I have a 1973 RD350 with DG expansion chambers, Y boot intake, boost tube, VAPE ignition and Mikuni VM28 carbs. I got it with a 170 main jets, 27.5 pilots and the needle on 3. I now have it with a 160 mains, 25 pilots and the needle on 3. It bogs down and stalls on takeoff (same thing it did when I got it). Elevation is 5000 feed. Does anybody know what I might need to do
1973 RD350
- Thread starter kbbecker
- Start date
I think you're going backwards on the pilot jetting. 180 is probably the smallest on the main jet with that setup- somewhere between 180 and 220 is typical. Maybe your elevation means going down to 160, but the 27.5 pilot should've been the one or possibly a 30 or 32.5, with your air screw 1 - 1.5 turns out. Also check your timing. Mine has pretty aggressive porting, so it's at 1.8mm, but if you're mostly stock it should probably be around 2mm with DG pipes, the Y boot and pump gas.
Thanks for the reply. I got it with a 170 main, 27.5 pilots and 3 on the needle without the DG expansion chambers and then moved to 160, 25, needle at 3 with the chambers. No difference. the plugs are oily which is disconcerting. I'm not sure about the igintion timing, still trying to figure that out with the VAPE ignition. Thinking maybe autolube pump?
Wet plugs can be several things. Start with your ignition and ensure you have fat blue sparks. Vape requires 5k of resistance, which means you need either resistor plugs with non-resistor caps or resistor caps with non-resistor plugs. Can't have both. Ensure timing is correct and ensure the flywheel isn't slipping since it's not keyed. Make sure float bowls are right and that the float needle is seating. I've had issues with the fuel getting into the intake because the float needle allowed the bowls to overflow. Have you done a leak down test? If the right side crank case seal leaks, it can pull oil from the transmission. A reed petal not seating can also cause wet plugs.
Thanks, that helps a lot. One comment, if the floats are not right I should see gas coming out of the overflows which I do not. The eingine was recently rebuilt by HVC so I don't think I have a crank seal issue or a reed valve issue. Maybe wishful thinking. I have not done a leak down test but am looking at the timing.
I've had an XR500 crankcase fill with fuel and not a drop came out of the overflow. I am obsessive about leak down tests, personally. I probably do them way more than I should, but with a 2T I worry about holing pistons. I'd still lean towards a timing issue or something related to spark. It's not abnormal to get shiny glazing on a bike that mostly sees idling, so you may also want to do a high rpm plug chop before digging too deep.
Sounds like you've gone around a few times chasing this problem down. When I've experienced this, I start with what I think is the most likely problem, go through that system competely to eliminate it as the issue, and then move on to the next system having confidence my first guess wasn't the problem.
I would start with thoroughly going through the carbs. There's no telling what parts may be in these old carbs, having been worked on by who knows how many people. I once had a customer bring me his 350 . Running terrible, barely rideable. He thought it a carburetor problem, so he cleaned the carbs in an ultrasonic cleaner (they looked imaculate on the outside), and put all new parts in both from aftermarket rebuild kits. according to him, it ran even worse after all that. I asked him if he kept all the old parts. He did, and gave them to me.
All I did was to completely disassemble and clean everything manually, and reassemble with all the old original oem parts. Bike ran perfectly with no other work.
So I suggest you rebuild the carbs very carefully. Take out the emulsion tubes and make sure they are perfect. Make sure you can achieve a perfect clean stream of carb cleaner through every passageway and drilling. Try to get all original Mikuni parts always! It is extremely rare to need to replace anything in your carbs unless you damage something. Needles can wear, but an RD with that many miles still runs better with the ruined factory needles than the crappy "new" rebuild kit needles I've seen.
Those old oem needle ans seat assemblies will work better than new aftermarket if you clean and polish the bores with some fine steel wool and a toothpick. And, make sure you don't swap the fuel bowels. Only one has the drilling for the enricher on the left carb.
Set the float arms parallel to the gasget surface. look closely at the little tab that presses on the needle. If there is any mark, polish it 100% smooth with #600 sandpaper.
Start with #25 pilot, #180 main, needle clip in the middle. If the carbs are clean with all Mikuni parts and no damage, the bike will start and drive well enough to figure out if you need to change the mains. If it doesn't, leave the carbs alone and look elswhere.
I would start with thoroughly going through the carbs. There's no telling what parts may be in these old carbs, having been worked on by who knows how many people. I once had a customer bring me his 350 . Running terrible, barely rideable. He thought it a carburetor problem, so he cleaned the carbs in an ultrasonic cleaner (they looked imaculate on the outside), and put all new parts in both from aftermarket rebuild kits. according to him, it ran even worse after all that. I asked him if he kept all the old parts. He did, and gave them to me.
All I did was to completely disassemble and clean everything manually, and reassemble with all the old original oem parts. Bike ran perfectly with no other work.
So I suggest you rebuild the carbs very carefully. Take out the emulsion tubes and make sure they are perfect. Make sure you can achieve a perfect clean stream of carb cleaner through every passageway and drilling. Try to get all original Mikuni parts always! It is extremely rare to need to replace anything in your carbs unless you damage something. Needles can wear, but an RD with that many miles still runs better with the ruined factory needles than the crappy "new" rebuild kit needles I've seen.
Those old oem needle ans seat assemblies will work better than new aftermarket if you clean and polish the bores with some fine steel wool and a toothpick. And, make sure you don't swap the fuel bowels. Only one has the drilling for the enricher on the left carb.
Set the float arms parallel to the gasget surface. look closely at the little tab that presses on the needle. If there is any mark, polish it 100% smooth with #600 sandpaper.
Start with #25 pilot, #180 main, needle clip in the middle. If the carbs are clean with all Mikuni parts and no damage, the bike will start and drive well enough to figure out if you need to change the mains. If it doesn't, leave the carbs alone and look elswhere.
Thanks for the reply. Supposedly they have been rebuilt and they looked fine when I changed the pilots, mains and needles. I'll go back thru the carbs one more time. The thing that concerns me is that both plugs look the same, both carbs screwed up.? Possible. I've also noticed weeping where the cylinders mount to the ccase and drips oil (black spots) under the engine after running. I had a 400F that I ran with a stock ignition and stock air box with expansion chambers with 27.5, needle on 3 and 170 main at sea level, ran like hell, nice brown plugs?
Really nice RD on the pic.
The carbs are pretty simple to go through, and if you are thorough, it's an easy thing to eliminate as the source of a problem. Also, both cylinders running the same would indicate something alse, though often hard to tell.
Weeping at the cylinder base gasket is not unusual for a few reasons, principly the frustrating difficulty in keeping the head bolts tight. They take very little torque to bend the cylinder heads causing them to leak and end up getting loose with heat cycling. Then, there is often base gasket problems. Head bolt torque needs to be checked very frequently on new engines. Yamaha changed the head design to help this, but all the 350's suffer. The head gasket design was changed on the 400's which helped a lot.
If the base gasket actually blows out, it definitely will cause problems, but is pretty rare in my experience, and doesnt sound like your issues.
Expansion chamber spigots pretty much all leak dreadful black goo, some worse than others and depending on how you ride, how you set up the oil pump (or pre mix) and jetting.
There's a build thread for the bike in the pic in the link below if you want a better pic.
Weeping at the cylinder base gasket is not unusual for a few reasons, principly the frustrating difficulty in keeping the head bolts tight. They take very little torque to bend the cylinder heads causing them to leak and end up getting loose with heat cycling. Then, there is often base gasket problems. Head bolt torque needs to be checked very frequently on new engines. Yamaha changed the head design to help this, but all the 350's suffer. The head gasket design was changed on the 400's which helped a lot.
If the base gasket actually blows out, it definitely will cause problems, but is pretty rare in my experience, and doesnt sound like your issues.
Expansion chamber spigots pretty much all leak dreadful black goo, some worse than others and depending on how you ride, how you set up the oil pump (or pre mix) and jetting.
There's a build thread for the bike in the pic in the link below if you want a better pic.