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Throttle response inconsistent, CM400T

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Mar 18, 2021
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Location
Pittsburgh, PA, USA
Hello!
Throttle sometimes seems unresponsive or weak but overall ok.
Carbs super clean, float height spot on, new boots, new exhaust gaskets from top to bottom, new plugs, air box on with new filter.

Any advice would be greatly appreciated.

Thanks
 
Haven’t gone there yet.
If I roll the throttle slowly everything seems fine. It’s when I roll it quickly that things get ugly. Seems to take a while to catch up.
I’ll try ignition test next.
Thanks.
 
Which carbs are we dealing with? VB21 or VB22?
It always helps to post the year with the model. OR you can add that to your signature line when you edit your profile
 
Sorry still learning.
VB22.
1980 CM400T.
Replaced all the gaskets and diaphragms.
Vacuum leak maybe?
Will do an ignition test today but I think maybe it’s carb related somehow?

Thanks everyone for the help.
 
Do both of the accelerator port spray when you twist the throttle fast? Those actually cover up the dead spot on hard accel.
Cleaning them is a booger since the outlet port is on the back side facing the engine.
 
Not all carbs had accelerator pumps but they all had air cut valves (the 1" diaphragms on the side of carb body)
These carbs really hate not having factory air box fitted. Blocked pilot jets are another issue on any bike left standing a few years
 
Will check accelerator pump tomorrow.
I did change the air pump and accelerator diaphragms, aftermarket.
Airbox is on, new filter, aftermarket.

Thanks again.
 
You did assemble air cut valves correctly? I've seen 'dozens' where the spring was in the wrong place holding things 'open'
BTW, I've never had a problem with the after market diaphragms for air curt valves or accelerator pump although accelerator pump pushrod length can be 'inconsistent'. I have had to shorten one or two and 'adjust' the cam arm once or twice where rod was ~2mm 'short' (many years ago, probably fixed by now?)
 
So I fine tuned the accelerator pump to make sure it was thoroughly pushing gas into the chamber. Bent the arms a little. Seems to work much better now.
I installed a fuel line filter and think it may be a bad idea. Seems like the fuel flow stops on occasion which killed the engine. The petcock is aftermarket so I don’t think it has a filter inside of it. Any suggestions? Lose the filter maybe?
 
In theory you still have the petcock filter, you have to take the petcock off to check by looking. If there is, remove it with a screw extractor since it'll be stuck in there with sediment. Clean it and reinstall. I know lots of people like the inline filters but I have never used one and the road bike has toured the lower 48 states twice now.
 
Yeah the first petcock had the additional filter on the bottom.
I removed the inline filter and switched to regular fuel line. Works like it should.
These bikes seem to handle pretty well.
Do they get passed 4500 RPMs? Seems like it struggles after that. Another issue?
 
The filter on 400~450 shuulod be a nylon mesh tunbe around 1/2" diameter about 3" long and fitted into bung in tank.
The OEM fuel tap was usually riveted together but I think some were similar to the earlier CB's with a filter bowl on the bottom.
The filter bowl was also a water trap.
The CM's had pretty 'slack' steering compared to the CB's. The only one I'm really familiar with is the CB250/400N.
The 250 was pretty overweight as it was basically a sleeved down 400.
The 400 did handle better than earlier bikes, they were designed by a different set of people
Honda pretty much moved everyone from motorcycle division to the automotive side in 1976~78 (also why some engines were such a disaster in what should have been trivial area;s, CX500 cam chain tensioners and water pump seals, CB250N generators, CB900 Cam chain tensioners,ALL early 'Com-Star' wheels, etc )
They should be able to rev way past red-line, 4,500 is where slide should be lifting.
Usual cause on the bikes with rubber diaphragms is the transition circuit makes motor run rich without the stock intake system (you can remove the filter foam but not the air-box assembly or even lid)
On later carbs with solid piston the inside of the vacuum chamber could just need a good cleaning (along with the piston part of slide)
They are also much more accurate on fuel delivery running a bit on the lean side even when set up 100% by the book
Have you checked the electronic advance with a timing light? I've never heard of a high speed coil failing but anything could be happening after30 years or so
The main reason 3 valve parallel twins are scrapped is failure of the generator/ignition system as there were no replacement for probably 35 years?
 
Do they get passed 4500 RPMs? Seems like it struggles after that. Another issue?
It should; mine has always accelerated well past 4500, even before I went through the carbs. I’d say it accelerates better from 4500 on up to redline. If your stumbling at 4500, I’d say your back to ignition/carbs. Darn.


Tom - 1982 CM450E / Midwest USA
 
The filter on 400~450 should be a nylon mesh tube around 1/2" diameter about 3" long and fitted into bung in tank.
The OEM fuel tap was usually riveted together but I think some were similar to the earlier CB's with a filter bowl on the bottom.
The filter bowl was also a water trap.
The CM's had pretty 'slack' steering compared to the CB's. The only one I'm really familiar with is the CB250/400N.
The 250 was pretty overweight as it was basically a sleeved down 400.
The 400 did handle better than earlier bikes, they were designed by a different set of people
Honda pretty much moved everyone from motorcycle division to the automotive side in 1976~78 (also why some engines were such a disaster in what should have been trivial areas, CX500 cam chain tensioners and water pump seals, CB250N generators, CB900 Cam chain tensioners, ALL early 'Com-Star' wheels, etc. )
They should be able to rev way past red-line, 4,500 is where slide should be lifting.
Usual cause on the bikes with rubber diaphragms is the transition circuit makes motor run rich without the stock intake system (you can remove the filter foam but not the air-box assembly or even lid)
On later carbs with solid piston the inside of the vacuum chamber could just need a good cleaning (along with the piston part of slide)
They are also much more accurate on fuel delivery running a bit on the lean side even when set up 100% by the book
Have you checked the electronic advance with a timing light? I've never heard of a high speed coil failing but anything could be happening after30 years or so
The main reason 3 valve parallel twins are scrapped is failure of the generator/ignition system as there were no replacement for probably 35 years?
The 1982 CM's came with the water trap type petcock, worth getting if one is needed, NOT the 85/86 CB450SC since those are vacuum operated.
CB's have less rake than the CM's which tightens up the steering nicely
Because the engine will rev way past redline I set my rev limiter to 10,500 which is still way past redline.
I have 3 bad stators here, all with a bad high speed coil.
Full advance of 43 degrees +/-2 should happen at 4500-5350 rpm but every one I've checked is hitting that at 6K plus which is probably due to age.
Rex's Speed Shop in the UK makes a nice replacement CDI or if you want to have control of the advance curve there's the Ignitech DCCDI-P2 which is also how to get a rev limiter.
 
Thank you all for the info.
Super helpful.
Jim, are you thinking it’s my CDI then? Is this something I can check before spending money on the part?
The ignitech seems cool but “tedious”. I just want to get her running properly I think. I miss those high revs.
 
Thank you all for the info.
Super helpful.
Jim, are you thinking it’s my CDI then? Is this something I can check before spending money on the part?
The ignitech seems cool but “tedious”. I just want to get her running properly I think. I miss those high revs.
I think it's very likely that the CDI is failing BUT run thru all these tests first to be sure everything else in the ignition circuit is good. https://www.vintagehondatwins.com/f...gnosis-for-1978-86-CB-CM-400-450-Manual-Trans
 
Interesting. I never kept up with developments on the 400/450 three valve motors after I left Honda dealers, had enough issues with Suzuki electrical systems to keep me busy - real bad regulators on almost everything so they usually got CB250N reg/rect (plus working on V-4 Honda's that no one else would or could do - I still hate VF750's and V-45 Magna's and don't care how 'good' the later models were)
 
Ok. Assuming I did this right.
Seems like more than one area is “off”.

Primary- 1.2
Secondary- nothing from each wire
Plug caps- 5.44, 5.17
Stator:
GR and Pink- 132.5
Blue & white- 81.8
L blue & brown- nothing
Ignition switches I think are all ok.
Alternator- all 1.5-1.6.

Seems like I’ll be spending more money.

Also, I noticed the other day that my side stand was out and didn’t kill the engine. Is there a possible connection here with the CDI? Possibly shorting out the CDI?

Thank you.
 
Replace the VOM battery unless it's new and retest, clean the spade connectors also.
No reading on the Brown and Lt. Blue means the engine won't even fire but we know it does.

There is no side stand switch on the manual transmission models, only the A model.
 
It’s a miracle but yes.
No reading on spark plug wires is odd.
Also, I’m confused about the colors of the wires for the primary pickup coil. They also seem to be green and pink. Unless they’re faded and only appear that way. Couldn’t get a reading either so new battery for DMM and try again.
At some point, I’m probably just going to replace all the ignition components anyway. Give it an upgrade.
 
Ok, new readings with a brand new, hopefully better, multimeter.

Coil: primary - .7
secondary- still no reading
Plug caps - 5.44, 5.17
Stator: green pink - 133.7
Brown Lt blu - 211
white blue - 82.3
Ignition switches: ign on, run- OL
ign on, off- OL
ign off, off- OL
on- OL
Alternator - .9, 1, 1.1

What are your thoughts?

Thank you!
 
I have both spark plug caps off each wire, then a probe is inserted into each wire simultaneously.
I find it odd there is no reading.
Assuming I’m doing it right.
 
You're doing it correctly. Trim a 1/4" off each plug wire and retest. Be sure you VOM is set for 10K or higher since factory spec on the stock coil is up to 8.8K ohms and if it's aftermarket it could exceed 10K easily.
Your stator is good.
Coil is a question mark
CDI ground circuit is bad.

I've edited terminology
Ignition switches: Ign on, Run on- OL ​ Good
Ign on,
run off/Kill on- OL Bad
Ign off,
run off/Kill on- OL Bad
on- OL Bad

So start the test with Ignition and Run/Kill switches set to Off and Kill. This should give close to zero resistance.
Turn the Ignition to ON with Run/Kill in the Kill position. This should give you close to zero resistance
Turn the Ignition OFF and Run/Kill to the Run position. This should give you close to zero resistance
Set the Ignition On and Run/Kill to Run. This should give you OL or infinite resistance

 
Well, it’s starting to sound like I should just begin upgrading all these electrical components.
I think I’ll try and go the Ignitech, GM and GL1000 route.
 
Thanks again for all the help.
Lastly, what’s the bare minimum required to get my bike running well?
If I replace the coils and CDI for now, will that be enough?
 
Just an update.
Replaced the coil to a GM coil (ACDelco) and it definitely makes a difference.
Although, bike still won’t rev passed 4500-5000 RPMS.
Next step, new CDI.
Debating between Rex’s and Ignitech.
I’ll try and post photos in the near future.
 
Curious if you used aftermarket AC type coils vs the real McCoy AC Delco 10472401 / D555 one? I'm considering this change.
Alt
 
Winters are long and depressing here but it’s coming around so I need to get things back up and running.
My problem before was the bike wouldn’t run over 4500 RPM. This is what led me to doing an ignition test, replacing the coil and debating on whether or not to upgrade the CDI. BUT, I noticed one thing I did wrong when assembling the carbs- I actually put the needle jet holder into the carb’s venturi side instead of inside the main jet. I’m thinking this may be what was causing my issues. I’m assuming that the needle would raise but there wasn’t enough fuel flowing into the venturi for the bike to reach higher RPMs. If this solves my problem, great. Otherwise, I may be looking at a new CDI still.
 
I purchased an ACDelco Professional E530C Ignition Coil. I don’t know if it’s the best, but it was definitely an improvement over stock.
 
Sounds good to me!
@LongDistanceRider (or anybody), can I ask what your thoughts are on my earlier post concerning the needle jet holder? Not sure if that would cause the problem or if I’m back to CDI issue.
 
Sounds good to me!
@LongDistanceRider (or anybody), can I ask what your thoughts are on my earlier post concerning the needle jet holder? Not sure if that would cause the problem or if I’m back to CDI issue.
I'm not sure what you did there. For the jet holder you've got the nozzle that protrudes into the venturi, concave side up, which is held in place by the jet holder/emulsion tube and the secondary main jet screws into that.


Clarification edit: The needle jet (tubular part) installs from the bottom and is pressed/held up into place by the jet holder/emulsion tube ......
The internally chamfered (concave) side must face up at the needle and slide in the completed assembly......66
 
Last edited by a moderator:
Right.
But, instead of having it inside the emulsion tube, I had it outside the emulsion tube placed in the hole that’s located in the venturi. So the needle was still going into the holder but the holder was extended too far up which would probably restrict the amount of fuel being released.
 
The needle nozzle does go in 1st and does protrude into the venturi
This is the Primary Jet emulsion tube that also protrudes into the venturi and fits the hole right below it in the picture.
201_4736.jpg

Here you can see the needle nozzle protruding
201_4705.jpg
 
Yes! Thank you for the super helpful photos.
Previously, I incorrectly pressed the nozzle into the hole from the venturi side. This could be why my bike would bog around 4500 RPM. We’ll see once the weather turns.
 
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