cut-outs after reving above 4500rpm

[GinettaG15]

i´m an owner of a 2500s Series 2 with HS6 carbs and PI camshaft.engine has around 2000miles since a complete re-built.
engine is running with the correct airfilter and the SU Needles are rather on the rich side.

car drives nice, starts perfectly, idles smoothly, engine is quiet, does not smoke, neither consumes any oil.

the ignition has an accuspark module fitted and a correct lucas sports-coil.

so what is my problem?

i can drive the car all day long, no matter if outside-tempeatures are 10 or 35 degrees, it runs well and troublefree....but once i rev it over 4500 it starts to cut-out (from the sympthoms it sounds /looks more fuel related)...it recovers after a few seconds and runs normals again when keeping the rev´s below 4500.

the car had a mechanical fuel pump fitted and now an electric. with both pumps the problem is / was present
by the way: the electric pump fitted was succesfully tested in another triumph with 2500 engine.
fuel-filter is clear and always full of fuel. tank outlet is clean. tank vent is not blocked, even made a test-drive with removed fuel-cap.
accuspark module was changed to another module , just to exclude it comes from tis side
HT leads are new (as the old ones looked a bit tired) but problem remained
as pre-causion i also changed the spark-plugs, without any effect to my problem.
fuel-level in floats is correct, Carbs are also refurbished.

and i also tested different igntion settings: from 8BTDC, to 11, to 14, to 16 and even to 18 (it stated piniging just with 18...but was running much better...lol)---> result? cutouts with rev´s higher than 4500

[Charles H]

Have had this before. What plugs are you using? I used NGK plugs for years, then changed to Denso plugs. Problem solved and never come back!

[GinettaG15]

to be honest...i dont know which plugs are actually fitted.....but i know that i tested different brands of plugs and the problem is always the same....even i doubt that its ignition related....."feels" more fuel/air related.

[wild bill]

Had THIS before! Next time you run the car and it starts acting up, go round to the back of the car with the engine running.Put your hand over the exhaust tail pipe and it should get "blown off" If so, all is well with the exhaust, if not, you have a collapsed baffle or blockage in the system.Once you clear that, splitting off the boxes one by one the car should be fine. Had this on holiday with a Saab 99.Found the fault after stripping carbs to component form( twice) checking all ignition and valve timing, etc,etc,on a campsite in York! Good Luck!

[GinettaG15]

so...its definately clear now: it IS fuel-related:

i recently drove to my GF (50miles distance, mainly motorway) and by mistake i forgot to push-in the choke completely...and the car drove brilliant, even on high rev´s and high speed.

so i guess i need to search for new carb-needles.

BAM´s are fitted....as the car can be started nearly without choke (at least in spring/smmer) the mixture on the low-rev side looks to be quite rich and on top end it looks to get too lean...so will try BDN needles now....if you have other ideas /recommendations, please post them.

[johnnydog]

I have a 1976 2500S with HS6 carbs and the engine is totally standard in every respect.
Initially it had Waxstats with poppet valves on the butterflies. Whilst it ran very well, I thought it ran out of puff at higher revs, although there was none of the symptons you describe. I changed it years ago to standard butterflies and red tipped jets.
I also decided to change the needles from the original BDB. I used the Minty SU website, which allows the comparison of different needles relative to each other. At the time, I basically inputted various needles from all different Triumph models until I got the best (in my opinion) mixture curve against BDB needles, trying to get the mixture the same at the bottom end to retain the smooth tick over, but richer at higher revs.
I settled with BDM needles. I fitted them in 2006 and they have been in ever since with no issues, with no detriment to fuel consumption, but a marked difference in 'uumph' and 'pull' / 'eagerness'.
CW recommends BPZ needles for a 2.5PI engine fitted with HS6 carbs which I didn't consider at the time for some reason.
But here is a screenshot of the mixture range of the three needles for comparison. As can be seen, the BDM needles are very similar at low revs, but get then get slightly richer up the rev range. They actually produce a consistent increase of mixture in the upper rev range as do BPZ, are slightly richer but give a more even increase up the rev range.
Dave B has a standard 2500 engine on HS6 carbs, but fitted with BPZ needles from the days when the car had a PI engine fitted. It is now on a later 2500 engine, still with BPZ and it also pulls very well.
I think you need to experiment to find the best for your engine, but these are some options based on my experiences with my 2500S and Dave B's....

[GinettaG15]

thanks for your reply...i´m already a user of the minty SU homepage.

yesterday i have received my BDN Needles...i havefitted them without touching the jet-settings:
and they are worse in every aspect:

Idle is a bit lower, until 2500rpm i can only drive with a wide open throttle , otherwise the engine dies and the cutouts on high revs have moved 300rpm higher....pulling the choke and the sympthoms (on low as on hig revs) are gone.

so i´m now (temporay) back to my BAM´s as with those the engine runs well up to 4500rpm

by the way: my carbs dont have the butterfly valves, nor waxstats.

BDP compared to BAM´s are much leaner....so they cannot work at all in my specific case
BDP compared to BDM´s are slightly richer
BPZ´s cannot be found in the minty table, only BP...and they are constantly richer from low to high compared to the BAM´s , surplus a sudden increase of richness on quite high revs.


i would need to find a neele which is identical to BAM up to 60% of the revs and than it should get richer.

i already searched for hours at minty but cannot find anything.

BCG looks not bad but gets richer quite early.

[Alec]

Ginetta,

why didn't you adjust the jets?

I would imagine that as your last needles were not entirely suitable that you set the jets as a compromise. (All needles have a near identical dimension at idle.)
Burlen sell a needle chart listing dimensions of all their needles. Also remember the spring affects the running as well, and there are alternatives available.
Finally with changes in fuel settings you also need to alter the ignition timing and may require a different distributer set up?
Hence why such tuning is normally done on a dynamometer.
I take it although it has a P.I. camshaft that the comprssion ratio is standard 'S' specification. A bit of a mixture?

Alec

[GinettaG15]

Ginetta,

why didn't you adjust the jets?

I would imagine that as your last needles were not entirely suitable that you set the jets as a compromise. (All needles have a near identical dimension at idle.)
Burlen sell a needle chart listing dimensions of all their needles. Also remember the spring affects the running as well, and there are alternatives available.
Finally with changes in fuel settings you also need to alter the ignition timing and may require a different distributer set up?
Hence why such tuning is normally done on a dynamometer.
I take it although it has a P.I. camshaft that the comprssion ratio is standard 'S' specification. A bit of a mixture?

Alec

i think this does not make sense to adjust the jets...why?

if idle is "just" accetable, mid range a total disaster with flat-sots etc and top-end just a slight improvement...this cannot be corrected by the jet.

as said, my BAM´s are absolutely perfect up to ~4500rpm....so best would be needles with a similar profile than the BAM´s , but getting richer at top end...until now i cannot find any which fits into this request.

[Alec]

Hello Ginetta,

the reason I said that is relative to my point about all needles having the same idle dimensions (within about 1/2 a thou) which indicated to me that they had previously been set to compensate for the needles fitted and were far from the basic setting?

Alec