problems with rd400g wont rev in lower rpms

[NivagRD]

The extra intake tract volume (I.e. with the spacers) reduces the signal that the carbs see from the moving pistons. This is of little concern on a race bike, ridden a lot at wide throttle openings, but not so good on a road bike mostly ridden at part throttle openings. Gavin

[NivagRD]

https://www.klemmvintage.com/cztech3.html
Scroll down to "inlet signal" .

[5port]

The extra intake tract volume (I.e. with the spacers) reduces the signal that the carbs see from the moving pistons. This is of little concern on a race bike, ridden a lot at wide throttle openings, but not so good on a road bike mostly ridden at part throttle openings. Gavin

Yes, agree.
There are also big differences with engines designed for lower Primary compression and those designed for higher Primary compression. I think the lower Primary compression is more to match modern designs with modern transfer porting that can better use a modern expansion pipe to help with transfer, and even intake.
The RD is a bit of an older school with less optimised transfer/porting and it is designed to work with a bit more Primary compression to help with transfer. Also, the 7th port can allow a bit of late inlet boost, helped by the reeds.
The reduction of signal strength to the carbs will always make carb set-up more difficult. Some more modern carbs are designed to function well with lower signal strength and so, may also function well in larger sizes.  The old Mikuni VM on the RD is a good older style carb, with great range of jetting available, but it doesn't generally help the carburation to reduce the signal strength with extra inlet volume.
The difference between the inlet design of the RD250/350 and the later RD250/400 reflects the improvements that Yamaha identified to suit the 7th port, and was introduced on the TZ700/750 so, it is probably a good compromise. The same effect is made on the earlier RD250/350 by spacing the reeds back about 5mm. However, the actual difference is probably minor on a standard road bike.

Cheers

5port 

[NivagRD]

5port - I agree all points.
The transfer ports on an Aprilia start at a further distance from the cylinder wall and have a much more generous short side radius where they turn the "corner" into the cylinder (compared to an air-cooled RD). The transfers would flow better and need less primary compression to work well.  Best, Gavin

[manueljose]

Ok, good news your suggestions are correct and the bike works wayyyy better. Thanks a lot for the help.

please watch the video and check what has been done.

https://www.youtube.com/watch?v=n3AnlfUjNCU

OK, good so far. You have to bear in mind that diagnosing your problems is not simple. Often, owners have multiple problems and, just saying what is wrong is difficult. I don't want to waste your time and money either.

The air filter is correct.

The balls-out job is good. Air gets into the air-jet from the small tubes on the airbox, down to the floatbowl nipples on the inside edges, then up through tunnels to the air jet chamber where there is a hole at top rear. Check air jet is 0.5mm.
There is no air from Idle air screw adjustment to air jet. The air from the Idle air screw goes only to the Pilot jet.
You check the tiny angled orifice with the pilot jet removed, screw the Idle air adjuster lightly seated (closed), put a finger down the slide housing and over the bypass orifice and put a carb cleaner spray tube up into the pilot jet housing and squirt. A good solid jet should spray out of the tiny angled orifice. If the jet is weak or blocked, it needs clearing with 0.5mm wire.

I cleaned the carbs as best as i could following your suggestions, i had never cleaned them this way as i always had the air screw removed. One of the carbs was spraying badly, as soon as i cleaned as suggested it sprayed really nice, so i think one of this passages was not in optimum condition.

I suggest sticking with the 145 Main jets at the moment.

Overall, I think your symptoms could still be the reed problem(s). Possibly, you have a reed petal sticking on the side that causes the intermittent issue at 2000.  You should find that the YZ85 reeds are good if the housing is opened up properly and used without a spacer. This has to be done with a clearance at the side for the reeds to open, not just for the reed to fit in. Obviously, the clearance needs to be enough to allow the reed cage to fit wherever it sits, they don't sit perfectly in the middle.

I said before, 5mm spacers can be used on the older 250/350 that had shorter reed inlets. Your bike does not need spacers because Yamaha made the inlets better and incorporated the spacing.

Highly tuned race bikes might need spacers to suit their engine. You have found that old style 2T race-bike engine character is no good for the road!

Removed the spacers and changed to oem reeds with 40 year old metal petals, as you can see on the video the bike is way better, i put 30 miles on the bike yesterday and its working really good down low, I'm still in the break in period so i tried to stay below 4000rpms, and occasionally revving her to 7000 and letting go of the throttle.  i did notice that before with the yz85 reed valves the bike would pull really good after 4000 rpms, now it does pull but not as effortlessly as with the yz85 reeds and spacers.

Since i need to replace the metal petals, one suggestion by Ed Erlenbach is to use either yz125 reed petals or yz85 reed petals.

I have my yz85 petals in perfect condition from the cages i removed, and i if i ever use the yz85 reed cage i can always purchase new petals for it, since they are cheaper than the yz125 petals.

reeds by manuel caicedo, on Flickr

My only doubt is on Ed Erlenbach's webpage he suggests the following: - I like to use YZ85 reed petals part # 4ES-13613-00-00 they are a single petal so you buy 4, they bolt right on but you do have to trim the front edges to narrow them just a bit at the front easy peasy.
But comparing the yz85 with the oem petals, the yz85's are wider, i understand removing the lenght at the top since they are longer, but should i also remove the width? or just leave the width alone? this has me scratching my head.
I'm doing this in the hopes i can regain some of that acceleration i noticed with the yz85 reed cages.

yz85 reed by manuel caicedo, on Flickr


Heads look OK.

Bike looks good.

Your English is great!

Cheers

5port

PS.  Check the short rubber link-pipe between the carbs on the two choke stubs. It is just 6mm id. They can split and cause air leaks, also, can be too big dia and leak or, can be caught on the stub and not fitted properly. Splits are difficult to find/see, so maybe replace that rubber pipe with new.  Also, make sure you have good spring clips on the x-over tube where it goes into the rubbers.

   

[5port]

Good, getting somewhere.  Yes, you trim the sides. I have done a set recently. I assembled them and marked the overlap with marker pen, dismantled and cut carefully with sharp strong scissors. I also smoothed the cut edges on 1200 grade carbide paper.
IMO, You don't need to be worried about the steel petals. If in good condition on standard cages, they last.  However, those modern Yamaha petals are good and you may get some improvement.

Cheers

5port

[Cocoablini]

I had a couple of bad VRRs on my BMW(longer commutes and overcharging constantly) and the heat killed the VRR. Battery, if water-based, could be dry if the VRR puked. That would explain weird lighting though I though the usual failure was electronics blowing out(lights) and other weird events

[Sloath]

Hi manueljose, nice looking bike hope you managed to get it sorted.
Interested to know  where you managed to get your 5j6 needles from pretty much impossible to find here in the UK.
Cheers sloath

[manueljose]

Hi manueljose, nice looking bike hope you managed to get it sorted.
Interested to know  where you managed to get your 5j6 needles from pretty much impossible to find here in the UK.
Cheers sloath

i have somehow sorted it, it still hangs below 1000rpms sometimes specially when cold but not like before, before it was undrivable in slow traffic, but it has been raining here like crazy and I'm still in the process of breaking the engine in. i have not reached 200 miles below 4000rpm i yet have 25 to go, hopefully the weather allows it this weekend. then i can increase to 5000 rpms for 100 miles more.

because i think this is happening because it still needs better jetting, but wont be able to jet ii correctly until fully broken in. i don't know if I'm being anal about the break in period, but too much money invested on this bike to just floor it from the begining.



the 5j6 needles i got from https://hodaka-parts.com/item.asp?PID=6227 in the USA.  it took a while to get them, but finally became available they still have them.

[5port]

i have somehow sorted it,

Great.  Nothing to do with the advice you have had then. 

5port

[Cocoablini]

If it hasn't been said, is throttle cable OEM and right length? I had an off name cable and it sucked. The connector would kink and then snap to attention. It would bog on roll past idle and then near wheelie

[manueljose]

i have somehow sorted it,

Great.  Nothing to do with the advice you have had then. 

5port

hi, no, your advice was great, the bike feels way better, i'm still trying to get miles on the bike,  i have clocked 200 miles below 4000rpm and i'm on 50 miles below 5000rpms.

between the weather and little kids its really hard to drive the bike.  since I'm doing the driving only on weekends on a mountain road that has virtually no traffic, trying to break the engine commuting is really bad since there is too much traffic and too little distance, and weather is really unpredictable this time of the year and again i hate driving on the rain.

now that i have been getting the bike past 4000 rpms,  i do feel a flat spot in power from 4000 to 5000, it picks up strong after 6000rpms but i'm avoiding those rpms until i do the last 100 miles below 5000,  then i will do 100 miles more below 6000 rpms.

at this stage i might be done breaking this engine on the year 2024

i have made a crude powercurve of what i feel the bike is doing, you can feel it the most after third gear, first and second blast trough the flat spot but you can still feel it.

please check the attachment.

but i figured i need to break the engine in first, before doing any jet changes from the ones you have recommended.

one thing i forgot to mention, i have been using motul 510 2 stroke oil, which is not synthetic,  should i turn to motul 710 which is fully synthetic? 

also i have the baffles on, but the packing material was removed and i never installed it again, should i put new packing back on?