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MG MGB Technical - SU jet depth

hi
Got stock engine I believe with a pair of
1 1/2" and have fitted K&N filters with AAA needles. Done the usual tuning and checked the plugs seems ok, but the depth of the jests are approx .080" down, does this seem OK?
Gary
G Roberts

What do your spark plugs look like? That should tell you if you are at least in the ballpark on mixture and general tune.

1) The fact that your jets end up at .080" down may mean that your needles are sticking out of the air piston more than they should?? As you are probably aware, the fixed needles should be set such that the shoulder of the needle is even with the bottom surface of the piston. Swing needles are set so that the sinter needle guide is flush with the bottom of the piston. If they stick out .020" then their relationship with the jet will be offset by .020" causing the jet to end up at .080", rather than at the more typical .060" (approx.).

2) Perhaps your float level is a bit off?

I think I would double check your tune up - valves, timing, float level - and then recheck the running of the engine. If you have to adjust any of these items, you may find that you also have to readjust the mixture, possibly raising your jets. A quick and easy check is with a vacuum gauge connected to a tap on the balance pipe on the intake manifold. You should be getting a vacuum reading of somewhere between 15.5" ~ 16.5" Hg. if you are pretty much correct. If way lower or higher, then your timing is probably off.

Good luck!
Bob Muenchausen

An even better Q to ask yourself is "How is my car running with this setting?" If you are happy with it, if it is in tune, if it is getting 30 to the gallon,highway figures, and takes off smoothly, doesn't jerk and stumble, why worry about a decimal figure? If it isn't, set the needles flush, as Bob said, and then simply follow the time -hallowed procedure in the workshop manual.
Ken R.
Ken Rich

How does .080 relate to the starting position of 12 flats/two turns?
Paul Hunt

.080", or any other distance in inches, is precise and measurable - 12 flats, not so much. The starting point I have always used is .040" for 1 1/2" carbs and .050" for 1 3/4", remember Gary, that's only a starting point. As Ken said, what actually matters is how the car is running. Have fun, Wade
Wade Keene

Hi
Just to say a big thanks to everyone. Finally makes sense, when I fitted the AAA needles last week it was pouring down with rain, checking today ( in glorious sunshine) I can see that the cintered bush is below the piston floor by approx .020". Resetting the needles and a quick tune and run around the depth of the jets is now .065" which is about normal and about the two and a bit turns in on the adjusting screw.
Where would we be without the BBS?
G Roberts

G- Ya know the feeling when you get a really good deal on buying a used car??? Thats how I've felt discovering this and the other boards. Some of the guys are so generous. And you get guys like Peter, Sean, and SteveS providing nuggets that could not be found outside. Feels like a real "Brotherhood in B's". I always make an extra effort to complete post and -when I can- include prices or expectation. I'll tell you where I'd be....wondering how to keep em all running at the same time is where. God save the BBS! VicPS BTW I've been using a colortune- alternately- on 1 & 4, O2 sensor( K&N), and the gunson C) sensor up the wazoo, for my SU tunes on both the H4 and H6...way overkill and about as good as it gets at 6700 feet above sea level.
vem myers

This thread was discussed between 06/06/2003 and 07/06/2003

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This thread is from the archive. The Live MG MGB Technical BBS is active now.