Spit seems to run weak at higher revs

[cliff.b]

While generally, I feel my car is running well, with plenty of go at lower revs, it seems to be a bit flat around 4000RPM, picking up again approaching 5000. Not really a flat spot, just less urge, and exactly where I want it to be pulling well if overtaking in 3rd gear.

The plugs look ok but I decided to check the mixture out at higher revs by driving 10 miles or so at around 4k revs, coasting into a layby, and on removing the plugs, they were almost completely white. 

I presume this shouldn't be the case and if so, am I right in thinking this is likely to be worn or incorrect needles?

Or is there anything else that might be causing this?

[johny]

Not worn needles as they would tend to make it richer. Engine, intake and exhaust all standard I take it?

[cliff.b]

3 minutes ago, johny said:

Not worn needles as they would tend to make it richer. Engine, intake and exhaust all standard I take it?

Unfortunately I don't know what's actually inside the engine as the person I bought it off didn't know.

Carbs & inlet manifold look standard but haven't dismantled yet.

Exhaust manifold is standard but the downpipe & rest of exhaust are non standard. 2" dia pipe and I think the silencer has been "modified" (evidence of cut open and welded)

[cliff.b]

And by worn needles I was wondering if that would be making it too rich at lower revs and if I am over weakening it trying to compensate for that.

[Pete Lewis]

you  could advance a degree or two  

and richen  by up to half a turn  ( thats quite a drastic change ) 

see what that does by comparison 

small changes and test

can you master the piston lift pins on the carbs ???

Pete

 

[johny]

hmmm a freer flowing exhaust might give you a weaker mixture at higher revs.

How does the mixture setting compare with the manual - have you had to set it very different to the normal position? The only way to set it up perfectly is on a rolling road...

[Pete Lewis]

do you have std needles , no waxstats ?????

here you can compare the needle tapers www.mintylamb.co.uk/suneedle/

pete

 

[johny]

Another thing Ive just thought of is carb spindle wear - is there any play in them that is allowing air to be drawn in?

[cliff.b]

Several questions so I will try and cover all of them.

When I bought the car last year it wasn't running and once I sorted the ignition side, it was running very rich. Black plugs & exhaust stank of petrol. I used a colourtune to get it somewhere close then kept checking the plugs until I was happy with the colour.

Waxstats were fitted but I removed them, applying the 1p fix, after which it was running weak, so I went through the above process again.

Ever since, it has all looked good when checking at home. Plugs look good, slight rise in revs then back to normal when lifting the pins or my personal test, removing the oil filler cap to get the same effect. There is zero smell or anything visible from the exhaust. If I hadn't carried out today's testing I would be none the wiser.

I haven't stripped the carbs yet so don't know what needles are fitted at the moment

[cliff.b]

23 minutes ago, johny said:

Another thing Ive just thought of is carb spindle wear - is there any play in them that is allowing air to be drawn in?

Can you determine that before you strip it down? Wiggling the spindle while on the car there is a slight movement side to side but I don't know what is normal 

[johny]

Dont know much about SUs but my Strombergs became difficult to set up and ran lean at higher revs. It took me too long to twig (so much so that Ive probably damaged my valve seats) that the spindles were worn but replacing them did the trick. There was quite a bit of movement (one carb more than the other) detectable on the car when a spindle end was rocked back n forth or up and down while new it would have been almost imperceptible...

[cliff.b]

15 minutes ago, johny said:

Dont know much about SUs but my Strombergs became difficult to set up and ran lean at higher revs. It took me too long to twig (so much so that Ive probably damaged my valve seats) that the spindles were worn but replacing them did the trick. There was quite a bit of movement (one carb more than the other) detectable on the car when a spindle end was rocked back n forth or up and down while new it would have been almost imperceptible...

It sounds a plausible explanation, but difficult to diagnose if you don't know what normal is. 

I think, as suggested, I need to make some adjustments and test at different engine speeds to try and understand what is happening. 

I can see me needing to strip them down though, which I have never done before, but I'm sure I will manage. Would just rather be driving though 😒

[johny]

Must admit there was no doubt with the play in my spindles especially the one where I had installed an extra spring years ago to give the accelerator pedal more resistance😖

[cliff.b]

35 minutes ago, johny said:

Must admit there was no doubt with the play in my spindles especially the one where I had installed an extra spring years ago to give the accelerator pedal more resistance😖

Ah yes, the law of unintended consequences. Caught out by that sort of thing many a time, but experience has taught me to try and think things through now. Not that it always works out lol

[Iain T]

Cliff, as you've identified a problem I would first take a look at what needles are actually installed. If you've a modified engine it may be you need non standard needles. 

Iain 

[cliff.b]

19 minutes ago, Iain T said:

Cliff, as you've identified a problem I would first take a look at what needles are actually installed. If you've a modified engine it may be you need non standard needles. 

Iain 

Yes, it makes it difficult not knowing if the engine is standard but I can at least see what needles I actually have. But I still won't know if they are suitable for the engine 🤔

[NonMember]

1 hour ago, Iain T said:

If you've a modified engine it may be you need non standard needles. 

Even if your engine is standard but the exhaust isn't, then you might need different needles.

[dannyb]

The best bet is to have it tuned on a rolling road. I did with my Spit and it was like a different car. The guy knew his stuff and reprofiled the needles and took the dizzy apart and gave it more advance. 

But you do need someone that can deal with SU's. 

Danny

[Pete Lewis]

before you go  opening the wallet  get the basics sorted first   

basic needles for a std 1500 are ABT or late ADN

a quick check of the spindle for wear is worthwhile 

if you are not after utopia  stick with the compromise 

you dont need to spend £££s to go tootling around or even giviing it the beans 

even worn parts can give acceptable settings with a bit of thought  

it she basically runs fair  dont jump into spending £££££s  with little return 

Pete

 

[cliff.b]

16 minutes ago, Pete Lewis said:

before you go  opening the wallet  get the basics sorted first   

basic needles for a std 1500 are ABT or late ADN

a quick check of the spindle for wear is worthwhile 

if you are not after utopia  stick with the compromise 

you dont need to spend £££s to go tootling around or even giviing it the beans 

even worn parts can give acceptable settings with a bit of thought  

it she basically runs fair  dont jump into spending £££££s  with little return 

Pete

 

Do you know why they changed to ADN needles for later cars?

[Pete Lewis]

nope 

heres some of the torque curves for 1500 the best is between 3k 4k then it drops a bit 

many would not rev a 1500 as they are renowned for rociprocating failures 

Horsepower/Torque Curve for 1975 Triumph Spitfire 1500 (man. 4) (model for Europe ). Detailed engine characteristics. (automobile-catalog.com)

[cliff.b]

31 minutes ago, Pete Lewis said:

nope 

heres some of the torque curves for 1500 the best is between 3k 4k then it drops a bit 

many would not rev a 1500 as they are renowned for rociprocating failures 

Horsepower/Torque Curve for 1975 Triumph Spitfire 1500 (man. 4) (model for Europe ). Detailed engine characteristics. (automobile-catalog.com)

Interesting. The torque drops off quite suddenly after about 4000RPM so maybe the original issue I was investigating is normal. Even when accelerating hard I don't rev more than 5k and that's not very often. There's not really any point with this engine as it's all torque which in my later years I quite like. I only rev it that far so it will be at about 3k after I change up and pulling well.

 

[Anglefire]

I was trying to work out what was different on the two graphs then I realised they were metric and colonial.

[Pete Lewis]

its always best to stick with the basics   its easy to get carried away and the problem is not a problem

happens alot 

drive her and get to know her foibles and think Ive just saved  a good few £££ss   Ha !

Pete

[cliff.b]

Ok, an unexpected lack of rain today has allowed some further testing.

So off for a drive, slower this time with more varied speed (revs in the 2 to 3k range). After a few miles I coasted to a stop and the plugs still looked very lean, so it would appear this is happening to some extent at lower revs.

So tried making half a turn richer, as Pete suggested, but then the exhaust smelt of petrol, it wouldn't tickover nicely and once on the move, it felt sluggish. Going back one flat improved things so back on the road again.

Stopped again after a few miles and plugs were as shown here.

Not as white as yesterday.

Didn't change anything and drove straight home, the last couple of miles in town traffic. Looked at plugs again and the same plug was like this 

 

I have also remembered that a few months ago I took the plugs out and they were black, whereas the previous year they had looked perfect. So I reset the mixture again but I'm wondering now, is the problem that it's too rich at low revs and I am over weakening to compensate 🤔

Over the Winter I replaced the dash which meant removing the choke cable. If that is not adjusted correctly, could that make it rich at low revs/throttle openings but not affect the mixture much at other times?