Over throttling

Vintagewannabe · August 22, 2024, 08:58:12 PM

So I have gotten my new to me 73 rd350 going. I have done the y pipe and jetted for it. I am at a 160 main and could probably go back to a 155, 27.5 pilot. My one puzzling part is the bike really wants the needle on the highest clip, no matter what main I have. Has anyone else had this? The bike also seems pretty sensitive to givving too much throttle before about 7000rpm
This may be my inexperience with these old 2strokes, but I had also may mean my reeds are trashed. As far as I know they are original.

1976RD400C · August 23, 2024, 06:01:16 AM

If you mean by giving it too much throttle it gasps and acts like it is shutting off, then the needle is set too lean. Move the clip down on the needle to raise it to richen it.

m in sc · August 23, 2024, 07:19:20 AM

what jet tube and needle are you running? if its a p-0 and a 5dp7 it should be in the 180 range. if its stock internals should be around 220-240

Vintagewannabe · August 23, 2024, 12:53:09 PM

Quote from: m in sc on August 23, 2024, 07:19:20 AMwhat jet tube and needle are you running? if its a p-0 and a 5dp7 it should be in the 180 range. if its stock internals should be around 220-240

they are stock needle and tube, and it has the stock pipes. Also the needle is as high as it will go.

IR8D8R · August 23, 2024, 03:29:40 PM

We are going to need to know the main jet size, the pilot jet size, the emulsion tube number and the needle number. If someone has installed one of those Keyster carb kits there may not be numbers. Then you'll need to start over with all new Mikuni brass. It's a good idea anyway, including the needle and seat. Use the viton tipped needle.

"stock sizes" at this point could be anything unless you've owned the bike since new and never changed them. Yamaha used several different combinations. Some of the "stock" combinations are NLA. Mark is referring to the current "best sizes" that are available now. 5DP7 needle, P-0 emulsion tube, 30-35 pilot jet, and 180 main with a 2.5 slide. That is the recommended setup for starting over if your jetting is bad and you've got mismatched pieces in there. ...Or Keyster. Same thing. If your bike came from Denver it may have different jetting than where you are currently requires. Someone on here knows if your combination of these parts is correct. Some old guy told me to always use new brass. I believe him!

You'll probably want to do the Dave F. mod. Search the site for that.

IR8D8R

m in sc · August 23, 2024, 03:36:51 PM

as an fyi, the carb kit needle is the same as a UK spec 75 350B, which is totally different carb setting than the us models.

back to your setup: assuming the 73 has the stock carbs and guts etc, you need to make sure the jet tube was taken out and cleaned, and that the air jet was also cleaned. you're describing an overly rich condition on the needle and this will absolutely affect it. and, it will also affect the main jet sizing.

here's a cutaway i did forever ago. it shows the air jet and (stock) 73-75 us spec jet tube.

Vintagewannabe · August 24, 2024, 02:31:29 PM

I was going off of the HVC numbers. I'm already well above them though. I thought you didn't get up to 180 or so unless you had some mild chambers like DG. I used new mikuni brass, but didn't note the numbers on the tube or needle. I know the emulsion tube was clean but I could definitely double check the air jet. The bike has been bone stock so I would be shocked to find swapped internals. It has also bee. Local to the area for at least 30 years from the title that I saw.
Where do I find a viton tipped needle?
What do you mean 2.5 slide?
I have seen the F mod but thought it was useful for running chambers.
The plugs are also a very nice dark brown. They indicate a bit rich. Also there is no lean popping, it starts very well with no throttle also. I followed instructions and the bike idles best at exactly 2 turns out on the air screw.

m in sc · August 24, 2024, 06:38:53 PM

2,5 slide is the slide cutout, if its stock it has one.

I mean, another possibility is ignition getting weak at rpm. might be losing a coil, pretty common especially if original. old saying w rds: jetting is usually electrical. lol. the needle int he leanest position is .. a point of interest to say the least. it should at least be in the middle.

m in sc · August 24, 2024, 06:44:39 PM

.. also, when you say 'high' is the clip on the needle closest to the pointy part or furthest away? for reference, the top clip (furthest away and leanest) is #1, closest to point part is #5.

Vintagewannabe · August 24, 2024, 10:58:06 PM

It would be at the number 5 position. I am going to pull them tomorrow and look at all the numbers.
Coils could be an issue. They are likely original.

Vintagewannabe · August 25, 2024, 10:03:50 AM

So it looks like my emulsion tube is 175 0-8
The tube had some crud around it, and on the left carb had clogged 2 holes.
The slide is a 2.5, and I really think the needle is original. But cant find any numbers so I'll include a pic.
All of my circuits looked cleared but I still sprayed them all out.
I was wrong on my mains, I have 170s. So you say it may need 180s? I also find that I need to be a step richer than most for whatever reason. Elevation maybe?

m in sc · August 25, 2024, 11:14:32 AM

needle should be a 5l1. if there's no numbers... its not a stock one and may be the main cause of jetting/tuning woes. If it silver, its stock. if its copper/brass: aftermarket. it is imperative you make sure the air jet in the bellmouth at 6 o clock is also clear. a good spray thru w the jet tube removed will verify.

Vintagewannabe · August 25, 2024, 11:59:23 AM

Quote from: m in sc on August 25, 2024, 11:14:32 AMneedle should be a 5l1. if there's no numbers... its not a stock one and may be the main cause of jetting/tuning woes. If it silver, its stock. if its copper/brass: aftermarket. it is imperative you make sure the air jet in the bellmouth at 6 o clock is also clear. a good spray thru w the jet tube removed will verify.

That just seems so shocking. Even with the proper needle,like a 5dp7, would I still need to be at a 180 main?
And all of my air passages are definitely clear

m in sc · August 25, 2024, 01:07:34 PM

the 5dp7 is only used in the dave f mod or stock r5 carbs and in conjunction with a 0-0 or 0-4 jet tube (or p2 depending on state of tune, etc), and then, it has a 2.0 air jet vs the .8 mm one on the stock rd carbs.

yes, it makes that much of a difference in conjunction w the jet tube used.

heres the 5dp7 needle and an r5 style jet tube vs a stock rd jet tube and a 514 needle. these are what all the main jet fuel runs thru.

Vintagewannabe · August 25, 2024, 01:33:16 PM

Quote from: m in sc on August 25, 2024, 01:07:34 PMthe 5dp7 is only used in the dave f mod or stock r5 carbs and in conjunction with a 0-0 or 0-4 jet tube (or p2 depending on state of tune, etc), and then, it has a 2.0 air jet vs the .8 mm one on the stock rd carbs.

yes, it makes that much of a difference in conjunction w the jet tube used.

heres the 5dp7 needle and an r5 style jet tube vs a stock rd jet tube and a 514 needle. these are what all the main jet fuel runs thru.

So it seems like its more worth it to just get the dave F mod kit from economy cycle and use what I need for now, then do the rest later when I get some chambers.

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Over throttling