MGB: THANKS, PROBLEM SOLVED -Jet sticks in SU carb

Looks like I fixed the problem of the sticking jet. The cam surface was smooth so that wasn't the problem. Before pulling the carbs, I worked the jet up and down by hand several times and it seemed to bind slightly. I pushed on the link arm to relieve the bind and that seemed to do the trick. When I now open and close the choke both jets close as they should, even when I close the choke slowly.

Thanks to all for the help. Another problem solved thanks to the MG forum.

Herb
:beer:

The car: 71 MGB with twin HS-4's; smooth idle at 1,100 - 1,200 RPM

The problem: jet in front carb does not always completely retract when choke is released

The symptoms: Occasionally after the engine is warmed and the choke is released, the engine will barely idle (500-600 RPM). I increase the idle via the adjusting screws and then all of a sudden the engine revs up well above the set idle and I have to readjust the screws back down to the proper idle RPM.

I finally noticed when this problem occurs that the front jet was not fully retracting to the base of the carb after the choke was released. I tightened the choke cable to remove any slack, but it still occurs. It doesn't happen every time, it's sporadic.

Can anyone tell me what needs to be done to correct this?

Thanks,
:beer:
Herb

Being a sporatic problem my 'guess' is that the link from the pivot to the jet is misaligned or the pivot point is dirty. It can be a little hard to see any bends in the link with the carbs on the car. It should extend straight down and then make two sharp bends and come in behind the jet. It is easy to straighten, if you can see the bends. A little WD40 to the pivot point will clean the area. It could be gum or dirt in the jet bearing - that is normally a consistant problem. Chester

Chester - thanks for your input. I removed the air cleaner housing and checked out the linkage. It has the two sharp bends and looks correct. However, I did notice that the screw that attaches the linkage to the jet was only partially screwed in. I thought that might be providing some "slop" and causing the problem. I snugged up the screw, sprayed carb cleaner in all likely places, but still have the problem.

I did notice that if I close the choke slowly, the jet tends to hang up, but if I forcefully close it, the jet seats completely. At least I know that if the problem occurs, that I can slam the choke and it should fix it. But something is still causing the jet to hang up.

Any other suggestions?

Herb

I'd go ahead and remove the carb, clean it up, and then operate the choke. This way it's a lot easier to clean, and will give you a better vantage point to see what might be hanging up. Sometimes the arm gets bent and misaligned just enough that it puts side pressure on the jet, and then it can't slide freely. Check the brass for corrosion.

I run into this problem very often when assembling carbs. The rapid closing gives some momentum to overcome the slight stick, the spring does not quite have enough pull to get past the 'stick' when moving slow. It virtually always comes down to the two things I mentioned - once in a while the spring will get pinned between the body and the metal arm that pivots. The screw should go into the jet - and the boss that the screw goes through as well - at a direct 90 degree angle. The link should be almost perfectly parallel to the center line of the jet as it goes down. The part of the link that crosses behind the jet should have complimentary angles so that the final down turn is again parallel to the jet center line. You can change the angle of that link to increase spring tension - that should not be necesarry, changing it too much can bring the fast idle cam into play when you don't want it. Rod is right, easier to do off the car.

Herb,

The one other thing to check is the top of the choke cam. This surface is always worn, but in varying degree. Sometimes the fast idle screw will be snagging on a depression, or gouge.

Sorta apropo: John Twist giving tech session at MG2009 "use 90 wt in your SUs"!!! Jimmy?

JimmyHilton Wrote:

Hummm .... I have used different wts to tune with, 90 wt sounds extreme. My biggest concern would not be enrichment on accell, I would be concerned with the piston falling on decell and part throttle response. Be interested to hear if anyone has tired this and what results they got.

OldBloke Wrote:

All good info here, but the number 1 reason, I find, for a sticking jet on enrichment, (side bowl carbs, HIFs have their own issues) is the poor quality of the jets now being supplied by SU. Older jet barrels are smooth and parallel, while all of the SU jets I now receive (OE Master kits and also Joe Curto) have spiral machine marks. They are very subtle and may not be seen at first glance, but run them up and down in the jet bushing and they magically become visible!

Reason number 2 is that the jet may not quite be centered with the jet and if the needle has any contact with the jet it can cause it to stick, being aided by number 1 reason, in the last little bit before the the jet returns to its proper resting place.

Most jet sticking seems to occur at the full, or near full, choke position. Make sure that the linkages are properly aligned and installed correctly. Lube all load bearing surfaces and check for free movement. I use 90 wt for this as it is thicker and tends to stay in place better.

Now here is the number 1 task to do before you reassemble your carbs................
Use crocus cloth to polish and remove the spiral marks on the jet. Do this and your problems should be over. I caveat though, do not use fresh crocus cloth without first drawing the surface of it over a rounded metal surface first. This removes the "highs" and lets you get a smoother surface finish.

But also be aware that evaporating fuel will leave a varnish on the jet surface and should be cleaned periodically, especially if your car sets idle for long periods of time. I take the piston out annually and clean it and the jets with carb spray. My favorite is Berrymans, which can be had at any Wal-Mart.