Throttle Pot sourcing

[Technoholic]

KevtheRev said:

Regarding shunting, I listed everything I could find on the subject then started working through cheapest/easiest first. I got as far as renewing the HT ignition side & pretty much sorted it, but this is my full list.............

1. Check using RG that I/O giving expected readings
2. Check/replace Distributor Cap & rotor arm – cracks or tracking
3. Clean Stepper Motor (idle control valve)
4. New plugs Iridium plugs - NGK BPR6EIX
5. Junk the Spark Plug Extenders. I fitted Accel 9002C Ceramic Leads
6. Check for vacuum leaks, particularly on breather system
7. Check under bonnet electrical connectors & go through plugs & clean/spray
8. Check timing - set to 28 Deg BTDC @<3500rpm
9. Ported v full vacuum advance system see “chimpongas” threads on this subject
10. Ignition Coil
11. Ignition Amplifier
12. Some forum quotes on Plenum spacer having an improvement
13. Remove & get injectors cleaned
14. Modify crankcase ventilation system, fit PCV - would need further understanding of benefits
15. Aftermarket ECU for better control of fuelling & timing

Thanks for the comprehensive list, here's what I've done so far

1. Check using RG that I/O giving expected readings - I think they are correct but would like Blitzracing to check my logs out
2. Check/replace Distributor Cap & rotor arm – cracks or tracking - I have a new rotor arm and cap to fit but not sure how easy this is, does it mess up timing?
3. Clean Stepper Motor (idle control valve) - New stepper motor fitted as old one was pretty stuck
4. New plugs Iridium plugs - NGK BPR6EIX - New plugs fitted, BPR6ES, not the iridium, type but these are still well recommended
5. Junk the Spark Plug Extenders. I fitted Accel 9002C Ceramic Leads - already got rid of the extenders, need to fit new leads when I change the dizzy cap
6. Check for vacuum leaks, particularly on breather system - not sure where to check this one
7. Check under bonnet electrical connectors & go through plugs & clean/spray - Any particular you'd recommend checking?
8. Check timing - set to 28 Deg BTDC @<3500rpm - Not something I have the ability to do I don't think.
9. Ported v full vacuum advance system see “chimpongas” threads on this subject - My vacuum advance module is leaking I found today, I have a new one arriving tomorrow
10. Ignition Coil - Changed
11. Ignition Amplifier - Changed

The below are the next level of things to try which I'm not yet thinking about
12. Some forum quotes on Plenum spacer having an improvement
13. Remove & get injectors cleaned
14. Modify crankcase ventilation system, fit PCV - would need further understanding of benefits
15. Aftermarket ECU for better control of fuelling & timing

[Technoholic]

With regards to the cap and rotor, I just changed it, couldn't have been easier actually. Waiting for the vacuum unit to arrive which I guess will be more tricky.

Can the vacuum unit be replaced without removing or messing with the distributor?

[blitzracing]

Shunting is associated with a poor combustion burn. Its a problem in TVRs because they run a long duration cam, so you get quite a lot of exhaust gas blown back into the plenum at valve overlap, with contaminates the fresh air in the plenum. This does not allow the mixture to burn as well, but anything that gives a better combustion or flame front will help improve things- hence the long list of to do things. Personally I found the best fix was to make the mixture richer at around 1700 rpm, as this masks it. The problem is a catalyst car will always try and stick at 14.7:1 AFR whatever you do with the lambda feedback, and the somewhat sluggish mixture control just makes things worse. Best bet is to switch to the green tune, and the low speed mixture can then be tweaked with the CO trim setting. Problem is 1) you need a suitable green tune chip 2) it will fail emissions testing 3) it might cook the catalyst.

None of this is unsolvable however. There are suitable maps on Steve Sprints 14CUX pages that can be downloaded into an Eprom, and you can switch between the green and white map with just a switch and couple of resistors. As for the catalyst, it would be suck it and see- if it glows red its too hot! Mind you the catalyst cycle chucks in excess fuel anyway to keep it hot, and I don't think a green map would be any worse than this, but Ive not had a car to test this on........

[Technoholic]

blitzracing said:

Shunting is associated with a poor combustion burn. Its a problem in TVRs because they run a long duration cam, so you get quite a lot of exhaust gas blown back into the plenum at valve overlap, with contaminates the fresh air in the plenum. This does not allow the mixture to burn as well, but anything that gives a better combustion or flame front will help improve things- hence the long list of to do things. Personally I found the best fix was to make the mixture richer at around 1700 rpm, as this masks it. The problem is a catalyst car will always try and stick at 14.7:1 AFR whatever you do with the lambda feedback, and the somewhat sluggish mixture control just makes things worse. Best bet is to switch to the green tune, and the low speed mixture can then be tweaked with the CO trim setting. Problem is 1) you need a suitable green tune chip 2) it will fail emissions testing 3) it might cook the catalyst.

None of this is unsolvable however. There are suitable maps on Steve Sprints 14CUX pages that can be downloaded into an Eprom, and you can switch between the green and white map with just a switch and couple of resistors. As for the catalyst, it would be suck it and see- if it glows red its too hot! Mind you the catalyst cycle chucks in excess fuel anyway to keep it hot, and I don't think a green map would be any worse than this, but Ive not had a car to test this on........

Fancy testing on my car?!

When it gets to messing with maps and AFRs etc, I think it's best left to the experts, and while I understand it all, im definitely not an expert in that regard!

[Belle427]

Technoholic said:

With regards to the cap and rotor, I just changed it, couldn't have been easier actually. Waiting for the vacuum unit to arrive which I guess will be more tricky.

Can the vacuum unit be replaced without removing or messing with the distributor?

Fiddly job but the distributor does not have to be disturbed.

[Technoholic]

Belle427 said:

Fiddly job but the distributor does not have to be disturbed.

Ok that’s good, do you know if there is a guide anywhere?

[Belle427]

Not really but remove cap, remove plastic cover underneath that says "Do not remove"
Take out 2 screws holding vac unit on and unhook operating arm from baseplate, bit of wiggling required here.

[Technoholic]

Ah ok so wiggling around under the cap won’t mess up the timing then? That’s the bit I was worried about.

[Belle427]

If the vacuum unit is the same you should have no problems.

[blitzracing]

Technoholic said:

blitzracing said:

Shunting is associated with a poor combustion burn. Its a problem in TVRs because they run a long duration cam, so you get quite a lot of exhaust gas blown back into the plenum at valve overlap, with contaminates the fresh air in the plenum. This does not allow the mixture to burn as well, but anything that gives a better combustion or flame front will help improve things- hence the long list of to do things. Personally I found the best fix was to make the mixture richer at around 1700 rpm, as this masks it. The problem is a catalyst car will always try and stick at 14.7:1 AFR whatever you do with the lambda feedback, and the somewhat sluggish mixture control just makes things worse. Best bet is to switch to the green tune, and the low speed mixture can then be tweaked with the CO trim setting. Problem is 1) you need a suitable green tune chip 2) it will fail emissions testing 3) it might cook the catalyst.

None of this is unsolvable however. There are suitable maps on Steve Sprints 14CUX pages that can be downloaded into an Eprom, and you can switch between the green and white map with just a switch and couple of resistors. As for the catalyst, it would be suck it and see- if it glows red its too hot! Mind you the catalyst cycle chucks in excess fuel anyway to keep it hot, and I don't think a green map would be any worse than this, but Ive not had a car to test this on........

Fancy testing on my car?!

When it gets to messing with maps and AFRs etc, I think it's best left to the experts, and while I understand it all, im definitely not an expert in that regard!

You could simply try switching to the green map as a simple test- you just need a 470 Ohm resistor for the tune resistor, and then unplug and re plug the ECU or it will throw a tune resistor error. The default TVR green tune is the Range Rover map, which is fine for testing at low speed and this has the big advantage that you can frig about with the mixture up to about 2400 rpm with the screw on the side of the AFM- but you do need to start by setting the DC voltage on the CO trim wire output on the AFM. This is typically around 1.3 volts as a starting point.

Use a voltmeter between red and black wire and blue and red wire on the AFM plug and check the voltage with the ignition on, but the engine not running. Turn the adjustment screw clockwise to richen the mixture, and anticlockwise to lean the mixture. The screw has multiple turns, that will go from 0 volts to over 3.5 volts.

The Range Rover map is about 20 BHP down on the TVR map on a 3.9 as it leans out badly at higher RPM,
so if running green does fix or reduce the shunting, you would then have to get the correct TVR map in green. I think Steve Sprints site has green maps for all engine sizes now.

Edited by blitzracing on Friday 14th December 09:57

[Technoholic]

Happy to do that but I actually meant bringing the car to you to have a play with?

[blitzracing]

Where are you based?

[Technoholic]

I'm down near High Wycombe, Bucks

[blitzracing]

Before you set of down the M4, ill pop a switchable tune resistor in the post to you, so at least you can see the driving / shunting difference between the maps for starters, just send me your address.

[Technoholic]

OK I appreciate that! I'll pm you my address now

[Belle427]

Top work there, thanks for your continual help.

[Technoholic]

so, yesterday I changed my dizzy cap, rotor arm and leads. I've just taken my car out for a spin, I don’t want to speak too soon as I’ve only been for one drive but it feels like the shunting has magically disappeared. Like, it’s a completely different car. I was driving everywhere at 1500 rpm in 5th and I couldn’t get it to shunt all. Normally slow moving traffic in 2nd would have been impossible but it was as smooth as anything. I don’t want to count my chickens yet but so far it feels likes a different car! I didn’t even get round to the vacuum advance yet!

If the shunting really is fixed, what should I do about the vacuum advance? Leave it disconnected or replace it?

[Belle427]

It should improve low speed driveability and fuel economy but i do know some choose to leave it disconnected if its not a daily driver.
Have a read of some of the links on this thread, be warned its a slippery slope now you have started playing!
https://www.pistonheads.com/gassing/topic.asp?t=17...

[Technoholic]

Ok, so while I can't bear to read anymore of CoG's essays, his points are valid. So basically connecting up the vac advance can only be positive. The question is do I connect it to the original ported vacuum and risk introducing the shunting again, or do I go for manifold vacuum which seemingly has no downsides except emissions. Thoughts?

[Belle427]

Test them both and see how it drives, I moved over to full manifold vacuum and it’s all been very positive.
Lower under bonnet/running temperatures and better low speed driveability.