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Discussion Starter #1 (Edited)
Hello.
First post here. Just coming to the end of my CX500 complete rebuild into a custom scrambler style. Using a Rae San ignition and pod filters so just installed 90/120 jets. Pops and occasionally backfires and has trouble idling. Revs increase when carb pushed in which is strange. I’m thinking it’s just a case of fuel mixture screw. Any ideas? Cheers

 

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Discussion Starter #3
I only unscrewed the jubilee clips and moved them enough to remove the reservoirs to change the jets. Is there a way to better seal the manifolds, as the jubilee clips are tight?
 

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Just to be clear - as those filters can cause running problems - the filters were already fitted and the bike running properly before the jet change and the jets were the only thing changed at this point?

We'll come back to the possibility of a vac leak.

And are you sure the new jets went in to the carbs in the correct locations - ie, the 120 jet screwed into the brass emulsion tube and the 90 screwed into the aluminium carb body?
 

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Discussion Starter #5
Yes the Jets are definitely in the correct places.

It was running ok (always needed a bit of choke) with those pods and standard jets except at high throttle requests the bike lost power. I assume, as the old jets couldn’t provide the fuel required with those pods.

The engine was purchased separately from the bike and so I’ve never seen it running perfectly yet.
 

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OK, it seems that it's most likely a vac leak.

It's likely that the non standard clamps don't fit properly as the stock clamps are {from memory} only 9 mm wide. I'm guessing yours are wider and won't fit into the registers in the boot.

Also, your boots are likely rock hard and the carbs may not be seated in all the way. A heat gun can help with this. There are also means of softening the boots {%10 methyl salycilate/%90 xylene}but it generally takes several days. The boots are also available new now from a variety of sources on ebay and David Silvers.

Can you take a couple of pics of your carbs seating into the manifold boots?

Alternatively you can fog fuel {or starting fluid, ether} over the suspect area with the engine idling. If the idle changes you have found your leak.
 

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Discussion Starter #8
Thanks for the tips.

I've tightened the clips and sprayed Easy Start into the gaps with no noticeable change in revs.

I've attached another video which shows how it reacts to throttle movement. Notice how even after I've released the throttle it stays at higher revs for a little while.

 

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Routing of the accelerator cables can cause the idle to hang. Did on my bike. Also, is there a chance that the pod filters are blocking the atmospheric ports?
 

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Spray starter fluid around the carb boots and the links between the carbs to check for vacuum leaks. You'll know you've found one when the idle speed changes.

Sounds like you're running lean, which is either a tuning issue or a vacuum issue. Take the pod filters off entirely and see if it runs any better, they may be blocking a port on the intake. Honestly I don't recommend using anything other than a K&N filter. Just asking for trouble.

Sounds like you have one cylinder running way different than the other. Unplug a spark plug and note how it runs. Then plug it back in and unplug the other one and note how it runs. If one side thumpa-thumpas different than the other or dies entirely you have either a severe carb balance issue, clogged jet, or similar.
 

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Discussion Starter #13
Thanks for all the advice. I'm away now for a few weeks. On my return I'll replace the pods and try your suggestions. Cheers
 

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Discussion Starter #14
Thanks for all the suggestions. I've double checked the jets are in the correct place. Both mixture screws are about 2.5 turns out. The atmospheric ports are clear of obstructions. The carbs will need balancing but I don't have the equipment to do it yet. Below is a video showing how it sounds when each cylinder fires individually. Cheers!

 

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2.5 turns out is the starting point for adjustment, not the absolute setting. Depending on things like the jets, air filter setup, exhaust system &c the optimum can be as much as a turn different. Google home made carb manometer and you will find thousands of pages describing how to build a manometer for balancing motorcycle carbs for less than $10.

BTW: Welcome to the forum. Please add your location to your profile and your bike's model and model year (NOT year first registered) to your signature (see Forum Settings link in my signature) so that you don't have to remember to tell us every time and we don't have to keep asking when you forget.

And welcome to the world of antique vehicle ownership. Your bike looks nice in the videos but it has had about 4 decades of Previous Owners who may or may not have done the maintenance necessary to keep it safe & reliable and even though you may have done a lot of work on it I'm sure you missed a few things (the brake line is the obvious one) so it is highly recommended to download the Factory Shop Manual for your model (available through the CX Wiki - link in my signature) and go through all of the service procedures, regardless of whether your bike has reached the specified mileage. I also recommend looking on all rubber parts with suspicion because rubber does not age gracefully. Check the date codes on your tires and replace them if they are over 5 years old no matter how good they look & feel because old rubber simply cannot flow around the irregularities in the asphalt well enough to grip, especially if it is cool or wet. It looks like your bike still has the original rubber brake line, which should have been replaced every 2 or 3 fluid changes (= 5 or 6 years) so I would start shopping for a modern stainless braided one (they last practically forever and double the life of the fluid).
 
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