'Backfire' through #1 carburettor...

[Guest]

G'day all,

I've got a baffling little problem with my GS1000 resurrection project that I'm wondering you may be able to shed some light on.

Basically I appear to have some sort of backfire originating from carbie #1, which sounds like a "PFFFT" sound that comes out of the pod air filter at random intervals.

It is most pronounced on warm-up, but still happens when you are cruising along at a moderate speed even when the engine is fully warmed-up. When you hear the "PFFFT" sound from the carbie, the bike momentarily will lose power and then in an instant carry on like nothing has happened, until the next "PFFFT", and so on.

A few items of further information before you all start asking questions:
1. The bike, as I mentioned, has pleated pod filters installed. These were cleaned with warm soapy water, dried, and lightly oiled before installation.
2. The bike has a set of Yoshi cams installed, which appear to give about 0.7 to 0.8mm extra lift. The valve clearances are currently set at 0.15mm for both inlet and exhaust, which is the best info I could find on the internet, and asking questions here on the GSR forum.
3. I have a 4-into-1 exhaust system installed, with a long 'straight through' muffler of my own devising. This is a quiet enough system, with a 2" opening at the end of the muffler.
4. I have new spark plugs installed, and tried swapping out the plug on cylinder #1, but that has not changed a thing.
5. There are new NGK spark plug caps installed, the type with resistors built-in.
6. I have tried setting the air screws and pilot fuel screws to the 'factory settings' (ie. 1 1/4 and 3/4 turns out from lightly seated, respectively). And then I have tried richening the mixture on cylinder #1 up to 2 turns out on the fuel screw, but this hasn't changed anything.
7. The valves were freshly lapped-in by myself during the rebuild.
8. The cylinders have been freshly honed, and a fresh set of piston rings installed. Everything was well within tolerances.
9. All the jets in the carburettor and the needles are all the correct size, and appear to be in good visual condition. I have new main jets and new needle jets installed.
10. The carbies were cleaned out using carbie cleaning fluid and compressed air through all the passages and nooks and crannies.
11. I have not yet changed any of the small O-rings for the air or fuel screws.
12. The rubber inlet manifolds are brand new Suzuki items, installed with new O-rings and clamps.

I'm not even sure that 'backfiring' is the correct term for what I have happening with cylinder #1, but there is definitely a forceful 'pulse' or 'puff' of pressure coming back through the carb in instantaneous but irregular bursts. I mean, you can SEE the pod filters pleats flexing outwards whenever it happens and makes that accompanying "PFFFT" sound.

I have a Gunson 'color tune' kit arriving sometime in the next couple of weeks, which will help me get the pilot mixtures bang-on. Until then, it's all guesswork, and I've never had any joy with the 'highest idle speed' method, I'm afraid.

So, what do you think may be causing my problem on #1 cylinder?!

Thanks in advance,
Mike.

[Big T]

Your valves are loose- they should be .03-.08 mm

Your #1 carb pilot circuit is clogged - did you follow the VM carb cleaning tutorial?

When you say your jets are the correct size, the correct size for what? Certainly, you should be about 125 main and 20 pilot, with the needles raised one notch

[tfb]

G'day BigT,

Thanks for the advice there.

With the valve clearances, I'm just taking advice on what the correct clearances should be for these Yoshi cams... if they were stock cams, I'd go for the stock clearances with no hesitation.

So I guess I want to ask the GSR folks inc. yourself, Have I got the clearances correct for these *Yoshimura* camshafts? I emailed Yoshimura themselves on this question, but received no reply, and haven't been able to unearth any definitive information on the internet either. So if some folks want to add to BigT's doubt over this, go right ahead... I guess at the end of the day it's one more thing I could try.

I did indeed follow the VM rebuild instructions that are provided as a PDF here on the GSR. I've used these excellent instructions a number of times over the years, and over the years have pointed many a GS punter towards them, too.

But I will recheck the pilot circuit as you suggest -- one has to be thorough, if nothing else.

BUT I am starting to wonder if this is a timing problem. Using the strobe lamp the timing checks out OK, and I have taken care to dismantle and check the ATU as well while I was doing the rebuild, so yes it is advancing correctly. But I'm wondering if the spark timing is either too advanced or too retarded...

... OR maybe the valve timing is out? I did use the correct 20 cam chain links between the exhaust and intake camshaft sprockets as per the manual for the GS1000SN... but maybe the Yoshimura cams use the 19 cam chain links as per the GS1000ST?

Or maybe I should just try a stock pair of cams and see what happens??

Finally, re. the jet sizes being correct, I am referring to the stock jet sizes as per the Suzuki factory manual for the GS1000SN (1979).

Thanks again for the feedback BigT, I will keep your advice in mind as I ponder things in my sleep (which is when a lot of solutions present themselves ).

Cheers,
Mike.

[hjfisk]

You can't use the stock jetting with what you have done to the bike. The pods and pipe require a jetting change never mind a cam change. Try jumping you main jet up about 5 sizes from stock as a starter, then work from there. Then you will need to do the plug chops ect. Also have you done a carb sync on this bike? Also get a O-Ring kit in those carbs, one bad o-ring can cause problems

[rustybronco]

What sort of ignition system is installed on the bike and are you sure it's only #1 cylinder doing it?
Have you measured the secondary resistance of the #1~4 coil.

[chuck hahn]

First things first I would say. He specifically stated that the cleaning consisted of carbs spray and some air..with NO NEW ORINGS in the carbs.


Aint diddly squat gonna do him a bit of good until he cleans and rebuilds the carbs 100% to the letter and gets the nvalves adjusted right.

Next he needs to start it and check the timing and vacuum sync the carbs. And clean the pods again and ver so LIGHTLY oil them.


Either get the ENTIRE servicing done or walk away...plain and simple.

[mvalenti]

cleans and rebuilds the carbs 100% to the letter and gets the nvalves adjusted right.

Probably a thread or two on the subject I would imagine...

[chef1366]

Sounds like a leaky valve
Either it's bent, the guide is worn out and it's getting stuck open just enough to let the compression out.

[tfb]

Either get the ENTIRE servicing done or walk away...plain and simple.

Hmmm, you come across a little harsh there, chuck... as you can see from my initial posts (long careful list of all I have done) and all that, I am a fairly thorough man and I like to have things done properly. Does anyone who has an issue with their machine cop the same brusque treatment from you?!

On a forum, folks can't see each other and it's just words on a screen, and so expression is pretty important. No one likes having it implied they're a careless dill when all they're doing is trying their best. I just need some advice or pointers, so a simple "Take care that you don't ignore those O-rings!" would have done the trick.

Ditto with that advice about the pod filters. Did I not say I had cleaned and re-oiled them? Move on. "Aint diddly squat gonna do him a bit of good...". Maybe work on your delivery a little more? Just sayin'...

Meanwhile, on to some of the other suggestions...

Yes chef1366, I too would think that backfiring into the carb would betray something amiss with the valves, but I once had a similar problem on my 1100 Katana and it turned out that the pilot mixture on that cylinder was a bit too lean. Richened it up a touch and the problem disappeared. Bent/ leaky valves weren't part of the problem, as I have had the engine apart before and since, and all has been good in the valve department. It's the same with this motor; all the valve gear has been carefully inspected and I *know* it is good. So it has to be something with the carbs or maybe the timing or such-like. It's just got me a bit bamboozled but folks are putting up stuff for me to check out and that's what I'm after.

rustybronco, you raise the q. of what ignition system is in the machine; the answer is, the original set up with points and condensers. I have dressed and set the points, there is no visible pitting and there is a fresh pair of condensers in the setup too. I'd like to switch over to a DynaS system at some stage, but funds are tight at the moment so the old setup will have to stay for the time being.

Thanks for your suggestions so far folks; I will keep on sleuthing and maybe dart off and get some O-rings this afternoon. One step at a time and we'll get there.

Cheers,
Mike.

[tfb]

OK, I've just spent a bit of time lifting the jet needles another notch, balancing the carbs, and visually checking the O-rings in the carbs. They are pretty tired-looking, I must admit.

But I took the bike for another short ride and it's much improved... the backfiring on #1 has almost gone, as has the mid-range hole ('flat spot' was too mild a term, heh).

But I will have to get my mitts on some O-rings for the fuel screws... we don't want extra air leaking into the circuit there, and leaning the mixture out.

Question: I notice on various parts microfiches (eg. Robinsons UK) that the O-rings for the air screws are available, but the whole pilot mixture needle + O-ring + spring assembly simply ISN'T THERE on the diagrams!?!

So does that mean I have no alternative but buy 4 entire carbie kits just to get hold of the 4 fuel-screw O-rings? I can get the air-screw O-rings as spare parts easily enough... but it's the availability of the fuel-screw O-rings that has me wondering.

Cheers,
Mike.

[hjfisk]

Order a kit from Robert Barr @ cycle O-rings here on the site , you get the entire set for like $20.

[Big T]

Order a kit from Robert Barr @ cycle O-rings here on the site , you get the entire set for like $20.

Slightly higher to Perth

The material for the O rings is important - you want Viton or similar, which Robert uses in his kits

The fuel screws were made nontamper because us Yanks couldn't be trusted not to befoul the skies

If you find your fuel screw tips are broken, Z1 makes a nice replacement set

[OldVet66]

Too lean will cause the carburetors to spit. With pipes and pods on my GS1000E I'm running the equivalent of a Mikuni 132.5 main jet, Dynojet needles third clip from the top and Pilot fuel needles a tweak passed 1/2 turn out air screws approximately two turns out. It gets 44MPG without flogging it too much. If you get much more, it is definitely a lean condition. I had my first set of carbs built by someone else before I learned the ropes. I was so excited that I was getting 52MPG. The carbs were also spitting. It seemed to be #1 but it's kind of hard to pinpoint. Needless to say, that motor overheated badly one evening, stop and go traffic going up the mountain in 30 degree weather. Even properly jetted, VM's will give a small spit at times, but nothing like the lean spit that acts like a misfire.

[rustybronco]

2. The bike has a set of Yoshi cams installed, which appear to give about 0.7 to 0.8mm extra lift. The valve clearances are currently set at 0.15mm for both inlet and exhaust, which is the best info I could find on the internet, and asking questions here on the GSR forum.

5. There are new NGK spark plug caps installed, the type with resistors built-in.

7. The valves were freshly lapped-in by myself during the rebuild.

8. The cylinders have been freshly honed, and a fresh set of piston rings installed. Everything was well within tolerances.

9. All the jets in the carburettor and the needles are all the correct size, and appear to be in good visual condition. I have new main jets and new needle jets installed.

12. The rubber inlet manifolds are brand new Suzuki items, installed with new O-rings and clamps.

I'm not even sure that 'backfiring' is the correct term for what I have happening with cylinder #1, but there is definitely a forceful 'pulse' or 'puff' of pressure coming back through the carb in instantaneous but irregular bursts.

So, what do you think may be causing my problem on #1 cylinder?!

Thanks in advance,
Mike.

OK, I've just spent a bit of time lifting the jet needles another notch, balancing the carbs, and visually checking the O-rings in the carbs. They are pretty tired-looking, I must admit.

But I took the bike for another short ride and it's much improved... the backfiring on #1 has almost gone, as has the mid-range hole ('flat spot' was too mild a term, heh).

But I will have to get my mitts on some O-rings for the fuel screws... we don't want extra air leaking into the circuit there, and leaning the mixture out.

Question: I notice on various parts microfiches (eg. Robinsons UK) that the O-rings for the air screws are available, but the whole pilot mixture needle + O-ring + spring assembly simply ISN'T THERE on the diagrams!?!

So does that mean I have no alternative but buy 4 entire carbie kits just to get hold of the 4 fuel-screw O-rings? I can get the air-screw O-rings as spare parts easily enough... but it's the availability of the fuel-screw O-rings that has me wondering.

Cheers,
Mike.

rustybronco, you raise the q. of what ignition system is in the machine; the answer is, the original set up with points and condensers. I have dressed and set the points, there is no visible pitting and there is a fresh pair of condensers in the setup too. I'd like to switch over to a DynaS system at some stage, but funds are tight at the moment so the old setup will have to stay for the time being.

I don't believe there's an immediate need to update the ignition system to resolve the issue you are seeing. Your ignition system could be marginal for the mods you have done, but in my mind, not the issue.

You've answered the questions I had except for checking the resistance on the coils secondary. It probably won't be the the reason for your trouble either but it won't hurt to check, plus the cost to check it is minimal as well.

The next thing you could check into is a leak down test, but I wouldn't go to any great lenghts to check it as you have already lapped the valves and honed the cylinders and I doubt they are the root (route) cause either.

No my mind's eye thinks it's a lean condition that's causing your troubles (as was stated before), although it's odd #1 is the only one showing evidence of a backfire.

My suggestion is to first do a leakdown test, then a compression test, carefully inspect the plugs for any indications in regards to the issue and then start tearing into #1 carb for evidence.

Leakdown tester...

http://www.thegsresources.com/_forum/showthread.php?p=1559962#post1559962

[tfb]

Thanks for all the good advice there folks! Like all problems, this one may well be a combination of a few things, and (as is often the case) I end up working towards the solution, rather than finding that quick fix.

Oldvet66, describing the carbs as "spitting" is a very good way of putting it. Incidentally, the reason I know it's carbie #1, is because you can see the pleats of the filter flex outwards whenever it spits.

So it's definitely carbie no. 1. I think I will pay closer attention to the idle mixture for sure. I have a Gunson Colortune kit on the way, so that should help me nail the pilot mixtures for each carburettor.

BigT and hjfisk, thanks HEAPS for the tip-offs where to get new O-rings and fuel screws. I took a really close look at tips of the fuel screws, and of course someone in the past has been doing a bit more than "lightly seating" them. So they will need changing out too.

rustybronco, hopefully today I'll get some time to check out the resistance on the coils. They are a 2nd-hand pair off my old Katana, and were working fine when removed, but who knows what time sitting on the shelf may have done. Re. compression and leakdown testing, I'll do in due course if the problem persists.

Thanks all for the good advice. Reminds me of a saying, "It takes a village to raise a child." Maybe can adapt that to, "It takes a GSR forum to sort a GS!"

Many thanks,
Mike.