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Discussion Starter · #1 ·
Hi All!

I'm trying to understand and put together a list of all the possible causes for why my bike is backfiring.

1980 Honda CM400T

I've cleaned the carbs several times. Everything on the bike is stock meaning that the airbox is still in and no jet kits have been added. Air filter is new and the rubber boots on both ends are in great shape. Also adjustment screws are at factory req. - 2 turns out.

What I'm experiencing is at around 3700-4000 rpms while decelerating (overrun is the term I believe, correct?) the bike backfires. Occasionally it will backfire at idle and/or during start-up. I also noticed that once and awhile it backfires in the carb.

I tried spraying carb cleaner around the boot clamps, gasket and exhaust clamp areas to see if the rpms change, nothing noticeable.

From what I have read online I should be checking the following:

* Valve clearance
* Carbs "pushed" in all the way with the boots - mounted on heads
* Rings - may be damaged
* Replace pilot jet with a larger one
* Raise jet needle a notch or two
* Synch carbs
* Exhaust leak
* Air leak - manifold
* Air cut valves may be damaged (appears fine)

What am I missing?

I'm trying to learn how to do these things but unfortunately I don't have the required tools for all these checks and can't financially afford to get them either at this point.

It may have to go to the shop but I'd like to do what I can before I send it in and find out it was something stupid I should have noticed.
 

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There are some real experts here who will likely be along here shortly with some good advice.

In the meantime, have a look at this. It's a big help in getting the right help to the right places.

viewtopic.php?f=4&t=3797
 

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backfiring is caused by unburnt fuel, I'd focus on things that cause that condition:
float level too high
leaking fuel inlet valve
valve or ignition timing out
hole in the slide diaphram
clogged air jet
is your engine in good shape?...done a compression test lately?
 

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Discussion Starter · #4 ·
Thnx for the replies, I'll check out that link!

The behaviour has changed a bit, not sure why... currently it is NOT backfiring during deceleration but rather in first gear when I'm trying to take-off. In 1st I loss power and the bike sputters/bogs which leads to it backfiring. If I keep the chock or rev it to stay above +/- 3000 rpms it will go fine, if not it dies and I have to use the chock to get it going again.

I experienced this before but after I cleaned the pilot jets (again) it was fine... perhaps I dislodged something and it's found its way back..? :?:

I have done a compression test and come out to be 140-left 150-right, I believe 177-190 is factory
 

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Discussion Starter · #5 ·
Have you ever heard of and/or used a product called "seafoam"? I've heard mixrd things about it and whether to use it on old bikes...
 

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I've used seafoam on four different cars I've owned. The only vehicle of the four that ran better afterwards was my Isuzu pickup. Everything else (after the giant smoke show) ran exactly the same. If your vehicle intake system is clean internally it's not going to help much.

Since I have some sitting around the garage, I had a notion to pull the float bowls on my CB350 and fill them with seafoam to draw it through the engine. However, my carbs are clean, so there's no reason to treat the intake. I'd do better to use it in the oil as a detergent to clean out all the old oil sludge. But ATF or Marvel Mystery Oil is cheaper than Seafoam to do that ;)

All of the backfiring I've ever had happen was due to an air leak in the exhaust system. Air gets in and mixes with the unburned hot gasses and ignites. In fact, my 2000 Kawasaki W650 has little one-way valves in the exhaust system to let air in to cause the bike to backfire on purpose as an emissions control. I put blocking plates in there so it wouldn't pop so much as I coasted down a hill or into town, etc...

So check your exhaust flange at the engine. Any air sucking in there is liable to make things pop. When you let off of the throttle to decelerate the engine is still pumping away and sucking a bunch of gas through the pilot circuit at the very least since the throttle butterfly is closed you have very little air getting in.)

On Kawasaki it's just normal for it to do that. I kind of like the sound of it now that I've got it under control a bit.
 

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foto4peter said:
I have done a compression test and come out to be 140-left 150-right, I believe 177-190 is factory
yep 185 is factory with 150 being minimum. Did you do the test with the throttle wide open? What was the condition/color of the plugs? Black? If so, this is likely the cause as the fuel isn't being compressed fully for a complete burn. You can verify compression by squirting in a bit of oil. If the compression rises the rings are your culprit. If not, time to check valves for leaks.
 

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Seafoam is fine in my book to use as an upper intake/combustion area cleaner but I'd NEVER put any in my oil. Just a bad idea.. As for filling the float bowls with it, the bike would simply die (fail to run). You have to have regular fuel running through the system while slowly putting the Seafoam in through a vacuum port or slowly trickling it into the intakes. You'll also have to keep the revs up quite a bit to keep it from dying. Seafoam just doesn't like to burn like fuel does. I also recommend that you change your plugs afterwards, and change your oil if you dump much Seafoam through her. If you plan on doing a future plug reading, or not being stranded on the side of the road, then change the plugs afterwards.


GB :mrgreen:
 

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Discussion Starter · #9 ·
Here's an update to the situation...

I did some parts replacing on the carb:
new accel pump assembly
new aircut off diaphragms
new float bowl gaskets
and another good cleaning (soaked it over night)

The bike runs "better" but it is having problems at idle and starting off in 1st gear.

Usually takes several attempts to start and when I'm riding, come to a stop and start off in first the bike losses "power" which usually causes it to die. When I restart it it will take several attempts and usually backfires. The rpms "bounce" +/-500 until it either dies or I use the choke to hold it around 3000 rpms. If I hold the choke at 3000 then it will run fine.

I hope I've stated this clearly, any help is greatly appreciated.

I read somewhere that turning the pilot screw out a full turn or two could help.
Is this possible?
And if so which? I ask which because on these carbs there are two screw-able pieces in the pilot jet "area".

Also I didn't do the compression test, had a shop do it while they were doing some work on the brakes.
 

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Pretty obvious this is a fuel problem. The carbs probably need to be properly cleaned. This consists of soaking them in carb cleaner for several hours or a couple of days, after removing any rubber parts. Then use some fine wires like old guitar strings to gently clean out air bleeds, jets & orifices in carb body, throat and bowl area. Clean emulsion tube cross-drillings if it has them.

During and after using wires to clean orifices, flush the holes with spray carb cleaner, such as Berryman's Chemtool. Almost all of the carb jets & holes will blow right on through with carb spray cleaner, showing that they're open. While still wet, blow them all out with compressed air. If you have to take the carb somewhere to use air, take along the carb cleaner to wet them down again.

You don't want to ream the jets with wires, just gently clean. When you cut off a piece of wire run the cut end over some fine sandpaper or emery to remove burrs so you don't gouge jets.

When searching for an intake leak etc. it's a lot cleaner, less damaging and more effective to just use a propane torch. Turn on the gas but don't light the flame. Play gas jet around all areas and when engine speeds up you found your leak. That way there's no damage from caustic cleaners, no muss, no fuss.
 

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Too Lean!....
Are the float heights correct?.....Jets in correct positions?......
 

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Discussion Starter · #13 ·
float heights are not adjustable, the orginal floats are all plastic which by the way are in very good condition (no leaking) jets are good too but I'll double check...

tomorrow I hope to have time to check for any air/exhaust leaks

thanks again to everyone for chiming in, much appreciated!
 

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Discussion Starter · #14 ·
well... I took it to the extreme - I removed the pilot jet (testing purposes) and it starts right up and doesn't backfire
but as you would expect it's too rich! try to give it gas or adjust the choke and it dies.

so unless I'm mistaken I need to increase the jet size however I'm not sure by how much..? or where I can purchase them online. anyone?

cheers Peter
 

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Discussion Starter · #15 ·
hey all... well I've finally been able to play around a bit with the bike and found that whoever worked on the bike last time put the intake manifolds (boot between carbs and engine) upside down. So instead of having a "perfect" circle passage into the engine, it was blocked by about a quater inch.

Once I put them on correctly and some minor adjustments runs much better. Although now I replaced the stock mufflers with some 15" emgo universal slash-cut mufflers (still 2 into 2 system) and am back to running lean with backfiring.

Another thing I noticed is that the bike is very difficult to start with the electric starter but if I pop start it starts up fine... Why?
 

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This all points to being too lean, in the low speed circuits. Does it run a little better with the choke on just a bit? Try backing out the pilot jet screws a little bit, then ride it. Make sure you test things when it is fully warmed up. My CB360 takes longer to warm up than I thought, and I have to wean it off the choke slowly.

One other thing. I found that using premium gas really made a big difference in popping and backfiring. Try a tank full and report back.
 

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i second what wentwest is saying, i think this problem can probably be resolved by sorting out your air screws and idle adjustment. If the idle is too high and the air screws too far in the bike would run ok once started but be a bastard to start on the electric start. next time you go to start her up try taking the idle/throttle stop out several turns (anticlockwise) and starting her. if this is your problem the bike should start up fine like this, but immediatly die because of the low idle setting (unless you catch it with a bit of throttle). If so then try opening your air screws a half turn or so at a time until the stutter as you open the throttle goes away.

Setting the idle a little over the 1200rpm that honda state may improve things too, and remember to only adjust carbs with the engine warm!

(i just realised how old this thread is, sorry about that :oops: )
 

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Discussion Starter · #18 ·
no worries Hario! after the New England polar caps melt I'll be able to give an update. I did have a little success over the summer getting it running better but I'll hold off on the details until the warmer weather... cya
 
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