Honda Twins banner

1 - 17 of 17 Posts

·
Registered
Joined
·
12 Posts
Discussion Starter · #1 ·
I'm in the process of rebuilding a set of VB21A's for an 81 CB400T. I have two sets of VB22's with the accelerator pump, however they're in worse condition and the pump diaphragms are beyond saving.

I'm not making power right off of idle to mid-range. Once I break into the higher RPM's it pulls hard. It will idle without issue and the engine can rev fine with quick blips, however it will bog down if I give it more than 1/4 throttle. My tach doesn't work at the moment, so I can't give accurate RPM figures.

I've completely dis-assembled the carbs, brand new jets (in the correct locations), gaskets, needle valves - All of them are OEM replacements from Honda. The pistons slide smoothly. Carb sat in carb cleaner, all passages blown out with compressed air. New plugs, NGK N7EA (cold-ish weather up here) and fresh 93 grade gas. I have a stock airbox with a new mesh OEM filter. Mixture valve adjusted between 1.5 to 2.5 turns out without a change. No external vacuum leaks - checked with brake cleaner and didn't get any RPM rises. It revs up a good bit when I apply choke. If I hold my hand over some of the intake scoop on the top it will rev much better, so I think it's a lean condition on the low end. The plugs are slightly ashy and white at the tips, but otherwise look good. Float levels look OK, and it wouldn't be fueling properly up high if that weren't the case, correct?

Any suggestions? Are there some less-than-obvious air/fuel passages I might have missed?
 

·
Registered
Joined
·
12 Posts
Discussion Starter · #2 ·
Was able to grab a picture of the plugs. These were the old ones, the 8's instead of the 7's, however the new ones look similar.

Lower one is LH, upper one is RH.

20181218_094933.jpg
 

·
Registered
Joined
·
805 Posts
I've had the same experience, I ultrasonic cleaned the carbs and replaced every o-ring/diaphragm and still had a 3500rpm flat spot. Cleaning/replacing rubber made it a lot better, but that stumble was still there. It was worse when cold, warmed up it was better.

The advice I've seen is to sync the carbs, then up the jets (primary or secondary I can't remember, whichever is around 70) by one size. Go from 70 to 72 or 68 to 70. I haven't run my bike yet with my 72s but I'm hoping that with a carb synch I will have no flat spot
 

·
Premium Member
Joined
·
10,203 Posts
The primary main jet (smaller of the two, located closer to engine) is the one it is usually recommended be increased. With the VB21 carbs you might start with 75s.

Before syncing the carbs you need to properly adjust the idle mixture screws. Set idle speed at 1200 +/- 100. Adjust one mixture screw to achieve highest rpm. Reset idle speed. Adjust opposite mixture screw.

Since you won't have the accelerator pump you might improve acceleration off idle by setting the mixtures 1/4 turn or so richer (counterclockwise or out) than the above settings
 

·
Registered
Joined
·
12 Posts
Discussion Starter · #5 ·
Welp, took the carbs apart again. Got an ultrasonic cleaner on its way, should get here today. Turns out the jets are already 75 and 110. If this doesn't do it, I'm going to start going after the engine, it's possible something there could be causing it.
 

Attachments

·
Premium Member
Joined
·
24,208 Posts
I'd suggest a 75/115 jetting. I'd also suggest going thru the ignition diagnosis found in the Electrical section
 

·
Registered
Joined
·
12 Posts
Discussion Starter · #7 ·
Pulled apart the carbs again, did a few sessions in the US cleaner and made sure I cleared all passageways with compressed air. I was able to get a video of what it's doing. This was after letting it warm up for a few mins. The tachometer doesn't work, so I can't give any accurate RPMS. It'll start up no issue, idle just fine, quick blips off of idle are fine, however it sounds like it's fuel starving in the mid range. It'll get through it and the top end feels good. I blocked off some of the airbox intake to "richen" the mixture some and it'll rev through it.


With no RPM reference, I adjusted the mixture by ear as best as I could. They're both sitting ~2 turns out. The jets are 75/110 on both carbs.

I'll start doing CDI troubleshooting per longdistanceriders suggestion. I'll see about sourcing some 115 jets too.
 

·
Registered
Joined
·
12 Posts
Discussion Starter · #8 ·
So I cleaned up the original VB22's on a whim, threw them back in and they work great. I'm not sure what's up with the VB21's, but whatever. I swapped in different mixture needles so they don't hit the float bowl.

Never figured out what's up with the 21's, but oh well.
 

·
Premium Member
Joined
·
24,208 Posts
Oh, those. I just cut the tab off the float bowl and retain the know tab, gives me an easy reference point for each screw when adjusting
 

·
Registered
Joined
·
12 Posts
Discussion Starter · #12 ·
Per chance, does anyone know what the individual part #'s are for the O-Rings in the gasket kits? I just need the two that go on the fuel crossover tube for the VB21's. I would have to drop $65 for two gasket kits, but I only need those two O-Rings.
 

·
Registered
Joined
·
805 Posts
JIS P6 size or 1.9 x 5.8 mm, you can just order some or see if a local plumbing supply place has them. Just make sure they are nitrile or viton
 

·
Registered
Joined
·
12 Posts
Discussion Starter · #15 ·
I've run into more issues, even with the VB22's. It'll only idle on the right cyl with the choke in. With it pulled out, it'll idle really high on both ~4000. It takes forever to warm up, but once it does it runs fine except at idle. Ignition system diagnostics went good, nothing out of tolerance. Swapped the leads and still only idles on right. Left side plug was very sooty, right plug looked good. Messing with the mixture screw didnt affect it. Next step is to check valve clearance I guess. I got a compression tester coming in too. Engine only has 11k miles on it, hard to imagine the engine is bad.
 

·
Premium Member
Joined
·
24,208 Posts
I'd suggest that the sync of the carbs needs to be checked, sounds like the right side is open while the left side throttle plate is closed. Check that the adjuster screw is inserted properly in the link, there are 2 washers that go above and below the link.
 

·
Registered
Joined
·
805 Posts
I had the same thing once after reinstalling my carbs, one of the 40 year old boots hadn't survived the re-installation and had a big split. It was bad enough that spraying wd40 around the boots increased the idle.
 
1 - 17 of 17 Posts
Top