Setting carbs and timing - what sequence to follow?

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Setting carbs and timing - what sequence to follow?

#1 Post by Llessur » Tue Jan 08, 2019 1:37 am

I’m sure I read a post on a Triumph-related forum a while ago that had a really good guide to setting up the carbs and timing but sadly I can’t find it now. It wasn’t necessarily about the specific details of each process but the sequence in which they should be undertaken and the relationship which they have with each other.

For example – I’m setting up the engine in my 2500S after a long period of inactivity. The carbs have been rebuilt and are at their factory settings, the dizzy has been rebuilt too and static timing was set at 10 degrees. The car started and ran well enough to tune. So I have now balanced the carbs and set the mixture but car drove sluggishly, so I advanced the timing to a point where the car runs much better. But obviously this has now affected the carbs (certainly the idle speed) so they will need to be tweaked again.

Does anyone have any thoughts as to the best route to take to ensure both the carbs and timing are set optimally? How does everyone else do it? What is the sequence I should now follow to get things to be as good as they can be?
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Re: Setting carbs and timing - what sequence to follow?

#2 Post by tony » Tue Jan 08, 2019 2:53 am

You've done the basics right. Now a bit of fine tuning.
Wind the idle down till its right.If you can lay your hands on a carb balancer all well and good , if not use the lifting pins . Just a little is necessary , as per the book.
Best you can do with timing is drive it down the road in top gear, slow down to about 15kph then plant your foot down. If it pinks retard the dizzy until it no longer pinks. conversely if it don't pink advance it a bit. Trial and error I ve found :shock:
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Re: Setting carbs and timing - what sequence to follow?

#3 Post by Alec » Tue Jan 08, 2019 7:29 pm

Hello llessur,

the thing to remember is that the mixture affects the timing required so set the mixture first. They should not need much adjustment from the factory setting, if they do there is something wrong somewhere. Dynamic timing as Tony suggested is a good way to go. If you have a timing light it's worth checking the timing at various parts of the rev range and cross check what it should be from the workshop manual. Also confirm trhat the right distributor and vacuum module (The numbers are stamped on the body) is fitted, again refer to the manual. With old cars it's quite likely a correct type of distributor has been fitted in the past but of the incorrect specification, as there are very many different specifications used by Triumph.

Balancing can be done with a small bore tube used like a stephoscope to listenm to the intensity of the hiss comparing one to the other. It matters little at wide open throttle though :-)


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Re: Setting carbs and timing - what sequence to follow?

#4 Post by Clifford Pope » Fri Jan 25, 2019 12:12 pm

I think to some extent you have to go round again, because adjusting some things then requires a tweak to something else.
The mixture isn't an absolute setting - if you then adjust the balance then you alter the vacuum on each carburettor, because before adjustment each carburettor will be drawing air influenced by the average of the two, so both will be wrong to a small extent.

I remember instructions for tuning old single carburettors with a mixture screw and an idling screw. You get the idling approx right, adjust the mixture. That will alter the idling, so you correct that. But correcting the idling will also change the mixture setting, and so on.

Then factor in a second carburettor, and of course the engine can only idle at one speed, not the speed each carburettor might be trying to set. One will be running faster then it wants, so sucking more mixture, one slower.

For a final test on the mixture I find inspecting the plugs the best guide. I know some people swear by the lifting pin method, but for me it always seems a bit vague and uncertain.

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