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All OK to 4,500rpm, then really hesitant (like running on one pot), then at 7500rpm takes off like a scalded cat and pulls hard.
Any ideas?
I have already cleaned the carbs twice and have an inline filter.
The midrange improves when I put the choke on, so think it is fuel supply related.if choke improves the bike then try bigger jets,also which clip are u using for the carb needles?i think the needle position is at fault,or maybe the filter wants bigger jets as gas ratio leans out in the midrange.is the bike on cdi?
1987 Yamaha RD 350B
In first gear whack the throttle.Maybe you should stick with the standard jets and rectify the fault first. Changing to non-standard bits, you will never know what the original fault was and could get a bit lost. Needle position is likely as Rajnish says.
quote:
All OK to 4,500rpm, then really hesitant (like running on one pot), then at 7500rpm takes off like a scalded cat and pulls hard.Perhaps you have an RD400 engine in there?
How was it running before you changed the carbs?
Did you put the same ones back on or did you replace them with some others?
I’ve donated to the forum, have you?Good stuff guys,
Just a bit more info on my ill fated bike.
It’s a 1991 GPz500s, very clean, and only with 10,000miles on the clock.
It had been off the road for 2 years, with terminal engine trouble [no oil pressure, sop I have replace the engine but used the same carbs.
The fuel and carbs were obviously showing signs of varnishing and hence the problems I thought, so cleaning has been the order of the day so far.Thanks for the tips & advice chaps, I’ll check the carbs again & let you know. On the bright side, I’m getting quick at taking the carbs on & off!!
[
]yes u need to check the carbs,i’ve a feeling tht the free flow filter is leaning things in the mid range,try to bring the needle to richest,if bike runs better in mid range,u need bigger pilot,if mid is poor and top goes better then may be mains.
1987 Yamaha RD 350B
In first gear whack the throttle.
What does the free flow filter do?
Andy,
I had a similar problem after fitting a dynojet kit to my bike.
Most carbs fitted to bikes have 3 fuel circuits:
1. the idle circuit – controlled by the size of the idle jet
2. Full throttle – controlled by the size of the main jet
3. Mid range – controlled by the needle position in the main jetAs your bike is idling OK and is also OK on full throttle then the needle position is the likely culprit. First make sure that the carbs are clean and check that all the floats and float valves are not sticking. Then balance the carbs. If you still have the issue then run the engine into the rev range where the problem is at it’s worst and “chop” the engine (hit the kill switch and pull in the clutch to stop the engine as quickly as possible). Then take out the plugs, you should be able to see by the electrode colour if it is running rich or lean. If its running lean,you need to raise the needle position and if its running rich then you need to lower it. I’m not sure how you alter the needle position on your carbs, its normally done with a circlip in grooves at the top of the needle or by shims. Be careful not to make big changes and change all the carbs the same way. Then repeat the “chop” procedure until it runs correctly. Be careful not to go to lean as this can cause the engine to run hot and in extreme cases melt holes in pistons. Basically you’ve just got to be patient and play with it a bit – always make sure that you note the original positions so if all else fails you can go back to the beginning.
Hope this helps
Katana
Thanks for that info, and the time taken to help me.
I never imagined I’d be so keen to check the most intimate nooks and crannies of my carbs!
Ah! Plug chops, takes me back to the RD250’s & power valve too!Thanks again, I’ll keep you posted.
Always wondered what a plug chop was. And I was doing them for years!
I’ve donated to the forum, have you?yup chops take u back to the good ol’RD days.they r very useful to know the status of the fuel supply.
1987 Yamaha RD 350B
In first gear whack the throttle.UPDATE,
Thanks for all the tips and help on carburettor subject guys.
I have tried to raise the needle height on the main jet on these carbs.
On doing so, I discovered there was no provision for adjusting circlip height, but I did use a spacer under each needle shoulder to raise them (approx. 2mm) for a quick try.Unfortunately, the bike felt exactly the same, and I did not do enough miles to change the colour of the spark plugs significantly so I have to try 2 (last?) things a friend noted a couple of ideas,
1/ Another carburettor clean!
Including complete removal and cleaning the mixture screw which I have not yet done.
Not sure how to balance them them though!2/ Check the Float Height, as Katana mentioned
I am told the if the height of the fuel is too low, their can be insufficient fuel drawn through the jet, so I will check this. Still surprised this setting can change, but who knows!
I’ll have a go the weekend & keep you updated.
Cheers, Andy SpinEUREKA petrol after you’ve stripped your carbs as many times as me!
But it’s finally worked!
Checked Float level – within spec
Stripped down carbs to the last bit, a la Haynes manual,
I think the key this time was blowing cleaner through the pilot screw circuit over & over again.I returned the needle heights to standard, and like magic….. I’d lost a screw to hold the diaphram tops on!!! Oh bother I said [thought something else]
Replaced screw, and carbs to bike.
One of the coldest days of winter, I was one glowing with the super results.
The bike is transformed !!Thank you to all who responded to the posts, great to have a bit of moral support, it has certainly helped me persevere.
Isle of Man here we come!!!!
Regards
Andy Spin
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