Mystery flat spot

[JJohn]

My TR6 hill climb car has suffered, all season, from a drop off of power between 4 & 5,000 rpm.  It accelerates well up to 4,000 at which point it stops pulling and, according to one listener, misfires.  At 5,000 rpm all hell breaks loose as though the after burners have been lit and the tacho flies round the dial to 6,500.  This characteristic is clearly shown on the graph from the rolling road.  It was also shown on every occasion the car was on the rolling road since 2014. It was not apparent when driving and acceleration was smooth, constant and without any drop off.   However, during last winter's rebuild, the opportunity was taken to look for more performance by changing the camshaft and the exhaust manifold.  The graph from this year's rolling road shows exactly the same drop off at exactly the same engine speed.  The only difference is that the drop off is all too apparent.  This has resulted in a very difficult and frustrating time and despite many efforts put into the fuel and ignition systems the problem persists.  Has anyone got any ideas?

[TR NIALL]

20 minutes ago, JJohn said:

My TR6 hill climb car has suffered, all season, from a drop off of power between 4 & 5,000 rpm.  It accelerates well up to 4,000 at which point it stops pulling and, according to one listener, misfires.  At 5,000 rpm all hell breaks loose as though the after burners have been lit and the tacho flies round the dial to 6,500.  This characteristic is clearly shown on the graph from the rolling road.  It was also shown on every occasion the car was on the rolling road since 2014. It was not apparent when driving and acceleration was smooth, constant and without any drop off.   However, during last winter's rebuild, the opportunity was taken to look for more performance by changing the camshaft and the exhaust manifold.  The graph from this year's rolling road shows exactly the same drop off at exactly the same engine speed.  The only difference is that the drop off is all too apparent.  This has resulted in a very difficult and frustrating time and despite many efforts put into the fuel and ignition systems the problem persists.  Has anyone got any ideas?

Webbers or Injection.

[BlueTR3A-5EKT]

Are you saying you changed the cam and exhaust manifold and since then it has had the flat spot?

After that work what did the rolling road test exhibit?

 

Good but in depth reading on cam design and technology here:-

https://tildentechnologies.com/Cams/index.html

[TRTOM2498PI]

1 hour ago, JJohn said:

My TR6 hill climb car has suffered, all season, from a drop off of power between 4 & 5,000 rpm.  It accelerates well up to 4,000 at which point it stops pulling and, according to one listener, misfires.  At 5,000 rpm all hell breaks loose as though the after burners have been lit and the tacho flies round the dial to 6,500.  This characteristic is clearly shown on the graph from the rolling road.  It was also shown on every occasion the car was on the rolling road since 2014. It was not apparent when driving and acceleration was smooth, constant and without any drop off.   However, during last winter's rebuild, the opportunity was taken to look for more performance by changing the camshaft and the exhaust manifold.  The graph from this year's rolling road shows exactly the same drop off at exactly the same engine speed.  The only difference is that the drop off is all too apparent.  This has resulted in a very difficult and frustrating time and despite many efforts put into the fuel and ignition systems the problem persists.  Has anyone got any ideas?

Hi Jim,

Hope you are well ?

Out of curiosity, which cam did you install ?

 

 

Cheers.

[TRTOM2498PI]

57 minutes ago, TR NIALL said:

Webbers or Injection.

Believe Lucas PI.

[JJohn]

Thanks for your responses.  Yes, it has the original Lucas PI.  The "flat spot" shows on the rolling road graphs both before the change of cam & exhaust manifold and after but, it was only noticeable after the rebuild with the new cam and manifold.  Before that, it drove as though it had seamless, smooth acceleration right across the rev range.

[JJohn]

The cam is one of Jon Wood's which has given excellent results with the same manifold in other engines he has built.  Expecting +20 bhp after the mods, this year's rolling road showed it is down 20 bhp.

[Tim D.]

2 hours ago, JJohn said:

Thanks for your responses.  Yes, it has the original Lucas PI.  The "flat spot" shows on the rolling road graphs both before the change of cam & exhaust manifold and after but, it was only noticeable after the rebuild with the new cam and manifold.  Before that, it drove as though it had seamless, smooth acceleration right across the rev range.

If you saw it at the rolling road did you get an AFR trace at the same time. That would tell you if you have a mid range mixture issue. 

Tim

[roy53]

Have you changed any of the electrics ? coil ,dizzy etc. 

The rolling road i use like the 123 bluetooth as they can remotely program it suit any mid range flutters 

Roy

[JJohn]

Thanks Tim and Roy.  I did not get an AFR trace.  It will be on the rolling road again soon so I will ask for one.  Swapped the coil, dizzy advance curve adjusted but no change.

Jim

[roy53]

I would try a different dizzy then

 

[John L]

Might be worth taking the MU to Neil Ferguson, he has said its possible there is a mark on the fuel ramp, that the roller has a job to pass over.  This should show up on an AFR on the rolling road.

One other thought, how is the MU timed, like a CP or a CR?  They are different.

John

Edited October 20, 2023 by John L

[Tim D.]

20 hours ago, Tim D. said:

If you saw it at the rolling road did you get an AFR trace at the same time. That would tell you if you have a mid range mixture issue. 

Tim

Perfect. It is possible  that the MU is not properly setup for your particular car. I had my MU setup to standard calibration by Neil but when I checked the AFR it was too rich at low load low revs  and too weak at high load high revs. Bit of a tweak of the adjustment dials on the MU and it was much better although these PIs must be set a little rich as there is no acceleration enrichment. 

Tim

[Andy Moltu]

The PI needs to run a bit rich.

By 4k rpm you should be at max advance so the dizzy shouldn't make a huge difference at that point (unless completely knackered) 30-32 btdc at 4k.

If the power is dying at 4k but fine over 5k you probably need a mixture check.on a rolling road. Running lean would in general cause pinking, too rich might explain a misfire.

[Brent C]

Hi

I have had experience of a similarish flat spot, but at a much lower rev range. Mine came in at about 2200 RPM and would clear at about 2500 RPM, but it is worth a try. It took me a very long time to pin it down and like you I went through all the usual potentials.
It turned out my problem was caused by a resonance problem. The culprit was the flexible hose on the fuel supply onto the metering unit. I changed it out and it immediately cleared. You could give it a try, by altering it’s length, by adding an extra fitting, or even borrow a hose off another car, before paying out for a new one, if it is the culprit. The hose had been on the car since new and there was no visible fault with it, so I still don’t know why it suddenly started to have the flat spot, but I have since found out that it has happened on other cars.

Brent

[stuart]

29 minutes ago, Brent C said:

Hi

I have had experience of a similarish flat spot, but at a much lower rev range. Mine came in at about 2200 RPM and would clear at about 2500 RPM, but it is worth a try. It took me a very long time to pin it down and like you I went through all the usual potentials.
It turned out my problem was caused by a resonance problem. The culprit was the flexible hose on the fuel supply onto the metering unit. I changed it out and it immediately cleared. You could give it a try, by altering it’s length, by adding an extra fitting, or even borrow a hose off another car, before paying out for a new one, if it is the culprit. The hose had been on the car since new and there was no visible fault with it, so I still don’t know why it suddenly started to have the flat spot, but I have since found out that it has happened on other cars.

Brent

Yes, we had the same on one, took a long time to find it too.

Stuart.

[JJohn]

Thanks everyone for your input.  The resonance one is very interesting.  I have had the car on the rolling road.  The diagnosis was that the new cam has so increased the engine's appetite for fuel that the MU cannot keep up with it.  It showed  dangerously lean after 5.000, where there is no vacuum to increase the supply.  Electronic Injection or Webers . . . . . . .?

 

Jim

[stuart]

7 minutes ago, JJohn said:

Thanks everyone for your input.  The resonance one is very interesting.  I have had the car on the rolling road.  The diagnosis was that the new cam has so increased the engine's appetite for fuel that the MU cannot keep up with it.  It showed  dangerously lean after 5.000, where there is no vacuum to increase the supply.  Electronic Injection or Webers . . . . . . .?

 

Jim

Metering unit would need to be adjusted to suit. Dennis Vessey over at Barnetby could sort that https://www.vessey-classic-car-services.co.uk/

Stuart.

[JohnC]

Out of interest, what manifold are you using? I used to have a TT1200 6:2 manifold which gave a horrible loss of torque in the mid range but came on song around 5000rpm. Apparently that was caused by pulses interfering with extraction at mid-range rpm. The manifold was designed for high-end power, so it was working as designed, just not as I wanted it! I changed back to the original cast iron manifold and things got better straight away.

Cheers,
JC

[JohnC]

On 12/6/2023 at 3:51 AM, JJohn said:

The diagnosis was that the new cam has so increased the engine's appetite for fuel that the MU cannot keep up with it. 

What power are you seeing at the rear wheels?

[JJohn]

Hi John,

 

i have only just seen your question.  Thanks for the response.  Before the installation of the new cam, there was 160 bhp at the wheels.  The new cam, with the "flat spot" from 4 to 5K rpm gave 150 bhp.  Replacement of the PI system by 3 x 45 DCOE Webers has fixed the problem. So I now have a PI system and the earlier camshaft for sale.

Jim Johnstone

[Andy Moltu]

On 12/5/2023 at 4:51 PM, JJohn said:

Thanks everyone for your input.  The resonance one is very interesting.  I have had the car on the rolling road.  The diagnosis was that the new cam has so increased the engine's appetite for fuel that the MU cannot keep up with it.  It showed  dangerously lean after 5.000, where there is no vacuum to increase the supply.  Electronic Injection or Webers . . . . . . .?

 

Jim

Ultimately the Lucas PI should comfortably deliver enough fuel for an engine putting out 160 bp at the wheels (with adjustment)

If it was going lean over 5k WOT, there are the realistic scenarios: 

1) The fuel pump isn’t able to deliver enough fuel. (Pump inadequate or PRV misbehaving)

2) The maximum fuel settings are too low.

On the rolling road pull the choke out a little, this has the effect of increasing the maximum fuel settings. If it richens the mixture, then it’s an adjustment to the maximum flow on the metering unit. If it doesn’t get richer then the problem is with fuel supply (Pump/PRV) 

The problem is most RR operators will have diddly squat of a clue with Lucas PI and probably assume twiddling with the rings under the conical black cap is the only place to adjust the MU. However the maximum fuel is determined by the screw beneath.

That said a standard MU won’t be far off for fast road spec and significant adjustments probably for 3/4 race and above.  However, and this is a subjective opinion) is that many metering units supplied are set a bit lean to satisfy the current “demand” from from owners who are mislead into thinking that low tick over CO levels are a good thing. In reality if you get this from a PI 6 pot you have probably compromised elsewhere in the rev  range. 

Obviously power is not just determined by fuel delivery. Airflow, exhaust scavenging and ignition timing all complicate matters so putting components together and making them work in harmony is a dark art. Simpler for those using efi where the fuel and ignition parameters are easier to adjust if near infinite in number.

 

 

 

[astontr6]

On 10/19/2023 at 10:23 PM, Tim D. said:

If you saw it at the rolling road did you get an AFR trace at the same time. That would tell you if you have a mid range mixture issue. 

Tim

 I suspect that the fuel cam in the M/U needs modifying? The only people that I knew that could do that were SAH and they are long gone! Is there any one out there who still can do this?

Bruce.

[Andy Moltu]

You don’t necessarily need to the cam itself, much can be achieved by adjusting the maximum fuel settings and the concentric rings that exert pressure on the springs (which can also be changed) that control which point on the fuel cam the metering unit is fuelling.

The trick is finding folk who know how to do this and an almost indeterminate amount of rolling road time. That’s an ever diminishing number. 

The other thing to check is the tapped gaps. If they are too small (for the cam) you might be able to supply enough fuel only to find it hasn’t completed combustion before the exhaust valve opens. Perhaps not your issue as it works fine with side draught carbs.

 

[JohnC]

22 hours ago, Andy Moltu said:

The trick is finding folk who know how to do this and an almost indeterminate amount of rolling road time. That’s an ever diminishing number.

Or buy a wideband AFR sensor with associated gauge & logging kit, and make a vacuum calibration setup. It may cost less than a single rolling road session and will give you endless hours of fun! I have learnt so much about the PI system by going down that road (was that a pun?). 
Cheers,
JC