1978 XS750 high speed problems

[IanDMacDonald]

Carb Syncing

Okay fellas, I think it finally comes down to my carbs needing to be synced. When I stick the 135's in, she lags at higher RPM's, not exceeding 70+ mph. When I stick the bigger jets in (142.5's, 145's in), the middle carbs runs to rich, causing the spark plug to become saturated, shutting the cylinder down. So, my unprofessional opinion is that the carbs need to be synced. I will check the web/ebay for a vacuum gauge and do this hopefully within the next week. Any suggestions is much appreciated.

Thanks,

Mac

[IanDMacDonald]

Carb Syncing, Part 2

Okay, so I read the post regarding jetting recommendations, and they say to go with 135's with pod filters. I think I will do that, and then work on carb tuning. I will order one off ebay now. Will I have to set float height before or after I sync. the carbs? Any help is much appreciated.

Thanks,

Mac

[DGXSER]

Before you go into tuning and synching, you do want to set your float height.

Before you even mount htem on the engine, you should make sure you have checked and rechecked that all of your carb parts and passages are very clean. Spray carb cleaner and compressed air through all of the holes, openings, orifices in the carbs. Then set them on a bench nice and level and check float heights. Check both sides of the float are the same setting and they are within spec.

After they are assembled, raise each slide with one hand and put your finger over the moon chaped opening in the inlet bell. See that the lside drops very slowly, then like a rock when you remove your finger. I also attach some fuel hose to the fuel T and blow into it as verification your float valve is sealing.

Now, use a bread tie with the plastic stripped off to "preset" the butterfly valves to one set opening. Put the wire in the throat of the carb #3 and use the idle screw to set the butterfly tight to the wire. Then use the synch screws to get the other three to match it.

Now...mount them on the bike. Pull the choke out, prime with gas and start your engine. It should run pretty well and go up well into the 7-8k rpm level with no issues. Depending how good your compression is, you may need to elevate the idle before it will idle with no choke.

[IanDMacDonald]

Bread Tie Method

So, checked my float levels and was astonished to find them below a quarter inch off from spec . So, readjusted them (more later), and tried the "bread tie" method of adjusting the butterflys. Got the carbs back on and started the bike in order to get it to op temp so I could use my new carb sync tool I just bought. As soon as I started the bike, the tach shot way up to almost 5k, and I immediately pushed the choke in halfway to reduce this. I pulled the tank off and backed the screws out a couple of turns, still high. I will have to back them out some more, I ran out of time. Then, the acceleration is running horribly rough. To be honest, I'm not sure if I am running too lean, or too rich. I forgot to drop down to the 135's, I was in such a rush (I have the 142.5's in). I will do that before I re-adjust the tang height. Not sure if I should even mess with the needle seat height.

[DGXSER]

Based upon the float heights being off, I'd reset the carbs back to stock and then adjust your main idle setting down before you start playing with other stuff.

[IanDMacDonald]

Actually had a little time today to play with the bike. First, I dropped the idle down to 1k via the air/fuel mixture screws, as the idle screw was not doing it well enough. After that, I put the 135 main jets back in. The bike when warmed up seemed to rev a lot smoother in neutral. Once I put it in gear, she rode horribly! Not sure if I have the float tangs set too high, or too low. I guess I would have to pull the plugs to check that. After I get that set, I will then proceed to syncing the carbs.

[cywelchjr]

Actually had a little time today to play with the bike. First, I dropped the idle down to 1k via the air/fuel mixture screws, as the idle screw was not doing it well enough. After that, I put the 135 main jets back in. The bike when warmed up seemed to rev a lot smoother in neutral. Once I put it in gear, she rode horribly! Not sure if I have the float tangs set too high, or too low. I guess I would have to pull the plugs to check that. After I get that set, I will then proceed to syncing the carbs.

If you had to use the mixture screws to lower the idle speed there is something else wrong with it, either a leak or the throttle hanging up somehow. If things are right, you adjust the mixture screws for highest idle speed and then typically turn them in till the idle speed starts to drop slightly.

[DGXSER]

Hey Ian,

Here is a link to some very good guidelines for tuning the CV type carbs.

I am not certain what the 750 had on it for carbs though.

[rpgoerlich]

what type of carb's are on it MIC1 or 2's? The MIC1's will have a lift lever for the choke are a pain to sync and the jets are mounted in the bowl. The MIC2's will have the pull out lever(like the 11's). I'd say you found your biggest problem with the diaphragms. When you cleaned them did you pull the emulsion tubes(MIC2)? It is possible but unlikely some one swapped the carb's for 850 Hitachi's.

What kind of carbs ore on it?

What do the plugs look like?

[IanDMacDonald]

Yeah, have not had much time to touch the bike between school and work. I will run it tomorrow and then pull the plugs to determine whether the floats are set to high (rich), or too low (lean). The carbs are factory MCII's. I pulled the carbs apart and found them to be in almost pristine condition, with the exception of the crushed diaphragm. I have read the various postings on this site with regard to carb tuning, etc. That's where I got the idea to go back down to the 135's, as I have pod filters. The guy who was running great with 145's+ must have some greatly adjusted floats! Anyways, I hope to get the float height corrected to a point of this bike running smooth. And after that, comes the synchronizing of the carbs. Thanks for the help fellas. I'll keep you updated.

Mac

[IanDMacDonald]

Okay, dropped my float height to about 31/32 of an inch. Bike turned over on Full choke. Attempted to kick it again and no response. Placed it on intermediate choke, and she started right up. Very smooth revs, I could tell instantly that the lean/rich mixture was very close. Took it out and had great revs up to 4k rpm, and she started lagging. So, reading my carb tune procedure, it says that I need to adjust my needle clip position for mid-range rpm's. Is it possible that I am just not running the floats leans enough? Should I lower the tangs a little more and see if it makes a difference? If you are supposed to select the main jet first, how do you really know if the main jet is the proper size, if you cannot get proper full throttle opening in gear? She revs great in neutral, and when the clutch is pulled in, but as soon as I put the engine under load, she bogs down mid-range now. I'll have time tomorrow to play with it some more.

[rpgoerlich]

I've had a similar problem with one bike. My problem was old hard shrunk diaphragms that I couldn't get seated properly in the carb body. Finally soaked them in warm water and stretched them so they would seat. Once re-installed it ran fine.

One other time was a 79XS750SF with PODs and MIC II's with stock jetting, which sold before I could correct it. It would rev to 6k and sit there.

[Ken Talbot]

And after that, comes the synchronizing of the carbs.

FWIW - my suggestion would be to stop messing with your carbs, leave everything the way it is now, and do a proper sync. Run it for a bit, say enough to see how it is running at various speeds, etc.

Then, and only then, choose ONE thing to do, whether that is move the needles, adjust the pilot air screws, reset the floats, change out jets, or whatever. The emphasis is on doing only one thing. Sync it again, then run it again to see what is happening.

By changing one or two things, then something else, then thinking about yet another thing to change, all the while thinking the sync will come last, you are just chasing your tail and your chances of getting it all together properly at once are like hitting the big lottery win - i.e. pretty much impossible.

If you really want to get serious, write down exactly what it is doing before you start. Then write down exactly what you do at each stage, and again - write down how that changes the performance.

JAT, YMMV, etc, etc.

[mack]

Sorry been away for awhile

Got a new grandson in the last couple weeks so it's been a bit busy around here lately. If I remember correctly you changed the C clip positions before you rejetted. Did you put them back to the factyory position when you rejetted?

[trdriver71]

Yeah, have not had much time to touch the bike between school and work. I will run it tomorrow and then pull the plugs to determine whether the floats are set to high (rich), or too low (lean). The carbs are factory MCII's. I pulled the carbs apart and found them to be in almost pristine condition, with the exception of the crushed diaphragm. I have read the various postings on this site with regard to carb tuning, etc. That's where I got the idea to go back down to the 135's, as I have pod filters. The guy who was running great with 145's+ must have some greatly adjusted floats! Anyways, I hope to get the float height corrected to a point of this bike running smooth. And after that, comes the synchronizing of the carbs. Thanks for the help fellas. I'll keep you updated.

Mac

Just to note you do not adjust your floats to adjust your mixture you just make sure they are at the proper height so the float bowl does not over flow or have too little fuel in it so when you run at high rpms you don't starve you engine of fuel.