TR2/3/3A Periods of Intermittent Missing

[Geo Hahn]

I think you're going to have to 'catch it in the act' -- IOW, check some things at the moment when you are experiencing the problem.

One thing I would examine is what the floats/bowls are doing when it happens. Possibly a valve is sticking open and flooding two cylinders with fuel. This can usually be spotted by just looking in the throats of the carbs (air cleaners off) to see if lots of fuel is getting sucked into one carb.

One my cars was doing this -- but only on hard left-handers. Would bog down and stumble, then clear shortly after coming out of the turn. In that case it was just a float that had lost most of its float.

 

[CJD]

Is it engine hot, cold, or both?

Since you don't mention backfiring, it's not ignition, but at least you have a full ignition tune up now!

Check the fuel pump...and if it's only hot, then wrap the fuel line between the pump and carbs to see if it helps (vapor lock).

 

[charleyf]

I have not seen a mention of the possibility of a problem in the fuel lines at the float bowl valve. I had sme debris in the fuel line immediately above the valve. It would temporarily starve the one fuel bowl, which then starved two cylinders. Then it would start flowing again and refill the bowl.
Charley

 

[RedTR3]

Is it engine hot, cold, or both?

Since you don't mention backfiring, it's not ignition, but at least you have a full ignition tune up now!

Check the fuel pump...and if it's only hot, then wrap the fuel line between the pump and carbs to see if it helps (vapor lock).

John,

It is both hot and cold....And yes I already tried insulating the fuel lines

 

[RedTR3]

OK, well, this afternoon I removed the carb jets to inspect them, to my untrained eye they looked OK. I started the car and it was doing its "stumbling" episode. Not being an expert at anything, I decided to "fiddle" with the carbs, rapping them with the plastic end of my screwdriver...the stumbling went away and I was able to drive the car for a mile or so (as far as I wanted to go). I'm not sure if the rapping actually solved the problem or if it was lucky timing. I did notice that the car seems to be using a lot of fuel also according to the gas gauge.

Maybe we are on to something with the carbs, I am just not experienced enough to troubleshoot them. Any suggestions on my next step?

 

[Geo Hahn]

In my experience, way too much fuel and way too little fuel can produce similar results, making it hard to know which way to look.

The reason I suspected that the needle (or Grose just if you have that) was sticking open (rather than closed) was the "noticeable black smoke" you mentioned.

As I said above, just looking in the carb throats when the problem is occurring may reveal excess fuel in one. You may even be able to examine the plugs and see if two are decidedly more fouled/black than the other two.

If you can't reproduce the problem right now you might at least remove & clean the needle valve and assure that the action is free & loose.

 

[RedTR3]

Thanks George, now I guess I need a little education...I always assumed that if the needle valve stuck open then the excess gas would flow out of the carb overflow tubes. What you are saying is that it can also flow to the engine and foul things up?

 

[Geo Hahn]

Thanks George, now I guess I need a little education...I always assumed that if the needle valve stuck open then the excess gas would flow out of the carb overflow tubes...

My bad. The incidence I had of this was on the Strombergs - the 'bowls' and floats are under the carb and no overflow, just flooding. You are correct, stuck open on an SU would send fuel out the tube.

 

[CJD]

I think you are onto something. If rapping the carbs helped (assumining it wasn't just a coincidence), then I would look at dirt or water in the carb bowls. That would make it run fine till a bit of dirt, or a gulp of water, leans out the mixture.

It's not too hard to remove the bowls and clean them out...maybe 5 minutes each, and then see how she runs.

If that is non-productive, then I think you are back to checking the fuel pump output. Dirt or rust in the line can make the pump valves stick and reduce pressure.

 

[charleyf]

My bad. The incidence I had of this was on the Strombergs - the 'bowls' and floats are under the carb and no overflow, just flooding. You are correct, stuck open on an SU would send fuel out the tube.

I do believe that a stuck valve would do both as it would raise the fuel level in the carb causing some flooding and fuel would also go out the overflow.
If tapping on the carb helped then my idea of debris in the fuel line before the float bowl needle valve tends to fit. Try removing the needle valves and blow or clean out the fuel lines.
Charley

 

[RedTR3]

OK, I am starting to investigate the carbs now...I removed the air cleaners and first thing I noticed is that on the front carb, the piston does not always drop down cleanly to its lowest position...it sometimes hangs up a little perhaps 1/8 -1/16 inch from bottoming. I don't know if this is "the" problem but certainly it is not right. What is the procedure for fixing this so that the piston moves freely all the way?

Tim

 

[charleyf]

OK, I am starting to investigate the carbs now...I removed the air cleaners and first thing I noticed is that on the front carb, the piston does not always drop down cleanly to its lowest position...it sometimes hangs up a little perhaps 1/8 -1/16 inch from bottoming. I don't know if this is "the" problem but certainly it is not right. What is the procedure for fixing this so that the piston moves freely all the way?

Tim

If you have a manual the procedure for this is called "centering the jet". And yes this could well be the cause of your problem. You can also find this online.
Charley

 

[sp53]

Looking at your picture it looks like you have the banjo bolts on your carbs. If you did not know, they have screen filters right and the banjo bolt. The union that connects the 2 cabs together might be loose also. Plus like john suggested the bowls could easily be dirty. I had a similar problem and it turned out to be the needle and jet/seat and it was new. What I do with stuff like this is pull all the plugs every time it acts ups and look at them for carbon, raw fuel, or oil. I keep 3 sets of clean plugs around for problems like this. Sometime I will replace the plugs or plug that are fowled with one that did not fowl from another spot and put the fowled in another hole just to see if that cylinder will burn the plug off. This gives me an idea about the condition of a given cylinder or valve.

 

[TR3driver]

I'd suggest first removing the dome & piston, to check that the piston moves absolutely freely in the dome. You shouldn't even hear a rubbing noise. Also pull the jet out and check that it is clear inside. It's possible (tho unlikely) that there is a bit of FOD (foreign object or debris) in the bottom of the jet that sometimes jams the needle. Then center the jet as Charley suggests. In my experience, jet centering problems are consistent, not "sometimes".

But it's definitely not right and worth attending to before investigating anything else.

If you do find that the piston rubs on the dome, try swapping pistons between domes. They are supposed to be a matched pair, but sometimes people get them mixed up. I believe there is a Charlie Twist video on the University Motors site that addresses this, but don't have the link handy. He also shows how to do the "drop test", to check how well matched they are.

BTW, the jet centering needs to be "right on the money". If it is just slightly off, the needle and jet will wear from rubbing on each other, which messes up the mixture profile (making it go rich at idle) even after you correct the centering. Since the mixture is adjusted at idle, but the adjustment affects the entire range, you can wind up with the mixture going too lean under cruise conditions. Which in turn can cause poor performance and overheating.

Also, I've discovered that on my carbs, there is enough play in the connection of the dome to the body to affect the jet centering. To make the dome attachment repeatable, I install the 3 screws loosely, then gently turn the dome clockwise while snugging the screws down. This might not apply to all carbs, but certainly won't hurt.

PS, to check the centering, lift the piston a bit and let it fall. It should land with a clearly audible 'click'. If not, the centering is probably off (although it could be binding as above). I prefer to do this test with the jet in the full up position (mixture nut full lean) to help ensure a bit of clearance with the jet in the normal position. But, it makes the centering adjustment really finicky.

PPS, the "SU tool kit" includes a centering tool that is junk in my opinion. It centers the jet to the body, not the needle. If the hole in the body could be made perfectly centered to the needle, then there would be no point in providing an adjustment! But some people seem to like the tool, so maybe their carb bodies just happen to have the hole in the right place.

 

[RedTR3]

OK, I spent the morning working on the front carb, making sure that all parts move freely and I centered the jet. The centering took quite a bit of trial and error, but I finally got the piston to move up and down freely (When it was sticking, it was difficult to move up also). The engine runs quite well now, although I have not put it all back together yet for driving...I want to clean the float bowls as suggested - but in doing that I am going to replace the float bowl gaskets which should arrive tomorrow. I am also replacing the float bowl needle valves as cheap insurance.

In a few days I should have it back in driving condition and I should be able to report back in a couple of weeks.

Thanks for all your help so far, and Go Giants!

Tim