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Discussion Starter #1
Hi,

I have a perfectly stock bike, a real beauty in showroom condition. Bought it a few months ago and have only done a few trips on it. European model, with VM29 carbs. It idles really smoothly, starts easily and pulls hard when opened up. The issue is that when cruising it will splutter and bog before it accelerates. I marked the throttle handle with masking tape and identified it to occur between 1/8 and 1/4 opening. It is not so noticeable when the engine is cold.. gets worse when it warms up. This suggests to me that the issue is caused by richness in that range. If I hold it exactly at 1/4 for a few secs it will clean up and happily rev when opened further, also in top gear.

This issue prevents me from enjoying an otherwise super sweet bike. Going through turns can be sketchy when I can't rely on it pulling.. overtaking is also less enjoyable with that sputtering and hesitation.

I checked the pilot screws and they are all at 1/2 a turn out, as they should be. I took the top off one end carb but couldn't see how to access the jet needle. It sits somewhere below the slide lifting mechanism...

Could it be wear in the needle jet? The bike has done 49000kms. I assume the slide is the stock one and unaltered.

Any experiences with this and/or ideas on what to check? I'll see if i can find some thin clear tubing to check the float levels.

Thanks in advance!

Kristian
 

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SuperMod of the North
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Welcome. This is a good thread, tons of info on carb adjustment. It is in the top of the Tech and Performance section. I am not familiar with your exact carb but gently lifting that rubber boot you see will lift out the slide with the needle attached. You can inspect an/or adjust the needle and drop it back in. Make sure to get the boot sitting nicely for a good seal when you put it back together.

 

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Thanks for the reply! Sounds like you are referring to the CV carbs fitted to US bikes. This is a VM29 (smoothbore) flat slide carburetor, a bit similar to the Mikuni RS series. Fitted on european 750s until I think 1988. The 1100s all had CV carbs.
 

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Discussion Starter #4
Reading the post you linked to it seems lowering the fuel level in the bowls could cure the issue. I'll start by checking the level 😁
 

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I must say, I would love to see a picture of this machine. Also, I would like to see a picture of your carbs, as I thought they stopped using the VM's in the late 70's...
 

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SuperMod of the North
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Ho Lee Shit. That is beautiful. By the look of those hills in the background you have some good roads to let her do her thing too! Thanks for posting.

Ignore everything I posted, those carbs are from a different planet than we live on over here. The principles in the carbs 101 thread will apply in regards to jets and needles though.
 

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Oh!!! So THAT'S where those carbs come from! I belong to a GS forum, and there are many racers that use those. Never knew much about them, as I have never had to tune them. All of the fuel circuits will be the same as all motorcycle carb (as far as tuning them go). Start with the basics, and work your way through. Tin's is correct, the guide can help you understand where you need to be adjusting in your throttle range. The floats are a good place to start. While you're in there have a look at your pilot jets, and note the size#. Do some research and compare that to what should be in there stock. A bike that old, you can't rely on it being stock. You never know what some previous yahoo may have done to tune it in the past. Good luck, and keep us posted.
She's a beautiful bike...
 

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Discussion Starter #10
Thanks guys!

I'll definitely check those float heights. Hoping it's all stock in there... But as you say, can not be sure on a 33 year old bike 😅 It is in very good original condition though.
My second guess is wear in the needle jets. Hopefully it won't be too expensive replacing them if that's the case...

May not happen straight away.. End up doing a lot of work on the house instead... 😏

Yes, plenty of winding mountain roads in the area where I live 👌
 
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