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1980 CM400T running lean/won't idle

Still no change at all in the refusal to idle, and the horrible intermittent bog/complete lack of power from 4000-6000 rpm is just as bad as ever.
I've run out of things to check or clean on mine, they seem perfect!
I don't have a way to measure advance right now, but I could always pick up a timing light to check it.

Thinking out load after reading this, and most of your original post…
Pilot/mixture screws are good?
Are you getting fuel?
Certain there’s no vacuum leak?
Certain the air cut & accel pump diaphragms are good - cracks in mine were very hard to see.
All carb gaskets are in place and seated?
Air filter is clean?
You said all ignition tests were good.
Don’t remember what you said about your compression test?
I don’t have an extra set of carbs to swap, sorry.
There’s no FI system I know of unless you build your own.
There are strobe apps which pulse the flashlight on your phone you may be able to use to check your timing?
I feel for you, there is a magic answer, you/we just haven’t found it yet. Stay on, you’ll get it.


Tom - 1982 CM450E
 
[Admin edit: thread was (accidentally?) closed for no apparent reason, now merged with new posts. Carry on :) ]

How strange, I do admit I was quite confused. Thanks for sorting that out!


Please explain how EFI will eliminate the ignition system and/or how a more complex electronic controls system will be less problematic or easier to diagnose/adjust/maintain.......???

Most (all?) aftermarket EFI systems include ignition control (megasquirt, microsquirt, various arduino projects, all standalone EMS's). An EFI system is dead simple compared to a CV carb and 1980's CDI...the whole system contains around 3 moving parts, and a simple microcontroller with some I/O circuitry to run it. There's really nothing at all to go wrong over time, or go wrong at all really...and when something does go wrong, the system will generally tell you what it is or at least indicate which system has a fault. A closed loop fuel map is very easy to tune, and one would simply mimic the OEM advance "map" for ignition (for starters, anyways). Really I don't see how it wouldn't be easier to diagnose/adjust/maintain than 40 year old carbs and mystery CDI's, especially since my experience with every old carb has been terrible, while my FI engines run perfectly all the time, regardless of weather or age.

After all, what's easier: hooking up a laptop and changing a few table cells, or spending an hour pulling the carbs to change jets, needles or set float height?


Thinking out load after reading this, and most of your original post…
Pilot/mixture screws are good?
Are you getting fuel?
Certain there’s no vacuum leak?
Certain the air cut & accel pump diaphragms are good - cracks in mine were very hard to see.
All carb gaskets are in place and seated?
Air filter is clean?
You said all ignition tests were good.
Don’t remember what you said about your compression test?
I don’t have an extra set of carbs to swap, sorry.
There’s no FI system I know of unless you build your own.
There are strobe apps which pulse the flashlight on your phone you may be able to use to check your timing?
I feel for you, there is a magic answer, you/we just haven’t found it yet. Stay on, you’ll get it.


Tom - 1982 CM450E

Thanks for the reply! I'll try to go line by line:

Pilot screws are new, along with o-rings, etc
I'm 95% sure there are no vacuum leaks, and least none I can detect with starting fluid or a little gasoline. The ONLY exception here is the throttle shaft, I do get some idle increase when I spray fluid at it...but that goes for both sides, not just the problem side.
Air cut and accel diaphragms are brand new, though I did just swap to the old air cuts with no change.
All carb gaskets (all one of them) are new and in place.
Air filter is brand new.
Compression does test low on my gauge, but I have no idea how accurate it is and both cylinders are exactly the same reading. Piston rings are brand new, cylinder was honed with a ball hone.
The app idea is interesting, might have to try one of them. Not sure how they sync up with the spark though, interesting...

I keep hoping for an aha! moment, where I find I made a dumb mistake or missed something, or something is obviously damaged somehow....but so far it is not forthcoming :(


I'm also looking into other, non-CV pumper carbs that I can potentially find new...something like a Keihin FCR or a Mikuni flat-slide pumper (I believe it's called a TM33?). I'm positive there must be another carb out there that works, just a question of figuring out what it came on. I know that a non-CV carb will not perform as well, but right now I'll settle for running at all!


Also, if anyone fancies themselves a carb rebuilding guru, I'd be happy to send the carbs out and pay someone to get them working! I'm happy to admit defeat here and break out the wallet, I only ask that you tell me what the issue is so I don't lay awake in bed wondering...
 
Also, if anyone fancies themselves a carb rebuilding guru, I'd be happy to send the carbs out and pay someone to get them working! I'm happy to admit defeat here and break out the wallet, I only ask that you tell me what the issue is so I don't lay awake in bed wondering...

First, I absolutely do not know how the thread got locked, LDR and I are generally the only ones who will do that though any of the Mods can but most are not here as often and don't get that involved... it had to have happened by accident during a menu click by one of us.

As to your thought above about the carbs - you do realize that LDR is THE resident expert on these carbs, right? He wrote the very in-depth rebuild thread that most follow on them when rebuilding. Whether or not he'd like to do your carbs for cash is debatable and of course, entirely up to him... :)
 
I'm also looking into other, non-CV pumper carbs that I can potentially find new...something like a Keihin FCR or a Mikuni flat-slide pumper (I believe it's called a TM33?). I'm positive there must be another carb out there that works, just a question of figuring out what it came on. I know that a non-CV carb will not perform as well, but right now I'll settle for running at all

There’s these:

Mikuni Carbs VM30 with everything needed to install
https://r.tapatalk.com/shareLink/to...hread.php?t=1676&share_type=t&link_source=app

Or these:

https://speedmotoco.com/honda-cm400-carburetor-kit-mikuni-vm30/

Or eBay. I haven’t looked.

I still keep thinking it’s something crazy simple, we’re all missing for whatever reason; I’d forgotten you had the top end apart. Or that it’s not carb related at all.
I thought someone on here was working on EFI and had a thread started? Or maybe it was electronic ignition. Anyhow, if you can manage to stick with it, I bet you find it(even if you have to pretend you just bought the bike and know nothing, and start over).


Tom - 1982 CM450E
 
I got to thinking, go back to basics: An engine needs fuel, air and spark to run. I have spark, I have air (cylinder runs on starting fluid sprayed into air box with filter out), so therefor I MUST be missing fuel. How could it be missing fuel, only at idle, if the tiny holes are clear (double and triple checked)? Well, those holes must get fuel from somewhere....so I looked at the very helpful diagram posted in the sticky, and determined to check the ONE jet that cannot be removed: the one under the rubber cap.

LO AND BEHOLD.....it's plugged solid, fml. Which explains at least some of my problems, so that's good...at least I can sleep at night, haha. Next question is...how on earth can I clean it?!? Blowing through it with compressed air doesn't do anything, so clearly something mechanical is needed, but I don't want to damage anything.

Progress, I guess?


There’s these:

Mikuni Carbs VM30 with everything needed to install
https://r.tapatalk.com/shareLink/to...hread.php?t=1676&share_type=t&link_source=app

Or these:

https://speedmotoco.com/honda-cm400-carburetor-kit-mikuni-vm30/

Or eBay. I haven’t looked.

I still keep thinking it’s something crazy simple, we’re all missing for whatever reason; I’d forgotten you had the top end apart. Or that it’s not carb related at all.
I thought someone on here was working on EFI and had a thread started? Or maybe it was electronic ignition. Anyhow, if you can manage to stick with it, I bet you find it(even if you have to pretend you just bought the bike and know nothing, and start over).


Tom - 1982 CM450E

Thanks for the info! I'll check those out for sure, I do love VM Mikunis from working on sleds, they're so simple and easy to work with. I did find a reasonably long thread about EFI with GSXR throttle bodies and a megasquirt, but I think that may have bumped into lack of stator power for the fuel pump and nothing came of it (at least not posted).
 
LO AND BEHOLD.....it's plugged solid, fml. Which explains at least some of my problems, so that's good...at least I can sleep at night, haha. Next question is...how on earth can I clean it?!? Blowing through it with compressed air doesn't do anything, so clearly something mechanical is needed, but I don't want to damage anything.

Progress, I guess?

Definitely progress! Man, I hope that’s it… isn’t this the jet you have to file down a screwdriver tip for to make an exact fit? I’ll have to check LDR’s carb bible sticky again, maybe it’s a press fit and I’m confused? No, please don’t damage anything at this point!


Tom - 1982 CM450E
 
Definitely progress! Man, I hope that’s it… isn’t this the jet you have to file down a screwdriver tip for to make an exact fit? I’ll have to check LDR’s carb bible sticky again, maybe it’s a press fit and I’m confused? No, please don’t damage anything at this point!


Tom - 1982 CM450E

It's the pressed in one under the rubber plug, the one that needs a special screwdriver is the one under the jet that screws into the carb body. I wish it was the other one, that one is removable! Looks like I might have to find another carb, but if that fixes it it's totally worth it.
 
Looks like I might have to find another carb, but if that fixes it it's totally worth it.

Figures. I’d wait on LDR myself, to be darn sure it can’t be cleaned/fixed. Although I completely understand why you may want a “new” carb at this point! Good luck.


Tom - 1982 CM450E
 
I managed to get the passage clean, with the help of some Acid Magic (muratic acid for pools) and a small, stiff wire.

Bike now idles perfectly! Carbs perfectly sync'd, mixture screws adjusted to about 2-3/4 turns. Idle set to around 1200 rpm, no issues at all.

HOWEVER.

Above 2000 rpm, the bikes starts to break up, and a 3000 rpm it hits a wall. It will NOT rev past it with any load on the bike whatsoever, and in fact one it hits said "wall" it barely runs at all until it gets back down to idle, where half the time it dies and takes significant cranking to restart. Then it idles fine again, and the cycle of idle -> 3000 rpm gentle acceleration -> falls on face and dies repeats. Downhill, I got it up to 5000 and it seemed as if the kill switch was hit, engine completely stopped firing and had to be restarted. The choke has no effect to speak of on this issue, which I assume is some quirk of CV carbs.

I did manage to try running it with the sync ports uncovered (forgot the caps) and had the same issue but not as bad...in that case I could coax the engine up past the 3-6k dead range and it would run ok. In that instance pumping the throttle helped, which I assume means it was lean (makes sense, if the ports were open!). With the caps on, the behavior is as above, the engine just stops at 3k.

I'm a little hesitant to label it an ignition issue, since when I can coax it up to speed the engine does RUN at higher rpms...it just REALLY doesn't want to get there.


So now it seems like out of the frying pan and into the fire...solve one issue, and a worse one pops up!
 
I managed to get the passage clean, with the help of some Acid Magic (muratic acid for pools) and a small, stiff wire.
Bike now idles perfectly! Carbs perfectly sync'd, mixture screws adjusted to about 2-3/4 turns. Idle set to around 1200 rpm, no issues at all.

Absolutely fantastic news, and CONGRATULATIONS!

HOWEVER.
I'm a little hesitant to label it an ignition issue, since when I can coax it up to speed the engine does RUN at higher rpms...it just REALLY doesn't want to get there. So now it seems like out of the frying pan and into the fire...solve one issue, and a worse one pops up!

Did you ever verify your timing? That's all I got except wait for LDR and others more knowing than I. Hoping this is simpler, whatever it is.
 
The air box boots, air box and air filter were all correctly in place and attached (much to my annoyance, since they're a royal pain to deal with).

I did run through all the ignition tests quite a while ago, back in the beginning when it wasn't running right...everything checked out within spec. I may recheck just for my own satisfaction, but I found no issues previously.
 
The pistons move freely with no sticking, and the needles are loose as well. I also flipped the carbs upside down and both pistons return to closed when depressed, though one spring seems a TINY bit stronger than the other...as I said though, both return to closed when upside down.
 
This info leads me to think you're starving for fuel. I would be looking at the emulsion tubes under the main jets and at the air bleeds in the top of the carbs. Passages as well. The rubber plug is installed on both carbs by the main jets, tight?
The only other thought is the timing is not advancing at all.
 
After last night's refusal to run, I pulled the carbs yet again and verified all passages are clean and open (including the ones going from the emulsion tubes to under the black cap). I swapped back to the new jets and needles, and tried again. Now it does run and get up to speed, but it seems to run lean (maybe?) at times and other times it just drops a cylinder entirely.

It will always run great under 4000 rpm, but above that it gets dicey - sometimes you have to coax it up with just a tiny bit of throttle, other times it will bog but slowly climb with the throttle at WOT. Once it gets above 6000, it runs decently but seems slightly rough, and starts breaking up and stuttering around 8500.

It WILL NOT cruise in top gear AT ALL, since top gear at 60mph puts it right around 6000...and there is a hole there, almost like the ignition shuts off from 6000-5000, then turns back on. If I try to decelerate through that range, or upshift into that range, the bike just bogs and dies and no amount of throttle, lack of throttle, choke or downshifting will get it fired up again until it gets under 4500 or so, then it comes back to life just fine. 5000-6000 is also the worst of the acceleration bog and cylinder dropping, really the bike just really hates being between 4000 and 6000 rpm no matter the conditions.

I'm not sure how much power this bike is supposed to have, but the fastest I got it to hit was around 75 down a long hill which seems pretty slow to me. I was also JUST able to accelerate from 30 to 65 up said hill, and it was really starting to struggle after 50.


Things I've checked/verified:
All ignition checks pass 100%, some readings exactly matching those in the sticky and elsewhere.
The carbs are as clean as I can get them unless there is some mystery passage somewhere that I don't know about - idle passages are clean, jets brand new, pistons totally free, caps sanded flat, etc.
Gas is fresh.
Tank is clean, verified with borescope.
Jets are in the correct locations and correct size.
Intake tract is assembled and intact
Plugs cleaned last night with wire brush and solvent.
Compression rechecked at 130 psi on my gauge, both cylinders identical readings.

Conditions today were about 80F and moderately humid.


Does this sound like a bad/failing CDI? It certainly seems that way, and I really wish I had a way to test it.
 
I picked up a timing light and 2 spark lights (the ones inline with the plug), so I'll check the ignition at some point.

However, I just picked up very nice '05 Rebel 250 for her, so this project is lower priority now. She did comment that the Rebel is smaller than the CM, and when you get them next to each other the Rebel really is tiny!


Thanks for all the help so far everyone, I really appreciate it.
 
Just spent an hour plus reading this entire thread and wonder if the actual issue was ever found and resolved. I must say this has to have been one heck of a learning experience for Galaxywide!
 
I picked up a timing light and 2 spark lights (the ones inline with the plug), so I'll check the ignition at some point.

However, I just picked up very nice '05 Rebel 250 for her, so this project is lower priority now. She did comment that the Rebel is smaller than the CM, and when you get them next to each other the Rebel really is tiny!


Thanks for all the help so far everyone, I really appreciate it.

Just spent an hour plus reading this entire thread and wonder if the actual issue was ever found and resolved. I must say this has to have been one heck of a learning experience for Galaxywide!

His post (above yours in this reply) is his last time seen here at VHT, so unless he comes back we'll never know. Seems like he found something more appropriate for his wife and the CM went on the back burner.
 
His post (above yours in this reply) is his last time seen here at VHT, so unless he comes back we'll never know. Seems like he found something more appropriate for his wife and the CM went on the back burner.

To Bad I would have loved to hear what the solution to this one might have been. As I mentioned a real "learning opportunity" now (apparently) lost.
 
To Bad I would have loved to hear what the solution to this one might have been. As I mentioned a real "learning opportunity" now (apparently) lost.

Maybe not, depends on if/when he comes back. If his alert settings are such that he gets notified when someone replies to one of his posts, he might come back to respond... and he did get another smaller Honda twin for his wife, so he might come back for that as well at some point. I do hate it when a thread goes unfinished though.
 
Sorry to leave everyone hanging like that! Unfortunately I'm no closer than I was last year, still refuses to run correctly. I didn't get much more testing done, since she got a bike with fewer issues (though now the front caliper is stuck, yay) and then the weather went cold. I checked the timing and spark, and everything checked out within spec...I continue to believe the ignition system is fine, and it's these ridiculous CV carbs that are giving me a hard time.

I've since moved to somewhere that's less easy to do test rides at, but I still really want to get this thing working right. It's such a perfect little bike and my wife absolutely loves it, just has to be reliable!

I did have a thought a while ago: all my testing has been done without the side panels/fairings attached, I wonder if somehow those might be affecting airflow in/around the carbs? It would be totally absurd if so, but at this point I'm ready to believe anything. So I may try putting them on and testing with no other changes, you never know I suppose.

I also remain interested in going to EFI, if I knew of some throttle bodies and injectors that would work I could easily figure out the electronics/tuning side. From the research I've done it seems that small individual throttle bodies are quite hard to come by, since most twins appear to us TBI and inline 4 TB's are usually integrated together.
 
Please refresh mine and other memories about the specific issues. Sounds like I had you check the ignition system to verify everything there was good so we have a good foundation to work from.
To do an EFI system you have to run an electric fuel pump and we're a little short on power there, 170w. However that's fixable by using a GL1000 stator rewound to produce 350w+. BUT this also means the stock CDI system gets scrapped because there's no CDI components in a GL stator. So now you need the Ignitech DCCDI-P2 module and you should change the voltage regulator as well since it charges at a higher rate than is good for batteries and electronics.
 
This is an old thread, but have you tried the "clear tube trick"? Basically, you take clear tube, put it on the drain of the carb, turn the gas on and loosen one of the drain screws. Hold it up against the carb and you will see the actual level in the fuel bowl. It should ideally be like a mm or two under where the gasket would meet the bowl. Make sure it's about the same for both sides. Are you using aftermarket float needles? If yes, get rid of them. I've wasted literally a full week in the evenings trying to make aftermarket carb parts work. They're all junk. I made some aftermarket float needles work before in a set of carbs but I had to adjust it so far out of book spec to make it work and verifying with the clear tube. The fact it idles well at idle but bogs a bit when you need the power makes me think it's the float height adjustment. Sometimes, you can try going down a big hill and see if the power comes back and/or trying to use the choke at the same time.
 
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