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Hi all my R90 will not rev past 4k unless I have the choke on, any ideas? carbs have fresh rebuild with new diaphragms,o-rings and gaskets. Timing is spot on, bike has only 21k but had been sitting for 10+ years. Thanks to all who can offer some insight.

beemersanducs wrote:Hi all my R90 will not rev past 4k unless I have the choke on, any ideas? carbs have fresh rebuild with new diaphragms,o-rings and gaskets. Timing is spot on, bike has only 21k but had been sitting for 10+ years. Thanks to all who can offer some insight.

I'm afraid I have questions, not answers, and a couple suspicions. All the history and detail of the problem that we can get will help.

Are both cylinders affected in the same manner?

Did the carb rebuild include spraying carb cleaner into each of the orifices in the cabs (especially those around the butterfly) including watching the spray exit where it should?

Have you measured the flow from the petcocks? Your problem smacks of fuel starvation. Have you opened the cap with the problem happening at speed? (vent hole in cap could be plugged).

Has the engine ever run right in your experience, and did the problem show up just after the carb rebuild?

You get the idea. The more clues we have the better our guesses are going to be. (And the less frustrated you will be since we won't be scatter shooting the problem.) How about a paragraph or two telling us about the resurrection; the bike's condition before, a summary of the things that were done, and so forth.



Ken

If a fresh carb rebuild means that the enricheners were disassembled, likely things have been switched or assembled wrong on them.

Kurt in S.A.

Is there any possibility the cam at the throttle is in the wrong position?

I would bet the farm that your problem is too low float height, very easy to get this wrong with a carb rebuild.

To measure, remove float bowl, raise float with your finger, turn fuel back on, fuel should just start to well out as the float moulding line is parralel with the bottom of the carb. Bend tab to adjust.


This is from another forum........Posted: Tue May 07, 2013 10:53 am    Post subject: Level Setting
The level is checked by removing the float chamber, holding the float fully up, and turning on the fuel. Now slowly lower the float until fuel JUST begins to drip - at this point the metal float bridge should be parallel with the joint face.

Adjustment is made by carefully bending the tab which presses on the pin at the base of the float needle - I use a watchmakers' screwdriver and lever it against the float pivot pin.

Use of the Haynes manual instruction to set at the point where fuel stops dripping will give a slightly higher level.

Use of the early (/5 Series) method, involving removal and inverting of the carburettor, on later carbs with a spring-loaded pin in the base of the float needle is not advisable at all. This will give a very low float chamber level, which will usually limit maximum speed to about 80 mph in top gear, even though the engine will rev well in lower gears.

Dripping carbs - usually every ten years of so - are usually caused by the rubber tip of the float needle hardening, and often manifest themselves after the bike has not been in use for a while. After some use the tips often soften, and the problem seems to go away - only to reappear later.

In such cases replacement is the only real answer - unless there is evidence of dirt, the floats are becoming porous, or the petrol tank internal coating is coming off, after use of ethanol-debased petrol.

G'day BnD
This is an outside chance, but worth mentioning. We don't know how familiar you are with your bike ...

If you are using the Clymer manual to learn about your bike, there are some inaccuracies, including the following bit in the Troubleshooting chapter that had me scratching my head for a while -



It's not just a typo on this page. The narrative continues over the page to emphatically get everything arse-up.

The correct info is that the choke (enricher, actually) is ON when the lever is DOWN and OFF when the lever is UP.
As I said, an outside chance ...

ps: If this post provides the solution, do I get Chasbmw's farm?
I've always wanted a farm.

I had a similar problem on an R65 with a bad diaphragm. I know you mentioned replacing the diaphragms but here are two things to consider. 1) did the 'tab' on the diaphragm get seated properly in the top; and 2) there are apparently some diaphragms available, non OEM, that are of a heavier material. These non OEM items are not as flexible and it may be possible they are not allowing the slide to raise to the proper height.

OK sorry for the slow response but for some reason I have trouble logging in, I disassembled the carbs last night and found some tank lining in the main jet on the OS carb blocking about 50% OD carb was clean, I have another set of diaphragms (new) to try. I got the first ones from a dealer that I have received inferior parts from before. Anyway thanks for everyone's help. I will figure this out!!!

Looks like I might lose the farm!

If your tank has been lined with petseal, then it might be affected by ethanol in petrol.

OK all, problem solved..... you know, some times the most its the esoteric thing your mind comes up with....and other times you just don't have enough gas in the tank.......I'm so embarrassed.... I'm not worthy please be kind.....