TR2/3/3A TR3 Running rough..Cam or Carbs?How to diagnose?

[karls59tr]

I've done some mods to my 3 engine but it seems to run rough thruought the range other than when I get into the higher rpms.Have done plug checks and found the plugs black and sooty so I leaned out the carbs a couple flats but havent tested the car yet for improvement. I read somewhere that if you put a header on these cars you should change the SU needles to "RH"anyone heard this.....but wouldn't that just worsen a running rich problem? The engine is freshly rebuilt with a couple hundred miles on it.It has a mild street cam and I realize that a somewhat rough idle is standard for a modified cam. The car has a header and the later TR4 curved intake manifold.The carbs have been rebuilt and Petronix ignition installed.I have opened up the valve gap(I'd have to find a previous thread here to find what I set that at) I'm in Winnipeg so its still winter here and cant get the car on the road yet but I'm interested in any feedback as to how to proceed to get the engine to run smoother(if its not just the cam). Cheers Karl

 

[Moseso]

That's the trouble with doing several mods at once: Which one is causing the problem?
A "mild street" cam should idle OK by 1000 RPM. What is your duration and lift?
You're right about jets and needles. If it's running rich now, you don't want a richer needle.
I have no experience with the Pertronix ignition, so I can't help you there, but don't rule out ignition problems until you've redone and rechecked all that.
Generally rough running is often a sign of a vacuum leak -- but that causes lean mixture, not rich.
Have you double checked your float level in the carbs?

 

[billspit]

This is where dyno time would be invaluable. Do you have one nearby?

 

[MGTF1250Dave]

Aloha Karl,

You mention that you installed a Petronix ignition system. Did you have else done to the distributor? If it is the original stock Lucas distributor, it may need to be refurbished. The bushings and shafts will suffer wear over time, the mechanical advance springs weaken and the vacuum advance may also develop a leak. The advance curve may also need to be altered based on the engine modification you've made. Jeff at Advanced Distributors can probably renew your distributor and recurve the advance in a week or two (including shipping) for one hundred dollars (US) or less. Here is a link:

https://www.advanceddistributors.com/

I was very satisfied with his work and price.

 

[karls59tr]

I think the float level is Ok but I'll double check. Interesting that you mention vacuum leak.....if the vacuum diaghram on the distributor was malfunctioning would I get similar symptoms?

 

[karls59tr]

Billspit.... Would a dyno check pinpoint the problem? What is the ballpark cost for that? MGTF Dave....I was under the impression that bushing and shaft wear were problems that the Petronix unit was supposed to solve? Maybe I was misinformed.I believe I replaced the dist springs at some point but maybe the wrong ones? Also the vacuum advance may be leaking. You may be on to something. What is involved in "recurving" the distributor?

 

[vivdownunder]

Karl,

First I would fit a new set of plugs. If yours have gone sooty black, often that's a sign they are history.

I used to swear at Champion but now swear by NGK. Unleaded is a strange brew to allow cat converters a long life, but it doesn't do much for our cars. I use one grade hotter plug on my 3A, NGK BP5HS, and they stay really clean compared to the standard heat range.

Regarding SU needles, the standard SM is OK for up to 86mm pistons, but for 87mm pistons TW is specified by Triumphtune. Needle RH is a just a richer version of SM for standard 83mm pistons. 86mm pistons can use either SM or TW - whichever tunes up best.

For vacuum leaks, the most likely area for trouble is the vacuum tube fitting beneath the front carbie.

Check that both carbies are sucking equally with a Unisyn, or listen using a length of heater hose.

Rich running can't be stopped if the carbie needles and jets are old and worn. Renewing these works wonders to overcome tuning problems and run on. Check that the carbie dampers drop fully down onto the bridge with a clunk. If they stay up then the car will run rich.

If the needle valve and seat in the fuel bowls are old, then new ones will stop rich running due to fuel seeping past and overfilling the bowls.

Finally your fuel might have gone stale over your winter lay off. A drum of fresh might be needed.

Best regards,

Viv.

 

[MGTF1250Dave]

Aloha Karl,

The Pertronix solves the problems associated with points and condensers in the ignition circuit. Mechanical and vacuum advance mechanisms work independently of either the points or the electronic firing circuit. If the bushings and shafts are worn, there may be some lateral end play in the shaft with the cam lobes. This could induce a timing error as the cam lobes wobble around in the distributor and this error would be inconsistent.

 

[DougF]

Has the car run at temperature for any time with the chokes off? If the carbon doesn't have a chance to burn off during a good drive, the plugs will be black.
Can you wobble the distributor shaft from side to side at all? If so, it needs rebuilt.
Are your rotor and cap new? Is the rivet that secures the contact tip of the rotor loose? If so, replace the rotor.
Has the engine run well at all since these modifications were done?
Do you have an SU shop catalog? Did you follow the directions on where to set the jets for initial start up? Did you make note of the settings before the carbs were rebuilt? It will probably run a little lean at these settings, but either is a good place to start.
Are the plug wires new?
In the future, after you get the engine running well, if it starts running poorly at some point, do not attact the carbs. Unless you have a bad o-ring, cork washer, or sticking float, that are causing fuel leaks, the problem is most likely something else.
Carb performance degrades so slowly that you don't notice. Most carb problems are caused by the last person to touch them.
First, make sure your ignition is set up properly. Don't mess with the Pertronix, it is a very good system, but very easy to damage by crossing wires, etc.
Find a good starting point for richness for the carbs. Set both to the same point. Work on carbs making same adjustments to eachalong the way. Keep in mind though, that no two carbs are the same in the end. If you have a Colortune, use it to set the air/fuel. Once that is set, balance the two.
Any questions along the way will be happily answered.

 

[billspit]

Billspit.... Would a dyno check pinpoint the problem? What is the ballpark cost for that? MGTF Dave....I was under the impression that bushing and shaft wear were problems that the Petronix unit was supposed to solve? Maybe I was misinformed.I believe I replaced the dist springs at some point but maybe the wrong ones? Also the vacuum advance may be leaking. You may be on to something. What is involved in "recurving" the distributor?

I'm not sure what they charge as i've never used one, but believe it could be $200-300 for enough time to find a problem.

It may well be worth the $100 to send the distributor to Adanvced to get it "tuned up".

I would also look for a vacuum leak.

 

[WA1KWA]

"Most carb problems are caused by the last person to touch them."

Absolutely sage advice, Doug.

Colin