Jetting time

[scully]

One of the reasons I try to always wear gloves...ouch, but it'll heal :)

[Czakky]

God damn! That could've been much worse!

[m in sc]

YOW! that suuucks.

[SoCal250]

OUCH!

[1976RD400C]

15 minutes on the wrong charger. Thank God it was on a steel table in my garage. The flames and smoke was unreal.

[jmendoza]

My 1976 RD400 C is completely stock with the paper air filter, and a new clean one at that.  Please be advised that a paper filter is more restrictive than a foam filter in the stock airbox. This means the bike will need slightly richer jetting with an aftermarket foam filter.

That said, my bike now has 22,000 miles on the stock bores and it has stock jetting. Timing is stock at 2.4MM BTDC. Movable timing mark on stator has been verified and calibrated with a Sanginamiya (Suzuki) dial timing gauge.

Here is the foolproof test for determining if your pilot jet is the correct size: With a warmed up engine, screw in the pilot air screw all the way. If it does not kill the engine, go up 2.5 in pilot jet size until it will kill the engine when screwed in all the way.   Always start with the needle clip in the stock position or this test is not valid and if you try to tune the pilot jet with the clip in the wrong position, you will go around in circles and wind up dialing yourself out instead of in.

Now to tune the pilot air screw: Take off in 1st gear and notice if the bike grabs and pulls, or if you have to slip the clutch to keep it from stalling. If it wants to stall and you have to give it gas and slip the clutch, turn in the Pilot Air screw 1/4 turn and try again.   Once you are satisfied with the pull away from a dead stop, now go and try decelerating from 3rd or 2nd gear and listen to the pipes for backfires; a few  occasional pops is  about right, no popping is too rich, and a bunch of burbling and popping is too lean. Another test for this is to simply rev the engine in neutral and see how quickly the tach needle goes back into an idle. If it floats slowly back to idle with backfiring and popping, turn in the Pilot Airscrew 1/4 turn.  No backfiring at all and a gassy smell to the exhaust calls for opening the airscrew 1/4 turn.

You should never have to adjust the pilot airscrew more than one full turn either way, from 1.5 turns and if you find you have to, either the pilot is clogged, or the wrong size, or the needle clip is in the wrong position, or your fuel level in the float bowl is off.

RD400 C, D, E have a notch on the bottom of the piston skirt on the exhaust side to "free-port" the crankcase to the exhaust system to smooth out the "bucking" at part throttle. One way to reduce "surging/bucking" when decelerating is to set the idle speed as low as practical, like around 1000 RPM. Also having both slides set so they close equally also helps. If the pilot air screw is a tad lean, turning it in 1/8th of a turn will reduce surging. A good battery and proper timing is also key to getting good idle and reducing surging, they all work together and have interaction, so it's not just the carbs and jetting.

One other tuning note: It is far easier to tune a bike when it is all stock, because if it will not tune with the proper factory settings, there is a problem. Once you have it behaving, in stock trim, THEN go and modify and re-tune. This way you have a baseline because you know the bike will run properly if it is all stock. Without that baseline, you can chase your tail until the cows come home!

[rd400canuck]

Ya thankfully I didnt lose my RD after all the money and effort I've put into over the winter. With the cost of the bike and parts I'm in for minimum $6500. I have to say I thoroughly loved riding the bike before it tried to castrate me with flames. It was everything I hoped a completely stock bike would be. Smooth and reasonably quiet, sounded wonderful.. somewhat torquey down low and not a struggle to get going. Mid range is defo where this bike comes on. Its doesnt have the torque off idle I remember the RD400G having, though but I seem to remember that bike signing off faster up top than this one. Im fairly certain the exhaust ports were lower on the G so that would make sense it it's the case. The throttle pull is a little much, though. That oil pump spring really pulls on it.

Thanks for the advice on pilot tuning, jmendoza. I did notice that stock air filter looked obstructive. It felt almost like a felt to me instead of paper. I have the timing at 1.8. Will I see much off an advantage going to 2.3? I can't remember why I think this but I thought with modern gas it was wise to back off on the advance a little.

[Striker1423]

Ya thankfully I didnt lose my RD after all the money and effort I've put into over the winter. With the cost of the bike and parts I'm in for minimum $6500. I have to say I thoroughly loved riding the bike before it tried to castrate me with flames. It was everything I hoped a completely stock bike would be. Smooth and reasonably quiet, sounded wonderful.. somewhat torquey down low and not a struggle to get going. Mid range is defo where this bike comes on. Its doesnt have the torque off idle I remember the RD400G having, though but I seem to remember that bike signing off faster up top than this one. Im fairly certain the exhaust ports were lower on the G so that would make sense it it's the case. The throttle pull is a little much, though. That oil pump spring really pulls on it.

Thanks for the advice on pilot tuning, jmendoza. I did notice that stock air filter looked obstructive. It felt almost like a felt to me instead of paper. I have the timing at 1.8. Will I see much off an advantage going to 2.3? I can't remember why I think this but I thought with modern gas it was wise to back off on the advance a little.

I'm unsure in the 400, the info is up here somewhere. But on the 350, stock is 2.0 and we back it off to 1.8 for today's corn fuel.

.5 seems a bit much. But Like I said the info is here. Don't base the 350 timing for your 400, it isn't the same.

[SoCal250]

Follow the jetting advice Chuck gave you and the procedure that is posted in the Tech Section that was mentioned previously. Do NOT set your timing to 2.3mm!

FYI, the stock timing for a RD400 is 2.3mm, not 2.4mm (as that is for the RD400F Daytona). Less advance should be used for today's fuel if you want your engine to survive. For a 400 you should be using 2.0mm BTDC for today's fuel, and for a 350 you should set to 1.8. Refer to Chuck's advice on this that he posted elsewhere on this site. He knows what he's talking about. 

[rd400canuck]

Hey guys,

I'm a little concerned with my pilot jetting. Bike is all stock aside from being on the 2nd over bore and I am on middle of stock needle clip. Floats are at 25mm.

The bike seemed to prefer a 27.5 (25 stock) at idle and 1 out but to stop it from surging when slowing down I put it at 1/2 and it really seemed to like that. Out of curiosity I wanted to check how it behaved with 30's in it so put them in put it at 1 turn out and set out for a 1/8th plug chop. Once the bike was warm and ridden for a few I pulled over and put the new plugs in. I did a a steady 1/8 throttle never going over for about a minute. Pulled the plugs and they were bone white 

So I thought maybe the needle is responsible for more than I thought so I put them on pos #4 to richen them a level. Did the exact same thing and the plugs where still white... MAYBE a little less so but they were white. Also, after I put the 30's in the idle was lower and I had to give 1/4 turn on the idle screws to get it back to normal. So... it's defo getting more fuel from the 30's

Am I doing something wrong here? How can a stock bike that came with 25's be so damn lean on 30's?

[rodneya]

Did you do a leak down test when you put the motor back together?

Just looking at the top of your plug after a short run is not a plug chop, you need to look at the base of the ceramic part and it only really works for checking your main jet at WOT
With modern unleaded fuels my plugs only start to show color after 15 to 20 minutes or so riding time.

[rd400canuck]

Hi,

I did yes, multiple leakdowns throughout the build. Last one I did was after putting the motor in the bike and the needle didnt budge. I can do it again to make sure nothing changed. Probably a good idea because I noticed the bike seemed to surge more when I would close the throttle with the 30s. It dodnt do it as badly with the 27.5's

[rd400canuck]

well... for fun I went from the 30's to the 35's at 2 turns out. bucked like a bastard at close throttle and low rpm. I went to 1 turn out, was a little better but still surged. I did my usual 1/8 throttle run and pulled the plugs. They are verging on caramel color but still too hot looking.

I'm going to do a leak down again just to rule that out but both plugs behave identically.

I'm just not comfortable doing extended 1/8 throttle with the plus coming out bone white.... and 1/8 is exactly where I am most of the time while steady cruising. The bike really seemed to behave best with the 27.5 at 1/2 turn out but damn! Those white plugs are scaring me.

Could timing cause what I'm seeing?  I'm sure I have it at 1.8 ~ 1.9

[rd400canuck]

ok, spent the afternoon checking things over. Did a leakdown test and its still sealed up tight. Held 6 psi for minutes. I got out the dial gauge and check for 1.8 mm btdc and I had the notches lined up perfectly. I got a timing gun but it was only giving out the weakest of reddish light and it was impossible to see the notch on the flywheel through the tiny little window on the stick ign system. So I turned it by hand and each plug caused it to flash on the line or a hair after. So that all checks fine.

Thanks jmendoza for the right up to jet pilot. I put the 27.5's back in and put the air screws to 1/2 out and the bike ran the best it has since I started dicking with larger pilots to try and stop the plugs from coming out white. It sounded perfect idling and revved out nice at 1/2 to 3/4 throttle. It even stopped the aggressive surging that got worse and worse with each increase in pilot. I guess... I'll just stop worrying about 1/8 throttle plug chops. This pilot was what was originally suggested by Supertune I think it was... and that's the one it seems to like best. I just had to prove to myself I guess that it didnt need more fuel and something weird wasnt going on.

Now I will start with needle and main jet chops.   1/2 throttle for needle position, correct?

[Alain2]

1/2 throttle for needle position, correct?

In theory it's 1/4 to 3/4 throttle.

My experience on street rd400 is start rich and stop raising the needle as soon as you are able to "clear" 1/2 throttle between 4500-5500 rpm on flat road. I'm not saying this is best power, but it worked safely for me.

I tried the plugs thing, but it never worked for me.

You may want to buy a cylinder head temperature gauge, they are cheap and will tell you if something is going sour. Even better if you get one for each cylinder, than you get a better picture.