Welcome Aboard !

By "warm up", I take it you mean the engine has run long enough to no longer need the chokes/ enricheners being in the ON position.

That is very typical of a bike that's been in storage. When carbs get clogged due to being in storage with fuel in place, a varnish or other crud begins to develop, which clogs smaller passages. And it always clogs the smaller passages and fixed jets first. The smaller jets are the lower RPM jets the engine runs on when the chokes are turned OFF.

When the engine is cold and the chokes are being used, the bike is running on the excess fuel being flooded into the carb by the chokes. But when the chokes are turned OFF, this supply is taken away and the true state of the carb is revealed.

Pulling in the clutch lever simply adds a load, which the poorly supplied engine is not making enough power to overcome. The load is so light that it tells me your carbs are in fairly dire condition.

• Remove both pilot screws and blow carb cleaner through those passages.

• Metering of fuel is so finely tuned, that I highly advise you to replace both small "mid-range" jets. Any wire or other device pushed though these tiny jets may damage the soft brass and change the metering.

• Also inspect the needle for corrosion which will give you a flat spot during acceleration. As with the fixed jets, replace them, don't clean them.

Photo of carb needle corrosion

• Your issues are NOT contained simply to the smaller jets. They are a symptom of issues throughout the entire fuel system. Start using a carb cleaner, such as StarTron, to clean the entire fuel system.

Hope this helps.

Thanks for responding. The restoration is a complete frame-off including new DellOrtos so clogging isn't an issue. It's been a nearly 2 year project.

I've checked the jets on multiple occasions to be sure and they're clean. Same with the depths of the needles. Maybe I should try raising the needles one level and see if they might help alleviate the problem. My experience with DellOrtos is very limited and I've tried to tune them in via recommendations from here and other sources. The extra load on the engine I understand but I've never seen it so pronounced once a bike warms up. I didn't mention the choke since it's only used briefly for less than 20 seconds before the bike settles in at a low idle around 800-900. Pulling the clutch in at that point has little to no impact.

► I'm in the same "experience boat" as you concerning DellOrtos. But I've worked on a slew of others extensively, and on all of them, the needle only comes into play starting at low mid-range. Your issue is well blow that RPM range. So "No !", nothing you do to the needle will have the slightest impact on your issue. If its carburetor related, then it's in the pilot, slide cutaway, and/or the low speed jet.

► Are the pilot screws both set to 1-1/2 turns out ?

► Are you sure about the float levels ?

► What air filter element is in there ?

► The funny thing about carbs is it's the last thing to be tuned. Every other tuning step has a metric; a measurement you can check. Carb adjustment depends upon the sum of all those other adjustments being absolutely correct. Therefore, check the valves clearances and then the ignition timing with a strobe lamp at high RPM.

► Are you sure fuel is able to rush out of the petcock ? One time I had a R75 where the internal seal rotated and ON was really somewhere between ON and OFF.

All the best.

I wholehearedly agree. The carb tuning is always the last thing to get settled. Never had this problem with my other R90S. Maybe I was just "lucky" to hit on things right off.

I've played with the pilot screw and the setting make a difference only at high idle. Fuel flow is good. I cleaned the tank and re-lined it last summer before starting the reassembly. I checked the petcocks and they flow nicely in both settings. Both are brand new but still checked them out.

As to the air filter it's a new K&N all properly oiled up. Timing is spot on at idle and advances correctly at higher rpms. Using the Dyna III electronic ignition setup.

Based on your suggestions I'm leaning towards the low speed jets being the culprits. When riding at normal speeds everything is nice and smooth and balanced.

Going to check the low speed jets and float levels again this weekend. Would be nice to get this worked out to get out with the good weather this holiday weekend.

Some new ideas....

• A lean condition could also be caused by using old, hard rubber tubes on the intake (connecting the carb to the head).

• Bings also have small screws that are taken out to attach the vacuum gauges used for balancing the carbs. When the gauges are removed, it's essential that those small screws be re-installed or you'll get a tremendous air leak. I'm not sure what set up the DelOrto uses, but the reported issue would be the same.

• DelOrtos, not being a CV type carb, would be even more sensitive to being out of sync. Vacuum gauges need to be used to sync the carbs not only at idle (~1000 rpm) with the idle speed screws, but also at mid-engine speed (~3000 rpm) using the cable adjustments.

• Since the DelOrto is a slide-type carb, it may be prone to a user error common on Amals. I don't know this, but it's easy to check. When installing the carb slide on Amals, if extreme care is not used, the needle can be pushed to another needle clip position. The resulting mis-match between dual carb needle positions can be hard to diagnose.

Hope this helps.

After tearing both Dellortos down again I found the culprit. On Dellortos there is a 3rd jet. It's neither the idle or the main jet. It protrudes down into the bottom of the float chamber and is always within gas. There is an o-ring that seals it into an indentation in the float chamber. Even though these were fresh build carbs with all new o-rings, seals, etc, the o-ring on the right side had chunked and that chunk had been swallowed into this tiny jet. It could not provide gas. I cleared it out with reverse air stream and put a new o-ring on it. I did the same with the left carb even though it showed no indication of having the same issue. Luckily I had an additional rebuild gasket kit here or I would have been wanting for a simple o-ring. I also double checked the float levels and both were spot on. Double checked the screens on the gas intake and no issues there. Reassembled them and did testing today. The R90S now idols correctly at 950-1050 with the clutch out and there's only a minute drop to around 900 with the clutch in. Same both after initial start and full warmup.

Like you I have many years (more than I'd like to remember at times) with Bings going back to my R67, /2's, /5's and forward. In the last 35 plus years this is only the 2nd time I've dealt with Dellortos. Great for speed but quite a different animal from the Bings. Never too old to gain new experience with these fantastic and challenging Airheads!

Thanks for the responses and suggestions. Time to go give this restoration a real workout!

larry m

Just goes to show what Sherlock Holmes said 100 years ago is still true... "When you have eliminated the impossible, whatever remains, however improbable, must be the truth."

That jet is the slow speed jet, and the engine was incapable of idling properly with it blocked.

Glad you found it. Enjoy your R90S !!