Carb running rich

[arcturus]

Hi, H4. 1.1/2"on B16 running rich The plug is really sooty after a long run. I have the jet adjustment nut up as far as it will go. Will this be the result of worn Needle/jet assembly or should I be looking elsewhere?
Needle as spec' GT.

Sorted! the jet wasn't going up all the way as you can see in photos.

[Ron Kwas]

Arcturus;

That Jet is still waaaay below flush with Jet Housing, and raised to flush is where I would adjustment to to allow it to be adjusted to...

I'd first check why Jet is not able to be adjusted to flush, but just for reference, other possible causes for a rich mixture: Metering Needle mounted recessed into Dashpot, worn MN.

Good Hunting!

[blueosprey90]

Well, it seems that for some reason, the adjuster nut is not moving the jet. My humble guess is that the entire jet assembly is spinning in the housing as you turn the adjuster nut. There is a locking nut higher up, above the adjusting nut, that may be loose.

This John Twist video may give you the confidence to disassemble.

I run dual SUs and set the jets with a caliper (the end part that slides in and out). I usually set the jets at .09" below the bridge, but this is probably too lean for you. I would target in the range of .010" to .012". I agree with Ron. It looks like your jet in in the range of .030 to .040.

https://youtu.be/sxVFCY_1aSA

[arcturus]

I think that there is a problem with the whole jet assembly on the carb,no matter what I do I cant get it right. Striped it down again and double checked every thing and now it' running lean. I think that I will replace. No problem at all with rear carb.before you ask the throttle shaft isn't worn.

Checked everything out again and set jet height as suggested. 0.10". No problem with rear carb but front wouldn't go below 0.35" before whole assembly turning.Also spring week and not pulling jet up firmly.
Will order all the bits and wait,probably three or four weeks for delivery. Burlens are really slow.

[Derek UK]

The jet block/guide is screwed into the body and locked firmly there by the thin lock nut. The block should be just below the face of the bridge. If you can see a few thou of the carb body that's fine. It shouldn't be flush. The jet should then be able to go up flush with the top of that. Baseline for tuning should have the jet wound up to that position. Screw down your favourite choice of flats to start with. Usually 12 with the HS6, it may be different with the HS4. If the jet guide nut is tight it shouldn't move when you wind up the adjuster nut.
This thread and the associated ones with it might prove useful. The issue discussed is a washer. Might this be your problem?
https://www.mgexp.com/forum/mgb-and-...stion.3801168/

[arcturus]

Originally Posted by blueosprey90

Well, it seems that for some reason, the adjuster nut is not moving the jet. My humble guess is that the entire jet assembly is spinning in the housing as you turn the adjuster nut. There is a locking nut higher up, above the adjusting nut, that may be loose.

This John Twist video may give you the confidence to disassemble.

I run dual SUs and set the jets with a caliper (the end part that slides in and out). I usually set the jets at .09" below the bridge, but this is probably too lean for you. I would target in the range of .010" to .012". I agree with Ron. It looks like your jet in in the range of .030 to .040.

https://youtu.be/sxVFCY_1aSA

Won't run at 0.010" two flats rich and no problem at 0.025". Is this measurement your starting point or final setting?

[arcturus]

Originally Posted by Derek UK

The jet block/guide is screwed into the body and locked firmly there by the thin lock nut. The block should be just below the face of the bridge. If you can see a few thou of the carb body that's fine. It shouldn't be flush. The jet should then be able to go up flush with the top of that. Baseline for tuning should have the jet wound up to that position. Screw down your favourite choice of flats to start with. Usually 12 with the HS6, it may be different with the HS4. If the jet guide nut is tight it shouldn't move when you wind up the adjuster nut.
This thread and the associated ones with it might prove useful. The issue discussed is a washer. Might this be your problem?
https://www.mgexp.com/forum/mgb-and-...stion.3801168/

No, I have the correct copper washers in place.I can assure that the nut is tight.I just think that the face is worn. By replacing the whole assembly I won't be chasing my tail.

[blueosprey90]

I've found that 1 flat is good for about .01" of adjustment.

I'm currently running my MGA with H6 SUs at .095" and .097", but I've run them as low as .008" and the engine was sweet!

I'm running the Volvo with H4 SUs at .010 plus two flats, so say .012". When I first got the car, the carbs were set at .013" and .0135" and I was getting a lot of fumes. Of course, that could have been the overall tune of the car, and I also reset distributor and timing to address.

I like to run lean, but there is a danger that the car runs too hot and burns a valve and causes valve recession. On the MGA I have hardened seats and also run a dual exhaust gas temperature gauge to monitor exhaust temperatures. On the Volvo, I don't know whether the head has hardened seats and I don't monitor temperature, so I run a bit more rich.

Normally I think the starting point for any SU carb tune is 2 1/2 turns.

You might want to analyze your needle profile. Seems like your needle might be too thin. Is it worn??? The GT needle profile is on page 41 here. http://mgaguru.com/mgtech/books/pdf/...arburetors.pdf

[Burdekin]

Originally Posted by arcturus

I think that there is a problem with the whole jet assembly on the carb,no matter what I do I cant get it right. Striped it down again and double checked every thing and now it' running lean. I think that I will replace. No problem at all with rear carb.before you ask the throttle shaft isn't worn.

Checked everything out again and set jet height as suggested. 0.10". No problem with rear carb but front wouldn't go below 0.35" before whole assembly turning.Also spring week and not pulling jet up firmly.
Will order all the bits and wait,probably three or four weeks for delivery. Burlens are really slow.

The H4 starting point setting is 0.04” or 1mm below the bridge.

[Burdekin]

Originally Posted by arcturus

I think that there is a problem with the whole jet assembly on the carb,no matter what I do I cant get it right. Striped it down again and double checked every thing and now it' running lean. I think that I will replace. No problem at all with rear carb.before you ask the throttle shaft isn't worn.

Checked everything out again and set jet height as suggested. 0.10". No problem with rear carb but front wouldn't go below 0.35" before whole assembly turning.Also spring week and not pulling jet up firmly.
Will order all the bits and wait,probably three or four weeks for delivery. Burlens are really slow.

Also both carbs jet height should be set the same.