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

[Mack]

Thanks Ian, further details would be appreciated. That is a very neat set up you have there. 

I have seen the isolators mentioned elsewhere, is their function to isolate the carb from heat from the manifold? 

The manifold on mine has indeed been modified so I am keen to stick with the SU's, especially as the car was running well previously. I plan on getting it on a rolling road at some point when this is all sorted to get a better idea of how they can be improved. 

The heat shield seems like a great idea! 

 

Matt

Edited September 27, 2023 by Mack

[Ian Foster]

Matt

Yes, isolators are there to limit heat transfer from the (heated) manifold.

I have taken a couple more photos of my set up and have attached some CAD drawings I did at the time I was working on it.

Basically the manifold flange needs to be built up with aluminium (approx 18mm thick) to match the profile of the HS6. The rear of the manifold is already almost the right profile, but the front one is the same lozenge shape as the standard 150 Strom. The existing holes need to be filled in and new holes drilled and tapped to suit the HS6 bolt pattern. Ideally the studs should have UNC threads in the aluminium and UNF for the nuts onto the carb flanges. I used a local fabricator that I knew through my former career as a Structural Engineer, but any good shop should be able to do it.

Next onto the manifold is the heat shield, which is made in two parts. The first part is made from 3m thick aluminium and is sandwiched between the isolator and the carburetor and has a small outstanding angled flange below the carb, onto which the main shield fixes.  I did it this way so I could more easily get behind the heat shield if required eg-starter motor and jet adjustment), rather than having to take the carbs off.  The removable lower part is made from carbon fibre faced with reflective aluminium foil, just because I had some spare carbon and epoxy left over from a boat project.  They could just as well be made in folded aluminium.

Then comes the black plastic isolator. Mine are 13mm thick and IIRC were sourced from Burlen Carburetor Services. (you will also need 6No. gaskets, 3 per carb).

I have used the K&N filters for many years, but am planning to fabricate a bespoke air box to allow a cold feed. I am hoping to use my existing filter elements within the new airbox.

One further mod has been to add stub stacks to aid the flow into the carbs. I had some turned up using Delrin, but I believe other options are available commercially. Fitting them immediately increased the idle speed, which suggested to me that flow had been improved.

Hope this helps. Shout if you need anything else or are struggling to find a fabricator (you never know my contact might be prepared to do another one).

Ian

 

Manifold-spacer-heatshield.pdf Stub stacks.pdf

Edited September 29, 2023 by Ian Foster
spelling

[Iain T]

1 hour ago, Ian Foster said:

One further mod has been to add stub stacks to aid the flow into the carbs. I had some turned up using Delrin, but I believe other options are available commercially. Fitting them immediately increased the idle speed, which suggested to me that flow had been improved.

I agree I bought mine from MED engineering. They are made to fit SUs so I elongated the fixing holes and machined a groove in the back to fit Strombergs.

Iain 

[Mack]

Thanks for that Ian(and Iain) Looks to be a well engineered solution.  

The PO already fitted stubstacks so that's one job done already. I will definitely look into modifying the manifold further over winter, the current set up needs changing. The gasket material arrived so hopefully a new gasket will solve the problem temporarily. I will report back!

 

Matt 

[Ian Foster]

8 hours ago, Mack said:

gasket material arrived so hopefully a new gasket will solve the problem temporarily.

Let's hope so.

[Iain T]

9 hours ago, Mack said:

I will definitely look into modifying the manifold further over winter,

If you have a mk2 manifold there is an internal casting air flow restriction to one cylinder on each bank. Take a look at Chris Witors website and it shows what to do. On mine I ground out 3-4mm of said casting to help even the flow to numbers 2 and 4 (I think but don't quote me). The experienced guy who sorted my head and inlet manifold looked at it and commented the mod had already been done. Yes by me! 

Good luck

Iain 

[Mack]

Update.

The adapters bolted to the manifold with some countersunk allen headed bolts and the carbs then bolted to them. I have made up new gaskets for all four connections. I have added some pics to vaguely show the manifold adaptions already present.  Unfortunely I didn't take one of the adapter plate! Also shown is the stub stack fitted.

I think getting the manifold altered properly is the way forward in the future and they definitely need balancing and the cabling sorting out. 

Having got the carbs back on it seems to have sorted out the low power issue. It will idle off the choke pretty quickly, however when going to rev it, it will then stall. This seemed to clear itself quite quickly though. After a quick test drive it seemed mostly back to ''normal''.

However there seems to be a distinct whistling sound coming from somewhere still. I presumed this to be the vacuum leak but now I am not sure, unless there is another one somewhere! 

I will try and post two videos to try and demonstrate it. It might just be my inexperience shining through and that might be normal?? 

At least it will be driveable to it's new garage and I can get properly stuck in! 

[Mack]

Apologies about the scale of the video 😑

[Iain T]

The whistling is almost certainly from a leak somewhere in the intake system. Could be the engine side or carbs. I pinpointed my leak quite easily to a carb/manifold area. Only a miniscule leak but it completely mucked up the tuning and the reason why one carb had to be set differently to the other. 

I may be wrong but I think the 'doughnut' type of stubstacks work better. 

Iain 

[Iain T]

That whistle could be your alternator. To check take the fan belt off and rev for a few seconds. But not too long. 

Note put belt back on! 

Iain 

[Mack]

Thanks Iain, I tried the old hose stethoscope technique but couldn't find the source of the whistle. I will try the alternator but believe it is relatively new. 

 

Matt

[Pete Lewis]

apso check the rocker cover and gasket  if they leak air you upset the manifold /mixture on these semi sealed systems 

pete

[Iain T]

57 minutes ago, Mack said:

will try the alternator but believe it is relatively new.

I had a new alternator that was so bad it vibrated the car! New means nuffink! 

Iain 

[Ian Foster]

4 hours ago, Iain T said:

I may be wrong but I think the 'doughnut' type of stubstacks work better. 

Took me a while to find it, but this compares various profiles. As Iain suggests the simple doughnut seems to score well.

[Iain T]

I did a lot of research regarding which type of stub stack to fit. It seems I got this call correct! 

Iain 

Edited September 30, 2023 by Iain T