81 cb650c backfire on downshift/cruise at 4k rpm

If it's broken or just needs tweaked

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jdip16
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Joined: Fri Jul 26, 2019 10:42 pm

81 cb650c backfire on downshift/cruise at 4k rpm

Postby jdip16 » Fri Jul 26, 2019 11:26 pm

Hello new friends,

ISSUES: backfiring on the downshift or around 4k rpm, a little lag when you want to give her the beans, and when the bike is hot idle is at or above 2k.

BACKSTORY
After two years of owning my bike (name tbd, suggestions welcomed) I finally got her on the road this May. I received the bike with a rusted out tank and carbs split into 4 ziplock baggies. After a very frustrating winter, carbs we're back together with all genuine Honda rebuild kits and factory setting Jets. Tank was treated pretty successfully with a por-15 kit and some aircraft grade fuel puddy to fill the pinholes.

Spark advance was replaced and gapped to specs. Intake boots, spark plugs, and pretty much every gasket you could think of was replaced. After we figured out the idle screw was turned in almost all the way, the bike finally idled normally.

After stripping a screw on the cylinder head cover, and replacing said cylinder head cover, valves we're gapped, timing and carb sync were accomplished.

Essentially, two aircraft maintainers went head to toe and replaced everything we could think of.

The bike starts great cold, runs through the gears like a champ, and rips the New England back roads like you read about.

Only issues thus far are: backfiring on the downshift or around 4k rpm, and there's a little lag or studder when you want to give her the beans. And the high idle after riding 30mins

The bike came with a MAC 4into1 and k&n air filter. Filter has been cleaned but not reoiled.

I was thinking of tinkering with pilot screws a little before going to bigger Jets.

Thoughts...

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GeorgeSweety
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Location: UK.

Re: 81 cb650c backfire on downshift/cruise at 4k rpm

Postby GeorgeSweety » Sat Jul 27, 2019 10:56 am

Hi jdip and welcome. You have rebuilt the carbs with genuine Honda jets and set them up to factory settings then use K&N air filter and an aftermarket exhaust, I think that may be the cause of your problem. What type of carbs are on your bike, PD 51 slide or CV VB44 carbs? If it has PD 51 slide carbs did you replace the air cut off valves?

jdip16
Posts: 4
Joined: Fri Jul 26, 2019 10:42 pm

Re: 81 cb650c backfire on downshift/cruise at 4k rpm

Postby jdip16 » Sat Jul 27, 2019 11:02 am

CV carbs; exhaust and filter came with the bike. I get that you should tune with everything factory, but I don't have the option at the time. Ballin on a budget if you know what I mean hah.

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GeorgeSweety
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Re: 81 cb650c backfire on downshift/cruise at 4k rpm

Postby GeorgeSweety » Sun Jul 28, 2019 2:05 am

Yes I get the tight budget thing :( ! Have you checked your spark plugs to see how your mixture is?

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Volker_P
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Re: 81 cb650c backfire on downshift/cruise at 4k rpm

Postby Volker_P » Wed Jul 31, 2019 1:02 am

Yes, start with the look at the spark plugs. Typically for stock they look a little bit white as setting is already on the lean side. K&N without oil might be too lean then, but CV carbs are not that sensitive to the air side as the PDs. Moreover all four plugs should look equal.
Sounds a bit like carb synchronization might be off or an air leak at the carb boots.
Cosky's great (free) online manual: http://cosky0.tripod.com

forum links to common technical issues

jdip16
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Joined: Fri Jul 26, 2019 10:42 pm

Re: 81 cb650c backfire on downshift/cruise at 4k rpm

Postby jdip16 » Wed Jul 31, 2019 9:04 pm

Clamps for boots were tightened down after a long ride last week. Might be taking the bike to work tomorrow. Will check plugs and maybe fiddle with pilot screws after work. Here's a dark photo of my bike with my buddies recently resurrected '74 360t. And also a pic of my winter project picked up today '77 550kImageImage

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Volker_P
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Re: 81 cb650c backfire on downshift/cruise at 4k rpm

Postby Volker_P » Thu Aug 01, 2019 4:50 am

Nice 550 Project, just lots of dust but fine basis. :D
Guess there is no issue happening when riding at load at at 4000K that can be solved just by pilot screws. Except maybe to finally install them because they were missing. :lol:
Cosky's great (free) online manual: http://cosky0.tripod.com

forum links to common technical issues

jdip16
Posts: 4
Joined: Fri Jul 26, 2019 10:42 pm

Re: 81 cb650c backfire on downshift/cruise at 4k rpm

Postby jdip16 » Tue Aug 06, 2019 7:16 pm

Sorry it's been a minute. Since my last post I've probably put another 200-250 miles on the bike (named POPeye).

Plugs were checked after last post showing a lean mixture. Fiddled with pilot screws and first went out ¼ turn (clymer says 2¼ as a baseline for rebuilt carbs). Adjusted idle accordingly and lost the high rpm at idle with a hot bike. Bike was still popping at the low end, but on the decel at high gear no popping. Went out another ¼ and adjusted the idle accordingly and had rpms hanging.

Brought pilot screws back in (2½ turns) and ran it for 100ish miles with same results after the first adjustment. Today (5th day in a row on the bike), for gits and shiggles went back in to 2 turns out from lightly seated. Performance went down, brought back the high idle and more pops.

Went back to 2½ turns to get the same results as yesterday, and then an extra ⅛ turn out. POPeye is running the best it ever has. Throttle response is awesome, idle is on the money, and only popping on a hard decel or downshift to high rpm.

I'm going to run the bike as is for a long ride and see what happens. Maybe go another ⅛ a turn out.

Ultimately, going to go up in main jet size when the bike goes in the basement for winter.

Opinions, questions and concerns are welcomed.


PS- sorry for the long post


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Volker_P
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Re: 81 cb650c backfire on downshift/cruise at 4k rpm

Postby Volker_P » Wed Aug 07, 2019 12:35 am

jdip16 wrote:I'm going to run the bike as is for a long ride and see what happens. Maybe go another ⅛ a turn out.

Good idea. In the times I was riding my CB over winter the engine required an extended ride at changing revs and loads in spring to run really fine again. :wink:
Popping issue might be missing exhaust seals, too. These are copper rings. Typically they appear as black, undefined stuff sticking at the head but they still seem to work like that. Problems only when missing.
Cosky's great (free) online manual: http://cosky0.tripod.com

forum links to common technical issues


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