TR6 lack of top end speed

[Neil T]

Hello all,

I'm looking for some advise and suggestions as I am fast exhausting my own investigations... 

My TR6 (1970) has had this issue for quite a while (well for a few years at least) and to be honest I cannot remember when it first started, which is really annoying as I may have been able to work out the cause at that time !

The car starts well and runs really well, no misfire etc etc.

However when I reach about 65 / 70mph the car really does not want to go any faster. It doesn't misfire at that speed, but just feels as that it is at its limit, either in or out of overdrive.  In the distant past I have had the car up to well over that. 

I verify this speed by using Google Maps with the actual speed showing. 

I must also add that my Speedo is also running slow, for example when I am doing 30mph (real time), the needle is on 40mph etc

I have had the following works done since this has occurred, but it still has the same top end issue. 

Totally rebuilt engine, new fuel injection, new gearbox with A type OD (by Tom Cox), new prop, distributer recon by the Distributer Doctor.

Only the Diff remains unchanged (3.45), as checked my myself by counting revolutions of the Prop. 

Timing is also ok.

Tyre size was mentioned, but although running slightly different tyres, I really don't think they would be the cause for such a speed drop.

The injection system is Bosch that has been running well for years (supplied by Malcom's Prestige Injection).

I am however now wondering if by any chance it could be related to the fuel pressure valve in the boot as I am running out of options !

Sorry to rabble on but any input would be really appreciated. 

Neil 

 

 

 

 

 

 

 

Edited November 9, 2023 by Neil T

[Drewmotty]

What is the timing at 3000 or so rpm?….needs to be close to 32deg btdc with modern fuel  

Are the hose sizes on the fuel pump feed big enough?…..pump could be cavitating  

What is it like with the intake plenum removed?….air restriction  

What are the plugs like after an ignition cut at 70mph?….mixture lean at higher rpm. 

[Mk2 Chopper]

I would check to see the fuel pressure is correct. 

Gareth

[Mike C]

Blocked air filter? Blocked or damaged fuel line between the PRV and MU?

[Rob Y]

Maybe a silly thing, but surely worth checking, you haven’t run out of throttle cable movement? I mean has the accelerator pedal hit the floor at your maximum speed or maybe there just isn’t any more movement possible on the throttle cable. Simple fix of course if this is the problem.

Recalibrating the Speedo is pretty easy if it’s just an offset/zero error, but you need to remove it from the car first.

 

Cheers, Rob

[stuart]

5 minutes ago, Rob Y said:

Maybe a silly thing, but surely worth checking, you haven’t run out of throttle cable movement? I mean has the accelerator pedal hit the floor at your maximum speed or maybe there just isn’t any more movement possible on the throttle cable. Simple fix of course if this is the problem.

Cheers, Rob

Seen that problem before on an aftermarket linkage set, luckily no longer sold but a very poor design.

Stuart.

[mleadbeater]

sounds like it’s running out of power, if there’s no misssfire ie. fuel starvation, or ignition issues I would suggest checking compressions,  timing advance, air filter (getting blocked), exhaust system restrictions.

Try freewheeling in neutral at high speed, see if the car coasts or rapidly slows, suggesting drag somewhere in the transmission and running gear (tyres, brakes, etc)

Good luck

Mike

[harlequin]

37 minutes ago, Rob Y said:

Maybe a silly thing, but surely worth checking, you haven’t run out of throttle cable movement? I mean has the accelerator pedal hit the floor at your maximum speed or maybe there just isn’t any more movement possible on the throttle cable. Simple fix of course if this is the problem.

Recalibrating the Speedo is pretty easy if it’s just an offset/zero error, but you need to remove it from the car first.

 

Cheers, Rob

I had a similar experience which turned out to be to much carpet and sound deadening stopping full travel of the throttle pedal 

George 

Edited November 9, 2023 by harlequin

[Andy Moltu]

Assuming that you haven’t had the engine apart when this problem started(and got the cam timing wrong) checks include.

Timing max advance to 32 btdc.

Throttles fully open when pedal fully applied. 

When at 70mph try pulling out the choke a little, this should richens the mixture in the event the metering unit is set too lean. 

Pull the choke out further at full throttle 70mph - if it goes mega rich and slows, the mixture probably isn’t being starved of fuel delivery alternatively Check fuel pressure, ideally under load, if it holds up then probably not a fuel pump or restriction issue.

 

[Neil T]

Thanks to everyone for the responses 

You have at least given me some more options to try and look out for.

I can straight away rule out bulky carpets / underlay under the throttle as there is nothing in the car apart from the drivers seat at the moment due to just having fitted the new gearbox.

I'll look at the easy wins first such as throttle cable travel / linkages / air filter / plenham etc, then move on to the other suggestions accordingly.  

I will try start looking at this over the next couple of weeks or so and report back with any findings. 

Cheers

Neil 

 

 

[John L]

Do you have a cast iron exhaust manifold?, have you had the lower exhaust pipe off from this manifold, It is possible to get the gasket fitted incorrectly and so blocking the exhaust flow, such a silly item that can be fitted right and wrong!

John

[astontr6]

3 hours ago, stuart said:

Seen that problem before on an aftermarket linkage set, luckily no longer sold but a very poor design.

Stuart.

As Stuart says, check that the butterflies are fully open to the horizonal when accelerator pedal is pushed to the floor? I have seen this problem a number of times on all sorts of cars with cable operation. I have a cable made up of motor cycle throttle parts to over come this problem that has got plenty of adjustment.

Bruce.

[marko]

As others have said check the throttle body butterflies are opening fully by taking off the big black plenum tube and getting someone to press the accelerator pedal to the floor - sounds like you're only getting half throttle somehow.

 

[harrytr5]

3 hours ago, John L said:

Do you have a cast iron exhaust manifold?, have you had the lower exhaust pipe off from this manifold, It is possible to get the gasket fitted incorrectly and so blocking the exhaust flow, such a silly item that can be fitted right and wrong!

John

A member in his teenage years and a TR6 owner who used garages to service it fitted the exhaust gasket the wrong way round on the lower cast iron Manifold and wondered

why he was down on power till it burnt a hole in the gasket. So easily done even by the pros.

As been said, butterflies not fully open and or throttle cable not working right.

Trust you will find it

Regards Harry

[DRD]

The accelerator pedal can slip restricting movement. Check it sits in line with the other pedals and full depression opens the throttles properly. If it's slipping you need to adjust it and tighten the bolt on the shaft on the firewall in the engine bay.

[BlueTR3A-5EKT]

The air filters are known to collapse    That is why genuine pi ones had a steel mesh inside them

The rubber insulator 517411 used to go missing.   Good seller on the day.     Happened when the filter housing was removed for service.      When put back on the rubber insulator would get squeezed into the air intake hole and end up jammed on the plenum.   Item 90

 

[Waldi]

Check that the crank pulley timing mark (TDC) has not slipped, this has been the case on several pulleys. Cause is the rubber bond between the inner hub and outer ring of the pulley can slip.

To virufy: TDC mark on pulley has to be checked against the real TDC of piston #1.

Waldi

[Brent C]

Does it rev freely in the other gears?

Brent

[Dave McDonald]

Check that the adjustable throttle stop bolt against the bulkhead, items 10&11 on Moss diagram, isn’t too long preventing full butterfly openings. 
Also check that, if that bolt has been set too short, it hasn’t caused the locating ferrule on the throttle cable to slip up the cable where it sits in the flat horizontal bulkhead alongside the clutch master cylinder, which can also prevent full throttle opening. 
Fave McDonald

[Roadrunner Paul]

Hi Neil

I was just about to say the same as Dave above but he beat me to it!

My TR5 has an aftermarket underslung throttle linkage supplied and fitted by one of the TR specialists. The acceleration did not seem what I expected from the fuel injected six so I set about checking the simple things first. Plenum off - I could not understand why the throttles would open no further than half way at full accelerator travel. I checked the cable and then found that the adjustable throttle stop bolt was adjusted as far it would go - I had to replace it with a bolt that was a half inch shorter to achieve full throttle opening!

Best of luck with finding the problem. Please let us know what you find.

Paul

[Neil T]

Thanks again for even more suggestions.... 

I will look at the 'quick wins' first, and I will report back on here once I have (or have not) found the culprit !

I may go quiet for a little while as my normal everyday work gets in the way of this more essential work 

for now, thanks again ...................... 

 

Edited November 11, 2023 by Neil T

[Peter Cobbold]

On 11/10/2023 at 6:15 AM, Waldi said:

Check that the crank pulley timing mark (TDC) has not slipped, this has been the case on several pulleys. Cause is the rubber bond between the inner hub and outer ring of the pulley can slip.

To virufy: TDC mark on pulley has to be checked against the real TDC of piston #1.

Waldi

Waldi, I agree, the engine is running retarded.  Simplest check is to set the static, say 20 BTDC instead of  12 and see if it perks up. Peter

[Waldi]

Hi Peter,

that is a really easy method for a coarse check.
If it starts to heavily pink under load after temporary advancing the ignition, this is likely not the issue.

All the best,

Waldi

[Neil T]

Hello All,

Just a quick update. not fully solved yet as still retrimming the car BUT I have found out why the Speedo is running slow.

I seem to have a Speedo in the car with the numbers SN-6409/08A, which I now know is for a 3.71 Diff.

Looking through my spares cupboards I have four spare Speedos, two of which are marked SN-6409/04 and 04A....these are the correct ones for the 3.45 Diff.

I think the 04 one is TR5/TR6

So that will be one issue sorted.

Further updates when the car is tested on the road.