1978 GS750 Idles Fine, Stumbles/Loses Power While Accelerating - Esp. Above 4000 RPMs

[Guest]

I have a 1978 GS750 with 4770 original miles. It is all stock (all original except for the consumables; tires, brake lines/pads, battery, hoses, K&N air filter, fork seals, plugs, chain, halogen headlight assembly). I overhauled the bike when I first got it and it ran GREAT for about 5 months. I used it primarily for commuting, never driving it more than 20 miles at a time. Then, about 30 miles into my first long ride, it started losing power under a load (I realized when I tried to crank on the throttle to accelerate and I was at WOT and struggling to do 55). It idles fine, but it struggles to build speed, most noticeably at 4000 rpms and beyond. The best way I can explain it is that if I hold the throttle consistently, the bike seems like it is bogging down, but will get a burst of power every couple of seconds. It seems to me like a fuel starvation issue.

When the bike had ~3500 miles on it, I rebuilt the carbs (completely disassembled each one and dipped for 24+ hours; replaced all o-rings with Robert Barr’s kit), replaced the intake boot o-rings, synced the carbs with the Morgan Carbtune, colortuned each cylinder to get the best fuel/air mixture, used weatherstripping to ensure a good seal between the air filter and the airbox). I did not replace the intake boots as they seemed to be in excellent condition.

Since this problem began, I have checked the timing/points, checked the valve clearances (all good), checked the petcock (works perfectly, no leaks), and ran the bike on prime & loosened the fuel cap (to eliminate the possibility of it being a vacuum issue). In every instance the problem persisted. When I replaced the vacuum line to the fuel tank and the fuel line, the problem went away for about half a day, but returned when I went back to work after lunch. I am thinking that this was just a coincidence. I pulled and inspected the carbs again, ensuring that the main jets looked good and that there were no clogged passages. At the same time, I also raised each needle jet (by lowering the clip to the center position) (based on recommendation from Clymer's manual for this symptom). Problem still persisted. I did not have any WD-40, but I sprayed carb cleaner on the intake boots while the bike was warmed up and idling and there was no change in engine speed (leading me to believe that the intake boots are fine).

Most of these things I simply checked for good measure, but I am thinking that it is most likely something simple since the problem began while I was driving it rather than after I had tinkered with it in some way.

My next step is to pull and dip the carbs again. Other than that, I am at a complete loss. Any tips/ideas would be extremely helpful.

[Nessism]

Sounds like you have been quite detailed in your analysis (thank you for that) so shotgunning a few ideas that pop into my head...

- voltage at the coils (should be within .5 volts of battery voltage)

- spark plug cap resistance (measure resistance and snip back a small section of coil wire to assure good contact)

- spark strength in general (check for a strong blue spark)

- measure fuel level in the float bowls (you need to make a tool)

- debris in the carbs, and tank

- charging system output (long shot)


Hope you figure it out soon.

[1_v8_merc]

I also think you should measure the floats, just to be sure.

Many people over-look them, and they can easily cause fuel flow problems.

[Dave8338]

Pull the petcock and have a look at the filter screen. Are you running an inline fuel filter as well?

How much oil did you spray on the K&N?

[rustybronco]

I have a 1978 GS750 with 4770 original miles. It is all stock (all original except for the consumables; tires, brake lines/pads, battery, hoses, K&N air filter, fork seals, plugs, chain, halogen headlight assembly). I overhauled the bike when I first got it and it ran GREAT for about 5 months. I used it primarily for commuting, never driving it more than 20 miles at a time. Then, about 30 miles into my first long ride, it started losing power under a load (I realized when I tried to crank on the throttle to accelerate and I was at WOT and struggling to do 55). It idles fine, but it struggles to build speed, most noticeably at 4000 rpms and beyond. The best way I can explain it is that if I hold the throttle consistently, the bike seems like it is bogging down, but will get a burst of power every couple of seconds. It seems to me like a fuel starvation issue.

Didn't read all of your post.

I would check for proper fuel flow out of the petcock before tearing into those carbs again.

[MisterCinders]

When you rode it on shorter trips, did you ever get to WOT?

If not, then maybe you have main jets that are too big (rich) or too small (lean). Try a plug chop at WOT if you can.

[laverda1200]

if it is actually a carb problem, engine rpm is irrelevant to diagnosis, especially with a direct throttle slide carb.

what you want to observe is throttle opening position.

closed throttle is the pilot/idle circuit, as is the immediate transition onto the slide needle as you start opening the throttle. From about quarter throttle you are fully on the needle, being fed through the main jet. The main jet has only a very small effect from closed throttle to about throttle 3/4's open.

from 3/4's throttle to wide open, the needle is pulled pretty much out of the needle jet, so the fuel feed regulation is all the main jet combined with proper float height, properly operating float valve, and the needle jet opening size.

as you have listed the changes from stock, the first thing I would do, oddly enough, is wash out you K & N oiled air filter, which if overoiled will totally choke off air flow. I notice someone else has already suggested you check the K & N air filter.

with valve clearances set and timing set, both properly, your air filter cleaned (don't re oil for your initial test ride) then the throttle position where it bogs will then tell you which carb circuit to look at more closely.

if the engine bogs at wide open throttle, then it is a main jet issue, either too big or too small.

if the engine bogs at wide open throttle but picks up sharply when you roll the throttle back to say 3/4 throttle, then the bike is running too large a main jet and the bike is on a proper mixture on the neddle at 3/4 throttle.

if the engine bogs at full throttle and doesn't improve when you vback off the throttle onto the needle, the main jet is too small AND the needle clip position needs to be richened some more.

if the engine is bogging at 1/4 throttle to 3/4 throttle, it is the needle and needle jet circuit that need attention.

Hope that helps. I often use a marker to actually mark the edge of the throttle grip so I know exactly how wide or how closed the throttle is when I am troubleshooting carb setup.

Paul

[tom203]

Didn't read all of your post.

I would check for proper fuel flow out of the petcock before tearing into those carbs again.

Well, I read it all and I suspect the petcock too. I know you said it " works perfectly, no leaks", and you ran it on prime as well, but maybe there is some debris in there that randomly blocks the outlet tube no matter where the lever is. This sounds like fuel starvation and since you did a good job on carbs, the 33 year old petcock gets my vote, You can try disassembling it, but it will probably not like this.

[1_v8_merc]

if it is actually a carb problem, engine rpm is irrelevant to diagnosis, especially with a direct throttle slide carb.

what you want to observe is throttle opening position.

closed throttle is the pilot/idle circuit, as is the immediate transition onto the slide needle as you start opening the throttle. From about quarter throttle you are fully on the needle, being fed through the main jet. The main jet has only a very small effect from closed throttle to about throttle 3/4's open.

from 3/4's throttle to wide open, the needle is pulled pretty much out of the needle jet, so the fuel feed regulation is all the main jet combined with proper float height, properly operating float valve, and the needle jet opening size.

as you have listed the changes from stock, the first thing I would do, oddly enough, is wash out you K & N oiled air filter, which if overoiled will totally choke off air flow. I notice someone else has already suggested you check the K & N air filter.

with valve clearances set and timing set, both properly, your air filter cleaned (don't re oil for your initial test ride) then the throttle position where it bogs will then tell you which carb circuit to look at more closely.

if the engine bogs at wide open throttle, then it is a main jet issue, either too big or too small.

if the engine bogs at wide open throttle but picks up sharply when you roll the throttle back to say 3/4 throttle, then the bike is running too large a main jet and the bike is on a proper mixture on the neddle at 3/4 throttle.

if the engine bogs at full throttle and doesn't improve when you vback off the throttle onto the needle, the main jet is too small AND the needle clip position needs to be richened some more.

if the engine is bogging at 1/4 throttle to 3/4 throttle, it is the needle and needle jet circuit that need attention.

Hope that helps. I often use a marker to actually mark the edge of the throttle grip so I know exactly how wide or how closed the throttle is when I am troubleshooting carb setup.

Paul

Wow great informative advice!

I'm guessing you've wrenched on a bike or two, lol.

[cwscooge]

Thanks everyone for the feedback. It’s good to have more stuff to take a look at.

As far as the petcock, I failed to mention that I have drained the tank, pulled the petcock and inspected the screen and there was no debris or residue on the screen or in the tank. I also completely disassembled the petcock and it was in perfect condition. When reassembled, the petcock worked perfectly: does not flow when in the center position (run?) until suction is applied to the vacuum line, at which time fuel flows very well. Fuel flows immediately when turned to the prime position.

I am not running an inline fuel filter.

I cleaned and oiled the K&N filter yesterday and there was no change. I applied a light amount of oil (per the directions). I have even run the bike (just once around the block) without an air filter but with the airbox fully assembled and the problem still existed.

I did get to WOT on the shorter trips. Even at the lower rpms, there is a noticeable difference but it’s like it just doesn’t have any balls when I crank the throttle whereas when it’s running right, it’ll take off like a scalded dog.

Laverda, your post reminded me of one thing. With the clutch pulled in, or while in neutral, I can rev the engine up to 9000 and it doesn’t hesitate at all. It’s only when I get on the bike and try to ride it that the problem is noticeable. Does that make sense?

Thanks again for all the feedback.

[Dave8338]

What does the voltage look like at the battery, static?
At 4000 RPMs?

[Nessism]

Did you verify the jet's are correct? Many aftermarket "carb kits" are generic across several bike models which can cause problems. Even if you didn't install any new jets, a previous owner could have.

[KEITH KRAUSE]

One of the classic signs of fuel starvation is when a bike will rev fine in neutral (no load) but will not rev well under load/acceleration.

So you say the bike ran fine for 5 months after you "overhauled" it? On trips of 20 miles or less it ran fine. You rebuilt and cleaned the carbs about 1,200 miles ago? It first showed the problem on a longer ride and now shows the problem at all times? Just want to be clear.

How long since the carb cleaning? Has the bike sat at all? Does it idle at approx' 1,100 rpms once fully warmed up? Does the idle rise significantly from cold to fully warm/hot?

[laverda1200]

Thanks everyone for the feedback. It’s good to have more stuff to take a look at.

As far as the petcock, I failed to mention that I have drained the tank, pulled the petcock and inspected the screen and there was no debris or residue on the screen or in the tank. I also completely disassembled the petcock and it was in perfect condition. When reassembled, the petcock worked perfectly: does not flow when in the center position (run?) until suction is applied to the vacuum line, at which time fuel flows very well. Fuel flows immediately when turned to the prime position.

SNIP
Laverda, your post reminded me of one thing. With the clutch pulled in, or while in neutral, I can rev the engine up to 9000 and it doesn’t hesitate at all. It’s only when I get on the bike and try to ride it that the problem is noticeable. Does that make sense?

.

that sounds an awful lot like fuel starvation if it will go to full revs with no load, but not under load.

float valves sticking/or closing too soon?
how are you measuring float height? Also, isn't stock float height on those carbs 22 mm +/- 1mm? If so, after checking and cleaning the float valves again maybe set the float height to 21mm to keep a little more fuel in the bowls.

gas tank cap not venting properly? - ride the bike with the gas cap left off to test.

do these carbs on your bike have an accelerator pump? If so, are the pump diaphragms in good shape and the pump output voluime adjusted properly?

have you had a chance to test ride and observe the throttle position at which the engine bogs? If so, what throttle position?

Paul

[cwscooge]

Okay, here's a follow up.

Keith - You are clear on everything you asked. I cleaned the carbs in October of 2010 and the problem manifested itself in February 2011 (I was in college and work full time and didn't have time to troubleshoot it in depth and am just now getting around to it). The bike sat (never ran) from about April - August. Prior to my adjusting the clip on the needle jet, the bike idled just above 1000 rpms. There was no difference in idle between cold (barely warmed up and can run without choke) and fully warm.

laverda1200 - I have tried running the bike with the gas cap off and the problem still exists. The problem isn't so much that it bogs, but more like struggles to gain speed, almost like a sputter. This occurs through the entire throttle range, but is most noticeable at and beyond 4000 rpms. I am not sure if these bikes have an accelerator pump or not.

The jets are original to the bike (when overhauling this bike, it was apparent that I was the first person to loosen/remove almost every bolt/screw).

Nessism - The voltage at the coils (orange/white wire) was 11.6V and the voltage on the battery was 12.72. I'm guessing this is not right since it greater than .5V difference from the battery.

The spark plug cap resistance is, by cylinder: 1 - 8.79, 2 - 8.28, 3 & 4 - 7.72.

I checked the float bowl fuel level but I am not sure if I did it right (Clymer says to put the bike on centerstand and let it idle, then check). I checked it without the engine running and they were all flush with the mating surface between the float bowl and the carb body.

I may be pulling the carbs to verify that the float valve is functioning properly and that there is nothing clogging it. I will also verify what sizes the jets are.

Also, I have not check spark strength recently, but when the problem originally occurred, I checked that and they all seemed weak to me. I am not sure it that is normal on these bikes though....