Two bikes, two different problems

[Mahalaleel]

I finally finished doing the complete cleaning and replacement of o-rings on my carbs and intake boots on my 1978 Suzuki GS750.

Before reworking the carbs it would not start easily, it took a ton of work and spray to get it going. Now, it starts right up first kick. Woohoo!

The bad news is that it won't idle right, or return to idle. I start it up, and it will sit around 1200 RPM, sometimes higher. I have the idle adjustment knob all the way out, so it's doing nothing.

When I open the throttle a bit and then close it the engine continues to run at about 3000 RPM. If I reach my hand in and push down on the throttle pulley it slowly goes down a little bit, but not back to a healthy idle.

My theories are that I need to tune the carbs using the two pilot air and jet screws, re-synch the carbs, or something. Also maybe the spring on the throttle pulley is too weak, although this isn't the entire problem because even with the pulley all the way down it doesn't return to a normal idle.

What are your tips?

TIA.


Second bike! Honda VF 750 S V45 Sabre 1982 or 1983 don't remember (it's a friend's bike).

The bike starts right up and idles great. It runs really well until it's warm. When I hear a fan kick in it starts to stall when idling at stop signs. As long as I keep the RPM up it stays going, but if I allow it to idle after running for a bit it will stall. It starts right back up, but will repeat the problem at the next stoplight or stop sign.

Theories?

 

[hampshirehog]

No slack on the throttle cable? Either that or blockages in the pilot jets, an air leak somwehere (vacuum hose fitted?) or your airscrews are too far out / fuels screws too far in. I assume it's fitted with a proper airbox.

 

[Mahalaleel]

Yes, a little bit of slack on the cable closer to the engine.

I really hope the pilot jets don't have any blockages, I went through it very thoroughly twice with dip and cleaned out the passages. Shouldn't be the problem. Which vacuum hose are you talking about?

And here's a newb question. Right to left or left to right, how are the carbs numbered? There are three things that look as though they could be vacuum ports. One is connected to the petcock (and the petcock is working, so I figured I got this part right), the other two are?

There are two screws, which one is air, which one is fuel?

Oh, and no, it has 4 pod filters from the PO.

 

[hampshirehog]

Carbs are numbered from left to right as you sit on the bike (1 is outermost left, 4 is outermost right).

I'm blowed if I can remember which carb has what hose on it but you have the main fuel line attached to a tee between 2 and 3, two breather hoses which sit airbox side of the carb slides and a vacuum hose that sits on one either 2 or 3 and sits head side of the slides. If you're missing that hose and the nipples isn't blocked off there's your massive airleak that will give you your problems.

That's assuming the PO jetted correctly for the pods.

 

[Mahalaleel]

Ok, I have the vacuum hose from the petcock to carb three, and the petcock is working fine, supplying fuel so that seems right to me. The other two are blocked off.

I have absolutely no idea when the pods were turned on, one of the POs put them on, so I don't know if it's jetted right.

 

[Dave8338]

Ok, I have the vacuum hose from the petcock to carb three, and the petcock is working fine, supplying fuel so that seems right to me. The other two are blocked off.

I have absolutely no idea when the pods were turned on, one of the POs put them on, so I don't know if it's jetted right.

Well that is the first place to start. What jets are in the carbs, currently?
It is showing ALL the signs of running lean on the pilot circuit.

Jet size is first. It needs to be correct before we continue. Sounds like a Stage 1 or 2 would be in order, for starters.

Next, IIRC...the 750 is a 16V engine and has NO intake rubber o-rings, so that rules THAT out.

How are the intake rubbers?

Stock exhaust?

 

[Mahalaleel]

This one is an 8v, and I already changed the o-rings on the intake boots. I don't think it's an intake leak. The boots are a little older, but don't seem too worn.

If I can do anything without changing the jets I would much prefer it, I'm trying to keep the budget as low as possible, however I'm willing to do 20 hours of work on it to save money.

 

[Dave8338]

If everything is tight (intake wise) you will have to do some serious plug chops to determine where things are lean. If on the pilots, you will have few options left, but to rejet. I have soldered jets closed and re drilled them to get the "burn" right however, that assumes YOU have the correct size drill bits and either a drill press OR a VERY steady hand. I used a drill press...

 

[Guest]

I had the exact same symptoms when my needles were a notch too lean. This also made the carbs really hard to sync. If you've eliminated intake leaks then it does sound like you need to check jetting.

 

[Mahalaleel]

Ok, from what you're saying I will try adjusting the fuel and air screws first hoping to avoid buying new jets.

Is there a guide to doing this properly some place? I actually don't even know which screw is air which one is fuel, so this will be my first time trying this.

I also read it's possible to adjust the huge needle's height. Would that have any effect?

 

[Steve]

Ok, from what you're saying I will try adjusting the fuel and air screws first hoping to avoid buying new jets.

Is there a guide to doing this properly some place? I actually don't even know which screw is air which one is fuel, so this will be my first time trying this.

I also read it's possible to adjust the huge needle's height. Would that have any effect?

Check your main jets FIRST. If they are not right, nothing else you do will get it even close, so don't bother to try any shortcuts.

The screw on the bottom is the pilot fuel screw. On a stock bike, it will be 5/8 to 3/4 turn out from lightly seated.
On a bike with pods and a pipe, it might be 3/4 to 1 full turn out from lightly seated.

As a rule, a good starting point for the pilot air screws (the ones on the sides) will be double of what the fuel screws are, then you will tweak from there.

Adjusting the height of the needle will affect the jetting in the mid-throttle range, from about 1/4 to almost 3/4 throttle. To raise the needle to richen the mixture (the usual thing), you will need to lower the clip one notch.

.

 

[Mahalaleel]

Thank you sir!

How do I go about "checking" my main jet? What am I looking for?

 

[Mahalaleel]

As they stand all of the screws are all the way in. Could this be the problem?

 

[Steve]

As they stand all of the screws are all the way in. Could this be the problem?

Only if you want the bike to run. :-\\\

If you are saying that the pilot fuel screws (on the bottom) and the pilot air screws (on the sides) are all turned in until they are bottomed out, the bike will simply have no fuel to run at idle speeds.

Look at my previous post (just three before this one) to see the approximate settings.

.

 

[Mahalaleel]

Ok!
Good news (kind of). I discovered that the problem is that the slides in the carb, or maybe the rod that runs through everything, or maybe even just the throttle pulley are getting stuck, and not returning to the minimum open space.

Is there some way to lubricate things without sucking lube into the combustion chambers?

 

[Steve]

They don't need to be lubricated, they need to be CLEANED.

The gunk that is making things stick needs to be removed by cleaning.

.

 

[rustybronco]

Ok, I have the vacuum hose from the petcock to carb three, and the petcock is working fine, supplying fuel so that seems right to me. The other two are blocked off.

The other two nipples need to be open to atmospheric pressure. either through a vent hose, vent hose and filters or left unconnected.

 

[Mahalaleel]

Ok! Today I fiddled with it some more. I found that when I pushed the throttle pulley back down, the bike would idle just fine at 1200-1400 RPM.

So I figured it would be the cables, seeing as they are kind of old. I lubed 'em up a bit and put them back on. It was better, but things still don't return to idle without a push down on the pulley by hand. It gets all the way down to 2000 RPM instead of 3000.

Someone suggested I may have put the slides back in the wrong carbs (sounds very likely to me) causing undue friction and unnatural fit.

My next step then would be to switch them around and then resynch, right?

Oh, and I tried with and without covers on the other two nipples, and it seemed to make no difference anywhere between 1200-3000 RPM.

 

[Mahalaleel]

Well!

After a hiatus I went back to work on the bike and found a few things in addition to having a very interesting time.

Having pondered for weeks on what could have been the problem i decided to see if I had screwed up reinstalling the intake boots. Well I had! One of them was not screwed in all the way and was causing a pretty large air leak.

So I fixed that.

The throttle, however, still insists on getting stuck and I have three theories.

Theory one: This spring is too weak.

Theory two: This thing wiggles and the return throttle pulley gets stuck. It gets pulled down instead of the pulley getting pulled up. The top rivet is loose.

Theory three: God knows that if I fix the bike I'll ride it, and he's protecting me or something...

Anyone know what I could do? Where replacements might be?

When I do get the pulley down by hand I can get it to idle consistently at 1400 RPM. Is that too high? Any lower and it decides to slowly stall out.