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  This topic is about my 1973 Triumph GT6 MkIII
F1000RACER Avatar
F1000RACER Platinum Member Gary H
Alpine, CA, USA   USA
Just finished a body off restoration on a GT6 Mk3 Post KF20,000 chassis.

The Post KF20,000 GT6 came with a ZS 150 CDSEV carb. These were a one year carb for the GT6 as B/L was battling emissions at the tail end of the production. Since I'm dead set on making this car as close to original as possible I'm making the vent valve on these carbs functional.

P/O had for some reason kept the stock CDSEV on the rear but had a mismatched CDSE on the front. After watching ebay for a few months I saw a matched pair come up for sale so I can now put the correct matched pair on.

My question to the forum members is....I read on several other blogs that the Bypass valves on these carbs were originally blocked off. Does anyone on this forum know if this is correct? I talked to Joe Curto and he says he blocks off the Bypass valve on all the carbs he does. I couldn't get him to give me a good explanation as to why he likes to block them off.

Any help from the forum is appreciated.


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trrdster Avatar
trrdster Wayne Tate
Spencer, NC, USA   USA
Gary, can't help as to why, but nice car. That took a lot of work.

1970 TR6
2000 Jaguar XK8
1949 Triumph Roadster 2000
1978 Spitfire (rust victim)
1971 GT6 (tarp covered for 12 years, rusted inside out)
1980 Spitfire (getting all the good GT6 parts, all poly suspension and Spax shocks)

tmpass Avatar
tmpass Tim P
Medway, MA, USA   USA
1970 Triumph GT6+ (MkII) "Capo"
1970 Triumph GT6+ (MkII) "Blue Oxide"
I believe the bypass valves, as well as the temp compensator, are both idle circuit emissions pieces. Basically the are manipulating mixture while the car warms ups and during idle.

By blocking the bypass valve you eliminate a variable in the mixture adjustment. It is usually blocked by cutting thin aluminum sheet to match the gasket or just putting a gasket with no holes cut into it. You wouldn't be able to even see that it was blocked and the carb would look completely original.

A lot of members post about having a racing idle (2500+ RPM) after rebuilding carbs due mainly to maladjustment of the bypass valves.

I am no expert tho... and will probably be corrected smoking smiley

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65or66 Jim B
Lake village, IN, USA   USA
1965 Triumph Spitfire MkII
1973 Triumph Spitfire 1500 "Jusanudda Munny Pit"
The complete name for it is "Deceleration Bypass Valve". When the throttle is closed on an engine at higher rpm (like when shifting), the high vacuum condition can pull excessive fuel into the engine, causing a rich condition. This raises emissions. The DBV opens a small passage when this happens, and reduces that high vacuum/rich condition. It also makes the engine a little slower to drop back down to idle when adjusted properly. It needs to be adjusted properly to work correctly, and the valve components themselves go bad over time, and can leak or stop functioning. That can cause other issues needing to be chased down. I think that's why some people block them off. Earlier cars did not have them, so the cars do not run worse if properly blocked off. Just a higher emission number when lifting off the throttle.

Your computer-controlled EFI engine will do the same thing when you rev it up and release the throttle. That's why some people complain about the engine speed staying high between shifts on manual trans cars. But most drivers these days have never driven a manual trans car without that 'feature'.

Bobs78Spit Avatar
Bobs78Spit Silver Member Bob Berger
St. Louis, MO, USA   USA
About halfway down this page from the Buckeye Triumphs Tech Site is a good write up on the DBV and how to set it.

F1000RACER Avatar
F1000RACER Platinum Member Gary H
Alpine, CA, USA   USA
Thanks for the feedback. I found a copy of the 1973 Triumph Emissions control systems today on the web. It clearly has a section in the factory manual on how to set the Bypass Valve. I've concluded that it was indeed functional not blocked off.

The buckeye triumph post is very helpful. Some nice tips on how things should be. I know these carbs inside and out now.

For those into GT6 MK3 I'll post some build pics of this car. It started as an original Blue Plate California low miles car. Never wrecked and zero rust. Had a low buy it now price on ebay last year so I jumped on it. Took about 12 months to restore it. I have a bit of an unfair advantage, I own a CNC machine shop that is well equipped.

I have 6 TR250's that I plan to restore over the next few years.


Edited 1 time(s). Last edit at 2017-11-30 02:28 PM by F1000RACER.

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