'77 GS750 Bogs on throttle

[Aerik]

Okay, so I've been tinkering around with this problem for a few months, and I'm finally getting annoyed enough to ask for help.

Siouxsie, my '77 Suzuki GS750, accelerates smoothly at up to a quarter throttle or so. If you apply more than that, however, she makes a noise like you'd make if you attempted to gargle without water, and will actually stall if you let it keep doing that at idle. You can 'roll up' to full throttle by increasing it gradually or feathering, but there's no real acceleration at that point.

Here's what I've tried so far:

The jets, airbox, and pipes are all stock.

The air cleaner is clean and in good shape (it's a Uni, and has been freshly oiled).

The mixture screws (this model has separate air and fuel screws) are at spec.

The carbs have been taken off, inspected, and cleaned several times, with no change.

The plugs are pretty new, and made no difference. Two were somewhat rich at last inspection, but that's been more or less adjusted out.

These carbs don't have the normal synch ports (I've been told I have to get something to thread into them somehow), so I've had to settle for bench-synching.

Can anyone think of anything else I should try? Or is my bike just being a 33-year-old Suzuki? We work on stuff all the time, but we're not mechanics, so don't hesitate to point out anything I might have missed.

[siggywiggy]

is it just on 1/8 throttle? 1/4 throttle? etc? if so it sounds like jetting. if its on lower throttle try bigger main jet if its in full open throttle try smaller main jet. Your altitude and humidity could be effecting it as well. Is the filter open or is it encased? could possibly try drilling holes in the case which would allow more air flow. just some ideas

[Bubba]

Not sure what kind'a carbs yer runnin' but if they're CV carbs, check the integrity of the slide diaphragms, looking for cracks in the rubber or leaks around the outer seal. If the diaphragm(s) is(are) leaking, you lose vacuum and the slide won't open normally as you crack the throttle, leaning your fuel/air mixture. If I was a guessing man, I'd look at the two carbs that weren't running rich first. Prolly the two cylinders that looked rich were carrying the load on acceleration.

Some other things to check: if your carbs have squirters (most bike carbs don't) make sure they're working; re-check the orifices on your main jets for blockage.

Edit: Also, check yer other vacuum hoses, crackcase breather hose, etc for cracks/leaks.

Edited July 2, 2010 by Bubba

[max power]

^ what he said. I'd check the diaphragms thoroughly first.

[Aerik]

No diaphragms-- old-school vm slide carbs. I just pulled them completely apart, re-cleaned everything, soaked the bodies in a bucket, and swapped back to the jets recommended in the Clymer manual (I thought I had the stock jets, but there were waaay too many different specs for this bike). It was better, but still lean in the mid-throttle, according to a plug chop reading. I'm thinking of raising the needles (the stock needles are adjustable clip type) to see if that changes anything.

Unfortunately, I decided to check the valve clearances. One was too tight, so now I'm waiting for ASK to order in a 2.55 mm shim. After that arrives, I'll find out if the needle thing helped.

[Bubba]

Sounds like yer on the right track. However, it'd be kind'a unusual that you would need to raise the needle. As the older slide carbs age, the needles normally wear and get loose with the years of vibration and flow more--not less--fuel. That being said, you sound like you know what yer doin' and if it looks lean in the mid-range, raising the needle (lowering the clip) might help.

I still think I'd look for a partially clogged passage (I think you said you checked your float bowl fuel levels) or some other issue causing air infiltration behind the slides. How are your rubber carb mount boots? Check those by spraying either WD-40 or a solvent on each boot and see if you get a change in idle or smoke from the exhaust.

[Aerik]

I have been meaning to do the wd-40 test-- I'm fairly certain I did that last fall, but it's entirely possible that it was on another bike. We've had too many weird projects going on over here. After the valve shim arrives and I get it back together, I'll try it.

[Exit Wounds]

Did this problem come on all of a sudden or has it been this way? Fuel line plugged? Being that old there may be debris in the tank blocking fuel flow through the petcock too, float needles sticking maybe... sounds more like fuel than fire, but I've had coil wires come loose when the bike is under a load. It didn't show its ass till it was on the road moving.

Edited July 3, 2010 by Exit Wounds

[ReconRat]

I'm here to say that a too tight valve adjustment (especially an exhaust valve), will cause a bike to bog down, just off idle throttle, when it warms up. Air-cooled are touchy about this.

Been there, done that. Some factory valve specs on old bikes are too tight for air-cooled. Generally I won't set an air-cooled exhaust valve at minimum gap. That never works out well for me. I'd rather set it at maximum, or a middle.

[Strictly Street]

Hmm...

Check for 12 volts at the coils.

Odd as it sounds I had a similar problem with my 1980 Kaw. Turned out I was only getting about 8 volts to one coil and 10 to the other. Midrange rpm's sucked ass.

Replaced the wiring, re-did the spark plug wires and all was well. Something about 30 year old wires and corrosion.

I initially thought it just - had - to be a carb issue.

If you cant fix it with a hammer it must be an electrical issue - Hardly Daveson manual

[ReconRat]

Hmm...

Check for 12 volts at the coils.

Odd as it sounds I had a similar problem with my 1980 Kaw. Turned out I was only getting about 8 volts to one coil and 10 to the other. Midrange rpm's sucked ass.

Replaced the wiring, re-did the spark plug wires and all was well. Something about 30 year old wires and corrosion.

I initially thought it just - had - to be a carb issue.

If you cant fix it with a hammer it must be an electrical issue - Hardly Daveson manual

This also. But with poor electrical at the plugs, sometimes it bogs, but for me it mostly crackled and dimmed the headlight. The headlight going dim is a distinct clue that the amp output/battery is bad. And 8 volts at the coil is weird... Where was the voltage drop? Something got hot... I once had an old plug wire come off the plug cap and wave around at me. Replaced them all with home made NAPA gear. Worked a lot better.

Actually, 6 to 12 years is a long time for plug wires. A 1977 bike is... 33 years... They should be replaced. It will run better.

Edited July 3, 2010 by ReconRat

[ReconRat]

The plugs are pretty new, and made no difference. Two were somewhat rich at last inspection, but that's been more or less adjusted out.

This is a clue. If the two that are rich (or lean) are running off the same ignition coil, you probably have a primary or secondary ignition problem. Regardless of what was adjusted out. Electrical should always be checked and verified first, when hunting for engine problems.

[Strictly Street]

This also. But with poor electrical at the plugs, sometimes it bogs, but for me it mostly crackled and dimmed the headlight. The headlight going dim is a distinct clue that the amp output/battery is bad. And 8 volts at the coil is weird... Where was the voltage drop? Something got hot... I once had an old plug wire come off the plug cap and wave around at me. Replaced them all with home made NAPA gear. Worked a lot better.

Actually, 6 to 12 years is a long time for plug wires. A 1977 bike is... 33 years... They should be replaced. It will run better.

Voltage drop was at the end connector that attached to the coil. Replaced the entire wire with a new run straight from the battery [with a fuse] which I could do because of the type of ignition I have. This won't work on all bikes.

http://www.wgcarbs.com/forum/index.php?topic=263.0

[max power]

If you cant fix it with a hammer it must be an electrical issue - Hardly Davidson manual

Awesome.

[Aerik]

Good ideas on the electrical stuff. I'll have to test that once I get it back together. Of course, the downside is that the spark plug wires aren't changeable by themselves-- I'll have to do the coils if the wires aren't good.

Earlier, when I had two that were rich and two that were lean, there was one each per coil, so that may not be it, but we'll find out.

At any rate, I'm still waiting for my valve shim. Grrrr.

At least we have another project bike to keep me busy while I wait.