150CDSEs. Can't get rich enough. More advice please! GT6 with 2.5 engine.

[chrishawley]

With much Forum help previously I had successfully rebuilt the 150CDSEs. Or so I thought.

But on installing them I can't get a rich enough mixture on the adjustment. By whatever means I use (piston lift, Colourtune) the mixture is weak at idle and across the whole range. Car won't pull above 3500rpm. Temperature compensators are blanked off. These were originally GT6 carbs and on rebuild the original jets and needles were reused. With these carbs the engine starts and idles nicely but feeble power beyond that. Nothing will induce the C.Tune to show anything other than blue.

I have a knackered pair of Triumph 2000 CDSEs and if I fit these overall performance of the vehicle is pretty reasonable. So although these carbs are beyond saving they work just well enough to rule out other (non-carb) causes of poor running.

I'd appreciate any suggestions on how to investigate further. Obviously having had the carbs apart 'technician error' is a likely part of the equation - but just what is beyond me at the moment.

Thnx

C

[Clive]

Odd you can't get it rich enough at idle, but I am no expert on Stromberg models. CDSE sounds like off a late car, so maybe restrictive? You may have more luck with early type carbs that allowed a wider range of adjustment. std GT6 needles will be the wrong profile for a 2.5.

 

However, 1 1/2" carbs are on the small side for a 2.5, but should only be restrictive at higher RPM (4-5k??) The usual "fix" is a pair of short dashpot HS6 carbs off a sprint, the needles are about right too. Inlet manifold needs opening out to suit, and the carb mounting holes made oval to fit on the existing studs (or use less-than-ideal adaptors)

[dougbgt6]

Chris,

Late USA TR6's have Stromberg CDSE 175s on the 2.5, so as Clive says, not big enough?  Maybe not the answer you want to hear.

Doug

[Pete Lewis]

using CDSE150 the only ref i can find is a australian spec used neddles   B5DG

pETE

[chrishawley]

Thanks for the replies which have got me asking the right sort of questions, although answers are as yet elusive.

I'd had it handed down to me that 1.5 inch carbs (be they ZS or SU) were never adequate fittings for 2.5L engines. But picking up on the Aussy spec I found reference that European 2.5 saloons had 1.5 inch ZS carb (CD3) also with B5DG needles. Meanwhile TRs always had 1.75 inch carbs. So I infer from that that BL found that a 150 series carb satisfactory on applications with a 'slow' cam and lower compression. My engine is 'slow' with the 10/50 cam.

Maybe I'll have to consider moving up to 1.75 carbs but will explore the lower cost options first. So, if it seems reasonable, I'll put some some B5DG needles in (or if unavailable B5CB) and see what happens.

QUESTION: What determines the height of the jet (in relation to the bridge) on a CDSE? The needle can only go in one place on the shoulder down the bore of the air valve. But the jet - does that have a fixed position or is it something I should be checking the measurement on?

I was hoping to make the Knebworth show in this car - but that hope appears to be receding (sad face thingy).

Thnx

C

[Nick Jones]

What piston springs are fitted, if any?

The mixture the engine sees is partly determined by the open jet area (in turn determined by the needle profile and piston position) and partly determined by the vacuum above the jet (determined my airflow and piston position).

One of the most basic design calibrations of the CD carb design, SU or Stromberg, is to set the piston weighting (combination of actual weight and sometimes additional spring pressure) so that the piston is fully raised only at the engines maximum airflow demand. If the weight is insufficient, the main effect is that the piston will rise too much, vacuum above the jet will be low, and the mixture lean across the range. The secondary effect is that should you ever manage to find a rich enough needle that it appears to more or less work, you will then find that once you get to a certain rev/throttle combination, the carb piston is prematurely fully raised, and it looses its ability to control, the vacuum increases and the mixture goes uncontrollably rich.

150CD carbs are just big enough for a 2L @6000rpm,  with not much margin for extra flow. They’ll support maybe 120bhp without resorting to “restricted inlet” type tuning tactics.

This doesn’t automatically mean they are too small for any 2.5. If you are not expecting to rev your 2.5 much beyond 4.5 - 5k rpm they’ll be fine, provided they are correctly calibrated.

Nick

[dougbgt6]

Chris,

There are many variations of CDSEs, but there are two forms of adjustment. With top adjusters the needle is turned up and down with a special tool through the top of the carb. And with bottom adjusters the jet is wound up and down from below the reservoir.  With bottom adjusters the neck of the fixed needle should be flush with the bottom of the piston. On the top adjusters the fixed jet should be just below the bridge, 1/32".

Doug

[Pete Lewis]

agree the jet must be a little below the throat of the choke bore this makes for better atomisation from the jet

have abrowse at Buckeye triumphs   ignore this is TR based the basics are well shown on here 

https://www.buckeyetriumphs.org/carburetors

Pete

[thescrapman]

Some 2.5 are fitted with HS4 as standard. The low HP / higher torque one.

I have driven one 3 times now, on front of an BW65 as well just to make things worse!

First drive it wouldn't pull the skin off a rice pudding, Topped out at about 50. I did 75 miles like that. 😞

Found rocker gaps were 200 thou, that is what happens when someone nicks the correct pushrods to fit to a customers engine and fits a set of Vitesse ones he has to hand and then just puts the rocker cover back on after tightening adjusters as tight as he can.

Once sorted it was not bad, and would rev quite nicely, but you could not hear yourself for the knocking from the engine,

Anyway, back to the point, it has a 2.5S engine now, on the same HS4, revs cleanly up to 5000, pulls like a train, and you can spin the wheels pulling out of a junction if the owner isn't with you, so 1.5" carbs can work fine on a 2.5.

[Clive]

8 minutes ago, thescrapman said:

Some 2.5 are fitted with HS4 as standard. The low HP / higher torque one.

I have driven one 3 times now, on front of an BW65 as well just to make things worse!

First drive it wouldn't pull the skin off a rice pudding, Topped out at about 50. I did 75 miles like that. 😞

Found rocker gaps were 200 thou, that is what happens when someone nicks the correct pushrods to fit to a customers engine and fits a set of Vitesse ones he has to hand and then just puts the rocker cover back on after tightening adjusters as tight as he can.

Once sorted it was not bad, and would rev quite nicely, but you could not hear yourself for the knocking from the engine,

Anyway, back to the point, it has a 2.5S engine now, on the same HS4, revs cleanly up to 5000, pulls like a train, and you can spin the wheels pulling out of a junction if the owner isn't with you, so 1.5" carbs can work fine on a 2.5.

No disagreement, I did say restrictive at higher RPM!

I think the actual carbs are the issue, as I think they have some emission control system? Are they the ones with the allen key jet adjustment? 

Whatever it is, it appears some other worn similar carbs work better. And the HS6 are the obvious "fix", I don't think there is a short dashpot HS4? 

Or an early pair of CD150's and a trip to the rolling road to get the needles reprofiled. I wonder is a pair of CD175 off imported TR4 etc may work? Didn't Nuala use some on her rally 2000?? Just throwing alternative ideas out there.

[Iain T]

22 hours ago, chrishawley said:

I'll put some some B5DG needles in (or if unavailable B5CB) and see what happens.

I don't know the taper on these needles but I assume due to being used on a 2.5 they would need to go down to 50 thou or less or you might not get enough fuel in. 

Iain 

[Clive]

On 05/08/2021 at 10:06, chrishawley said:

 

QUESTION: What determines the height of the jet (in relation to the bridge) on a CDSE? The needle can only go in one place on the shoulder down the bore of the air valve. But the jet - does that have a fixed position or is it something I should be checking the measurement on?

 

How have you been adjusting the mixture? You need the special adjusting tool (I gave mine away about 20 years ago to a local member...)

 

All needles (for a given jet size) are about the same around idle. So yu need to start by getting that correct via jet height. Then you can work on the profile.

[chrishawley]

I can't be the first one to have encountered so I guess I just need to stay with it and absorb all possible suggestions and possibilities.

• The carbs have springs - but the grade is indeterminate. No witness as to colour so could be natural or blue or red with no colour left.

• The adjusting tool is a pertinent point in that yes I have one: With the needle on its max upward adjustment the idle mix is still lean (ignoring what happens at load for the moment). Purely impressionistically I get the feel that 'if only' there were one extra turn of adjustment I might be in a workable range.

* Buckeye site v.good

* I'd be very happy for starters to a just get a decent idle mixture - would at least be a positive starting point.

Happened across a Lotus site with much discussion of jet height on ZS carbs. Owners report various heights below bridge of 0.060 inches to 0.1250 inches. Don't know that that relates directly to Triumphs but does suggest that even minor discrepancy from OE spec in refitting the jet could have a major effect on operation. Since (i know now) one turn of the adjuster moves the needle by about 1/30th inch then if the jet is (say) 1/16th) too high the needle adjustment would run out of range.

So as well as trying some different needles I guess I need to be measuring the jet height. I don't have a shallow depth micrometer so is there a way to get an accurate measurement by some other means? And am I right to assume that the jet is a moderate interference fit which should move without major force if I judiciously warm up the body of the carb (e.g 100 degrees C)?

No further hands on work on this today but hopefully tomorrow.

Thanx

 

[Iain T]

21 minutes ago, chrishawley said:

I don't have a shallow depth micrometer so is there a way to get an accurate measurement by some other means?

I have the bottom adjustable jet and use the depth gauge which is on most vernier calipers to measure the depth. Just poke the arm down and straddle the bridge. 

I don't have any experience with top adjusters but it seems you will have to lower the fixed jet so you can get a decent idle. 

As Clive said most needles for a 90 thou jet have virtually the same taper at the first two graduations. 

It's much easier with a bottom adjuster! 

Iain 

[Pete Lewis]

club HQ have a needle lathe in the museum 

no idea if it works 

Pete

[Iain T]

1 hour ago, Pete Lewis said:

club HQ have a needle lathe in the museum 

With only thou's to take off and knowing where on the taper it must take a lot of practice. Not something I would play with. Plenty of needles out there to get it sort of right or even spot on. I'm accumulating quite a selection! 

[chrishawley]

Problems not solved yet but possibly a working hypothesis.

On the reconditioned carbs the bridge to jet height was (minus) 42 and 54 thou. But I also have the deteriorated Triumph 2000 CDSEs on the vehicle at the moment. AFAIK these have never been 'worked on' and provide satisfactory performance. On these the jet height measures at 120 thou (of 1/8th inch I suppose).

So I've adjust the jets on the other carbs to 100 thou and will test them next week.

I can't find any ZS documentation from the time relating to jet height or the setting of it. It never occurred to me that fixed jets are, in a sense, adjustable. Were they always set the same? Or did the jet adjustment vary by application?

Until I crack this I remain open to all insights and suggestions.

[Paul H]

You might find this link helpful 

Paul 

[Pete Lewis]

the jet will move with a light press with   eg  a vice and some drifts to make up the spaces 

warming wont do much as they must hold in place when pretty hot on the engine 

but a gentle shove should shift the jet insert  ok

and as with Pauls experience there are reconditioners and............then there are others 

Pete