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I am having a problem with my VFR750. It has a problem revving up under load past 6000 rpm. I have been told that if I adjusted screws that are circled that it would fix the problem. Here is an explanation of what is happening. When I am riding it will do fine in the lower rpms up to about 5500-6000 rpms. After that when open up the throttle (from half throttle to full throttle) it surges a bit and the rpms will actually start falling slowly while losing acceleration/torque and it sounds as if it bogging. I worry messing with the carbs. I am hoping someone has had this issue and has a fix. Thanks. If anyone has any questions please let me know.   

IMG_20170915_131825.jpg

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  • Member Contributer

Those are your synchronizing adjuster screws. They have to be adjusted with the engine running with a set of vacuum gauges. From what you are describing I don't believe this is your problem. While you have the carbs off I would remove the float bowls and make sure the jets are clean and float level is good. I would also remove the tops and inspect the diaphragms for any holes and that the slides move freely up and down. One last area to inspect is the fuel tank valve screen and make sure it is free of rust or other that would restrict fuel flow. Oh! and the fuel filter and pump. I'm sure others will chime in on things I don't know about. That is what makes forum what makes this forum great. Good luck.  

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  • Member Contributer

From your symptoms I would also suggest this is related to fuel starvation.

 

Check the following:

- float bowl needle valves can open fully and are not partially blocked with crud

- main jets are clean

- check any filter that is in the system between the carbs and the fuel tank

- check the fuel tank vent to ensure the fuel can flow freely to the carbs

- if you have a fuel stop cock, check that this free of any crud and allows full fuel flow.

 

Good luck and let us know how you get on.

 

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Thanks to all for your responses. Here are the things I have done already, Which I should have mentioned in my original post.

 

1. Replaced fuel pump.

2. Replaced fuel filter.

3. New petcock. 

4. New fuel lines. 

5. Replaced spark plugs.

6. Removed fuel tank and removed debris that was in there.

7. Ran two full tanks with Seafoam.

 I am going to try what was suggested in the above posts. If i can figure it out I will post what it was. 

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You clearly have no idea what you're doing with carbs, as most don't. So I'd advise the following VERY seriously: Send the entire carb unit to someone with significant experience in stripping down, cleaning and reassembling the carbs.

 

Don't attempt this yourself, regardless of whatever guides you come across, you will f*ck them up. 

 

They should be 'bench synched' which means they're adjusted off the bike, on a bench, so that the gap when the throttle is closed will match cylinder one (which should be bottom left in your photo...it's the one with no screw). Most of the time this will be enough but a sync while on the bike may be required, which again I'd suggest you have someone with experience do for you. Getting at and adjusting those screws on the bike is a massive headache, trust me.

 

While you're there, get some new seals from Lite-tek, they're cheap, far superior to stock AND ethanol proof.

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  • 3 weeks later...

^^^

 

yes, you can send those carbs to me for a professional carb cleaning, inspection,  adjustments,  a very accurate synch and a tweak of the factory-lean mixture screws and a leak test too.

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