x flow carburettor problem

[Jam Mad]

i have to start by pleading for your patience. i have posted some fairly basic questions to this site, and they have always been answered in a most helpful and friendly fashion. in the event that i over-step the mark, and you get to the stage where you just wanna say " crikey ! why doesn't he just get the book and learn, rather than waste my time typing, " then please just tell me. oh, but please give me the title of the book too.. wink.gif

 

anyway... driving out of london this evening, the engine started chugging, like it was missing a breath, before racing to about 4500 revs at idle. i pulled over. now this wasn't the chugging of a cylinder not firing, it sounded like a carb problem.

 

the new air filters that i fitted last week gave the first clue. at the front of the rear filter, it had buckled under heat. where the last one had almost burned through in fact. touching the rear metal air filter cover, it was quite hot... while the front one was cool. apart from a little smoke that was from the heat of the carb, very little else.

 

i started the engine and it was fine... plus it got us back ok too... but i noticed when i was hunched over the engine that the rear carb is making a funny gulping noise.

 

any ideas ???? is this the 'carb balancing' thing that i have read about and always thought.. " oh yeah, i'll learn that one day " or do you suspect it may be something more serious ?

 

while it got us the 20minute/ 4mile drive through london back to our pad ok, i didn't feel safe driving off to our friend's house for dinner ( sad.gif ) 'cos of the engine racing, and tiny whisp of smoke as it happened...

 

anyway.. the forum is open... please let me know if you have any ideas....

 

j wink.gif

[Michael Peterson]

This sounds like one for Roger King. I'm stumped. Must be the first time a Seven has done somthing odd wink.gif

 

Edited by - Michael Peterson on 24 Nov 2001 04:06:18

[Fast Westie]

Jam MAd

Sounds like the rear carb is too weak and backfiring

 

The car in front is a Westfie1d wink.gif

[Carburet]

Yep, this does sound like the rear carb is very lean causeing back-fireing, I'd have thought you may have noticed the characteristic coughing/sneezing noise. It could be caused by the O-ring seal between the carb and manifold not seating properly and that carb or barrel gulping too much air and therefore running lean. The chugging is also entirely consistent with out of balance carbs. I can't quite picture what's melted as I'm assuming you have K&N filters which don't have any plastic to melt. I'm sure someone like Roger K could sort this or if you're in North London Ratrace are open saturdays until one-ish they're on o208 830 5677

[Roger King]

There's a good chance this is more than one problem. If the idle was at 4500rpm!!!!!!!, it sounds as if something got stuck. The throttle cable is quite good at this, but doesn't usually make that many revs. It may even be that one of the internal return springs has broken and jammed the throttle open. If the latter is the case, it must be dealt with immediately before you can't stop the car at some critical moment.

 

It could also be that you have an air leak.

 

The other symptoms might be due to poor carb balance or idle screw settings.

 

If all is working properly the engine should idle nicely at around 900rpm and not have any significant backfiring, spitting, flat spots, etc.

 

It is impossible to diagnose this one for certain without seeing the car, but in the absence of anything obvious becoming apparent, I would recommend that you remount the carbs with new mounts, change or service the throttle cable and get the carb balance/mixture screws set up properly.

 

All the above assumes that the carbs are in fundamentally good condition and the jetting is correct.

 

P.S. Make sure that the external throttle return spring(s) aren't fouling on any lugs of the carb bodies.

 

Edited by - roger king on 24 Nov 2001 11:17:09

[Jam Mad]

thanks guys.... so i think the best thing is i teach myself how to balance carbs first of all.. ( any advice welcome on which book i can find that in ) and then get my carburettors looked at by a good mechanic that i know near here to check the o-rings and stuff...

 

the idle didn't seem, to my untrained senses, to be something sticking; as the chugging got worse for about 15-20 seconds, i decided to pull over... so i had the car at about 1500rpm trundling along. as i depressed the clutch again to change down, or 'cos i was coming to a stop ( i don't remember exactly which ) the engine ran up to around 4500.. but it wasn't an instant thing that i would think would occur if something had stuck... it was more like a gradual 4-5 second run up to the higher engine speed that led me to think it was more likely to be a fuel/ carb related problem rather than a mechanical one like a caught component. if that makes any sense at all !

 

as i said, after sitting there for the few minutes it took to call our friends, it started fine for the drive back.

 

fast westie, when you say the carb is 'weak' what do you mean ?

 

carburet, the bit that seems to have been damaged by the heat is part of the air filter... the wire mesh has warped, and on the last filter some of the padding in the filter had burnt away inside the mesh.

 

anyway, thanks again for all your help folks.. i'm off to find some scales to balance my carbs... wink.gif

 

j

[oilyhands]

There is a section on balancing webers and setting the idle mixture at

 

http://members.aol.com/DVAndrews/webers.htm.

 

Weak means that the idle mixture is weak, I.E. too much air and not enough fuel, provided there is no air leask this can be corrected using the volume adjustment screw on the top of the barrel.

 

All is explained in the above page.

 

Oily

[Jam Mad]

well, after a soggy afternoon with tony pashley, we know a few things, but still don't know others !!

 

the important thing was that on taking off the air filter, we noticed that the leading trumpet of the rear carb is covered with caked on soot. when you look into the trumpet and pump the accelerator you could see fuel dribbling down from the jet at the top of the tunnel. something that doesn't happen in the others.

 

i'm going to have my local garage owner take a look... but i think it's a carb thing... tony and i did check that balance was fine, the jets in each half of the carb were the same, and the spark plugs all looked ok... so we're stumped !!!

 

many thanks for your help guys... introduce yourselves when we're ever near a bar and i'll buy...

 

j