TR2/3/3A Missfire and Backfire

[LarryK]

Especially I find over the counter ones bad. Order mine from England.

 

[sp53]

There is a connecting rod that clamps both carbs together. When you move the throttle, do both carbs move together? The clamp could be slipping on the rod and your car might be running on one carb.

If you can see the springs on the valves close and open, you lobes are probably fine. When the lobe is gone, there is no movement.

steve

 

[Ken H]

I've been lurking for a while and finally decided to join the thread.

My immediate reaction, on reading the initial post, was valve timing. I agree with Graham on that. It's difficult to check this without a degree wheel. One thing you might try--and I don't know for sure if this will work--measure the amount that the #1 valves are open at TDC on the exhaust stroke. They should be the same. The intake valve will be opening and the exhaust closing, but at that point they should be the same distance from fully closed. Be sure to approach this point turning the engine CW as seen from the front.

Another possibility: adjusting the carbs should be done with the engine fully hot. Just warming it up in the driveway generally is not enough. But, I suspect that would leave it too rich, and the popping indicates a lean mixture.

I like this approach! Sorry for the delay. I had to make a holder for the drop dial since I don't have a magnetic holder. I'll attach a pic of the setup. I ran each test three times to show variability in the test setup. At the point of balance on number one cylinder the exhaust was open. .025, .023, .025. The intake valve was open .033, .032, .033. The engine was cold and tappets set at .012. Looking back at it, I probably should have set tappets at zero clearance. All rotations were clockwise.

The difference is about .010 inch. I rotated the crank to move the dial .010 and I saw the pulley mark move about 1/8 of an inch. I rotated the crank to move the dial .010 and I saw the pulley mark move about 1/8 of an inch. If I did the math right this is about 2 deg. of rotation. Let me know your thoughts. In the meantime, while I have the setup, I'll check valve lift on the other cylinders to see if I have even wear on the cam lobes.

 

[Ken H]

There is a connecting rod that clamps both carbs together. When you move the throttle, do both carbs move together? The clamp could be slipping on the rod and your car might be running on one carb.

If you can see the springs on the valves close and open, you lobes are probably fine. When the lobe is gone, there is no movement.

steve

When I'm adjusting the carbs, the connecting rod between carbs is loose per the manual. After adjusting, I tighten the clamp and check for slippage.

 

[Ken H]

I like this approach! Sorry for the delay. I had to make a holder for the drop dial since I don't have a magnetic holder. I'll attach a pic of the setup. I ran each test three times to show variability in the test setup. At the point of balance on number one cylinder the exhaust was open. .025, .023, .025. The intake valve was open .033, .032, .033. The engine was cold and tappets set at .012. Looking back at it, I probably should have set tappets at zero clearance. All rotations were clockwise.

The difference is about .010 inch. I rotated the crank to move the dial .010 and I saw the pulley mark move about 1/8 of an inch. I rotated the crank to move the dial .010 and I saw the pulley mark move about 1/8 of an inch. If I did the math right this is about 2 deg. of rotation. Let me know your thoughts. In the meantime, while I have the setup, I'll check valve lift on the other cylinders to see if I have even wear on the cam lobes.

I checked valve lift on all valves and the lift ranges from .356 to .365 and everywhere in between. It appears like there isn't a worn lobe and all are wearing pretty evenly. Do you know how much the valves should open?

 

[sp53]

The only other thing I can think of now is the distributor mounting plate for holding the points is unwantedly moving inside the distributor and causing the timing to float around and not advancing . I had it happen once with a broken rod that hooks the plate to the vacuum advance. I would probably purchase a new distributor because the bearing and other moving parts would be new. BPNW had them on sale sometime back for a 125.00. There are also rebuilders here on the forum and it is not that hard to fix DIY if you have another one for parts.

steve

 

[CJD]

I think you're getting way to deep. You have an off-idle lean mixture.

Check for vacuum leaks.
Ensure correct needles.
Check the float levels.
Check for actual timing at idle and off idle.
check connections to and from the coil that would cause a weal spark.

You would not have a good idle like you do if anything mechanical was off. And, I understand you have new parts and have checked these items. But the problem will lie in one of the above.

 

[Ken H]

I think you're getting way to deep. You have an off-idle lean mixture.

Check for vacuum leaks.
Ensure correct needles.
Check the float levels.
Check for actual timing at idle and off idle.
check connections to and from the coil that would cause a weal spark.

You would not have a good idle like you do if anything mechanical was off. And, I understand you have new parts and have checked these items. But the problem will lie in one of the above.

 

[Ken H]

Thanks for the input and I agree with your comments.

I wouldn't call the idle good, but it will idle at 600 with some misfiring. Floats have been checked and rechecked. I have new CM needles from Moss. They are stamped CM, but I haven't measured them against the SU specs so it's possible they could be out of spec. I can borrow a micrometer from my son to measure them.

Today I received a new timing light. (Broke the old one) and was able to measure spark advance. The goal is to compare my advance curve to what the distributor should exhibit. I thought this would tell me if the centrifugal advance was working properly and hopefully something about the vacuum. While running the test, I noticed that the misfire seems to be associated with number one cylinder although I didn't test other cylinders. I'll need to put some other marks on the pulley to check other cylinders. While watching the pulley with the strobe on, I could witness each misfire. Number one plug fired well for five or six times and then it fired early. The early sparks would show up about one inch (on the pulley) btdc. each time I heard a misfire. I'm trying to understand what could cause that. As I've mentioned before the cap, rotor, and wires are new. I examined the cap, and it looks good. I checked the number one wire with ohm meter and tightened the spiked screw a little, but the misfire still exists.

I've attached a spark advance curve generated with the vacuum line connected. I was going to repeat the test with vacuum line off, but the engine overheated, so I'm waiting for it to cool. I'll test some more in the morning. If you have a good source of info for advance curves for the distributor, please let me know. thanks, Ken

 

[LarryK]

You put in new radiator. Was block flushed? Should not overheat on idle. Especially with the low presssure cap. Too lean can overheat, but something is missing in this equation.

 

[CJD]

Well, the curve starts a bit more advanced than I like, and not exactly like shown in the manual...but I do not see anything that would cause a misfire or loss of power.

I'm at a loss. With a vacuum leak you would not get it to idle at 600rpm.

Would you happen to have a spare coil to try out?

 

[charleyf]

Ken,
In your recent post you referred to the needles as CM. I am sure you meant SM as you previously stated.

Your large number of flats to adjust the jets got me to thinking. Why? My best guess is that your needles are not properly fastened into the piston. Each needle has a "shoulder". The bottom edge of that shoulder should just be exposed or even with the bottom of the piston. When the needle is to low with the shoulder exposed below the piston your needle settings are all off. Which would require the extra flats to adjust the jet. Additionally this also means that the float settings are off. The needle/ shoulder height is like the first domino to fall and then each of the other adjustments must be altered.
Charley

 

[Graham H]

Charley, refer to reply 18?

Graham

 

[charleyf]

Graham,
Have you ever heard of needing to turn 20 flats down to adjust the jets? I believe there is a reason .
Ken's answer was not after he checked. I am suggesting what he thought was correct may not be.
Charley

 

[Ken H]

Ken,
In your recent post you referred to the needles as CM. I am sure you meant SM as you previously stated.

Your large number of flats to adjust the jets got me to thinking. Why? My best guess is that your needles are not properly fastened into the piston. Each needle has a "shoulder". The bottom edge of that shoulder should just be exposed or even with the bottom of the piston. When the needle is to low with the shoulder exposed below the piston your needle settings are all off. Which would require the extra flats to adjust the jet. Additionally this also means that the float settings are off. The needle/ shoulder height is like the first domino to fall and then each of the other adjustments must be altered.
Charley

Thanks for the input. I was extra careful when putting the needles in the piston and the shoulder is flush with the piston bottom. Screws are tight. I've rechecked them to make sure they aren't moving. My latest "discovery" is with the timing light showing an occasional spark occurring in number one cylinder about one inch on the flywheel btdc. The spark fires six or eight times at 5 deg btdc and once much earlier. I don't understand how this is possible.

 

[Ken H]

You put in new radiator. Was block flushed? Should not overheat on idle. Especially with the low presssure cap. Too lean can overheat, but something is missing in this equation.

No, the radiator isn't new but was recored about 25 years ago and I have the original fan which isn't very efficient. I live in Florida and the temp is around 90F. I've always had overheating when the car idles for 15 minutes or more.

 

[sp53]

I would get the SM needles-----plus The top part of the whole distributor------- cap, wires, points, condenser, and coil comes on and off easily with 2 nuts and does not effect the geared parts of the timing that stays in place. I have a distributor set up like that that I can change out in minutes and essentially change the whole ignition system to see if that makes any difference with performance;. you could have 2 problems working against you. The black powder you saw was carbon from either rich mixture or weak spark

Steve

 

[Ken H]

Graham,
Have you ever heard of needing to turn 20 flats down to adjust the jets? I believe there is a reason .
Ken's answer was not after he checked. I am suggesting what he thought was correct may not be.
Charley

I'm thinking that I may need to revisit the passage between the float chamber and the jet to see if something is blocking flow. I ran a wire through it once before, but it's been a while.

 

[Ken H]

More thoughts about the occasional spark fifteen or twenty degrees early. Could a short or excess resistance in the wire running from ignition switch to coil cause the coil to discharge early? Voltage in that wire measured at the coil is ok.

 

[CJD]

Ken, the jumping spark could be cause by cross-fire within the distributor cap...or...a worn out distributor. On other engines it could be secondary wires run too closely together, but that would not really be an issue on the TR3 with such short runs.

I would first check that the distributor point plate is solid. You should not be able to move it back and forth or up and down. Also check that the rotor shaft is solid, with little or no ability to turn it with gentle pressure, and spring loaded with moderate pressure. Any slop in the above checks should be addressed. Then, remove the distributor cap and wires from the cap. Study the cap extremely closely for cracks or signs of arcing.