Wednesday, 4 July 2018

A change of carburettors

I had a few days of from work so decided to fit this set of refurbished SU HS6 carburettors to the DHC's engine. At last I should say, as they have been collecting dust for almost a year now!
The job itself proved rather straightforward with a few random challenges thrown in! To start with the old carburettors had to be removed. Pretty straightforward, despite the lower nuts being rather awkward to reach. And in no time the old carburettors were removed from the engine:

But while disconnecting all hoses and cables I again found a very porous piece of fuel hose, between the fuel pump and the carburettors this time. So had to clamp another hose to prevent the fuel tank from emptying itself on the drive! And time to visit my local automotive supplier for some 6mm and 8 mm fuel hose. At least the various gaskets remained in place and were undamaged.

After returning home with a fresh supply of fuel hoses, fitting the new carburettors proved rather uneventful.

Except for the moment when I had connected the choke cable to the carburettors linkage. When I was checking the choke's action, the cable snapped ...

Time for another unscheduled trip, to the shed this time. Initial plan was to pick up one of the used choke cables but in the end I decided against it and took the new choke cable which was meant for the '76 FHC. As this car won't see any action within the next one or two years, there's enough time to order a new one. The joys of parts exchangeability.

With everything connected it was time for a basic set up of the carburettors, and some final checks. Or the final preparation for the first start attempt. And for this I used mostly simple tools:

After this check came the always dreaded moment of truth! Will it start and will there be a fuel leak? I shouldn't have worried after a dose of quick start the engine fired up immediately, accompanied by a nice fuel fountain! Luckily easy to rectify by using a different hose clip. After that it was time to set up and balance the carburettors a bit.

I had set the jets slightly leaner (=higher) compared to the old carburettors. And this has everything to do with the fact that I fitted different needles, which are overall richer. Which brings me to the last stage of this job.
When the carburettors were refurbished last year I thought I could use the old needle set up. But Club Triumph's 10 Countries Run 2017 proved that the breathing modifications I've carried out over the past few years were more effective than expected. As a result of this the fuelling of the engine was on the lean side.
When this engine was rebuilt in 1994 it was set up on a rolling road, where we found out that BDL needles gave the best performance (approximately 135 BHP @ 5500 rpm). But not anymore! Time to get behind the computer for some needle comparison. Here I found out the joys of the internet (digital SU needle charts) and the computer, to compare the various needles quickly and clearly. And for comparison I used The TR7's standard needle (BDM) and a popular choice for slightly upgraded engines (BAL).

As the BCE needle is slightly richer overall, I opted for a set of those. Hopefully I will be able to do a test run this Saturday morning and see how she behaves now!

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