Think I might be losing my mind - they're Dells but they're 40s not 44s, don't know why I started thinking that. Still on the big side for an 1800 but maybe not too huge? Thanks for all the advice by the way, I'm certainly no expert and it would be great to have the engine running really well.
Basically you want the air flow in the venturis to be fast at the maximum RPM of the bus, so the vacuum sucking up fuel is strong, so the jets are working smoothly. Carburettors that are too big tend to be tricky to tune because the air is not going fast enough through them. If you make the venturis too small, they act as a choke on the flow of air into the engine and limit the maximum RPM. But on a bus with a 4000-4500 rpm max, and carburettors that will do for a 10000 rpm engine you will have no problem with air starvation.
40's on an 1800 should be ok as above if your venture's are 28's ish. think @zedders had some 29 'specials' someone might check the numbers on..
Considering that the original carbs for bus 1700, 1800 and 2000cc have 26mm venturis you'd be a boy racer with 28mm. 28mm do seem to work on most sizes bus engines... but ICTs are 27mm. It's a mistake to go big. I'm presently trying to force my oversize engine to like 32mm but it messes the fuelling up too much and I'll be going back to 30's when I finally give up.
Whatever idle jets you fit they stop supplying fuel at the same throttle opening no matter what the venturi size. But the bigger the venturi, the more air flow it requires to operate the mains. Go too big and there will be a gap where fueling is poor. In simple terms, for every engine there will be a sweet spot venturi size that pulls the mains in just as the idles are dying. Too big leaves a lean hole, too small - a rich overlap.
All points well worth taking into account but sadly the compression test suggests that there is some other work to be done before the venturis. Got 80, 110, 130 and 50 so it's off for a bit of engine out exploration to see what needs doing and then get it done. Fingers crossed there isn't any major damage or it could turn out to be a very expensive oil draining plug...
You already had the cylinder compression issue though, the oil drain thing has just prodded you into addressing it.
How does a sump extension as sold by all the usual suspects attach .. would that solve the op's original issue, or does it utilise the strainer bolt ?
No, it bolts onto a bar that spans the strainer hole I have one going spare if needed, fitted by PO, removed by me
Dont forget those original carbs fed two cylinders through that single vent . But the air chuffs twice to make up for it.
Are you sure they are hydraulic followers. Where in the world are you ... I have a sump plug repair kit, has great drill bits and taps to resize with new plugs. There are others on eBay similar and if you are going to pull the motor to look at low compression it will be heads off anyway. Might as well drill out and recut a new thread and sort it all in one go
An 1800 Type 4 wouldn’t have had hydraulic followers originally but @Rosko seems to think they are. If they’re solid and have been adjusted with hydraulic preload… It appears it’s off to a garage to fix so we’ll probably never know.
