Topic: 81 CM400T No more backfire!

[snowcase]

Well this is my first post. Gotta say, I love this place so far. A lot of info out there but I can't seem to find anything that deals with the revs not returning to idle and a bogging around 3k - 4k. I *think* this is an easy fix but I would like to hear your guys thoughts. The guy I bought it from did some pretty shoddy repair work on the carb boot to the engine. While under there I noticed that the intake boots were completely gone. I've never worked on a bike or owned one before but it seems that engines that are finicky with a slightly off carb sync would not take kindly to huge amounts of unrestrained air.

Am I correct in thinking that both of my problems are caused by the lack of these boots? I'm going to buy the parts anyway but I haven't found a place to do so yet other than used ones on ebay. Anyone know where I could pick them up online?

And here's a picture just for fun.





Thanks for any help!

[tonycb650]

yes!

[Johnie]

I have bought and sold a number of CM400's. In fact, I just sold my last restored one a 1980 CM400E last Thursday. I do have some used CM400 parts I planned on putting on eBay. If you need something let me know. I did find a number of NOS parts from Western Hills Honda in OH. Check them out or let me know what you may need and I will see if I have it. Those carbs are easy to work on. There are a few philips screws holding the float bowls on. With a small driver with the proper fit you can get those bowls off. Just be sure not to strip those screws. Once off you can easily take off the float and jets to see just how clean everything is. Same thing when you take out the jets. They are brass and you will need a strong reliable screwdriver as you do not want to damage the heads of the jets. I also used a mirror to see up in there once I had the bowls off.  I have yet to see a CM400 carb that does not need cleaning. The top of the carbs also comes off easy to let you get at the slide and needle. Take the tank off to get at the top part. Fairly simple procedure. Good luck with it.

[snowcase]

yes!

Great to know! Thanks!

[snowcase]

So I got my parts today including a new boot for the carb to head. The bike runs much better now although I still have yet to receive the air filter. It did stay at 6k right after I started it but I took it for a ride and it ran great. No more bogging. Very, very fun to ride. I'll be changing the oil and getting a new battery this week.

[snowcase]

Speaking of which, anyone know a good place to get a battery? The napa website says they have them in local stores but I highly doubt my store is one of those.

[tonycb650]

got mine at pep boys. if there is a cycle gear around you, they have them also. walmart should have it also.

[snowcase]

I'm still waiting on the air filter but I still have a high idle. It was much higher. Now it's 4k-5k. When it first starts it runs great. After it warms up is when I have the problem. With it warm and the choke out it idles where it should be but will stall after pulling the clutch in. Is it stupid to post this before I get the air filter and see if that remedies the problem?

[snowcase]

Got my air filter today. Didn't fix the problem. I was thinking the sketchy fuel filter the PO put in may be causing it to run lean. Any suggestions?

[woodardhsd]

Got my air filter today. Didn't fix the problem. I was thinking the sketchy fuel filter the PO put in may be causing it to run lean. Any suggestions?

I doubt the air filter is making it run that badly.

Either the air screw or the idle speed screw may be out of whack.  I imagine that with a large air leak, the PO had to fiddle with something to make it run, probably enriching the mixture in the process.  The air-screws may be set way too rich, or the idle speed knob is adjusted way too high.  

Mike

[snowcase]

I've messed with the idle knob. It's the only way to bring the idle down if it gets stuck at 5k. You're probably right about the PO messing with the mixture. Hopefully it's something the Clymer can help with.

[woodardhsd]

Here's a link to a PDF copy of the factory manual for the 400 twins.

http://www.mediafire.com/?cnkmdznfjgz

Good luck,

Mike

[snowcase]

Already have the actual version thanks. I was just out working on it and it runs almost perfect with the choke 9/10ths of the way out but as soon as you push it in the revs spike.

[snowcase]

Noticed another missing part.





See the hoses coming off the "top" of the carbs in this picture? They're not there on my bike. Where do they go and could this also contribute to the high idle? Anyone know the size and length so I can replace them? I also thought that the fuel filter the PO put in might be contributing to the lean mixture. It also revs very high when I turn the gas off then dies.

[Johnie]

Those 2 black hoses are coming off the float bowls and are over flow tubes. And when you drain the carbs for winter storage the gas in the float will exit these tubes under the bike. The ends do not connect to anything but just hang under the bike in the center stand area. Are you sure the gas tank is clean? And did you ever clean out the carbs and jets as discussed earlier? These problems can go all the way back to proper cleaning of the carbs, jets, floats, etc. You said the previous owner did some shotty work on the bike. Probably never touched the carbs either. I have not seen a an older twin just like yours that does not need the carbs cleaned.

[snowcase]

So nothing I should worry about then.

[snowcase]

Well now I have another problem. It's starting to backfire into the left carb around 2k rpms. It also has a slight gas leak in the same carb but I haven't found the source of it yet.

I ran seafoam in it yesterday and today and it has brought the high idle from 5k to 4k and less frequently. It will no longer run with the choke out when it's warm and starts instantly without it cold. I also noticed that I can sometime get the idle to come back to normal if I wiggle the throttle cables. I'm still hesitant to take the carbs off because it seems like I'm making progress.

[BlindJoe]

have you checked the tappet clearance?

[snowcase]

I have not and have no clue how to go about doing it.

[BlindJoe]

You have to take the top engine cover off first, then you'll need a feeler guage, a 10mm wrench, and a small screwdriver. Its pretty easy to do on these bikes, just follow the manual and you should be ok

[Johnie]

OK, so you ran some Seafoam through the carbs and it brought the idle down a bit. That should tell you something. This will be a process of elimination. And keep in mind like I mentioned many posts ago, you do not have to take the carbs off to clean the jets. You can keep fiddling with it, but in the end you will be cleaning the carbs. Did you mention if the tank was clean? I can not recall...

[snowcase]

I assumed it was a float or jet problem. Any suggestions on working on them? I have the clymer manual to guide me through it, just looking for some tips that might not be in there.

The tank looks clean and the filter the PO installed is also clean. I'm thinking there may be something blocking it though because even with 1/4 tank I don't see any flow in the line.

[snowcase]

I have the float bowls off and the jets out. I was very surprised at how clean everything was. The jets are 72 and 118. I think I saw somewhere they should be 120. If it's running lean now which jets should I have? Anything I should do while I have them open? There was just a small bit of crud in the bottom of the bowls. I also noticed on the left carb there is no pilot (I think) screw. Is this correct?

[Johnie]

The smallest bit of crud will plug a jet. Take a mirror and look up under the carb. You will see the brass jets with a screwdriver slit in it. I use carb cleaner and air. Hold them to a light and you should see a very small hole there. There are 2 jets...slow and fast jet. Thinking back you should have an air screw for each carb. I wonder if you are missing one there? Hard to believe it would run at all with one missing though. Any chance you are missing the sync port screw? As far as what jets you are supposed to have...somewhere on that carb body should be a ID number. That would tell you what jets should be there. If I can find my ID sheet I will tell you what I find. Last, you could take the top caps off the carbs. There is a spring under there and a piston slide with the jet needle on the end. I usually clean the needle with metal polish to get the crap off it. You can also spray some carb cleaner down there. I did not mention taking the floats off. But you could do that too. Then the float needle will drop out and you can clean that too. I hate to toss a lot of stuff at you as I know there is a lot to learn here. But that 400 was the first bike I restored and I learned the same way you are. Just keep an eye on everything you take off and when it goes. I even used a camera to keep tabs on where stuff went.

[Johnie]

Below is a pic of the 400 Honda Shop manual I used. It is a very valuable tool and you should consider getting one. You might find one on eBay. I think I got mine at www.helminc.com It has a great carb section too. That pilot screw with the spring in it should be 1 to 1 1/2 turns out from gently seated. It screws up from the bottom.