Need help with low speed surging

[tz375]

Usually it's the pilot circuit and your quest is to discover if it's a LEAN surge or a RICH surge.

Try turning the air screws in 1/4 turn at a time and run the bike. Then turn it another 1/4 turn in. Try to ignore the fact that actual idle may get really lumpy. Repeat with another 1/4 turn. Did the surge get better or progressively worse - or no change.

Then go back to 1 1/4 turns out and repeat this time turning 1/4 turn and ride it and then another 1/4 turn.

What difference did that make?

Hopefully that will at least indicate which way to go. Large capacity 2 strokes do tend to surge at low throttle openings. I had one RD400 that was terrible at small throttle - especially at low revs. I tried everything and eventually learned to stop trying to ride it like a car and give it more revs & throttle.

[Ndulucky13]

I turned the screws form 1.25 turns to just .75 turns out and it did get better. I ordered some number 30 pilot jets last night and I was also able to set the timing properly thanks to the video shared. I noticed my timing mark matched perfectly with the correct .132 inches before tdc I saw some peoples didn't actually line up at all? I also pulled the head off last night to inspect the pistons and head itself I saw a little milky oil at the top of the bore, thought that was strange. I took a compression test before pulling it off and only got 100 100 and 105 psi. Is it time for new rings?

[GTandcbr]

I think your 30 pilots will improve the running of the bike. I experimented with turns out but settled at the manual 1.25 out it gave me much smoother running. Can't imagine what the milky oil was but could have been moisture from condensation mixed with the 2T oil ? With regards to your compression test its very difficult these gauges can be very inaccurate but I would be encouraged that they are all the similar reading. Incidently they are the same figures I get from my cheapo gauge and I just had a rebore and new rings!

[Vintageman]

My 2 stroke Yams (350 - 400) lean surged and larger pilot jet cured them. To me lean surge means you are real close on correct pilot size and one larger usually does it unless the clutch dampeners soggy, loose chain, binding chain sprocket, bad swing arm bushing or wheel bearing...

My 73 GT550 had (past tense) the same problem only if I understand what you are saying. I increased pilot jet size to fix the small throttle lean surge, but at in town riding it was too rich. I put up with that for a while .... the final thing I did was New needle jets, stock needle and stock needle clip pos, and back to stock pilot all just like mama intended worked best when all these carb components are as new.

If you have carbs apart and you haven't look at needle jet (pop out and check) as advised well then?

Compression low. I don't think that is root case of small throttle surge if bike runs OK otherwise. Surge very well is carb jetting or some reason it is not synced at small opening ... use your fingers to feel if all three slides crack open same time versus using slide dot which is at 3/4 or so of WOT

If your top end has low compression you will see a nice improvement in power across the RPM range if you bring the top end back into spec.

With head off you are almost there.

[Ndulucky13]

Ok so the clutch does slip when the bike is cold but not when warm, also the PO but 80wt gear oil in the trans is that correct? The bike surges with the clutch out and in gear slowing down, as though one of the carbs is slightly fueling and the others are not. With clutch in slowing down it does not surge. The slides do all move at the exact same time and are all set identical. Also I wouldn't be apposed to buying new needle jets but I cant seem to find any. Anyone have a good idea where to get them?

[GTandcbr]

Here is the link for the 1972 manual
http://www.3cyl.com/mraxl/gt/manuals/gt ... ntents.htm" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;
You will see gearbox oil is 20\40 but I just use 10\50 supermarket oil and change it regularly. Clutch slip is common when the engine is cold and in my opinion is down to the oil viscosity.I would check to see if the needles are defective because replacements do come up on eBay from time to time but are very expensive

[Vintageman]

Also I wouldn't be apposed to buying new needle jets but I cant seem to find any. Anyone have a good idea where to get them?

If the needle jet are fine than no worries. So check but should be smooth inside where it meters. But it is common they are pitted due oxidation over time. Yes check check jet needle too.

You are correct- there are no 192-O5 or 192-O6 available. It took me a long time to find these new

Here what is available for 192 series
https://www.jetsrus.com/a_jets_by_carbu ... mikuni.htm" onclick="window.open(this.href);return false;

If me here are ideas i would try

a) buy 192-O2 and hand ream to 192-O5/6
http://www.carbideplus.com/0_1035_Carbi ... 001035.htm" onclick="window.open(this.href);return false;

b) buy 192-P0 and run jet needle on 5th clip and use 25 pilot jet versus 27.5

c) find what other series is the same shape on top and correct orifice size. Most will be too long for the 192 is very short. But, if the top is the same you might be able to hack off the bottom and find a way to put the main jet into it.

Clutch slip is common when the engine is cold and in my opinion is down to the oil viscosity.

+1

[Ndulucky13]

I found these are the my the right ones? Is 1977 the same as 1972?
https://www.ebay.com/ulk/itm/310519442161

[Alan H]

Completely different carbs.

[Vintageman]

I found these are the my the right ones? Is 1977 the same as 1972?
https://www.ebay.com/ulk/itm/310519442161" onclick="window.open(this.href);return false;

Right 192 series but too large at P4 and think too rich.

b) buy 192-P0 and run jet needle on 5th clip and use 25 pilot jet versus 27.5

I mean 1st clip from top to make needle lower and leaner to compensate for larger orifice needle jet. Smaller pilot one step as well.

I reamed my O5s to like a O9/P0 and ran OK but too rich as could be seen by poor mpg (31 MPG) now in the 40s. Back then I did not know you could get an 192-O2 and that reamer I showed makes its very near what you need stock ... if you know how to ream without over sizing it.

Again if what you have is pitted inside

[Ndulucky13]

Turns out they were not pitted, they just needed a good dunk in a carb cleaner bucket. I stripped the carb body's as well and dunked those. Found quite a bit of crap came out amazing where that stuff hides. Seemed to run much better. I also installed my 30 pilot jets and that seemed to help couldn't tell for sure as I took the head and jugs off after running it to check ring end gap. it was at the threshold so its re ringing time. So I ordered some new rings. I also replaced the (I think) original tires with some new sticky rubber.

[GTandcbr]

**this is a personal opinion** these carbs are full of little internal channels and take several goes to get them clean. Its the same with the adjustments. The manual settings are a starting point only and as every owner rides the bike differently and external conditions ie weather and altitude also affect the running of the motor. So it takes a while to clean and getting it adjusted to your needs