AberdeenBikers

A friend from work has a Yamaha Thundercat (1998) and she says it's not running right.

Her husband had a quick look at it and thinks it's only running on 2 or 3 cylinders.

How can this be confirmed and how can it be fixed?

You will definitely know if its on 2 or three just by the rough sound. Probably ignition related. Just check all the leads to start with. Maybe use a spare plug on each lead to see if it is sparking.

Obviously dont touch the plug while doing this.

I'm not all that clued up on bike mechanics especially these old school carbed machines but my YZFR6 (similar engine to the T-Cats) was running like a bag of shizzle start of the year; wouldn't idle, kept missing and throttle response was pretty piss poor.

Got it booked in for a service and plugs were replaced. Came back a totally different bike.

When riding, if its firing in 3 or less cylinders the bike will feel as if it is holding back. It is very obvious when it happens. You open the throttle and you get a kind of "Drrrrrrrrrr" noise as well under load. Very very obvious.

Check the battery condition. I had a lot of trouble with my CBR misfiring and running on 3 cylinders for ages. Could not work it out. Once a new battery was fitted its never occured since!

After this the most likely culprits are plugs, blocked carb, ignition leads. Try running the bike and spraying carb cleaner into the air intake. the bike will start to die as it goes through, but then will pick up again afterwards.

Come to think of it my old SRAD did this. Turned out to be some dodgy fuel. I just took it out and caned the crap out of it and it cleared up :I can't believe it's not butter! .

Grant wrote:A friend from work has a Yamaha Thundercat (1998) and she says it's not running right.

Her husband had a quick look at it and thinks it's only running on 2 or 3 cylinders.

How can this be confirmed and how can it be fixed?

I'm by no means an expert but when my first Bandit started playing up last year, it turned out only to be running on 3 cylinders. . .

The problem was just a build up of crap in the carbs. . . a good clean out and re-balancing sorted it out perfectly

Might not be the same problem thats causing her bike to run soft but it would be a good place to start

Spit on your finger tips and quickly touch each header pipe about a foot down from the head, you will soon find out which cylinder which is not pulling its weight, if it is missing on 2 (1 &4 or 2 & 3) then the coil for that pair is probbably the best place to start looking.

using a pair of plastic handled pliers, remove each spark plug lead (one off at a time and replaced before the next is removed). Be sure to grip the plug socket and not the actual HT lead as you well may pull the thin lead from it`s cheap manufacturers crimp.

Once each lead has been removed, observe for rough running and change of note. Obviously if there is no change in note or peformance then the removed lead / plug is questionable.

IZINBARD wrote:Spit on your finger tips and quickly touch each header pipe about a foot down from the head, you will soon find out which cylinder which is not pulling its weight, if it is missing on 2 (1 &4 or 2 & 3) then the coil for that pair is probbably the best place to start looking.

Quickest check without dismantling anything is to check which headers are cold as above theory but I wouldn't be sticking my fingers anywhere near them. a small spray of WD40 on to each header soon confirms this, the one thats still wet is not firing.
if its just one the next check is the drain on the float bowl to see if there's petrol in the carb, if not theres some poo in there blocking it. if theres fuel check the plug. take it out and check its firing on the same lead.
if its ok pull off the carb and clean out.
If the above doesn't work he could be in a world of valve trouble.

Ha ha the spit bit was for the wimps, the headers will not be that hot if it is done in the first 30 secs or so of starting up the bike, have to wait a lot longer for WD40 to start smoking unless you rev it hard from cold. Either or same effect.

Make sure the coils are connected to the correct cylinders. One coil should go to cylinders 1 and 3, the other to cylinders 2 and 4.

DON'T ask me how I know this

Also check the HT caps are in good nick, new NGK ones are only a couple of quid each if suspect.

If a coil has to be replaced it's a good idea to replace them in pairs as one will almost certainly go shortly after the other.

TheSaint wrote:Make sure the coils are connected to the correct cylinders. One coil should go to cylinders 1 and 3, the other to cylinders 2 and 4.

DON'T ask me how I know this

.

How do you know this ?

you must have a very strange crank :I can't believe it's not butter!

blackstorm wrote:

TheSaint wrote:Make sure the coils are connected to the correct cylinders. One coil should go to cylinders 1 and 3, the other to cylinders 2 and 4.

DON'T ask me how I know this

.

How do you know this ?

you must have a very strange crank :I can't believe it's not butter!

On a conventional japanese four, pistons 1 and 3 and 2 and 4 come to top dead centre as pairs, 1 will be on the compression stroke, 3 on the exhaust stroke. 2 and 4 lag 180 degrees behind and are on induction or power strokes. Each coil fires as the pistons on its "up" stroke approach top dead centre. The piston on the compression stroke goes bang, causing the power stroke. The spark in the cylider on exhaust stroke does nothing, other than, erm, spark. It's called a lost spark ignition system.

My first Japanese four was a 1982 Yamahaha XJ550 I bought in 1983.

As for how I found out the hard way, not telling

Nope sorry, that would be 1 and 4 as a pair and 2 and 3 as a pair, hence my post re two down pipes being cool then go straight to the coil that services that pair. Firing order is usually 1342 but not always, big bang V4's a point in case.

IZINBARD wrote:Nope sorry, that would be 1 and 4 as a pair and 2 and 3 as a pair, hence my post re two down pipes being cool then go straight to the coil that services that pair. Firing order is usually 1342 but not always, big bang V4's a point in case.

Depends on the crank configuration (whether 1 and 3 go up in unison or 1 and 4 do) but I take your point and I'm too tired to go through all the permutations of firing order and which coil feeds which plug right now. I agree with you a duff coil could cause 2 pots not to fire, and I think you'll agree with me that getting the HT leads on the wrong plugs could also be a cause.

Never come across that configuration before for an inline 4, the vibration would twist the crank and at high revs probably snap it in two, but that is not to say that it does not exist, its called a 180 degree crank when used in a V4 and I think the RC30's had it, special for race aplications only. Old british bikes use to use a 180 crank on some of the old twins but I know nothing about them, though Fido would.

As I understand it Big bang on inline fours have 1 and 4 firing at the same time and 2 and 3 firing at the same time.

Plug leads on the wrong plugs would give a miss fire but I had the impression from the original post that this just started happening, not as a result of somebody working on the bike.

:I can't believe it's not butter! I have nothing better to do at the moment so here is a list. I have worked on most types of engine over the years, air cooled, water cooled and oil cooled, singles, parallel twins, V twins, triples, inline 4's, V4's, inline 5's, straight 6's, V6's, straight 8's, V8's, V12's, V16's and V20's, the last 4 supercharged and the last 2 supercharged and turboed as well, not had the pleasure of working on a square 4 though. As far as fuel goes, petrol, diesel, gas and kerosene. The biggest was a 2 stroke which had 10' diameter vertically opposed pistons (no cylinderheads) running in 5 cylinders and the smallest had a piston 10mm diameter. I have serviced and rebuilt turbos and Roots screw superchargers, also done a bit of work on compressors, mopeds, bikes, cars, trucks, boats, ships, plant-diggers-dozers-graders-pipe laying stuff (funnily enough not much time on Cats) and industrial stationary engines (for hospitals and the like) and a bunch of stuff on rigs and platforms.

IZINBARD wrote: Never come across that configuration before for an inline 4, the vibration would twist the crank and at high revs probably snap it in two, but that is not to say that it does not exist, its called a 180 degree crank when used in a V4 and I think the RC30's had it, special for race aplications only. Old british bikes use to use a 180 crank on some of the old twins but I know nothing about them, though Fido would.

As I understand it Big bang on inline fours have 1 and 4 firing at the same time and 2 and 3 firing at the same time.

Plug leads on the wrong plugs would give a miss fire but I had the impression from the original post that this just started happening, not as a result of somebody working on the bike.

:I can't believe it's not butter! I have nothing better to do at the moment so here is a list. I have worked on most types of engine over the years, air cooled, water cooled and oil cooled, singles, parallel twins, V twins, triples, inline 4's, V4's, inline 5's, straight 6's, V6's, straight 8's, V8's, V12's, V16's and V20's, the last 4 supercharged and the last 2 supercharged and turboed as well, not had the pleasure of working on a square 4 though. As far as fuel goes, petrol, diesel, gas and kerosene. The biggest was a 2 stroke which had 10' diameter vertically opposed pistons (no cylinderheads) running in 5 cylinders and the smallest had a piston 10mm diameter. I have serviced and rebuilt turbos and Roots screw superchargers, also done a bit of work on compressors, mopeds, bikes, cars, trucks, boats, ships, plant-diggers-dozers-graders-pipe laying stuff (funnily enough not much time on Cats) and industrial stationary engines (for hospitals and the like) and a bunch of stuff on rigs and platforms.

I just break them. I bet I can still pee higher up the wall than you though

TheSaint wrote:

I bet I can still pee higher up the wall than you though

That shouldn't be difficult

:I can't believe it's not butter!

IZINBARD wrote:Spit on your finger tips and quickly touch each header pipe about a foot down from the head, you will soon find out which cylinder which is not pulling its weight, if it is missing on 2 (1 &4 or 2 & 3) then the coil for that pair is probbably the best place to start looking.

Finally got around to have a look at the T-Cat this afternoon.

Header pipes on the outside (presumably 1&4) were stone cold whereas 2&3 were hot as you'd expect.

How can I confirm if it's the coil, leads or plugs that are not working on 1&4?

I don't want to order a new coil until I know it's the coil that's knackered.

Take the coils from 1&4 and swap them with 2&3.
It's unlikely to be the leads and plugs if both 1&4 aren't firing.
If you can get it to breaking point they'll probably let you try another coil on it.
Good luck

As above but you may find that it will not start as the plugs on 1 & 4 could now fouled, it would be a good idea to clean them before you do anything else (and if possible replace them all as they are out anyway.) Clean with a wire brush, and cook with a blow torch or gas burner from a cooker until all signs of oil / petrol are gone, it will get hot so hold with pliers. 2 or 3 minutes held just above the flame should do. Don't forget to regap them before putting back in. Good idea to do that before getting them hot

I'm not a sparky but Re the coil in question, mark which wires go where with a label of some sort then remove the low voltage wires (should only be 2) put an avo on them +ive red probe into the connector and black -ive probe to earth one should show 12vdc or more, the other should show zero vdc.(earth wire) If you get those sort of readings look to the coil, if not, trace back the wires looking for a brakeage and ohm out the earth wire or replace with a temporary wire from the coil to earth. Best of luck Grant.