GS750B running problem

[Anonymous]

Ive just got home from a 400K ride and on the way home I noticed that my bike seemed to be running alittle odd.

On heavy acceleration, then deceleration (hills, corners etc) it would pOp really loud (backfire?) out of the exhaust.

Ive got 4-1's with a large alloy can, but ive never had it do this before.

I also noticed that it seemed to have less power.

Could this be an exhaust valve on its way out? How often do I do the valve clearances (shims arent they?)

Ok, I just checked the timing and its spot on, no probs there.

One other thought, any ideas if it could just be a carby issue (one carb flooding??)

thanks in advance

[Anonymous]

POP

(see last sentence)start by checking your plug condition if ok then do a compression check, if ok go to coils and Ignition system, if ok go to carbs, if not go to valves and check them, simple tree check out always but always look for the obvious, VACUUM LEAKS and loose connections FIRST!!!

[tfb]

Loud popping on deceleration means it's running lean somewhere (either one cylinder or all). Could be leaking inlet manifolds, or a blocked jet somewhere causing leanness. Check the plugs; if one is looking whiter than the others, that could be culprit cylinder, so check the carbs & manifold for that cylinder.

Another (common) possibility is that the vacuum diaphragm at the back of the fuel tap is playing up, and starving your engine of fuel, causing the lean running. Check the diaphragm, and the hose that connects it to #2 inlet manifold. An easy way to check if this is problem, is turn the tap to 'prime' and if the engine starts running properly, then the fuel tap or the vacuum hose is the problem.

Or, if you have an inline fuel filter, that could be all clogged up and likewise starving the engine of fuel. Or any other blockage in the fuel line.

The other things Geesis mentions are all well-worth checking.

One thing is for sure: it's not a carbie flooding, because then that cylinder would be rich or flooded. The popping indicates lean running.

Happy hunting,
Mike.

[Anonymous]

Ok, I checked plugs, They are iridium ones I installed just on 8000Km ago.

The 2 left cyls were slightly carboned? up with a whitish hard powder

The right 2 cyls were covered with a fine coating of a fluffy black powder

Compression test (cold) at w.o.t. indicated:
1: 82Psi
2: 90Psi
3: 94Psi
4: 100Psi

Checked carby flaots, mixture screws, jets, coils, ignition system, plugs, connectors etc (new ignition leads etc: all is Ok

Checked for vacume leaks with a spray bottle full of soapy water: no leaks detected

Vacume diaphragm on fuel tap is fine, even bypassed it by blocking it off and running on the PRIME position.

Inline fuel filter (and in-tank filter) are both clean, no water etc. Fuel line was fine but I still replaced it with new line.


Today I travelled 300Km to see my kids and then 300Km home, and I saw a dealer I know in the city, he mentioned that if the valves are not sealing then that could be my problem, and he suggested that I check the clearances and proceed from there.

Well its now 8Pm here and its dark outside, so I cant really do much (plus the neighbours would get upset LoL) so I will check the clearances in the morning.

Any indications of cold clearances because I really dotn want to work on it hot.... (its been 35c - 40c here through the days)

thanks for the help!

[tfb]

Andrew,

Those compression figures do seem pretty low (well, very low), but you should do the test with the engine warmed up, not cold, to get meaningful figures. What do you mean by "w.o.t."?

Whitish hard powder would seem to indicate lean running; black powdery carbon on the plugs is probably OK, but too much means it could be a bit rich and cause plug fouling and misfiring; ideally we're after a dark grey (on ULP) with no oilyness, etc.

I'm not sure on what valve clearances your engine should have; I assume it's shims. The main thing is that you don't have zero clearance, as that means those valves won't be closing properly.

Maybe others on the forum can help us nut things through?

Mike.

[KEITH KRAUSE]

Andrew, the valve clearances MUST be checked on a stone cold motor. Let it sit over night. I'm not sure what your clearances should be but I'm guessing between .03 and .08mm. VERIFY that though.
Re-check your compression on a warmed up motor. If it's OK, after setting the valve clearances correctly, and adjusting the side air screws using the "highest rpm" method, do a carb synch with a vacuum tool. Your VM carbs are very sensitive to vacuum imbalances between the cylinders. A bench synch is rarely good enough. Your two whitish plugs and two blackish plugs sound like a carb synch problem, though it could be other things such as dirty passages in the carbs or incorrect float heigths or poor sealing float valves.
The valve clearances, air screw adjustments and carb synch are standard maintenance anyway, so do them first if the compression is good.
The popping can also be caused by loose exhaust bolts/old gaskets or pin holes in the pipes.

[Anonymous]

Andrew, the valve clearances MUST be checked on a stone cold motor. Let it sit over night.

Hi, thanks for your replys everyone, they all help.

Keith, Well I dont know if I can get steve austin to bring me a motor over to Oz, I think he will be to busy wrestling LoL

Seriously though Keith, how cold do you say is cold, because at the moment here its been 26c at nights and upto 45c days :'(

Also checked the exhausts for leaks etc, none detected. The exhaust has a black powdery appearance too, and ive been told that is normal for unleaded 98 octane

[KEITH KRAUSE]

I don't know how much a warm overnight cool down compares to a cold overnight cool down in how much it effects clearances, I just know the bike must sit overnight so all the parts will cool as much as outside conditions will allow. A few hours is not enough. The cams and valves cool at a very slow rate compared to the cases, etc.

[Anonymous]

Thaks Keith, I will do this later today (tis too hot outside atm

[KEITH KRAUSE]

Just remember, as for your lack of power complaint, hot weather WILL ALWAYS result in a noticable decrease of power regardless of motor condition. So you should at least partially blame the hot weather for contributing to this.
This lack of power is hard to describe, but it effects all of us. On your VM carbs, the side air screw can help in hot weather by turning them out an additional 1/4 to 1/2 turn until the weather cools a bit. The screw adjustment has a limited effect, and sometimes doesn't help much, depending on conditions.
Turning the side air screw out can increase decel' popping though.