Hitchcocks Forum

Can anybody suggest what I need to do to stop carb spitting , I fitted the carb in the Australian summer with 260 main 25 pilot and number 3 slide , over the summer I got the running and mixture bang on with a dark tan on the spark plug tip .
Now the summer is over and it's much cooler my engine is spitting back even when hot at the traffic lights should I try a different slide ? shallow or deep ?

Phil,

If it is spitting back just off idle a slide with a smaller cutaway should help.

Paul.

This is the reverse of what I'd expect, now the weather is cooler it appears to be running weak when I'd expect rich to be the case. I suggest you look elsewhere, check for air leaks and also check the timing hasn't slipped, running too advanced.
REgards, Mark

Mark, A cooler motor needs a richer mix - like at start up. However the issue may lay elsewhere but given the info posted and the (big) assumption all else is well that's my stab.

Phil, you could experiment by making the pilot mix over rich and see if it still spits if so I'd be double checking for air leaks between card and motor.

Yes Phil, you're right of course (struggling with back pain this morning,) but what worries me is that the 3 slide is already fairly rich (I think a 2.5 may be available,) which is why I suggested looking elsewhere. You know what they say about carb problems being 90% ignition! REgards, Mark

Hi guys------- PHIL, hang on there mate !! How depressing to read your Summer is now over--- ours hasn't even started yet ( No, I tell a lie--- the sun did shine for more that one consecutive day just recently, LoL.....so you're correct then ---- Boo hoo )... Just thinking your carb. problem through}---- any change in the bikes' environmental temp. or altitude affects the WHOLE of the carbs' operation ie it's not selective... Is the rubber tip on the choke slide sound ??-- Just where/when does this spitting occur ?? ie at start-up, mid-range, approaching full throttle or when you "roll-off" the throttle just back from the fully-open position ??------------- Finally, did you do ANY work involving the ignition system at all ??

Fuel/air mixture goes by weight.
Cold air weighs more than hot air.
This could give you a weak mixture now that the temp has dropped.
A weak mixture can cause spitting back in the carb.
But as AlanR says - if you can tell us more it will be easier to assess what's wrong.

Hi guys ( Notice I resisted the temptation to say}---Hey Presto!!)------------Yes, agreed the calculations are done on a weight basis but a carb. can only work by passing VOLUMES of fluid and air, the designer having made allowances for converting these vols. to mass, and thus achieving the required mixture................... Cool fuel weighs more than hot fuel also, but I agree that the overall effect of reduced ambient temp. will be a weaker mixture......For those trying to follow the tech. points have a look at this }------------- http://racingarticles.com/article_racing-33.html ---------------------- All valid reasons to have a FULLY COMPENSATING fuel injection system fitted I think ??

Phil, Please forgive the implication you may not have done this correctly but how did you adjust your pilot mix? Now I'm not saying this has zero effect but how many of us change our carb settings between summer and winter because cold air is denser? Also I'd be checking ignition first.

Hey everybody ..thanks for the tips , as the spit back was on idle and just above . Australia have just had hottest modern summer on record and since my first post it's got all hot again , when it gets down to 19 degrees average with the cold rain I hope to have the winter slide number 2 and it's an area that brings the MK 2 into the flexibility needed for an engine heavly ported out , high comp , sport cams so getting a tick over was a job well done but having to prime the engine with a kick over 1/4 throttle with a rubber glove over the air filter..this just wets the plug , big ports = smaller cutaway on the slide , my number 3 was just standard .
Reminds me of the days changing from summer to winter thermostat on minis .
It's an engine part for just 4-5 months of a year but anoyance of a big spit that cuts the engine out sitting at the city traffic lights with no electric start ..it can be pretty dodgy pushing that bike to get to the curb ..so many monster double B trucks and trailers in the city now .
This has happened 3 times now , again thanks for advice on the cutaway number scale in relation to air density . As to the non function of the choke ..the rubber glove !in normal running everything is bang on for mixture and ignition timing , it has digi boyer kit fitted and set at full retard at tick over , I found it to be tricky in micro ajustments with gear slop taken into account but it never pinks.
Plug colour was dark tan center to black at rim .
Could poor fuel be to blame ? I keep it on 98 proof unleaded with valve safe juice ,when the slide arrives it will only take 10 mins and my troubles will be gone I hope .