Well, here’s one I haven’t seen before. My middle carb (1963 OTS 3.8) is sucking significant air through the suction chamber breather holes. And I don’t mean just a little air movement. As you open the accelerator, the sucking through the breather holes is quite strong. And at idle, if you plug both holes, the engine starts to stumble and die. None of these symptoms are showing up in the front and rear carbs.
No obvious issues that I can find. Just took off the suction chamber bell and piston and other than needing a little cleaning after a year since carb rebuild, I can’t see anything obvious that would be causing this. There is no air restriction of the carb throat. Piston rises same as the other two under accelerator opening. Rises and falls at the same rate as the other two carbs. All dampers have same oil at same level. Butterfly is not sticking and is seating properly.
I’m going to go get some carb cleaner and spray everything and clean it all up and reassemble and see what happens. I’m just stumped on why this would be happening. And yes, I’m being very careful not to scuff up any of the metal and not hit the needle.
Anyone run into this before? I tried a search, didn’t spot anything in the archives, but I certainly could have missed something.
Are the idle adjustment screws all the same on the three carburetors? If memory serves me correctly they should all be run down to their stop, and then backed off 3 turns or so.
No change after cleaning everything of the middle carb. on closer scrutiny, I do think the middle carb piston rises farther than the other two and drops slightly quicker. That could be because it’s sucking so much air through the breather holes.
You could try a drop test. Also, swap piston/bell assembly with another carb (not permanently) and see if the problem follows the piston/bell or stays with the carb. IMHO.
In the end, assuming the suction chamber/piston swap doesn’t change anything, I’m going to put new plugs in and do a full carb tune procedure and see if that cures things.
Well, did as Rob suggested…and the problem followed the suction chamber and piston. The middle carb now is fine and the rear carb is sucking air through the breather holes. So it’s definitely something to do with the suction chamber/piston combination. So, now I have to figure out what exactly that problem is. The jet needle looks perfect, so there’s something going on with the interaction between the piston and chamber. Next I’m going to swap the chambers only and see what happens.
OK, so it’s the suction chamber bell that is the problem. Damper swaps don’t make a difference. I left the middle carb piston on the rear carb and moved the middle dome/bell back to the middle carb and the problem followed the dome/bell.
So, now I’m really confused. Why would the dome be an issue? Possibly a crack? Not seating properly on the carb? But neither of those would explain sucking significant air through the breather holes.
I’m stumped. I’ll carefully inspect the domes but I know they are the same part number, so that’s not it. I will compare them carefully and see if I can spot a difference.
I assume it’s nothing to do with the dampers. There are two types as you know, but I can’t think of that being an issue. I’d try a drop test–takes just a minute.
I’m not sure what the drop test will tell me. I first moved the entire middle carb piston, chamber, and damper to the rear carb and put the rear carb equivalent part on the middle carb. The problem followed the piston/ dome to the rear carb. I then left the middle piston in the rear, and rear piston in the middle and swapped just the domes. The problem followed the dome. I then tried moving the dampers around. The problem stayed with the middle dome.
So if the middle dome causes the problem even with a different piston (and also different dampers), what is the drop test going to tell me?
Don’t get me wrong. I’m happy to do the test, but since the dome seems to be the problem irrespective of which piston is paired with it, I don’t understand what the drop test will tell me. I’m probably missing something here and happy to be corrected and happy to do more testing to see if something changes.
So, let’s say the dome is the problem (and in the morning I’ll do similar swaps with the front carb just to be sure), what do I do with that info? Can one purchase a new dome? I would think it would make sense to purchase and done, piston, damper set if that’s even possible?
I still don’t understand how this problem suddenly cropped up? And still not sure of the mechanism causing the breather hole suction?
I’m befuddled and bemused at this point. Par for this course, I think.
I don’t know either. But I was thinking that leakage between piston and bell, if excessive, might cause your symptom. That’s what’s measured with the drop test–the seal between piston and vacuum chamber. It should be tight, but with no physical contact. In your case a faulty (perhaps warped?) chamber should have a fast drop time with either piston–compared to spec and/or compared to another vacuum chamber. Just grasping at straws, really.
Another possibility might be something different about the vent holes? But I can’t see how they could get bigger–only plugged with dirt.
I assume that you are talking about the two holes in the front flange of the carb - where the three pipes to the plenum are bolted. These go to the underside of the piston to allow it to rise and fall. If you are getting air flowing throu them as you suggest it could only be from the manifold vacuum - that is vacuum is getting by the round lower section of the piston. The fit of the piston into the bell should have nothing to do with it. The fit of the lower piston into the carb body would but you say you have eliminated that and the problem is caused by the bell.
How do you know that a significant amount of air is entering these holes? Blocking the holes would, on a normal carb, result in the piston being sucked down and poor running as some vacuum must get by the lower piston carb interface into that area. If you remove the plenum and expose the carb intake to the atmosphere you will equalize pressure in the areas of the carb outside the butterfly, and in particular the area under the piston that gets air from the holes. That should cause the extra airflow to cease. Not a solution but perhaps an additional clue.
I have been running these tests with the air cleaner plenum off. The manifold is wide open to the carb throats with no restrictions. I can remove it, but not sure how that changes anything.
Yes, I’m talking about the two open breather holes at the top of the flanges.
The two holes on the carb with the dome in question suck enough air at idle to be loud and you can really feel it if you plug the holes with your finger. It’s almost as if the carb throat is not pulling enough air on its own even though the throat is wide open. As throttle increases, suction at the breather holes increases significantly and you can hear and feel the increased suction through the breather holes. This is not the case with the other two carbs. And when I swap around the suction chamber (or dome or bell…whatever you want to call it), the suction at the breather holes moves to the carb with the suspect dome.
The car will run and the pistons…all three…will rise and fall as one would expect. I can see them doing it. If I plug the two breather holes on the carb sucking significant air through them, the engine will stumble and try to stall. That’s NOT the case on the other two carbs. While certainly there is some airflow through those breather holes, there is not the big suction effect as on the one carb. And when I move the dome to another carb, then that carb starts having the same suction issues and the other two act normally. It has something to do with the dome.
This all started because I was trying to chase down a stumble at low idle when the engine is hot. The car has been running fine. Then out of nowhere, when I’d come back from an hour long run, the idle would be low and stumbling. I can’t think of any reason the dome would suddenly be causing this problem. But until I can solve the dome related issue, it makes no sense to try to retune the carbs.
Oh, and moving the mixture screw on the carb with the problem dome make no discernible difference to the idle speed.
Again, the breather hole suction issue moves to the carb with this specific dome when I swap domes. The problem does not follow the piston, it follows the dome. I can swap just the domes without moving the pistons and the problem moves with the dome.
I fell like I’m repeating myself here, so I’m going to stop now. Frustration is setting in. And it too early in the day (5:45am) to solve it with a nice single malt.
I have vented dampers, this no internal chamber vents.
Here’s some verbiage that goes with that figure:
“Many of the SU carbs I see have had dampers replaced with the wrong type. Referring to figure 10 it should be noted that the area above the hollow piston rod must be vented. otherwise pressure will build up on the upward movement of the piston andvacuum will occur with downward movement. This will restrict normal piston travel. Venting may be done in two ways. The cap may be drilled to allow venting to atmosphere or the web or gusset on the chamber neck may be drilled to allow internalventing back into the suction chamber. You must have one or the other but not both. If you have a solid cap and no internal drilling there is no vent and pressure/vacuum conditions will occur as aforementioned. If the cap is drilled and the web is alsodrilled. then there is a direct air leak into the suction chamber. If the chamber neck, such as found on 1 1/4" Sprite carbs, has no web then it cannot be drilled internally and must have a vented cap.”
It’s not the carb, per se…it’s whichever carb has the questionable chamber on it at any given moment. The problem is isolated to the individual chamber, not an individual carb.
Thanks for all the responses and ideas so far. Still trying to decide the best next step.
Well, I just bought a used matched HD8 piston and suction chamber set on eBay. Should have it sometime next week and will see if that solves the problem with the suspect suction chamber. Will let you know how it works out. Assuming the new dome solves the breather hole suction issue, then I can go ahead and retune the three carbs. Thanks for all the suggestions and ideas.
Bought a used chamber and matched piston. Arrived today. Needle on the “new” piston was larger than the old piston, and I was too lazy to switch needles, so simply switched the “new” dome with the problem dome. Presto chango! No more massive air suction though the two breather holes in the flange. Now it is exactly like the other two carbs.
Awaiting my tach/dwell multimeter to arrive on Friday so I can go ahead and rebalance/tune the carbs. I’m an old school guy, so I bought this:
I couldn’t help myself. Similar to what I used to tune my ‘73 MGB and ‘71 E-type coupe back in the late ‘70’s/early ‘80s. And even if it’s off a few percent it will be significantly more accurate than the Jag’s tach.
Anyway, problem appears to be solved for now. We’ll see how she holds a rebalance/tune.
I have one of those! Just used it to initially balance the carbs. I finished up using my Acton multimeter that has an RPM function. Either is fine, digital readout vs. analog needle.
Accuracy really isn’t important in terms of absolute number. You’re just trying for the highest/smoothest rpm at idle and then setting the speed around 750. I balance the carbs with a Synchrometer.
