Manky Monkey Motors

I presume that it's an intermittent electrical fault somewhere on my GS250T. Here's the thing. It seems to me that occasionally (twice today) I can be riding along quite happily and suddenly the bike slows sharply and seems to be running on a single cylinder, then just as suddenly the engine will pick up again and run fine. It seems to do it most when I'm riding in 5Th of 6Th gear at over 7500rpm. I changed the Coils the other week because the previous pair's HT's were breaking down where they had been joined and the moisture was getting in, causing the high voltage to escape to the engine casing, admittedly although the plugs are only a month or two old, they should be replaced also due to running them on knackered HT leads, so I'll do that. But the question still stands, where do I look to try and locate this gremlin, I know that there are a whole bunch of things that I can be changing to try to eradicate this issue.

If the ATU (Signal generator) is slightly off its correct position, will that cause it?

Would it be caused by the CDI being faulty?

Or should I be looking for a wire that has a break in it but still makes contact most of the time.

I know this is probably stupid question coming from an electronics/electrical biased person but when it comes to vehicle electrics a tad lost and confused.

Thanks for any ideas and help put forward, the more suggestion the better in my eyes.

Thanks again chaps Mr_G.

hi it sounds like a cdi fault to me   but you best ask the mighty knowledgeable one ========TOAD

 if he dont know no one will

Toad do you know?  :D And luckily I do have a spare CDI unit.

i reckon its the cdi unit i had a 400t that did exactly the same. as you,ve got a spare swap it over and give it a try. that should sort it ;)

So I got home from work and went straight out and put my spare CDI on and it's not made a blind bit of difference. What about the position of the ATU then, there are 2 locating/securing screws that hold it on to the engine casing and the locating holes have a few degrees of adjustment spacing I.E. I can rotate it slightly to ensure correct alignment.

What about if it's not actually an electrial fault? what else could it be? fueling issues? That's what my Boss seemed to think when I told him about the symptoms.

Like I said, I really should change the plugs again to be sure, and I really can't afford to send it to the shop and have them charge me extortionate amounts for something that I really should be able to do myself, I mean it's not like I haven't taken it appart enough in the past to know what to do.

I'm just stuck on where the fault lies. Hair loss imminent!

All comments and suggestions are welcomed with great appreciation, as this is my main commuting vehicle I need it sorted asap.

Cheers Folks.

Why did the Irish man wear 2 condoms? To be sure to be sure.

without being there it sounds like something is shorting somewere, if you say its intermittent, do free stuff first eg carb bowls for signs of poopy stuff etc, then check elec connections for coils, kill switch etc

finally plug caps and plugs..

report back, try again  ;D

Plug caps were replaced when I replaced the coils.

On the way home from work today I stopped at some lights and could smell petrol very strongly, took a peek down at the engine (as you do) and noticed Fuel p*ssing from the bottom of both carbs. Switched off the engine and pushed it about 1/4 mile to the nearest bike repair shop, went in and had a chat with the tender (who knows me quite well by now). Told him about the issue i was having and he was adamant that it is not an electrical fault. He said that it sounds like it's fuel starvation. He managed to sell me some fuel pipe and while I was there I got a pair of plugs. Rode the bike home still p*ssing fuel and endeavored to take it appart.

I found that the short length (1.5 inches) between the carbs and the fuel filter was more holy than Mr God himself, I replaced the pipe and put it back together. Had a job to start it at first (probably due to length/path the new pipe is and takes) but after jumping it from the car I eventually got it running. I think that the initial symptoms may be corrected now but now the bike will rev up and ride fine (short test up and down the road a couple of times)  but now when I idle for abit, unless I rev it every few seconds while I am waiting, after a short time of idling nicely the revs get less and less until it dies. It will start again but something is obviously not right. I called Doc earlier to ask him if the 4mm bore pipe that I replaced the old 6mm pipe with would be restricting the flow of fuel too much at idle due to the decreased vacuum pressure in the carb, but he seems to think that if that was the case the bike wouldn't ride well either. I still want to replace the 4mm with new 6mm to be absolutely sure that that is not the problem and before I take the carbs appart I was hoping that some of you folks might have a few comments/suggestions on the novel that I have just written.

Cheers Peeps.

change the pipe for 6mm, check float bowls,

see if yours has the small gauze filter just as it goes into carbs.

double check none of pipes are crimped/squashed with tank on..

I will try to get some 6mm pipe tomorrow from a different vendor, as for a small gauze filter, where are you talking about? actually in the carb or the in line fuel filter? If it's the latter then it's a small clear plastic cylinder with a cone of filter paper inside, there are a few specs of dirt in it but nothing major that could be causing a problem in my opinion anyway.

Correction. Bike is unridable today. I just got it out, started it eventually, there's no power and will only reach 4k rpm. In my eyes it has to be the 4mm pipe thats the problem! Will replace it with 6mm asap and try again. Grrrrrrrrrrrrr

Altho' the pipe won't help it should have started, I think you have a blockage in either the carbs or the fuel tap.

couple of things i was thinking about, make sure pipe is not closing when open throttle, plus check vacuum pipes to tap for cracks..

try it on prime if you have and check if better and how it flows..

Just been to bitza bikes and got me some 5.5mm pipe, the vacuum pipe is fine. This is another shop that knows my and my bike issues quite well, I didn't even need to say what I had, he just said that they meaning my bike has a small filter in the carb above the needle, I was unaware of that and I don't recall cleaning it the last time I stripped the carbs. I'll try that along with your suggestions.

Many thanks, keep them coming.

OK so I took today off work in order to get this issue sorted. I stripped out the carbs, took them appart and made sure that they were all spanking and clean, which they were, no shyte is the gauze filters and clear every where else. Removed the 4mm bore pipe and replaced it with 5.5mm put it all back together, took it out to the road and gave it a blast. Seems to have cured it. Now everyone said that the reduced fuel pipe shouldn't matter but what about length? I have used as little pipe as I could get away with and the bike seems to be doing as its told now.

So my electrical gremlins were non existant and the problem was fuel.  ;D

Thanks for all suggestions and information on here guys you have been a great help.

I would offer to buy you all a beer but I'm not that rich.

Cheers again folks Mr_G.

Aaaaaaaaaaaaaaaaaaaargh, After going for a spin on the bike and all was fine, I went out again to go somewhere and the bloody heap of doodoo is at it again! I have tested the fuel tap, I know there is plenty of fuel getting to the float bowls, all the jets are clean and so are the emulsifiers, the diaphragms are in good order, everything is clean clean clean. What else can it be? could valves be sticking or something? I'm so hacked off, I thought I had fixed it and not its back to square one!!!! It looks like I'm going to have to take it to the shop and get them to do something with it cos it's obviously far beyond me!

:(

Do'nt know if this will help,or not.Have you tried running it with the "points" cover off ?
If the gasket,has been replaced,or ommitted,you can get arcing,very common fault on 70's,80's stuff,especially XS 250/400's.
Try running it in the dark,you will see more clearly if you have arcing due to bared wire.

Petrol cap venting ?
Carb diaphragm ?
Petrol tap diaphragm ?
Perforated float ?
Worn float needle ?

OK so I just came in from the garden after spending all evening working on the bike. Tonight I have :-

Taken the fuel tap off and apart (no faults found there) Re assembled and fitted.
Tested that the fuel cap is not at fault, Apparently it vents through the key hole. I can blow through it and after I refitted it and did something I heard a sucking noise coming from the cap so I know that's working.
Re made all earth connections and added a couple to ensure that that's not a problem.
Had a furkle with some wiring.

I then took it out on the road and tried it. I then discovered that only the left exhaust was hot so I wheeled it back and stuck my strobe on the right HT and found that when the problem occurs the Right plug is not firing. After a few moments scratching my head and deliberating I called my Boss and had a chat with him about my findings. We came to the conclusion that the chap at Bitzabikes was more than likely correct in that he said it sounded like an electrical fault. We discussed that I had replaced the coils and swapped the CDI unit but the fault was still occurring. We then talked through the ignition system and ended at the conclusion that the Signal Generator was most likely at fault. I then endeavoured to investigate, I then found that the brown signal wire PVC coating was warn away in places where it had been rubbing against the ATU and causing an intermittent earth fault, i.e. the small signal voltage created by the SG was going to earth and not to the CDI, I swapped this for my spare SG and tried it again, this time the bike would not even start. I guessed that my spare SG was dead so I wrapped the chaffed wire on the original with insulating tape and put it all back together (I also noted that my spare SG had a wire retaining clasp screwed to it and the original one did not so I nabbed it and stuck it on). Tested it up and down the road a couple of times, tidyed up and then went for a 10 mile ride. The fault seems to have reduced now but it is still happening.

So I suppose that I am getting somewhere with it now, but am wondering if I need to replace the ATU and SG as this is the second time that I have bodged the SG back together.

Thanks for all suggestions and please keep them coming, If I don't need to spend £3k on a new bike I would rather not but time is running out rapidly.

Thanks again Mr_G

If one bit of wiring is chafed it may be worth checking as much of the wiring as you can cos it could be happening elsewhere as well.  Electrickery is funny stuff, it drains away or ends up in the wrong place if your wires aren't all proofed!