Topic: CB175 Misfiring

[rafanomenon]

Okay. Where do I start.

I bought a 1971 CB175 in June. Found out that the starter clutch and the kickstart were blown, and the only way to get it running was to bump start it. As soon as it ran, I found out the motor needed some major work. Troubles shifting, backfiring, clanking, herking, jerking, etc. I found another motor on Craigslist for $200 shipped from a 1972. That's what's currently sitting in my '71 frame now.

The engine has compression and runs fine. Starter motor works, kickstart, everything turns out okay. The current problem I'm having right now is the backfiring and misfiring, which I'm pointing to either a carb issue, or timing.

Steps to repeat problem:
1. Start bike
2. Idles fine around 1,200 rpm
3. Warms up, has problems idling
4. Once taken on the road, it pulls through all the gears
5. After about a mile, the backfiring starts
6. Then misfiring
7. Then only runs on both cylinders every 3 seconds with a 'pop' from the second, then back to one
8. Once at a stop, the idle creeps itself to 6,000 then down to 1,200, then back up, then down, etc.
9. Plug reading shows the right cylinder good/little lean, and the left cylinder tan/white (lean)

What I've done:
- Rebuilt both carbs (no floats) and throttle needles
- Filed/cleaned points
- Cleaned tank and petcock 3 times
- Cleaned carbs 10498109 times
- Greased and rerouted throttle cable

What I need to do:
- Check timing (how do I do this with a light?)
- Set static timing (how do i do this when oil spills out of the left crankcase when the cover is removed?)
- Set points gap
- Check voltage from battery (new)/coils
- Charge battery (it died after leaving the ignition on. Oops. I charged it for about an hour just enough to crank the starter.)

Thanks in advance!!

[Bill Vaughan]

Does it have the original spark plug boots?  If so, do yourself a favor and order new ones.  They're still available from Honda and easy to replace (screw on/off).

Most Honda models used a resistor type boot and the resistor goes bad over time.  The problem first manefests itself in an intermittent spark condition; sometimes it fine, sometimes it's not.  Eventually, the boot(s) can just fail altogether.

Answers to a couple of your questions:

How to check timing - Here's a link to a free download of the Honda Factory Service Manual for your bike (it's a big file): http://www.mediafire.com/?tej2xyizztm

Static Timing/Oil Leaking - This is normal.  A dribble of oil will come out of the crankcase when the cover is removed.  The trick is to level the bike as best you can.  You actually want it to have a very slight lean in the opposite direction in order to contain the oil.

[Bill/BentON Racing]

You can lean bike slightly the other way,then can static time it,Honda made a cover w hole drilled so you could see index marks and static or time dynnamically,you could make one out of spare cvr.Pt gap .012-.016 in.,you can pick up cheap lights most any auto store.I suspect your coil.Bill

[rafanomenon]

I'm a little hesitant to say I fixed it, but I'll take the instant gratification - whether temporary or not.

I fixed it.

Things I did:
- Filed points
- Set gap to .014
- Checked and adjusted float height
- New plugs

Going back to the previous symptoms, one of the plugs was reading a bit rich. I can now blame one of the float tangs being bent off spec - about 4mm-5mm off from where it should be. The spec says to adjust the bottom of the float to the gasket surface, but this was WAY too high. As soon as I put the carbs back on and turned the petcock on, fuel started pouring out of every crevice that the carb bodies had. Turned it off, took the carbs back off, and reassessed my float heights. I used the stock spec (19.5mm = ~.75") and measured using a micrometer to the outer carb body lip, as opposed to the gasket this time around. This seemed to work. Who knows - it could also be off a little do to the new float needles in the carb rebuild kit. Whatever it is, it's now working for me. No leaking fuel, and they seem 'balanced'.

I then changed the spark plugs, and cleaned and set the points gap. I also blame a fouled plug for the inconsistent firing, which could be the result of one of cylinders running too rich.

I didn't check the timing, but this is something that I still want to do before the final road-ready checklist, along with checking volts at the coil, condenser (if possible?), rejetting and tuning and finally - carb synchronizing.

Whew. Thanks for the help guys. Here's a sound bite I took from today. Idle still sounds a little off, and that will be fixed once I get the main jets in, and move the needles up on clip. Fingers crossed...

[OakBehringer]

Things I did:
- Filed points
- Set gap to .014
- Checked and adjusted float height
- New plugs

All these things can make a big difference. In regards to the gap, which could be the most mystical at first: If you had too large of a gap, the condensers may have not had enough time to charge, which could cause a misfire. Or I could be wrong about that

it could also be off a little do to the new float needles in the carb rebuild kit. Whatever it is, it's now working for me. No leaking fuel, and they seem 'balanced'.

You can adapt a drain screw like this to check how much your bowls are filling up. You want the same height for both carbs:



I've actually done this with just a tube by tapering the end and threading it into the drain screw hole.

I then changed the spark plugs, and cleaned and set the points gap. I also blame a fouled plug for the inconsistent firing, which could be the result of one of cylinders running too rich.

Check again in a week- new plugs aren't the most accurate to read.

I didn't check the timing, but this is something that I still want to do before the final road-ready checklist

Do you have a strobe light gun? If not, get one. They're cheap, awesome, and make the job easier and more accurate. Just put the battery leads on the battery and the magic clip around the wire for the plug for which you want to set the timing. Point it at the timing marks, pull the trigger. Any shop manual will explain this. Your timing changed when you changed the point gaps, but it was probably off in the first place. Getting the timing right should make a big difference, and must be done next.

I guess you should take the side cover from your old motor and cut it like this:



I wouldn't know from experience b/c I've never worked on this bike, but I'll find out soon enough, as you know.

[rafanomenon]

Looks like I'll be timing and carb sync-ing this weekend 

One thing the manual doesn't give reference to are the timing of both the left and right cylinder/spark timing. All the manual tells you to do is to remove the left crankcase cover and align the 'F' marking on the generator rotor to the timing index mark on the stator. This is where the spark should be produced. I remember someone saying that these small Honda twins produce spark at the same time due to a 360 degree centrifugal force advancer? If not, and there IS actually a way to test the timing of individual cylinder/sparks, how do I go about doing this?

I also plan on checking and setting the valve clearances, which also involves the removal of the left crankcase cover. Might as well kill two birds with one stone 

[Bill Vaughan]

You're right.  Because the 175 engine employs a 360 degree crankshaft design (pistons rise and fall at the same time instead of being opposed), you use the same timing mark for both cylinders.  The coils fire on every revolution even though one spark per cylinder per cycle is essentially wasted.

For what it's worth, the manual's way of setting the timing is fairly crude.  I'd use a timing light.