Start up & Warming Behavior

[hudson]

Hey guys - need some guidance on my R75/5 SWB with Mikuni VM carbs.

After opening the petcocks & hitting the ignition switch, bike will not start. It will turn over, but seems like it is either flooded or starving for gas. Blipping the throttle while starting doesn't do it. Sounds like it is bogging out. I press the choke levers down on each carb, it starts up...then smokes like crazy out the exhausts. After about 30 seconds or so, I can flip the choke switches up and it wants to stall, but now the the throttle blips work and it stays running as a I blip for a few more times. Now engine stays running, settles down and smoke is finally gone after a good minute or two.

I go for a nice ride, runs great...until about 20 minutes into it - starts to rev the rpms on its own. I reach down and back of the idle screws just a touch. Better, at least for a few more minutes. May rev slightly again, throttle bogging slightly again, and smoke is pouring out of the exhausts again at the next few stoplights. Fortunately I was right near home and got it back in the garage.

It seemed like the engine was running hot and the odor of the smoke seemed I want to say fuel over oil, but not sure. I know it burned some oil, as the level dropped a little from its "normal" level that it likes. The smoke seemed a lighter whitish gray, not necessarily dark black gray, but it was dark out, so I didn't have much light to see.

To sum it up, starts hard, then rides good, then eventually acts up again...

I have spark, battery is new, carbs cleaned and rebuilt, fuel flows freely from tank to carb. Carbs synched and idle adjusted after warm up. But it seems like I have to tweak each time I start it the next time. Like I'm chasing the proper idle setting. The mixture screws are about 2 - 2 1/2 turns out from fully turned all the way in. When choke levers are flipped up to off, the revs drop back, so it seems like they are disengaging properly, but maybe not all the way?

Fuel related? Too lean or too rich? Something else?

Any thoughts?

Greatly appreciated. Thanks.

[George Ryals]

Too lean would explain both the overheating and revving up without throttle opening. You could also have a sticky advance mechanism.

http://www.mikuni.com/pdf/vmmanual.pdf This Mikuni manual might help.

[hudson]

Just wanted to give an update - took carbs apart again. On the right, the needle was stuck and the float level was too high. I think this is the culprit - getting flooded and burning all this excess fuel. But not starting up until I do an oil change and making sure there is no fuel mixed in. Plan on doing that in the next few days. Once I start her up, I'll hit it with my timing light just to be sure.

[Wobbly]

► You are correct. When you have multiple carbs, all carbs need identical setup and adjustment for the bike to run correctly. Good find.

► I have started numerous brands of bikes with the Mikuni carb "upgrade" and they all behave as your BMW. They won't start without the chokes ON. Once they start, they run ragged until the engine fully warms up. I credit this to the "OFF/ON" choke system of the Mikuni, versus the stock Bings which have infinitely variable choke settings. It's just part of what you get with a Mikuni.

► You have a secondary issue with the oil burning, which is not related to the carbs. First you should install a new air filter. Clogged air filters will allow the intake to pull oil from the engine. Then you should check the oil return hole and PCV flapper, both of which live right next to the starter under those small covers. A /5 is bad to get a nick or crack in the PCV flapper disc. They can be replaced with the more reliable reed valve off a /7. The other small cover hides an oil return well. The very small oil return hole in the bottom of the well can get clogged. When that happens, oil mist in the breather system cannot return to the engine and gets shuttled to the carb intakes.

Hope this helps.

[Kurt in S.A.]

Carbs shouldn't be set up exactly the same...they should be set up so each performs the same way under the same throttle conditions. Certainly jets, needles, etc., are the same...if that's what you meant by exactly the same. But the synching operations are needed to make individual adjustments outside the carb so each performs in harmony to the other.

Only a small run of the /7 bikes have the small oil return oil just forward of the engine breather valve.

Kurt in S.A.

[Wobbly]

Let's not split hairs, brother. What I'm saying is that before carb synchronizing can be attempted on a new-to-you bike or while fitting new carbs, all the static carburetor settings need to be exactly the same. All jets matching, same float levels, pilot settings, needles, needle clip positions, same (hopefully new) fuel source, etc, etc. Only then you can see if your manometer can fine tune those settings.

All the best.

[Kurt in S.A.]

I agree!

Kurt in S.A.

[Wobbly]

Actually, it goes well beyond simple carb "mirroring". Since the carb adjustments are dynamic in nature (engine running), they must be built on the foundation that all static adjustments of the engine and ignition system are "perfect". So before any carb adjustment can start, you must know that all other pieces (valve adjustment, timing, ignition advance, compression, plug wires, plug caps, etc, etc) are set or operating within normal parameters.

That's where a lot of home mechanics fail. They overlook the condition of (say for instance) the plug wires, and instead commence to changing carb settings. This leaves the bike with 2 variables "out of whack", which is infinitely harder to troubleshoot than the original single issue.

[Rob]

I loved renewing my plug wires and caps every few years.
Couldn't tune a carb to save my life.
I sucked in way too much ATF.

[SteveD]

I loved renewing my plug wires and caps every few years.
Couldn't tune a carb to save my life.
I sucked in way too much ATF.

The Harmoniser electronic tuning tool cost more than $4, but it sure makes tuning much easier.