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TVR 2500M Triple Weber 40 DCOEs

PDIGIO_6x3

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I have a question for the TVR/Triumph guys out there. I own a 1974 TVR 2500m and it has three weber 40 DCOE carburetors on it. These carbs look great, but the problem is that there is a stumble in the acceleration of the car. I have to take off very slow until the RPMs are higher in range. It is sort of manageable if I toggle the choke while driving...It definatly feels like a lean out situation (not enough gas or too much air). The problem never really goes away though.

I have made sure that all of the jets in the carbs are clean and I recently put a fuel regulator on the car which helped a little.

Has anyone else experienced this problem? I need help with this aggrevating problem, I can't find the solution!

Any help or suggestions would be great!

Thank You in Advance,

Matthew
Not sure if you're still using points in your ignition, but I have triple CDs ZS, and had similar performance issues. It was my points. Give that a look before you lose your mind on the fuel system.
 

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Grantura_MKI

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I thought that was the case. Would be sure that the blue hose is sufficient for the vacuum load and will not collapse.
 

velloti

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I have a question for the TVR/Triumph guys out there. I own a 1974 TVR 2500m and it has three weber 40 DCOE carburetors on it. These carbs look great, but the problem is that there is a stumble in the acceleration of the car. I have to take off very slow until the RPMs are higher in range. It is sort of manageable if I toggle the choke while driving...It definatly feels like a lean out situation (not enough gas or too much air). The problem never really goes away though.

I have made sure that all of the jets in the carbs are clean and I recently put a fuel regulator on the car which helped a little.

Has anyone else experienced this problem? I need help with this aggrevating problem, I can't find the solution!

Any help or suggestions would be great!

Thank You in Advance,

Matthew
I'm with the good Dr on this. The later 40 DCOE units (40 DCOE 18 IIRC) fitted to the TR6 conversion kits have a 30mm venturi as opposed to the 28mm venturi fitted to the earlier kits that used the 40 DCOE 2 (IIRC). When I was going through the set up calculations I kept coming up with the primary veturis being 27mm or 28mm depending on exactly what RPM I used for the maximum horsepower point. It was relatively easy to find 28mm venturis, difficult to find 27mm but, hey were made so there are some out there somewhere. Odds are unless you are running the early TR6 US spec camshaft, the 28mm is probably the better choice. The venturi size would be the the first place I would look if those are anything other than the 40 DCOE 2.

TRF used to offer the John Passini book on Webers, I don't know if it is still available from them or not. Brooklands out of the UK did a reissue of the Passini book about a dozen years ago, but again don't know about availability. Plus there is the Haynes Weber book and a Pat Braden book on Webers that go through the various circuits in these carburettors as well as graphs and general calculation information for the various jets as a function of venturi size.
 

velloti

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there is not enough info into the throttle position to give a accurate diagnosis , if this is a light throttle stumble , first where are the idle mixture screws set at ? two what are the idle jet numbers , is this stumble just off of idle just as you try to pull away ,or is it while cruising in a gear ,, i can give you more ideas ,with the info ,call me on 561 909 7149 tx first im 79 started working with webbers in 64 knew john passini at radbourne , got a lot of info from him in those days
 

Rocky_LC

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Hey buddy…. I’ll get the info, and give you a call in a couple days, just to touch base.

Thanks for reaching out!

Rocky
 

Rocky_LC

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After significant troubleshooting & tuning with the help of Alan V, I believe I have eliminated the stumble…. It is was a case of not enough ignition timing!

There was a bit of idle adjustment screw tuning, and airflow balancing, but in general…. It was the timing!

I am now running about 28* of advance at idle, and mechanically limited the centrifugal advance to about 6* with a spacer in the stop pin (so about 34* all in before 3000 RPM). Here’s my thread on the distributor, which also got disassembled, cleaned and lubricated…

Distributor Mechanical Advance

I know that’s quite a deviation from “stock”, but everyone says “Webers like a lot of of timing”.

The car runs well, and is easier to drive! Plugs look great!

Also got around to checking the crank thrust clearance, which I happily measured at .010”, not in spec (.006 - .008”) but good for a few years of happy motoring!

Thanks, Alan for your help and advice!

Rocky
DDF9ED7E-4CCA-47C6-ADC0-0D8B2CF334B0.jpeg
302D50B8-A3FF-4C83-A1C4-55416B699F84.jpeg
 
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PDIGIO_6x3

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Maybe some things to look at.
I have the triple ZS setup, and experienced similar issues. Look at your spark plugs after a good long drive. This will tell a good story about lean or rich mixture. Also, with those carbs, I wonder if there's enough push of fuel. Are you using a mechanical fuel pump? Although that logic would seem to give you an opposite effect, where not enough fuel at higher rpm. Also, a vacuum leak in the system, accelerator shafts seals, etc... I had this issue where the alignment of the three carbs were slightly uneven, and when tightening the shaft coupling introduced enough of a force that caused seal leakage, making a lean mix. If you still have points, you can NEVER rule out bad contacts. I've been bitten there as well.
 

Grantura_MKI

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I assumed that you had ”curved” the dizzy already? If not, then thats is what I would do. The vacuum advance is not really up for the job. Running points is fine in my book.
People think that these carbs require tones of fuel and pressure…not correct.
 

Rocky_LC

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I have a mechanical pump (and a regulator), with about 2 psi of fuel pressure. I’m pretty confident fuel supply is adequate, as my problem was “off idle”, initial acceleration, when the Weber bowls would have been full.

I had looked at my advance curve, and it came in relatively quickly, I was setting my timing a little higher than what the books suggested at 12-14* of static timing…. I previously running about 16-17* at idle.

Now static timing (at idle) is about 28* BTDC. That’s a lot more than you can see recommended in any manual or postings.
I limited the max advance so I’m not going much over 35* total. I actually drove the car yesterday with 25* BTDC, and had the stumble, a little more (28*) cleared it up!

I’m not running vacuum advance on my car as there is no source of it off the Weber carb bodies.

I have a good electronic ignition (Pertronics Igniter & matching coil…. Plugs are gapped at .050”.

I’m very happy with how it’s running now!

I was just posting my solution for posterity, not asking for advice!

Rocky
 
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Rocky_LC

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No problem, just wanted to close out my part of this discussion with my solution…. A lot of threads ask a question, but never give out the results or solutions….

Just wanted to let people know what worked for me.

If it had been suggested that I run my timing at 28* vs the suggested 16* somewhere in these troubleshooting suggestions, then I missed it, and apologize….

I appreciate everyone who helped me!

Rocky
 
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