Stumble at low rpms under load

[mrb19572001]

Hey, This is Mike and this is my first post! I have a pristine 79 with 14000 miles that has been storage since 1998. I am having issue with the bike stumbling from idle to about 2800 rpms under load[ riding], after that the bikes runs very well as it should. In neutral, the bike starts and idles fine. I can rev the bike up and it runs clean , no backfires and comes right down to idle. I have had the carbs apart, cleaned, replaced the idle screws, slow jet, both mains and fuel valve with seat. I also synced the carbs to almost prefect between each other. The plugs were new when the bike went into storage. I cleaned the connections to the coils and trimmed back 1/4" of the plug wires and made sure the connection to the plug was good. The plugs burn a light brown to tan in color. Valve clearances are all in specs as is the compression readings. Idle screws are 2 turns out, I have tried 1/2 turn either way and this seems best. A kn air filter is used and Ontario moto tech exhaust system with #70 primary main { recommended by omt and used with good luck before storage, #98 secondary main with #35 pilot and floats set to 15mm. The only thing I questioned during reassembly was the rubber plugs that cover the slow speed circuit were a loose fit but stayed in place. Do they need to be a tight fit? I hope I haven't rambled on with to much info, but the more up front the better. Any suggestions on what I can do to clean up the low speed throttle drivability would be greatly appreciated. Thanks, Mike

[steve murdoch icoa #5322]

Welcome to the site, Mike.
Sounds like you have been very diligent in prepping your bike.
Your problem MIGHT be the K&N filter. I had a stumble, albeit higher than yours at 4500 rpm, that was eliminated when i went back to the stock foam set-up.
Here is a bit of info from one of the best guys for CBX carbs.
viewtopic.php?f=20&t=10018

[daves79x]

Second the 'ditch the K&N' sentiment. If you still have the stock wire basket, you can get a foam sock from Louis or overseas, or just buy the late model paper one from Honda. Apart from that, hope you used genuine Keihin jets and float valves, if not-go back to them. It only takes one partially plugged idle/slow-speed circuit to give you that stumble. With a stock filter, I use #68 primary mains. Many rubber plugs available today are not a tight fit, but the little nub in the float bowl holds them in place anyway, as long as they aren't hard and brittle.

Dave

[mrb19572001]

Thanks for your replies. The bike ran fine great with the kn before the bike went into storage. I covered the air filter 50 % with a plastic bag and that made no difference. And yes, I did use the slow speed jet and pilot out of a kit. Also, using the choke made no difference at all. While checking things over, I found a lot of water under the pulsar cover. May pull the carbs off and switch those jets out with the stock ones as they were good, just needed cleaned.

[Jeff Bennetts]

Second the 'ditch the K&N' sentiment. If you still have the stock wire basket, you can get a foam sock from Louis or overseas, or just buy the late model paper one from Honda. Apart from that, hope you used genuine Keihin jets and float valves, if not-go back to them. It only takes one partially plugged idle/slow-speed circuit to give you that stumble. With a stock filter, I use #68 primary mains. Many rubber plugs available today are not a tight fit, but the little nub in the float bowl holds them in place anyway, as long as they aren't hard and brittle.

Dave

A quick tip concerning the stock foam sleeve/sock, if your's has turned to dust or missing, I have had good luck using the replacement A/C filter material that you can buy in sheets for a few bucks at any DYI store. Just cut it to the dimensions you need to wrap around the wire cage of the air filter and use a piece of Velcro (the hook side) to hold the seam closed.

It works for any dual stage filter that has an outer foam filter and the A/C filter material is easier to clean and will last forever.

Did you bench test the accel pump and verify that all the carbs were squirting fuel through the posts in the carb throats?

[steve murdoch icoa #5322]

If the K&N is at least 20 years old, certainly it could have deteriorated in that time.
The choke having no effect on operation might mean sacked springs or an improper installation.
There is a short video provided by Mike Nixon on the correct installation of the springs.
http://www.motorcycleproject.com/text/tech_videos.html

[mrb19572001]

Today I pulled the tops off of the carbs [the ones I could get off] and found #6 slide stuck wide open. Cleaned, reassembled and took out for 25 mile ride. That took care of 95% of the stumble. Now there is only a very small hesitation coming right off of idle and stops at 1200 rpm. Completely ridable now, But I went ahead and pulled the carbs. They are disassembled and tomorrow I will get some parts ordered. I will post the out come when everything is buttoned up test rode. Thanks for everyone's input.

[daves79x]

Well, looks like you found something! This is an oft-overlooked part of carb cleaning. The dome receptacles and corresponding slide towers accumulate varnish and can get very sticky. Q-tips and carb cleaner will fix you there. Don't agonize over keeping them together as sets - they are all the same and if all parts are in good shape, any slide will work with any tower.

If you are looking for genuine Keihin jets and float valves, try PJ Motorsports in Oregon. They carry all you will need. If you do have aftermarket float valves, and don't have the originals in good shape, you must get the genuine item. Use the original Honda brass float valve seats and aluminum washers also if you still have them.

Dave

[Larry Zimmer]

Well, shame on me for not posting this earlier for the benefit of all. So much is made of clogged jets, etc. with carbs. And, justly so. HOWEVER, I expect that what Mike found, and Dave commented, might well be a common issue with the older bikes that may not have had a total carb clean/rebuild. While doing some other work, recently, I had the carbs off. Always watched the jets, synch, etc. But, never pulled the carb slide vacuum domes. (Don't know why neot??!!) Anyhow, the insides and around the vacuum pistons were all black 'slime'. All of them. Might very well explain slightly sluggish response. While it does require pulling the carbs to check, might well be worth the effort if you haven't done it within the past 10 years. All it takes is a shop towel and some carb cleaner or brake cleaner.

cheers,

[mrb19572001]

Well , several weeks have past since my last post and here's what has happened since then. I removed the "kit" jets and float needles and replaced them with the cleaned originals, minus the fuel screws. Also, new passage plugs were installed. The bike fired up and idled fine, but still had some hesitation right off of idle. I was able to clean that up by turning in the fuel screws to about 3/8 to 1/2 turn open. That didn't seem right, so I removed one of the fuel screws and compared it to a stock one. The "kit" screw measured .034 next to the shoulder and .019 at the tip. The stock screw measured .030 and .023. That is why the "kit" screw had to be turned so far! I replaced all the fuel screw with the stock ones set at 1 3/4 turns out and now the bike runs beautiful. The restriction ratio is not comparable to the originals and caused a lot of unnecessary issues. Lesson learned, use oem or comparable quality jets only! I compared the ids of the slow jet and secondary main, and there was some differences there also. Any way, thanks for everyone's ideas and suggestions. Now I am loading the CBX two Z1s and a CB 750 into the toy hauler and we are heading south to find some warmer weather to ride the next couple of weeks. Again, thanks Mike

[steve murdoch icoa #5322]

Glad you got it sorted, Mike.
How about a pic or two of your CBX in the warm sun just to torture those of us in the north.

[daves79x]

A great testimony to the trials of using aftermarket carb parts. I hope many read this and heed the advice! Have a great trip!

Dave

[Rick Pope]

Well , several weeks have past since my ( snip ). Now I am loading the CBX two Z1s and a CB 750 into the toy hauler and we are heading south to find some warmer weather to ride the next couple of weeks. Again, thanks Mike

The Big Bend area of Texas has some great riding and the weather is nice. We've been in Terlingua for a week now.....

[cross]

A great testimony to the trials of using aftermarket carb parts. I hope many read this and heed the advice! Have a great trip!

Dave

How can I figure out if my float valves are original Keihin?
I do not see K stamped on them like I do on the jets.

Thank you
Sasha

[daves79x]

No 'K' on the Keihin float valves. The differences are pretty hard to describe. If you show me a photo of your float valve, I can tell you. Generally, the Keihin float valve body is a dull gray, compared to the shiny aftermarket ones. The spring-loaded needle is more rounded on the Keihin, aftermarket is pretty squared-off.

Dave