GS750 poor acceleration

[casimage]

Thanks to all the member of this board, I was able to rebuild the carbs on my 78 GS750, bench sync them and probably half assed sync'd them using a carb tuner mercury stick. Also replaced the O rings on the manifold boots.

I was also able to time the Dyna S ignition with Dyna coils and spark plug wires. Put in new plugs gapped at .028.

The bike starts right up, and idles smoothly. I let it warm up and took it around the block before doing any serious riding.

It runs great up to about 3K RPM and hits a flat spot. It will eventually get up to 6K but it is extremely sluggish.

Any suggestions?

Thanks in advance.

[chef1366]

What year is it?

[casimage]

1978 Gs750 Ec

It is a 1978 GS750EC with Dyna S ignition, Dyna Coils, Dyna wires, Jardine 4-1 headers, air pods. Carbs jetted with 110 jets.

[KEITH KRAUSE]

Did you inherit this problem or has the bike ran well for you recently?
Did it just have the problem after the carb rebuild?
How well did you clean the carbs? How did you clean them?
Besides the ignition/coil and manifold work, any other work? Valve clearances set?

What is the idle set at, cold? Once warmed up well, what is it idling at?
What color are the plugs after it's warmed up?
Have you verified that the petcock is flowing well?
Have you removed the two floatbowl vent lines (at carbs 2 and 4)? If not, then remove them and leave the brass nipples open to breath.
What jetting, besides the mains, are you using? The 110 mains are too small by the way. Stock is 102.5, so a decent pipe and even cheaper pods will require larger mains. If using the stock jet needles, where are the e-clips positioned? What size pilot jet? Stock 15? Where are the pilot fuel screws set at? How did you set the side air screws?
Lots of questions, but every one of them matters.

[casimage]

Answers to Keith's questions

Keith, first of all, thanks for the advice you have given me throughout this project.

The bike had sat or over a year when I decided to put it back into riding shape. I had forgotten to drain the gas tank so the gas in the tank probably deteriorated and mucked up the carbs. Hard to start, ran rough, so I pulled the carbs and rebuilt them with the help of you and other members of this forum. The bike had 4-1 Jardine exhaust with 110 jets intalled by the dealer. When I rebuilt the carbs, I replaced the air box with separate pods, not K&N, just four individual pods.

The carbs were taken apart and cleaned with carb cleaner, compressed air, and I had thought that I had checked every passage and opening. I could not take apart the internal carb slide as the three screws holding the plate to the mechanism were frozen so I have no way of knowing where the needle clip is positioned at.

When I put them back together, I still had a problem and you suggested replacing the O rings. I pulled the carbs, replaced the boot manifold O rings and used Allen screws. I now have a complete set of screw extractors as those phillips screws were frozen.

I bench sync'd the carbs and bought a Motion pro economy carb stick. At 1100 rpm idle, all of the columns are within a 1/4 inch of each other.

It starts right up with the choke lever at 1/4 if cold. Idles at 1100 then goes to 1200 when warm.

Timing was set using an inductive timing light at 2,500 RPM. I used the mark approximately .5 inch to the right of the F mark. Both 1/4 and 2/3 appear to right on that mark.

I bought a Mikuni rejetting kit with 100, 110, 120, 130 and 140 jets. Pilot screws (I assume you mean the ones on the bottom of the carb), are set 1.5 turns out. Side air screws are 1.5 to 2 turns out.

I had the valve clearances checked about 10 years and 4,000 miles ago. Four years ago I had the cylinder head gaskets replaced but no work was done the valves or carbs at that time.

Thanks again for all of your help. I bought this bike new back in '78 and my wife and I have a lot of memories from that time period before reality and responsibilities reared their ugly little heads. Getting it back into shape has been a labor of love but time well spent.

[Unclesol]

Clip location

If memory serves from the time I re-jetted my 78 (22 years ago...), you don't need to take those three screws out to adjust the clip location. Since the slide is essentially spring-loaded, simply compress the spring and this should allow you access to the clip at the top of the needle.
I hope this helps. Even more, I hope my memory is not totally gone yet...

[KEITH KRAUSE]

OK. So I'm reading that the bike ran fine BEFORE you let it sit a year? Since then you've cleaned the carbs and installed pods.
Some of my questions still need answering and a few new ones.
> Do you know that the petcock is flowing 100%?
> Have you removed the two floatbowl vent lines? They must be removed and the nipples left open when running pods. Fuel starvation will result if the lines are left on.
> What color are the plugs after it was warmed up?
> Did you adjust the floats?
> Did you install ALL new o-rings in the carbs? From Robert Barr?
It's always a challenge trying to diagnose a problem from basic descriptions like "a flat spot". You say "It will eventually get up to 6K but is extremely sluggish".
So, besides being sluggish from 3 to 6K, it will not rev any higher than 6K??
I'm just trying to figure if it's fuel flow related, or a rich or a lean mixture.

Besides my questions above:
>does the problem seem to get worse as it warms up, or a little better?
>After warm up, does the bike rev better with less problems while in neutral (no load)?
>What happens if you remove the pods? Any change at all?
Can you give a more detailed description of what it does when this flat spot hits and it runs sluggish? Any other symptoms such as dark exhaust when revving, slow to return to idle, "spitting" out the filters, popping, smell fumes, fuel dripping, etc?

I'm not narrowing it down at all without answers to these questions, but the info you've given so far makes me tend to think it's not a rich mixture problem. But nothing's for sure. That leaves a fuel flow problem and a lean mixture.
A fuel flow problem is easiest to track. If the gas cap vent is clean, the vacuum line new and tight, the diaphragm opening fully, the petcock screen clean, the petcock clean inside, the fuel line is 5/16" cut the correct length and not kinked, the carb passages to the float bowls/float valves clean, the float operating correctly and the vent lines removed, you should be good as for fuel flow. Any change if you warmed up/tested the bike while the petcock is at prime?
If a lean mixture, that's possibly a bit more complicated. You say the carbs are clean and there's no intake leaks. What's bugging me is the jetting you have now. Past experience tells me your mains are too small. I'd try the 120 mains at least. Your Mikuni kit is limited in sizes but the 130 may work too, once you reach that tuning point. I doubt you had the bike at 3/4 plus throttle while you noticed these problems and the mains shouldn't be causing your 3 to 6K problem, but I just want to mention them.
What could be your problem is the jet needles position. Stock, your e-clips are in the 3rd (middle) position. With your pipe/pods, experience tells me you need to AT LEAST place the e-clip in the bottom position. This would be the richest position (5) for the jet needles. I don't know if a jetting spacer would be needed (position 4 1/2), but my point is the needles should be raised, based on my experience. In fact, many times the stock needles just don't work well and a jet kit (with supplied jet needles/spacers) such as DJ or K&N must be used.
Your pilot fuel and pilot air screws adjustments sound "ballpark" for now, but the side air screws should be tuned using the highest rpm method. 1 1/2 turns out on your pilot fuel screws seems OK for now.
It sounds to me like you can't(?) remove the screws that hold the throttle arm to the slide assembly? Are the heads damaged? They must be removed to check/change the needles. After that, the carbs must be re-bench synched and vacuum synched. The jet needles are by far the most jetting work on the VM carbs.
If you can't check/adjust the needles, all I can say is to try our other suggestions and maybe you'll fix the problem. But skipping the needle check is skipping a basic tuning item and it really should be done if at all possible. If you do fix the problem, I still suggest taking 1/3 throttle position plug reads to see if the plugs are lean. They almost certainly are. Very lean actually. We can walk you through the jet needle adjustments if you can get to the needles.
I know the stock Phillips screws can be a little tight but if you have a GOOD fitting tool, not just the best one in your toolbox, the screws usually crack loose with a firm downward twist. I've never stripped one yet. If the screws are damaged, you obviously have a problem but they may still be taken out.
Let us know about these screws and we'll try to help there too.

[KEITH KRAUSE]

Just noticed you're on line now. Hope my reply helps if you still have the problem.
I've got to sign off because I have to get up at 2:00 (over-time \\/) and I work with high voltage. I don't know how I'm gonna make myself go to sleep this early but I have to try.
Wish I could read one of Earls ramblings now. That always puts me to sleep.

[casimage]

Thanks again

Keith, thanks again for your time and suggestions. I will spend time this afternoon getting the answers that you requested.

I did remove the lines from the vents as suggested.
The petcock was rebuilt using a kit.
Plugs are tan in color, no signs of carbon after idle.
Floats were adjusted with a caliper to 24mm
All new O rings from a Suzuki dealer were installed.
No change in behaviour when bike is warmed up. Revs up to around 3K, hits a lag and, with throttle open, will eventually get up to 6K and beyond but not in a smooth even manner.
Haven't removed pods yet, will do and let you know.
No other symptoms like dark exhaust, fuel spills, etc. but will check again this afternoon.
Will try running with petcock at prime.
The three screws holding the throttle arm to the slide assembly seem to be frozen on all four carbs. None of the phillips screw drivers I have were able to budge them. The screws holding the boots to the carbs have been replaced by allen screws as the heads got messed up by the phillips screw driver and I used screw extractors to get four of them out. They weren't stripped and the allen screws went right in.
The 110 mains were installed by the dealer years ago when I put the pipes on. I can go to a higher jet once I get this straightened out.
I will try again this afternoon and evening and get back to you.
Would you recommend that I consider a DJ or K&N jetting kit? I noticed they come in stages so would a stage 3 be appropriate?
Thanks again and I will post additional answers later.

[casimage]

Totally weird

Had another go at trying to figure this out. Pulled the air pods off and set the petcock to prime. Bike started right up, idled at 1100RPM and I let it warm up. I put a piece of masking tape around the throttle so that I could measure trottle travel while trying different levels.

At 0.0" full stop, idle to 3/8" smooth up to 3K rpm. From 3/8" to 3/4", stumbling, slow, RPMs rise and fall up to about 6K. From 3/4" to 1 1/4" stumbling, slow RPMs rise to about 8K with backfiring. Above 1 1/4" the bike will slowly rev up to at least 10K but definite backfiring.

Totally unexpected was that the horn started blaring at 7K and I had to disconnect the horn wire. This is an aftermarket Fiamm horn with separate relay installed by the dealer. Shut the bike off, restarted and the wire to horn constantly reads about 11 volts without depressing the horn button.

I will go back to the stock horn set up just in case there were electrical problems adding to the situation.

I assume that Keith is correct in saying the 110 jets are too small for the upgraded exhaust and air pods. Can someone recommend a jetting kit or I can use the 120 or 130 jets in the Mikuni kit. The K&N web site shows a bunch of different kits but is very short on information as to what kit fits what bike.

[KEITH KRAUSE]

For pods and pipe, you want a stage 3 kit. I like the DJ kit. Works for me.
There's a chance that the stock jet needles will work but I prefer the jet kit. You're going to have to remove those tight screws to access the needles though.
Reading back, it appears that the change to pods is the biggest change. This change WILL require a re-jet.
Basically, it appears the bike ran fine, then sat a year, then had this problem upon re-start. I have to assume the carb cleaning/rebuild was done right, also the new ignition, coils and leads. If so, then the pods are the only change left that still needs to be tuned for.
Based on your latest info that suggests the petcock and fuel flow is good and the electrical conections should be new/good, I have to think the problem is a lean mixture. Your throttle position tests are described a bit differently than I'm used to (inches rather than actual throttle position) but I get the idea.
A lean mixture will cause uneven rpm's (surging), spitting/popping, etc.
A faulty ignition can cause popping/backfiring but I have to assume the new ignition/coils should be good and you checked the advancer mechanism for correct operation and the advance timing is spot on. A rich mixture can dump raw fuel into the header and cause backfiring but your limited plug tests/symptoms don't suggest a rich mixture at all. Don't see how the current jetting could be rich anyway.
So that further suggests the problem is a lean mixture, especially the jet needles being in the stock position. Like I said, this is based on all the other work described earlier being done right.
Problem is how to remove those screws. If I was close by I'd like to try removing them. But I'm not. Shipping them here and back would cost about $40? Still no gaurantee I could remove them...it depends on the screws condition. If they come out, I could of course re-jet/install the new needles at the same time because it's easy but you could do that too if you can remove the screws. If you can't remove them, you'd have to replace the slides/throttle arms as units. That may open a new can of worms.
Can you show a pic of the screws?

[KEITH KRAUSE]

Oh, and it sounds like the vibrations from testing wiggled something loose on the horn wiring. Wonder what connection method the dealer used and if it has other effects? I'd go back to the stock set up too unless you have a desire to keep the aftermarket horn.

[casimage]

Thanks again Keith

Keith, thanks for all of the time and advice you have given me.

Since I have to remove the carbs to install new jets, I will have another go at those screws. I used liquid wrench on the first attempt with no luck but I will admit that the problems I encountered with the screws on the manifold boots probably caused me to be over cautious as these screws are really small.

I will try and post a picture if I am still having problems.

With regard to the Dynojet kit, the only kit that I have found appears to be for a 1980 to 1985 GS750. Mine is a 1978 GS750EC. The kit number is 3315 I was not sure if it is the right one before I order it. In the interim, I thought that I would use 140 jets from the Mikuni kit that I already have.

I'll go over the electric stuff just to be sure and go back to stock on the horn.

For timing I used the "!" mark about 1/2" to the right of the F mark with the bike idling at 2,500RPM with an inductive timing light. Was this the right mark to use? Also, how do you check the advance mechanism?

Thanks again for everything.

[Footy.]

Just a thought, and its something that is easy to check out.

I also have a 78 GS750E, that i fitted with Dyna S ignition to replace the points. You have to replace the original points cam on the advancer unit, with the Dyna rotor.

When i did this, i found the advancer unit locked up. There are two slots in the Dyna rotor that engage with two lugs on the advancer arms.

It turned out one of the lugs was a tight fit in the slot, not allowing it to pivot. This could only have been because the slot in the new Dyna rotor was not big enough, and i had to carefully file the lug until it would swivel in the rotor.

It is easy to check the advance function by strobe. If its not advancing as it should to the marks, check the fit of the Dyna rotor.

Footy.

[KEITH KRAUSE]

You used the correct mark to adjust your advance. 2,500 rpm is full advance too.
It's easy to see if the advancer is working smoothly. Your timing gun will show the advance mark moving with increased rpm. I assume you have it lined up perfect at 2,500, then just lower the rpm and you'll see the mark move off.

As for a jet kit, you can't use an '80 or later kit. Those kits are for CV carbs. You have VM carbs.
The main reason for buying this kit, besides having the correct main jets, is to use their jet needles. It is POSSIBLE that your stock needles will work but it's a gamble. The gamble is the possibility of adjusting the needles either once or twice to found out if they'll work. If not, then you will have to locate a jet kit. If you have to try the stock needles, I'd try the needles with the e-clip in the bottom position (richest) first and test. If the plugs are too lean, you know right there you need the jet kit. But if the plugs are rich, it figures we can get the stockers to work. I believe you wouldn't have to make more than two attempts with the stock needles. You may have to get some small (approx' .022") washer/spacers to make a 1/2 position change but Radio Shack, etc, can supply washers if needed. Because you have non-K&N filters, your stock needles have a better chance of working.
The jet needle adjustment is the most jetting work because besides the adjustment itself, you must re-bench synch and vacuum synch. But it won't cost you to try, except for your time. I generally go with jet kits, but if you can't get one, then you don't have a choice.
As for the main jet choice, I really doubt your 140's will work. Too rich in my opinion. I suggested trying the 120's in your collection. The 130's may work too, but they're on the high end. That doesn't mean the 130's won't work. If I was just guessing, I'd go with 125's. Full throttle tests will tell what mains to use.
You could call DJ too and ask if they have a kit for your bike. They've always been helpful to me. It's possible the '78/'79 GS1000 kit uses the same jet needle. Then I would think they'd just swap you two choices of mains and you'd be set. I have their number in the garage if you can't locate the number. Might be worth a try? The other option is K&N. Did you look them up?
One more thing to mention is the pilot jets. You shouldn't need larger pilot jets if you use a kit. But you may need 17.5 if you run the stock needles. There's a good chance that richer pilot fuel screws adjustments will be enough for your pilot circuit though. Tests will tell.
To sum up, I'd try for the kit first. But if no luck, I can help you adjust the needles and it wouldn't take that long to know if we can get the stock needles to work. As for mains, you have some already that may work. If it turns out you do need a size you don't have, I would think we would have it narrowed down to the point where you would only have to buy 1 set, or two sets at the most. Example: if your 130's are rich and the 120's lean, then you'd most likely buy 125's. If those were still off a bit, rich or lean, it would be obvious you need 122.5 or 127.5 based on your plug/performance record/result keeping. I can't gaurantee this of course, but it's based on my past experiences. The cheaper pods is what causes some extra work sometimes because the popular jet kits are designed for use with K&N filters specifically. In my opinion, your Jardine pipe doesn't flow as well as some, but it's a decent pipe and it's flow characteristics may help your stock jet needles work too.
Let us know about those tight screws. Get a good fitting tool and maybe some liquid wrench or ? overnight will help. If the heads are still in good shape, the correct fitting tool WILL remove them.