Revs Stickin at 4,000

[VermontNewbie]

I just bought my first motorcycle, Suzuki GS400 1977. It has a problem I'm trying to overcome. The rev's will stick sometimes at 3000-4000 rpms and if they don't it descends to idle very slowly from that point. I bought new diaphrams off ebay, and though its helped it hasn't solved the problem 100%. Recently I took off the fuel cock and cleaned it out and found that the vacuum hose was clogged, once more air got in from the tank it seemed to work even better. I rarely get the RPM's stuck anymore but it still descends very slowly. Could air be getting in through the fuel line or whats going on here?

Thanks!

Oh and this bike does not have an airbox cover, but apparently the local suzuki dealer cannot order me one, what do I need to do to the carb if I get the conical clamp on filters that go strait on the carb?


EDIT: It pops occasionally also, I think it just might need to be synced though.

[VermontNewbie]

Just learned that Land Air Suzuki here in vermont wants 400 to pod filter, rejet sync and such =| there has to be a cheaper way

[VermontNewbie]

I found it works alot better if I keep the idle adjustment lower than normal, it falls almost as fast as it should and doesn't get stuck, this gets weirder and weider

[KEITH KRAUSE]

What are these diaphragms you said you installed? A '77 400 has VM carbs, correct? Do you have aftermarket carbs or stock?

[VermontNewbie]

I think they are stock... I know they have a carb slide with a diaphram in them.

I'll try and get some pics.

[KEITH KRAUSE]

I'm sorry. You do have CV carbs. My mistake.
First thing, are you sure the throttle cable(s) are routed correctly and have a little slack? Any way the throttle cable is hanging up?
If that's OK, then you may have an intake leak. But first, you should make a temporary cover for your airbox. I imagine you can use some duct tape for now. Without the cover, you'll run quite a bit leaner. This may be causing your problem and can also be related to the popping.
Try making a cover that you know is sealing well and see what happens.
Also, check the throttle cable. If you still have a problem, then we'll discuss intake leaks.

[VermontNewbie]

The throttle cable is in and has a little slack and I have duct tape covering the airbox. Funny you mention that because I made that little cover today. The thing works fine when the idle is set low, but when it set to idle at about 1.3-1.5k it starts to get stuck at 3-4k

[VermontNewbie]

Checked plugs thismornin and they were a bit black but no deposits or anything on them.

[KEITH KRAUSE]

I'm assuming you received the bike with this problem and it's not something you did to it when you worked on the carbs? I know a lot of people have trouble with re-installing the diaphragms.
I'm thinking intake leaks and carb synching. Here's what I would try.
Make sure there are no possible intake leaks with the airbox and tighten the clamps for the rubber boots and manifolds. Then start the bike and warm it up. Spray a fine water mist on the manifolds and see if the rpm's drop for a moment. If the rpm's drop, then you need to replace the o-rings inside or possibly the manifolds themselves, assuming the manifold bolts to the head are tight to begin with.
This test does not always show the leaks though. If the o-rings have been in service for quite a while, then they most likely need replacing. They are the most common reason for intake leaks resulting in "high idles" and rpm's hanging up. These should be replaced as part of any carb rebuild. They're cheap. If you change them, put some hi-temp' bearing grease on them and torque to 6 ft/lb. If you pass the water test and decide to not look at your o-rings, then you can try the carb synch. Remember, if you do have any intake leaks you can't synch the carbs. A vacuum tool carb synch is best. Do you have a tool? Motion Pro makes a simple but good tool for $40. If not, you can still do a manual synch if you have your manual. A manual synch may get you running close enough if your problem is synch related.
As for your earlier question about installing pod filters, you'll have to buy a jet kit in my opinion, if they make one for your model. You can also try larger Mikuni jets and "shimming" the needles but I always go with kits.
Let me know how it goes. I'll be busy today and I'm not sure when I can check back with you. So if I don't answer soon, you know why. There are others here who can help too. Good luck.

[VermontNewbie]

Did that wet test and got no results, did get some new weirdness for you though. when its idling if you crank the throttle hard, it will either pop loudly delay a bit and rev up or stall. I think ill make an appointment to the local garage for a synch, I do not have the manual

[VermontNewbie]

New fun thing wrong, left spark plug wire isn't delivering a charge to the plug, bleh.

EDIT: the plug on the right side had black ashy stuff on it and the plug on the left hand no deposits at all, but was a little black.

[KEITH KRAUSE]

You really should get a good manual/tools and learn to do these maintenance things. You'll save $ in the end, but I know sometimes there's no time for it. If you're capable of taking off the carbs you could still check the o-rings. I would still suggest the carb synch even if the o-rings are good or replaced. If you take it to a good shop, they should be able to find any problem(s). Please let us know how it goes so we can help the next guy.

[VermontNewbie]

Right now its less of a time more of a money issue So far this thing keeps falling all over itself and im almost out of cash. I bought the thing with the aide of a mechanic who assured me it would be running fine after a carb cleaning and maybe a 20 dollar kit. I can afford one small trip to the garage then I might have to sell it again and take the loss

[KEITH KRAUSE]

Just saw your last post. I was in the middle of my last reply.
Now you've got spark problems? Black plugs mean your mixture is too rich or a weak spark. If the bike was sitting, then a carb clean up is necessary in any case. Then at least check the coil leads and plug caps for clean/good connections. Make sure the battery is charged and filled correctly with distilled water and all connections are clean/tight. The plugs may be old and need replacing too and gapped correctly. The air filter needs to be clean. These are easy checks to do and might help.
I'm also wondering if the previous owner made some problems for you.

[VermontNewbie]

The left spark plug wire kills sparkplugs I guess, I've replaced the left one twice and the right one when i bought the bike. I don't know anything about the coils and such unfortunatly. I'll see if a mechanic friend of mine can look at it.

EDIT: THanks for all the help by the way =]