[Closed] Internal combustion engine gurus needed. Motorcycle content

From generic experience with Japanese bikes of this vintage, check that the throttle spindles / linkages on the carb bank are moving freely and returning to the idle stop smartly. If it has mechanical balance adjusters in the linkage check those too.

I would have tentatively thought that you might be able to tell from the engine sound if it was one carb slide hanging and carrying the rest, or whether it was just the throttle not closing fully (for whatever reason).

When it hangs, the engine sounds very clean. I mean, it doesn't sound like it's only running on 3 or anything.

The throttle stop seems to snap back very cleanly and all the butterfly valves look to be in the same position.

air leak? Perished carb rubbers? worn needles? dunno really

Is it getting worse after you've got it hot? If so the air leak in the inlet rubbers might only be there once the tiny little cracks in ther perished rubber have expanded.

What state were the carb gaskets and o-rings in when you stripped them down - or did you replace with new?

I've tested for vacuum leaks with the bike hot and no change in revs.
The gaskets/manifolds look pretty new actually so I'm fairly confident it's not them.

It did initially seem to get worse when it's hot but actually, yesterday it was hanging after only a short while so maybe that theory is out.

The float bowl gaskets seemed in good shape. I replaced one o-ring on one of the float valve seats that was pretty much a black goo, cleaned everything out well. All the others seemed good.

Just had sliders out again, checked all bores and smoothed down slides a little with 1000 grit wet and dry. They slide freely.

Could it be connected to the charging system in some way?
When I took it for a long ride after rebuilding, the battery had been on the charger a long time.
I wonder if now that it's likely dropped that it's somehow related to the idle issues...

Just to rule out try disconnecting the throttle cable at the carb. It's an old bike, rust and tension in the cable can cause the revs to creep

Throttle cable .. as above dooh, try stretching the return spring also

It's probably not this but it's easily discounted; does it have a fast idle on the choke? Is this sticking? I've seen this cause cause issue even when the throttle cable appears fine.

does it have a fast idle on the choke?

When the revs are really high, pulling the choke on makes them plummet, pretty much so the bike will stall.

craigxxl - Member
Just to rule out try disconnecting the throttle cable at the carb. It's an old bike, rust and tension in the cable can cause the revs to creep

I'm 99.9% sure that the throttle cable is totally fine. When disconnected it runs silky smooth. The throttle return spring snaps back all the butterfly valves very promptly too.
However, I'll try what you suggest anyway.

Here's a little vid of my poor bike going mental...

I think Smokey Joe is onto one possible cause: ie the perished rubber of inlet hoses only being exposed to air leaks once engine hot/expanded. Have you removed the inlet rubbers and pulled them around to expose cracks? I would replace with new just for elimination purposes. All clips new? Same symptoms.

It's a puzzle why you're not getting overreving when spraying carb cleaner in when hot thus proving air leak hypothesis.

Sounds like you know what you are doing with the carbs: any minor glitches would show up probably in rev range, power losses etc rather than hanging idle. Ditto diaphragms. I would also replace them as well, tiny gaps might not show up.

Keep going, you will get there in the end.

Nice bike by the way, worth the effort I'm sure.

Sorry just re read the op. The suspect area is the air inlet tubes: ie from the air filter box to the carb. The rubber stubs that connect the carb to the engine on the 'other side' are probably irrelevant to this. Also, this is not electrical nothing to do with charging. Trace back whole system from air filter box. Air filter ok? Rubber seals not missing? Spray carb cleaner in that area and I suspect you may find culprit.

I thought that air coming in from the filter side wasn't really such an issue or at least doesn't cause such strong running issues?

Air leaking in from between the carb and cylinder was more of a problem I thought. That seems to be ok.

I still think it's something mechanical - the throttle only has to be open a fraction of a mm to give you those revs.

Looking at it from a different angle, is there something on the choke circuit that is working wrong and the idle speed has been set to compensate for it?

I thought that air coming in from the filter side wasn't really such an issue or at least doesn't cause such strong running issues?

Sorry this is mistaken. In theoretical terms the carburettor is a device for combining the fuel/air mixture. If there is unregulated variation in the supply of either it will result in malfunction. Thus air supply is critical.

One theory is that what is happening is this: there are hairline cracks in the air inlet pipes. Most of the time these remain sealed. As the engine heats and the metal expands, the cracks in the pipes (hoses really) are exposed leading to air being sucked into the air fuel mixture in an unregulated way, leading to an increase in revs. This has the same effect as opening the throttle.
There are two ways to test the hypothesis:
-spray carb cleaner over the air inlet tubes. When cold and warm. If revs increase or vary then proves hypothesis. Squeeze hoses about a bit to verify'
-remove air inlet tubes and wrap carefully in duck tape (aka duct tape). If this solves problem then proves hypothesis.
-thorough visual inspection of the hoses, remove from bike, pull them around thoroughly, will reveal brittle cracking etc. This is likely with an old bike like yours. May not be visible from superficial inspection on bike.
-there may be other causes.
Tillydog I think has a point and they could also be a factor. Generally though choke issues will result in a richening of the mixture (too much fuel) which leads to 'bogging down'. A lean (too much air) mixture will lead to high revving, which is your problem.
I have assumed you have ruled out mechanical issues with throttle mech etc. The fact that the symptom is variable ie intermittent suggests its not mechanical to me.
Back to the bike: it has the look of the Katana about it doesn't it? The golden age of suzuki styling..

 

Back to the bike: it has the look of the Katana about it doesn't it? The golden age of suzuki styling

It does. it's actually called a Katana I think in the marketing of the time. I think it sits on the cusp of the transition to the Katana proper which was much more sort of stylized. I actually used to have a 550 Katana like below too.

[img] [/img]

Thanks for all the suggestions. I'll be trying them out. I really feel like I've eliminated the possibility that most people suspect in the rubber manifolds on the cylinder side of the carbs, which don't seem to be leaking.

I ran it initially with the throttle cable detached and it seemed better but the engine had only just started so I'll try that again but for longer.
When I left it yesterday, the revs were climbing so much I felt I couldn't leave the choke in order to spray the carb cleaner. Didn't have enough hands...

Another one is the exhaust. Some suggest that exhaust leaks can cause it and there is a very slight blow where the four goes into one, but it's pretty minimal.

Grrr.... I just want to ride the thing.... 🙄

How were the butterflys when you had the carbs off? obviously they should all snap shut crisply if you manualy open them on the pulley & allow them to snap shut. (idle adjust fully wound off).

I have had plenty issues in the past with corrosion on the butterfly spindals,it can also be caused by gummy deposits. I would think that if the butterflys close properly , the rev's could not stay up.

Yeah the butterflies all snap back like a mousetrap!
I also took the time to set them all in exactly the same position, just touching the bypass hole above them as per the manual. All snapping shut and positioned together now.

I've just checked all that removing the carbs AGAIN. I've noted down what the pilot screws were set at, and some were quite different.

Turns out from seated= 2, 3.4, 1.75, 1.75

I've seated them all and wound them all out by the same, 2 turns to see if that does owt when I start it.... 😕

I'm going to disconnect the throttle cable once more just to keep it out of the equation and see how I go.

A guy on the GS forums keeps saying replace inlet O-rings to rule them out. I could do this but the boots look relatively new and the carb spray test when running shows no change in rpm. Bit reluctant to pull them apart for possibly no reason....plus I have to order them too...

Adjusting mixture screw properly on a 4 can be a pita. The factory recomendations are fine when everythings nice & new, but with the sands of time, strange things happen. Dont be afraid to wind them in to see what gets a different result. In = lean Out = rich.

Also when it comes to carb cleaner , Honda do some that is streets ahead of anything else I've used, quite brilliant stuff. I dont think it would help you though, just saying.

any updates?

Well, it's a weird one. Sometimes it seems to run ok, others not so much.

After testing for vacuum leaks several times with carb cleaner, I believed that I found a suspect O-ring on one of the intake boots.
I ordered new offerings and fitted them last night. Subsequent testing and I seemed not to have any more leaks.
Took it for a razz and it did seem to be running better though I still found it hard to pin down a reliable idle. It's not seeming to race like it did but it's changeable.

It also still doesn't start again that readily when hot, although it has been blummin warm lately.

As far as I can tell the charging circuit is ok as I'm getting a strong turnover. I've ordered a wotsit to test the spark and the compression and also a carb balancing tool.

We'll see what happens.
It's defo running better than it was after all my fiddling but very much not 100%

I've noted down what the pilot screws were set at, and some were quite different.

Turns out from seated= 2, 3.4, 1.75, 1.75

I've seated them all and wound them all out by the same, 2 turns to see if that does owt when I start it....

That was a bit rash - you could try and return them to their original settings, otherwise you need something like a [url= http://www.gunson.co.uk/product/G4074 ]Gunsons Colortune[/url] to set the idle mixture properly. You'll also need to check / adjust the carb balance, but get the mixture somewhere close first (then go around the loop a few more times). When you get it right, you'll have a smooth idle and a very controllable, silky smooth response to small amounts of throttle.

It can be a PITA getting there though, because every time you adjust one carb it affects the other 3.

I've got the carb balancing kit on order. I've seen those colourtune kits but I'm trying to cap my excessive spending on it this month.

I suppose the alternative is have a garage look at it which might cost loads anyway.

Hopefully the compression tester will indicate if the valves want seating or worse, rings are going.

Just ordered a colourtune kit too...
This bike is costing... 😕

Oh well.

is there a gasket or similar between the butterfly and the carb jets that air could be leaking in through?

So the bike generally has been running ok and I don't appear to be having the same hanging idle issue as before I fitted the new intake O-rings.

Along with some other kit to tune it up and get it running better I bought a compression tester.

Testing each cylinder I got 90,40,65,75... 😯

I'm thinking that there is something weird going on as you'd think the bike probably wouldn't even start on those numbers!

Warm engine, throttle wide open.

I put teaspoon of oil in the worst cylinder and it bought it up to about 75, suggesting the rings might be bad.

Thing is, this bike has only done 16k. Surely the rings wouldn't have given up just yet? 😐

Next step is maybe get a 'leakdown' testing kit to further isolate any issues.

The strange thing in all of this is that it actually runs relatively well except an unreliable idle.

Frustrating stuff 😛

keep us updated please as interested ..particularly in those compression readings ..have you checked valve clearances? if too tight might affect compression when hot ?

.have you checked valve clearances? if too tight might affect compression when hot ?

If it's only got 16k that indicates long periods of standing.

Gummed up rings and or buggered valve stem seals a distinct possibility with those compression results.

How does the oil look I'd expect it to separate when you drain it out into a clear container if it's the rings

is it smokey on cold start ?

My advice is to go and give it a good hard thrashing, bounce it off the read line, scrape the pegs and when you stop you will know you''ve done enough when the engine goes pink tink

Leak down tester on the way so I'll see.

I'm not sure if it's buckets or tappets actually as I've not ventured in there yet. Ordered a gasket for when I do though.

It is very low mileage for its age so yes, I guess it must have done a fair bit of standing around. Bloke I bought it off said the original owner bought it new and sold it in his eighties. I guess he didn't get out much.

My advice is to go and give it a good hard thrashing, bounce it off the read line, scrape the pegs and when you stop you will know you''ve done enough when the engine goes pink tink

I generally don't go over 60mph on it as it's so nice to just chill and enjoy surroundings. 😀
Maybe I need a longer, boring motorway ride.

Sounds like possible gummed rings. Old british motorbike trick was to fill the top end with solvent / diesel turn it over a few times then drain and change the oil. Dunno if this is wise on a more modern engine. Certainly you could try petrol.

Valve guides would not cause low compression unless the valves were sticking open

Kayak said:

I suppose the alternative is have a garage look at it which might cost loads anyway.

Have to say I agree. I'm all in favour of DIY but sometimes when it gets baffling have to throw in towel and get expert advice. Sometimes finding an old school mechanic (village garages are good for this i.m.e) might give you a diagnosis/starting point. I find if you throw yourself on their mercy the right mechanic can be very helpful. Or try parts department of local Suzuki dealers? Surely there's a forum for these puppies?
I know I keep banging on about the air leak side of things but have you done basics like checking air filter, seals in air supply side etc. Sometimes there is a little valve at the bottom of the air box that gets gummed up and messes with your head, man...

If it's running OK, just ride it. Don't go taking it to bits. Motorway miles won't help. If you don't go over 60, you need to make sure you get to 60 in 4 or 5 seconds - give it some exercise! 🙂