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1980 Honda CM400T
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Discussion Starter · #1 · (Edited)
Howdy.

I recently purchased a 1980 CM400T as a project while I have some downtime from work. The bike was ok when I bought it, I ran through the "Basic Checklist for that new-to-you old bike" published by longdistancerider. Specifically the carbs were disassembled, cleaned, and reassembled with new jets and gaskets. After about 300 miles of riding, the bike runs well, idles smooth, and has great throttle response over the whole RPM range.

After calculating my fuel economy at about 29 mpg (averaging 55-65 mph) I discovered the bike is running rich. The spark plugs tips are black. The bike has a noticeable deceleration backfire. I executed the idle-drop mixture adjustment procedure in the service manual to the best of my abilities. The right cylinder seemed responsive in this process, the left did not. The bike sits happily now at what seems about 3/4 turn out, as opposed to the factory 2 turns out setting, though the backfire won't go away. Unsure on how this adjustment will affect mpg until further testing completed.

Any suggestions on trouble shooting backfire and lower-than-normal MPG due to rich mixture? Anything helps. Thanks in advance. Happy to be part of the group.
 

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Also for the rich running as they control the air bleed for the main emulsion tube, they are an economy diaphragm/power diaphragm.....

How are the rubber bungs in the float chamber/main body of carb ?

I assume float hieght/fuel level is correct and air filter element is not over oiled, and, no rag under the seat ?
 

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The right cylinder seemed responsive in this process, the left did not.
400T calls for 185psi. Circuit still shows it's clogged.

The bike sits happily now at what seems about 3/4 turn out, as opposed to the factory 2 turns out setting
I did not design the carb, nor the 'initial' turn out. But I do not go off on my own if the bike is bone stock.

though the backfire won't go away. Unsure on how this adjustment will affect mpg until further testing completed.
I've experienced backfire and it's more a combo of sync and lean-drop. Every intake stroke, no matter the rpm, all circuits are having a blend of all 3 circuits of gas moving out of those holes. So just the lean drop alone out of spec will effect the mileage. Valve set is where? Compression is what? Sync is how off? New jets are larger?
 

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Discussion Starter · #7 · (Edited)
Also for the rich running as they control the air bleed for the main emulsion tube, they are an economy diaphragm/power diaphragm.....

How are the rubber bungs in the float chamber/main body of carb ?

I assume float hieght/fuel level is correct and air filter element is not over oiled, and, no rag under the seat ?
Checked the air cut valves and they're actually in pretty good shape. Rubber still malleable, no holes/tears/etc. The rubber bungs were replaced during the rebuild.

Air filter currently is not oiled, I'll consult the manual on how to do that. There was a rag under the seat I just removed and after 30 miles test ride didn't seem to make a huge difference.

Float level is another question. I thought I had them properly adjusted. I used the clear tube method using the drain on the float bowl to determine the fuel level in the bowl. The following picture describes the fuel level in the bowl with respect to the top of the bowl.

Light Automotive tire Gas Machine Automotive wheel system


maybe a tad high? I’m not sure. If it is that’s probably what’s causing the problem.
400T calls for 185psi. Circuit still shows it's clogged.


I did not design the carb, nor the 'initial' turn out. But I do not go off on my own if the bike is bone stock.


I've experienced backfire and it's more a combo of sync and lean-drop. Every intake stroke, no matter the rpm, all circuits are having a blend of all 3 circuits of gas moving out of those holes. So just the lean drop alone out of spec will effect the mileage. Valve set is where? Compression is what? Sync is how off? New jets are larger?
Valves are set within spec, i'd have to pull the manual to tell you exactly the spacing. Compression is unknown but doesn't seem to be a huge issue. Bike starts easy, idles well, revs well, plenty of power, just sucking down gas. I probably should get it checked eventually though.

Both carbs have been synced. Definitely within spec per the manual. New jets the same size.

If it helps any more, the wind down to idle after revving is on the slower side. Just pulled the plugs and here’s a pic for reference.


Thanks for the help all. Let me know if this gives you more info to base your expertise off of.
 

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Plugs says it's running quite rich...

Fuel level does look a tad high, I don't have the fuel level spec to hand relative to float bowl to main body interface datum point...

Will look up fuel level during my very busy day...

Since it performing well ign timing and advance would be ok but always worth checking to rule that out...

Air filter foam not oiled so should be running a bit weak...
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Choke ffla f
 

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Compression is unknown but doesn't seem to be a huge issue... the wind down to idle after revving is on the slower side.
Ever hear a top fuel engine idle and then they blip the throttle and the rpm comes down real fast? That's the high compression bringing it down.

Here is how you read book spec of 185psi. Though it does not give the low side, other spec's say between 153 - 187psi. So if it's 152 or below, there goes the kinetic or the heat helper to light the gas off. So low kinetic would read rich, being it can't fire off more fuel, so it adds more soot the plugs.

The one camp says it's OK it runs. The other camp says I don't guess at compression that way. Get my drift about one who chases their tail figuring out 3 variables that are fuel, spark, and compression? Before I throw a dime at a strange bike, I check the compression first thing. Learned my lesson a long time ago.
 

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Discussion Starter · #14 ·
Here is how you read book spec of 185psi. Though it does not give the low side, other spec's say between 153 - 187psi. So if it's 152 or below, there goes the kinetic or the heat helper to light the gas off. So low kinetic would read rich, being it can't fire off more fuel, so it adds more soot the plugs.
Each tested around 135, there's some measurement error in there due to the schrader valve being slightly offset from the cylinder head (more volume = less pressure obviously), but even correcting for that, it's still low. I'll try another valve adjustment and retest. Most likely have found the culprit. Thank you for your help. Still surprised considering the bike runs so well in all aspects but fuel consumption, but can't argue with the science.
 

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Discussion Starter · #15 ·
Each tested around 135, there's some measurement error in there due to the schrader valve being slightly offset from the cylinder head (more volume = less pressure obviously), but even correcting for that, it's still low. I'll try another valve adjustment and retest. Most likely have found the culprit. Thank you for your help. Still surprised considering the bike runs so well in all aspects but fuel consumption, but can't argue with the science.
Doing the math to correct for the increase volume of the chamber with compression tester attached makes 135 psi equivalent to 150. Still lower than spec by about 20 psi. Will do valve adjustment and retest.
 

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Did you add about 10/15cc of oil down the plug hole and check again ?

Also wide open throttle...

Much chuffing of smoke from the cam cover breather when up to temp ?

Be warned, the 4 centre head bolts can be problematic....

Think twice before taking the head off !

Oil consumption and blue exhaust smoke ?
 

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Discussion Starter · #17 · (Edited)
Did you add about 10/15cc of oil down the plug hole and check again ?

Also wide open throttle...

Much chuffing of smoke from the cam cover breather when up to temp ?

Be warned, the 4 centre head bolts can be problematic....

Think twice before taking the head off !

Oil consumption and blue exhaust smoke ?
Tested again after valve adjustment. This time compression topping out at 185 on each cylinder at wide open throttle. Not sure if increase due to valve adjustment or holding throttle open this time. Engine seems to run a little stronger but won't know for sure until I can ride it tomorrow.

No smoke from the breather or exhaust, no oil consumption.

If nothing else, glad the engine is in good shape. Now to see if problem still persists. I'll pull the plugs/check MPG after a test ride tomorrow.
 

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Modern mobiles have great microphones picking up every rattle, thud, knock, exhaust chuff and sounds....

Engine sounds at little tappety and knocky/rattly....

Do you have any exhaust leaks ?

How's the tachometer cable ?

Coffee break over and back to the grind stone ( lathe )...
 
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