Mikuni Carbs Mystery Solved.....Somewhat

[mpinkorea]

Hey sorry havent posted in a long time. I have a 1978 Suzuki GS750E. When I bought the bike I could not figure out what everyone here was talking about with the vaccuum operated petcock since my carbs only had 2 outlets (both vents). Come to find out when i pulled them off of the bike and rebuilt them (new o-rings, gaskets, 102.5 and 15 jets and all other moving carb parts) that 1 carb was incorrect or these carbs were put together hastely. I also replaced the rubber boots from the engine to the carbs as well as the carbs to the airbox.

Sometimes when I am sitting at a stoplight the engine wants to die, but I think this is from flooding the carbs. If it does choke off, letting it sit for a minute and it will fire right up which is why I assume it is flooding. I also wonder if the single fuel line in that splits between the 1 and 2 and the 3 and 4 cards is causing too much fuel to flow (this was another thing I thought was wrong) as all other bikes like this I have seen with only 1 fuel line providing fuel in between the 2 and 3 carbs. I currently have to run on the prime setting all the time. This is because I am lacking that 3rd port for vaccuum on the #3 carb.

I have a used set of carbs coming that have all of the correct parts and are in better condition. I have a stock airbox with a UNI airfilter and the bike has Vance and Hines 4 into 1 pipe. The plugs seem to be the correct color and the bike does not smoke, so I assume that the slide needle setting of 1 notch raise that I have is sufficient. I am going to clean up this other used set of carbs and finally have what is supposed to be on the bike. I would like to know if the consensus of what I have done is correct.

Thanks for any help or suggestions. I would like to know when I get the new old carbs so when I rebuild them it will be correct.

 

[mpinkorea]

Any ideas on this?

 

[Guest]

Are you sure you're not putting a fuel line on your vacuum port for your petcock. Where are you running your vacuum line from the petcock? Gas will not flow without it unless you're on prime.

 

[ptm]

Here's a shot of completely stock '78 GS750E carbs with the hoses identified. #1 doesn't have any - no vent or vacuum. It shouldn't flow too much fuel if the needle valves are doing their job and the float heights are set properly.

As far as dying at idle, the only time that happens to me is when I forget to take the choke off after it's warmed up.

Good luck...

 

[Steve]

PTM has a good picture there for reference. Apparently the hoses are considerably different between your carbs and the CV carbs on my '82s. As the picture clearly shows, the fuel line comes in the middle, then distributes the fuel through a common gallery that you can see very nicely. If your fuel is connected to the other hoses, I don't see why it would work at all. There would be NO way to regulate the fuel level in the float bowl, which just might be why you feel like it is flooding. IT IS!!!. I don't understand why there isn't a vent hose on #1, but apparently it works.


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[ptm]

I don't understand why there isn't a vent hose on #1, but apparently it works.

The vent hose on #2 actually vents both 1&2 - you can just see the hose that connects 1&2 right next to where the long vent hose attaches (above and parallel to the fuel line between them). The same with 3&4, the vent hose on #4 actually vents them both.

 

[Steve]

OK, I just looked at it a bit closer. What I thought was a vent hose coming from #3 is actually the vacuum line to the petcock.

So...the plumbing is not so different from the CV carbs, after all.


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[KEITH KRAUSE]

He says his #3 carb is lacking the vacuum port.
If he has no port at all, then that would mean he's using another #1 carb where a #3 should be.
At that point, the bowl venting between 3 and 4 would work but no fuel would flow into carb #3 or 4.

OK. Just re-read his first post. He has a fuel line splitting so that one line feeds 1 and 2, and the other 3 and 4. So that explains how he's getting fuel to all four carbs. Though I wonder what fitting he used to go between 1 and 2, and 3 and 4. I also wonder about bowl venting, which will cause fuel starvation if inadequate.
With a #1 carb being substituted for a #3, I have to wonder what else has been done wrong.
I'd check that other set of carbs out well and use them if they're good.

 

[KEITH KRAUSE]

OK, I just looked at it a bit closer. What I thought was a vent hose coming from #3 is actually the vacuum line to the petcock.

So...the plumbing is not so different from the CV carbs, after all.


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Main hose connection difference between VM and CV carbs is the vacuum is at #3 on VM's, and at #2 on CV's.

 

[mpinkorea]

To answer some of the questions that I have seen. Thanks for the input thus far and the photo too..BIG help. The previous owner had no idea what he was doing with this bike. The carbs have obviously been pirated to and from. To explain this easily without attaching a photo of the carbs. I have no vaccuum line running from the #3 carb, it is blanked out like the number 1 carb in the photo. So yes I have to run the bike on prime all the time, unless it is off. This is why I believe the flooding type response once i stop from high engine speed. The carbs that I have coming look exactly like the photo above. My current fuel line runs from the stock fuel petcock, which was replaced with a rebuilt and operational one. The fuel line extends down to a "T" fitting and splits off to 2 "T" fittings that look similar to the center "T" fitting in the photo, but there is one between 1 &2 and 3&4. I believe that there would be some excess fuel problems because of this however I rebuilt the carbs that are on the bike currently by dipping them and replacing everything. The float levels were checked and adjusted and all of the seals, o-rings, gaskets and jets were replaced. I plan on dipping the new set of carbs and putting in the same parts I did on the first set. Does everyone concur with the settings that I have for the parts that are on the bike. I checked and I have no airleaks that I can detect. I think that the correct carb set up will correct alot. Thanks again.

 

[KEITH KRAUSE]

The float levels were checked and adjusted and all of the seals, o-rings, gaskets and jets were replaced. I plan on dipping the new set of carbs and putting in the same parts I did on the first set. Does everyone concur with the settings that I have for the parts that are on the bike. I think that the correct carb set up will correct alot. Thanks again.

102.5 mains and 15 pilot jet are stock. Only other question is where the jet needle e-clip should be. I've seen '78 carb specs that contradict each other. One says the factory places the e-clip in the 2nd position from the top, and another says the e-clip is in the 3rd position from the top. On your stock bike, one position is a considerable change. So you need to get the correct info where the e-clip really goes.
The floats should be set at .94", unless you have factory info stating otherwise.
Bench synch for both fully closed and fully open slide positions. Know how?
That just leaves the side air screws and pilot fuel screws.
The side air screws should be set using the highest rpm method. They'll probably end up around 1 1/2 to 2 turns out. I set them at about 1 3/4 turns before starting the bike.
The pilot fuel screws (underneath) are sensitive to adjustments and will probably take some time to get just right. I'd start them at 1 to 1 1/4 turns out and go from there. Factory settings vary from as little as 1/2 turn to maybe 1 1/2 turns out max. Be very careful with the sharp tipped pilot fuel screws. Seat them only lightly or the tips can break off in the carb body.
After you set the pilot fuel screws initially and the carbs are bench synched, warm up the bike completely and set the side air screws using the highest rpm method. Then vacuum synch. Then a final fine tuning of the side air screws if needed.
PS: if you decide to split the "new" carbs you have coming for whatever reason, be aware that removing the main fuel "T" often results in a leak upon re-assembly if you have the inferior "T" fitting with the rubber coating/built in o-rings. Less likely but still possible, the two smaller rubber fuel tubes can leak too. It may be wise to leave the carbs mounted to the bracket and don't separate them completely. I know this may compromise 100% cleaning of the main fuel passage between the carbs but it should come clean. Just a warning about that "T" and those two tubes.

 

[mpinkorea]

Just got the carbs today and I am already dipping the number 1 carb. Everything was removed and all in all the carbs look really good. I was amazed at what parts I have swapped onto my current carbs. Still wondering about the setting though, I am pretty sure that they are correct since the bike in its present state is running well despite the constant gas on thing.

 

[mpinkorea]

Well just got the new used carbs about 3 days ago.. All were dipped and ALL internal parts were replaced. They look good and better yet they have all the correct pieces and look like photo that was attached to this post. Thanks for that. I will most likely be putting them on tonight and adjusting them tomorrow. I do have 1 question though. The mixture screw that is on the bottom of the card closest to the cylinder when installed on the bike. Of course the Clymer's manual says not to remove this, but you cant dip them with it in so it was removed as I did with the other set of carbs that are now off of the bike. It took me a bit of time to adjust these correctly the last time and was wondering with my set-up how many turns out from bottom should I start.

 

[ptm]

The mixture screw that is on the bottom of the card closest to the cylinder when installed on the bike. Of course the Clymer's manual says not to remove this, but you cant dip them with it in so it was removed as I did with the other set of carbs that are now off of the bike. It took me a bit of time to adjust these correctly the last time and was wondering with my set-up how many turns out from bottom should I start.

Glad you got a good set. See Keith's post above for initial setting recommendations.