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You have a couple of issues going on...try and address them separately, and not get confused into thinking that they are related.

Confirm that you have good compression first...if you don't, then we don't need to worry about carbs or ignition, or anything else until we correct that.
If you have low compression numbers, then drop a spoon full of oil into the low cylinder's spark plug hole...or insert the straw from your favorite spray lube, WD40 etc...it really doesn't matter...get some oil in there.
Crank it over again, and see if your readings improve.

When did you last check the valve clearances?
 

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Discussion Starter #22
Sounds good! I'm not sure when valve clearances were last checked - I'll check again while I'm at it.

What would good compression numbers be? If you look at the compression test results above you can see the numbers seem PRETTY good when the carbs are off
 

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Here's the text book answer...Healthy engines should have compression over 100 psi per cylinder, with no more than 10 percent variation between the highest and lowest readings.

I'm confused by your troubleshooting logic...I don't understand why you are doing compression tests on a cold engine with the carbs off, or when the carbs are on, the throttle is closed. What do you hope to gain by this?

Your readings appear to be inconsistent, so that is why I'm suggesting that you do it again properly with a warm engine, and the throttle open.
 

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Discussion Starter #24
That totally makes sense. Honestly there wasn't an immense amount of logic behind it. The engine was running great until I messed with the carbs. I tested the spark plugs to be sure they were fine, and checked compression while I was at it. I picked the bike up from a local shop and they had done a full compression test (values were quite good) before selling.

Like I said bike was running great before I fiddled with the carbs, so I feel like I'm chasing a false trail a bit. BUT, I will totally happily put the carbs back on and warm the bike up to do a compression test.

Mainly I'm concerned about the carbs, though.
 

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Discussion Starter #26
Awesome. I seem to have SOMEWHAT fixed the problem, which is excellent news. Turned out that it was the pilot air circuit that was clogged on the #1 carb. Idle now seems pretty solid, though exhaust temps at the head are still somewhat varied. I'll get solid numbers for y'all asap (just started dumping rain). Under throttle it's sounding a bit unhappy now, but I'm going to warm it up completely and take it for a ride and hopefully diagnose it further that way.

So my next steps are:
1) Start bike, take temps at exhaust headers at idle
2) Throttle for 10 seconds, take temps at exhaust headers
3) Ride around for a bit, pop out spark plugs and see what they look like

Thoughts? Now that the idle issue seems to be mostly solved, what would you suggest? I might just ride it around for a few weeks, as long as it doesn't suck to ride, then work on it when it's not summer.
 

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I wouldn't even screw around with any of that. The carbs need to be gone through properly, after which you won't have any issues. 95% of running problems are caused by carb issues and most of the time people just try to fix one or two things, as they don't want to do a full rebuild. These bikes are more than 30 years old and most of them haven't gotten the maintenance they need by now.

Just bite the bullet and do the work. You can do it in a day, easy, once you have the parts on hand. The process has been described here dozens of times and the factory manuals are pretty good.
 

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Discussion Starter #28
Oh I know, but the carbs were fully gone through about 1 month ago at a local shop. Full rebuild, clean, everything.
 

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Oh I know, but the carbs were fully gone through about 1 month ago at a local shop. Full rebuild, clean, everything.
Wanna bet?

Did they ultrasonically clean them?
Replace all the o-rings with the correct ones from Suzuki?
Assemble them properly?
Bench sync them?
You (or whoever put them back on) do a vacuum synch and adjust the idle mixture?

Face it, freshly rebuilt carbs don't make an engine run badly, as yours does. I've been doing this a long time and I've learned to not do things halfway. You may have paid for work you didn't get, or they didn't do everything that's in a "full" rebuild, or they guy that did the work didn't do a good job.

Trust me on this one...
 

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Discussion Starter #30
Hahahah, that's fair. I do know that they did the first 4 (clean, replace o-rings, assemble properly, bench sync). I have not done the vacuum sync or idle mix adjust. I honestly think they did a good job, though, because they ran great UNTIL I took them off the bike to rejet. My guess is that in doing that I got some dust or something into the system.

In that case, my plan will be to ride it around, and set aside some time this week to do the full rebuild. If in riding it around I find that it's something that is not bad for the summer (clunking etc is gone), then I'll roll with it for now.

When doing the rebuild, do you recommend I follow every step on the owner's manual? (Completely separate the carbs, throttle shaft, choke, etc)? Considering that all of the o-rings were replaced within the past month, do I need to order new ones or can I keep the ones I have?
 

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I would not separate the carbs. It take a bigger cleaner bath to hold them as an assembled rack but you'd more than likely screw them up if you removed the butterflies and pulled the shaft. Not worth it.

As for the o-rings, if the shop didn't use the Suzuki-supplied o-rings, all bets are off. There are a lot of aftermarket versions, many are "close but not same-same." I know the ones supplied by Suzuki are correct. The ones supplied by a Mikuni distributor are correct, IF you specify the right sizes. As they do not focus on OEM carbs, good luck with that. The ones from K&L, or whomever, aren't always the correct size. Usually companies like K&L give an assortment of "Mikuni sizes" or "Keihin sizes" or whatever. They aren't specific to a certain type of carb, just some sizes commonly used in that brand of carb. Or close to those sizes... and made of what, I don't know.

You need to do a full synch and adjust the idle mixtures when you put them back on.

As for the blocked idle air passage, how do you explain that? That's not one that usually is an issue and dust isn't likely to plug it.

Tackle this however you wish. Just don't tun the bike until the issue is sorted. If you are re-jetting it, too, you can cause problems there, as well.
 

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Discussion Starter #32
Definitely was Suzuki O-Rings. Keep them together, keep the butterflies and shaft on, pull the choke plunger?

I honestly have no clue how the heck the air passage got blocked. That completely confuses me.

I won't mess with the jets any more - I already put them back to how they were when it was working.

I'll obviously check on the forums, but is there a guide for how to adjust the idle mixtures? Should they all be the same number of turns out?
 

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Idle mixture screws on carbs are generally set to the same position, unless you have a CO meter and can measure the exhaust CO of the individual cylinders. Bottom them out gently, mark the position of each screw when closed and open them up the prescribed amount.

If you have no intake or exhaust leaks and if you have the valves all adjusted the same, you can sometimes use a digital temp gauge to measure the individual header temps and tweek the mixtures slightly.

You might let us all know what modifications have been made to the bike and what jet kit you were putting in it. Should we assume you have stock jets back in it now?

Do you know if the float levels were all set correctly? Even minor differences from one carb to another can make the mixtures vary from cylinder to cylinder. You can always test the fuel heights with the carbs on the bike, if nothing else. They need to be all the same.
 

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Discussion Starter #34
OK good to know - I think the idle mix screws are actually all off from each other. I'll start them at all the same, because there's a good chance that's what is causing the discrepancy at this point. There are no intake or exhaust leaks.

Mods are:
1) yoshimura exhaust with race headers etc
2) individual pod filters
3) mikuni jets that were tuned for this setup - not sure what the needle is or the pilot air jet, but needle clip is at bottom position, and the main jets were 142.5
4) I swapped out the main jets to 150 because at WOT I was getting no power. It would give me power perfectly until 3/4 throttle or so, and then flatten out. 150 gave me more power, but still didn't completely solve it, so I was putting 152.5 in. At the advice of the guy at the bike shop, I tried putting 152.5 in the 2 middle carbs and 155 on the outer 2. When I put the carbs back on, they started acting up like this, so I put it back to 150.


SO, currently, we have 150 in all main jets.

I do not know if the float levels were set correctly. How do I test the fuel heights? Manual says to use a specific tool for that.
 

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The idle mixture screws will never be all in the same position relative to a clock face when you look at them. That's because the threaded holes are not all threaded the same. You have to mark their position when they are fully (lightly) screwed in, then back them out the same number of turns. They'll likely be pointing four different directions.

The first thing I'd do is try to figure out what parts are in it. Mikuni jets and needles will be stamped with a number and sometimes the Mikuni logo. The needles are probably aftermarket. Most likely they'll be DynoJet, as that was probably the most common, back in the day. I prefer Factory Pro, I probably won't ever use Dynojet products again. Your call.

150 mains are pretty big for a 750. That's 13 jet sizes larger than stock. Granted you're flowing more air, but...

Are you running CV carbs or aftermarket carbs?

The float levels are easy to check. I'd verify they are correct before going further. You can buy the Suzuki fuel height tool from any dealer for 10-12 bucks. Checking the float heights is easier than checking the fuel levels.

If you are tuning CV carbs, the throttle position is not relevant without also knowing the revs. It's the slides that dictate the needle position.
 

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Discussion Starter #36
Ahhh good to know. I'll check the needles and jets for sure - it has been pissing rain here, but finally got a bit of sun today so I am going to take it for a ride first.

I know somebody earlier was saying that 142.5s were solid for them. CV carbs - I'm pretty darn good at cleaning and tuning non-cv Mikunis, but these are my first times with CV.

What would you say the issue is if I'm getting stutter at WOT (particularly if I'm going to WOT from not very open)?

Is this the guy I want?
https://www.ebay.com/itm/Fuel-Level-Gauge/233250641725?epid=3011448607&hash=item364ed2573d:g:SIsAAOSwiB5c-ubb
 

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Yeah, that's the fuel height tool. I'd still recommend that you yank the carbs off and check the float heights. It's easier.

So, the stock CV carb for '86 was the BST31SS. It will be stamped 28A00 or 28A10 on the side.

How are you testing this bike? On a dyno? Riding it? WOT means nothing, really, except at high revs. At lower rev ranges, you can be wide open with the throttle but the slides can be only partially open. How open the slides are will dictate the mixture. The needle controls the fuel flow until the slides are open most of the way, at which point the main jet plays a role.

Here's a good guide to tuning CV carbs:

CV Carb Tuning Procedures
 
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