compression test

[Guest]

Hi guys and girls firstly thanks for all your replys on my previous posts regarding my bandit 12 im still trying to find out why the bloody thing wont run properly im starting from scratch and i did a compression test and i got 175/180/175/180 are these good readings i cant seem to get any positive figures from anywhere ive just been told its the differential between cylinders thats important.                                             

 

[Screwriverracing]

Figures seem OK,

Put a little oil down the bores and see if it goes up.

Cheers SRR

[Guest]

i forgot to say i could only do a cold test as i cant get it to run properly 

[Bandit31]

would a leek-down test help?

[banoffee]

10 hours ago, Bandit31 said:

would a leek-down test help?

Don't forget to slice the end off before sticking it down the spark plug hole.

[Oilyspanner]

Very good Banoffee ! there's an old proverb - better a leek in the hand than a leak in your pants.

Re. bike.

Could  have shrunken 'o' ring seals on your fuel pilot screws, some carb cleaners aren't seal friendly.

Compression seems good and even, so engine seems fit.

Check valve clearances too, before trying to balance the carbs.

Have you returned the carbs to stock jets ?

[Guest]

It was always on standard jets the bike is totally standard as well I'm going to check valve clearances next 

Edited March 2, 2016 by yellowcakeuf6

[Oilyspanner]

4 hours ago, yellowcakeuf6 said:

It was always on standard jets the bike is totally standard as well I'm going to check valve clearances next 

That's good YCUF6, reduces the possibilities !

[Bandit31]

38 minutes ago, Oilyspannerlyspanner said:

That's good YCUF6, reduces the possibilities !

what possibilities ?

[Oilyspanner]

Dynojet jets are differently sized to Mikuni and jet needle wouldn't work with Mikuni needle jet etc. If everything is standard it leaves : dirt in carbs/worn parts/not set correctly, Check the v. clearances though, then work back through if required .

[wraith]

Are the carb rubber's in good condition? Try putting some washing up liquid over them and see if it runs better then.

[Dr Jon]

Hello yellow,

Can you describe the running problems in more detail? Your compression test is excellent so i doubt there's much  serious wrong with the moto.  You say the bike is all standard, does this include jets/needles etc?

I gather from a previous thread it stood for a while?

Was it running well before it was parked up?

Does it fire up quickly when you spin it over?

Will it tick over or run at low revs with careful throttle control?

Does it run well at particular throttle openings, i.e. ok at low revs but misses as you open it up? Pay very close attention to how wide the throttle is open when it begins to run badly, this is very informative. Different throttle openings relate to different jets.

You mentioned popping through the exhaust. You need to supply more detail, is this when you are trying to start it or when revving it hard? These symptoms could easily be ignition based not fuel.

If you can get it to start then quickly (and carefully) feel the downpipes and see if they are all warming up. Once they are hot spit on your finger and wipe it on each pipe. Notice how quickly the spit burns off, this will show if you have problems on individual cylinders. If you have problems on pairs 1+4 or 2+3 then this shows you definitely have ignition problems, most likely but not definitely a coil. The ECU can fail on a single pair too. 

You mention a rattle. When we say rattle we mean a higher pitch kind of sound such as is made by a loose chain or small pieces of metal tapping such as wide valve clearences. When valves and seats wear on these engines they get quieter not louder, they wear up into the head and close the gap. Valve clearances are very critical on B12 but only in fine tuning. They would need to have closed up enough to have caused a significant loss of compression which you know is not the case. To be wide enough to cause serious running problems i would think either a tappet locknut had come loose or snapped a rocker which would be seriously rattly.

Carbs out of balance do not cause rattles they cause a much lower pitch knocking  from the bottom end on tickover, this because the irregular firing pulses cause the clutch basket to shake. Very common on these motors (GSXRs and Fs anyway). Carbs out of balance are unlikely to cause serious running problems, again just fine tuning. If they're far enough apart to cause missing or backfiring they need repairing not balancing.

Check the carb diaphragms, easy way is to take air box off and just lift the slides one at a time with your finger and watch how quickly they come back down. Absolute speed isn't important it's the comparison between carbs, any differences will show up problems. The diaphragms can come loose where they join the slides which will cause serious imbalance and running problems.

I'm assuming you've checked the bleeding obvious, plenty of petrol flows from the tap. The fuel pipes don't squash up awkwardly when you fit the tank down onto the frame. The petrol is fresh and not full of rust and water. If it's got an inline fuel filter added to it this shifts fuel ok and isn't gettin airlocked. You removed all HT caps, cut back the leads a little and refitted them so contact between HT and plug caps is good.  Plugs all in good condition, clean and gapped correctly etc. etc. 

It's really difficult diagnosing at a distance but if you can give some very specific details about the symptoms we will stand a better chance. It all comes down to logic in the end, though that logic can be pretty fkin tortured sometimes!

 

 

 

Edited March 3, 2016 by Dr Jon
Typos

[Guest]

i went through everything again and i borrowed some vacuum gauges from a friend i balanced the carbs 1-2 were out of sinc and 3-4 were about right but they were all out of sinc with each other by quite a bit  and its running pretty good now i also took the pilot screws out one extra turn,it smells a little rich on tick-over but i really need to give it a good run and see how it goes i think the top end rattle is valve clearances but to be honest i think im being a little bit fussy as my old slabby rattled a lot more i cant comment on how it was running before as it was a non runner when i got it.the bike starts fine and the exhausts all heat up about the same.the standard exhaust sounds like shit and i think its strangling it somewhat ive got a quill race can to go on it which i think will help it breath better.thanks for the info i will report back in a few days  ( if the weather ever gets better )