Bogging/Hesitating HS4 Help!

[dunketh]

Righto, last night I fitted a HS4 carb (from a 1500 spitfire) onto the mog.
I used a 30mm spacer pinched form an MG owning mate and a mini manifold to do this.
I drilled a tiny hole in the spacer to accept the vacuum advance pipe from the dizzy. This is sealed in the drilled hole with silicon.

Anyway, when driving in 'low' revs the carb has made a HUGE difference and I've found myself climbing huge hills without gearing down.

But.. in high revs the car dies. Not completely, it starts bogging down and hesitating. I can't drive through it either it just gets worse.

Example: I cannot go above 40mph in 3rd gear, that seams to be where it dies. If I shift into fourth I can slowly achieve 70mph but its painfully slow to get there.

Its doing my head in!
Somebody please say "Aha, its this... " or 'Aha, its that..."

I don't want to go back to the HS2 as I feel thats a step backwards. I've tried adjusting the fuelling on the new carb but when its 'good' according to the damper lift test its chucking petrol smoke out the exhaust.
When its not chucking fuel out the exhaust lifting the damper stalls the car instantly.

HELP!!

Could it be a needle problem? I'm still using the needle the carb came with?
The dashpot is filled with auto-trans fluid - is this the problem?

[Kevin]

I fitted a HS4 carb (from a 1500 spitfire) onto the mog.

Well I have just looked at the Burlen site and the 1500 spitfire carb is probably a
AUD665 with a needle of ABT & a jet AUD9451
For the 1300 Marina it could be
AUD354 with a needle of AAQ & a jet AUD9450 or
AUD670 with a needle of ADG & a jet JLZX1110
So as has been said before the carb is not set for the right engine you need to get one from a 1275cc engine, also a 30mm spacer seems quite large although I done know much about spacer differences, if you are going to stick with this carb you are going to have to get to a rolling road to get it set up correctly.

The dashpot is filled with auto-trans fluid - is this the problem?

Not heard of that being used before most often engine oil is fine for the summer and if a poor response is found in the colder weather 3 in 1 or similar is often used.
It going to be much simpler to get the right carb and probably cheaper in the 1st place.

[Innovator]

There is one simple answer that will solve your problem..........you need a rolling road tune by an operator who knows what he is doing.

The mixture will be incorrect and the easiest way to get it correct is a rolling road. If you do a search the subject of rolling roads has been covered many times.

My advice is always the same. If you do engine mods (carb, air filter, exhaust etc) you need a rolling road tune up. If a rolling road tune up is not done then you may as well leave things standard.

I have never come away from a rolling road tune up disappointed and would nearly consider paying for anybodies rolling road tune up who didnt get benefits after engine mods.

The local Mini people are always good for recommending rolling roads.

My local rolling road operator always told me to use ATF in my dash pots.

John

[Ratbag]

Righto, last night I fitted a HS4 carb (from a 1500 spitfire) onto the mog.
I used a 30mm spacer pinched form an MG owning mate and a mini manifold to do this.
I drilled a tiny hole in the spacer to accept the vacuum advance pipe from the dizzy. This is sealed in the drilled hole with silicon.

Anyway, when driving in 'low' revs the carb has made a HUGE difference and I've found myself climbing huge hills without gearing down.

But.. in high revs the car dies. Not completely, it starts bogging down and hesitating. I can't drive through it either it just gets worse.

Example: I cannot go above 40mph in 3rd gear, that seams to be where it dies. If I shift into fourth I can slowly achieve 70mph but its painfully slow to get there.

Its doing my head in!
Somebody please say "Aha, its this... " or 'Aha, its that..."

I don't want to go back to the HS2 as I feel thats a step backwards. I've tried adjusting the fuelling on the new carb but when its 'good' according to the damper lift test its chucking petrol smoke out the exhaust.
When its not chucking fuel out the exhaust lifting the damper stalls the car instantly.

HELP!!

Could it be a needle problem? I'm still using the needle the carb came with?
The dashpot is filled with auto-trans fluid - is this the problem?

You might try the bigger needle (less fuel on opening the venturi). Sounds like over-fuelling on acceleration (engine vacuum increasing). What spring is fitted? Could try a red spring, or two springs. Thicker oil in the dashpot helps fuel atomisation on acceleration. I've always found a Gunson's Colourtune spot-on for checking fueling.Cheaper than a rolling road, but not as accurate over all conditions - you may get lucky!

Phil.

[Packedup]

As everyone else says, try different needles and springs, and see what happens. Do you get any more go when it's bogging down if you use the choke? If so, then you need a richer needle at a certain part of teh taper, if it gets worse then you need weaker.

And the Triumph twins may well have a different spring to an A series single setup, which may or may not be confusing the issue a little more.

The spacer, IIRC, should improve torque at that length. But I could very well be wrong, never did quite understand all that!

Oh, also, what air filter are you using? could the engine simply be getting too much air conpared to what it expects?

[bmcecosse]

Your acceleration etc is fine - just high speed running that's not right. Pup has the answer - try easing the choke out when this problem occurs - if the car picks up then that's it! Otherwise - try the car with the air cleaner and casing completely removed - I find this restricts the flow causing exactly your symptoms. If the choke trick works then a needle with a thinner section towards the sharp end is required. You can modify one yourself with a leccy drill and swiss files - in fact some do it by simply filing a 'flat' down the needle - this is fine if you have a selection of needles to play with - but probably better to look up one of the many SU needle sites - and select something slimmer - Burlen will supply. Of course - it could be something else - try cranking the ignition timing up until you get pinking - then back it down again till it stops. Could also be fuel supply restriction - the pump simply not able to keep up at the higher demand rate.

[Stig]

I've always found a Gunson's Colourtune spot-on for checking fueling. Cheaper than a rolling road, but not as accurate over all conditions - you may get lucky!

Colourtune is great for checking the mixture at idling, but how are you going to check it under load at high revs without a rolling road?

If you want to avoid the hassle and expense of a rolling road tune-up then by all means experiment with different needles as suggested. Why not see if you can find the needle & spring used in a single HS4 on the same size/spec engine as yours -maybe a Mini? IIRC there was a short bit in Vizard's book giving his best guess needle/spring for various engine specs (with the caveat that it's only a starting point before a rolling road setup).

Good point from bmc about the air cleaner thogh, worth trying that before pulling the carb apart.

[bmcecosse]

Colourtune is a useless thing on a car with an SU carb. The little lift pin lets you check and set the idle mixture - which has only the tiniest relationship to the running mixture!

[dunketh]

Cheers guys.
I'm sure it is fuelling but I just can't get to the bottom of it.

The ABT needle in there at the moment shows an almost identical curve to the BEJ needle recommend by WinSU, thats the only reason I 'bungled' straight into it. Had it been wildly different I'd have waited.

Only other thing I've noticed is engine braking is greatly reduced - smoother ride but not sure if I like it or not yet...

The colortune plug I've got doesn't seem to work - which is odd. Looking into the cylinder you see no flame burn, just the occasional sparking.

[bmcecosse]

BEJ would be for an HIF 44 carb !

[Packedup]

Only other thing I've noticed is engine braking is greatly reduced - smoother ride but not sure if I like it or not yet...

Shouldn't be vastly different

Unless you've got an air leak somewhere, but I'd expect that to show up at tickover.

The colortune plug I've got doesn't seem to work - which is odd. Looking into the cylinder you see no flame burn, just the occasional sparking.

Is it definately firing on all four when you see the spark? Cause if you can see the spark, you can see the flame, unless I suppose the glass is really crudded up (the spark being very bright compared to the flame). If you're only seeing a spark then maybe you're simply not getting combustion fo some reason. Have you tried it on all four, or just the same cylinder?

I reckon if the car starts, ticks over and pulls OK to a point, then as the only thing you've changed is the carb it must be a fuelling issue - Best to check for air leaks, make sure the piston can rise and fall smoothly, make sure the pump can cope (which it really should) etc.

[Stig]

Unless you've got an air leak somewhere, but I'd expect that to show up at tickover.

Ah, unless there is an air leak and it's running really rich which coincidentally gets the idle mixture about right.

Nah, sounds a bit contrived.

[Alec]

Hello Dunketh,

a very weak mixture will be very difficult to see in the colourtune. Incidentally try it in more than just one cylinder. It does sound as if the general mixture is far too weak. Colourtune is not only for the idle either, blip the throttle and you should see an initial orange flame then blueing out if your needle is pretty close, but get the idle right first.

Alec.

[Packedup]

Ah, unless there is an air leak and it's running really rich which coincidentally gets the idle mixture about right.

Nah, sounds a bit contrived.

It's possible though - I think I've had it happen a couple of times, but it's fairly unlikely so long as pristine (or preferably new) gaskets were used and the manifold/ head nuts have the right washers and have been nipped up enough (I've fluffed that in the past, usually on seperate inlet ones).

[Innovator]

Would it be possible to use the colour tune with a video camera on it. Then you could check the mixture under load at higher revs?

John

[bmcecosse]

NO - it's only designed for idling - anything more and it will blow apart! You can check running mixture from the plug colours.
The revving up thing is useless too - what's that telling you ? The SU is designed to enrich the mix on acceleration - it's the constant load running that matters.

[Innovator]

I think there is a misunderstanding. When I said under load at higher revs I mean constant load at engine revs above tick over ie real world driving, not sat in the drive at tickover.

I have never seen a Colour Tune (always used a rolling road) but did not know they could not be used at anything more than tickover.

John

[doobry]

NO - it's only designed for idling - anything more and it will blow apart! You can check running mixture from the plug colours.

I bought one many years ago - the instructions stated that it could be used also at higher revs, not just tickover, however in reality it is useless as this is not going to be under load.

I found the colourtune useless altogether and never tried using it a second time. It may be ok on some cars but on mine it was very inconsistent.

[Alec]

Hello all,

I know that it is not on load, but it does give an indication that the mixture is going rich when the throttle is opened, which should happen on all carburettors, not just S.U.'s and that the mixture is correct at higher revs. If it's not at rest then it won't be under load. By the way, they do not blow apart, I've used them for years.

Doobry,
if you get inconsistent results then it's the car. Colourtune is no more than a window, but one of it's drawbacks is that any oil being drawn in will indicate rich.
There is another DIY method of checking mixture under load but this entails installing an oxygen sensor in the exhaust and monitoring it's output with a voltmeter as you drive (2 man operation really) or you buy a fuel\air ratio indicator.

Alec