Carb tuning question

[ShawnNYC]

Thanks Peanut. Makes sense. I ordered the next two sizes up for the mains and another size up for the pilots (47.5 & 50) last night, so hopefully I have enough options to get it right. I was able to go for a quick ride today and pulled some choke while off the throttle at a relatively high RPM. It seemed to quiet down the popping. Safely...lol
I should have everything this weekend and hopefully some time. Thanks for the info.

 

[ShawnNYC]

Finally got a chance to try the larger jets and get out on the road in-between rain storms. The 47.5 pilots nearly eliminated the decel popping, but I'm still breaking up at higher rpms. I don't have a tach so I can't be specific. I ordered the next two sizes up for the mains (147.5 & 150). Is it normal to go that much higher than stock? I realize the exhaust is a big factor, the pods as well. Just want to make sure this isn't crazy big and I should be looking at something else.

 

[peanut]

popping on decel is caused by unburnt fuel being ignited in the exhaust .....
You may well be too rich already in the idle circuit.

Air leaks in the exhaust can allow oxygen in... which will fire any unburnt fuel especially with an over rich mixture .

You might try refit the headers with new gaskets and use exhaust sealant on the silencer joints .
Make sure you set the idle mixture screws as per spec .
What colour are your plugs ?

 

[ShawnNYC]

Thanks Peanut. I put in the bigger pilots on your suggestion and it seemed to help. The exhaust gaskets were replaced a few weeks ago, although I can still feel some exhaust escaping...
I should have some time this weekend to take a quick spin and check the plugs, It's been raining constantly which sucks.

 

[peanut]

Thanks Peanut. I put in the bigger pilots on your suggestion and it seemed to help. The exhaust gaskets were replaced a few weeks ago, although I can still feel some exhaust escaping...
I should have some time this weekend to take a quick spin and check the plugs, It's been raining constantly which sucks.

I suggested upping the pilot jets because it is the pilot jets that supply the fuel at deceleration as well as at idle . As you are running open air filters and exhausts it is generally considered essential to up your pilots at least one size. On my bike I have needed to up mine 3x sizes from 27.5 to 35 . There is no magic jet size Its individual to each engine and must be determined by testing.

There is a simple setting up procedure to check whether your pilot jets are correct..... you'll find it in the excellent carb tuning guide in the tech section..

We are extremely limited as to the advice and suggestions we can make because we cannot see or hear your engine running . We only have your description and observations to go on. From your description it seemed prudent to check that your exhaust is not leaking . There are other causes of popping in the exhaust on deceleration . Try increasing your exhaust valve settings to ensure that the exhaust valves are seating properly and do a compression test .
edited with reference to 5T's post
Engine tuning is done primarily by throttle position not just rpms . There are 4x basic throttle positions for tuning .There are many excellent guides around this forum and the net .
Each throttle sector employs a different set of components so it is helpful to state which throttle position they are occurring .If the breaking up is occurring at 3/4 to WOT then its likely that your main jets need increasing but thats provided there are no battery/ electrical /ignition issues obviously

 

[5twins]

With these CV type carbs, you can't really rely on throttle position alone for tuning, you must consider the RPMs as well. This is because the slide isn't directly connected to the throttle cable. For instance, half throttle doesn't always mean the slide will be half open like on a normal slide carb where the cable connects directly to the slide. CV carb slides are operated by vacuum and are load or demand sensitive. They will only open as much as the engine needs or can use.

 

[peanut]

With these CV type carbs, you can't really rely on throttle position alone for tuning, you must consider the RPMs as well. This is because the slide isn't directly connected to the throttle cable. For instance, half throttle doesn't always mean the slide will be half open like on a normal slide carb where the cable connects directly to the slide. CV carb slides are operated by vacuum and are load or demand sensitive. They will only open as much as the engine needs or can use.

yes of course agreed..... I was just suggesting a starting point . I wasn't suggesting using just throttle position but having to explain oneself in detail all the time takes a considerable amount of unecessary typing. Surely the right way to approach carb tuning even on cv carbs is by throttle position because most of the throttle range is fixed jet dependant no ?
The slide is a transitional component and it would be near impossible to tune a carb by that alone.

I just find it easier to explain carb tuning to someone without your considerable experience by using throttle position

. Applying some masking tape to the handlebars and marking the various throttle positions gives a consistant starting point and also gives a means to monitor and record changes and see at a glance at which throttle positions any issues are occuring when test riding.

Now I'm going to lie down in a darkened room

 

[ShawnNYC]

Thanks for all the great info. I'm waiting on some larger mains to be delivered. My issue is definitely between 3/4 and WOT. I'll see where that goes. I have one more set of pilots to go up to 150 I believe, but I'll up the mains and see what happens first. I just wanted a sanity check. Exhaust valves were recently adjusted. .004 if I remember correctly. I appreciate all the responses. This place is amazing.

 

[peanut]

Thanks for all the great info. I'm waiting on some larger mains to be delivered. My issue is definitely between 3/4 and WOT. I'll see where that goes. I have one more set of pilots to go up to 150 I believe, but I'll up the mains and see what happens first. I just wanted a sanity check. Exhaust valves were recently adjusted. .004 if I remember correctly. I appreciate all the responses. This place is amazing.

Its a 2 way street buddy ...... we're all learning from these sort of threads all the time ............ even the most experience of us .

Remind me ... do you have points or electronic ignition and advance ?

 

[ShawnNYC]

Hey Peanut, it's stock ignition. Just got back from a month long trip and I've only had a short ride, but I think I'm pretty close to getting it dialed in.
The weather has gotten really cold, really fast but I'll continue to tune it as best I can.
I just wanted to thank everyone for the replies and info.