Poor idle

[John Quilter]

I tend to agree.

[cass3958]

Over the last couple of weeks I have had the air cleaner parts off the car giving them a polish and today with them off I started the car and noticed a seeping of fuel at the base of the AED solenoid. Very slight seeping causing a wet patch which once the engine was off it evaporates very quickly. Any fuel leak is bad so I had to remove the carbs as you cannot get to the bolt without the carbs off. Sorted the leak which was a faulty fibre washer and put all the carbs back on and ran the fuel pump and all fixed. I did something else for a couple of minutes before I realised I had left the pump running so switched it off and now I can see a small amount of fuel pooling in the mouth of the right hand carb. Left carb is dry.

IMG_9059.JPG (1.31 MiB) Viewed 1886 times

I pulled the tops of the carbs so I could look down the jet holes and I could clearly see that the fuel level in the left carb jet hole was below the level of the top but in the right carb the fuel level was running over the top of the hole and pooling in the mouth of the carb. This is obviously a problem with the float levels as discussed in this post earlier. I pulled the float chamber top off and adjusted the tangs on the shut off valve so they sat lower so the float sat lower and the the fuel level in the jet was below the top. I had to adjust it a couple of times to get the right fuel height but now all is well.
Not had a chance to test it yet but ever since I have been running the car I have always had a problem with run on after I switch off the ignition especially when hot. People have told me this was due to bad ignition timing or using poor octane level fuel but I am now wondering if my problem all along has been excess fuel overflowing into the carb after shut down caused by this float not being adjusted correctly. The amount of fuel was tiny and would not last too long until the level of fuel in the float chamber had dropped to the correct level after the fuel pump had stopped running.

[Glyn Ruck]

Excess fuel will cause run on.

[JCS]

Rob

If you have been suffering fuel overflow, by the nature of the carburation system you would expect an idling synchronisation issue. In the situation you experienced the opposite carburetter must be set weak, or the engine was / is idling erratically.

Perhaps it is time to check if the jet drops are equal. If they are equal and the engine idles smoothly then that overflow issue can only have been recent.

Norman

[awiedie]

Can't believe how time goes by, but especially in these crazy times it does. I finally backed the old S type out today to see if my carb rebuild had an effect on my poor/no idle problem, and I am happy to report that the car started right up on choke and when that kicked out, the engine settled in to about a 650rpm idle and stayed there. Wow, what a relief. Next I have to fill up my leaky DG box and get it out on the road.

Here is a separate question about carburetors that maybe someone has run into and could offer an opinion. I restored a '56 MGA which I just recently sold to a very nice fellow about 600 miles away. He had it checked out by the shop that services his vintage Porches and they could only find a few niggly things that they suggested be attended to. The fellow called a few days ago and said that he noticed that the damper on the rear carb, an H4, was not screwed all the way in and that when he tried to tighten it, it did at first and then loosened up, like a stripped thread maybe. He had to drive the car about a 100 miles to a summer home the next day, so he decided to put some duct tape around the top of the damper and we would sort it out later (the car ran great during his trip). So, my question is, has anyone seen the threads getting stripped either on the damper cap (brass) or in the suction chamber body? I can't imagine how it is possible and he has not checked to see if there are really bad threads in either location. He is totally unfamiliar with these carbs so I would like to help him out.

[cass3958]

Can't help Alan as the threads on my damper caps are bakelite plastic.

[Glyn Ruck]

Alan. I have never seen carb body threads stripped or plastic or brass damper threads. Hopefully the problem lies with the dampers as thy are cheap. But full spares are available from Burlen.

[JCS]

As you will be aware the damper cap threads are very fine, and the damper rod on a 56 MGA are fixed rigidly in the cap. Recent replacements are plastic and have a floating ball joint arrangement in an attempt to prevent cross threading and a bent damper rod.

Drivers and in fact mechanics, not familiar with these dampers often cross thread the first thread. In fact it is very common to find the first one, or even two threads are damaged. This does not cause too much trouble IF the cross threaded damper has been backed off and refitted again, many were damaged years ago and are still serviceable.

However, once in a while an unfamiliar driver will try to use a spanner or tool to tighten the cap, the presence of any hexagon cap especially misleading them to use a spanner. Normally this will take the edge off the first thread on the damper rod and it can be unscrewed, if the operator persists with the spanner/tool the dashpot threads get damaged. On some models the damper cap has a serrated circular cap to encourage finger tightening, but motor movers still often use all methods to tighten.

With care, the threads on the two threaded parts can be recovered by squarely lining up the two parts before engaging the threads manually. This can be done easier if the oil is either emptied out of the dashpots and the damper rod is checked to ensure it is not bent, or the dashpot is removed for ease of thread recovery.

The threads must have been really abused if they are not recoverable, and usually because a spanner/tool has been used. In fact, it is virtually impossible to strip the threads or even heavily damage one, if the damper has been replaced by finger tightening in the correct way.

The damper is only intended to be lightly nipped after the threads have been FULLY engaged, when the first threads are damaged it points immediately to initial cross threading.

One word of warning, if it is necessary to replace a damper always ensure both dampers are renewed, as it is very probable the damper valve settings have been modified since the car was produced.

[awiedie]

JCS,

Thanks for the comprehensive reply. Since I can't see the problem myself, and am relying on the new owner's description, it does sound like his mechanic may have done something he shouldn't. I'm going to have him send me some closeup photos of both the damper and the suction chamber threads.

I am also going to send the new owner a copy of this thread and maybe we can sort the thing out over the phone. One thing I don't get from your post though is the comment about replacing the dampers in pairs. Please explain "damper valve settings", it's new to me.

Alan

[JCS]

The valve at the bottom of a damper rod can, and often does vary, between engines. There are a number of different reasons for this variation from engine size, manifold volume, valve timing, valve size, exhaust back pressure and number of carburetters etc.

For example the largest engine to use S.U carburetters is the R-R 6.75ltr V8 and you would not expect the damper specification to be say, identical to the small Morris 1000 cc engine. The damper specification for the Jaguar XK engines also differs across the range.

A common variation is the damper valve lift, some dampers have very small valve lifts of a few thousandths, while a post war six cylinder Bentley for example has a 0.080 inch valve movement.

Originally there were dozens of different dampers but in recent years it appears that many have been standardised.

It is generally known that a secondary function of the damper is that it performs the same function as an accelerator pump on a variable depression carburetter. This is quite commonly discussed on the various forums. But it is never understood that the damper can restrict movement both up and down in some installations.

It is because of the more recent standardisation that I warned against renewing singular dampers. I trust that you will now realise why I made that point that the damper setting alter, and are set by S.U and the engine maker.