Lets Trouble Shoot My Old Triumph

I have been having an issue on my Triumph Stag over the last month. Was out for a drive and went to get on it and found a nasty miss when I got the car above 3-4k rpm. I figured that it was an ignition issue as I had been driving the car more and more and hadn't had any fuel side issues. So did the old throw parts at it trick:

•           Spark Plugs
•           New cap and rotor 
•           New plug wires

Had the same issue after all of that was done, but it improved the way the car was running otherwise. I checked the timing and found that is was all with in spec for the car. So that brings me to now. I'm not sure what to start looking at next. The coil tested out all good as well. Symptoms:

• Missing and back fires above 3k rpm under load
• Seems to get worse as the car is driven more so some heat relation
• Revs fine in neutral

The current suspects are:

• the vacuum timing advance: has been a degenerative failure and an old diaphragm makes scense to me
• Carburetor Secondaries issue: The car has a 4 barrel Holley carb conversion. Could the secondary circuit in the carb be gummed up and causing issues?
• Something else?

What does the hive mind think? 

basic engine specs:

• 3.0 L V8 Single Overhead Cam
• Holley 390 Conversion 
• Pertronix Electronic ignition Conversion

Picture because everyone likes a picture.

Have you looked at the fuel side of things? Maybe a plugged filter or pump going bad? Vaporization?

Something else?

Are all your grounds grounded? Shiny metal tightly contacting shiny metal.

Firing order error?  I have seen this cause problems  only at higher RPM's.

It's been a good 10/15 years since I played with the slant 4 ( half a Stag), but I'd go with the diaphragm, or the advance springs in the dizzy.  Another option could be play in the dizzy driveshaft- I had all of those issues on my Dolomites and/ or TR7.

Equally the electronic ignition *could* be failing- I reverted back to points ignition at one point, due to unreliability with the electronic ignition conversions..

What fuel pump does it have?  If it only affects high RPM could it be fuel??

I will have to investigate the Distributor more to see if I can find any issues with it. I havn't had an issue with the fuel pump, but could it be having trouble under load? What is a good way to test that? 

With it running, feel the coil. If it's hot, there's your problem. Simple to check and eliminate as the issue.

I serviced and repaired Stags when they were new. I was always skeptical of those aftermarket ignitions systems. Saw quite a few failures in the 70s. 

Does the miss and backfire start before or after opening the secondaries? I am assuming it has vacuum secondaries.

If you're running a carbureted engine and it is sputtering/backfiring and missing in a certain RPM range, it could quite possibly be a fueling issue. Check your fuel filter, see if the secondaries are opening at the correct time and in the correct fashion. A miss/backfire means lean; you're probably dumping too much air and not enough fuel. Too much fuel will make it fall flat and bog. 
 

Update! 

So went back and checked the fuel filter again, was nastier than I thought so that got replaced! 

Felt like it improved the issue, but it's still present...

Next on to testing the secondaries. I used a zip tie around the rod to test to see if there was actual movement and then went out and drove the car. Got back and found that the zip tie had moved so looks like we have secondaries action. 

I'm leaning toward a Fueling issue at this point, with more and more suspicion directed at the fuel pump. The issue is still:

• High RPM sputtering and backfires
• Seems to get worse as I drive more so most likely heat related
• Feels worse in the higher gears ( more load?)

So what are some good tests for a fuel pump? Seems to start up fine but could it be falling behind with the load?

Also miss and sputtering around when the secondaries open up normally.

Check the fuel pressure or get a cheapie electric pump and temporarily hook it up.

I'd also check the coil along with see if the distributor shaft is worn.

Did you check the coil as Don49 and Tom1200 suggested? An overheating coil will cause the sort of thing you're getting.

That said, see if you can track down a fuel flow spec for your carb - so much fuel in such an amount of time. Also check the pressure and make sure you're getting enough, as well as that you're not getting too much. If the problem is related to the secondaries opening, that's definitely a place to start digger deeper into the carb. I don't know the Holley 390 at all, but if it were me I'd probably just pull the thing off and rebuild it if it hasn't been done in 5+ years.

Another update: I checked the coil today. Left the ignition on for about 10  min and did not feel any heat in the coil. I checked in previously with a multimeter according to the suggestion on MSDs website and it appeared in spec. 

Flattened cam lobe is possible. Were you running a good cam and ZDDP additive?

I have been running Valvoline VR1 racing oil full of zinc to help the engine but have not been running ZDDP addative. 

I have been going down the learning about carbs rabbit hole and ordered a rebuild kit and am wondering if I just have some gummed up secondaries that are running the car rich. If anything this is a good opportunity to finally learn about carburetors! Will say that I have been missing O2 sensors and the like after playing with all of this

In reply to VonSmallhausen :

What do your spark plugs look like? Pull a couple, take a picture, and post it. If you know what to look for, the plugs will tell you everything you need to know.

Black and sooty? Rich. White and ashy? Lean. Burned up with melted and cracked insulator/electrode? Detonation (timing or fuel mixture). Evenly tan or light brown? Good.

Easiest way to check the fuel pump is with a fuel pressure gauge. Of course, the thing about fuel pressure gauges is you have to watch them WHILE the issue is occurring, not just at idle. That means you need to stick it up on the windshield and go drive. The idea is that when the carb's needle/seat are closed or fluttering near closed because the pump is able to keep the fuel bowl full, you will see pressure in the line. When the carb is using more gas than the fuel pump is able to push into it, the needle will be open all the time and you will see basically no pressure between the pump and carb. 

Usually you can do some testing of this 'empty fuel bowl' thing, even without a gauge, just by driving. For example, floor it from idle and by the time you get to 3500rpm you might have already emptied the bowl. Accelerate gently to 3500 rpm and THEN floor it and you should get a noticeably longer time (maybe a second) before the bowl empties and the symptoms come back. Even stronger version of that test, accelerate to a high speed, downshift to a gear that has you way above 3500 (rpm is just an example), decel/coast to 3500 with closed throttle, then floor it. If you get a second or two of normal operation before the lean-out, that all suggests the pump flow is not keeping up with demand and the fuel bowl is emptying.