Morgan +4 Forum

SU Choke advice needed

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avecplayzeer Avatar
avecplayzeer Mark M
San Mateo, CA, USA   USA
Hi Guys (and Ladies if any are on the List - I hope so)
My earlier adventure with the fuel pump having concluded, I'm now moving on to the SUs.

(A brief aside; I switched from Marvel to 20W50 in the piston dampers and was rewarded with a huge improvement in throttle response.)

Anyway, since I bought the car it has been very hard to pull the choke cable. I have eliminated the cable itself. Diving further yesterday, I discovered that the jet levers have enlarged holes at the pivot points. This seems wrong to me especially after comparing with a picture from Joe Curto's site. Although both jet levers are damaged to the same extent, the problem seems to be in the rear carb jet, which does not move very freely.

Has anyone else had the same problem and if so, what did you do to correct it?

Thanks and best regards,
Mark Morgan

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Rear SU 2a.jpg

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ahaugland Avatar
ahaugland Silver Member Alex Haugland
Eugene, OR, USA   USA
I can't remember whether the holes on the H series carb levers should be perfectly round or elongated off the top of my head. I can check a set I have disassembled at home if I remember today. That said, have you tried separating the cable from the carbs and checking the components separately? How easily does the cable pull and push when it isn't actually pulling on the carb levers? Likewise, how easily can you move the jet levers by hand rather than simply by pulling the cable? The cable should slide very easily in the sheath when it isn't attached to the carbs. Likewise, the jets on the carbs should move fairly easily by hand. Watch the action and look for binding and feel for any catch points in the system. The carb lever should move smoothly with no point in its motion having significantly more effort to get past than any other. If you need new arms, they should be the same as a Triumph TR3 or early 4 and are available from Moss Motors.

--Alex Haugland
Eugene, Oregon

Fred Winterburn Avatar
Ripley, ON, Canada   CAN
Mark, This is a late reply so you have probably already figured it out. But, if I remember correctly, the one hole is supposed to be sloppy so that the throttle buttterfly opens just before the jets are enrichened. Keeping that hole sloppy also ensures that the throttle is back to the normal idle position if the choke isn't pushed in fully after the engine has warmed up. The holes in the linkages in my 56 Plus4 H6 carbs were so badly worn that all of the holes were sloppy. I used antimony based plumbing solder to fill the holes and then re-drilled them to the sizes I wanted. I then elongated the hole that is supposed to be elongated the right amount. This was several years ago. The plumbing solder isn't as hard as the original brass, but makes a sturdy enough bearing surface as long as they are lubricated occasionally. The other thing I did was replace the stranded choke cable with a solid wire to make it easier to push the choke (really an enrichener with SUs) off. That way the return springs don't need to be stiff. I managed to salvage the old choke knob and incorporate the solid cable into it so it looks original.
If you inspect your choke linkages, it should be easy to tell which hole should be elongated. ( I simply drilled it a larger diameter after filling with solder) It's the left hand one shown in the picture you posted from Joe Curto's website. Fred

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BarryTR3A Barry Brown
Crossville, TN, USA   USA
If you haven't found the answer yet, also check the SU needle for wear to see if the jet needs centering. The jet should also have a fairly highly polished appearance. If the jet is corroded, use a VERY fine polishing compound {toothpaste may work} and gently polish the outside of the jet.

Good Luck!

avecplayzeer Avatar
avecplayzeer Mark M
San Mateo, CA, USA   USA
Alex, Fred, Barry;
First of all, thanks very much for your responses. I had not checked the site for a long time due to life changes, etc. (I retired in July '15). Hopefully I will now be able to return to getting my car back on the road.

The choke cable is solid, so that's good. Disconnected from the linkage it moves very freely in its sheath. My gut tells me the sloppiness in the linkage holes is not the source of the tightness of the jet movement. It seems to be the jets themselves which are tight. I bought new washers in cork, soaked them overnight in engine oil and installed them. No improvement. I bought the SU jet centering tool and tried that many times too, also without positive results.

(After I get this solved I'll be asking for help about a loss of power at above 3000 rpms.)

Thanks again for your advice and for participating on this forum.

Best regards,

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