Missing/popping when hot?

[jono035]

Hoping someone can help with a little bit of trouble shooting...

Bike: GSX250F Across, inline 4 250cc, 2x dual-throat mikuni carbs.

I haven't been having any problems over my 10km 20 minute commute in the morning (bike runs well, revs freely, decent power) but if I go anywhere further afield (particularly on a warm, sunny afternoon) then after about 20 minutes riding the bike starts to misbehave. Basically it seems to lose low-rpm (4-7k) power and starts missing/popping if I try to cruise at that rpm. I can still get the bike to accelerate but it feels like it requires more throttle and higher revs (10k) to increase my speed at all.

I've tried spraying a little bit of wd40 around the carbs when I got home after I had the problem to check for a vaccuum leak. I didn't hear any changes to engine rpm but a little bit sprayed in the air intake made it pick up noticeably (to verify the wd40 would do anything). I've also had the carbs apart and taken a look. All the jets looked clean and round, the rest of the carbs looked clean, needles straight and unworn, nothing nasty looking in the float bowl.

I don't know what condition the valve clearances are in (sounds a bit rattly). I've done a compression test while cold with the results being 3 cyls at 180 psi and 1 at 130 psi... Pulled one of the spark plug caps and the spark was good and solid, although that was while idling, not with it missing.

Also, since I got the bike it has never wanted to idle on the pilot circuit, even when hot. It either needs the enrichener ('choke') open to idle or the fast idle screw wound in until it sits at around 3k. This is the same whether cold or hot. If this isn't done then it runs for a little bit, then sounds like it misses once and the rpm drops slightly... It keeps doing this until it stops completely. It sounds pretty rattly under 2k rpm but I don't know if this is normal for when you're sitting on top of the head.

I'm going to pull the carbs off again either tonight or tomorrow to give them a going over with carb cleaner just to make sure...

[slofox]

One cylinder at 130 would be worrying me a little - that's quite down on the others...

Does the temp fluctuate at all?

[jono035]

One cylinder at 130 would be worrying me a little - that's quite down on the others...

Does the temp fluctuate at all?

you mean according to the gauge? doesn't go up off the stops really unless i'm sitting idle...

[Viscount Montgomery]

Won't idle properly hot or cold and needs the enrichener circuit open to idle, means fuel starvation and it's running lean on the pilot circuits. Strip and check the petcock(s), also check the breather hole in the tank lid, could be partially gunked.

Back to the carbs, totally strip and remove everything, jets tubes and float valves. Blow thru all the circuits and passages you can find. Blast CRC thru them. Peel the plastic off some electrical wire and poke an appropriately sized strand of wire thru the circuits to see where they go and where they connect/exit.

For the valves, whip off the cam covers and bung in feeler gauges when the motor's cold. Check em' all. The correct clearance figures found in manuals or on the net. Sit the cam lobes vertical to the valves on each when measuring. Peek thru the ports and look for dark oil seepage on any valve stems. Means buggered valve seals if there is.

For compression, tip some oil down the low reading cylinder. Check it's compression reading again, if it rises somewhat then the ring(s) are headin' south in that pot.

Seems funny how the motor runs fine for 20 minutes then craps out after that though..

Don't really want to delve into electrics.. possibly an overheating regulator/rectifier..

Check the tank breather though, ride with the lid up and see what happens

[Viscount Montgomery]

ps check vac hoses for pinholes

[jono035]

Won't idle properly hot or cold and needs the enrichener circuit open to idle, means fuel starvation and it's running lean on the pilot circuits. Strip and check the petcock(s), also check the breather hole in the tank lid, could be partially gunked.

Back to the carbs, totally strip and remove everything, jets tubes and float valves. Blow thru all the circuits and passages you can find. Blast CRC thru them. Peel the plastic off some electrical wire and poke an appropriately sized strand of wire thru the circuits to see where they go and where they connect/exit.

For the valves, whip off the cam covers and bung in feeler gauges when the motor's cold. Check em' all. The correct clearance figures found in manuals or on the net. Sit the cam lobes vertical to the valves on each when measuring. Peek thru the ports and look for dark oil seepage on any valve stems. Means buggered valve seals if there is.

For compression, tip some oil down the low reading cylinder. Check it's compression reading again, if it rises somewhat then the ring(s) are headin' south in that pot.

Seems funny how the motor runs fine for 20 minutes then craps out after that though..

Don't really want to delve into electrics.. possibly an overheating regulator/rectifier..

Check the tank breather though, ride with the lid up and see what happens

Cheers for the reply there. Stripped the carbs right back last night, took everything that could come out, out (got them back together around midnight) and gave them a good going over with cleaner and a toothbrush. Found a possibly gunked up pilot jet and main jet and one of the floats was on the absolute edge of service spec. Fixed those and it idles ok when hot now with the pilot screws wound right out (fixed pilot jets control the mixture I think, screw controls air+fuel rate).

The carbs are pump-fed (the tank is under the seat) and I couldn't feel the pump relay clicking more than about once per second when riding (it goes bonkers if I have empty float bowls or the tank disconnected).

I'm not sure whether it is electrics or not... The voltages still read fine when it is hot and idling, the lights are still strong and not flickering etc. I had the cap off while hot and idling and there was tons of spark (easily bridging an inch and no misses) but then again it doesn't miss when idling, only when under load apparently.

I've sprayed some wd40 around the vac hoses when the bike was hot and the engine rpm stayed constant...

One thing that I remember reading somewhere with these carbs is that the throttle slide holders wear out causing mixture issues. I didn't notice that they were all a little worn and 3 of the 4 had a bit of play. It looks like it allows the slide to drop past the needle a little bit which would richen things up. When I pulled the plugs 3 were black, 1 was white, so this could be the problem... Might see how hard new ones are to get, otherwise it may be a case of adjusting the needle height to try and compensate (eurgh.)

Edit: It looks like a real prick trying to get the head off and I don't have allen keys that are up to the task and the moment so that is on hold. Now that I've got the plugs out again I might try taking another shot at the compression test and see if that cylinder is a ring issue.

Thanks for the advice mate, any feedback on any of this info would be appreciated. In terms of the spraying wd40 around the vac hoses to check for vacuum leaks I don't know how reliable that method is, but given that 3 of the plugs look pretty dark and sooty then I'm thinking it is more likely that it is running rich? At least on those 3 cylinders... #4 is a mystery...

[jono035]

Took the slides out of the carbs... Sure enough, the 3 rich cylinders have loose/worn slide holders and the lean cylinder is still nice and tight... Lowered the needle on the 3 rich cylinders and raised it on the lean cylinder. Went for a ride around for an hour starting from a hot engine without the problem showing up. That'll get me around until I can get new slide holders or fix the existing ones with epoxy at least...

[FruitLooPs]

I've got a very similar problem with my GSXR750, I've got metal slides with plastic holders.... hmmm might've helped me out! Had a vac leak or two I sorted, fully stripped and cleaned mine twice over - got a new jet kit + adjustable needle and synch'd. It still has a small miss I cannot pin point, and like you if I idle it'll be okay for 30sec or so with the odd miss, then it'll start to choke and eventually stall left alone.

As for the white plug, if thats the low on compression cylinder you'll want to watch out. Eventually you could burn an exhaust valve and it will run intermittently or not at all on that cylinder. Ask me how I know

[jono035]

I've got a very similar problem with my GSXR750, I've got metal slides with plastic holders.... hmmm might've helped me out! Had a vac leak or two I sorted, fully stripped and cleaned mine twice over - got a new jet kit + adjustable needle and synch'd. It still has a small miss I cannot pin point, and like you if I idle it'll be okay for 30sec or so with the odd miss, then it'll start to choke and eventually stall left alone.

As for the white plug, if thats the low on compression cylinder you'll want to watch out. Eventually you could burn an exhaust valve and it will run intermittently or not at all on that cylinder. Ask me how I know

When I saw that white plug I did have a bit of an oh-shit moment but thankfully I wrote everything down, cylinder #2 was the low compression cylinder and #4 was the one running lean. I haven't pulled the plugs again since changing the needle positions because it is running plenty well enough now. Will go through the process again once the replacement throttle slide holders show up ($11.50 each from Coleman's, 7-10 days lead time air-freighted from Japan, replacing all 4).

It was quite easy to tell that the throttle slide holders were worn. When I had the slides out with the holders (and needles etc.) still in them you could move the holders up and down inside the slide about a mm or 2 just with your fingers. Pulling them out and comparing them side by side showed that they were all different profiles to each other, with the one that had no movement being the obviously least worn.

I don't know how similar the holders would be in that thing but it'd definitely be interesting to see if you can notice any play in them if you have them out...

[jono035]

Thanks for the advice Danae, had checked the plugs previously while idling but not while the problem was occurring under load.

Also the plugs are friggin annoying to get out, I have to remove the battery, holder, starter relay, fuse box etc. and then balance that somewhere so it can stay connected to test the spark! A few seconds turning over on each plug head isn't going to do all that much.

Should really get a flexible extension for my socket set, can't get any of my other tools in there properly...

[motorbyclist]

It's refreshing to see a reasoned, non-destructive approach to mechanical work on KB

I can give you a hand with checking the clearances as it isn't hard. I've done the bandit before which is essentially the same motor and know what to watch out for... and have the tools.

PROTIP: the sparkplug is your fundamental diagnostic tool - we check this FIRST, before we go stripping carbs

the spark check is done with the removed plug grounded against the engine block/head. this tells us if the plug is infact creating a spark. spark erosion of the lead cap tell us nothing.

oh, and the "enrichener" is infact called a choke, as it works by "choking" the carburettor, and has been called this by manufacturers/industry for over a century now

[jono035]

EDIT: wtf kb fails me

Was that you who posted that rep, Andrew?

What can I say, I'm lazy and getting the plugs in and out is a bitch.

[motorbyclist]

Thanks for the advice Danae.

lol that was me, danae left herself logged into my computer, thus causing some epic posting fail

[motorbyclist]

What can I say, I'm lazy and getting the plugs in and out is a bitch.

so is stripping carbies... that said when I did jafa's bandit the bloody lead caps were seperate from the lead, so any effort to reinstall the plugs was fraught with uncertanity.

and some idiot at suzuki put a whole lot of plumbing directly above the motor

[jono035]

I figured it must have been you, but didn't want to assume anything! I'd like to think that I'm man enough to accept advice on engine repair/maintenance from a woman...

Yeah, the plugs are virtually impossible to get out when hot because the leads go soft and pull out of the caps... Then I got the pliers and tried to pull the cap off, but that resulted in one of the caps cracking apart... It's a horrendous pain, is there some better way of doing this? I'm going to grab some large heat shrink and stick that on there so I've got something to pull them out by.

The carbs needed cleaning anyway, so they were a good place to start. Found a couple of the pilot jets had some crap in them so that seemed to clean up my idling issues. Was also checking for any vacc leaks at the time but everything looked pretty solid.