TR6 Misfire at 2000 revs

[1973TR6]

My TR6 runs great apart from a consistent spluttering/misfire in all gears when passing through 2000 revs. I have had this problem for several years and tried many ways of trying to resolve it.

Under normal accelaration it is smooth up to around 1900 revs and then clears again at around 2100 revs. If I accelerate hard through all gears I don't get the problem. If I ease the throttle gently through this rev range when driving on the level I do not get the misfire but if easing it through going up an incline it splutters badly until I get past the magic 2100 mark.

Previously the problem occurred around 1700 revs and I had a mechanic check it over, he adjusted the tappets some of which were a long way out and some loose. After this adjustment the misfire moved to the current 2000 rev point.

Over the past couple of years I have had a new metering unit fitted and also a new Bosch fuel pump. The injectors give a clean spray and do not dribble. The condenser and plugs have been replaced and some 12 years ago I had new plug leads and have probably only driven 7,000 miles since. The compressions have been tested and show good readings on all cylinders. The car has a stainless steel sports exhaust fitted.

The car has never failed on me but this problem is always there and is not intermittent.

The only thought I have is if the throttle butterflies are worn could it give these characteristics?

This is a great car to drive but would be so much more enjoyable if I could sort out this misfire so I would appreciate any ideas on what I could try next?

[88V8]

The only thought I have is if the throttle butterflies are worn could it give these characteristics?

This is a great car to drive but would be so much more enjoyable if I could sort out this misfire so I would appreciate any ideas on what I could try next?

Hello and welcome.

 

It sounds like a transitory lean period, so a metering problem would be my bet.

Correcting the loose tappets effectively advances the valve timing, not sure how that ties in.

 

If the spindles are worn it would give a high idle. It would also upset the metering a bit due to reduction of the vacuum felt by the M/U, and it would certainly do no harm to have the throttle bodies respindled, but the air leaking through the spindles is insignificant relative to the air pouring into the engine once you open the throttles so I doubt it's the problem. Depends how worn....

 

Are you able to balance the throttles OK?

What's the idle speed?

How far open is the bleed valve?

What happens if you pull out the 'choke' a bit, does that affect it?

Have you the means to test the pump pressure at the M/U?

Any engine mods?

 

There are some real PI experts on here, I can't find a similar problem by searching the forum but I'm sure someone has been there done that

 

Ivor

[foster461]

My TR6 runs great apart from a consistent spluttering/misfire in all gears when passing through 2000 revs. I have had this problem for several years and tried many ways of trying to resolve it.

Under normal accelaration it is smooth up to around 1900 revs and then clears again at around 2100 revs. If I accelerate hard through all gears I don't get the problem. If I ease the throttle gently through this rev range when driving on the level I do not get the misfire but if easing it through going up an incline it splutters badly until I get past the magic 2100 mark.

Previously the problem occurred around 1700 revs and I had a mechanic check it over, he adjusted the tappets some of which were a long way out and some loose. After this adjustment the misfire moved to the current 2000 rev point.

Over the past couple of years I have had a new metering unit fitted and also a new Bosch fuel pump. The injectors give a clean spray and do not dribble. The condenser and plugs have been replaced and some 12 years ago I had new plug leads and have probably only driven 7,000 miles since. The compressions have been tested and show good readings on all cylinders. The car has a stainless steel sports exhaust fitted.

The car has never failed on me but this problem is always there and is not intermittent.

The only thought I have is if the throttle butterflies are worn could it give these characteristics?

This is a great car to drive but would be so much more enjoyable if I could sort out this misfire so I would appreciate any ideas on what I could try next?

 

Welcome to the forum. You mentioned plugs, leads and capacitor, what about the dizzy cap and rotor, points, point gap ?. Several of these can fail in non obvious ways and you cant rule them out until you try a new/different one.

 

Stan

[PILKIE]

Do you also have a 6 branch sports manifold?

As some makes apparently can cause a popping due to the exh gas flow not being smooth at certain points in the rev range!!

Do you get any popping when you decelerate at all!

Give it a little choke and see if it still does it! that may point to a slightly lean mix!

[Richard Crawley]

Welcome to the forum 1973TR6!

Problems with wear in the throttle bodies & poor adjustment of the butterflies will show up with poor/high tick over & poor initial pick up, unless really bad I wouldn’t expect it to make too much difference at 2000 rpm. Check/renew the metering unit control vacuum hose which is connected from the centre throttle body to the top of the metering unit; correct vacuum is critical to the operation of the M/U & fuel delivery across the range. You seem to have checked most other things except the dizzy & if a new vacuum hose doesn’t help, I would make checking the dizzy a priority; there are some really crappy repro caps & rotors around. Also check you actually have the correct dizzy fitted &, using a strobe, check what’s happening with the mechanical advance from 1300 up to around 2000 RPM, rising above this should see maximum advance occur & you could have a bob weight sticking. It could of course be any of the other things already mentioned!

[Mike G]

Hi

I had an identical problem, many years ago on my six and know of only two other cars that have had this same problem, one a five and the other a six. No matter what I tried or change I could not cure this dead band around the 2000-RPM mark.

The cause of the miss fire, was eventually traced to something so obscure, that I doubt any one on this forum would have had the answer. The flexible hose in the fuel line to the metering unit was eventually identified as the culprit. After fitting a new one, the miss fire was cured and has never returned. Inspection of the hose revealed nothing, it looked perfect, so I do not know what had happened to it to cause this most annoying problem.

Hope this helps.

 

Mike G

[1973TR6]

As a new member of this forum I am overwhelmed by the number of replies so quickly and with some excellent suggestions. A number of you mentioned the distributor which I have had checked recently by a mechanic but think it a good idea to check again and make sure all of the areas suggested are covered and perhaps fit a new cap just in case.

I like the idea of trying a bit of choke to see if that improves things.

Enough to keep me busy for the moment so I will work on all of these suggestions over the next few weeks and will let you know how I get on.

Many thanks to you all.

Roger

[smizgals]

Roger,

 

I had a similar problem for a while, however it occured on sudden deceleration, and was

eventually traced to the MU not being timed correctly.

 

Similar syptoms returned after changing the standard Champion plugs for equivalent NGK's on a recently re-conditioned/lead-free cylinder head. Reverting back to the Champion plugs cured it. The NGK's appeared to have been running too hot.

 

Stan

[Hamos]

I have exactly the same problem on My 1972 TR6.

I've just had the engine rebuilt with UL stage 2 head and a phoenix manifold and big bore exhaust. Mine splutters at about the same rev range 1900 to 2200 ish. It gets really unhappy if I hold it in that range.

I put it down to a lean mix as I've not had the MU recalibrated yet to account for the extra gas flow. The car was running OK before (except for the engine needing a rebuild). Mine is packed up in storage for the winter, until I move house and get a propper garage. Cant help with a cure, as I've not had a tinker yet, but just so you know you are not alone with this problem.

[Richard Crawley]

I have exactly the same problem on My 1972 TR6.

I've just had the engine rebuilt with UL stage 2 head and a phoenix manifold and big bore exhaust. Mine splutters at about the same rev range 1900 to 2200 ish. It gets really unhappy if I hold it in that range.

I put it down to a lean mix as I've not had the MU recalibrated yet to account for the extra gas flow. The car was running OK before (except for the engine needing a rebuild). Mine is packed up in storage for the winter, until I move house and get a propper garage. Cant help with a cure, as I've not had a tinker yet, but just so you know you are not alone with this problem.

I have a similar spec engine to you & it will need a recalibrated M/U. Mine came from Prestige & if you give Malcolm the details of your engine he can usually supply a calibration to suit.

Edited November 14, 2008 by Richard Crawley

[JOHN'S TR6]

Roger

 

I feel I have to add my bit as my 1973 125bhp CR does the same.

I have had my car nearly four years now and have changed numerous items over that period of time.

Re-conditioned M/U distributor, distributor & cap, leads, rotor arm, injectors and generally a lot I have forgotten about.

 

When I first got the car, I popped into a local garage in Newport who worked a lot on tr's and Stags.

His comment to me was that the problem could be one of many things and it has become apparent to me in the four years that I have owned the car, that he was right. The problem we have got, could be a result of one of many issues.

 

Toward the later part of the summer just gone, I homed in on the throttle linkages. After playing around with air traveling into the throttle bodies, I managed to obtain a balance of air that enabled the car to drive smoothly through the 1500-2100 range without a stutter. The best since I had the car.

 

The pleasure of driving only lasted about 150 miles though before the smoothness disappeared.

My original CR throttle linkages has a nylon roller that over the years has be warn down to reveal an uneven groove.

This meant it was impossible to set up and to retain the setting.

I think that this winter's investment will be a prestige throttle linkage upper slung set which I am told is much easier to set up.

 

Keep on battling, we will get there.

 

John

[1973TR6]

I have exactly the same problem on My 1972 TR6.

I've just had the engine rebuilt with UL stage 2 head and a phoenix manifold and big bore exhaust. Mine splutters at about the same rev range 1900 to 2200 ish. It gets really unhappy if I hold it in that range.

I put it down to a lean mix as I've not had the MU recalibrated yet to account for the extra gas flow. The car was running OK before (except for the engine needing a rebuild). Mine is packed up in storage for the winter, until I move house and get a propper garage. Cant help with a cure, as I've not had a tinker yet, but just so you know you are not alone with this problem.

[1973TR6]

John,

 

The throttle linkages and airflow are pretty much the only areas I haven't looked at although have had this in mind for some time. Glad to hear you had some success so I will try the same.

Thanks for your input

Roger

 

Roger

 

I feel I have to add my bit as my 1973 125bhp CR does the same.

I have had my car nearly four years now and have changed numerous items over that period of time.

Re-conditioned M/U distributor, distributor & cap, leads, rotor arm, injectors and generally a lot I have forgotten about.

 

When I first got the car, I popped into a local garage in Newport who worked a lot on tr's and Stags.

His comment to me was that the problem could be one of many things and it has become apparent to me in the four years that I have owned the car, that he was right. The problem we have got, could be a result of one of many issues.

 

Toward the later part of the summer just gone, I homed in on the throttle linkages. After playing around with air traveling into the throttle bodies, I managed to obtain a balance of air that enabled the car to drive smoothly through the 1500-2100 range without a stutter. The best since I had the car.

 

The pleasure of driving only lasted about 150 miles though before the smoothness disappeared.

My original CR throttle linkages has a nylon roller that over the years has be warn down to reveal an uneven groove.

This meant it was impossible to set up and to retain the setting.

I think that this winter's investment will be a prestige throttle linkage upper slung set which I am told is much easier to set up.

 

Keep on battling, we will get there.

 

John

[1973TR6]

I have a similar spec engine to you & it will need a recalibrated M/U. Mine came from Prestige & if you give Malcolm the details of your engine he can usually supply a calibration to suit.

As far as I am aware my engine is standard spec although did have a complete re-build just before I bought it 20 years ago! I have twice fitted new MUs the second one being about 2 years ago. Would I need to have someone check the calibration of the metering unit even though it is probably a standard spec engine?Thanks, Roger

[88V8]

As far as I am aware my engine is standard spec although did have a complete re-build just before I bought it 20 years ago! I have twice fitted new MUs the second one being about 2 years ago. Would I need to have someone check the calibration of the metering unit even though it is probably a standard spec engine?Thanks, Roger

Here's an idea.... not wishing to be defeatist... but several years of this problem you said .... give yourself an Xmas present, have a chat with Malcolm at Prestige - not on a mobile it will bankrupt you - who not only does injection equipment but has done a number of complete restos, and if necessary take or send the car to him and it will come back working

Ivor

[1973TR6]

Here's an idea.... not wishing to be defeatist... but several years of this problem you said .... give yourself an Xmas present, have a chat with Malcolm at Prestige - not on a mobile it will bankrupt you - who not only does injection equipment but has done a number of complete restos, and if necessary take or send the car to him and it will come back working

Ivor

 

Ivor,

That's a great idea. I had thought of investing some money in getting a TR6 specialist to look at it but have always held back as I dreaded the thought of going through the whole cycle, and cost, of replacing the metering unit etc again.

Are you referring to Prestige in Hemel Hempstead? Would I be better going to one of the specialist TR people?

Thanks,

Roger

[Hamos]

Prestige is in Wrexham according to the website.

 

Thank you Richard for the reply. I note from some of your previous posts that your engine is a similar spec to mine. I will be taking the car, or MU, to Malcolm, as I'm moving to the North West in April. The MU currently in the car is one of his. I have the problem mentioned by the OP, but my car does seem weaker than I expected after having all the engine work done. It revs freely but just seems to be held back, which is why I'm thinking a weak mixture is the fault throughout the rev range, with a particular lean spot around 1900 to 2000, could this be the natural sweet spot of the scavenging effect of the system? Just a pity I'll have to wait until April to get it sorted.

Hijack over!!!

 

Keep us updated on your progress Roger

Edited November 15, 2008 by Hamos