Bone stock bike jetting?

1976RD400C · May 10, 2023, 05:29:37 PM

Do I dare put a 120 main in it? It has a 130 now and stops revving at 8000, even in the low gears. Stock is 115. Everything is stock, air box with snorkel, OEM air filer with tubes inside, exhausts, reeds. Non ethanol gas being used. It runs great other than the top rpm. As I recall they should hit 8500 going through the gears. Man, a stock 400 sure has some grunt in the mid-range.

Striker1423 · May 11, 2023, 09:12:32 AM

I'd consider it, or if they make a half size jet (I don't think they do past 110).

SoCal250 · May 11, 2023, 12:04:44 PM

Never ridden a completely 100% stock 400 but as Striker mentioned you can go half steps on the main. I'd try a 125 first. The 4/042 main goes in increments of 5 all the way up to 200.

1976RD400C · May 11, 2023, 03:41:35 PM

Well I don't want to talk about it, but might as well give you guys some reading material and maybe save someone from the same experience.

I had the carbs off for a full checkup and jet change. I had some silicone sealer holding the oil line nipple on the left carb because that was kind of a loose fit. I fought with it to get that off. The sealer was holding in great. When I assembled it I put a dab of 3 Bond on it instead. That was the carb I did yesterday and didn't check it today.
Went for a test ride and it ran great with the 120 jet (didn't have any 125's). Still poops out at 7800-8000 rpm. Head down the test straightaway and right before I let off, wham, tire screech, pull clutch. There was a nice parking area right up ahead. Pull in, WTF IT SEIZED! First thing I do is push the kickstarter and it is free. Push it few more times and it has good compression on both sides. I'm a bit baffled. I know it seized and locked the wheel back there. It did start back up but I'm not going to ride it back on the back roads with no shoulders. Called wife and she picked me up.
Pulled plugs, look OK. Did compression check, 120 psi both sides. Then I look at that left carb oil nipple and it has pulled back a little. The 3 Bond sealer was still all gooey. The nipple vibrated back out some and I had kind of missed with the dap of sealer so it really wasn't doing anything.
Piston shot, cylinder is fine, just needs a quick hone. New piston on it's way.
Kind of embarrassed. I am going to get even with that nipple, I hope it likes a dap of JB Weld and then just pull the hose off the nipple from now on.
The GPS reading is when the tire let out the screech.
Peace, out.

Striker1423 · May 12, 2023, 08:18:31 AM

Knock on wood, but I've never tweaked, glued, or smashed my oil lines in and they've never fallen out. Strange enough as it sounds, they just seat and I go. A little twist and out they come too.

85RZwade · May 12, 2023, 09:36:06 AM

Part of the mystique of these old rides is that you just might land on your ass today. You don't get that with shiny new bikes

RDryan · May 13, 2023, 08:28:12 PM

Quote from: Striker1423 on May 12, 2023, 08:18:31 AM
Knock on wood, but I've never tweaked, glued, or smashed my oil lines in and they've never fallen out. Strange enough as it sounds, they just seat and I go. A little twist and out they come too.

Same here, in fact I wouldn't be surprised if they are the original lines and nipples feeding into the carbs. Heck I have an oil pump rebuild kit but I've never used it. At best if the bikes been sitting unused for awhile I pull the lines or sometimes both the lines with the nipple out of the carbs and manually actuate the oil pump by hand just to see if it works prior to starting the bike. So the lines do get handled but never seem to fall off.

1976RD400C · May 14, 2023, 06:02:38 AM

After thinking about it this, this is what I come up with. The nipple didn't actually come out of the carb. When I first looked at it, it was about halfway pushed back and oil was leaking down the side of the carb. I don't think vibration had anything to do with it. When I put the nipple in, I noticed it was not a press fit at all so I put some sealer on the outside to hold it, just like it had when I took it apart. The sealer never dried and I kind of missed the best spot to put it. If you look at the hole the oil comes out in the throat of the carb, it is tiny. I went for a ride to see max performance and the oil pump must have cranking full tilt with open throttle and high rpm. Since there was no press fit at all on the nipple, it pushed back from the oil flow trying to come out of the tiny hole in the carb. Using Yamalube 2R may have made matters worse because it may be thicker than 2S, I don't know. I put green Loctite on it so now I'll pull the hose off instead of removing the nipple. New $190 OEM piston and rings should be here next week.

Kawtriplefreak · May 14, 2023, 08:53:41 AM

I really hate those nipples too. I am considering doing away with them completely on my set of VM 28s by drilling out the orifice to fit a small (3 MM) if memory serves brass tube, which is what we used on the TM32s for my 75 RD350 to adapt them for oil injection. Then you can slide the hose on and use a cable tie to secure it. We made it a tight press fit and drove them in with a dab if red loctite and they have never moved. I am so sorry that happened. Glad it didn't do more serious damage.

rodneya · May 14, 2023, 10:47:27 AM

Quote from: 1976RD400C on May 14, 2023, 06:02:38 AM
Using Yamalube 2R may have made matters worse because it may be thicker than 2S, r="0"></a>

Pretty sure 2R is premix only

m in sc · May 14, 2023, 07:02:17 PM

also, never assume your tach is 100% accurate when its this old at that rpm. .02

1976RD400C · May 18, 2023, 03:24:12 PM

All good now. 20 mile run. Runs strong but I didn't try to rev it too high with the new piston. It does have that surging/bucking/chain snatch a little while coasting easy. Gobs of torque from 4000-6000 rpm. It handles good too.

oldguyRD1964 · May 25, 2023, 10:21:21 PM

Hey not sure if this is any help but I live by the beach in LA my bike 250 is stock and had 120 main jets, I really wanted it to run clean so I started researching jetting, I understand it changes based on weather/humidity etc. In my case, I found if I was on PCH out a ways there is a long downhill and then an uphill, I was wide open going up and noticed my bike would bog from what I was reading it said it was being starved. I started going up on the main jets and noticed at 165 I think it was running a lot better everywhere except 1/8 throttle but then I had an engine problem... Also I found my gas line size made a difference (with filters). You might consider trying going up to 135 and seeing what happens my plugs looked good with 165 about the same as 120 (yamalube) like I said I'm not an expert here but it did help.

SoCal250 · May 25, 2023, 10:32:54 PM

Quote from: oldguyRD1964 on May 25, 2023, 10:21:21 PMHey not sure if this is any help but I live by the beach in LA my bike 250 is stock and had 120 main jets, I really wanted it to run clean so I started researching jetting, I understand it changes based on weather/humidity etc. In my case, I found if I was on PCH out a ways there is a long downhill and then an uphill, I was wide open going up and noticed my bike would bog from what I was reading it said it was being starved. I started going up on the main jets and noticed at 165 I think it was running a lot better everywhere except 1/8 throttle but then I had an engine problem... Also I found my gas line size made a difference (with filters). You might consider trying going up to 135 and seeing what happens my plugs looked good with 165 about the same as 120 (yamalube) like I said I'm not an expert here but it did help.

If your RD250 is truly stock, a 165 main is WAY too big for sea level!

oldguyRD1964 · May 26, 2023, 01:45:59 PM

I am not sure how stock the engine was/is this tuner guy I hired might have done some port work but I don't recall 100%... I just know in my case 120 was not the right size... maybe it was 145 sort of forgot... my apologies and my engine issue was not related to the jetting....

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Bone stock bike jetting?