Looks like it is my labyrinth seal

[RDnuTZ]

I can't remember everything that Dave has tried so far, but if you haven't checked the reed cages to see if petals are sealing properly that could mimic some of what you are seeing with running good sometimes and excess fuel symptoms at other times.

[teazer]

Mark is absolutely correct about the lab seal.  It is not a seal as we know them but has clearance to the crank.  It has nothing to wear unless the main bearings are so worn that the crank is rubbing on the seal and crankwheels would probably be rubbing on the cases.

100psi is a bit low for compression.  Not so low it won't run, but it won't exactly be perky.

If the motor hasn't been apart since Pontius was a pilot, it's probably a good time to strip and rebuild the motor with new pistons in a fresh bore and a new crank with all new motor and transmission seal.

With the motor out, it's time to clean the carbs meticulously. The outsides are just for appearances, it's the insides that have to be clean. And a great opportunity to clean out the fuel tank and filter.

[RDDave]

Okay then, the lab seal is not the problem. The engine is quiet. No bottom end noise.

I have repeatedly tried the above suggestions. Original cylinders had about 100lbs compression on std bore. As part of the 'no stone unturned' path, I have new ebay cylinders sets on. Again, with about 100lbs comp.

I have completely disassembled the carbs including the emulsion tubes and have put on other carbs that I have.

Every single piece of the ignition system has been changed, swapped side to side, and changed again. I have even bypassed the bike's wiring harness and jumpered the ignition directly from the battery. I have the ignition on a relay as I was losing just over a volt through the harness. I can pull the carbs AGAIN and either inspect or swap the reed blocks, but they look fine. 

I have done just about everything I can imagine multiple times. We will spend some more time with it. I don't know how I feel about it not being the lab seal; On the one hand, I wasn't looking forward to spending the time and money to tear down the engine. But I sure would have been happy to put this saga behind me.

Oh well.............................

.

[m in sc]

for the same money spent on that, you can put a cdi on it .testing good and actually being good especially w old ignition components can be totally different

[RDDave]

I was an Evinrude mechanic years ago and I can attest to what you are saying. Using factory test equipment, many times I have seen ignition components that were in spec but wouldn't work and vice versa. The ONLY true test was with known good components.

If it were the ignition on my bike, I just don't understand how every piece that tests good from my stash would do the exact same thing on my bike. And I have tried several different pieces at each juncture. Now I don't have a way to run the pieces on another bike to test, but still, the same problem with different parts?

I am stumped. And have been for some time. But with this guy's help, I will keep trying different things.

.

[m in sc]

my 72 t500 was like that. i chased it forever. like, a few years. then just put a cdi on it. been perfect for the past 5-6 years.

the 71 got a regular points replacement elec ignition. all i'm saying is, it sounds like ignition/electrical. 

i chased this on an rd years ago, and on my buddies GT550 as well, both turned out to be intermittent charging issues.

an HPI is a more cost friendly option vs the vape. both replace charging and ignition system on the bike. just my 0.02

[RDDave]

As always; Thank you.

.

[RDDave]

I have read through some of the ignition threads. But I didn't find what the Vape does over the HPI given it's higher cost.

Are there recent threads with more details on each and maybe why one over the other?

I have worked on my fair share of ignitions both points and electronic and just cannot grasp how changing every single part of the system still leaves me with the same symptoms.

BUT! I greatly respect the experience here. So, i am seriously thinking about dropping the coin for an electronic system. Frankly, the biggest down side is the cost. Seems exorbitant to me given what I am used to spending for car ignition systems. If it weren't the cost AND the possibility that it won't fix my issue, I would be all over it. But, it is what it is.

What about the rr on the HPI? Other than MORE money. This is a street bike. Is there a recent thread that details? I guess I would stay with DC, right? So, I also need a rectifier?

And there are several threads on HPI failures. Is that a product problem or operator error?

I would very much prefer to just stay with the points. I understand what electronic ignition brings, but unless my point system is broken, for the cost, the electronic just doesn't bring enough to the party. But, if it is the ONLY way to resolve my bike's issue, then I have to go that route.

Thanks again for putting up with me.

.

[m in sc]

i have extensive experience with both,  for years. the vape is the stronger unit as far as charging. however, the hpi is half the money and can be great on a street bike. i have an hpi w a rr on my lightweight rd. bern working for years, documented well in that thread in the haus of projects.

[RDnuTZ]

what about the Dyna S from Economy Cycle? Seems like the most cost-effective solution to eliminate the 50 years old points system entirely for sanity check here. Also, the solid-state r/r are available multiple places- I think I paid $45 for mine off eBay. If you added those 2 parts and still doesn't fix problem I would go new IGN coils if not already done. OEM here can create open circuit condition when hot, then repair itself when they cool. Last thing I would do is replace original main wire harness. Those wires often turn brittle over time and create intermittent failures. I hate shot gunning for solutions, but I'm a much better parts switcher than debugger on electrical Gremlins 

https://www.economycycle.com/product-category/yamaha-rd250350400r5ds6ds7-parts/electrical-ignitions-lighting/ignition/dyna-s-ignitions/

[m in sc]

i would concur w above for the most part.

look in the supertune section he goes over installing both cdis.

as far as coils,  i have a poat in the tech library? about using sv650 cpils

[RDDave]

Converted to the SV coils last year. And have tested with the bike's wiring harness completely bypassed by using jumper wires.

I currently use a relay to power the coils. That way I have bypassed that part of the harness and keep full battery voltage to the coils. Before, I was losing just over a volt through the harness.

Now, one thing that I haven't done is to monitor the voltage to the coils ALL the time. The battery is new and the lights are bright so I don't think I am dealing with low voltage. It runs the same with the lights off as it does with them on.

Keep the ideas coming. I MUST be missing something.

.

[RDnuTZ]

I have read through some of the ignition threads. But I didn't find what the Vape does over the HPI given it's higher cost.

Are there recent threads with more details on each and maybe why one over the other?

I have worked on my fair share of ignitions both points and electronic and just cannot grasp how changing every single part of the system still leaves me with the same symptoms.

BUT! I greatly respect the experience here. So, i am seriously thinking about dropping the coin for an electronic system. Frankly, the biggest down side is the cost. Seems exorbitant to me given what I am used to spending for car ignition systems. If it weren't the cost AND the possibility that it won't fix my issue, I would be all over it. But, it is what it is.

What about the rr on the HPI? Other than MORE money. This is a street bike. Is there a recent thread that details? I guess I would stay with DC, right? So, I also need a rectifier?

And there are several threads on HPI failures. Is that a product problem or operator error?

I would very much prefer to just stay with the points. I understand what electronic ignition brings, but unless my point system is broken, for the cost, the electronic just doesn't bring enough to the party. But, if it is the ONLY way to resolve my bike's issue, then I have to go that route.

Thanks again for putting up with me.

.

Unless you had a known good complete OEM ignition to swap on there to confirm, I'm leaning towards your ignition is the source of failure. I had a Gremlin like this back in the 1970's - IIRC it was my 1975 RD250B. I ended up buying an aftermarket tune-up kit (Rocky I think) and that cured the problem. Pretty sure it was bad condenser. I think you have reached the point as you detailed above I bolded. If you haven't replaced points & condenser 1st as cheapest option- try that, otherwise if it was me it would be new ignition time. The time spent on this is worth more than the couple hundred $ at my stage of life.

[RDDave]

I have a lot of spare parts and have changed the points and condenser (more than once) and have also swapped the points between cylinders. I have tried probably 6 or 8 different combinations of points and condenser. I also test the ignition components. The point resistance is very low. I file and clean them, then check for resistance until I get it as low as I can. And the different condensers all hold a charge.  But I haven't tried new ones, and I don't have a way to test run them on another bike.

.

[1976RD400C]

I think one test that you can do is ride it until it doesn't run right, then install brand new spark plugs that are gapped a bit smaller and see if it improved at all. If so, then it would point towards a weak spark.