TR2/3/3A Missfire and Backfire

[Ken H]

I have a TR-3B which I've owned since the early 70's. It was a daily driver until a few years ago. Now I'm retired I'm trying to get it running again. I've rebuilt the carbs, new sprockets and chain, new plugs, points and condenser, new fuel filters and flushed the gas tank.

The engine will start and idle ok once it has warmed up. Currently the carbs are set at 15 and 18 flats open with SM needles. The timing is at 5 deg. btdc. I just replaced the vacuum advance on the distributor and tested the vacuum line to insure it's not leaking. There is fuel pressure and flow at the carbs. I have 10W30 oil in the dashpots. Compression is 175 psi in all cylinders. Years ago (but not many miles ago) I replaced the carb butterflies and shafts with new bushings, so I don't think shaft wear is an issue.

When I try to drive, the engine will misfire with slight acceleration. With more acceleration, it misfires badly. Hard acceleration causes a backfire through the carbs.

I'm running out of things to check. I have not checked the head for carbon buildup or the cam for wear.

Can anyone suggest what could be causing my misfire and backfires?

 

[charleyf]

[QUOTE

The engine will start and idle ok once it has warmed up. Currently the carbs are set at 15 and 18 flats open with SM needles.
[/QUOTE]
By the book you should start the jets at 12 flats and then you usually lean them down from there. That is often a couple of flats less.
Charley

 

[CJD]

Popping through the carbs is either a timing issue, extreme lean issue or a valve issue. Where is your timing at idle...and at what speed are you idling?

I saw your posts about needles, but don't know the answer. I do know that if you installed the wrong needle you could be way lean and causing the popping.

 

[poolboy]

Are you sure you have the spark plug wires in the correct firing order in the distributor cap ?

 

[Graham H]

You have replaced the timing chain and sprockets, I would say you have the valve timing out of wack?

Graham

 

[Ken H]

Are you sure you have the spark plug wires in the correct firing order in the distributor cap ?

Yes. Wires are new, copper core and in the right order.

 

[Ken H]

[QUOTE

The engine will start and idle ok once it has warmed up. Currently the carbs are set at 15 and 18 flats open with SM needles.

By the book you should start the jets at 12 flats and then you usually lean them down from there. That is often a couple of flats less.
Charley
[/QUOTE]
Thanks for your help. I started at 15 flats and after warming used the "lift the piston" method which eventually took me to the 18 and 15 flat combination. That made me wonder if the needle was correct. When I tried to lean the carbs, I reached a point when the engine suddenly stopped firing and stalled. I was idling around 600 rpm. I like to idle at 450 when adjusting carbs, but the engine was rough when below 600.

 

[Ken H]

You have replaced the timing chain and sprockets, I would say you have the valve timing out of wack?

Graham

The chain and sprockets were changed a few years ago and the process was complicated as you could reverse the sprockets and get subtle changes in timing. I did not replace the cam at that time, but wonder if I shouldn't have. Since then, while trying to fix the problem, I removed the timing cover and rechecked the sprocket positions. I don't have a degree wheel which would have been helpful. Last month, I rechecked by turning the engine to the balance point on the cam in number 4 cylinder and number one cylinder was at TDC.

 

[Ken H]

Popping through the carbs is either a timing issue, extreme lean issue or a valve issue. Where is your timing at idle...and at what speed are you idling?

I saw your posts about needles, but don't know the answer. I do know that if you installed the wrong needle you could be way lean and causing the popping.

At idle (600rpm) timing is 5 deg BTDC. I like to try to get the engine to idle at 450, but it wouldn't.

 

[sp53]

Nice idle at 600, so I think you are all tuned up. I would say the spark is cutting off and then coming back on and popping or exploding the unburned gas fumes somewhere some how. Perhaps the coil is grounding or arching off. I had a coil arching sporadically to the temp gauge tube then stop; it ate the tube, bummer. Sometimes if you turn the lights off or check at night in a dark place you can see the arching.

steve

 

[Ken H]

Nice idle at 600, so I think you are all tuned up. I would say the spark is cutting off and then coming back on and popping or exploding the unburned gas fumes somewhere some how. Perhaps the coil is grounding or arching off. I had a coil arching sporadically to the temp gauge tube then stop; it ate the tube, bummer. Sometimes if you turn the lights off or check at night in a dark place you can see the arching.

steve

Thanks Steve, I have all new wires and was pretty careful to not have them cross or come close to ground, but it's a possibility. I'll run the check just in case.

 

[Ken H]

At idle (600rpm) timing is 5 deg BTDC. I like to try to get the engine to idle at 450, but it wouldn't.

I have the richer of the two needles installed and will stick with that unless I discover I'm running rich.

 

[CJD]

the needle does not affect idle. You are definitely not rich. Lack of power and intake popping is a sign of lean…very, very lean. Changing needles has me concerned you got the wrong one…otherwise I would check for vacuum leaks. Also, do the needles lift when reving the engine?

 

[Ken H]

the needle does not affect idle. You are definitely not rich. Lack of power and intake popping is a sign of lean…very, very lean. Changing needles has me concerned you got the wrong one…otherwise I would check for vacuum leaks. Also, do the needles lift when reving the engine?

I spent quite a while centering the jets when reassembling the carbs last week after replacing the jet seals. Both pistons drop freely with a solid thud when hitting the carb bridge. The front carb's sound is crisper and slightly metallic when the piston drops. I've always thought that's the ideal center. The back carb drops solidly but with the metallic sound. I played with it for fifteen minutes trying to get a "cleaner" drop but couldn't. Both pistons move freely with very little friction and come fully to the bridge in the carbs with the jets screwed fully up.

I'm pretty sure I've fixed a vacuum leak I had in the line from carb to distributor by putting new fittings on both ends but have not put a gage in the line to see how much vacuum I have.

 

[CJD]

I understand the pistons drop...I wondered if they lift when the engine is revving. The vacuum should be enough to lift the pistons with the engine running, at least at about 2k or so. If they do not lift, that can cause a lean mixture, as the thick part of the needle blocks the jet.

 

[Graham H]

Are the jets tight in the pistons and the shoulder level with the bottom of the piston?

Graham

 

[Ken H]

I understand the pistons drop...I wondered if they lift when the engine is revving. The vacuum should be enough to lift the pistons with the engine running, at least at about 2k or so. If they do not lift, that can cause a lean mixture, as the thick part of the needle blocks the jet.

Sorry, yes they do. Both pistons lift equally and seem to be free. When the engine is shut off, the pistons return to bottom.

 

[Ken H]

Are the jets tight in the pistons and the shoulder level with the bottom of the piston?

Graham

Yes, they are.

 

[Snowkilts]

Have you ever looked at the centrifugal advance in the distributor? They are spring loaded lead weights with brass tubes in them sitting on a pin. Once the brass tube wears through the lead weights will disintegrate quickly. Mine were severely worn. This can cause timing to be erratic. It's fairly easy to check, just open the distributor and take the plate off.

 

[LarryK]

Have you checked you point gap?