TR4/4A Negative Ground Conversion Question

Quick question just to ease my mind...when switching to negative ground, does the white wire go from negative to positive on the coil or vice versa. When I pulled it off I don't remember and I've looked at the wiring schematic, but I'm psyching myself out and WANT to be sure. Thanks

When you switch to negative ground the white wire from the ignition switch goes to coil (+). In negative ground wiring a white/black wire goes from coil (-) to the points in the distributor.

If you have a really old coil it might be marked SW and CB instead of (+) and (-). With those coils, when you switch to negative ground the white wire from the ignition coil goes to CB and the white/black wire goes to SW.

One way to remember this is that the coil is seeking a ground via the white/black wire to the distributor. A ground which it 'finds' each time the points close.

Thus, whatever the polarity of the car, the other (white) wire is the polarity that is not the ground (+ in the case of your conversion).

I have had cars with the coil hooked up backwards and it was difficult to tell the difference which way was correct. For sure having it correct is better and the spark is better by like 20% or something like that. Someone wrote an article about. I think the MG guru guy that needs a Triumph. I bet it is out there somewhere Cyberspace.

Thanks guys! I did a quick google search while working on the car. Guess I should have just started with that! Sorry for the rookie mistake but appreciate the responses. Dad and I got all the fluids in, HVDA transmission bled, brakes bled and attempted to start it. But...I think timing is off or 180 out. This is kind of my first go at a carb'd car and initial startup. Gas is getting to the carbs and oil pressure is there. But I'm getting a backfire. I wonder if there is someone local that has done these before and knows these cars in and out. I'll keep plugging away on the door windows and trim and then it's pretty much just starting it so it can drive. Kind of bummed it didn't start today.

ghawk16 said:

But...I think timing is off or 180 out. This is kind of my first go at a carb'd car and initial startup. Gas is getting to the carbs and oil pressure is there. But I'm getting a backfire..

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Generally, a new topic would start a new thread, but since you hijacked your own thread, I will comment.

I think back firing -especially thru the carbs is a good sign the timing is off 180. If you have an early TR3 the oil filler cap is near the front but if it's a 3B, the oil filler cap is near the rear. In either case, you can remove the cap and see a pair of lifters. If you turn the engine over by hand (or bumping the car in gear) you will find a spot where the intake and exhaust are both closed fully (because you can reach inside the filler cap hole and wiggle both lifters back and forth since there is no tension exerted from the push rods at the spot where the piston is at TDC and expected to fire). This will also be in alignment with the hole in the crank pulley and pointer. So, on an early 3, the front (#1) piston at TDC (hole at pointer) and both #1 rockers loose (180 away from #4 in exact same crank position), with the dizzy cap removed, the rotor should be pointing at the same spot as the wire for #1 cylinder.

If not, you can swap wires 180, or pull the distributor and rotate 180. The timing can be way off and the engine still start, so start with the dizzy loose, and rotate it each direction about 10 or 20 degrees while someone is cranking the starter, and it will probably fire up if you are not out 180. Let us know how it goes, because the first start is always exciting for everyone.

If the backfire is out of the carbs, it can also be fuel starvation. Fuel getting to the carbs is good, but it has to get through the float valve and to the jets as well. If after checking the timing the problem persists, be sure to read up on how to flush out the carbs and set the float valves.

dklawson said:

If the backfire is out of the carbs, it can also be fuel starvation. Fuel getting to the carbs is good, but it has to get through the float valve and to the jets as well. If after checking the timing the problem persists, be sure to read up on how to flush out the carbs and set the float valves.

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Thanks for the feedback. Yes, some of it was out of the carbs! I mentioned the same thing to my dad as there is fuel in the bowls. But as I was initially pumping the lever on the pump to get fuel in the system it seemed like I wasn't getting suction after the 5th or 6th pump. So I'm wondering if I pumped it TOO much and ruined the new diaphragm I put in. The carbs were rebuilt by Joe Curto so I want to say everything was set correctly. I do need to put some oil in the dash pots tho. That is certain. We did put some fuel in the carbs after taking off the filters, kind of primering them. Just hope I didn't ruin any of my new components trying to crank it. Did have oil pressure and had Gibbs break in oil and paste on everything so should be OK.

Joe C. will have rebuilt the carbs and set them to a factory default for starting. You should read up on how to adjust the carbs and expect to have to tweak them. They will not be set to match your exact engine.

The fact that Joe C. rebuilt the carbs does infer that the float valves will be working and set correctly. That further suggests looking at the ignition timing. For some background on static ignition timing, visit the website linked below. Look for my PDF on static timing and download it.
Link:
https://sites.google.com/site/purlawson/home/files/

Everyone has their preference for the carb dashpot oil. I still use straight 20W oil as the factory did. 3-in-1 oil with the blue label is available in most home centers and it is 20W oil.

dklawson said:

Joe C. will have rebuilt the carbs and set them to a factory default for starting. You should read up on how to adjust the carbs and expect to have to tweak them. They will not be set to match your exact engine.

The fact that Joe C. rebuilt the carbs does infer that the float valves will be working and set correctly. That further suggests looking at the ignition timing. For some background on static ignition timing, visit the website linked below. Look for my PDF on static timing and download it.
Link:
https://sites.google.com/site/purlawson/home/files/

Everyone has their preference for the carb dashpot oil. I still use straight 20W oil as the factory did. 3-in-1 oil with the blue label is available in most home centers and it is 20W oil.

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Thank you! I was going to get some 20W oil to put in the dash pots. I also downloaded the Moss static timing instructions and will print those along with yours. I'm thinking I'm 180 out as I'm getting a backfire now out of the exhaust.

As for the carbs, he built them per the specs I gave him for my engine since I have 87.5mm sleeves and a 270 cam. And now that I'm questioning everything in regards to timing I'm questioning my rocker gaps since I may have gapped them to stock cam. But I'm 99% sure I gapped them to the 270 cam specs.

ghawk16 said:

As for the carbs, he built them per the specs I gave him for my engine....

And now that I'm questioning everything in regards to timing I'm questioning my rocker gaps since I may have gapped them to stock cam. But I'm 99% sure I gapped them to the 270 cam specs.

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Joe C. will have used your specs to select a needle profile for the carbs. Once those needles were installed he would set the "basic" starting adjustments for the carbs. Balancing the airflow and fine tuning of the mixture cannot be done until your engine is running. Be sure to read over the carb adjustment process in as many sources as you have access to in print and online. Wrap your head around the process and what each adjustment is for and you will be OK.

You are correct, you should check the specs for your cam regarding the rocker clearance. When discussing timing, particularly when an engine rebuild is involved, it is helpful to include keywords "ignition" and "valve" where appropriate. You will have to have set your valve timing correctly during the engine build. That's the part that involves proper orientation of the timing pulleys and chain. The static ignition timing needs to be set by eye or test lamp to get the engine to start. Once at temperature you can set the dynamic timing with a light.

When you are fitting the plugs, cap, and rotor, remember that the distributor rotor turns COUNTERCLOCKWISE. Therefore, your firing order will be 1-3-4-2 counterclockwise around the distributor cap with plug number 1 being at the water pump end of the engine.

dklawson said:

Joe C. will have used your specs to select a needle profile for the carbs. Once those needles were installed he would set the "basic" starting adjustments for the carbs. Balancing the airflow and fine tuning of the mixture cannot be done until your engine is running. Be sure to read over the carb adjustment process in as many sources as you have access to in print and online. Wrap your head around the process and what each adjustment is for and you will be OK.

You are correct, you should check the specs for your cam regarding the rocker clearance. When discussing timing, particularly when an engine rebuild is involved, it is helpful to include keywords "ignition" and "valve" where appropriate. You will have to have set your valve timing correctly during the engine build. That's the part that involves proper orientation of the timing pulleys and chain. The static ignition timing needs to be set by eye or test lamp to get the engine to start. Once at temperature you can set the dynamic timing with a light.

When you are fitting the plugs, cap, and rotor, remember that the distributor rotor turns COUNTERCLOCKWISE. Therefore, your firing order will be 1-3-4-2 counterclockwise around the distributor cap with plug number 1 being at the water pump end of the engine.

Click to expand...

Yeah, when I built the engine I bought a timing wheel and set the timing. I distinctly remember that since I remember reading that the pulley that goes on the cam itself can be orientated a couple of different ways based on hole locations and its front or rear orientation. But it's when I did the valve clearances I may have used the book and not what came with the cam. As far as ignition timing when I had it up to TDC on the compression stroke my rotor was facing 5 o' clock. So I removed the pedestal, turned the gear and set it back so it's facing the #1 plug. Other thing is, when I tried to start it I never gave it gas with the pedal. I just had the choke out so essentially wasn't getting much gas. I'm going to watch a lot more videos and read a lot more of the manuals I have. Just anxious to get it running.