Throttle Pot sourcing

[Technoholic]

Hi guys

I've been searching the old threads but I'm none the wiser. getting back to try to fix my shunting issue, my next step is the throttle pot. Reading 67% max in Rovergauge so I think it might need to be changed.

Is there a drop in replacement available these days? The Racetech one seems to be but it is a lot of money. Is there any alternative? Or I don't mind having to modify connectors if that's what needs to be done but I don't know which throttle pot I need in the first place to do this.

Thanks

[Technoholic]

spitfire4v8 said:

Before you go spending money on a throttle pot check you're actually getting full throttle first.
Either wedge the pedal at full travel, or get someone to press it for you .. remove the hose from the plenum and look where the butterfly is. It should be at right angles to the airflow IE least restriction.
I once had a Tuscan SP6 pick up 50hp on the dyno just by adjusting the throttle so it opened fully. Cheapest 50hp ever.
Check it.

well well well. I just tested it. The butterfly is opened about maybe 75-80% i.e. it's not fully horizontal when the pedal hits the stop.

Thanks for that tip, I think I saw that in an earlier thread and forgot to check.

How do I adjust this!?

[Technoholic]

Just done a test. I can get to 92% by manually operating the lever which I believe might be the maximum expected in RG?

I found the adjuster nut but it seems to be as tight as it will go. Is there any adjustment on the pedal end?

[Technoholic]

Ok so I’m effect I need to make the pedal travel further then, by winding that stop back a bit?

[Technoholic]

Right, I’ve found the stopper and adjusted it. I have managed to get it to the point where Rovergauge is reading 91% with foot to the floor. However the bolt is at the end of its travel, I couldn’t move it any more. I presume this is ok?

[Technoholic]

Steve_D said:

You could also have just adjusted the cable at the throttle body. The pedal has a limit stop when pressed but just hangs on the cable when at rest. Adjusting (shortening) the cable would give you the wider opening but at rest the pedal would sit a little higher.

Steve

Do you mean by adjusting the cable adjuster on the throttle mechanism? Because that’s at its maximum adjustment too

[Technoholic]

it could be a stretched cable I guess.


I have done a quick log with RG with the throttle being pressed so I need to import to excel to examine the throttle readings to see if there are any dead spots before changing it.

[Technoholic]

True, it’s fixed for now so will monitor it and put it on the list of things to replace.

[Technoholic]

blitzracing said:

Dont get too hung up on this- The TPS has two basic functions 1) to say the car is in idle mode (ie TPS as a low voltage) and 2) add extra fuel when the throttle is floored- fuel enrichment. This is done by looking at the rate of change of TPS voltage I believe, not its peak voltage. Problem is the 5 volt supply rail to the TPS may only be 4.5 volts, so even wide open the ECU wont see 100% at 5 volts. Id save your money on the TPS and look else where. Problem is shunting can be caused by a multitude of issues, so its important to make sure the ignition system is in good order, you dont have wrong temp readings, or very high fuel trims showing something is out of tolerance.

Yeah I think I’ve ruled the TPS out now as being a problem. Getting the full range as best it can now I think. Just took it for a drive. Nice little extra bit of power with the throttle fully open.

But the shunting is still a right pain in the neck. I’ve noticed my vacuum advance is faulty, let’s air through it so I guess that needs changing but not sure that could create shunting this bad?

If I send you a log file once I get it out for a drive with the laptop connected, would you mind taking a look?

[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?

[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!

[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?

[Technoholic]

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

[Technoholic]

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

[Technoholic]

I'm down near High Wycombe, Bucks

[Technoholic]

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

[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?

[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?

[Technoholic]

first step is to get the unit changed, I cannot see how to unscrew the right hand screw without removing the pas pump again. Im not doing that!

Which pipe did you tap for the manifold vacuum?