GT6 cold starting problem

[owensparrow]

I've checked the other posts on this issue and learned lots, but still can't get to the bottom of this one:

Late '73 GT6 with Stromberg 150CD carbs, all standard. Turns over well but won't start, quick blast of Easystart and it's up and running for the rest of the day, and runs super well. I've stripped and cleaned the carbs, all lovely and reassembled correctly I'm sure, but there is no fuel in the "choke pipe" between the carbs, so I believe this is part of the problem, but looking at it, I can't see what could go wrong with such a simple system.

Any ideas would be gratefully received

Ps, new points and condenser, timing spot on, plugs and leads all new, super unleaded \avgas mix in the tank, I'm out of ideas! Also when I pull on the newly adjusted choke when it's warm, it runs super rich and nearly stops.

 

[Pete Lewis]

with a  balasted ignition are you sure the coil is being supplied when you turn the key to crank it up ???

do make sure the air filter gaskets are not blocking the front face vent holes 

i take it you have temperature compensators  ... make sure the silly things are fully closed or blocked off  as if open the mixture at idle is weakened

set the delrin washer on the head of the needle level with the base of the air piston 

just some thoughts  

Pete

[NonMember]

10 hours ago, owensparrow said:

there is no fuel in the "choke pipe" between the carbs, so I believe this is part of the problem,

From your description, I'd say that's the whole of the problem - the choke mechanism isn't working. It is very easy to assemble that perforated washer bit wrongly, or get something blocked inside it. Pete's ideas are all good things to check but I'd expect them all to affect normal running, not just cold start.

[Stratton Jimmer]

You are not alone with this problem. My Mk3 GT6 has HS4 SU carbs and plays up just the same. Quick squirt of Bradex and she fires up a treat and does so for the rest of the day with no problems. Two days later, she won't start without a squirt of Bradex. I rebuilt the carbs which, when I bought the car only had a choke on the front carb but now has it on both. It made no difference. One thing is that once she has fired up, opening the choke does what it should. I use E5 Super and the car runs well on it.

The comments about the coil are valid - the six volt coil gets a twelve volt feed from the solenoid when starting. Please check that that is the case. It wasn't with my car but is now. It does start better than it did although, as I say, a shot of Bradex is still required but less than previously.

My Spitfire six, has had HS6 SU previously and had no starting problems and since swapping over to a pair of CD150SE and following Pete's guidance as above, has no problems. Definitely lock off the temperature compensators!

[johny]

Is it worth checking the compression on these engines? Not sure what effect low compression has on cold starting but cant imagine it helps...

[euan douglas]

My problem, as you've no doubt seen on an earlier post, was a split carb breather tube.  I had the same symptoms as you but replacing the tube and all was well

Euan

[Pete Lewis]

it is quite common for the choke disc valve to corrode and have some of the enrichment holes blocked  

and the pick up for the choke fueling is the short brass tube along side the jet housing  check its clear .

its only two screws to remove and view the disc if you then strip the simple disc assy down for a clean .

 

[johny]

Is it possible the mixture temperature compensation is stuck on the hot setting so that even with the choke working correctly its not enough to start? Or, if the choke enrichment device uses fuel from the pump is that not strong enough so alright when running but not enough on the starter?

As you can see I havent had much to do with the later design carbs so just spit balling as our American cousins might say🤪 

[dougbgt6]

7 hours ago, johny said:

Is it possible the mixture temperature compensation is stuck on the hot setting

Even with one dodgy carb the engine will try, so this would require both compensators open.

Problems with temperature compensators are more likely to be their "O" rings leaking, these are easily changed. Even if you decide to take the TC covers off and screw the mechanism  down, I think the "O" rings will still have to be changed. 

This sounds like e problem affecting both carbs, the choke disc valve as suggested?

Doug

Edited March 30, 2023 by dougbgt6

[Peter Truman]

My Mk2 Vitesse with Strom carbs was a bu++er to get to idle on choke I used to have to sit with the foot on throttle till the engine warmed up. this was until I saw a note provided by PeterL re putting a 5/16in drill bit between the carb rest boss and the choke cam, now I ave full control of the idle on choke. See attached last para.

[johny]

9 hours ago, dougbgt6 said:

This sounds like e problem affecting both carbs, the choke disc valve as suggested?

Doug

But OP says the choke works when engine hot and running?

[Ian Foster]

On 29/03/2023 at 21:42, owensparrow said:

there is no fuel in the "choke pipe" between the carbs,

Owen

I have 175CD2s on my my GT6 Mk2,  but they have the same choke arrangement.

I definitely see fuel in the transfer pipe when the choke is on. It is possible to assemble the choke disc incorrectly (180deg out) and there are quite a few small holes  and cross drillings in the disc which might just be blocked.

If you can get access to an ultrasonic cleaner that would be a great way to give it a really good clean out.

I have attached an extract from the Buckeye Triumph (IIRC) website which covers the choke mechanism, which might be of help.

Ian

 

Scan_0111.pdf

[Nick Jones]

I'm just about to investigate this on mine also.  It doesn't like to start from stone cold and the colder the weather, the less it likes it.  It does always start eventually, but takes alot of winding up. I believe that although it does get some choke/extra fuel, it doesn't get enough.  It's takes alot of extra fuel to get a quick start when cold. I'm going to be checking out the cold start disc, making sure all the holes are clear and that the disc is installed the right way up (see Buckeye info).

Something else to bear in mind is that with CD carbs (Strombergs and SUs) you are supposed to stay off the throttle until the engine fires.  Opening the throttle while cranking causes the carb piston to rise a little, reduces vacuum and weakens the mix.

[owensparrow]

Thank you very much to all of you, I've got some things to check out now, top of the pile seems to be the rotating enrichment disc, it's obvious from the mullering on the screws that someone with not much mechanical sympathy has been in there so perhaps it's assembled incorrectly, I checked it for FOD but didn't completely disassemble it, I will do that next.

The pictures will be a great help, cheers Ian.

[johny]

out of interest, where does the fuel come from to supply this later cold start device - is it sucked up from the float chamber or teed off from the fuel pump supply? 

[Pete Lewis]

there is a short brass tube down from the choke valve  enters the float chamber next to the jet body on all the CDS +  versions 

TR250_TR6+Carbs+Part+II.pdf

[johny]

so it just sucks it up and has to send some over to the other carb as well - think I prefer the old barrier arrangement...

[Pete Lewis]

yes  i seem to remember!!!!     the barrier device was unable to produce the cold start ratio of air fuel when leaner needles became Widley used 

so a starting valve was used instead  

makes sense but the old grey matter might have it wrong   my days in  emissions  was mainly on diesel smoke and downdrafts 

if they didnt chain the cooling fan down it would take off like a jet and exit the workshop at speed till the wire snapped 

we only played with strombergs on company hunters  etc 

Pete 

 

[Ian Foster]

Based on my limited investigations, it would appear that there are various flavours of both discs and cams, as reference to two of the three spare carbs I have (courtesy of the late Alan aka-Old Tuckunder), have different markings to the ones fitted in my carb.

One of the spares has a 'D' stamped on the cam, a 'P' stamped on the body and an 'R' on the disc. Another has a 'W' on the cam and an 'F' on the body,  so there are obviously different combinations for each car application.

If I compare the 'R' disc of the spare with the photo in the Buckeye article, it has no first drilling and the fourth drilling looks larger. The Buckeye article (last para) suggests that TR6s (TR250) were fitted with starter boxes on both carbs.

There are also a pair of brass plugs or valves on the inside of the inner part of the body, which may be there as a result of the manufacturing process. They look more like plugs than valves to me so are probably not service items like fuel pump valves.

If carbs are original to the car, then the combination of parts should be correct, but if the unit has been disassembled before it is quite possible that is has been put together wrongly.

Ian 

[Pete Lewis]

pleased you mention the pair of poppet valves  in the starter valve body  yes they can get gummed up 

Pete

[Ian Foster]

Can you explain what they do Pete?

Ian

[Ian Foster]

OK, so the rays of understanding are starting to penetrate the fog of ignorance.

If you look at the photo of the disc in the Buckeye photo, you can see that there is just one set of graduated holes. This is consistent with the starter box being used for one carb only (the other carb also has a starter box on the TR6).

For the GT6, which only has a starter box on the front carb, the disc has two sets of graduated holes (as per attached pic), with one set feeding the brass transfer pipe to the rear carb.

So returning to Owen's OP, if fuel cannot be seen in the transfer pipe, then it is entirely possible that the disc is assembled 180 deg out and fuel is not getting to the pipe.

Also looking at the disc in my photo, the first hole is blank, the second tiny, the third small and the fourth much larger, suggesting that you would need to be on full choke to get a reasonable amount of flow. If the cable adjustment is not allowing this, then it could also be an issue. 

Ian

[Pete Lewis]

and to add a little more

the brass push and twist knob allows more or less choke its a summer /winter adjustment as when you reset it it allows more or

restricts the choke fast idle cam and the starter valve max travel .

if you have this small device (showing in Ians pic  ) then if set to summer it wont be enough in the winter  

Have a look   not all have this depends on age and car maker 

Pete

[Iain T]

My choke unit, the same as yours, leaked and the engine was difficult to start. I lapped the disc in tried to seal it but the shaft was very worn and managed to get a new unit. I had to change the disc as the new unit had the single row of holes. Stopped the leak but still didn't start well. It took many attempts at tweaking the choke cam screw to get it so it's much better. If I remember I have a lot of disc rotation ie fuel to start. Actuating the choke does also slightly rotate the throttle disc to allow air through and it's a balancing game to get it right. Another thing to worry about is the throttle cam, is it the correct one and/or worn. If its correct stay off the throttle and don't depress the clutch. Note the 8mm drill bit setting didn't work for me it was trial and error. 

Good luck

Iain 

 

[Nick Jones]

21 minutes ago, Pete Lewis said:

the brass push and twist knob allows more or less choke its a summer /winter adjustment as when you reset it it allows more or restricts the choke fast idle cam and the starter valve max travel .

Discovered this yesterday. Don’t remember ever noticing it before.
 

Pic shows the winter setting. Mine was on the summer setting……

Probably all I needed to do was change this, but of course I’d taken everything to bits before worked out what it actually did. 
This did at least serve to prove that I have the right disc for the twin set-up with two sets of dosing holes, the holes were clear and the disc was fitted the right way up.

Also that the gasket is not one that I have in stock…..