RD350 sucking the float bowls dry with TM32s

[84rzrider]

My 75 RD350B stock porting with single petal reeds correctly jetted and well sorted TM32 Mikunis and Moto Carerra pipes is overwhelming the the flow capability of the stock fuel valve. Though I rarely run it hard enough for it to be a factor occasionally I do.Yesterday I took off hard in 1st picked up front wheel a couple feet. Shifted to 2nd at 9000 and floated the front wheel just a bit, shifted at 9000. Shifted to 3rd and started up a hill and she nosed over at 8500 and I got out if it as soon as it lost power. I short shifted to 4th and rode it the 1/2 mile home. Pulled plugs coffee and creme. No rattling. Idles pefectly. Free revs no load perfectly. General consensus from some pretty knowledgeable fellows on this side of the pond is that a larger fuel valve is in order. The problem is no one makes one for the RD350. 400 tank yes 350 no. Solutions?

[crogthomas]

Do you have an external fuel filter fitted?

It might be worth doing some simple flow tests, by draining the fuel pipe into a measuring jug against a stop watch. I found that filters made a big difference to the flow, but still gave plenty of fuel.
I don't know much about the AC bikes, but is the tap an unusual design, or a very small diameter?

[84rzrider]

No filters installed. The vent system is operating as well. Reproduction furl valves are cheap. I will likely try that first.

[reedpete]

Check the tank cap… it might not be anything to do with the carb or the tap, it could be the breather hole in the cap… well known to block with rust and cause fuel starvation

[84rzrider]

I will do that to eliminate that. But all that stuff was replaced, though that has been several years.

[reedpete]

Feb 7, 2024 1:55:19 GMT 1 84rzrider said:

I will do that to eliminate that. But all that stuff was replaced, though that has been several years.

It’s only an outside chance but probably my point is that these unsolved problems lurk where an incorrect assumption has been made…
so when checking fuel flow in the system it’s best to systematically check by starting at one end of the chain and working through.


by the way , a good working fuel valve (tap) is more than capable of providing all the fuel you could possibly need, so , if there a problem with the tap it’s because it’s compromised in some way.

a couple of tests you could try would be to run with the cap undone.. (being careful!!!)
also , if the engine doesn’t actually stop, you could run again after the carbs had a recovery time. 

another thought… if you only had a small amount of fuel in the tank and you are going up hill on one wheel… are you sure you just didn’t start sucking air ? A full tank of gas might give different results due to extra gravity pressure and eliminate risk of fuel pickup .. especially if you were on reserve ..

[84rzrider]

Thanks. Will do.

[84rzrider]

My good friend Frank B came over this afternoon and we did a fuel valve flow comparison test. We used one line into a container with a full tank and crimped the other line shut. Timed for 60 seconds with the fuel cap closed and 60 seconds with the fuel cap open. There was 2 ounces less with the fuel cap closed so that told us the vent on the tank is likely the issue and, in a day, or so when I have time to get back out to the shop, I will be taking the cap apart and figure out where the blockage is.

[84rzrider]

Well after fooling with it for a little over an hour and a call to M in SC for technical advice I am pretty certain it is fixed. Test ride to follow once the snow melts. We are getting snowed on right now, but it doesn't look like we are going to get the 5-8 inches forecast.
I took the cap apart and I didn't find any trash, but I did find the seal had rotated on the little inner washer and covered up the inner vent hole. I also took my scribe and ever so slightly opened up the main vent hole on the outer flange that the big seal goes on per M in SC. You have to keep everything in alignment while installing the big seal that goes against the tank itself and keep the three little nubs on the inner seal in the matching holes in the outer flange that orients the whole mess and then compress the spring to install the e-clip that holds it all together.
Next, I performed the flow test and here is where it gets weird. As before I used the same graduated container and pinched off the same line as before and opened the fuel valve to on for exactly 60 seconds. Before I got 3 cups of fuel in one minute with the fuel valve open and the cap on and two ounces more than that with the cap open. As stated before that told us we had a vent obstruction.
My 1st test with the correctly oriented parts for the vent and I got four cups of fuel in 60 seconds with the cap closed, vast improvement. I poured those four cups back into the tank as before. The follow up with the cap OPEN for 60 seconds should have been four cups but was only THREE. I thought briefly that I had lost my mind, so I repeated the test two more times, with the same results then called M in SC. Then I repeated the test while I had him on the phone, same results. Mark said to reverse the order and repeat the test and I did, same results. Mark said he can't explain it and nether can I. So, we agreed to take the win and ride it and see what happens. I observed the flow coming out of the line into the container and it is obviously stronger with the cap closed then open, which in my mind is the exact opposite of what it should be and furthermore the opposite if what it was before?

[charles13]

Feb 17, 2024 8:28:01 GMT 1 84rzrider said:

Well after fooling with it for a little over an hour and a call to M in SC for technical advice I am pretty certain it is fixed. Test ride to follow once the snow melts. We are getting snowed on right now, but it doesn't look like we are going to get the 5-8 inches forecast.
I took the cap apart and I didn't find any trash, but I did find the seal had rotated on the little inner washer and covered up the inner vent hole. I also took my scribe and ever so slightly opened up the main vent hole on the outer flange that the big seal goes on per M in SC. You have to keep everything in alignment while installing the big seal that goes against the tank itself and keep the three little nubs on the inner seal in the matching holes in the outer flange that orients the whole mess and then compress the spring to install the e-clip that holds it all together.
Next, I performed the flow test and here is where it gets weird. As before I used the same graduated container and pinched off the same line as before and opened the fuel valve to on for exactly 60 seconds. Before I got 3 cups of fuel in one minute with the fuel valve open and the cap on and two ounces more than that with the cap open. As stated before that told us we had a vent obstruction.
My 1st test with the correctly oriented parts for the vent and I got four cups of fuel in 60 seconds with the cap closed, vast improvement. I poured those four cups back into the tank as before. The follow up with the cap OPEN for 60 seconds should have been four cups but was only THREE. I thought briefly that I had lost my mind, so I repeated the test two more times, with the same results then called M in SC. Then I repeated the test while I had him on the phone, same results. Mark said to reverse the order and repeat the test and I did, same results. Mark said he can't explain it and nether can I. So, we agreed to take the win and ride it and see what happens. I observed the flow coming out of the line into the container and it is obviously stronger with the cap closed then open, which in my mind is the exact opposite of what it should be and furthermore the opposite if what it was before?

That defies physics and gravity, I don’t understand.

[84rzrider]

Nor do I. The M in SC mentioned in the post,a dear friend of mine is an engineer by trade with extensive training in fluid dynamics and he can't explain it either. I have thought about it until my old head hurts so I have resigned myself to ride it when the snow melts and see what happens.

[84rzrider]

As it turns out, gravity and physics don't change. Today I did the test again. Same results. Then I shot a little puff of air into the line I had off. Repeated the tests and got 4 cups exactly in 60 seconds with the cap on and off. So I am draining the tank now to further investigate. Apparently something is blocking the stand pipe intermittently.

[84rzrider]

I took the tank off and inspected it with my borescope. I didn't find anything. Took the fuel valve completely apart and other than the already known no screen or stand pipe filters didnt find anything amiss. I will be ordering a new fuel valve from John at Economy Cycle along with some wiring stuff I need for the Zeeltronic setup on my Gray Ghost RZ.

[84rzrider]

I did electrolysis on this tank when the bike was first resurrected years ago. The inside still looks good. I filled it up today with Evapo-Rust full strength and I will leave it in there until my Economy Cycle order arrives. I figure it is a good idea to make sure and already had the 5 gallons of Evapo-Rust on hand since the Gray Ghost tank job.

[crogthomas]

From the confusing results of your flow testing it certainly seems like dirt in the tap and/or filter is the reason for the inconsistent flow. I'd be tempted to jet wash the tank out first to get rid of any sediment hiding in the corners.

[tony2stroke]

Don't know if this is helpful or not,  but I had a problem exactly the same on an rd200, my problem was that I had 2 inline filters, 1 for each line, for some reason it did the same as you are saying happens to you, I scratched my head for ages trying to find out what was happening, for some unknown reason an air lock would form pretty quickly after pulling away, after taking the filters off the bike ran spot on again, apparently a number of other people had the same problem on the air cooled site, but not all, I don't know why or what makes this happen to some and not others, but if you are using 2 inline filters, try removing them and go for a test ride.

[84rzrider]

I will wash it out thoroughly. Thanks to all. Not using any filters.

[84rzrider]

The tank after a week filled with Evapo-Rust full strength. I did a water rinse then put a 1/2 gallon of WD 40 in the tank and coated it thoroughly, drained that and blew inside the tank with the air hose.

New petc**k from John at Economy Cycle prepped and just a touch of Gasoila on the threads, sealing washers and tank gasket. A bit of trivia, M in SC took me to the hardware store in Charlotte if memory serves in 2004 and we bought that can of Gasoila to seal the threads on the Pingel for the Phoenix Project. Still good after all these years

Another quality product from John at Economy Cycle Thanks John!!
Filled it up with gas. No leaks. Fired up on the 2nd kick. It was raining and cold so no test ride until later in the week. More to follow. Thanks M in SC for your guidance as always.

[muttsnuts]

I seem to remember that the fuel spigots on the TM32s are 5.5mm ID, the fuel tap spigot ID's are probably 4.5mm (maybe smaller), you need to double check, the point is that you have mistmatched flow rates, the tap can't keep the carbs supplied of the fuel they require

We get this alot on some of the big bore bikes, the solution is to fit 2 taps, one for each carb, or a high flow tap like a pingel etc

Check the ID of the outlets on the fuel tap and then measure the carb spigots, then do your maths to compare the flow rate, as a rough rule of thumb, for every doubling of a diameter, you quadruple the flow.

Don't be fooled by the usual it was fine with the old carbs on, carbs require different amounts of fuel depending upon their design, you can't assume the engine will be using the same amount of fuel, if you've got a performance increase with the TM's (I suspect you have), then that normally equates to the engine using/needing more fuel - the carbs meter the fuel, the float height in the carbs is designed to maintain the fuel level at WOT and that is designed in conjunction with the size of the needle valve (float valve) and of course the fuel inlet spigot, all these things must match and of course, the fuel being fed to the carbs must be able to meet demand

HTH

[84rzrider]

Will do. Thank you!