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Author Topic: RD350 troubleshooting - fuel overflow into engine, could sound like piston slap?  (Read 364 times)

Offline zack20cb

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I posted this on reddit this morning, but I thought I'd post it here as well. I'll try to sync the replies in both places. Here's the reddit page:
Code: [Select]
[instagram]https://www.reddit.com/r/motorcycles/comments/70a5rg/working_through_rd350_issues/
Hey all- so, I have this 1975 RD350 that I got a few years ago, non running, and it's taken me a while to get through different systems and get it up and running. Tank rust had fouled the carbs, and the previous owner eventually gave up and sold it to me. The mileage is high, over 40,000, but the bike is all stock and seems to have been well cared for. Apart from the rusty tank and a seized front brake caliper, everything has been in really impressive shape.

Rather than sort out the rust issues (so many options, all sort of scary), I made a small tank from a stainless steel water bottle with open fuel lines running straight down to the carbs (no petcock or valve). I decoupled the oil feed system and I'm just running premixed fuel for now.

Two weeks ago I pulled the carbs again, futzed with them a little, remounted then and finally got it running! After adjusting the idle screws, it idled nicely around 1300rpm (not sure if this is the correct idle rpm, but it felt nice and comfortable). It makes all the characteristic pops, and revs beautifully. Here's a video:
Code: [Select]
https://www.instagram.com/p/BYW41lhhbX[/instagram]Z/?hl=en
So, I was pretty stoked, the thing sounds amazing. But I noticed a problem. The fuel level in the homemade fuel tank drops WAY too fast. Where was the fuel going?
Also, after a few sessions like the one in the video, I started to notice some percussive ringing sounds from the cylinders. Piston slap, I thought.

Thinking it through, the missing fuel would have to be in the crank case. I'm thinking the homemade gas tank setup just flows too much fuel, or maybe my float valves need replacing, something like that.

This weekend I pulled the engine from the bike, took off the cylinders and tipped the engine over. Sure enough, a good amount of fuel poured out.
So, here's what I'm wondering:

Does it make sense that oversized fuel lines and lack of a petcock could overpower the float valves and cause the fuel to overflow into the engine?
With the heads off and the cylinders still on, the pistons seems comfortably snug in the cylinders, no play to speak of. So, why the piston slap? Does it make sense that the fuel in the crank case would cause the piston slap sound?

With the cylinders off, I did notice some slop in the conrod bearings. They're definitely not rock solid. Is this normal/acceptable? Given that the piston and crankshaft are retained firmly by the cylinder and case, maybe it's ok for the bearings to have some play.

I tried to pull the wrist pins, but there's some chowdering of the piston casting where the spring clips fit in, and although wrist pins slide back and forth and can spin, they won't come out, they interfere with the spring clip seat in the edge of the piston. I have some bolts and nuts that I could use for mechanical advantage to pull harder on the wrist pin, but I think they might get scored in the process when they drag past that lip where they interfere. Should I try anyway, so that I can inspect the small end bearings? New wrist pins are pretty cheap, probably just $50 for a pair.

One thing I could try, to stop the fuel overflow, is to bend the brass tab on the carb float arm where the float valve plunger seats. I know there are some engines where this is a normal thing to do. I'm not sure its meant to be done with these carbs.

If I start replacing carb components, where can I get good parts, particularly the float valve elements? I've heard that the aftermarket jets for these carbs are no good, and that if you do buy a carb rebuild kit, you should just throw the new jets away.

Thanks for taking a look! I'd appreciate any advice.

(why the \[code\] tags for links? I couldn't get the URL tag to work properly :\)
« Last Edit: Sep 15, 2017, 12:41:36 by zack20cb »

Offline Redbird

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Does it make sense that oversized fuel lines and lack of a petcock could overpower the float valves and cause the fuel to overflow into the engine?
No, it doesn't. It's all gravity fed. Most likely you have trash on the needle(s), the float(s) are sticking, or they're improperly set.
I'd get that figured out first and see if the "piston slap" noise persists.
When you are Dead, you don't know that you are Dead. It is difficult only for Others.

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Offline clem

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Air cooled RD's are loud, especially with aftermarket piston. Are you sure it's piston slap and not just cooling fin vibration?
Don't bend those tangs on the float to try to stop overflow. Those are used only to set float height at a specific height determined by the manufacturer. I believe you should have a float height of 19mm. Clean the carbs by totally disassembling them first then go from there.

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"After every war there are soldiers who refuse to surrender. To this day there are still thousands who cling to their 30+ year old motorcycles, thinking that the war is still on, refusing to concede that the four-strokes have won"

1972 DS7 http://www.dotheton.com/forum/index.php?topic=45886.msg505995#msg505995
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Offline clem

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Oh and the rods will have side to side play at the big end. The service manual gives the spec for side to side play. From there you can determine if you need to rebuild the crank.

Sent from my SAMSUNG-SM-G930A using DO THE TON mobile app

"After every war there are soldiers who refuse to surrender. To this day there are still thousands who cling to their 30+ year old motorcycles, thinking that the war is still on, refusing to concede that the four-strokes have won"

1972 DS7 http://www.dotheton.com/forum/index.php?topic=45886.msg505995#msg505995
1983 CB550sc bobber http://www.dotheton.com/forum/index.php?topic=30599.30

Offline AimlessMoto

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Piston "slap" can be other noises. Rattle in the clutch, big end bearing, small end bearing, fin vibration, detonation, pre-ignition, unbalanced carbs, twisted crank shaft, etc etc.

If you want to solve the running issue. Take the carbs off, clean them well with compressed air and carb clean (even better to soak overnight in carb cleaner, or boil for 15 minutes in 50/50 lemon juice and water). You have to take the WHOLE carb apart. The emulsion tubes need to come out (remove the main jet and washer then push the jet from the bottom through the venturi and out the top of the carb) you may have to use a small, blunt punch. Make sure your O-rings on the air screws are in good condition too. Balance the carbs to stock settings. Ensure they close at the same height (via idle screws) and top-out at the same height (via cable/cap adjusters), and ensure the idle air mix screws are turned out 1.25 turns from bottomed out on both carbs. Try to buy Mikuni parts if you need to replace jets/needle valve and seat/needle/etc... emulsion tubes are near impossible to find OEM so be careful with them.

While you're at it.. check your reeds if your bike has torque induction (it should).

You'll have to time the bike as well... you can do this rudimentary/approximately with a timing light and ensure your timing marks match on the flywheel/stator. The stock setting is around 2.0mm before top dead center. This is a good place to start. To be REAL exact you need a dial-gauge shoved into the spark plug hole and a LOT of patience. (Look up videos on YouTube on how to set timing on an RD350.)

If, when it's running you still have a bit of a rattle... and it goes away when you rev it... that's "normal" but could be a sign of your cylinder/piston beginning to get past the 1/2 way point in it's life. If you have a rattle that DOESN'T go away when you rev the bike up... that's not "normal" and can be indicative of the cylinder being out-of-round (ask me how I know).

The only way to solve that problem is to get the cylinders machined and have the shop buy you the right size pistons. Expect to pay somewhere upwards of $300-400 for a quality job with the price of new pistons.

I just had my RD400 top end redone by Chuck Quinzler from Team Scream Racing out of Clearwater, FL. I don't know if he's on this board... but I can send you his information if you want to chat with him. He's an RD guru.

Side-to-Side con-rod play is normal, I think .030" is in spec.

850-1050 RPM at idle is where you should be.

40,000 miles on a top end?!?! Are you SURE it's stock? No stampings in the tops of the pistons? .25, .50, 1.00, 1.25? That would indicate it was re-bored sometime in it's life. If you SERIOUSLY have 40k on the original stock top end... just get it bored and honed and throw some fresh pistons in there. Plus, piston kits normally come with new wrist-pins, circlips, rings, everything you need.

Remove your circlips with needle-nose, use a 5/16 or 3/8 socket on the end of an extension. Put the top of the piston in the cylinder and block the cylinder up on the base gasket with a couple pieces of wood... then carefully knock out the wrist pin with the socket and extension and a rubber mallet. You HAVE to remove  both cylinders to take the inner circlips off the wristpin... kind of a PITA.
« Last Edit: Oct 11, 2017, 16:08:55 by AimlessMoto »

Offline VonYinzer

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