer 30 although this might just be that it corrects for the damper wear. When stamping on the pedal there shouldn't be much delay but the carbs do have to readjust pretty quick. A fast but steady push always better. Oil level should be 1/2" below the top of the centre tube with the damper in so if topping them up with the damper out the level will likely be out of sight before adding oil.
There are dozens of how to SU tune-ups out there so I've lucky-dipped this one.
http://britishclassicmotors.com/3301.html

If you reset the points as part of your tune-up, measure them again after you notice the drop in performance. Cheap points often have quick wear fibre heels even with the cam greased properly. Check them in each of the cam positions as wear in the distributor will often make them different.
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Old Jan 29th, 2015, 22:15   #5
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Guys;

I fail to see how damping oil can be lost to anything but having miniscule droplets which have become liberated from the damper valve, being consumed by being ingested by the vacuum at the two ports on bottom of dashpot...it can't leak...because the bottom of the damping tube is closed and not open in any way to the metering needle cavity, which is open from below only. Am I missing something here?

Cheers
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Old Jan 29th, 2015, 22:54   #6
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Technically I'm sure you're right Ron. Back in the '60's (!) I always seemed to be topping up the carbs on my B16 122S. It could just have been that pulling out the dampers quickly, also pulled oil up and over the top. Most people check the levels with the dampers out and then put too much in hence my comment about checking the level with the plunger submerged in the oil. Filling the tube to 1/2" from the top before fitting the damper will give a too high oil level. Realistically, if the damper has a damped feel as soon as you start to pull on it after unscrewing it there's plenty of oil in there.
Note: Early dashpots had a breather hole in the lip just below the threads for the damper, later ones had the breather in the cap itself. I don't know if the change came about with the change from brass tops to the black plastic. A dashpot with an internal breather hole should have a cap with no hole. I think the vacuum that pulls the piston up sucks via the internal hole hence the need for a sealed cap. Not sure how the vacuum is applied when there is a breather in the cap. Any idea Ron?
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Old Jan 30th, 2015, 04:27   #7
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All good suggestions. Thank you.
I've often wondered where the damper fluid goes. Every car I've owned with SU cabs has needed frequent top ups. I use ATF oil per the owners manual and top it up once a week.

As for the bump, I'm sorry I didn't give the full story. I set it to have a slight rise in RPM and then come back where it was. The last couple of tuneups I've done, the front carb needed to crank up the jet 5 or 6 flats and maybe 1 flat on the rear carb. That's why I think I may have jet creep, at least on the front one.
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Old Jan 30th, 2015, 10:16   #8
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How old are the carbs, has some hamfisted gorilla previously stripped the threads? I would suggest removing the carb and giving it a good examination
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Old Jan 30th, 2015, 10:42   #9
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It may be totally unconnected, but years ago I experienced similar symptoms with twin SUs on a mini - I finally traced the problem to a spilt advance-retard vacuum pipe. I don't know if it would have the same effect as you have experienced i.e. apparently going rapidly out of tune - as far as I remember the tune/balance of the carbs, which had rich needles fitted, was actually maintained quite well, what was out was the degree of ignition advance available on acceleration from normal speeds, which did feel as though the points had closed up i.e. rise in revs but no power then cough and stall.

(For what its worth, the dashpots were checked and topped up every 2 or 3 weeks - around every 5 to 600 miles)
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Old Jan 30th, 2015, 13:01   #10
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"I use ATF oil per the owners manual and top it up once a week."

Which owners manual recommended ATF? Volvo Green Books say 20 SAE. I know Ron is an advocate but IMO it's too thin and allows the pistons to rise too quickly. This reduces the enriching effect of a thicker oil and that's what helps with the engine pick up. It's doing the work of an accelerator pump as in Webers etc. The SU type carb is unique in how it works.

If you having to crank UP the jets when you retune, I would say that they are sticking DOWN, perhaps due to the choke mechanism not allowing them to slide back to their set positions. This isn't uncommon. It might be that the jet holder is a bit out of line but you'd really need the carbs on the bench to check and adjust that.
You have to have the jets up against the adjuster nut and the jet moving normally up and down while you are adjusting the mixture. When you have the carbs set to your liking, how many flats are they down. e.g. 13 front and 11 rear. After they've gone out of tune as per your description, recount the number of flats. No change? I think that might be so.
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