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Sprint idle issue
http://forum.triumphdolomite.co.uk/viewtopic.php?t=34862		 Page 1 of 1
Author:  gmsclassics [ Mon Apr 08, 2019 9:06 am ]
Post subject:  Sprint idle issue
This also relates to the Sprint rolling shell I acquired and built up using standard engine and drivetrain from various different cars. I then fitted an 1850 auto and replaced the 2” SUs with standard set up back in 2013. Car has done 12500km without much issue in the 6 years since.

Recently this car has developed an intermittent misfire at lower revs, particularly when not up to temperature. Also the idle needs readjusting frequently, at least daily, by as much as a quarter turn of each carb adjusting screw as it seems to go up or down by as much as 150-200 rpm at random. I have also noticed that when letting the engine drop to idle, the revs drop below and then increase back up to a regular idle. This may be by 100-150rpm

I have always needed to set the idle at 950rpm as it drops down to 650 rpm once drive (or reverse) is selected. Anything less and engine is likely to falter and die. I know this is a significantly greater drop than should occur, but I have simply put this down to a worn auto box.

I noticed the distributor mounting bracket was loose so have fixed that. Timing by strobe at 650rpm is 10 degrees BTDC, all leads are firing, plugs are gapped 25thou and all look (colour) as though mixture is correct , carbs are balanced and mixtures adjusted by CO/CO2 sensor (?). Ignition is an old Lucas electronic system. Even though there is an in-line fuel filter, I’ve checked for debris in the float chambers and there is none. I’ve not taken the carbs apart as not sure I would know what to look for.

Car runs faultlessly above 2000rpm

Any advice much appreciated

Geoff
Author:  dursley92 [ Mon Apr 08, 2019 5:49 pm ]
Post subject:  Re: Sprint idle issue
Have you got an air leak somewhere on the inlet side?
Author:  gmsclassics [ Wed Apr 10, 2019 8:48 am ]
Post subject:  Re: Sprint idle issue
Thanks, can’t see / detect any sign of an air leak and mounts are in reasonable condition.

Decided to investigate further so took the dashpots off both carbs. The springs are yellow which seems correct but the needles are both BAW. Only reference I can see on other threads is to BBT or BDQ. No idea what the difference is? I had a surprise when I looked at the jets as they are at very different heights, the front is just below the level, yet the front is quite above. Both holes seemed oval as opposed to round with the greatest dimension side to side.

I think the carbs came off VA9803/9980HE but can’t be sure. Even if they were, that car had covered almost 200,000km so they may have been changed along the way. Certainly would explain the wear on the jets. I can’t find any ID on the carbs that corresponds with any of the part numbers in the parts catalogue (or Rimmers website), so unsure as to what parts to order. The carb casing has AUD9 on the side and AUD2874 (rear) on top of the body with AUD2875 on the front one.

Can anyone assist in identifying these and recommending best needle / jet combination? Anything else recommended to change at the same time?

Also where else can I get these from, bearing in mind they need to send these to NZ.

Appreciate someone’s help!

EDIT: Just managed to get onto SUcarb.co.uk and found all these types. They also list a BCM needle used between BBT and BDQ

Triumph Dolomite Sprint 1998 cc 1973 to 1974 AUD 545F, AUD 545R
Triumph Dolomite Sprint 1998 cc 1974 onwards AUD 661F, AUD 661R
Triumph Dolomite Sprint 1998 cc 1975 to 1976 AUD 680F, AUD 680R
Triumph Dolomite Sprint 1998 cc 1976 onwards AUD 663F, AUD 663R
Triumph Dolomite Sprint 1998 cc 1976 onwards FZX 1257F, FZX 1257R

Geoff

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Author:  roadster [ Wed Apr 10, 2019 9:31 am ]
Post subject:  Re: Sprint idle issue
A couple of ideas :-

Breather pipes letting in variable amounts of air due to loose connections or collapsing flexible pipes.
Intermittent flooding or incorrect levels in float chamber(s) , would account for big variation of jet heights and could be caused by damaged floats.
Author:  xvivalve [ Wed Apr 10, 2019 10:42 am ]
Post subject:  Re: Sprint idle issue
For identification, what colour is the plastic bit on the bottom of the jet? They changed this along with the tag numbers...except for the very last waxstat type that have a completely different visual jet arrangement entirely.

Your intermittent misfire until warm could be electrical, rather than fuel. I have this on my Alfa at the moment and I'm currently testing to determine whether it's plug or coil pack.

Never heard of BAW being used on a Sprint! Are the filter and cam standard?
Author:  gmsclassics [ Wed Apr 10, 2019 10:20 pm ]
Post subject:  Re: Sprint idle issue
Thanks for suggestions.

Breather pipes seem okay and have had float chambers apart to check for debris. Differing height jets are a concern, but I thought best to replace given holes seem worn and are not round.

Existing jets are black plastic covered with what looks like rubber hose connecting to float chambers. Nothing like the waxstats on the late 78 blue car. There are no tags on either carb unfortunately.

Is running with a standard type paper air filter which isn't new but is quite clean. Don't know the origin of the cam but I suspect it is a mild improvement on standard, given the engine still has good low torque with the auto 'box, yet when I used to have the 2" SUs on the car it went extremely well above 80mph!

Agree that it is possibly / probably electrical given the age of the old electronic unit / plug leads. However that is expensive change out and I'm not sure how to test the components without spare (other than a coil).

Existing unit:

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Author:  xvivalve [ Thu Apr 11, 2019 12:26 am ]
Post subject:  Re: Sprint idle issue
I thought each was colour coded, but I've just checked my spares (I only have 8 sets of carbs sitting doing nothing!!!) and whilst I could swear I thought one of them was yellow, the 545, 661 and 663 I have are all red, only the 680 is black.

Does anywhere local to you still have a 'Sun' diagnostic machine, the type that clips onto the HT leads and gives a graph for each; that would show if you were getting an intermittent electrical breakdown, and where it was located...
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