Fuel vaporisation

[Oldmogman]

Has anyone had problems with fuel vaporisation? At least I think that's the problem; my 1275-powered Mog gets grumpy in traffic with uneven revs, and sometimes cuts out.

However, it will re-start, usually quite easily, and is absolutely fine on the open road (apart from occasional hesitation/cough when pulling out of a low-speed bend at low revs).

The engine has an HSF4 carb on a Metro water-heated manifold, a K&N filter and a Janspeed tubular exhaust manifold. Jet needle is the stock BAZ (I think), plugs are NGK; points, rotor arm, condenser and coil are all new.

It used to run fine even in the worst traffic with good old leaded four star.

I've extended the stock steel heatshield with an aluminium one that reaches right under the carb, lagged the fuel pipe with asbestos cloth, and tried feeding it Castrol lead replacement/octane booster - even tho it has an unleaded head.

Apparently, the Grumpy heatshield kit will not fit this manifold/carb set-up.

Any suggestions?

[bmcecosse]

It's a problem that seems to hit some cars - when it does this - is the fuel pump ticking away 19 to the dozen ?
One cure is to move the fuel pump to a low level in the engine bay - then it has 'flooded suction' and will work much better with no vapours - other option is to fit a mechanical pumpfrom a Mini /Marina etc , the hole etc is there in the block and that will eliminate the problem. For best performance you should NOT connect water heating to that manifold - but for best economy - leave it on there. However - it's worth a trial without the heating in case that solves your 'in traffic' problem.
The splutter out of corners is fuel surge - the level in the float chamber may be a bit low - but really the answer is to fit the much better HIF38 carb which knows no surge!

[mike.perry]

Unlikely to be fuel vapourisation this weather! I assume that you have an SU HIF 44 carb off an Ital as that uses a .100" spring loaded needle.
The BAZ needle is considerably richer than the BER which is the standard needle for the Ital 1300 engine. I can't find an application in the book for a BAZ needle.
Uneven running and cutting out in slow traffic is more likely to be fine adjustment to the mixture or the tickover screws. I get the same problem on my 1275, complicated by trying to keep the twin carbs in balance. Have you tried pulling the choke out slightly to speed up the tickover?

[eastona]

does sound like rich mixture.

the tubular manifolds do introduce more heat into the engine bay, right under the carb, where you don't want it. wouldn't fuel vaporisation tend to be tricky to restart?

try leaning it off or put it on a mixture tester. As mentioned above, it may be a wrong needle.

Andrew

[MarkyB]

You can get heat resistant woven ribbon and stainless steel clips to wrap the exhaust in.
Ideal for a tubular steel exhaust (not recommended for cast iron manifold)
It really helps reduce the engine bay temperature and quietens things down a little too.

[bmcecosse]

Ah yes - didn't notice the BAZ needle which indeed indicates an HIF44 carb - and not the HSF(?)4 mentioned. Could well just be rich at idle - what colour are the plugs - both after a brisk run, and after a run through traffic ?

[Oldmogman]

Thanks guys.

Thanks for your feedback and suggestions.

Firstly, my apologies, it is indeed an HIF 44 carb!

And in answer to your questions:

The (I think) BAZ needle was fitted by the company that rebuilt the carb, I would guess to suit the new engine it was being fitted to.

Because the engine used to run fine with this needle in the days of four star, I'm assuming it's the right one.

I've adjusted the mixture on a basis of trial and error, and the plugs are now a light brown/grey colour, so I reckon it's about right (although I've not checked them after separate brisk/traffic runs).

However, very occasionally it will run on after switching off the ignition, which I think suggests a rich mixture?

I do have a BDL needle, which I'm told is a 'sports' one but haven't tried it because of the extra fuel it's sure to use.

I've upped the idle a tad but haven't yet been able to recreate the conditions of a traffic jam on a hot day to see if this has helped. (And ditto for turning off the heated manifold.)

No, the fuel pump is not going crazy.

As for the restarting, it will usually restart easily if it cuts out in traffic. However, if it's been left switched off for a few minutes after getting hot in traffic, it takes a little spinning on the starter to persuade it to go again.

Hope this helps the diagnosis!


Many thanks

[mike.perry]

The BDL needle is the standard fitted to the 1275 Ital and is far weeker than the BAZ which you have currently so you are likely the get an improvement in fuel consumption. Nearly every 1275 seems to use a different needle so there are plenty to choose from if a BDL doesn't suit.
See the thread on Needle for HIF 44 on "Other" section.