'61 MK2 Carbs flooding after rebuild, but float level believed to be correct

Hi folks -

I’m working on the MK2 today, because I like to punish myself unnecessarily.

I recently rebuilt both carburetors and the thermo-choke / starting carb. The reason for the rebuilt was significant particulate (rust, etc) that had gotten up into the carburetors and the car would no longer start. The car had not been using any fuel filter and its anyone’s guess how long it had been that way.

I rebuilt the carbs and thermo-choke unit, pretty easy really, and reinstalled. I ran the fuel pump to get fuel up into the bowl and then onto the individual fuel bowls of each carb, and noted the following (engine and pump off):

  1. Fuel was gathering and ultimately dripping out of the front carburetor - from the “throat” or place you’d attach the air filter.

  2. Upon pulling the piston and needle out, fuel literally spouted out of the jet, I’d say it got about a half inch in height.

  3. No fuel was leaking from the rear carb (at first)

I dropped the needle and piston back in and put everything back together, the flooding seemed to subside somewhat, so I started the car - It started almost immediately but had trouble running. I then noted that the Thermo-carb was also leaking, this time fuel was escaping from where the solenoid meets the body of the thermo-choke - not pouring, but a noticeable leak that dampened the surrounding parts and ultimately created a puddle

Due to a low battery, I was unable to get the car started again - so I’m charging the battery as I type.

I left it for the day and came back today to find that both carbs have a flooding condition, noted by fuel gathered at the mouth of each carb. No fuel ever came from the overflow tubes, tubes are clear.

  • I checked and double checked the 7/16 bar method of setting the floats - but will obviously check again. The only other thing I really changed was that the valve in the float chamber was the check-ball style Grose-Jet, and I replaced those with the traditional needle and seat design that came in the kit - I’m considering reinstalling the Grose-Jets…good idea?

The carbs did not leak nor flood previously, and I am using the same floats (rubber) and even the same float setting (I’d set them previously, and really just verified it during rebuild)

Any thoughts here? Fuel pump is the same, floats are the same, I’m 98% sure of the float height and the only thing changed was the Grose-Jets to the regular needle and seat - but I read some poor reviews of the Grose-Jet, which is why i changed it. My rebuild kit was not an SU kit (none were available) but another brand kit from Moss Motors. Car did not have flooding issues before, carbs just leaked a bit from bad gaskets and were full of debris.

I did of course change the main jets, but did not change the needles (old ones looked great, and would not come out anyway) but I don’t recall fuel spouting out of the jet, even with the needle and piston removed, so I don’t think that’s the issue.



If the fuel is leaking from the base of the carb throat, (jet area), it can pretty much only be caused by a high float level. If the float height is set correctly, and it sounds like it is, did you check that the needle valves were actually shutting off. Hold the float bowl top upside down and blow through the inlet. The weight of the float arm should be enough to close the valve. Sometimes the tang on the arm is bent enough to let the arm drop down so far it jams, even though the float height is correct, the single tang limits downward travel. Also make sure the floats actually float.

@OldJagNut “did you check that the needle valves were actually shutting off”?

Embarrassed to say…no, I didn’t. Didn’t even think about it - just installed them and that was that. So I’ll give that a go.

Can you please check that you have the correct fibre washer on the top of the fuel bowls. The washers are scalloped to allow the bowls to vent to atmosphere. Without the vent the floats cannot operate correctly

Hi @Phil.Dobson . So yes, the red fiber washers are the scalloped type. The kit provided a new aluminum washer for the top-side, then the red fiber washer with the three “teeth” or “notches” for the bottom. So assembly went as follows - Banjo bolt > green tag>aluminum washer>overflow fitting>red fiber washer> fuel bowl lid.

When I rebuilt my HD8s I discovered just what Kevin describes above. In one of my carbs, the float arm was free swing down and bent just far enough when the bowl was totally empty and the float sitting at the bottom it was possible for the arm and valve to jam rather than the arm push the valve up. I bent the arm back to proper shape back in the very slightest of increments until it was positively not jamming even with the float sitting at the bottom of a dry bowl. I did this with bowl not yet bolted on to the rest of the carb so that I could test it with water as opposed to gas. Different density but obviously if the float floats in gas it will float in water too. And water all over my hands is less bothersome than gas ha!

Thank goodness the carbs aren’t very hard to remove from the engine at all! At least they aren’t on my Mark X. How is it with the Mk2?


……“In one of my carbs, the float arm was free swing down and bent just far enough when the bowl was totally empty and the float sitting at the bottom it was possible for the arm and valve to jam rather than the arm push the valve up. I bent the arm back to proper shape back in the very slightest of increments until it was positively not jamming even with the float sitting at the bottom of a dry bowl”……

That is one of the most common causes of S.U carburettors flooding and was recognised way back by the company. The cure is to fit a modified arm, which I believe is now the standard replacement, and not to bend the arm, that was inclined to introduce any other issues. If my memory is correct the modified arms had an extra adjustable tang specially to limit the downward drop of the float.


……”So assembly went as follows - Banjo bolt > green tag>aluminum washer>overflow fitting>red fiber washer> fuel bowl lid”………

During the 50’s and 60’s there was a change to the call up of the S.U AUC 1928, (Jaguar part 1298) serrated fiber washers. The call up went to TWO washers instead of ONE per carburettor.

If you were to look at the parts manuals for the 3.4ltr /3.8 ltr MK2 and S type you will see 2 x AUC 1928 washers specified per carburettor, unfortunately Jaguar missed this modification in earlier parts lists, including E-Types. When I was at the S.U service school in the early 60’s we were warned that various maker’s service parts lists were not up to date on that issue. Unfortunately there is still confusion today, even in this thread. Fitting only one serrated fibre washer can cause restricted breathing.

You also need to examine the underside of any shielded type of metal top cap which has pressed in depressions, because they can become misshapen and restrict breathing.

Other points that may help, always fit spring loaded float needles and consider making sure that each and every float chamber lid is fitted with a “tickling” pin. Some of the MK1 / MK2’s have no pins on the rear carburettor. The pins are useful to judge the depth of fuel in each chamber without any dismantling and are useful to temporary flood carburettors in low temperatures.

Well folks, I fixed it. Here is what I found:

I had two main observations:

  1. As mentioned by both @OldJagNut and @dmaggs, upon removal of the front fuel bowl lid, the float arm did appear to be jammed in the open position. It seemed to me that the needle and seat “slide” action may have been at fault in my case, but it could have been arm jamb condition as described above just as easy. Everything else looked correct, and the float was indeed floating, but this lead to observation number 2:

2a) I noted that the float, while yes floating, wasn’t overly buoyant. Its hard to judge with a tiny float, but it didn’t really have much in the way of upward force - certainly less than I expected, though again, small float with very little fuel. That said, this got me thinking - a stock brass float filled with air, I suspect, would be more buoyant than a rubber float. I came to this completely unsupported conclusion because 1) The brass float is probably lighter overall 2) The brass float has a thinner structure 3) The brass float would have a higher volume of air to support it, versus the nitrophyl, which I am not even sure uses air at all. This grand assumption caused me to experiment and observe part B:

B) The force on the float arm required to close off flow (of air in this case) was greater when using the standard needle and seat, that it was with the Grose-Jets design. This was 100% seat-of-the-pants experimentation with no control whatsoever, but…in as best consistent conditions as I cared to create, it took a little more effort to close the valve with the needle and seat, than it did with the Grose-Jets.

When I combined this finding, with the idea that the floats themselves may have less buoyancy than standard, I wondered if the needle and seat was closing completely at all, regardless of sticking arms (the rear carb was also leaking, but did not have a stuck arm). If I accept that the floats are perhaps not providing the same force as a standard brass float, and compound that with a needle and seat that seems to require greater force to close, the issue seemed clear. SO…I replaced the needle and seats with the Grose-Jets, and tested it out.

Result - Car fired up first time, no delay, no leaks whatsoever. Conclusion? Certainly it was a over-fuel condition in the fuel bowls, the front most noticeably. Cause? Perhaps a stuck arm, maybe poor fit between needle and seat, a float that isnt “floaty” enough or a needle and seat that require more force to close than the float provided. In either of those cases however, the Grose-Jets fixed it.

Knowing my luck on this car, the carbs will leak somewhere new tomorrow, but for today, all is as it should be. Thanks all for the help in narrowing this down.


Good testing and troubleshooting! In case you want brass floats I believe I’ve seen them available from SNG and Moss! It makes sense to me that the hollow brass floats might be more floaty than rubber floats as you say but I also do not have any special knowledge of your particular rubber floats’ density. So the rubber floats just came in the car when you acquired it? Or was them some other reason you are using the rubber ones? Just curious! My carbs have the original brass floats and they aren’t leaking (knock on wood) so I left them alone.

The other advantage of brass floats is that, if they form a leak, you can repair them with solder.

So yes, the car came equipped with the rubber / nitrophyl floats. When I bought the car, I was told the carburetors had been rebuilt recently, but “rebuilt” must mean something a little different that I think, because the carbs were not really in good shape internally. Certainly work had been done, as evidenced by the Grose-Jets and nitrophyl floats, but I suspect it was either a half-hearted replacement of parts, or a rebuild that was done a very long time ago. I got a similar story on the brakes - rebuilt, yet at least two calipers have lost all ability to retain pressure. The “box o’parts” that came with the car does have old calipers in there, so someone replaced at least one or two, but again, I suspect it was not a total rebuild and probably done quite a while ago.

Yeah so you might want to consider going back to brass floats! Especially since you can get brand new ones so reasonably! Curious what benefits the rubber offered (other than maybe price) that made the previous owner choose to use them!

“…Curious what benefits the rubber offered (other than maybe price) that made the previous owner choose to use them!..”

The so called “rubber” or “non-sinkable” float type, sold by Burlen owners of S.U, have been used successfully for years. They were introduced to combat attack of the solder on brass floats, by modern fuel.

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I believe the main claim to fame was what @NWG points out - they physically cannot sink or leak.