carb.sync. problems/help...

mtflyr

Joined
Jul 23, 2012
Messages
36
Location
western Montana
Bike
'91 ST1100
I am in the process of syncing my carbs. after replacing insulators+pair removal. I am using Motion Pro sync. calibrated to #4.

I have good sync. on 1 & 3 matched to 4. However, I have no vacuum to #2. I have taken off the air filter to expose the velocity stack. I can place my hand over stacks 1,3 & 4 with nothing happening. However, when I do the same to #2 engine dies. Checked #2 plug and is dry. There is a noticeable vacuum noise coming from #2 stack nothing from the others. Also, the engine is running rough.

Please help...

Thanks
 
mtflyr said:
I am in the process of syncing my carbs. after replacing insulators+pair removal. I am using Motion Pro sync. calibrated to #4.

I have good sync. on 1 & 3 matched to 4. However, I have no vacuum to #2. I have taken off the air filter to expose the velocity stack. I can place my hand over stacks 1,3 & 4 with nothing happening. However, when I do the same to #2 engine dies. Checked #2 plug and is dry. There is a noticeable vacuum noise coming from #2 stack nothing from the others. Also, the engine is running rough.

Please help...

Thanks
Click to expand...

Was it running rough before the PAIR removal or did it just start happening after removing it? Perhaps someone else can suggest something obvious that you may have done during your insulator replacement/PAIR removal. It seems odd that you can hear a vacuum noise at #2, yet you are measuring no vacuum there, are you sure you've got the tubes connected to the proper cylinder orientation? It also seems odd that you don't hear anything at the other 3 cylinders. Not sure if your recent mods could have caused this symptom, as I've never done the job myself.

If the engine was running rough before the mods, then you may have had other problems prior to the mods and you need to do a compression check. If you don't have a gauge, an AutoZone in your area will loan you a compression gauge for free. You need to know the state of each cylinder before you can decide what else to check next. If you're sure that your compression was good prior to the mods, then look for some kind of plumbing error from the recent mods.
 
No, the bike was running just fine before. Now there is an appreciable "lope". I would not have done anything if it wasn't that someone who was in here before me either disconnected or forgot the complete carb. overflow circuit. Two of the hoses weren't even there the other 2 not connected. Through that inspection, it was recommended to replace the insulators and the general opinion to eliminate the pair.

Yes, the tubes are in order which probably wouldn't matter as there seems to an issue aside from the sync. tool. That being said, I am not sure if the person(s) who was there before or I happen to loose it, but, one of the locator "pins" at the base of the "stack" was/is missing. I haven't removed the stack yet to see which carb. stack it goes to. Before I go through that again I would like to see what others suggest. My inclination(hope) is that these are just locators that serve no other purpose.
 
No, not now. Since these treads I have disconnected the MP and taken the stack off the bike. I did notice the sealing surface on #2's tube has one section missing(cut off) around the outer edge and one of the holes, probably from misalignment (not a mounting hole). I did noticed this before, but, assumed this to be possibly ok since the bike ran ok before me going into it. Not so sure now.

At any rate, I recently ordered a nearly complete "air box" unit w/tubes from Ebay (used). From what I can tell, the sealing surfaces of the tubes look to better than what I have, so the plan is to mix & match when it gets here.

That being said, I did see a "very" similar thread/problem in the archive and it started me to wonder. When I first took off the carbs the butterfly spring on number 2 fell off. It was difficult to replace as I did it when the carbs were back on. I had to turn the adjuster nearly completely out to get it on. I then just guessed how far to tighten from looking at the other 2 as this was before getting the MP. This is how the engine was set when I started with the sync. From this archive thread, could it be the adjustment on #2 was too far in not allowing the butterfly to be fully closed? If so, would this cause a "no" vacuum indicated on #2 when I did the initial sync? Like this thread, I too put my hand over each inlet and had no effect on #1,#3 and#4. But, it killed the engine on #2.
 
mtflyr said:
I am in the process of syncing my carbs. after replacing insulators+pair removal. I am using Motion Pro sync. calibrated to #4.

I have good sync. on 1 & 3 matched to 4. However, I have no vacuum to #2. I have taken off the air filter to expose the velocity stack. I can place my hand over stacks 1,3 & 4 with nothing happening. However, when I do the same to #2 engine dies. Checked #2 plug and is dry. There is a noticeable vacuum noise coming from #2 stack nothing from the others. Also, the engine is running rough.

Please help...

Thanks
Click to expand...

First off double check all of your connections and make sure you are using the sync tool correctly. Once that is verified you said you were able to sync 1 and 3 to number 4. what was your reading at what RPM for these cylinders? If really low I am guessing that you are not getting a correct reading on number 2 (zero) if that is the only cylinder that you are feeling a suction at. If the number 2 is wide open and you have adjusted your idle to spec (1100) it could be that the number 2 is the only carb working at idle and the others are not pulling anything . Have you checked the sync at 2500-3000 RPM? at that point even if #2 was adjusted way too high they all should be reading something. If you can get your hands on an SK sync tool it is really easy to use and will get you close. Some have said they do not believe the SK tool is accurate enough, even if this is true it will get you to the point you can then fine tune with the Motion Pro.
Good Luck
 
Not unusual to "read" the snorkels backwards. #2 face right, #1 faces left. Double check all that.

When/if everything is balanced, covering any of the snorkles will cause the engine to "lope" as that cylinder shuts down.

Recheck all the vacuum hose connections and get back to us. Good luck.
 
mtflyr said:
When I first took off the carbs the butterfly spring on number 2 fell off. It was difficult to replace as I did it when the carbs were back on. I had to turn the adjuster nearly completely out to get it on. I then just guessed how far to tighten from looking at the other 2 as this was before getting the MP. This is how the engine was set when I started with the sync. From this archive thread, could it be the adjustment on #2 was too far in not allowing the butterfly to be fully closed? If so, would this cause a "no" vacuum indicated on #2 when I did the initial sync? Like this thread, I too put my hand over each inlet and had no effect on #1,#3 and#4. But, it killed the engine on #2.
Click to expand...

Well, that certainly explains a lot of the mystery, thanks for adding that to the story. If you have a significantly wider throttle opening on #2, then the engine is probably running primarily on that cylinder at idle, and might explain why cutting off either of the other 3 has no visible effect. I've never done this, so I'm just guessing, but it would seem that you'd have to be pretty far off in order for that to happen, and I would have thought it would raise your idle speed quite noticeably. But, you should see some vacuum on all cylinders no matter what, even if you twist the grip and open all the throttle bodies to raise the RPMs to a few thousand when doing a carb sync (not that you should do that, but I know from experience that if you do it the vacuum changes noticeably, but doesn't drop to zero). So I'm still at a loss as to your zero vacuum reading on the gauge, unless that's simply a hose connection issue.

At this point, you should probably experiment with the #2 adjustment screw until you start seeing the cylinder contributions equalizing out with your hand over the intakes method. I don't know if you can just look down into the stacks and see the butterfly positions or not as a guide, or if you have to just use trial and error and adjust blindly. Until you get things closer to normal running conditions I'd think the gauges are pretty much useless at this point.
 
I am not sure if my posts were posted so I'll give it another go.

First, thank you all for your input. It is greatly appreciated.

Secondly, with the replacement air box/tubes "in the mail" I will be starting over. The plan is to pick out the best tubes and replace what I have even though this may not be the problem(eliminating possibilities). I have turned #2's adjuster way out and will hopefully get a reading by doing so. I have brought back the adjustment on the MP to zero, so all is set...

I will let you know the outcome.

Thanks again.
 
You must log in or register to reply here.
Back
Top Bottom