1992 GSF400 Carb Conundrum

[lknbandit400]

Carbs are disassembled. It's a good thing I pulled the slide holder. Here's some pics so you know what I'm dealing with.

[lknbandit400]

Here's what I meant about being glad I pulled the slide holders. Check out what's left of these o-rings.



I hadn't removed these before. I also have one slide holder with a broken tab at the top. I doubt this is hurting anything, though.

The top has two ribs (or should have). The broken one looks like this:

[lknbandit400]

Just had another thought about the "hard to start" problem. Is it possible the bike has jumped time? The PO said that the bike fell in his driveway (while warming up, worked its way off the kickstand) and landed on the left side. Is it possible that a tooth jumped time on the impact?

I'd hate to have to remove the valve cover to check all of that, but if that's something that is a possibility then I'm thinking I should check that.

[SevenThreeSeven]

I would seriously bet against that scenario (a "jumped timing" situation).

In your very first post of this forum thread you said the bike started and then revved to the 5,000 to 9,000 range. I doubt the engine could do that with a mistiming issue.

I think you're working on the real cause: the carbs. You have a set of carbs with exactly the combination of factors (problems) that would cause the over-rev at startup: 1. the carbs were intentionally tuned to run over-rich (Dynojet "Stage Whatever") which was probably allowing too much fuel in, even at "0" throttle, and 2. all of the rubber bits in the carbs are in various stages of erosion/embrittlement/failure which is probably admitting too much air, even at "0" throttle.

Together these two things, too much air and fuel, almost certainly add up to an untunable idle situation.

[Variablevalves suck]

The clipped springs are dynojet springs and the clipped end goes up top, also dynojet needles are much thinner than mikuni and must be matched to the dynojet main jets, actually you can use any make but the sizes are not the same by quite some margin.
Sorry ive not read everything you posted as so much but sure you and Greg can get there.

[lknbandit400]

I would seriously bet against that scenario (a "jumped timing" situation).

In your very first post of this forum thread you said the bike started and then revved to the 5,000 to 9,000 range. I doubt the engine could do that with a mistiming issue.

I think you're working on the real cause: the carbs. You have a set of carbs with exactly the combination of factors (problems) that would cause the over-rev at startup: 1. the carbs were intentionally tuned to run over-rich (Dynojet "Stage Whatever") which was probably allowing too much fuel in, even at "0" throttle, and 2. all of the rubber bits in the carbs are in various stages of erosion/embrittlement/failure which is probably admitting too much air, even at "0" throttle.

Together these two things, too much air and fuel, almost certainly add up to an untunable idle situation.

Thanks for that - surprisingly comforting. I sure don't want to do that if I don't have to.

[lknbandit400]

The clipped springs are dynojet springs and the clipped end goes up top, also dynojet needles are much thinner than mikuni and must be matched to the dynojet main jets, actually you can use any make but the sizes are not the same by quite some margin.
Sorry ive not read everything you posted as so much but sure you and Greg can get there.

Welcome to the fray :) what you said about the Dynojet needles matches what Greg was saying earlier. He was saying that the Dynojet needles may have some more "wiggle" in the seat than the OEM.

Greg, looks like someone may have tried to make their own jet kit on your bike!

I read somewhere that Keihin jets are actual dimensions, Mikuni is based on flow rate and Dynojet is out in the wild blue yonder. Going off of memory.

[Variablevalves suck]

You should be able to tell if they are dj needles as they have a gentle taper to a fairly sharp point.
Stock needles are very thick and quite flat at the end.
I will have time to read the full story over the weekend so if I can help I will.

[lknbandit400]

You should be able to tell if they are dj needles as they have a gentle taper to a fairly sharp point.
Stock needles are very thick and quite flat at the end.
I will have time to read the full story over the weekend so if I can help I will.

Thanks! I've also been told that DJ needles have adjustable positions for the e-clip and the Mikuni do not (or not as many). That may be able to help distinguish also.

But so far the springs are cut, the EPA plug was pulled, and the mains are marked 100 with no "square within a square" mikuni branding so I'm betting I have the full kit. It's costing more to reverse back to OEM carb setup than to buy the DJ kit in the first place

[SevenThreeSeven]

Greg, looks like someone may have tried to make their own jet kit on your bike!

You're absolutely right. One of the 3 prior owners of my B4 was way out in the weeds, a carb witch-doctor. The carbs had Dynojet springs combined with some unknown brand of E-clip adjustable aftermarket needles (maybe Factory Pro or Dynojet?) but they still had the OEM Mukuni 102.5 Main Jets installed on the Emulsion Tubes.

The surprising thing is that when I bought it I had to ride it home, 425 miles, from Portland OR to Spokane WA, on a cold night in mid October of 2014. In spite of the bike's scruffy condition and the weird carb tune it ran great all the way (I froze). I rolled up to my garage at 3:30am feeling totally exhilarated.

After that ride to bring it home I only rode my B4 two other times with carbs before I took it off the road for my fuel injection conversion and general refurbishment/upgrades.