Help! Yamaha QT50 Yamahopper - 18mph max & Slow off mark

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Help! Yamaha QT50 Yamahopper - 18mph max & Slow off mark

Postby bikerboy » Sat Oct 24, 2009 6:59 pm

Hi there, I've not posted on this site before but found it on google & hope you guys can help me.

I've got a 1986 Yamaha QT50 that has a low top speed (normally 18mph max) and is slow off the mark (can take 10 seconds or more to get to 18mph, which is annoying at traffic lights).
The strange thing is that they are intermittent problems that have been happening on-and-off since I bought it around 4 years ago, and both problems are linked/happen at the same time. I can go out on it for 2 or 3 times in a row and it will go fine (good acceleration & 35mph top speed), then the next time I go on it it's back to 18mph top speed. Or sometimes it will start off at 18mph then after 10 min or so will go to 25/30mph for 1 or 2 mins then back to 15/18mph max. I've had it apart several times to try to fix this and other problems, sometimes it worked properly afterwards (for a short while) other times it didn't, but sooner or later it went back to 18mph top.
Another thing is that when it tops out at 18mph it doesn't matter if the throttle is 1/2 open or fully open, the top speed is still 18mph; the only difference is that when fully open it uses more fuel (uses around 25mpg, compared to 90mpg when it runs properly).

Also when the bike is limited to 18mph the spark plug gets covered in soot (can be brushed off easily, but gets covered again when i next use the bike) see pic below.

I'm not sure what to do but the things I've tried are:
-disassembled & blew out the carb with a compressor
-cleaned & checked the float in the carb (seemed ok, didn't leak or take in fuel as far as I could tell)
-moved the needle up 2 notches.. then down 2 notches (before putting it back in the middle)
-fitted an inline fuel filter
-changed the engine/gear oil
-looked at the reed valve (couldn't get it out but it didn't look cracked or anything)

-There's a gap between the carb & the air box so I tried fitting them together with tape (didn't help so i took tape off)
-Cleaned the air filter
-Put a thinner air filter in
-Removed the air filter completely

-Adjusted the oil pump using the long brass hex nut
-disconnected the oil line and premixed the oil to aprox 30:1, then 50:1, 100:1, 200:1 no luck

-cleaned piston, cylinder body & head, resealed with blue hylomar (had very little dirt on the surfaces tbh)
-cleaned exhaust port (around 1mm of black gunk but the hole was still clear & exhaust was new only 1000 miles ago so doubt its blocked)

-changed the indicator/headlight bulbs to lower watt ones in case they were draining power
-recharged & topped up the battery
-looked at the timing/magneto flywheel but didn't have a timing light or tachometer so couldn't test it properly (it doesn't look adjustable anyway)
-checked & cleaned the spark plug (BPR 4HS with 0.7mm gap)
-tried a BP6HS plug (new plug covered in soot after 30 min ride, top speed still 18mph but did go to 23mph briefly before going back to 18mph)
-checked clutch pads (2mm or so left)

The last time it worked properly was about 2 months ago just before a gudgeon pin circlip came loose while I was riding and seized the engine (gouged the piston rings & got jammed between the cylinder & exhaust port). I fixed the damage caused by this by replacing the circlip & the piston rings, & the bike worked when put back together but the 18mph top speed problem was back again.
The configuration for the bike the last time it worked was:
-Air box loosely connected & Thin filter in it
-oil line disconnected & premixed in tank at around 200:1
-BPR 4HS spark plug (looked fine when removed)

I'm pretty sure that all of the parts are original except that the cylinder has been rebored to 42mm & fitted with a 42mm piston & rings (standard is 40mm), and it had a new exhaust/silencer fitted around 1000 miles ago.
Hopefully I have provided enough info & any help would really be appreciated as I don't know what else to try!
If you need more info or pics just ask.
Many thanks,
RD
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re: Help! Yamaha QT50 Yamahopper - 18mph max & Slow off

Postby limes » Sat Oct 24, 2009 9:24 pm

looks like you are running rich by the plug. the fuel air mix screw on my qt50 is about 5 1/2 turns out. pre mix about 50:1 for fuel and make sure your choke is'nt stuck shut.
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re: Help! Yamaha QT50 Yamahopper - 18mph max & Slow off

Postby xrongor » Sun Oct 25, 2009 5:08 pm

probably not the magic fix all, but possibly the throttle cable has stretched and opening it all the way doesnt open the carb all the way.

also it is common for people to not take the pilot jet out of the carb. you cannot see it. its down a hole next to the main jet and requires a small flat bladed screwdriver to remove. you clean it with a tiny wire like a guitar string. a twist tie wire is too big.

not sure exactly how new your exhaust system is, but clogged exhausts are also a common QT problem. toss it in a fire for a couple hours and burn it out.

good luck!

randy
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re: Help! Yamaha QT50 Yamahopper - 18mph max & Slow off

Postby bikerboy » Wed Oct 28, 2009 7:55 pm

Hiya, thanks for the help. I tried turning the fuel air mix screw out 5 turns yesterday, but when I started the bike it didn't idle like normal but revved right up like crazy as if the throttle was full open.
The back wheel span like mad, & the engine got so hot I could feel the heat & smell burning from the engine after only 3 mins. I checked the throttle cable to see if it was stuck & adjusted the cable tightness using the hex nut & brass bit on the top of the carb to make the cable as loose as possible but it didn't help. I also tried adjusting the idle screw, but a little bit one way & it cut the engine, or the other way & it idled way too fast - I couldn't get it in the middle. The only way to get it to idle properly was to turn the fuel air mix screw back to where it was (about 1 turn out).

I also added a bit more oil a few days ago (it's now about 100:1) but it didn't help. I'm a bit worried about adding anymore in case it's the oil that causes the engine to get so hot & the thick soot on the spark plug.

Checked the choke too but it's ok, not stuck.

Also blew out all parts of the carb again with a compressor (I didn't remove the pilot jet but blasted it with the high pressure air & I could easily see light through it and the main jet when held up to the sky).

I looked at the throttle cable when I had the carb apart to see if it had stretched but it seemed ok & the needle went right to the top of the carb when I opened the throttle.

I took it on a 10 mile ride a few hours ago to see if there was any improvement but there didn't seem to be any - it was still really slow at pulling away when stopped & the top speed is still around 18 mph (although it did once briefly get to 21mph for about 30 seconds).

I had seen other posts on this site say that the QT exhausts tend to block, but the exhaust was new about 4 years ago (around 1000 miles ago) & when I took it off the hole was pretty clear with only 2mm / 1/16 inch of gunk around the pipe & exhaust port.
I've read that filling the exhaust with caustic soda & leaving it overnight works quite well though & I have some in the shed so will try that tomorrow.
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re: Help! Yamaha QT50 Yamahopper - 18mph max & Slow off

Postby a_robotham » Mon Jan 18, 2010 10:18 pm

I'm no expert but I believe your CDI is not allowing the spark to advance all the time. Sounds similar to what a car will do if the ignition does not advance. Try finding a cheap CDI and swap it out.
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re: Help! Yamaha QT50 Yamahopper - 18mph max & Slow off

Postby supercub » Tue Jan 19, 2010 7:54 am

What is your compression? If it is a variated transmission, is it going into top gear ratio when it goes 18mph, or is revving wildly?
200/1 mix is way too low. I would think 32/1 is closer to what it should be but there are people here that are more familiar with the bike.
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re: Help! Yamaha QT50 Yamahopper - 18mph max & Slow off

Postby jayel » Sat Jan 23, 2010 4:19 pm

the QT's came with about a 74 main jet for break in purposes needs about 68/70 to run good, check your main jet size and check that the choke (enricher) is workng right it may be holding open resulting in a overly rich condition
Where are we going, and why are we in this hand basket?
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re: Help! Yamaha QT50 Yamahopper - 18mph max & Slow off

Postby bikerboy » Sun Feb 07, 2010 8:34 pm

Really appreciate the help, thanks.
Here's an update on what I've been doing to the bike.
I took off the exhaust/muffler to clean it and had some luck. I mixed around 1 part caustic soda to 4 parts water and filled it with the mixture after blocking one end off. I left it for 24 hours & the next day emptied the solution & loads of black gunk came out! I removed the remainder of the soda & gunk by flushing the exhaust with a hose pipe several times, & shook it + whacked it with a rubber mallet.
I wasn't expecting the gunk as the exhaust was new less than 1000 miles ago, but the caustic soda cleaned it out great & I could see the baffles shining inside.
[A note of caution I was told in case anyone else is thinking of trying this: caustic soda is nasty stuff to get on your skin so handle it carefully, and always mix the soda to the water - not the other way round as that is dangerous due to the chemical reaction that happens!]
Also if you have a painted exhaust/muffler try not to get it on the outside, like I did, as it removed some of the paint & I had to respray it.

After cleaning & re-fitting the exhaust I went on several rides over the next few weeks and there was definitely an improvement. The standard top speed has increased from 18mph to 24mph, and it is quicker off the mark than before. Also I checked the spark plug after a 20 min ride & it looks normal (not sooty anymore).
However the bike is still not going as quickly as it once did, and it is still behaving a bit strangely.. every so often it would go from 24mph max to 35mph while I was riding, even though I didn't move the throttle. It would stay at 35mph for 1-2 mins then suddenly reduce in speed, then increase decrease increase etc as if I was breaking (pretty worrying if there were any cars behind me). After doing this for 30 - 60 seconds the engine would then either go back to a steady top speed of 24mph, or die and I'd have to roll to the side of the road and wait a few mins before I could get it going again.
I cleaned every last bit of gunk/soot out of the exhaust/muffler so I don't think it can be that anymore.
I'm wondering if the engine is possibly over heating at higher speeds due to the low fuel/oil ratio or something, if this is the case though it confuses me as to why the bike doesn't go right up to 35mph when I start it ie when the engine is cold?
I will experiment with the fuel/oil premix again to see if bringing the ratio down helps.
The weather has been pretty bad in England this winter (ice/snow etc) so I've not gone out on the bike for a while, but it's been above freezing for the last few days so I'll re-charge the battery & might go for a ride later in the week to see how it's runs.
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re: Help! Yamaha QT50 Yamahopper - 18mph max & Slow off

Postby bikerboy » Sun Feb 07, 2010 8:40 pm

a_robotham wrote:I'm no expert but I believe your CDI is not allowing the spark to advance all the time. Sounds similar to what a car will do if the ignition does not advance. Try finding a cheap CDI and swap it out.

I have wondered if it is sparking incorrectly due to a fault in the electrical system (like the CDI, rectifier or ignition coil). I don't know much about the electrical system though; Is there any way to test these parts with only a cheap multimeter?
Also, would I be right in thinking that the CDI is the coils & stuff behind the round crankcase cover on the left hand side of the bike (when viewed sitting on it)? I'm only asking as I'm relatively new to bikes and the CDI seems to get referred to a lot in forums as a box/unit (ie. one piece) but it doesn't look like a box, and it is made up of several bits (2 coils and a few smaller bits). Have I got it wrong?
I've had a look for replacement electrical parts to try in the bike but can't find any for a price I can afford at the moment, I will keep looking though.


supercub wrote:What is your compression? If it is a variated transmission, is it going into top gear ratio when it goes 18mph, or is revving wildly?
200/1 mix is way too low. I would think 32/1 is closer to what it should be but there are people here that are more familiar with the bike.

I'm not exactly sure of the compression as I can't find my compression tester, but it seemed ok when I last tested it after the last time I took the engine apart. I'm pretty sure it's single speed transmission, not variated. I know what you mean about the 200:1 mix, it sounds far too high but when I had it at 50:1 the bike seemed to run even worse; this was before I cleaned out the exhaust though, so I will try lowering the mix back down to see if this helps.


jayel wrote:the QT's came with about a 74 main jet for break in purposes needs about 68/70 to run good, check your main jet size and check that the choke (enricher) is workng right it may be holding open resulting in a overly rich condition

I checked the choke and it's ok. It still has the same main jet that came with the bike when I bought it 4 years ago, and it had done around 8,000 miles when I got it. Also the bike worked ok for the first 300 miles or so. I will have a look at the jet to see what size it is though.
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Re: re: Help! Yamaha QT50 Yamahopper - 18mph max & Slow

Postby RebelMob » Sun Feb 07, 2010 11:03 pm

Do not play with the cdi. Going by the look of your spark plug I would suggest you look at the fuel system again. Looks like the enrichment circuit (choke) plunger is either stuck or faulty. Check in your manual on how it sits in the carb and make sure the spring pushes the plunger and the plunger can travel all the way. Perhaps your float valve is leaking so check that it does not leak and flood the carb.
Do not run the bike on 200:1 oil mix that is way too dry and you will end up having to put it into the dustbin. Check your compression by putting (place not press) your finger on the spark plug hole and have someone kick over the engine, if it farts your finger off the hole you have enough compression. A bike without compression will rarely blacken a plug as not enough of the combustible mixture will be drawn into the cylinder.
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re: Help! Yamaha QT50 Yamahopper - 18mph max & Slow off

Postby scout » Mon Feb 08, 2010 5:26 am

From your description and the picture I have several comments. First and foremost get rid of the fuel in the tank. Use the proper premix ratio or else all other efforts will be for naught. At 32/1 (or whatever the correct ratio) the oil mixture provides the correct amount of fuel for combustion and the proper amount of oil for lubrication. If you add more oil, that is put in a ration of 20/1 then you are essentially leaning out the fuel mixture by displacing some of the fuel with more oil. Vice versa, when you run a ratio of 100/1 then you are displacing the oil with more fuel and providing less lubrication than needed. Neither is good.
Secondly, the plug looks like there is too much fuel for the amount of air that is being mixed with it, in other words it is running very rich. I agree with the post that your choke is not operating properly. Either not opening fully or the cable is not adjusted or has slipped. Another possiblility is the float is sticking open or is gas soaked and needs replacing. That would aslo account for the intermittent running up to speed and then slowing down. If the float is not restricting the flow of fuel to the carb bowl it will provide more fuel than needed for combustion and cause very poor performance and mpg . A copy of Freds Moped Repair Guide is available on this site I think. You can probably get a copy of the QT 50 repair manual for free somewhere on line. That should set you right for the specifications. I would put the idle air screw back to the original specs and take a look at the choke and the float. Those two items could provide the unwanted fuel flow you are experiencing. I do not think you have a CDI problem nor an electrical problem.
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