Weird change after removing carbs

[The1970's]

Hi all, I just recently pulled my carbs to reseal my chain tensioner. When I put them back on the bike seems to run the exact same but since I have an o2 sensor I noticed mixtures are much leaner at part throttle cruise. The plugs confirm this with a bright white color on the ground strap. My question here is what could have changed? I checked for vacuum leaks with some carb cleaner and couldn't find any. Another little thing I noticed is that the choke doesn't seem to operate the same. Now if I give it full choke on a cold start it will fire and stay at an idle, when I go to ease off on the choke it brings the idle up to a high idle like it did before. Any ideas? Maybe I need to sync again? I should mention it is a 78 GS750 with 4 into 1 and stock airbox w/ uni filter.

Edit: After riding again just now and carefully keeping an eye on the mixture it seems as if it has a lean spot just below 1/4 throttle. It had this before but was at around 30mph in 5th and I never was really concerned. It seems to be the same thing just moved up some in the throttle movement if that makes any sense. Like same mixture but under a higher load

[Steve]

Running lean with no changes to the carbs tells me there is a leak. Check your boots carefully.

Also, comparing "1/4 throttle" to "around 30mph in 5th" means absolutely NOTHING. That is, unless it takes 1/4 throttle to go 30 mph in 5th. On my bike, it takes virtually no throttle to go 30 in 5th, unless I am going uphill, then it takes nearly FULL throttle. You have to go by throttle position ONLY. Make marks on the grip and the housing so you can SEE what your position is.



I made marks for IDLE and FULL, then measured the HALF, 1/4 and 1/8 by simply cutting the distance in half.

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[The1970's]

You're right the 30mph was not a good way to describe it. I guess it's somewhere between idle and 1/4 throttle. I've checked pretty well for vacuum leaks and can't find any. One thing to note is when I re assembled it one of the airbox side boots came out of the airbox. I popped it back in but am wondering if there's a leak there somewhere. It does seem all 4 are running leaner

[bwringer]

Agreed -- it's very easy to get one of the airbox boots folded over where you can't see it.

Also, I've seen some bikes where the plastic airbox starts to sort of bow in in the center, so the inner two boots won't seal correctly. I've used a heat gun and some judicious pressure with a high tech tool called a "wooden stick" to straighten these out. It helps to remove the boots first.

Might be time for new airbox/carb boots, too.


You don't always get positive results from testing for air leaks with spray fluids. Sometimes the leak only shows up at higher RPM, or it's not enough of an air leak to suck in an obvious amount of the fluid.

[The1970's]

I live right near z1 and they had the airbox boots in stock. I picked up a set and I'll update when I get them on

[The1970's]

Well I got the new boots on and it doesn't seem to fix the problem. I notice the mixture seems more stable at cruise speeds but no real difference where my issue is. I recently upgraded to a SH775 R/R and my new theory is that the bike is now getting a better spark because of a pretty significantly higher voltage. In addition my cam chain tensioner was on the loose side when I rebuilt it. So I started thinking that maybe with all these changes that the bike is running slightly better and requires slightly less throttle to stay at cruising speed. This is turn brings me into the throttle position where the bike runs lean. Whereas before it was such low throttle position that I never really ran the bike there. Does this sound reasonable or crazy?

Either way my carbs and everything are all stock except the main jet is 1 size larger due to my 4 into 1. When I try to set the mixture screws by chucks instructions I end up with the fuel screws at about 7/8 and the side mixture screws at 1 turn. Anything more and the bike runs terribly lean. The bike seems to run better with the fuel screws at about 1.5 and side mixtures at 2. Would this indicate that maybe I need the next size up in pilot jet? If I can get this lean spot to go away the thing would run fantastic

[Steve]

As far as jetting goes, you will have to do some plug chops to see which circuit needs the help.

You say you have a 4-into1 header. What kind, what baffle? Quick clue: how loud is it?
The louder a 'muffler' is, the more air it is flowing. The more air the flows, the more jetting you need. A stock system flows remarkably well, in spite of being rather quiet. Your real limiting factor is the stock airbox, which you say you still have. It's possible that one size up on the main jet might be enough, but it's just a possible that you might need to go two or three sizes. You are running a bit rich on the pilot, that might be compensating for running lean on the main.

Plug chops will let you read the color of the plugs after you have held the throttle at specific openings that will determine which circuit is in use. You can then make changes to that circuit and test again. Make only ONE change at a time, and record all your results.

Usually, there is no need to use a larger pilot jet. The need for bigger jets is because of more air flow. At wide open throttle, your air flow is limited by the stock airbox, the size of the carbs and the restriction of the mufflers. You still have the stock airbox, you still have the same carbs, your 'muffler' might flow a little bit better than stock. At anything less than wide open throttle, your air flow is limited by the slides in the carbs. That means that, at idle and up to about half throttle, you are flowing no more air than a stock bike, so there is no need to increase jet sizes. The stock jetting is a bit lean, which is why we suggest increasing from about 5/8 to about 3/4 or 7/8, but there is no need to go more than one turn, unless someone has installed smaller jets.

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[The1970's]

Usually, there is no need to use a larger pilot jet. The need for bigger jets is because of more air flow. At wide open throttle, your air flow is limited by the stock airbox, the size of the carbs and the restriction of the mufflers. You still have the stock airbox, you still have the same carbs, your 'muffler' might flow a little bit better than stock. At anything less than wide open throttle, your air flow is limited by the slides in the carbs. That means that, at idle and up to about half throttle, you are flowing no more air than a stock bike, so there is no need to increase jet sizes. The stock jetting is a bit lean, which is why we suggest increasing from about 5/8 to about 3/4 or 7/8, but there is no need to go more than one turn, unless someone has installed smaller jets.
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This is kinda what I was thinking that it really shouldn't be flowing more air than a stock bike until I get up into the power band. I do not know the 4 into 1 brand but it is moderately loud, megaphone muffler with about a 6-10 inch baffle. I do have a O2 sensor so I know what the engine is doing and I have been using plug readings to confirm this so I have reasonable suspicion that the o2 is correct. I know the main circuit is pretty much perfect, maybe slightly on the rich side. And I know that I get a lean spot right where I don't want it to be. So what I'm trying to do is figure out how to dial out the lean spot. But from everyone else's posts it seems my side mixture screws should be out more than 1 turn with the fuel screws at 7/8. This lean spot occurs at just barely over 1/8th throttle. So would that be a pilot circuit or maybe a needle issue? I have tried with the needles on the next clip down (raising the needle) and it seems too rich at everything but full throttle. Is there a way to go a half step using shims?

[Steve]

Yes, you can do half a step with shims, but at 1/8 throttle, your issue is not with the needle.

The suggestions you are getting are based on a couple of assumptions. One is that you are going by 'feel' and possibly looking at the plugs. You actually have a meter installed, so you can verify what is actually happening. That is GREAT. A second assumption is based on what has worked for most of the others that have tried this. You have to keep in mind that every carb is different. Not just every carb set, every individual carb. The screw settings for each carb in your rack might be ever so slightly different. You can't just set all the screws the same and call it 'good'.

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[The1970's]

Yes, you can do half a step with shims, but at 1/8 throttle, your issue is not with the needle.

The suggestions you are getting are based on a couple of assumptions. One is that you are going by 'feel' and possibly looking at the plugs. You actually have a meter installed, so you can verify what is actually happening. That is GREAT. A second assumption is based on what has worked for most of the others that have tried this. You have to keep in mind that every carb is different. Not just every carb set, every individual carb. The screw settings for each carb in your rack might be ever so slightly different. You can't just set all the screws the same and call it 'good'.

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Understood, when I say 1 turn what I really mean is that they all end up around 1 turn +/- 1/8 turn for each carb. So if you don't think its needle would it be the pilot? Just some more turning of the screws?