Planet Minis

malibusteve

1978 z50 here. i got this bike out of storage and cleaned the carb, changed the plug back in december/january and got it running. Ran pretty good initially but performance has gradually deteriorated. Then started backfiring occasionally and gradually losing power....Gradually meaning over the course of a few months (every other week riding 1/2-1 hour at a time). After each use, i'm always consistent about turning the tank valve off and letting it run out of gas so that the gas doesn't gum up the carb again. Rode it 3 days ago and it left me 1 mile from home. Just stalled out, backfired, cut off and hasn't really run since. I have good spark, but for good measure, cleaned the plug and regapped. I sprayed some starting fluid into the spark plug hole, re-plugged and it started long enough to burn off the starter fluid before dying again. I checked to see that I had fuel flow to the carb from the line out of the tank and I do have flow to there. I next removed the thumb screw at the bottom of the carb bowl and nothing came out...isn't this telling me that there's a blockage somewhere in the carb? Anyway, i disassembled the carb and the float valve was stuck. It wouldn't compress. I freed that up, visually checked the other stuff and put it back together and on the bike...still nothing. I currently have completely disassembled and am letting it sit overnight in the carb cleaner. As i removed the two adjustment screws I was distracted and didn't take note as to there position. They have not been touched since the bike was running good. Can you all offer some advice as to where to start with them when i reassemble?

Can you offer advice as to what went wrong to start with? What causes backfiring? Why would the carb be gummed up anyway? Do you all think I'm going about this the right way? I'm obviously not a mechanic...thanks for your time.

Xare50

your on the right track it seems maybe check the fuel line

arlindsay1992

If you run the motor until it dies to empty the bowl, there is still fuel left. This is because the fuel pickup tubes don't go to the bottom of the bowl. They are a couple mm's off the bottom. So when the motor dies, there is still a couple mm's of fuel at the bottom. This makes it MORE likely to gum up. I wouldn't worry about the carb gumming up if you store it for less than 3 weeks. For storage longer than about a month you should probably drain the gas and remove the carb to drain it. Blow out the passages to make the carburetor completely dry. For normal storage between weekends, leave the fuel in the bowl and turn off the fuel from the tank to keep it from leaking if the float gets stuck or something.

For the air screw, 1.5 turns out is a good start, needle in the middle, idle screw 1 turn out. The first two settings will be close, the idle will be really high but that's okay, it makes it easier to start then you can adjust it. If it's too low, you'll never get it to start.

malibusteve

cleaned and reassembled the carb and put it back on the bike and it cranked and ran pretty good on the first kick. these bikes rule. having some trouble with adjustment though. Here's my method. Please advise. I'm starting by adjusting the idle screw to the lowest possible idle speed. I'm then trying to turn the air screw clockwise until the engine speed decreases or misses....making a mental note of where that position is.....turning the air screw counterclockwise until the engine speed decreases or misses and then splitting the differnce between the two positions. The problem is, i can make slow the engine down with the clockwise rotation of the air screw, but turning the screw counterclockwise seems to do nothing. I can almost turn that airscrew all the way out with it doing nothing to the engine speed. I did get it pretty close last night though by decreasing engine speed with air screw (from clockwise rotation) and then turning back counterclockwise about 1/2 turn. The only problem I had then though was that when i revved the engine it seemed to take excessively long to return to normal idle speed...I went out this morning and gave it a kick and it cranked the first time. Ran for a minute or so and then stalled. Kicked a few more times but unable to get it started. Suggestions? BTW, I'm using a Clymer manual. It shows a number of different carbs and according to the manual, mine should be using the Type 1 carb, but the carb on the bike is the Type V which (according to the manual was used on the 76-79 CT90) is original to the bike. I'm certain of this as I'm the original owner. Please Help!

arlindsay1992

Honda used a couple different carbs on the Z50 in early years. The method for adjusting them is still the same. The air screw adjusts how much air goes into the idle mixture. MIXTURE not SPEED. So screwing the screw in means less air and richer mixture. Turning it out means more air and leaner mixture. 1.5 turns out is a good starting point. Once the bike is warmed up, give it full throttle. If the bike blubbers then it's a bit rich, if it just goes bwaaah bwaaah without blubbering, it's a bit lean. Also it will be easier to start when it's rich. And sometimes, if you get a hesitation off idle, it's just because the idle is too low. Bumping up the idle a bit might solve all your problems, just make sure it's not up so high that the bike want's to move when in gear.

I hope this helps. It's kind of hard to explain the sounds through text.

mexicanyella

Arlindsay's right; I would add that you'll get a steadier idle and easier starting and sharper response off idle if you're pilot mixture is just a bit rich. You can go too far and also get plugs that foul occasionally doing it this way, though, and it just takes a little trial and error. What I do is envision (with a stock carb) my pilot adjusting range to be between 3/4 and 2 turns out from seated. With the engine fully warmed up, set the bike's idle speed (via the throttle stop screw, not the pilot screw) high enough that the bike idles without lurching too hard when you go into gear then adjust the pilot screw to where it idles fastest. Now crank it in toward seated just a bit; maybe 1/8 turn, not more than 1/4 turn. This should slow the idle down slightly, which you make back up by cranking the throttle stop screw in a bit. You want an even idle with only a slight jump when you go into gear and no trace of creeping when idling in gear.

On my '71 hardtail, creep happens as you approach 3000 rpm, and the clutch seems to be fully engaged by 3500 or so. Setting a slightly rich idle speed of around 1800-1900 allows easy starting and an idle that stays steady while I climb off to open farm gates or whatever, and immediate throttle response with no hiccups once warmed up. I run either 30W or 15W-40 oil in the summer, and if I try to set the idle cold I find the speed increases quite a bit after the engine warms up, tolerances close up and the oil thins from the heat. That's why I say set it warm. You might not get a great idle when cold, but it's more important to have it run right when warmed up.

I'll also add that this is with the early hardtail carb with the guillotine choke. The actual screw settings might vary for a later carb, but the procedure is that same. That procedure worked on my old YZ490 too, which had a 40mm mikuni. Obviously the "jump into gear, avoid creep" part was not an issue with the YZ, though.

If you can't get this procedure to work for you you probably have an air leak or a dirty pilot jet or some fuel-supply problem, in which case report back with more info.

malibusteve

man, I'm getting frustrated. I cannot get it to even run on starter fluid now. Took the carb back apart. Saw nothing that looked amiss. All looked clean and to be functioning as it should. I checked again for fuel flow through hose coming from tank. Good flow. I checked hose for holes. Found something that I thought might be suspect near the end that connects to the intake nipple. Had enough hose so I clipped off the suspect piece and then reconnected. Checked and re-checked the adjustment screws...still nothing. plug still firing. Also, the thumbscrew that is at the bottom of the bows. If i loosen that screw, gas pours out of the little hose that runs out of the bottom of the bowl. This indicates that I'm getting fuel flow to at least the float and valve right? what should i try next?

malibusteve

btw, mexicanyella. when you say throttle stop screw...what are you referring to? my carb/bike has two adjustments....idle screw and air screw

arlindsay1992

Throttle stop screw is idle screw. Idle screw just changes how far the throttle slide can go down. You screw it in, the slide goes up, the idle is higher. Air screw goes into an air circuit so in closes it more and lets in less air to richen the pilot circuit.

malibusteve

ok...the thing with it not wanting to run even with starter fluid got me thinking that this isn't a carb issue. remember that I said i had good spark and that the plug was gapped correctly. by squirting fluid into the spark plug hole, reinstalling the plug and then still not getting it to crank, i started thinking about other issues. it seems to me that the timing would not be the issue as it was runnign this morning and just cut off all with me not tinkering with timing. I've felt for some time even before this latest carb thing that this thing might have weak compression because of how it felt when you kicked it and because it just seems to have lost some get up and go over the years. so i just purchased a compression gauge and if I've checked it right, it shows less than 60psi compression. now it will push your finger off of the plug hole, but apparently 60 psi is way too low. clymer says 170 + or - 10psi. So as a nonmechanic i'm thinking blown head gasket or worn rings....what do ya'll think? Recall that this thing has never ever been worked on at all except for the carb....are rings and/or head gasket the next step?

mexicanyella

Squirt a little oil into the plug hole and kick it over a few times with the plug out. You may have washed your cylinder walls clear of oil with all this raw fuel and starter fluid going in there; via solvent action. This should get your compression back up enough for it to start, if you do indeed have a strong, steady spark and fuel getting in there. Never hurts to try pop-starting it in 2nd or 3rd when something's slightly amiss, either; if it gets it running the way it runs can give more clues, etc.

malibusteve

correction....compression is more like 110psi....not less than 60 as originally stated....i'll try the oil into the cylinder mexican...thanks

malibusteve

ok mexican, tried the oil in cylinder thing....kicked over a few times...replaced plug....still no start...did repeat the compression check and got it up to 120psi....closer to the spec but still off by at least 40psi...what would you check now?

mexicanyella

I think with the compression you've got, it should certainly run to some extent. I am beginning to doubt your commitment to the cause of spark evaluation. I feel your spark stimulation threshhold may be a bit too low, and you are getting all excited and self-congratulatory about a totally substandard spark.

It is possible for a weak spark to show at the plug at atmospheric pressure but not fire the mixture under cylinder pressure; usually this kind of spark looks like a tiny luminescent thread across the plug electrodes and is purplish. You want a bright whitish-blue snap. Check this on a new plug just to eliminate one variable.

Is your spark plug getting wet with fuel or oil, can you tell? If it appears to be dry, that could be a clue that even though you've got fuel in the bowl, it's not getting up the jets and into the intake stream. Did you see any traces of water blobs when you had the fuel bowl off? It takes very little water in the carb to cause starting problems. If it looks oily, that would be good to know, although if you're managing 120 lb. of compression, regardless of what the manual says, it can't be too bad in there. Sometimes that oil-squirt compression trick, if done a little too enthusiastically, can oil the plug up though and foul it.

Was there any exhaust smoke when it was still running?

malibusteve

Mexican, sentences 2 and 3 are a bit confusing to me, but I guess what your saying is that you don't think the spark I'm seeing is good enough and perhaps you're right. I'm not a mechanic so I'm up for trying anything from those with more mechanical experience than me.

I'll try a new plug and see what happens. As far as any smoke that was noticed...yeah, it did smoke just a little...white smoke. not much but some....as far as the plug condition....i did notice when i pulled the plug after it left me stranded that it was blackened and dry...in the last couple of days working with it, i haven't noticed that it was oily nor have i noticed that it has been excessively wet or dry...that probably doesn't help much, huh?
btw, thanks for your help so far

mexicanyella

I was just kidding around, but yeah, I'm wondering if your spark is weak and causing a misfire/no fire problem. A dry, black sooty plug can result from an ignition misfire in an otherwise healthy cylinder.

You're sure your points contact surfaces are flat, clean and meeting one another squarely? That plus incorrect gap can certainly weaken the spark and make the bike hard or impossible to start. Are you hip to points filing/cleaning/gapping? I am happy to write a several-thousand-word essay about it and have done so before, if you're into that sort of thing. But if you're confident you have done that right I don't want to preach.

malibusteve

i haven't even looked at the points. In fact, I don't even know where the points are located. How's that for honesty? I'd love to read your essay as long as you keep the words small..ha ha. I do have the clymer manual but as stated above, it covers different models so i have to piece some of the information together.

Again, really, really appreciate the help.

mexicanyella

Okay. Before getting too far into this, do make sure whether you have a fresh spark plug and a definite bright blue spark that is intermittent. It should fire once for every revolution of the crankshaft, not any kind of detectable random on/off sparking.

If you are confident you have a steady bright blue spark occuring once per revolution, as near as you can tell, then we need to think some more about this.

If you have a weak, thready purplish spark, an intermittent spark, or no spark, that's when you look at the points. They are under the flywheel, which is in turn under the cover on the side of the engine by the gear shifter. If you remove this cover you can see the points through holes in the flywheel on the older motors, but to really do a good job it's best to get a flywheel puller tool (Dr.ATV, TB, CHP racing, Dennis Kirk, throught the Tucker-Rocky catalog at your local bike dealer) and remove the flywheel.

The tool's less than $20 and is well worth it if you plan to keep a 6V points engine running. Also seek out an auto parts store with a good stock and maybe an older staff and ask for a points file. In my area the NAPA store is the one like this. While you're there get a can of electrical contact cleaner, which is an aerosol solvent like carb cleaner or brake cleaner that dissolves crap off of contacts and leaves no oily residue.

Points tend to get pitted on one side and peaked on the other from running as the contact face material gets blasted off one face onto the other. You need to file them flat again with the points file. You can use your old lady's emery board too but it's messier and it's a pretty lame tool. You file them flat and try hard to keep them meeting one another squarely (this is why you want the flywheel out of the way) and then you clean the filings and dirt off the faces with the spray cleaner.

You have to put the flywheel on to set their gap because they are opened by a cam that's part of the flywheel's back side. You loosen the screw that holds them in place enough to move them; as you move them the pivot geometry changes so they open more or less off the cam. You need a feeler gauge so you can set this gap between .012" and .016". Anywhere in that range works okay, and probably even out of that range although I've never tried. Changing gap changes ignition timing slightly so keep it in the range specified. You want a slight drag as you pull the correct-sized feeler gauge out of the points. When you've got that, you tighten the hold-down screw to hold your adjustment and check that you didn't just change the gap again. You will probably end up doing this a bunch of times and feeling like kind of an a-hole before you get it.

Once you have that gap set on a set of clean, smooth, faces-meeting-squarely points, if everything else is in order in your ignition system you will have nice spark and can eliminate some possibilities.

I'm not sure if I'm telling you stuff you already know or not, so look this over and if anything seems unclear ask and I'll explain further. Might be tomorrow morning though.

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malibusteve

man, this is all good stuff....i knew some of it....not much, but some....all of that stuff is good. I'm going to take a much closer look at the spark i'm getting tomorrow and I'm going to try a new plug...your information is good and i appreciate it

mexicanyella

I meant to say, at the beginning of my last lengthy post, that you want a bright blue spark that is NOT intermittent, but that happens every revolution. Also that when you put the flywheel back on to check and set the points gap, you then rotate the flywheel by hand until the points are at their most-open position--their rubbing block is then on the top of the points-cam's lobe--and THEN do the adjustment.

Also, let me pull up my pro-points soapbox here...(Z and CrawlerD and others are groaning, saying "here he goes again" and powering down their computers here) but once you go through this process a few times, carefully, it becomes much easier and you'll be able to keep the ignition working with a few simple diagnostic steps. My point being, don't be intimidated by it. It's frustrating at first because it's fiddly and you have a couple of tools to buy to do it right, but none of them are expensive and hey, my daily rider bike's almost 40 years old, it's been used as a daily college commuter and a daily farm-chore hack, it's got the original engine in it unrebuilt, I'm the third owner (the first two generations of owners were kids who likely thrashed it and neglected it) and I've had to do this once a year some years, twice a year a few times, and not at all for a few years, I haven't actually had to replace the points or condensor yet, and I've owned it since 1992.

I don't know how well points hold up on a modified small Honda single-cylinder engine that develops more fire in the cylinder and revs higher, but on a stocker they just work and work...until they don't, at which point they're easy to sort out and continue riding, likely without having to buy anything once you've made the initial tool outlay. They just worked and worked on my various other bikes too...DS100, GT380, XS650, CB450...the DS cooked a few condensors and needed occasional points setting but that engine got wet a lot and the flywheel cover seal wasn't the greatest. The street bikes never once had an ignition problem. I'd adjust those once in awhile because I felt sort of guilty for never checking them. As I recall the GT, a two-stroke triple, had three separate sets of points in there. Now THAT would bum Z and CrawlerD out. All in the same bike! Isn't that great?

And no problem about passing on the info. It makes me feel important (heh) and I'm glad to help. I have received a lot of helpful tips here and intend to pass on what I know, whether you want to know it or not. I will force you to listen to me talk about points.

One other thing. You mentioned you have a Clymer manual...as far as I've seen, all the 6-volt 50cc and 70cc Hondas have the same basic procedure. The four-speed manual-clutch ones (CT70H, SL/CL/XL70, and I think the C-series "Passports" have a different flywheel with a centrifugal spark timing advance mechanism built into the flywheel, but they still have the same gap settings and the procedure's about the same. So if your manual covers the 50-110cc Hondas you shouldn't have to piece anything together, info-wise, as far as this procedure goes. Other electrical problems might take a little interpretation (coils are in different places, some bikes have batteries, etc.) I don't actually own a manual at the moment because I think one of my dogs ate it, but have done fine borrowing the ATC 70/90/110/125M manual from the local library for a few things.

I'll check back tomorrow and see if you have any spark-checking reports or points-setting questions.