Setting carbs and timing - what sequence to follow?

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Llessur
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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:
Good luck.
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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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Re: Setting carbs and timing - what sequence to follow?

#5 Post by claude626 » Wed Jan 01, 2020 6:36 am

I found that this Edelbrock uni-syn carb balancing tool is fantastic. quite cheap and readily available through ebay etc.

Remember to disconnect the linkage between the two carbies, so that you can independently set the idle mixture for each of them...

https://www.google.com/search?q=edelbro ... =858&dpr=1

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

#6 Post by Clifford Pope » Wed Jan 01, 2020 10:49 am

The carburettors are individually at their factory settings, but does that mean the linkages etc between them are mechanically linked and adjusted? When you pull the throttle cable and the connecting rod starts to turn, does it start to open each throttle at precisely the same moment? If you tweak either throttle stop screw to adjust the idling, that will alter this balance, so it will need checking again.
If you adjust either mixture, that will change the idling speed.

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

#7 Post by Mike Stevens » Wed Jan 08, 2020 12:19 pm

First of all I set the timing to the book spec - so it will run!

Then disconnect the link between the carbs so you can set each throttle individually. This allows the setting of each carb for idle speed (not mixture!). I have an old Gunson's balance thingy, but you can do it with a bit of tube (as Alec suggested). Now the mixture. I do this by ear, screwing each adjustment IN (to weaken the mixture) until it starts to falter, then about 1/2 a turn back out again. Do this slowly, one flat at a time, listening to the engine note change each time. Do this on both carbs. The idle speed will no doubt change, so you can set that again now or leave it until later.

Now re-connect the 2 throttles together, making sure they are still in synchronism. For timing, I do as Tony suggested - set it so that it just doesn't pink. Remember that the book spec was when we had old-style 4-star petrol. The blend we get now is far removed from the 60s/70s so that the book spec probably won't be the best, so set to just not pink.

The idle will have changed again, so set it to what you want. Adjust the screws on each carb carefully so that they remain synchronised.

It takes a bit of getting used to especially the mixture, but it works for me!

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

#8 Post by Clifford Pope » Fri Jan 10, 2020 10:15 am

That's the way I do it.
I've always had difficulty tweaking the mixture on this carburettor. The pin-lifting technique just doesn't work, and is far too vague. Basically the engine starts to falter when the pin is lifted whatever the mixture setting, so it's no guide to rich/weak.
I find the best way is progressively after runs by examining the state of the plugs, making one flat at a time adjustments on the appropriate carb to the smaller nut under the jet, cleaning the plugs, and repeating the check after the next hot run.
Plugs are always a bit sooty when the engine is cold, but it burns off after a moderate run at full temperature, so if you check at that stage any sootiness indicates rich mixture. If it sounds "splashy" and lacks acceleration then youv'e gone too weak and needs enriching until not quite getting sooty again.

I always regret not having the old Strombergs any more. They seemed so easy to set up, and responded very sensitively to lifing the pistons a fraction with a small screwdriver.

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