Engine "sneezing"

Post by **stuartB** » 17 Oct 2008 11:31 am

my engine is making random 'sneezing' noises:

The last thing i did was replace the brakepads and discs, and when i did this i just pushed the calipers back with a pair of grips, no bleeding.

the brakes seem ok, but thats the only thing i've done - now im getting this random noise. seems to be after i break hard and then accelerate again - it releases a burst of what sounds like air when i then put my foot down hard on the faster pedal.

dunno if its related to the brake servo or something... anyone else had similar?

the engine popping is still very loud - i read somewhere that there is a cutoff valve in the carb that stops fuelling on deccelleration, which stops popping - is it possible the two problems are related? Might be backfiring through the carb or something. is this possible?

at first i thought ignition so i cleaned up all the sparks, and changed my leads etc, but no dice.. still sneezing.

its driveable, just embarrasing..

Post by **jtbo** » 17 Oct 2008 06:52 pm

Going lean when hitting loud pedal?

Does acceleration pump do it's job and there is large amount of fuel squirted to carb when you apply full throttle? You can test this if you leave engine off and twist throttle shaft from carb.

Post by **stuartB** » 20 Oct 2008 12:51 pm

hmm.. dunno.

im convinced its to do with the servo - whenever i brake, the problem happens again. juddery until i boot it a bit suddenly, at which point it sneezes. from then on its fine till i brake again.

im a bit over the level on brake fluid at the moment - a throw back of getting lower brake pads and filling the brake fluid rather than changing pads... d'oh. when i pushed the calipers back in, the level rose too high.

Im going to try a scoop out some of this fluid tonight. i bet it stops the problem..

Post by **jtbo** » 21 Oct 2008 10:57 am

Somehow vacuum get's messed up, how is one way valve in vacuum servo holding up? Can't think much else right now

Post by **stuartB** » 21 Oct 2008 12:26 pm

removed some of the brake fluid by dipping a rag in there and letting it soak out, but its still doing the same thing.

im thinking it has to be the carb. gonna rebuild it this weekend, see what happens. Failing that I'll be looking for a twin carb solution.

Any other things to look for? is it possible that the dizzy is skipping a beat or something? seems to happen at about 2.5-2.8k revs or so, then clears. long distance is fine, then i slow down and re-accelerate. THATS when it happens

EDIT: ill take a look at my fuel pump at the same time, good call. might be the cause. ALSO, afterthought: i have an in line lead replacement fuel thingy, any chance it could just be this that needs replaceing or would that not make any difference?

Post by **jtbo** » 25 Oct 2008 09:52 am

If it is fueling problem, then it could be inline filter too, that sure is cheap to replace and one should replace that at least once a year.

Why it does sneezes is that fuel air mixture is igniting when intake valve is open, so 'explosion' goes up to carb, or air fuel mixture ignites at intake manifold, before intake valve opens however I would say that latter is very rare.

Because it happens only after braking, it comes to my mind that what if renix gets wrong information of vacuum? Then ignition could be way off and that could perhaps cause the problem by shooting spark way too early?

Post by **classicswede** » 25 Oct 2008 10:31 am

stuartB wrote:ALSO, afterthought: i have an in line lead replacement fuel thingy, any chance it could just be this that needs replaceing or would that not make any difference?

Rip the useless peice of junk off and lob it in the bin. They are good for nothing as far as lead replacment goes. Your car does not need leaded fuel anyway.

Have you checked the manifold and servo hose for air leaks?

Post by **stuartB** » 25 Oct 2008 01:00 pm

exactly what i plan to do this afternoon actually...

RE: lead replacement thing.. consider it gone. cheers, i was wondering whether to get rid of it or not.

basically im gonna replace the vacuum hose, and give the carb a good cleanout, while at the same time methodically checking it's all bolted together properly

mainly i want to be sure theres no mechanical fault before the webers go on. if its just a carb / vacuum problem then no worries but if theres some other reason, involving moving parts and valves or the cam or whatever, im not likely to stick an extra 40brake on

cheers for the thoughts, i'll update later this weekend

Post by **stuartB** » 27 Oct 2008 04:00 pm

I took the carb apart, cleaned it out (all black inside!), replaced the leads, removed the lead replacement thing AND reolaced my vacuum hose (old and scraggy)

thanks for the ideas. Not sure which fix actually repaired the problem but its ok now!

(i have a sneaky suspicion it was one of the leads, but earlier in the thread i said "definately not the leads" so im keeping a bit shtum about that lol)