Re-starting The Tc

[berni29]

Hi There

I tune mine with a colourtune and a length of 5/8" tubing. And the pulling off the sparkplug lead check the engine revs method. For those not familiar with this method, the colourtune shows you the colour of the flame, and the tubing held against the ear lets you judge airflow by listening to hiss levels at each trumpet, and pulling the leads tells you how much work each cylinder is doing.

I get a fair result with this method. I do like the look of the airflow meter though. I will have to see if they sell them in the UK and the cost.

I am after a nice even idle. I do not think that I will be passing the cup of water test anytime soon though!

Nice to hear about how the other half live!

Berni

[type26owner]

Berni,
If Matt wants the guidance then you can follow along too and find out how to do it the best way. Notice I did not advise him to buy a Colortune. They are a gimmicky toy only and have little value to add to aid the tuning process. Sorry to burst your bubble. This just another instance of the Colortune being touted by experts which are not. I have too many fingers on one hand to count the number of persons who've published a web article or book which do know the correct way. Pitiful!

[type26owner]

Matt,
When you get the carbies back there is a quality assurance item to check out first to ensure the job was done correctly. So don't just slam them back on. You'll need a .001" thick feeler gauge shim to check the sealing alignment of the butterflys in the throats. It's critical this is done to perfection otherwise the idle and progressive hole running is compromised. The feeler gauge is used as a way to judge the gap down to the micron level. Place it between the closed butterfly and the throat top and bottom and on either side at 45 degrees. Each place put it in there and close the butterfly completely and gently tug on it. All is well if it gets caught in all six places. If it does not then it is misaligned or the butterflys is not the correct shape. Either way it's wrong and you'll never be able to tune the hesitation away when coming off idle onto the first progressive hole correctly. Suggest you do this at the repair place before walking out the door with them. The shim has to be cut so it's only about .1" wide but left several inches long. Aluminum cooking foil is too soft for this purpose and tears to easily.

If the butterflys are the stock 79 degree 30 minute ones then I would encourage you to have a leap of faith and have them install the 78 degree ones instead. Lotus installed those so they would not have to do any real tuning and screwed everyone in the process. Go into the archives on this forum because I explain it in depth there.

[type26owner]

The reason for using the 78 degree butterflys is to close the gap between the upper edge of the butterfly and the first progressive hole so there is no hesitation coming off idle because the mixture suddenly leans. To phase the butterflys into the correct position means having to drill a small air bleed hole through the each butterfly so it closes more then if it had no hole moving it away from the progressive hole on the micron level. When you first install the carbies it will most likely idle on the fuel coming out of the first progressive hole alone. You can tell this is the case by closing the idle mixture screw completely and observing it's still firing okay. The tricky bit in all this is the drilling process. The butterflys are brass and have to have the drill rake cutting angle removed from the drill bit or the drill will dig in and break off. Doubly important because the drills are tiny in diameter and very delicate. Consider this carefully before committing to this process because you need to be skillful drilling tiny holes.

Here's the really brillant part of this technique. The throttle shafts are NEVER twisted to balance the airflow from one throat to the other in a carb or you'll screw up when all the first progressive holes come into play and that should be all at the same time. It's only done by enlarging the bleed hole of the throat which has the lesser airflow. The balance adjustment on the linkage to phase the two carbies throttle shafts together is NOT used for the idle airflow adjustment. Only do this via the air bleed hole size, got it? This allows you to use that linkage interconnection adjustment farther up in the rpms where the engine is shaking around violently because the airflow between the two carbies is where it's now really out of whack. So just run the rpms up until the engine is shaking around the most and do the balancing there and you'll find out just how smoothly the pair of Webers can really behave. It makes a tremendous difference on the cruise power settings were they are prone to surge constantly and is most annoying if adjusted poorly. That's were the throttle is only partially open and the engine is only putting out about 15 horsepower to sustain the speed for about 90% of the time while driving. Suspect most folks have this all screwed up just like I did before figuring it out on my own. BEFORE DRILLING ANY AIR BLEED HOLES THE FIRST TIME MAKE SURE YOU DO THE BALANCING BETWEEN THE CARBIES AT THE RPM IT SHAKES AROUND MOST VIOLENTLY AT FIRST. Everytime the carbies are removed to modify the holes also do the higher rpm airflow balancing first before measuring the idling airflow rate. Tolerance I use for the airflow at idle is if they are all within the width of the STE orange indicator band then that's close enough. I get right around 3.5 kgrams per hour flow rate at 1000 rpms per cylinder. No way can you do this without the STE gauge. Don't even try!

A well adjusted set of Weber means the engine never spits, backfires, pops or sputters especially on maximum rpms overruns, not even once. It will run up and down the rpms just like it had fuel injection which is computer controlled. However, you're the computer which does the adjustments and the Webers can't react to a changing environment so the solution is a best fit compromise. If done properly the best fit really is pretty damn good though. Piece of cake!

[type26owner]

On the later 151 and 152 type Webers they added an air bleed bypass to make this adjustment at idle more convenient so the need to drill tiny holes is a thing of the past. Too bad Weber has gone the way of the Dodo bird also.

[berni29]

Hi There

Thank you for taking the time to explain all that. Very interesting. I will have to read it a few more times then give it a go when the time comes.

All the best

Berni

[type26owner]

Berni,
Let me add one other important item to the list. Before doing any idle tuning to the Webers you MUST make sure the exhaust system is in perfect repair. Any chuffing (the sound an exhaust gas leak makes) from the manifold to the pipe connection must be fixed and sealed up tight.

That's my task for today as it turns out. The female conical connection end of the exhaust pipe is bent out shape and will not seal when clamped anymore. This time I'm making that flange piece thicker and the sealing surface a spherical radius which matches the male end of the manifold. My exhaust system is the early style one and a real pain in the butt as it turns out. Only after I've corrected this leak am I going to modify the hole diameter in each of the butterflys.

Hey Matt,
Where are you in the process of repairs?

[Matt7c]

The TwinCam lives !!

Thanks for all the advice and guidance above, I'll certainly be refering to it when I come to tune the engine properly. However, for now, I have the engine running very smoothly. I just out out of the garage after refitting the carbs and have a huge smile

I cheated.

I checked out the price of refurbing my carbs by a proper expert and the full monty repair and refurb service was going to cost between ?300 and ?400 (lesser services were cheaper) for both. With up to that sum to play with, I had a look around and have acquired some newly refurbed and well-tuned (to another engine, I know!) 40 DCOE 18s for about ?250. Fitted them today, and they work superbly and the engine runs sweetly after 13 years

More work to do on tuning them, but that isn't so urgent to me at this stage.

I am extremely happy with life right now!

[type26owner]

Congratulations! Now you know why we're all wound so tight about our cars. They are a real blast to drive!

I cheated.

You didn't cheat. You just used good judgement. The repairs of the Webers are not for anyone that doesn't have years of hard earned engineering smarts. Even so equipped they can still humble you.