GT6 mk3 rough idle, occasional backfire from carb and lack of power.

[Mack]

Hello all,

This is my first post so please bear with me. Apologies for the wall of text backstory but I thought I would put as much info as possible in case any of it was pertinent.

3 weeks ago I bought myself an early 40th present, a beautiful mk3 GT6 that had been well cared for but also well used.

 The engine was rebuilt in 2014. Converted to unleaded and is running on SU Hs6 1.75 carbs with electronic ignition on Lucas distributor and a Huco electric fuel pump.

A bit of background to my problem. Picked up the car from the PO and drove it from Essex/Suffolk border home to Tunbridge Wells. 

No problems for about a week and then it refused to turn over in a supermarket car park. Walked home to pick up jump leads and my other car, only for it to start on it's own when I got back to it. Charged the battery.
 
A couple days later drove it down to Goodwood Revival, started first time but after an hour or so it started to idle at 2000 rpm. Then after stopping for lunch, refused to start again. Had to push start it, now thinking there was an issue with the starter motor/ starter solenoid. 

Tried various things over the weekend to test if it was the starter or the solenoid but ended up having to push start it a few times to get us to and from Goodwood. 

During this time I also noticed that the carb float bowl on the carb closest to the pump was overflowing after I shut the engine off. Took the bowl cover off and jostled the float needle and it seemed to sort it. 

The car had the occasional hesitate and backfire out of the front carb when warming up but that would mostly stop on the short drive to Goodwood. 

The car then ran well on the hour and half drive back home.

The next weekend I changed the battery for a new one having tested the starter out of the car. 

The car now starts ok but is idling quite roughly, with the carb backfire still present and what I can only describe as a hollow whistling coming from the engine. I opened the other float chamber and cleaned out gunk that was in the bottom. 

Warmed up the car, which was still idling roughly and tried to take it for a test drive. The car had no power and would try and cut out with the choke. Only just made it around the block.

So, bearing in mind that I am pretty unexperienced and not done any of this before, this my current plan for Saturday:

• Take off and check both float needles and floats. Replace gaskets as one is damaged. 
• Remove air filters and check the carb cylinders move freely, check Dash pot oil and top up if needed. 
• Check for vacuum leaks(?) It seems the way to do this is with a small unlit propane torch waved around the hoses and carb/inlet junctions, is that correct?
• Check fuel flow from pump by taking the hose going to the carb off and directing it into an empty petrol can. (Petrol gauge currently reads 1/2 full)
• Check timing with timing light and I have an inline spark tester to check for spark to the plugs.
• Check the plugs.

What else should I try? I will try and take a video if nothing I do helps initially. 

Also the PO said he usually ran it on E10 but I filled it up with 97 Ron E5, would this cause an issue with the timing?

Any help would be greatly appreciated!

Matt 

[Iain T]

Welcome Matt!

First off its not E5 fuel as E10 gnats pee shouldn't be used for our engines.

I had the same whistle sound and found the previous owner (PO) had used the wrong carb gasket and allowed air in. Sounded like a supercharger winding up! Fitting the correct one and the tickover was much better. 

Huco pump, I have one, are normally fit and forget. However! I found a soldered wire joint and heat sealed joint holding on by a strand and caused some 'ead scratching as the fault was intermittent. Check wire and connections. 

As it ran OK I think the timing is at least acceptable. However (again) the electronic ignition can play up. What make and type is fitted? 

That'll do for starters the guru's will be along sooooon! 

Iain 

PS

Don't all 2 litres use 1.5" carbs? Is your engine standard? 

Edited September 21, 2023 by Iain T

[Mack]

Thanks Iain, I will check the wiring and connections.

Maybe time for some new carb gaskets as well. I had planned to do a thorough service over winter but I am without a garage for another two weeks!

I believe the engine is mostly standard, but the head and inlet manifold have been gas flowed.

From the paperwork I have I believe the ignition is Pertronix Ignitor unit with a Flame thrower coil.

Matt

 

[Ian Foster]

Matt

Firstly welcome to the GT6 club. They are great cars and I'm sure you will grow to love yours once the teething problems are sorted. I've had my Mk2 since September 1977 and there's always something to do.

It sounds as though you have multiple issues.

My initial thoughts for what it's worth:-

• High idle sounds sounds like an air leak. If the larger carbs have been fitted using adapter plates, these are a possible source of leaks. My car has Stromberg 175CD2s and I have ditched the adapters and modified the manifold to give  4 stud fixings.
• Correct float levels and clean fuel supply is critical (SUs and Strombergs)
• Huco fuel pump is excellent.
• Use only E5 fuel and set the timing to 12 deg dynamic using the strobe (as WSM) as a base setting.
• Reluctance to start (or intermittent starting) could well be the starter solenoid. Mine played up and has now been by-passed, which I can do as I have a high-torque starter which doesn't need the solenoid.

I also have a Pertronix Ignitor 2 unit with a Flame Thrower coil. I think they have a good reputation for reliability.

Let us know how you get on with your checks.

Ian

Edited September 21, 2023 by Ian Foster

[Iain T]

I have Pertronix and Flamethrower coil. They are very reliable. 

It's a matter of going through each system and checking. A good way to learn about your car! 

It sounds like fuel starvation so could be the dreaded rubber slivers. What spec are your hoses? If the PO used E10 and didn't change the hoses that is another possible cause. Do you have in line filter? 

Do you also have a high lift cam? Normally 1.5" carbs work well on 2 litres including modified engines because the air speed is kept high. Your 1.75" have hopefully been set up with the correct needles for your engine. 

Iain 

Edited September 21, 2023 by Iain T

[Mack]

Thank you both for your replies. 

I don't believe the cam is different to stock but I will have a good look through the history file and see what I can find. There is an inline filter before the pump, which I will change on Saturday as well.  The hose from the pump to the first carb is rubber and then braided from there. 

I believe the carbs have been on there since the rebuild and the PO did multiple 10Countries runs and LeMans Classic runs since then so I imagine they are the correct needles. I will learn more when I rebuild the carbs over winter I guess!

I will have a good look at adapter plates for the carbs and try and order some new seals.

Cheers,

Matt 

[Iain T]

9 minutes ago, Mack said:

  The hose from the pump to the first carb is rubber and then braided from there. 

For E10 fuel you need R9 spec hose. To be safe although I only use E5 fuel I have replaced all mine with Gates Barricade available from the club shop and other good suppliers. Beware cheap Fleabay hoses! 

Iain 

 

[Pete Lewis]

agree E5 only   for factory performance  not mower fuel .

this has the ring of dreaded rubber slivers , bits of hose get sliced off when inserting the metal fuel pipes 

they float around and jam the back of the float needle valve   crafty little sods .

one thing you will read about on here is the 2ltr engine crank pulley damper ring has the timing marks these 

break down the damper bonding and the ring can move this is a bit out the box but it happens

so one day you need to confirm TDC  on No1 is as the marks show .

i dont like HS6 on the 2 ltr there are no real specifications for that set up 

high idles can be leaks in what ever breather system is fitted including rocker  cover  gaskets and filler caps 

i would replace the hoses with gates barricade (its tough stuff) and flush (pump ) out some fuel into a jar 

remove the float valves see if you catch some tiddlers as a first move 

to check for air leaks  you can use a pump oil can or old trigger bottle use engine oil   brake cleaner or petrol(it wont catch fire )  spray around the gaskets see if it gets sucked in 

Pete

Pete

 

[Mack]

Thanks for the replies and advice.

I changed the fuel filter, checked both float needles, which seemed fine. 

Removed air filters so I could see the barrels, both of which fell nicely when I lifted them but I topped up the dashpot oil regardless.

Some of the hoses were not as snug as they could be so I tightened those up. Some of those will need replacing soon I imagine, which I will do at the same time as the fuel lines. 

The car started but wouldn't idol without the choke and would falter when trying to rev it. 

Let it idle for a bit on the choke and then it seemed a bit happier, sprayed some carb cleaner around the carb to manifold junction, where there is an adapter plate. The revs picked up! Hopefully found my air leak. Tightened up the mounting bolts as best I could.  I will have find some replacement gaskets and fit them. 

Now seems to idle and rev quite happily but I haven't had the chance to take it for a test drive yet as I need to fiddle with the air filters. 

 A bit annoying to have a problem so soon after buying the car but I have learnt a lot already!

Cheers,

Matt 

[Josef]

17 hours ago, Mack said:

The car started but wouldn't idol without the choke and would falter when trying to rev it. 

Unless it’s a fairly hot day, this is normal. The choke is there to compensate for the fact the engine is cold. So it’s still needed until the engine can run normally without it. You will have to learn what your car needs though. 

[JohnD]

Mat,

All good advice above, and I'm glad to see from Pete that the late unreliability of the crank damper pulley is beginning to be part of Triumph lore!

Once you have confirmed that indicated TDC is true (best done with a degree wheel and piston stop, but a screwdriver in the plug hole will do it roughly), mark it on the pulley with Snopaque and get a stroboscopic timing light.   Inspect the ignition, for correct timing and the operation of the advance mechanism.   Also  look inside your dizzie cap for signs of 'tracking', as your backfiring from the carb could be missfires.

Good luck!

John (The prophet of damper degradation!     If you do suspect your damper, I can check it on my test rig!)

[Pete Lewis]

18 hours ago, Mack said:

A bit annoying to have a problem so soon after buying the car but I have learnt a lot already!

the joys of getting to know your classic 

we have all been there 

Pete

[Mack]

Evening all,

Replaced the air filters this evening and started her up. 

Took a good five minutes to idle without the choke, where on previous times I have hardly had to use the choke at all. Took her for a brief drive up the road and again had no power, however when I pulled the choke out it drove almost normally. I assume this confirms the lean condition? I am hoping that it is simply the air leak that I found before but I have yet had a chance to check the timing. 

Matt

[johny]

I wonder if the old filters were really restricted so making the mixture rich and that now youve replaced them its running much weaker so needs more choke? If this is the case you need to reset the carb mixtures to be correct for the new filters...

[Mack]

2 hours ago, johny said:

I wonder if the old filters were really restricted so making the mixture rich and that now youve replaced them its running much weaker so needs more choke? If this is the case you need to reset the carb mixtures to be correct for the new filters...

A good thought Johny but I actually just put the originals on there. 

Matt

[johny]

oh I see, previously it was running without filters? So you put filters on and it started to run weaker? Well I cant think of any explanation for more air then entering the engine unless you disturbed something else in the process😲

[Iain T]

Matt, go back to basics and take off the dash pot and air piston then measure (if you have a vernier) the depth of the jet to the bridge. Both carbs should be the same depth. Adjust if not. It seems you may be running lean so lower the jets 1/4 of a turn. 

If you don't have a vernier wind up the jets to be flush with the bridge then wind down both the same amount of turns. As to how many turns with your setup I don't know but take a look before you wind up and guess the depth! I then make a mark snowpake or similar on the adjuster so I don't get lost when fine tuning. 

The air leak needs to be fixed ASAP or it'll never idle or run correctly. 

Iain 

Edited September 26, 2023 by Iain T

[Pete Lewis]

and just what filters were on and what have you used to replace ??

the troubles with SU HS6 on a 2ltr is there is no spec for this set up  unless you compare to the 2500S 

and thats a very differing beast ,  1.5 on the 2ltr are more than adequate 

most end up replacing back to stock to get reliable running (as triumph engineered)

it just works  ,  "must have " ideas of bigger do  normally end up with a headache 

Pete

 

[Ian Foster]

Matt

Can you post a couple of pictures of your carb set up please. That might help us troubleshooting.

BTW have you had any dialogue with the PO regarding the current problems?

Ian

[Mark B]

I fitted Hs6 Sprint carbs to my Vitesse, only other mod is a 6-3-1 exhaust manifold that will help with flow, they needed mixture set, goes well, just keep an eye the colour of the plugs. You may have sprint hs6 fitted, because you need the sprint short dash pots to fit under the gt6 bonnet.

Very important to make sure the carbs are balanced, straight forward process using a carb balancer, I've used a Gunson version for 30 years, basic but works, but makes a world of difference if they are out balance

Edited September 26, 2023 by Mark B

[Mack]

Pete the filters are some quite tired K&N pancake filters. I gave them a good blow out with compressed air at work but they will need replacing. 

My first port of call is tracing the air leak. I have ordered some gasket material to make some new gaskets for the carb to inlet manifold junction as there is an adapter plate so twice the gaskets to go wrong! When I sprayed carb cleaner in this junction the revs changed so I am hoping this is the culprit. If not then I will go back to basics as advised by Iain. 

I haven't spoken with the PO since sorting the starting issue, he was as helpful as he could be on the phone when we were in on the way to Goodwood and advised about the initial float chamber problem. I will get back in touch if stopping the air leak doesn't sort it. 

Mark I also have a 6-3-1 manifold. I am going to start at the beginning with the carbs when the car is back in the garage. ( a couple of weeks from now when we move) so will check the balancing. 

I will take some pics on the weekend. 

Thanks all,

Matt 

[Mack]

Found a couple of pics on my phone. Filters removed here. I think the culprit is the adapter plate to manifold gasket.  I plan to sort out a better throttle return spring setup and feel like the adapters could be improved. 

 

[Iain T]

Aren't the heat isolators normally an asbestos type material? Mine are on my Stromberg but yours look like aluminium. 

Or is it an adaptor plate? 

Iain 

Edited September 27, 2023 by Iain T

[Mack]

The aluminium section is the adapter plate. I feel it could be improved with some studs to give the correct number of mounting bolts for the SU's but for now I just plan on making sure it is sealing correctly. 

[Ian Foster]

Matt thanks for the pictures. There does look to be quite a gap between the adapter and the face of the manifold and this is almost certainly your idle problem.

When I bought my Mk2 GT6 in 1977 it came with Stromberg 175 CD2 carbs, fitted onto steel adapters (probably SAH given the age). The PO had obviously just bought the bits and slapped them on, as the manifold hadn't even been opened out to suit (which I did PDQ).

I have persevered with the 175 carbs, but despite the stiffness of the steel adapters, I still didn't feel that the mounting was secure, so I had the flanges of the manifold built up and re-drilled to suit the four bolt mounting for the Strom 175 (which is the same as the SU HS6 BTW). This allows room for a decent thickness isolator (readily available from Burlen etc.), a heat shield and K&N filters. The adapters found a new home a while ago.

Despite this relatively unpopular (but not unknown) set up, using 2E needles the car actually runs very well and has good acceleration and economy (38mpg average on both the 2016 and 2018 RBRRs). One benefit of the 175s is that they have lipped seals in the carb body that makes them less susceptible to leakage around the throttle shafts.  For what it's worth, Gareth Thomas (always controversial) thought that with a good head and a 'corrected' manifold the 2L cars should have had 175 carbs from the factory.

If your inlet manifold has already been modified (I assume enlarged to suit the HS6s and modified internally to remove the imbalance on cylinders 2&5), then I suspect you are not going to want to start again with a 'standard' set up. I think the HS6s can be made to work, but would strongly suggest that you have the manifold altered further to sort out the mounting if you do. I can provided further details if it would help.

K&N filters can be cleaned and re-oiled.

Ian

Edited October 14, 2023 by Ian Foster
update