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GT6/2500 engine. Ignition timing. More advice about.


chrishawley
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Previous help through the Forum got me in the right direction regarding ignition timing. With the car nearly ready to hit the road I trying to firm up on something a bit more precise. Additional 'advice' in the meantime (not TSSC) has been conflicting.

Current situ':

GT6 with 2500 'MM' prefix engine with the softest of the cams

Lucas distributor (not sure which of the four) with accuspark

Stromberg 150 CDSEs - from a 2000 - fitted unmodified and seem fine. Carb balance good, mixture roughly in the right place

Engine starts, idles and revs nicely. Not driven yet.

 

ROM says 8  or 10 degrees BTDC static for an MM engine. But I can't do static because the accuspark. So...I've tried 8 degrees BTDC with strobe on the supposition (? correct) that below 900rpm the centrifugal will be giving negligible advance and the dynamic advance will be the same as the static. From there if  retard the ignition to say 6 degrees BTDC the idle revs drop off quite sharply. If I advance further to say 12 BTDC revs don't increase but the engine begins to fluff.

So does that add up to 8 BTDC (dynamic, idle, vac off))  being about the spot to go for?

My other supposition is that I can't really get optimum settings until it's on the road; perhaps using the vernier to tweak it up to the first point of pinking and then adjusting back from there.

I'd really appreciate some reality checking here so thanks in advance.

C

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a small pea bulb linked from the dizzy terminal on the coil and earthed will illuminate when the accuspark triggers just the same as with points 

if you do it by ear and advance the dizzy to get the fastest idle speed and back of form best , test if pinking back of a little more 

one thing to consider is  the damper ring on the front pulley can suffer from the bonding letting go and the outer ring moves so the timing marks are the useless this is not overly common but it happens , 

so if basic adjustments seem to be irregular you have to do a TDC piston and timming mark check hopefully this all stacks up ok

i am surprised that from 8 to 12 makes it misfire 16 to 25 yes  but 12 is not far off as a static it should run fairly well 

as you have CDSE you need to remove the temperature compensators (2 screws) on the side and turn the small nut to fully close the small

plunger fully shut being old many are open which you dont want at normal operating temperatures  this also upsets the idle speed as its a air bleed to bypass the throttles  and check the two sealing O rings on the unit are serviceable  you dont want stray air or you never set the mixture

this all affects the change in ldle speeds with changes to timing  its all connected 

check the carb adjustment so the small delrin washer fitted to the needle is level with the base of the air piston 

that will doo for a start its simple to do

to adjust the carb needle you need to either get the special allen key tooling or remove the piston and stick a 3mm allen key down the damper bore  to turn the inside adjusting nut 

 

 

Pete

 

 

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Thank you. Much appreciated as always.

I'll cross check the timing static now I know there's a method but it looks like 8BTDC idle on the strobe has got me in the right area (unlike 'other advice' which had me at 4ATDC dynamic at idle).

Next step carbs. Is your preference to shut down the TempComps and leave them that way permanently? Or just to shut them down to establish a better state of tune and then set them up correctly again?

Cheers

C

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shut down for good   one simple way is turn the carb manifold gasket upside down  it blocks the bypass drilling 

there is a delicate way to reset them but in essence to get the main mixture and idle setting to be correct they must be shut 

at up to normal temperature , the idea is as under bonnet high temps leads to a rich mixture being burnt that to aid emmissions 

they open , alows air bleed in behind the throttle plates  thus weakens the idle mix and the air bleed ups the idle speed an amount 

when you remove them you will see the small plunger is a rattling good fit  and the sealing 0 rings ( 2) are wasted flat.brocken 

so being without is more advisable than a gadget not doing its job 

a good site with nice pictures is all TR but the basics apply to all strombergs  https://www.buckeyetriumphs.org/carburetors

pete

 

 

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Super. Thanks. TCs were absolutely shot as predicted. Also note that I'm losing oil from the rear dashpot. So looks like a full scale strip/rebuild is in order. But I'm a lot further forward than I was so thanks for the advice.

Cheers

C

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losing oil  i presume this is a failed needle 0 ring and loss is from the bottom of the piston not some weird overflow out the top ??

the adjuster is held down by a sprag washer it will need a bit of a flatten to make it re grip on fitting back  ie a light squeeze in a vice 

not fully flat or you wont refit it  ....or pay a few ££ss for a kit 

most rebuild kits have more than you need , and can have multiple float chamber gaskets   dont get fooled  they change ofer the years 

and you need to check which gasket covers all the corners correctly or you make a big leaker .

Pete

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3 minutes ago, Paul H said:

 

With CDSE’s you will need to check the jet heights are set correctly and are not bent . 

 

Difficult to bend a jet I would think! 😆

Go to Buckeye Triumphs to get the ultimate rundown on Strombergs, how they work, how to fix them.

Doug

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5 minutes ago, dougbgt6 said:

Difficult to bend a jet I would think! 😆

Go to Buckeye Triumphs to get the ultimate rundown on Strombergs, how they work, how to fix them.

Doug

My highly polished refurbished CDSE’s came with jet height variance of 15 tho and a bent jet 

Paul 

 

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