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1981 CM400T
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Discussion Starter · #1 ·
Don't know where to start it, in carb or electric?

..long story short got some working/checking this weekend on the bike, checked compression around 150, oil/filter changed, valves set 10m in/14 mm ex, throttle cables changed, carbs cleaned and synced (pain in the arse), fuel lines/inline filter changed, plugs new (DR8EA?), getting good spark, new air filter, cam chain tightening...
been busy :)
got some manometers from fleabay - https://www.ebay.com/itm/New-Motorc...e=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649
and when syncing them, got reading for both carbs around 100 (left red margin) saying late timing!?..don't know if they work (seems pretty low) however seems they do the job as the engine is pretty steady now..couldn't tighten the 8mm screw very well when syncing :/ damn to tight to wrench :)
so hopefully they are in sync and wont come loose soon..

the problem is, there's a slight backfire through the left carb on regular intervals, my dad ex mech (helped me a bunch in the process) says it's the timing that should be set, hearing is a little late but now I read that timing cannot be set on these bikes, can it!?
don't have a strob lamp or so to check..
I think the air/fuel mix is set good, valves set per spec, checked twice, runs great, idles a little bit erratic at 1200 and between 4,500-5,300 RPM some hesitation, after that awesome... all in all better then before..
I made like 200km yesterday, was a nice ride..noticed when climbing hills (cca 4000 feet) that there was a lack of power!? mixture due to altitude maybe?also the idle RPMs increased when stopped on toll pay


SO my question really is, should I go through full electric check (CDI) or try to adjust the mixture better!? Or something else?
Maybe the carb idle circuit was not cleaned as it should be - comes to mind...
This timing issue is bothering me most as the manometers showed 100 x carb (should be 200-240 right?) plus this coughing through carb (heard especially on idle)
Thanks, cheers
I like to keep it cruising in 5th on 4tRPm at around 65 km/h - runs smoothest :))
 

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Just for piece of mind I would do a full electric check although you cant check the CDI box. As for tightening the nut on the carb sync when using manometers on my 400a I take off the left top motor mount and when they are set shut down the motor and go to full throttle then you can reach the nut with a 1/4 inch socket with a 6 inch extension. Yes a pain in the butt.

BIll
 

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Sorry, meant to say right side top motor mount. Also meant 1/4 inch drive socket . To early in the morning.

Bill
 

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It's true that there is no timing adjustment. It is possible for the key in the crankshaft to shear allowing the stator to rotate causing late timing, but not real common. Ignition advance can fail causing loss of power, you need a timing light to check that and, since power picks up beyond mid-range, that's probably not the problem.

Don't get too wrapped up about the numbers on the manometer, what's important is that they are even. Now that they're synced, adjust idle speed to 1200 (+/- 100) rpm. Adjust one mixture screw to achieve highest rpm. Re-set idle speed. Adjust opposite mixture screw to achieve highest rpm. Repeat. You'll need to remove the tabs on the mixture screws to do this.

You might want to take a look at this:

https://www.hondatwins.net/forums/63-fuel-supply-carburation/24191-rebuilding-vbulletin-carbs.html
 

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1981 CM400T
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Discussion Starter · #5 ·
HEh, been through this magnificent stickie when cleaning carbs, wouldn't make it without it..the mixture screws are 1 and 1/2 turn out, as per spec, maybe some 1/4 more, didn't want to mess too much with it, sounds right like it is :) ..power is good so hopefully is not the ignition..this coughing through left carb doesn't seem right when on idle, though..it's like it's gonna jump out of the boot sometimes :) idles ok, then this regular interval cough destabilizes it..maybe an air leak, but have checked for leaks before, no change in RPMs so I'm positive there aren't any..will check again..
also, i had some hard deceleration backfires before, now is waaay better after syincing..not gone but still
I think I'm gonna do the CDI test longdistancerider put together and just in case clean those old contacts/ground with contact cleaner...had to deal with the 8mm sync screw once again :) hate it..
 

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Consider the "turns out" spec as a mixture starting point and set the mixtures as described by J-T. Carb/intake backfire is related to too lean a mixture usually from a vacuum leak. I'm assuming that those adapters were removed after the sync and the screws put back in. That leaves the carb insulators, insulator O-rings or the carbs not fully installed in them. The carbs bottom out on the insulators when fully installed. The way the insulators leak isn't really detectable with carb spray or propane, it's at the rubber to mounting plate bonding surface. The O-ring can be detected.
Decel backfire is inherent to the beast but very minimal IF the mixture is rich enough, not set lean, and the air cut valves are operational.
Running the ignition tests takes 30 minutes at most and insures that system is correct with the exception of the CDI unit. CDI failures are related to loss of power in 5th gear or no spark.
 

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1981 CM400T
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Discussion Starter · #7 ·
Actually, I left the vacuum adapter on the left carb (cylinder) and put the screw on the right, the left one with the adapter is doing the cough...i think that should not be the problem/leak! I know people leave it on like that, no prob..i will put the screw back in just in case and re-check..and will re-tighten the boot once again (they are practicaly new, still gummy :))..and richen up the left mix a bit....there is spark and power on higher rpms so I hope it's not the CDI..the "Full advance" interval 4500-5300 is the hesitation zone,,,will be back with results after the test..dont have the strobe though to check the marks but do have a multimeter...just to be sure, basically, i run it like this: just put one probe in one wire and the other in the other (as per instructions) and check what is giving in ohm range, right?..where do I want the meter to be set, in kohm or !? raining these days so there will be time to check, any sunny period I try to take it out :)) thanks, toni
 

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2 of the stator readings are done in the less than 500 ohm scale and the 3rd is in the 1K scale. It really depends on the scales your meter has, they're all different to some degree. You're correct in the use of the probes to the wires.
Those vacuum cap plugs are known to cause vacuum leaks. If you want to leave both adapters in place then use a very tight fitting hose between them with a ball bearing inside the hose.
 

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1981 CM400T
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Discussion Starter · #9 ·
SO, went for a ride yesterday, the cough persisted and another thing happened that I haven't noticed before, idle RPMs were rising!, so at that point was certain there must be an air leak...went for a check with starter, and there you go, seems the airbox boot of this carb is leaking air, the rubber edge looks bad under the clamp...missed that before!!so have to find a way to seal it properly, since there are no spare parts in my country for this motorcycle (I'm the only owner of a CM400 in this area) I have to check on fleabay or patch it somehow (which I know is a temporarily solution) :/ hopefully will get rid of the problem....will do the CDI test anyway..happy is not the ignition though
 

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SO, went for a ride yesterday, the cough persisted and another thing happened that I haven't noticed before, idle RPMs were rising!, so at that point was certain there must be an air leak...went for a check with starter, and there you go, seems the airbox boot of this carb is leaking air, the rubber edge looks bad under the clamp...missed that before!!so have to find a way to seal it properly, since there are no spare parts in my country for this motorcycle (I'm the only owner of a CM400 in this area) I have to check on fleabay or patch it somehow (which I know is a temporarily solution) :/ hopefully will get rid of the problem....will do the CDI test anyway..happy is not the ignition though
Since you have no access to parts where you are, you could check with your local scuba supply store for solutions . Wet suits are made of rubber .
You may have a leak between the head and the carb insulator causing those issues.
Also, just for comparison, I use the 17 tooth sprocket instead of the stock 16 tooth sprocket and at 4000 rpm in 4th gear, I'm going 60 kph.
 

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1981 CM400T
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Discussion Starter · #11 ·
..one question, should I test the electric on cold or warm engine? i took off the left cover but the wires are so dirty i can't see what colour they are :) have to clean everything first..thanks, t
 

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1981 CM400T
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Discussion Starter · #13 · (Edited)
So tested only these stator parameters (not fully cold) and got these in red:

"specs for the stator tests
Pink to Green 129 Ohms - 129
Blue to White 84.2 Ohms - 93
Brown to Lt. Blue 203 Ohms - 227
"

is this bad? as I read that only +-1 Ohm within specs is okay!?
As per manual Ignition chapter, there are some instructions how to check it and what are the ranges that readings should fit in..something different than these EXACT ones in the sticky..
will do the rest of the test on bad weather days, now it's time to ride it and seems all ok with the ignition..
 
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