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Our RD350 weighs 242 ready to race but without water in the radiator.  The frame is about 60% stock and 40% chrome moly.  The almost stock rear hub weighs about a ton though.

And the GT750 weighs a portly 314.  probably 10 pounds could come out of the front forks and wheel with a little work.

Just thinking out loud but has anyone looked at a +10mm Banshee crank as a place to start.  That's a 64mm stroke but I assume that they use the smaller diameter banshee/350 sized crank wheels and 115mm rods, so it would need 350 cases and a 1mm lift plate and a little crankcase shaving to squeeze the crank in, but it has to be an option.

You could always weld up the cases to take a 68mm CPI top end but that's getting into silly money. I also don't know if there's enough metal in the cases to be trenched for that long pf a stroke.

Just thinking out loud again.  No.  No.  It's not time for my meds.  I have been a good boy. Honestly......  

There is a lot of conflicting information on the internet about pre-ignition and detonation.

Preignition simply means that a hot spot of some sort causes the ignition to happen before the spark is timed to fire.  That results in pressure rising earlier in the cycle than it should and can lead to a rise in operating temperature with potentially damaging results, but it is still a normal combustion event and the flame speed is more or less the same as a normal burn. Flame propagation is around 20-50 M/sec

Normal combustion creates peak pressure around degrees or so ATDC.  With early ignition that could potentially be before TDC as the piston is still rising.  That can create excessive pressure and heat and can lead to detonation, but it's the detonation that cause the damage.

Detonation by comparison, is exactly that.  The mixture detonates (explodes) and the speed of flame propagation is extremely fast and creates a sonic pressure wave which moves at somewhere around 1,500 M/S. It is that sonic boom that is heard and which causes the damage. The combination of excessive heat and the sonic wave combine to turn pistons into dust.

But back to jetting.  "It sounds lean".  Determine whether it is lean by doing what M in SC suggested.  Try a smaller and smaller air screw setting and see if it gets better or worse. The next thing you mention is that is gets better when the throttle is opened and that's usually where the side and then the needle start to have more of an effect. So it sounds like that may be OK.

Try adjusting the air screw and then let's think about the results of those tests.
 

520 chain is more than enough for any RD. Why carry the extra weight.

Though I'd put this here because of the topic; it would be a strength upgrade. This is for a TD3/TR3 or TZ250A/TZ350A, basically the same S/A as used on the Works RD56:

TD/TR/TZ Swingarm plus Axle

I swapped a Vesco arm for a stock arm like that one for a TZ250 restoration a few years ago.  OEM are amazingly stiff swingarms.

I believe you are correct.  Very mature response.

How did the car run with the new head design?

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.

I was under the impression that a 188 is the round jet equivalent of a 159 series. That's according to my measurements and notes.

For some reason I have a dyno chart that suggests that early carbs made more low end and later ones made more at peak, but I have no details of the tests.

Congrats on the new addition

Can you please post pictures of the top of both carbs so we can see the area where the choke lever sits.

It almost looks like someone fitted a chock (enricher) to the carb that doesn't have a brass tube and left the choke out of the other carb.  Or someone really messed up with mismatched carbs.
 

On the dyno, we removed the red sox and gained a hp or so.  Then we removed the filters completely and added another 1.

So I fitted the short rounded intakes 9 Too short to call them bell mouths and added another 2 HP at the top end.

I don't think I scanned the dyno chart, so I will have to look for them to see the effect on AFR of the changes.

AFAIK, that is an 87 FZR400 rear wheel.  I have an almost identical 88 FZR400 wheel here that I was planning to use in an RD350 project.

I worked it out 

Did someone mention TZ top ends on air cooled cases? Back in the day, TZ cases used to wear out and replacements were not always available, so racers took to modifying RD cases to take the TZ crank arrangement. Others bougt TZ top ends and fitted them to RD bottom ends.

This is a set of RD cases with a TZ crank and water pump with RD400 transmission and 40900 (64mm bore) top end with bigger reeds. 40970/40971 66.25 TZ750 C/D/E top ends also drop straight an to an Air cooled bottom end.


I forgot how to insert and image.  I'll be right back as soon as I work that out.