air leak

[zymon]

Hi all,

Well ever since i had rebult my bike a few years ago with the race built engine I have always had an idle running issue, i thought it may of been down to where it was tuned it wasnt ever going to run that smooth at tickover. Well i have dug out my beast again to get it ready and prepped for a trip to the ring soon and thought i will give the bike a carb balance and surely enough the poor running issue reared its head as i tweaked the screws it just ran all over the show.

It dips, runs smooth for for a few seconds then goes all over the show almost to point of stalling. Now it just dawned on me the possible symptom of an air leak - something which i would rule out instantly as the carb rubbers were brand new when i rebuilt the bike and they are the rvf ones which are supposed to be better.

As i was tweaking the carbs i sprayed some ultra 2040 ( wurth equiv to wd40 but doesnt attack rubber ) around the carb rubbers and after a second the revs started to pick up, which means its sucking in air around the carb rubbers

So my question to you peeps out there - do you get sealing issues when you have skimmed down heads ? Its a vfr lump with skimmed heads as just explained , rvf carbs with rvf carb manifold rubbers . I dont think i need to explain any more of a spec as rest is irrelevant?? Help please !!

[Drunkn Munky]

I dont have any issues with my stage 3 RLR tuned motor although i do run the tickover a little high on that one. are the rubbers old and perished?

Have you tried balancing the carbs with a drill bit? much quicker and although it may not be perfect its good enough.

[uber pikey]

Look, you must realise that you will just have to lift the carbs off inspect the rubbers and if they are ok re-seat the carbs and try the spray technique again, until you recover the leak you are pissing into the wind.
Try putting a little vasaline onto the rubbers before seating the carbs and make sure they are sat well onto the heads.
Good luck.

[zymon]

carb rubbers were new when i fitted the engine few yrs back ,

carbs have been on and off a few times already as it runs an air tray so balancing aint possible until its removed. I always use a bit a bit of rubber lube to fit the carbs on anyway - when i look at the carbs i can see they are right own onto the top of the rubber necks.

The only thing i can see is that on the back two rubbers the carbs seem a fraction offline but i cant see how this is possible ?

i have tried re seating them multiple times, they come off nice and easy and just press in nice on refit, was considering using some rtv sealant around the rubbers just to see if it seals it off just as a test, i know its a bodge but its not a permanent fix - just want to see where its getting in plus rtv sicilone isnt strong and peels off easy

Thanks for input so far - had a feeling there wasnt going to be my magic fix answer in my reply thread lol

[Drunkn Munky]

Shoot me down for stating the obvious here but you do have clamps on your rubbers dont you? its just ive never known a decent properly seated set of rubbers leak, difficult to fit when old and knackered yes but leak no.

[zymon]

yes , took them off to make sure they are as true as possible so they clamp up well - its really got me bamboozled ! going to look closer at the carbs tomorrow they may be the culprits for the poor seating etc

[Neosophist]

Hmm.

Lots of factors at play.. does spraying the WD40 equvalent always produce this rev change effect or was it just co-incidence.

Firstly, with the engine running rev the throttle upto about 4 - 5000 rpm and let it drop back to idle.

Do this a few times and note the symptoms.

Ideally it should drop cleanly back to idle and stay there.

Classic air-leak sign is that when you rev the throttle and loose it off the revs hang around 2000rpm before going back to idle.. this happens becuase the increase in pressure when the slides are closed causes the carbs to suck air through the air-leak, which keeps the idle higher than usual (as its like having the throttle open!) except you'll have little to no fuel goign through so it will be incredibly lean during this part.

An over-rich idle will cause the revs to drop back normally but drop below the set idle momentarily and then pick back up.

If it isnt an air-leak i'd look at float-heights, carb balancing and pilot screws, all of these will effect your idling. If your running an air-tray (no box?) then perhaps your pilots are too small..

Anyway.. lets start with the obvious and go from there.

[CMSMJ1]

Can I dive in on this? I reckon mine, with new rubbers, also has a bit of an issue. it hangs at 3k or so and then drops and can stall...

rubbers are tight, carbs have not changed in setup when they never used to do it....

[Neosophist]

hanging revs is either an air leak or too lean... mark have you tried turning the pilot screws out 1/2 a turn?

[CMSMJ1]

I haven't mate - this is RVF carbs btw - so half a turn in isn't it?