Hi folks,

I had another thread running "Is it a carb issue and how to fix it?" which was brought to a resolution (or so I thought) so I've opened this new thread but it's kind of linked to the previous one.

After running pretty well for a few days after a carb clean and a minor wiring repair, I appear to be down to only 2 cylinders. At normal running temperature the bike sounds very rough or stalls when at idle if the throttle is not touched. The "spray water on the exhaust downpipe" test indicates cyldrs 1 and 2 are not running. If I apply choke whilst idling the engine starts to race up to and beyond 5000rpm if I let it and then all the downpipes start to heat up. All 4 cylinders appear to be receiving a good spark and the battery is fine. When out on the road (which involves a bit of throttle/clutch dexterity at junctions) the bike picks up and runs well at 4000rpm and beyond, although frankly I haven't tested that too much until I'm a little more certain of what the problem could be. Before the problem developed into the severity it now is, I noticed the idle revs needing to be adjusted back to around 1000rpm once the engine had warmed up under running conditions. I cleaned the carbs a couple of weeks ago (that's what the other thread was mainly about) and since then it was running well until recently. I haven't balanced the carbs or done any other timing/ valve clearance/ mixture adjustments (knowingly!) since the carb cleaning (or in fact in the past 25 years). The 25 years of "doing nothing" might sound odd, but the bike's been unused for 12 of those years and has only 18000 miles on her now......and there I was thinking I was back on the road (lol). Just to complete the picture, after the carb clean I did put some Seafoam in the engine oil in preparation for an oil change, but I'm not inclined to think the problem is related to that, after all, cyldrs 3&4 are still fine (?). Due to the good sparking I'm thinking it's fuel related(?) but are there any more specific ideas out there about what could suddenly have caused these symptoms to kick in? Thanks in anticipation (and frustration).

 

Pull the plugs and see what they look like. They should tell you what each cylinder is doing. I would also run a compression test just to make sure that each cylinder is up to par.

 

Hi 74Demon and thanks for your comments,

All the plugs look very similar - good condition, dry with an even light tan colouration and all the electrode gaps are in spec and all delivering a good spark. There's no discernible difference between the plugs on 1&2 and those on 3&4. I also checked the plugs a week ago before these symptoms arose and they were the same then but to be fair, I haven't done many miles (approx 50) for these symptoms to have manifested themselves on the plugs. As far as compression goes, I don't have the wherewithal to check that (at the moment) although when I ran the engine with one of the plugs out it "seemed reasonable" - not very scientific I know - but in any case above 4000rpm the bike runs well.
I simply can't see what could have made such a significant difference over the space of a week and just 50 miles.

 

If the plugs are not fuel fouled on the 2 that are not firing, then it must be not getting fuel there. I didn't follow your carb thread, but I'd say that you still have carb issues at idle and low rpms.

 

My thoughts are that the carbs are out of sync, if you have good spark. You can pull them and slid a feeler gauge between the carb body and butterfly until you get the and drag. That will get you somewhere in the ballpark.

 

I'm interested in the various views as I'm conscious that pulling the carbs (again) is not risk free. Does anyone have a good pointer as to where I might get replacement carb parts if I find I need them as my googling hasn't found anything that looks promising. I've had the engine running again this afternoon in the garage and cyldr 1 was getting hot whilst cyldr 2 is still cold - rather odd...... I'm almost resigned to pulling the carbs but is there anything in particular I should pay attention to when I open them up that could cause these symptoms? I must say I'm pretty confused as to what could cause a sudden change in performance on 2 cylinders, and then possibly only 1 this afternoon(?). I'd I've thought "out of sync" would be unlikely to create such a sudden and dramatic change although the reason I'm on this forum is because I'm no expert! Timbuctoo - I'll check that once they're opened up, but your posting wasn't quite clear about how to do that. Would you be so kind as to clarify.

 

FWIW, I was able to get carb boots for my '87 from the dealership, but other parts are becoming scarce. The '90 had a longer production run, so hopefully there's more available. When I did mine I noticed that the airbox boots wanted to curl in rather than seat properly against the carb bodies, so with the sheer number of times that those carbs have been R&I'd, I'd give a look to make sure it isn't a massive vacuum leak. Ditto for the boots themselves

If it were me, before taking it apart, I'd get a vacuum gauge and measure them to confirm good vacuum. If you get one of the sets with four gauges, you can use them to balance the carbs while you're at it, but if it won't idle, you won't be able to get that far

When you did the carbs did you clean out those little strainers next to the float valve seats?

If you drain the carbs and the trouble cylinders have no fuel in the bowl, that would make floats/valves a suspect

You can also go back to factory pilot screw adjustment, but that's less likely to be the problem unless they're way out to lunch

A 'Hail Mary' might be to run the bike from a temporary tank to rule out debris in the petcock strainer or fuel filter, but those problems tend to manifest across all four cylinders, not just one or two. It's also possible that pilot jet problems are causing havoc yet again. I'm not exactly a carb expert (healthy respect, but also distrust) so take a few grains of salt

 

I agree with all that has been said, maybe add one more thing. Not sure of your carb fuel inlet configuration but my 91 has two, one for the 1-2 bank and one for the 3-4 bank. It seems something is going on with your 1-2 bank. I say "bank" you can call it whatever you want lol!

So there is a chance that your carbs are behaving differently due to the separate fuel inlets (If you have that). My 1 and 2 were running cooler when I checked a few days ago but haven't looked into it yet - Maybe somebody who has insight?

One more thing. I remember you can't get some pieces off due to stripped screws. Use a dremel (or similar) tool to grind slots in stripped screws and you will get a good bite on them. Make sure you order replacements!

 

Some more useful input from Tentacleslap and Gronkfries (Hi again guys, this is starting to feel a bit like a party, without the alcohol!).

So, a few responses to your suggestions {I hope it's not too long, but your input deserves a response}....firstly, I haven't yet pulled the carbs - just trying to summon up the courage and gather as much info before leaping in.....

The airbox and boots looked and felt OK when the carbs were off 2-3 weeks ago and I've just eyeballed them again (albeit with the carbs still mounted) and they seem OK, but I'll take a closer look if/when the carbs are pulled. {By the way, the carbs have only been off once in 25 years, and that was 2-3 weeks ago and that was due to my stupidity of leaving it sat for 10+ years with fuel in the tank, so I can't complain too much about that!).

As far as replacement parts are concerned, it's really the internals that I am most worried about. For example, float valve if the conical tip(s) is/are shot...... or a problematic diaphragm, without a replacement I'd be, well, stuck!

I haven't touched the pilot screws in the 25 years I've had the bike so I'm assuming there's nothing to suspect there (is that a reasonable assumption?).

I've run the engine from a temporary fuel source and the problem continues so it's not petcock strainer/filter (not fitted) related (but I'll return to the word "strainer" in a moment). Equally, when I suck on the vacuum line connected to the rear of the petcock, the fuel flows freely. Whilst on the subject of the petcock though, it is really "sticky" to turn onto off/reserve. Almost like there's a "notch" causing it to stick until I ease it into a position it will fully twist. That's a very minor issue for now but if there's a suggestion out there, I'm all ears.

On vacuum gauging.... I'm close to investing in a set of those as I've thought balancing might be a problem anyway, but it's probably not causing a total lack of running in the trouble cylinders. I think it might be contributing to the issue but my opinion is that it's unlikely to be the main cause of the issue - but I may well pay a set anyway so that I can do the balancing once all 4 cylinders are running.

Float bowl draining.....haven't done that yet but I will as soon as I have 5 minutes to do that. Very fair comment though.

Dremeling the tops of the screws.........yikes, that scares the hell out of me in case it goes **** up, but yes, I hear what you say. In fact, it's the 2 (or should that now be 1) cylinders I could get into last time that are problematic so I'm hoping it won't be a problem, if you see what I mean. 2 final things:-

Fuel supply lines (x2) - from memory there are 2 T-shaped supply lines providing fuel to the 2 "banks". That could certainly explain why one bank is OK and the other isn't. That's definitely worth more exploration on my part, although now that one of the 2 problem cylinders appears not to be a problem maybe that's not the issue either, but I will definitely investigate.

"Little strainers next to the float valve seats", no, no, no I didn't clean those last time around as I didn't know they existed or even saw them (please, no comments) until after I'd refitted the carbs. That could also explain what appears to be a cleared cylinder 1 as perhaps a bit of crd had got lodged and then released again(?). I'm trying to imagine the possible scenario that could cause the overall effect. I'm not trying to be "too" scientific about this diagnosis process, but often if you can imagine how the problem is being created, you can find the actual cause and avoid a lot of unnecessary effort and potential secondary effects - like rounded screwheads!

After all that, I think the next (hesitant) step after draining the bowls to see what additional evidence that provides is to take the carbs off again. The only thing that makes me reluctant to do that is the risk of collateral damage I referred to just now, not the work/time/money involved.

By the way, nobody's mentioned timing or valve clearances. Does that mean I can safely assume for the time being they're unlikely to be the source of the problem?

I will keep posting progress as it happens. And in the meantime if there are answers to my questions, well, you have my number! Thanks for all the inout so far.

 

Timing and/or valve clearance would not just go suddenly nor would carb sync.

Maybe lets start over. What year? MK1, maybe fuel pump? MK2 or 3 with vacuum petcock? I had a vacuum leak on my vacuum petcock that retarded fuel flow. Do you have good fuel flow? Did you pull the feed line to see if fuel is flowing to the carbs?

The coil on the left fires 1&4, the right one fires 2&3. If you still have trouble with 2&3, switch the coils to see if the problem follows the coil, maybe a bad coil?

I almost forgot to mention the stupid air filter that rots and gets sucked into the top of the carbs. There is a breather with foam that degrades and migrates into the top of the carbs. On my '94 its under the tank more up front and to the right. Inside the little black box was a foam filter that ended up on top of the carb pistons. I believed I replace it with some Scotch Brite that I cut to fit. I don't know if the foam parts would cause this kind of problem.