GS750 Stumbling

[mpinkorea]

Hey so I just posted and had some great input about the wrong carbs on my bike. They weren't wrong persay, just a jumble of parts. So I got a correct set, dipped them and replaced everything internal. I adjusted everything to the specs in the Clymer's manual as well as what the posts here had said in the past in reference to my bike. My problem now is that the bike stumbles when it is under load. I checked the petcock and it is flowing fuel well, both in prime and on. I pulled the vaccuum tube off the petcock and it is pulling vaccuum (it pulses--assuming this is what it does). The number 1 cylinder plug is good color however 2,3,4 are carbon encrusted. So based on that I assume that 2,3,4 are rich so I will be leaning them up a bit. I have the ignition spot on as far as timeing, new plugs and correct gaps. It just feels like when under load that it may not be getting the fuel it needs. All of the necessary o-rings are air tight and have sprayed them--no leaks. I do have a drain hose on the bottom of the airbox that is cracked and I may just plug this as it may be leaking some air. The filter is clean and the fuel is new. I did run some sea foam in the gas and I think I may have added a bit much (it is a fuel preservative designed for fuel injection so it should not hurt o-rings and seals). Maybe need to add some clean gas only. Any ideas or more info needed?

[KEITH KRAUSE]

What year?
Is the bike stock?
Stock jetting?
What exact throttle position(s) is the stumble at?
Don't plug the air box drain line. It's a drain.

[KEITH KRAUSE]

And spraying the manifolds doesn't always expose any intake leaks.
Does the bike idle significantly higher when warmed up compared to when it's cold?

[mpinkorea]

Keith sorry about that lack of info. It is a 1978 GS750E. The bike is stock until the exhaust which is a V&H 4 to 1. The jet needle in the slide assembly is raised 1 notch from stock location. The jets are stock jets and all float levels were good when installed back in bike. When the bike is idiling it is fine and has good throttle for about 1/5. Then it goes into a stumble like it is starving for fuel. Checked and double checked and it is getting fuel. Everything inside the carbs is new, so cant imagine anything is clogged. No inline filter just the filter on the petcock, but the tank is good--no rust and I use premium fuel. I think some of the problem is the carbs aren't adjusted completely as far as mixture goes yet and it may have something to do with the "Sea Foam" additive being so concentrated.

[mpinkorea]

Ok so just went for a ride with the bike and here are the results. Number 1 plug is clean. 2 plug is sooty. 3 plug is sooty. 4 plug is sooty. From idle to about 3K RPM the bike runs and sounds great.....this is about 1/5 throttle. From 3K to 4K..about 2/5 throttle bike stutters and bogs. Above 5K bike stutters and it seems like about 65MPH is the best I can do, even if I have the throttle at WOT. I am tending to think now that even though the carbs were dipped and reassembled with new components that something has made its way into the internal components and is causing a lack of fuel....and thus the stutter when I get on the throttle. It doesn't even help in the above ranges to ease the throttle on. This sound about right.....or wrong which ever way you look at it.

[roadragetrx]

I have a 1977 gs750 an was having a similar problem mine was solved by rejetting the bike the V&H exhaust i bought stated you will need to rejetand for my engine it was a 115 main and a 17.5pilot plus i lowered my needle 1 notch but every bike is a little different hope this helps.. Mine was starving for fuel about 5000rpm and would only go about 65-70 before now very crisp throttle and will do around 100mph

[mpinkorea]

Thanks that sounds really close to what is going on with mine. I also think that since I put the correct carb set-up on the bike things may be balanced out and the jets are too small. I think I will be ordering new jets. Any other suggestions from the GS'ers.

[KEITH KRAUSE]

Could be a lot of things. You will have to re-jet for the pipe as roadragetrx suggested.
I think the 115 mains (2 1/2 full sizes up from stock) are a good choice too. For the pilot circuit, I'd first try richer pilot fuel screw adjustments and if you don't like the idle/minimal throttle position performance, THEN go to a 17.5 pilot jet. Raising the needle by lowering the e-clip one position is usually enough for a pipe only mod. You DO have the airbox lid still on, correct? What position, from the top, are the e-clips now? Did the bike perform well at 1/3 throttle before the pipe mod? To see what the jet needles are doing, you MUST take plug reads after a 1/3 throttle chop.
Before getting any accurate plug reads, you must bench and then vacuum synch the carbs. Have you?
What have you done with the pilot fuel and side air screws?
I'd initially set the pilot fuel screws to 1 1/2 turns out from LIGHTLY seated. THEN, set the side air screws to about 1 3/4 turns out, start the bike and warm it up fully, then fine tune/adjust the air screws using the highest rpm method with a base idle of 1,000/1,100 rpm's, no higher. Test/get reads after some slower steady cruising around in 4th/5th gear about 35/40 mph on level or uphill roads. Chop off and read. Do what the plugs/performance say.
Be sure the factory jet needle plastic spacers were re-installed in correct order. Thicker spacer (ring) goes directly on top the e-clip and the thinner spacer goes under the e-clip. This could be the cause of your uneven plug reads, along with a poor synch, incorrect float levels, clogged carb passages, poor electrical connections...
What did you set the floats at? .94" is factory suggested I believe.
I assume you have the two float bowl vent lines attached (with stock airbox and lid) and they are not kinked and routed back and by the sides of the airbox? These two vent lines at carbs 2 and 4 must be clear to avoid fuel starvation. I've seen some owners use a fitting to connect them together. :shock: They can't breath like that.
Since you need to install larger mains anyway, I'd adjust the floats again if they're incorrect.
Verify that all passages are clear by using hi-pressure air after cleaning with carb cleaner.
Do you still run points? If so, what service did you perform on them?
Also, you said the bike idles fine? That means when cold it's around 1,000 rpm's and fully warmed up is maybe 100/200 rpm higher?
I assume you cleaned the gas cap vent also?
The fuel line is 5/16" and cut the correct length so it's not kinked/kinking?
All the tuning basics must be done first before re-jetting.
Please answer all questions to get the best help/suggestions.

[Cyg]

this man sound odd. but I have a gs 1978 750 that was only firing one three cylinders. I now have her firing on four, but I must of taken them carburators apart at least twenty times. and you know what the culprit was? the factory setting screw that the books says not too touch? well the person who owned the bike before me didnt read that part. and guess what he did. He took them out and screwed them in tighter than two turns. and then guess what happend he broke off the tip of the brass screw inside the carburator houseing. I took them apart for the last time. I used a wire brush strand too loosen it up and shot air thru it. Then it shot out that tiny piece of brass. Now all four run, at peak performance. So dont assume unless you have owned the bike since show room. that its all there the way its suppose too be.

OH this is rich, the mukini dealer I talked too about buying new o rings for it. Told me that they dont make them since the owner isnt suppose too mess with them. He said use this stuff called hylamar. Put that on the brass fitting, then use iron epoxy. or jb weld and seal them shut after turing them out two turns. That was his fix for not having the o ring too stop them from leaking gas out. And guess what it worked.

Follow the sheeton the v an h pipes you got. Jet according too their recodemendations. sounds like only one is getting the juice she needs. now a bike will run on three cylinders, but not in the higher rpms well. But she will idle, and go thru her gears, but she will not be in peak conditoin.

Spray the exhaust pipes with water and see if they all sizzle, that tells you alot right there which is getting fuel and fire.
another fix if they are not getting spark too the other three is too swap out coil leads. 1 for 4 and 2 for 3. That is an way too tell if you coil is bad or not.
Now this next part is more of a joke, so dont you sea foam users get uspet.

Seafoam. told u so.... now im joking here ok. couldnt pass it up.[-(

[Wandering Idiot]

I'm having the same problem but it's between 1200 and 3000rpm, it seems. The bike just bogs down, but once it creeps its' way over 3k rpm it takes off like a rocket. It didn't do this before I replaced the plugs the previous owner had put in (champions) with new NGKs. I also cleaned the carbs, but it ran fine for a bit afterwards. I'm checking the points today, while I'm going to do other maintenance, but I think I ****ed around too much with the carbs.

When I swapped plugs the first time I stepped up to a hotter plug, NGK 1111s. Big mistake. I wound up overtweaking my a/f mixture screws and my plugs were still burning too hot, so I bought the standard NGK plug for the bike, and now it's running way too rich and dumping carbon all over my plugs. It doesn't help that I forgot to turn off the choke for about 5 miles of riding. I'm thinking my points are fine and my carb play is what seriously ****ed me up, but the bike just hasn't run the same since I replaced the Champion plugs.

Also, I have a 4-1 pipe (don't know the brand). My bike is a '77 750E. What's the best way to adjust a/f mixture screw? Do I have to pull the plug all the time to check it or is there some other way my diesel wrenching ass doesn't know about?

[mpinkorea]

Keith to answer some of your questions. The bike has the stock airbox and all the rubber associated with the box and carbs is new, along with manifold o-rings. The e-clips are moved 1 notch to raise the needles from stock position. The pipe mod was completed by the previous ouwner and he did not do any adjustments. However this was with a hodge podge carb set-up. I now have the correct carbs for the bike and am starting from scratch on the adjustments. The carbs were dipped all internal pieces checked and replaced with new. The floats were adjusted to 24mm per the Clymers manual. They were then bench sync'd but not vaccuum synched yet because I am trying to get rid of this stumble. The air screws were adjusted using the high RPM method. The bikes idle doesn't seem to change much when these are turned in or out. If turned almost all the way in the bike will choke off. The pilot fuel screw were adjust on 4 plug readings. The #1 cylinder looks good but 2,3 and 4 are sooty and can not seem to get them to look clean. The vent lines are attached and are not kinked. The points that are on the bike were gapped to factory setting .013 and were timed using an inductive timeing light so they are dead on. I purchased a new gas cap from bike bandit and I don't know of a vent but I am assuming that it is some type of hole in the cap itself, so I assume that since it is new this should be open. Need any other info.

[KEITH KRAUSE]

Without a vacuum synch, the plug reads are hard to trust. I can only assume you did the bench synch very well. The uneven plug reads make me wonder though, since all jetting/work was done identically.
I understand that the bike bogs/stutters and this is stopping you from vacuum synching and from taking jet needle reads (1/3), pilot circuit reads and main jet reads. The three rich plugs and the assumed fuel starvation contradict each other.
Also, one position richer on the jet needles is enough for a pipe only mod, but it can also be too much. What bothers me is I've read info that says the stock e-clip position is #2, while other info says stock position is #3. Where are yours? Any idea what year the replacement carbs are?
It appears you have the bike prepared as best you know how for re-jetting.
Points and timing checked, newer plugs/gapped correctly, clean/solid connections at the plug caps, good leads/no cracks, good fuel flow, clean carbs with all new inner o-rings, floats set, clean air filter...
I also assume the compression is good, though it wasn't mentioned?
I have to wonder about the replacement carbs because the PO's could have changed things or you could have worn parts, such as needle jets that are causing problems. Lots of things can go wrong and be hard to see.
Are you positive the factory jet needle spacers were installed in correct order as I said earlier?
Considering the situation and all above are good, this is what I'd try/look at first.
Be sure the electrical connections, including grounds, are all good. Check coil terminals for clean/tight connections and check for wire strands breaking. Check for tight connections under the points cover. Clean the battery posts. Top off battery if low with distilled water.
Check the fuel line for any kinking.
Blow into the two float bowl vent lines. Can you?
I'd initially set the pilot fuel screws to 1 1/4 turns out.
Set the side air screws to 1 3/4 turns out.
Clean off the plugs with a little fuel and light wire brush and rag.
Make sure there's plenty of FRESH fuel in the tank.
Test.
If no improvement, before coming back, try putting the petcock on PRIME and re-test.
If no improvement, put petcock back to ON, then REMOVE the air box lid and re-test.
Let us know what happens.

[mpinkorea]

Keith some more responses to your questions. The carbs are 1978 model as far as I can judge, just manufactured before January 1st 1978, as far as the Clymer manual states. The difference between the carbs that were on the bike had no air mixture screws and the new used ones do. The needles are set at the 2nd notch from the bottom of the needle. Compression is fine and all electrical seems to be in order. Yes I can blow into the vent lines without restriction. I have placed the petcock on prime and did not notice a difference. I will however try and run the gas in the "ON" position with the airbox cover off and let you know what happens. Initially the bike had pods with the other carbs, but I have since replaced them with a correct airbox and a UNI filter (oiled) which was just cleaned. My plan is to place the needles back at the #3 position and install 115 main jets and 17.5 on the pilot jets. I think this will solve the problem, but will have to report back when I get the jets installed and adjustments made. This may just be a trial and error type thing.

[KEITH KRAUSE]

OK. The e-clip position. That's what I wanted to know.
I'm also thinking the e-clip should go in the middle position (3rd). I'm hoping the 115 mains are the right ones. I still think I'd try the stock 15 pilot jets with richer pilot fuel screw adjustments because the pilot jets are easy to change later if needed. Just use a short Phillips screwdriver to remove the bowls and swap them out with a short flat head. But 17.5 certainly make sense too.
Of course, changing those darn Phillips bowl screws to Allens is best.
I'm going to be going on vacation this Wednesday AM until August 14th, so if you need help I'll TRY to see what you're up to tomorow night. I just don't want you to ask for help and think I'm ignoring you. Others here should help you out.
I think I've suggested/covered enough things to help you. I hope there will be good news when I get back. Please let us know if it runs well because this will help others down the road.

[mpinkorea]

Thanks again Keith, I thought I would get the jets in the mail today but not such luck so possibly tomorrow. Thanks for the help thus far and I will try the 15 pilots first then if needed the 17.5's. Have a good vacation.