Is this a vacuum leak?

[StewRat]

I'm hoping to go racing this weekend and the StewRat is running ok - starts fine, idles, revs, sounds ok.

Only problem I have is that once revved up (separate post on defining that) it doesn't immediately drop to idle in either a fast or linear way.

Trying to link to a video, but in essence, if I rev up to say 5000 rpm, it takes several seconds to work its way back to 900rpm.

(this vid might work - http://stewrat.stewarthutton.co.uk/wp-content/uploads/2018/06/IMG_5554.mov )

I've sanded manifold interfaces on glass, made new gaskets, refreshed o rings, everything I can think of to eliminate a vacuum leak below the butterflies that would be sucking in extra air and causing higher revs.

But maybe - just maybe - this isn't that unusual. I dont know.

Anything obvious I'm missing?

Thanks

Stewart

[Diederick/DVK]

Are you sure the linkage is set up properly and the shafts in the carbs aren't too tight?
Do they snap back to the closed position if you manually open the shaft where the linkage attaches?

[StewRat]

Are you sure the linkage is set up properly and the shafts in the carbs aren't too tight?
Do they snap back to the closed position if you manually open the shaft where the linkage attaches?

Yes sorry - I should have said ... I've been wrangling with this for a while so the most obvious basics are covered.
The throttles snap back against their stops when released, and with the screws backed out fully the butterflies are fully closed.

this after burn s happening with the throttles back on their stops, which make me think air is coming in somewhere else.

but I've been round the manifold interfaces with carb cleaner, unlit blowtorch etc to try and see where it might be getting sucked in - no success.

timing checked, valve clearance etc

given its not a street engine, it feels like it wouldn't be unusable like this, so I wondered if I was chasing too much perfection ... or equally if my standards should be higher than this!

Stewart

[StewRat]

Just to wrap this up.

I'd just decided I was going to race on Sunday even if this was revving high, and was doing a wee job on the linkage when after blipping the throttle engine off, I saw a bead of fuel run down the manifold on LHS.

I went round the carb/manifold nuts one more time (NOT the first time) and since then the problem seems to have gone.

I think I need to grind down a 13mm spanner for this job, as there are a couple of nuts on DRLA bases that can get in a situation on their rotation where there is no room between them and the carb body to get decent purchase.

[leec]

You can get m8 nuts that have an 11mm head on them. Makes it easier to get a spanner/socket on

Lee

[Iryanu]

You can get m8 nuts that have an 11mm head on them. Makes it easier to get a spanner/socket on

Lee

They're better than 11! they're 10mm!

These things are life savers if you have all the tins and sheetmetal. I use them on the exhaust too where the merged header exits in a tighter bend (plus the larger diameter tubing can make a real pain of a job).

http://www.machine7.com/product.php?xProd=12217&xSec=2606

[StewRat]

I was about to say I havent been able to find them so thanks for the link.

It's a great find!

And currently on special offer so I've bought a bunch.

Thanks again

Stewart