Crossflow

[balsadoodle]

Hi,

I bought my first WF a few weeks ago, crossflow pre-lit. (Have just printed off the membership form; will be in post tomorrow with a cheque!

Can someone offer me some advice on a missfire? The car runs really well until I reach about 4,500 to 5,000 rpm at which point it suddenly starts to tail off in power and sound like a sort of chuffing / stuttering sound from the exhaust, occurs at different points between these rpm figures, but starts quite suddenly; spoiling all my fun!

The engine runs runs strongly and cleanly up to 4,500. Engine spec. is as follows:

Professionally rebuilt a couple of years ago +30 overbore, lightweight pistons, lightened flywheel, BV head, Kent FR cam, lumenition optronic and matching coil, Dellorto 40 DHLA's, big 421 manifold etc. Car was set up on rolling road earlier in year with previous owner so presumably this is a recent problem (hopefully not something serious, and why he sold the car???).

So far have changed plugs (BP6FS), tried adjusting fuel pressure from original 3.5 p.s.i., and checked for blocked jets, (jets fitted are as follows, excuse my lack of technical terminology!:

long jets: no. 7772.8, 200 top bit, 140 bottom bit.

short jets: no. 7850.7, no.on bottom bit obscured but looks like 07?

Done all usual ignition checks, cap, rotor, leads look OK; what about float height?, is it worth checking, and any ideas how to measure and adjust on DHLA's?

Any other suggestions please, if no joy I suppose rolling road is the answer; and recomendations near north Cheshire?

Many Thanks,

Rob

[stu999]

Do you have an electric fuel pump fitted? A tired manual one could possibly cause similar symptoms.

If you have an electric pump, try switching the ignition on with the fuel pipe to the cars routed into a suitable container, and observe for poor/weakening fuel flow.

What igniton system do you have - points or electronic?

[balsadoodle]

Running an old looking facet solid state 'leccy pump, with adjustable pressure regulator.

As regards ignition; electronic, optically triggered Lumenition system.

Rob

[Blatman]

tried adjusting fuel pressure from original 3.5 p.s.i

To what? More or less pressure?

[balsadoodle]

Tried 3, 3.5, 4 & 4.5 p.s.i. and no noticable difference.

[Blatman]

That's 'cos you're going the wrong way...

Turn it down to 2 to 2.5psi maximum...

For reference, my atmo Cossie (235bhp) would cough, spit and be generally unpleasant at high revs unless I kept it right on the boil. It would stall easily on a coast down if I didn't dip the clutch and give the throttle several good prods. Turns out I had 5psi of fuel pressure. Turning it down to 2.5psi cured it...

[balsadoodle]

Thanks; will try lower pressures, but if over pressure is the problem seems strange that it runs perfectly under all conditions below 4,500 rpm, yet at high revs, when there will be maximum demand for fuel flow, it starts playing up, as if it's beings starved of juice?

[Blatman]

Or it's flooding...

[Man On The Clapham Omnibus]

It doesn't have a rev limiter, does it?

[worzel]

Have you checked the ignition timing.  Most importantly, on a x/flow you should be acheiving about 36 deg BTDC (max) at higher RPM.  

Otherwise, as mentioned, a trip to a rolling road may be the only answer.

Good luck

[pookie]

Hi Rob,

If you get stuck try Autolink Services at Cheadle Hulme they are familiar with Westies and have done work for a few of the Manchester lads.(0161 485 5165 and ask for Simon). If you feel you need a rolling road then Mick Gilman at Congleton (01260 297115) is very good and quite reasonable.

HTH

Pookie

[balsadoodle]

Thanks everyone for the advice; will check out the things suggested tomorrow if its dry, and give Autolink in Cheadle a call if no joy.

BTW have noticed however that the DHLA's are rigidly mounted to the inlet manifold; there are some nylon washers with "o" rings fitted but these are clamped tight, but I think they're meant to be mounted so as to give a bit of flex??; what difference will this make, could be contributing to the hi rev missfire??

Thanks again,

Rob

[stu999]

BTW have noticed however that the DHLA's are rigidly mounted to the inlet manifold; there are some nylon washers with "o" rings fitted but these are clamped tight, but I think they're meant to be mounted so as to give a bit of flex??; what difference will this make, could be contributing to the hi rev missfire??

There should be some flex between the carbs and manifold. Some things to note:

The nylon washers/ 'O' rings are ****e... When tightened to allow the right amount of flexibility, the nylon washers/ 'O' rings have a nasty habit of dislodging, and causing bad air leaks. Ideally, they need to be replaced with aluminium plates with 'bonded on' O rings (known as Misab plates), which are far superior.

What is between carbs and fixing nuts? There should be some form of flexible system here too. The 'normal' thing to be found with nylon washers/ 'O' rings is what are known as Thackery washers, which basically look like spring washers, but with a couple of coils, if you see what I mean. Again, these are pants, the coils can snap with age, and again cause air leaks. Far superior are rubber bobbins and cup washers. The fixing nuts need to be nylocks (or other self locking nuts).

However, it wouldnt suprise me for you to say that there is no form of flexible mounting between carbs and securing nuts...  

[balsadoodle]

Mixture of broken thackery washers and plain washers between carbs and mounting nuts; all done up tight; no obvious air leaks but no flex either. Will get some misabs and rubber bobbins / cup washers as suggested. Could the current set up be contributing to my high rev missfire do you think?

[stu999]

Maybe, but if it has been 'OK' before, I doubt it...

If you can't be sure it has ran alright, then it is worth a try. the reason the carbs should have some flex, is to prevent the fuel 'frothing', due to engine vibration. It is just possible that this is what is occuring ATM - whcih will cause the mixture to go weak, and cause a misfire...