My bike is running very rich. I've cleaned the carbs multiple times, the jetting is all factory specs. Replaced coils, condensers, wires. The intake was replaced with cone filters which I would think would make it run lean if anything, but in any case it's running very rich no matter what.
I noticed when it's running the left cylinder feels much warmer. The left spark plug comes out black and dry, the right spark plug comes out with wet oil on it still. I get exhaust out of both exhaust pipes though.
I tested compression (borrowed a tusk brand, not sure if it's a good one) and it read around 65 psi in both sides so assumed something with the top end is wrong. All I have is a digital caliper but it 54.2mm at each measurement I top and bottom of each cylinder.
I'm guessing I should take the pistons and cylinders to someone who knows what they're looking at? A hone job and new rings, or do the pistons look like they should be replaced?
You could take the rings off, stick them in the cylinder, and measure the end gap to get an idea of wear.
yeah thats what I do as an initial check myself. from the pics nothing looks terrible. do the carbs have the stock bassinet thrm? you still have the stock needles in the slides? were the jet tubes removed completely and cleaned?
As the other guys have said, nothing alarming in the pics. Piston skirts show a little wear but not bad, and there really isn't much sign of blow-by from worn rings. Measuring the ring end gap as suggested above is easy.
To measure the cylinder correctly you would need a bore gauge and micrometer. Not sure where you're located but you can puck up something inexpensive like this to do the job:
HF bore gauge set (https://www.harborfreight.com/6-piece-telescoping-gauge-set-5649.html)
Are you sure you're testing compression correctly because if the bike was running previously it should have more than 65psi. Make sure you're using the spark plug adapter that matches the size (dia and length) of the spark plugs. Also make sure there is an O-ring on the threaded end. Thread it in by hand and snug it finger tight. Fully open the throttle and kick the engine over repeatedly until the needle stops climbing. Where it maxes out is your compression number.
You say you are running cone air filters - if they are the ones with a flat top opposite the carb that is a problem...
Thanks for the input
Quote from: 1976RD400C on August 07, 2025, 08:11:15 AMYou could take the rings off, stick them in the cylinder, and measure the end gap to get an idea of wear.
Ring gap measured between 0.97-0.98mm
Edit: Just noticed the manual says ring gap should be 0.15-0.35mm, so I guess it's safe to say they are long gone.
Quote from: m in sc on August 07, 2025, 11:17:40 AMyeah thats what I do as an initial check myself. from the pics nothing looks terrible. do the carbs have the stock bassinet thrm? you still have the stock needles in the slides? were the jet tubes removed completely and cleaned?
Previously I had removed all jet tubes and cleaned thoroughly, I took them out again and they were still all clear.
Interesting that I never noticed when cleaning before, the needles are definitely different, and I think there is damage on the choke side carb needle, and there's no working spring in it. I will have to get new needles to make sure I'm using the right ones.
I also measured the float arm measurement - manual says 15mm, I measured 14mm on right carb, 13.5mm left carb (one with damage).
Quote from: SoCal250 on August 07, 2025, 12:22:11 PMAs the other guys have said, nothing alarming in the pics. Piston skirts show a little wear but not bad, and there really isn't much sign of blow-by from worn rings. Measuring the ring end gap as suggested above is easy.
To measure the cylinder correctly you would need a bore gauge and micrometer. Not sure where you're located but you can puck up something inexpensive like this to do the job:
HF bore gauge set (https://www.harborfreight.com/6-piece-telescoping-gauge-set-5649.html)
Are you sure you're testing compression correctly because if the bike was running previously it should have more than 65psi. Make sure you're using the spark plug adapter that matches the size (dia and length) of the spark plugs. Also make sure there is an O-ring on the threaded end. Thread it in by hand and snug it finger tight. Fully open the throttle and kick the engine over repeatedly until the needle stops climbing. Where it maxes out is your compression number.
I tested compression after warming engine up and doing exactly as you mention, making sure plug adapter is correct size, there was an O-ring on it, tightened by finger, throttle open. I had removed both spark plugs as I thought they were both supposed to be out for compression test. Should I do it with the other plug still in?
Quote from: Brad-Man on August 07, 2025, 01:42:30 PMYou say you are running cone air filters - if they are the ones with a flat top opposite the carb that is a problem...
Yes, not doubting, but what problems could it cause?
I also did notice on the back of the right carb it looks kinda gnarly, not sure if it's an issue to have what looks like those holes plugged on the left side...
Quote from: ampzip on August 07, 2025, 10:42:59 PMThanks for the input
Quote from: 1976RD400C on August 07, 2025, 08:11:15 AMYou could take the rings off, stick them in the cylinder, and measure the end gap to get an idea of wear.
Ring gap measured between 0.97-0.98mm
Edit: Just noticed the manual says ring gap should be 0.15-0.35mm, so I guess it's safe to say they are long gone.
That's pretty big! Time to take some other measurements and see how involved of a top-end rebuild you need to do. With that large a gap there's probably a lot of wear in the bores, but who knows you could get lucky and find out that you can get away with just rings.
Quote from: ampzip on August 07, 2025, 10:42:59 PMQuote from: m in sc on August 07, 2025, 11:17:40 AMyeah thats what I do as an initial check myself. from the pics nothing looks terrible. do the carbs have the stock bassinet thrm? you still have the stock needles in the slides? were the jet tubes removed completely and cleaned?
Previously I had removed all jet tubes and cleaned thoroughly, I took them out again and they were still all clear.
Interesting that I never noticed when cleaning before, the needles are definitely different, and I think there is damage on the choke side carb needle, and there's no working spring in it. I will have to get new needles to make sure I'm using the right ones.
I also measured the float arm measurement - manual says 15mm, I measured 14mm on right carb, 13.5mm left carb (one with damage).
Those float needles should be replaced. Do the valve seats have a marking stamped in them "2.0"? Probably best to get them as a set and just replace them all. When buying new float valve sets be sure to get size #2.0 (Mikuni Type VM26/26). The DS7 & R5 run a #2.0, while the RD models run a #2.5.
Quote from: ampzip on August 07, 2025, 10:42:59 PMQuote from: SoCal250 on August 07, 2025, 12:22:11 PMAs the other guys have said, nothing alarming in the pics. Piston skirts show a little wear but not bad, and there really isn't much sign of blow-by from worn rings. Measuring the ring end gap as suggested above is easy.
To measure the cylinder correctly you would need a bore gauge and micrometer. Not sure where you're located but you can puck up something inexpensive like this to do the job:
HF bore gauge set (https://www.harborfreight.com/6-piece-telescoping-gauge-set-5649.html)
Are you sure you're testing compression correctly because if the bike was running previously it should have more than 65psi. Make sure you're using the spark plug adapter that matches the size (dia and length) of the spark plugs. Also make sure there is an O-ring on the threaded end. Thread it in by hand and snug it finger tight. Fully open the throttle and kick the engine over repeatedly until the needle stops climbing. Where it maxes out is your compression number.
I tested compression after warming engine up and doing exactly as you mention, making sure plug adapter is correct size, there was an O-ring on it, tightened by finger, throttle open. I had removed both spark plugs as I thought they were both supposed to be out for compression test. Should I do it with the other plug still in?
Shouldn't matter if plugs are out for the test. I usually run my tests with a cold engine.
Quote from: ampzip on August 07, 2025, 10:42:59 PMQuote from: Brad-Man on August 07, 2025, 01:42:30 PMYou say you are running cone air filters - if they are the ones with a flat top opposite the carb that is a problem...
Yes, not doubting, but what problems could it cause?
https://www.2strokeworld.net/forum/index.php?topic=6462.msg53591#msg53591
Quote from: ampzip on August 07, 2025, 10:42:59 PMI also did notice on the back of the right carb it looks kinda gnarly, not sure if it's an issue to have what looks like those holes plugged on the left side...
That right carb definitely needs to be cleaned up. Make sure all the passages are clean/clear.
aside from those float needles looking rough, i mean the jet tubes and needles.
https://www.2strokeworld.net/forum/index.php?topic=6624.0
Quote from: SoCal250 on August 07, 2025, 11:45:21 PMThose float needles should be replaced. Do the valve seats have a marking stamped in them "2.0"? Probably best to get them as a set and just replace them all. When buying new float valve sets be sure to get size #2.0 (Mikuni Type VM26/26). The DS7 & R5 run a #2.0, while the RD models run a #2.5.
Yes, 2.0. I noticed yambits just sells them as sets, so I'll go that route.
I will replace those air filters as well.
Quote from: m in sc on August 08, 2025, 07:52:42 AMaside from those float needles looking rough, i mean the jet tubes and needles.
https://www.2strokeworld.net/forum/index.php?topic=6624.0
Gotcha, although it is clean through the jet tubes, the tubes themselves weren't removed, I'm not sure how I would do that. The needles appear to be in good condition and are clean.
Other than that I will get the rest of the top end measured properly and figure out what needs to be done. Hopefully these are all of the remaining pieces of the puzzle.
take the main jet & washer out, turn carb upside down, gently tap out towards the top. I guarantee they are not clean
When tapping the jet tubes out be careful. Brass is soft. I keep an old plastic chopstick in my toolbox for this. A wooden pencil can also be used as a punch for driving it out.
And when reinstalling them make sure that the groove in the end of the tube is aligned with the small locating pin in the bore.
Pencil worked to push it out. They don't look too bad but do need some cleaning.
I took the cylinders to a local guy who does work for bike shops in town and he measured them. He said they were fairly wore and a .25 bore is necessary, so I will be replacing the pistons and rings.
Those cone filters cause weird reverberations when running at certain rpm ranges. Basically the pressure wave that comes back out of the carb when the reeds close hits the hard back of the filter and bounces through the carb and the reeds again causing them to flutter. It'll feel like a chopped up ignition and a loss of power.
Swap those air filters for the stock airbox of you can, or replace it with a foam UNI filter.
Quote from: Striker1423 on August 09, 2025, 08:16:41 AMSwap those air filters for the stock airbox of you can, or replace it with a foam UNI filter.
Simplest and cheapest would be to swap out the filters for a pair or UNIs. Alternatively, a Y-boot with a UNI or K&N could be used.
https://www.economycycle.com/product/uni-up-4182ast-2-stage-pod-filter/
https://www.economycycle.com/product/economy-y-boot-filter-kit-for-yamaha-rd250350400-r5-ds7-rz350/
Just a side note: There is a difference in the main jet if you run a UNI with the red sock on as opposed to taking it off.
An update on my comedy of errors.
Somehow I didn't realize to check if the engine had been bored previously. It had been already once before, so I decided to just hone it and install the new pistons and rings since previous ones were quite wore.
Finally got the parts and installed first cylinder fine. Was fitting piston into the cylinder for the second one and somehow snapped one of the piston rings...
Part numbers were identical on the boxes, both said 25 over. I'm only learning now (manual says nothing about it) that OEM rings need to be gapped? What is a safe way to do this?
Edit: and maybe dumb question, but could I use one of the old rings in the bottom temporarily?
It looks like the ring in the picture is for the next oversize bore size. How do the pistons fit in the bores without any rings on? You should check that first. Use a .002"-.005" feeler gauge alongside of the pistons to get an idea what's going on.
Well upon closer inspection, I noticed there is faint numbers on the rings... they are in fact 50 over rings. Unreal. I don't know which seller to go after as both boxes are identical, only the rings were different. Guess I'm just SOL on those.
As far as the piston in the bores, I can barely get .003 to fit.
its actually comon to file down one size set over to get a perfect gap. so, kind of a blessing. t have trimmed them using a fiber cotton wheel in a dremel, fixtured, and taking small increments off the side. do one side, then the other to make it even. be warned, it does FAST
Finally got the bike back together, starts really well, and seems to run pretty well, except that it's still very rich, getting lots of smoke, wet oil on plugs.
The guy that did the hone job for me thought it may be that it's over oiling with the pump.
I noticed that part of the oil pump cable is partially broken off (never knew what it was supposed to look like).
In any case, at zero throttle, the raised mark is supposed to line up with the pin, but it's way up from it. It takes about 1/4 throttle for them to line up.
I also measured the gap between the adjustment plate and the pump pulley after rotating the white plate to make the plunger move out. It measured 0.14mm, manual says minimum is 0.15mm and should be 0.20-0.25mm
Wouldn't these cause under oiling if anything?
I was thinking of just trying premix. How can I connect the output oil tubes from the pump so that the pump will still work without being attached to the cylinders? Could I just have a closed loop with all three hoses (one in, two out) connected together and a bit of oil in the line?
likely not over piling. that gets thrown around a lot by guesswork and is almost never the case. you can run premix, just cap inoffensive the inlet and the outlet lines. plug the lines to the cyls.
def get a new set of cables.
Be sure to upjet when you run premix. A lot of people say 32:1 but with today's oil you can probably get away with 40:1.
Based on what I'm seeing in your photos and your explanation of the issue, I'd say your mechanic's advice was 100% wrong. It would be under-oiling not over-oiling! Not sure what logic he was using.
I would leave the pump in place and use it. No need to try some mickey mouse bypassing.
Get a new pump cable. Your DS7 should be the "old style" as used on the DS7 & R5: 278-26321-00-00 CABLE, PUMP
(Not the later cable version 521-26321-00 used with mid-75 model year and up pumps, assuming your pump is original. Refer to Tech Bulletin M8-015C).
Once you've replaced the cable and adjusted it properly then test your pump stroke again. Adding a shim will fix it if it's under the spec. Shims are readily available in several thicknesses and super easy to install. Just take the nut off the pulley, remove the pulley from the shaft, add your shim to the stack under the pulley, and reinstall the pulley.
In addition to the cable damage where the housing fits into the adjuster, it also looks like someone added a new end on it. At least that's what it looks like in your photo. As it rotates that blob (circled in red) will interfere with the roll pin preventing the pulley from rotating as it should.
Also it looks like someone has been messing with the pin (yellow circle). Make sure it's in there tight. But don't push it in too far. I think 4 mm is max, or about 6mm showing (from memory) or the plunger will bind.
pump_detail.jpg
It looks like it is under oiling to me too, and you can adjust that with the cable adjuster in your picture but man you have to replace that cable. It looks like someone squeezed a nut on the end of it and it has to swing by that pin at full throttle.
Thanks for the replies and tips.
Quote from: SoCal250 on September 26, 2025, 10:50:08 PMBased on what I'm seeing in your photos and your explanation of the issue, I'd say your mechanic's advice was 100% wrong. It would be under-oiling not over-oiling! Not sure what logic he was using.
He hasn't seen the bike in person at all, I just had him hone the cylinders. He was guessing based on the smoking and wet oily plugs.
I just ordered a 278-26321-00-00 cable yesterday from ebay, shipping is taking forever lately and there is a canadapost strike now, so I'm not holding my breath on getting it any time soon. I was just hoping to try premix temporarily.
The fundamental problem at this point is still that it's running rich no matter what. The air screws are out 3 turns and idle screws are almost entirely in to get a good idle.
I really don't have a lot of experience with this. In my mind it's a carb issue, but I have brand now valve seats and and valves, I've cleaned it out, made sure the float arm is at the exact right height (15mm).
I guess next do a leakdown test?
it has to be a carb issue. make sure the airjets are cleared out. and that the choke isnt hanging open
Quote from: m in sc on September 28, 2025, 01:16:53 PMit has to be a carb issue. make sure the airjets are cleared out. and that the choke isnt hanging open
This. I'd honestly take the hardest look at that corroded mess of a carb from your prior pictures. A badly corroded carb can really throw off a lot of the carbs function.
This is all a learning experience for me, it's really the first bike I've ever worked on at all. That's my excuse for some really dumb mistakes I have made (or didn't know were made by the previous owner).
After cleaning the carbs thoroughly multiple times, it still didn't seem to help.
It's when I took apart the entire throttle cable assembly to replace the autolube cable that I realized that the retainer that goes under the spring and on top of the jet needle wasn't seating properly in both carbs. The needle must have been bouncing around under the crooked clip. I figured out how to properly install that.
Then I realized that it's possible the throttle cables / slides could easily be put in backwards into the two carbs... that's exactly what was happening, the cutaway was backwards, basically starving the carbs of air.
I put them in the correct way and immediately running well... throttle response so good now.
Thanks for all the help troubleshooting this. Managed to fix issues I didn't know I had in the process (like the under oiling). At least I'll finally get to enjoy it a bit this fall!
sweet! and we have all made the slide mistake at some point.
Quote from: m in sc on October 09, 2025, 01:37:09 PMsweet! and we have all made the slide mistake at some point.
Such a common occurrence with these things. It helped me diagnose my friends issue with his kz750 years ago. Quick and simple solution, but ran so much better.
Not sure if I should keep posting to this thread or start new ones with new issues.. In any case, for whatever reason I can't get the bike to reliably start on it's own now.
From cold start, it will start with choke on, but will die out before it warms up at all. Any throttle at all kills it. If I spray carb cleaner in the intakes, it'll rev up and run well, so I keep spraying till it runs without choke, and then it runs great. It'll run, idle and drive really well.
Again, carb issue? I've cleaned both several times. Idle jets, jet tubes, main jet are all spotless and spray well.
I've tried air screw down to 1/2 turn and it reacts the same.
Fuel delivery / carb issue
Does it die out even with the choke on? Do you have the washers on that fit under the main jet? Is that a tight fit?
Quote from: 1976RD400C on October 14, 2025, 07:08:28 AMDoes it die out even with the choke on? Do you have the washers on that fit under the main jet? Is that a tight fit?
Yes it will die with choke on. Yes, washers are on under the main jet. The washers appear to be factory, they fit perfectly under the main jet.
I measured float arm and it is at correct height as well.
Check for some air leaks. Sounds like its running really lean at a cold start. Strange that it will run after you do your method. Is the choke crossover tube installed when you put the carb in? Stock airbox? Double check if the carbs are synced?
Am I correct in stating that there should be a hole in the bowl of the carb with the pickup tube? Both bowls have a "2" stamped in them. It looks like there is supposed to be a "1" bowl which has a hole...
I guess that means the pickup tube isn't doing anything.
Would it be possible to drill that hole rather than buying a new bowl?
Ahhh thats a culprit of the choke circut not being engaged due to how its not getting any fuel! You could drill it, just be very careful. Do you need a drill.size and the depth? I can go get that info for you with my spare carb
Quote from: quocle603 on October 17, 2025, 04:34:40 PMAhhh thats a culprit of the choke circut not being engaged due to how its not getting any fuel! You could drill it, just be very careful. Do you need a drill.size and the depth? I can go get that info for you with my spare carb
That would be super helpful!
Either way it'll need to be replaced so I figure I'll give it a go.
Sorry for the delay. Having a newborn really put a damper on your ability to do things. I was able to fit a 3/32 drill bit into the hole and measured about 9.5mm down into the hole. It does not go into the port from what I can see, I tried to get other bits into it but couldn't see anything. I assume theres a smaller hole for the choke circuit to kick in.
Good luck!
The service manual lists the starter jet as a #70 so I would imagine that hole is equivalent to a #70 jet.
It looks like you have two of the same bowl. In a "choke" bowl, there is a cross drilling plus a small #70 jet in the vertical hole. If the small brass jet is in place, the port can be opened and the size is not critical.
That's how most Mikuni carbs are set up. https://pinkpossum.com/GT750/carb/BS40carbs.htm has nice cross section.
just carefully drill in a 1.5 to 2mm hole. dont go all the way thru the bowl though (ask how i know), and yes, that def explains it
Quote from: m in sc on October 19, 2025, 11:50:02 AMjust carefully drill in a 1.5 to 2mm hole. dont go all the way thru the bowl though (ask how i know), and yes, that def explains it
:whistle:
Thanks for the replies, got it exactly right. The bike starts perfect, idles perfect, runs perfect... been riding it a ton the last couple days, so satisfying.
Heyyyyy. Its the little things that leads into big victories. Definitely something that could happen to anyone. Thanks for reporting back and updating the community