[TR] Tuning Webers

[AMfoto1 at aol.com]

Hi David, 
 
Also take a look at _http://www.hottr6.com/triumph/tr6weber.html_ 
(http://www.hottr6.com/triumph/tr6weber.html) 
 
The short 12mm air horns might be part of your problem. Shorter horns are 
usually chosen for high rpm performance, longer ones for mid and lower rpm work 
(a bit more streetable, in other words). The main reason for this is standoff, 
or air fuel mixture escaping from the outer end of the carb. In simple terms, 
longer horns prevent standoff escaping when the air flow is slower, i.e., at 
lower and mid rpms, where it could tend to lean out the mid range rpms. 
 
You can go up to 35mm horns with the deeper (3.5" I think) K&N filters, but 
might have clearance issues at fenders or hood on your car.  Alternatively, 
"Sock" type filters can be used with most any length air horn. 
 
Or, are you running without filters? If so, Webers seem to work best in 
"still air", so some sort of filter or airbox is usually best. 
 
_http://www.teglerizer.com/dcoe/_ (http://www.teglerizer.com/dcoe/)  shows 
the standard jetting setup provided in the conversion kits sold for TR6 (and 
TR250, I'm sure). 
 
Also, which series DCOEs are you using? There are three different accelerator 
pump strokes, among other differences. TR6 are specified by Weber to use 
DCOE-18 (x3), which have the shortest accel. stroke length of 10mm. (Note: DCOE-18 
are no longer made, new kits for TR6 are provided with a different model, 
DCOE151 I think. Unfortunately I don't know what it's pump stroke is.) Haynes 
recommends DCOE-2 for TR250/6, which has a longer, 14mm accel. pump stroke. This 
might be a clue you can use. 
 
Your setup seems pretty much in line with the conversion kit specifications, 
except that your main jet is slightly larger (125 compared to 120), your air 
corrector is quite a bit smaller (145 compared to 160) and your idle jet is 
quite a bit larger (65f8 compared to 50f11). All else appears the same. 
 
One other key thing, the old Haynes manual specified 27mm chokes, while the 
Weber manual specified 30mm. The difference may be that the Weber was for the 
UK/European car model with higher HP rating (fuel injection, but also a diff. 
head), while the Haynes spec was for US versions of TR250/6. At any rate, 30mm 
seem to be the choice for race engines, while 27 or 28 are perhaps more for 
street. Smaller chokes will increase the speed of air moving through the carb at 
lower and mid rpms and might help your situation. (See Terri Ann Wakeman's 
site, although it's more pretinent to the 4 cylinder TRs. 
_http://www.tjwakeman.net/TR/WeberDCOEinfo.htm_ (http://www.tjwakeman.net/TR/WeberDCOEinfo.htm) )
 
Note too that there's some dispute whether the pump bleed jet should be 
closed (as shown in Haynes) or have a 50 jet in it (as shown in the Weber reference 
book). Similar situation to the difference in choke size, and you'll find 
some more explanation at the first link.  
 

You didn't mention the needle jet. A 2.00 is specified, but it appears some 
use a 1.75. The fact that higher RPMs seem good on your car points toward the 
needle jet, fuel supply and fuel pressure being correct.
 

 
There are several ways to tune and it means a lot of swapping of jets. 
Because of this it's usually best is to work with a shop that has supplies of jets 
on hand to try before you buy, as well as an exhaust gas analyzer and perhaps a 
dyno. That way you don' t end up buying many sets of jets whle trying to dial 
it in yourself with guess work. 
 
Keep in mind, the jetting provided in the conversion kit is always just a 
rough starting point. 
 
Every engine is different, as are driving habits and requirements, so the 
jetting is almost always going to vary from the "norm". Plus, what works for one 
car might not work for another car, even if it's very similarly set up 
otherwise. If you look at the list of what people are using in TR6 at the first link 
I mentioned above, you will see a range of variations. 
 
Webers are also sensitive to altitude and even weather conditions. For 
example colder and denser air calls for richer jetting and warmer air needs leaner. 
Most who use DCOE end up with a few extra sets of jets to swap out for fine 
tuning as ambient conditioins change. So, buying extra sets isn't necessarily 
bad (and any extras can usually be sold off on eBay). 
 
Note that there can't be much variation in compression, cylinder to cylinder.
 
As a freshly rebuilt engine breaks in over time, it will likely need 
re-tuning once or twice, too. 
 
Any additional modifications to the engine will call for re-tuning, too. 
 
Finally, what ignition system are you using? This can make quite a 
difference. Stock TR250/6 are generally not up to the job. Is it set up for mechanical 
advance only? Vacuum advance is not usable with Webers (I think mainly because 
tapping the intake manifold as a source for the vacuum throws off one of the 
carbs from the rest ). 
 
Hope this helps!

Alan Myers
San Jose, Calif.
_amfoto1 at aol.com_ (mailto:amfoto1 at aol.com) 
 
'62 TR4 CT17602L 
 
 
 
_http://www.triumphowners.com/640_ (http://www.triumphowners.com/640)  



 
 

I'm also running short, 12mm air horns and an electric
fuel with a pressure regulator set at 3psi and .375"
fuel lines.

David Brady
'68 TR250 CD8124L

David Brady wrote:
> Folks,
> 
> Thanks for your help. Here's what I'm running:
> 
> Three 40DCOE's,
> Idle: 65F8
> Air Corrector: 145
> Emulsion: F11
> Main: 125
> Accel Pump Jet: 40
> Aux Venturi: 45
> Choke: 30mm
> 
> The motor is reasonably high performance. A lot of
> work was done porting the intake runners and the head.
> It's dyno-ed at 170hp.
> 
> I'm far from Weber expert. I'll try to get my hands on some
> tutorial, but I don't really know where to start. It does idle
> good, so maybe we can assume that the idle jets and the
> progression circuits are good. It runs great above 3000rpm's
> so the main's are probably fine. My first inclination is accelerator
> pumps. Thanks for your help.
> 


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