Mid-range flatness or stumble

[BillB]

A new to me very low mileage 1981 CBX has a problem I have not had to deal with before. The bike starts and idles perfectly. Remarkably so. The previous owner always ran the carbs dry after each ride. Also, the response to rapidly opening the throttle under no load seems good and it returns to a rock stable idle extremely quickly. Remarkably so. Maybe the spin-up is not quite as fast as my other CBXs, but it's not bad. HOWEVER, when I head out for a ride there is a pronounced hesitation or flat spot as I gradually increase the throttle through the middle RPM range, say to about 3k, then it rather suddenly seems to have full power and pulls forward like it should. It might be mostly just a delay before it finally takes off. It makes the bike hard to ride smoothly.

Does this point to anything specific? Accelerator pump? Air valve? Something else? I posted her because I think it is probably carb-related. It has a stock air filter. In fact, the bike is bone stock and in near new condition due to dry storage and very low miles. It's almost as if the carbs have a plugged mid-range jet, but the CBX doesn't have such a jet, right?

TIA,

[daves79x]

A quick thing to check is the ignition advancer. If it is sticky, you can see what you're seeing. However, it would be correspondingly slow to spool down also.

As far as the carbs go, the accelerator pump likely is non-functional at this point. But most CBXs have poor or non-working pumps at this point unless the carbs have just been rebuilt, and most run OK that way. I can tell you that your bike will benefit from a complete carb rebuild. If they have never been touched, the o-rings are on borrowed time, at least.

Dave

[Larry Zimmer]

Another thought: Pull the vacuum pots off the tops of the carbs. Looking for 'gummy' stuff inside that could be slowing the slide piston response. Dave -- is this a possibility? And, certainly on the orings. Especially if they have seen any heat, even in storage.

[BillB]

Thanks guys. I'll let you know what I find. I should get into the carbs this weekend. This bike has only 3070 miles and that appears to be real from what I have seen. All the finishes are like new except for some storage dings. All the rubber I have seen so far looks pristine and pliable from dry storage and almost no heat exposure (super low use). It has a leaky valve cover gasket at the cam caps. No surprise there. I don't think the valves have ever been adjusted. It runs quietly and smoothly except for the dead area on acceleration.

[CBX-tras]

My 1100F, totally stock, had a flat spot mid range when I got it. Was still there after I rebuilt the rack.
I shimmed the needles 0.5mm and it performed way better. Shimmed them another 0.5 for a total of 1.0mm and it became a rocket.

However, ALL of my CBX's (several) never needed the needles shimmed. But, yours' might.
Make sure that everything else is functioning as designed. I've seen carb issues disguised as ignition and ignition issues disguised as carburetion.
Also, your '81 SHOULD have 2 circuit carbs but, check the body numbers to see if they have been swapped out to the earlier 3 circuit carbs. It wouldn't be the first time things didn't match up.
Last but not least: Check the clamps on the carb boots. If you just push on the screw head with a driver, they should NOT rotate. If you tighten them and they butt, end to end, they have shrunk and are leaking, replacement required.

[BillB]

I just received a logbook from the previous owner. Mike Nixon rebuilt the carbs 5 years ago. I still plan to inspect them, but that makes some things less likely.

[Larry Zimmer]

I might just comment of my own experience with my 82. Stock in every respect. Carbs have been rebuilt. (Pump works very fine!) It has always had a noticeable 'flat' spot between 3000 and 3400. Years back it had me thinking that I was unknowingly backing off the throttle when riding in that speed range. Finally realized that it just had a flat spot there. Just enough to notice it when cruising. I don't notice it when accelerating up through that range. Don't know why it's there. Just live with it.

[bobcat]

Remove the pilot/slow jets and drill with a #79 drill bit by hand. Then set the mixture screws at 1.5 to 2 turns
out from gently seated. That should take care of the low throttle cruise flat spot.

With that low mileage it is not likely the cam chains have been tensioned. It's the first few thousand miles that
most of the cam chain stretch takes place. Don't wait until you hear a rattle to adjust them. Happy trails !

[Jebbysan]

I am dealing with the exact same issue with a 1980 engine I just built.....
It has 79' cams, 79' advancer (422) that is like new, .005 lash on all, fresh NPR rings that sealed right up, 135-138psi cold , new A-Tensioner and cam chains from Tim's,.....carbs have the stock 110 main, I went 38 on the slow jet.....2.5 turns out from stop, 15.5mm float height just touching the spring shock, slides are free and in good shape....carbs are clean-clean...when the slow jets were out I managed to get everything out of that circuit....everything sprays out nice, all 6 squirters are functioning.....timing set engine off 10 degrees BTDC, has Dyna Ignition plate and coils, fresh battery.....intake rubber boots soft and pliable.

Same deal...60 degrees F, fires right up stone cold with the choke.....when warm free revs nice and falls nice, sync is very close, 1000rpm no problem....nice burble.....throttle blip super responsive but struggles at 4k....then revs past it......
On the road the problem is pronounced of course.....falls on its face at 4k under load....and will pull past it if coaxed.....just doesn't want to lift the slide......lean break-up...
I have narrowed it down to shimming the needles...they are 55a needles which I am not sure is stock.....
My question to others that have shimmed is how much difference does .020 of shim make?
If I am really stumbling...should I shim .040? I know I should just go try it and see but maybe I can do this once? LOL.....

Thanks,
Jebby

[Jeff Bennetts]

Jebby did you verify that the Dyna Ignition plate is in faze? Pulling 1 or 6 plug, ground it, ignition on, spin the crank slowly and make sure the plug fires at the ‘F’ mark on the split of the cases. Those CBX Dyna ignitions are known to fire early, you may need to slot the plate to get the adjustment you need, then you can set the timing again with a timing gun.

[Jebbysan]

Jebby did you verify that the Dyna Ignition plate is in faze? Pulling 1 or 6 plug, ground it, ignition on, spin the crank slowly and make sure the plug fires at the ‘F’ mark on the split of the cases. Those CBX Dyna ignitions are known to fire early, you may need to slot the plate to get the adjustment you need, then you can set the timing again with a timing gun.

What I did there to phase it was with the key on, I had a Milton spark tester on #6....as 1 and 6 fire together, and had the mark on F....which is 10 BTDC, and with the plate loose I rotated it back and forth until I saw the spark on the tester.....rotated it back and forth a good dozen times until I found the "sweet spot" (you come 'at' it counter clockwise to light it off) then tightened it down.....this has got to be close 10 BTDC +- 2 degrees....but I think it is closer than that.....but yes, I should check it with a light to verify the advancer is pulling the curve....one thing that struck me as odd, the 79' advancer is 10 degrees initial/31 degrees total....the 80' advancer is 10 degrees initial/ 41 total......strange that Honda would call out 10 more degrees total but pull it much slower....unless there is something I don't know....the engines are basically the same.....

I noticed after my first fire up that it didn't like to not have the foam filter in it......that cleaned it up a LOT.....so my feeling is that it needs fuel at the transition to the slide.....
I went ahead last night and shimmed all of the needles .040 but ran out of time.....I will fire it tonight and post results here.

Jebby

[daves79x]

.040 is probably too much. .025 makes a huge difference. The washers on the pilot screws are perfect for shimming. But see what you get. It's tough, but you can get the domes off with the carbs in place - is that what you did? 55A needles are correct for the VB62-64 carbs. Did you do a plug check to be sure it's not a rich stumble? With an all stock engine, intake and exhaust, I've found that #38 is a bit much and 2.5 turns out is also. Also hate to ask this, but you did not install rubber plugs in the idle jet towers? Don't laugh - I've seen it done.

All that said, you should not be having that much of a(or any) flat spot with stock (plus #38 idle jet) jetting. I'd check your advance with a timing light. You verified that the advancer is assembled correctly with both springs attached?

Did you try holding the choke on while testing the flat spot? Might give you a clue.

Back to the Dyna - these are notorious for intermittently dropping a pulse unit. Obviously you lose two cylinders briefly. No chance that's it? If your shimming test does not produce any results, I'd go back to the stock ignition and see what you get. I've pulled at least three Dynas and gone back to stock.

Dave

[Jebbysan]

.040 is probably too much. .025 makes a huge difference. The washers on the pilot screws are perfect for shimming. But see what you get. It's tough, but you can get the domes off with the carbs in place - is that what you did? 55A needles are correct for the VB62-64 carbs. Did you do a plug check to be sure it's not a rich stumble? With an all stock engine, intake and exhaust, I've found that #38 is a bit much and 2.5 turns out is also. Also hate to ask this, but you did not install rubber plugs in the idle jet towers? Don't laugh - I've seen it done.

All that said, you should not be having that much of a(or any) flat spot with stock (plus #38 idle jet) jetting. I'd check your advance with a timing light. You verified that the advancer is assembled correctly with both springs attached?

Did you try holding the choke on while testing the flat spot? Might give you a clue.

Back to the Dyna - these are notorious for intermittently dropping a pulse unit. Obviously you lose two cylinders briefly. No chance that's it? If your shimming test does not produce any results, I'd go back to the stock ignition and see what you get. I've pulled at least three Dynas and gone back to stock.

Dave

I used the pilot screw washers....I just lowered the engine a bit to get the caps off.....the 3 bolt removable frame tab is in the way of everything anyway, getting to the screws is the bitch....but I managed....been working on these a long time...maybe too long LOL....plugs are definitely on the lean side.....brainfart on the choke during the ride.....
Advancer is in like new condition.....springs intact and pull well....
No well plugs...LOL....79' only.....
Swapping out the Dyna could happen.....but this is not my machine and I think maybe the owner kept the stock stuff.....are you saying Dave that the triggers can go intermittent at high RPM's? I know it isn't under 4000rpm as the darn thing really runs smooth and torquey....it feels right.....
Shooting the stock pipes with an infrared shows 175 degrees amongst all......which is a popcorn number as the infrared doesn't like chrome but I am more concerned on consistency across cylinders.....

I am going to run this tonight at the .040 shim and see how it is......it just feels like the slides won't lift.....I will then set timing with a light.....
To do that I will make two marks, one on the rotor and one on the pulser or a stationary line lined on the case....might even make a quick pointer.....

I will post some results here. Sorry if I hijacked the thread but the problem I am having is pertinent to the discussion.....

Thanks for the reply.

Jebby

[daves79x]

I've seen two Dynas that would intermittently lose one pulser, and it eventually goes dead. Obviously turns the bike into a four cylinder briefly. But I think you would feel that.

Dave

[Jebbysan]

I've seen two Dynas that would intermittently lose one pulser, and it eventually goes dead. Obviously turns the bike into a four cylinder briefly. But I think you would feel that.

Dave

Yeah...and it would try to rev out as a four cylinder too....
I will see what happens here in a few hours.

Jebby