TR5/TR250 Car misses over 2500 rpm

[pdplot]

I posted last week about the smell of gas. Now a new problem has cropped up, probably connected to the gasoline smell. The car idles fine but begins to miss above 2500 rpm even when car is sitting in driveway and not under load. Front 3 plugs seem to be wet and gas fouled. Points fine, set to 15 thou. front carb mixture out 1 and 1/2 turns. Could it be a stuck float? This is the first running trouble I've experienced in 17 years of owning this car. I don't the carbs have ever been off the car.

PD

 

[This14u]

Stuck float or possibly water logged (gasoline logged?) float?

 

[tktrain]

Any black smoke from the tail pipe? Is the pipe black with soot? I have to believe that the front carb is to blame, start with that. Typically a rich miss will be load dependent, but you may be borderline. Could also be an ignition issue, carbon tracks on a cracked distributor cap can affect spark at the plugs. Is the gas smell all the time or only when the engine is running?

 

[Got_All_4]

points condenser could be bad. common for good idle and braking up under load

 

[poolboy]

Sounds like it might be a stuck needle valve, though.
Smell the oil on your dipstick. Does it have a gasoline odor ?

 

[pdplot]

Car was running fine last week. Just developed this miss.Timing ok. Points seem ok. Coil seems ok. Fuel pump? Who knows. I don't know how to test it.Just changed the oil and it's clear with no contamination by gasoline. Compression between 155 and 175 in all cylinders. Valve guides may be getting worn as some smoke on startup especially. Checked pistons in both carbs - idle almost stops when pistons lifted slightly indicating a weak mixture. Front 3 plugs - Bosch platinum - are sooty, tailpipe sooty. Rear 3 plugs light brown indicating normal mixture. Idles smoothly at 850 rpm and doesn't begin to break up until 2500 rpm. With all the British cars I've owned through the years - 6 in all - I never had any carburetor trouble with SUs - or these Strombergs for 17 years until now. I'm not familiar with them nor how to test them. Only clue -about a month ago, the front carb started leaking gas but it stopped right away.I suspect that's the culprit but who knows? Any more ideas from anyone would be appreciated.


PD

 

[mrv8q]

Carb diaphrams?

 

[hondo402000]

based on the information I have read, I would pull the front carb off, tear it apart, soak it in carb cleaner, blow it out with compressed air, check the float, check the rubber diaphram, check needle and seat, and put it back together, thats the cheapest fix, if that doesnt fix it then move toward the ignition

Hondo

 

[pdplot]

I checked the diaphragm. It was intact. I reinstalled the housing. I pulled off the fuel line and began to take off the front carb. Then I stopped. It looked like more work than I wanted to do so I pushed the fuel line back on and just for fun, I started the car. It ran perfectly with no sign of a miss at 2500 rpm or anywhere. What was the problem? Who knows.
I also drilled out the bonnet bolt hole and installed a 1/4x28 helicoil - actually 2 since they were small. When I put in the bolt, it still didn't seem to catch so I tried something else - a different (shorter) bolt. It seems to hold ok though I did not tighten it very much. Turns out there was a small plate welded behind the panel but it moved up and down a bit. I don't know if that was the way it was supposed to act but the bonnet latches perfectly so I'm leaving it alone. Two more jobs await. Installing a new window regulator and check strap in the driver door and installing pedal extensions on the clutch and brake pedals. Thanks for your suggestions.

PD

 

[poolboy]

Checking for a torn air valve diaphram was not a bad idea as per Kevin Browne


'Getting back to that fuel leak, if the oil change was before you had the leak, you need to check for gas odor in the oil.
The leak can come from only 2 places, either the O-ring seal around the float chamber plug or from the needle valve failing to cut off the flow of gas once the float chamber was full.
Even though the engine isn't running, there is still residual pressure in the fuel line as well as hydrostatic pressure from the level of gas in the fuel tank, both of which try to force gas past the needle valves

 

[pdplot]

When I pulled the fuel line off the carb, only a couple of drops came out. There was apparently no pressure buildup.I was prepared to block off the line with a bolt. Because this car takes so long to start when it's not been run for a few days, my TR6 expert friend (he's had, I believe, 14 of them) feels that the carbs are not getting enough fuel. That was why I was thinking about installing an electric fuel pump. How long should it take to start a TR6 that's been sitting for a few days?

PD

 

[poolboy]

A properly working mechanical fuel pump is all you need. It will provide more than enough fuel for 3 ZS carbs to to propel the TR6 over 100 mph for as long as you want to drive that fast.
You need to see if gas is making it's way to your pump and if it is figure out why gas is not getting to the carbs in proper amount.
But if the gas in the fuel lines has already leaked past the needle valves, I wouldn't expect to see much drip out when I pulled the line off the carbs.

You ask about how long it should take to start the engine ?
If the float chambers are run dry or the gas has evaporated and the float chambers need to fill up again, maybe 10 seconds of cranking.
If there is any fuel still in the float chamber after being set up for a period of time, starting should be instantaneous, provided that the proper amount of choke is applied and most importantly, provided that the engine is in proper tune.

 

[TR3TR6]

I'm running weber downdrafts on my 76. If it hasn't been run for 4-5 days, it starts right up using the mechanical fuel pump. If it been about a week or longer, I just give it full choke and it starts right up. I would check the flow from the fuel pump while cranking. If that's good, the front carb would be the suspect; maybe a stuck float.

 

[TR3driver]

After just a few days, there should still be enough fuel in the float bowls to start the engine (briefly). Sounds like you have a leak, or something wicking fuel out of the bowl.

It's not unusual for the valves in the pump to leak enough to bleed the pressure in the lines off, but it should take only 4-6 engine revolutions (remember the pump runs at half speed) to have fuel spurting strongly from the disconnected line. If not, you may have a stuck valve in the pump or something.

Which might also explain your problem with missing at higher rpm. The pump will still move some fuel with one valve stuck open, but not nearly as much as it should.

Electric pumps are better in some ways, but certainly not a panacea for other problems. And overall, I'd have to say I've had more trouble per 100,000 miles with electric pumps than with mechanicals. Of course, that may be just because the Stags used an SU electric pump

PS the book procedure for testing the pump on the car is to disconnect the line from the carbs and connect a pressure gauge instead. Start the engine (on the fuel in the float bowls) until the gauge stops rising, note the pressure, shut the engine off and watch how long it takes the pressure to start falling. The gauge should continue to read top pressure for "a short time" (I love how precise that is!) and the top pressure should be between 1.5 and 2.5 psi.

Most vacuum test gauges will also read fuel pressure.
https://www.harborfreight.com/fuel-pump-and-vacuum-tester-93547.html

 

[This14u]

If it's the fuel pump, why would the front 3 plugs be wet? Think you still have a problem with the front carb.

 

[pdplot]

They were only wet that once. The choke had been on for a time. Since then, I checked them twice. Sooty but dry. The float may have been sticking and may stick again. We'll just have to wait and see. This car has 103K miles on it and to my knowledge, the carbs are original to the car. Brass tag #3385R on the rear carb.

 

[poolboy]

If you have a 71 TR6, that carb is the version that was mounted.
The float is not likely to stick...the needle valve, however, well, that's another story.

 

[Got_All_4]

I know I'll start something here but I'd get a set of gross jets, clean them thoroughly with carb cleaner and blow them out with compressed air. Install them and never have any problems with sticking needles again. Had that problem with my TR3 a couple of years in a row. Each time I replaced the needles with new original style needles the problem reoccurred. It's been at lease 5 years now with the gross jets and no problems. Had the same systems as yours. Also when I turned to the left the car ran smooth until the gas ran out of the carb. 1st one too.

 

[TR3driver]

Funny thing about Grose Jets; some people swear by them and others swear at them. I have fixed several cars now by throwing the Grose Jets as hard as I could and putting back the stock needles. On my own TR3A, I went some 200,000 miles and only had trouble twice with stock-type valves ... both times I just wiped off the needle on the side of the road, put it back and went on. They don't "stick", but flakes from the inside of the fuel line can definitely cause problems. (I also replaced the soft lines, which solved the problem for another 10 years or so.)

A good friend let me drive his beautifully restored TR3A at VTR/SP 2003. Driving down the road, in the rain of course, I suddenly smelled fuel and could literally see the slick in the rearview mirror. Installed the original valves, fixed the problem.

 

[sp53]

If the gas tank is getting old or a new tank was put in with all its welding bits inside, and depending on the fuel filter system, the front carb will clog and foul first repeatedly.