Apologies, this will also be on kettleclinic where I usually hang out but those gents are likely abed soon .
GT550A, I rebuilt the carbs and went to 105/102.5/105 and 30 idle, all mikuni genuwine . Clip is at second from bottom (same as what I took apart). Pod filters (hey, it's VMs).
Idles perfectly, first test drive and its boggy from idle to 1/4 throttle, then it's fine throughout the rest of the slide range.
Recommendations for that idle to 1/4? I don't mind taking off like a bat outta hell, but when cruising speed drops to 1/4 throttle it's not the right song from the powerplant.
Smoketriples
current stable:
'75 Zooki GT750 (rider)
Sounds to me like your pilot jet or pilot jet circuits may not be as clear as you think. It is difficult to know if they are right. The pilot jet passage has two exits. The normal exit is in the throat of the carb engine side. The pilot jet bypass exits just under the throttle slide as it lifts. The best way to check they are clear is to spray carb cleaner in a watch it coming out of the exits.
If you dont already have it this may help http://www.3cyl.com/mraxl/gt/manuals/su ... /cover.htm" onclick="window.open(this.href);return false;
My own suspicion is its your jet needle clip if your throttle approximations are right. You may be too rich. Mine was 2nd from top clip slot and its better now at 3rd (GT550B).
I know of at least one other 550 owner who did this with same benefits - cleaner off idle+ to mid throttle.
Cheers,
Mike
That does seem to fit, as I'm running quite rich according to the mixture screw tweaking. I ran it in and out by 1/4s and it runs best at 2.5 turns out, not perfect, but it's the best possible with the mixture screw alone.
Smoketriples
current stable:
'75 Zooki GT750 (rider)
Edit: sorry, on my 550, I lowered the needle, ie raised the clip from 3rd to 2nd, so as to be less rich, and that has helped it run more cleanly through the throttle range,
Sorry,
Mike
I'm on second now, which was where the clip was when I tore down the old carbs. I did go back to the 25 idles since that's what I took out.
I threw a wrench in my testing though as one sparky was very loose and blowing compression past. I've no doubt it was sucking as well. With 25 idles (and loose plug) it was much better than 30 idles, but I doubt the validity of that test run.
Also took the pods off and went back to airbox per Alan H's recommendation on the kettleclinic board (and I've the bloody knuckles to prove it).
Going back out tonight with properly tightened plugs to test the 25s. Hopefully it's sorted, I really don't want to pull them out again to go to 27.5 but I suppose all my test runs yesterday are invalid.
Smoketriples
current stable:
'75 Zooki GT750 (rider)
Being "boggy" from off idle to 1/4 throttle, you have been through many carb circuits (Pilot, Pilot bypass, Slide cut, Needle Jet and into Jet needle). Is really bad in all these range? How does it behave if you go from just off idle to 3/4 or more throttle quickly?
If it runs crappy at ~1/4 throttle, I would suspect Needle Jet. Due to age there is a very high chance the bore of the needle jet is pitted inside. Doesn't take much. I emphasize inside. You have to take the needle jet out and look down into the barrel where it meters with the Jet Needle. You needle a good light and better than 20:20 vision to discern condition. It must be shinny or at least smooth. Any pitting or roughness is unacceptable and it will run as I think you are saying.
The 172 series is obsolete... some times on Ebay. It is stated state the 169 series Needle Jet is compatible but it's not exactly. I think the 182 series (or is it 188?) is identical but designed to take a Hex main. You can cross Round Jet to Hex jet, but the Round will screw in just fine and no crossing error if you are happy with the main jet tune.
vintageman is correct that if you need to replace needle jets, a 182 is the same but is threaded for hex main jets which are different thread to round jets. better to use hex jets and not damage the threads using rounds.
At 1/4 throttle the main things are pilot jet/air screw and slide cutaway. Assuming you don't want to mess with different slides, I'd stick to smaller pilot jets and see if that cleans it up enough. If it is then slightly weak at 1/4 to say a/2 then raise the needle put that back in balance.
It's odd to need smaller jets than stock with modern fuels containing ethanol. That tends to need larger jets. I think that Redzone ran with #25 pilot jets according to my notes from Chris (Coyote).
WOT it runs fine. I did change from the stock main jets of 97.5 95 97.5 to 105 102.5 105, don't know what the stock numbers were, but they were suzuki parts from Partzilla. Needles themselves are what came in the kit (Y49). EDIT: Didn't pull the needle jets, didn't actually realize those could come out. Any tips on popping them out and part numbers for the 97.5 and 95 hex jet appreciated.
I marked the throttle and did more testing yesterday, the 'odd feeling' acceleration/cruising lasts all the way until 1/2 throttle, then goes away.
I don't use ethanol fuel. Pumps right around the corner have non-eth 89 octane.
Of course it ran quite well with all original jets and needles, it just wouldn't idle (very clogged up) which prompted the tear down in the first place.
Smoketriples
current stable:
'75 Zooki GT750 (rider)
Kit needles are usually not the same taper as OEM. Close but not usually close enough. Hex jets are sized differently than Round. Hew are flow rated and rounds are diameter. If you go from 100 hex to 200 hex it doubles the flow. 100 round to 200 round would increase it to 4 times the flow.
I think I would go back to all stock parts, perhaps with new jets and see how that works
Back to all original (as in old jets and needles).
Runs better, still a little off song in low throttle. Part of the issue might be that these carbs are literally eaten away inside. The carbs were underwater from a hurricane flood for a few days, and then set with that water for a couple of weeks.
There were white (what looked like worms) growths inside that when removed left sections of carb gone.
It seemed to run okay after rudimentary cleaning (but no idle), although I only ran it a few blocks and back. Then I tore down the carbs for a proper rebuild. Really no clue what went away after a week in solvent.
Does anyone that sells parts by chance have a rebuilt set they want to move? I hesitate to do much more on these since the bodies were in such bad shape.
Smoketriples
current stable:
'75 Zooki GT750 (rider)
Using a round main jet does in a needle jet originally tapped for Hex does not ruin your needle jet : It won't compromise it role for the carb nor be unreliable. However, from that point on it will only accept the round and does so very well. Do not go back to Hex. In your case, since you are running pods and not the EOM size on the main anymore, I suppose it does not matter if you use Hex. You can search the planet for correct needle and if you do find hope they have no shelf damage .
However, if you were all stock and trying to cross the original round jet to a hex , it is not exact and you can be off by ~one step and in my experience it's more. IMO, it is better to run the correct size and correct Type OEM main jet. If you cross to hex don't assume precise, verify it is.
FYI, A needle jet intended for round main won’t accept a hex. Well, what matters is a needle jet with its metering office pristine and correct flowing main jet.
It sound like your carb bodies aluminum has oxidized and plugged l the Pilot hole and/or by pass hole. If you clean the two pilot circuit holes still may not be accurate diameter anymore or have too much friction to flow. But it may not be oxidation either, something else gumming it up, but check carefully for oxidation damage as well.
I think the pilot outlet is 0.8mm and the pilot bypass it 1.5mm. It is usually the pilot that plugs first. You I would start with a wire brush bristle smaller than 0.8mm just to free it and then try carb cleaner and nylon bristle. If real bad you could get a 0.8 mm drill/pin and ream it smooth. If you go a tad larger (it is easy to ruin it), but now have a nice smooth bore, you just may need to step down one size on pilot or a little more air via pilot air screw. Yes may be better to find new carb bodies on ebay etc.
I have had many a bad looking carbs work well, if the venturi is smooth and oxidation didn’t get into pilot holes you can usually have good luck with carbs. The air inlet to pilot and needle jet must be nice but only deeper inside where it meters.
Just as a frame of reference, Hex main jets have a 5.3mm x 0.9mm thread and Rounds are 5.0 x 0.75mm which is why a hex jet is too large to fit into a Round jet type needle jet, but the threads are way different and distorting the threads to take a round jet is obviously not recommended.
Vintageman is correct that hex jet to round jet conversions are the next size up or down, but since most carbs are pretty tolerant to fairly large over jetting without losing much if any power and with ethanol in teh fuel you will want slightly larger jets to maintain similar performance, I suggest that it's a bit of a moot point.
Corrosion in the carbs and/or partial blockages in those tiny (<1.0mm) passages can be a major PIA when trying to get a bike to run right.
I understand wanting to chose to be idealistic versus practical about brass threads needing to be exact versus does it work well. But in my experiences, I did not find it a moot point when I crossed. Me, I "hedge my bets" and go with the round main jet if stock build or starting with bike that currently well with a round in it. I always put new needle jet, if they are not as new. else it won't run as good as it can. Important for street riding.
That's a good article by H2Rick and it's largely correct. The only thing I would disagree with is that reference to numbering on reverse jets. He is correct that they are calibrated in reverse, but they are basically measured in diameter and not for flow. If you check the chart at the smallest and largest round jets and the smallest and largest hex jets you can see that almost double the round jet = 4 x the hex jet numbers. In fact his closing argument is that just because you can screw an inappropriate jet in, doesn't mean that you should.
In this case the issue is more than just damaging threads when it is totally unnecessary, the issue is also that the main jet holds the needle jet in position. There is a peg to stop it rotating, but what locates it vertically is the main jet and washer. In many carbs, there is enough friction/interference to locate a needle jet, but not always.
In addition, the washer acts as a seal so that fuel is not drawn in around the jet into the needle jet or through the locating slot.
There are times when we have to be expedient and stuck at the side of the road with limited spares or at a race track where we need to change jetting and the mismatch is the only way to get on track. In those situations, a bodge is appropriate, but where the correct parts are available there's less reason. And I am yet to see a street bike that is sensitive to a half jet size change or less.
We all find different ways of doing things and that's good and in this case, Vintageman and I agree to differ.
Much more to the point of this thread, I would spend a little more time making sure that the slow speed circuits are clean and change pilot jets (new ones only please) if necessary to get a clean idle and good transition off idle with the air screw in the right range.
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