Still ironing the bugs out my 400D, part 146
…and it still has a annoying stutter/blubber at very low throttle openings…
so when you want to give just a bit out of a corner, you have to give a bit too much… not ideal in the rain! Also with throttle closed, going down a slope it will not “zing” down but will shake at every piston stroke.
Time with a dial gauge at 2.3, carb resynched, new motor etc. Bog standard.
When I synched the carbs, it wouldn’t idle with the slides down, even with rubber bellow off, so I had to use the idle stops and screw them in quite a bit… the stumble comes back!
IS there a good way of making sure both carbs are synched and at same height AFTER setting the idle speed?
I used to set the slide height for idle with cable adjusters on top of carbs, but that means no free play… not too good.
Thanks for the input.
EDIT: ANSWERS:
Have you stripped those carbs and given them a good clean out. These carbs work well but every orifice must be clear.
Espescially the pilot jet with your problem.
RDLY:
I had a similar problem until recently and it was the carbs. One was lifting before the other and then falling before the other. It confused the hell outta me.
Anyway, I removed the carb to airbox tubes and without running the engine I listened to the ‘tap’ as the slides bottomed (with the idle screws fully out) One was falling way behind the other but when I adjusted the cable at the top the problem reversed itself!!
In the end I had to change the cable junction and splice in new carb sections as it was this that was the problem.
The bike now runs fine with no surging between 4-5k when cruising or slowing down.
I had set the carbs about 20 times before with no improvement
The correct way to adjust them is:
1. Back off or even remove the tickover screws.
2. Remove the air-box tubes.
3. Open throttle to lift the slides then close throttle and listen for 2 distinct taps.
4. Adjust the carb cable adjuster on the side that taps last. Repeat until they are in time with eachother.
5. Remove the inspection screws (10mm hex) on side of throttle slide bores. Wind the throttle until you see a small pimple appear on each slide through the inspection holes. They should be within a mm of eachother. If not recheck the above.
6. Adjust top cable adjuster to get a few mm slack.
7. Replace the tubes and screws and start the engine. Now adjust the throttle stop (tickover) screws for best idle (comparing the exhaust pressures is a good indicator)
8. Thrash Bike !!!
9. If it’s the same >>>> Thrash Something Else
Spike:
Thanks guys,
yes everything is clean and carbs synched… with the lolly stick/wd40 red nozzle/fingers in the carb trick. They hit the bottom of carbs together with a big CLUNK!
After doing that the bike won’t idle (like slides too low??) and no messing about with the airscrew seems to help. Slides are the correct type (as per Haynes) so I have to use the stop screw to make ‘em higher , right?
This is where it seems to lose synch… back comes the stumble… it’s a really low/almost no opening stumble… like when you shut the throttle coasting downhill. Stumble/bucking, etc .
Some people sugested upping the idle jet as it sounded too lean… but I’m not too sure about that.
Adjusting the airscrews has almost no effect if static, when you drive the bike you feel it. Setting them to 2 turns makes the bike having a hard time returning to idle. and closing them a bit makes the stumble worse.
So should I change my idle jets (27.5 mm were suggested) or should I get my timing (2.3 now) closer to 2.0 BTDC?
ANSWER: Seattle smitty.
Worn, loose-fitting slides can cause blubbering idle. Very common on older bikes with rigidly-mounted Amals and Bings, but could happen to rubber-mounted Mikunis if abrasive dust got past the filters. I have fixed these slides with a baked on moly coating. Order a small can of “Gear-Kote” from KG Industries (Google it). Works good on worn (or new) pistons, rotary valve housings, aluminum-on-aluminum stiction situations, and many other applications. Follow their instructions precisely.
Interesting I might try that.
