I have a 1976 R90S with a Boyer Brandsen ignition. I had been having carb problems -- I don't ride the bike often and every time I did the carbs would be clogged even though I drain the float bowls after every ride. So I took the shotgun approach: I sealed the tank with POR-15 (that seemed to go well), I Chem dipped the carbs and replaced all the seals, o-rings, pilot jets and accelerator pump pistons (I had replaced the needles and needle jets recently). The timing is good and so are the valve clearances. It starts right up on the choke and it runs fine at mid and full throttle. I took it for a 20 mile ride yesterday (it was hard to keep runnng at stop signs) and then synched the carbs using a flow meter. It would only idle with the idle screws turned all the way in. Also it ran best with the air screws turned all the way in. I've checked the float heights using Snowbum's method and they're good. I've checked the choke cables and they have play in them. I went out to try it today and gave up tying to go another twenty miles because it kept stalling when pulling away from a stop.
I'm at a bit of a loss. I'd appreciate any suggestions before I start tearing down the carbs again.
Been a while since I've had Dellortos, but in general, any carb that runs better with the air screws all the way in is because a vacuum leak is replacing the controlled vacuum leak that goes through the idle bleed circuit. As manifold pressure goes down as RPM goes up, a vacuum leak is less pronounced, so it runs better above idle. How are your slides? The reason 750s and up all used CV carbs is because the vacuum acting on the carb slide pulls it into the carb body and increases the force required to pull it up. Which increases wear, which increases the amount of air that goes around the slide. Spray starting fluid around the outside of the carb and intake boot while its running and see if the RPM increases. If it does you found your leak. I don't know of a way to check for air going around the slide. Maybe there is something in the Dellorto book?
Got Dels, '74 90S, always had trouble with idle, killing at stop lights, etc. On the main jet and with the accelerator pump, no problem. and then last spring figured out there was a leak on the intake manifold tubes. So I sealed them enough for idle, until it stumbles. Sometimes after a long ride the idle is right spot on, about 1500, however, sometimes it coughs a little like it's loading up. No biggy since I don't idle much.
The problem with sealing them too good, is you can't get them off at a later date. PO literally glued them on with what looked like red loctite. And they were locked. The carb top large o-ring has to be good too and seated correctly. Other than that they run just great. Also, like many other riders, when approaching stop lights I always back on down the throttle, try to synchronize my speed to no-stop roll throughs. I hate sitting at a stop light, idling, making those pipes turn red.
As for the crud in the float bowls, just make an aluminum tank, problem solved.
Thanks all. It may be a leak at the manifold but I'll have to wait for better weather to test that.
By the by, the Clymer manual identifies the starter jet as the accelerator jet and vice versa.
Thanks again. Apparently it was a leak at the cylinder spigot. Following the above suggestions I checked the mounting more closely (after pulling the carbs apart yet again and checking that all the circuits were clear). The carbs slid forward another 1/8 of an inch or so (sure thought I had them on all the way before). I also tightened the clamp more than before. It still seems to need the idle screws in more than I would expect (3-4 turns in from when the slide just begins to rise) but it runs best with the air screws at 1.5 turns out. Idling nicely at 1000 rpm with a slight stumble on take off if I'm not careful. I put that down to the higher compression and Italian carbs on a Bavarian bike.