Rpms stop at 5k when accelerating under load

[Schming]

From what I understand sync really only has to do with idle/low rpm performance. When the butterflies are all the way open it's not going to matter hardly at all if they are off by a small amount, especially because mine were bench synced fairly precisely using trbig's technique.

You can laser synch your carbs on the bench but what you can't do is know the condition of each intake port, valve, piston, combustion chamber, exhaust valve, port, header down tube and muffler and the slight differences in the weights of all the moving parts in the 4 cylinders.

Your carbs need to balance all those variables through a synchronization.

[DGXSER]

The bench synch gets all four butterflies open the same amount.

Under running conditions, each cylinder has little small differences. Spark plug, wires, carb condition and settings, compression, condition of the inside of the exhaust pipe and or intake boot, the amount of crap on the air filter section closest to that cylinder. All of which contributes to a difference in the amount of bang each cylinder produces when given the same amount of the same air fuel mixture. This in turn makes a difference to the amount of suction created when the piston rotates round on the non combustion stroke.

Therefore, the synchronization. The equalizer of all things vacuum. OK, so maybe just the amount of vacuum each cylinder draws, which is the best way we have to equalize the "production" of each cylinder.

Used to be, I used a single vacuum gage, on a single tube, and got my synch pretty dang close to what the Morgan Carbtune can produce. I even used to not bother synching if it ran decent already. And to be honest, my bike ran pretty well like that. Then I got the Carbtune, and I spent an hour or less synching, tuning, synching, and tuning my carbs. And I was surprised at the improvement and how noticeable it was.

Now, I do not think it is the single only issue of your running out of steam at 5k, not sure where else to look at the moment, other than spark. But it sure wil not hurt to synch them while running.

[skids]

I still think that vacuum synch does little or nothing for higher rpms operation. Ever see what happens to the vacuum levels after synching the carbs perfectly at idle and then raise the rpms? They will not be equal. That is because you are depending on the diaphragms at higher ranges. I think the way they work is with velocity pressure. The oblong hole at the carb inlet is the total pressure. The hole at the bottom of the diaphragm slide is the static pressure. Total pressure minus static pressure equals velocity pressure. The difference in pressure is mechanically made by those holes. Yes I know...yada yada.

[D0wn5h1ft]

Well I found part of my issue. My tank doesn't vent I'm pretty sure. Air won't come in or out of the vent hose. I popped the gas lid and ran it WOT and it definitely responded much better than before (still stock jetting, velocity stacks, and with the dual mac slip ons ON).

I have a funny feeling my tank "refurbishing" didn't go as well as I had planned. When I pulled the carb rack apart to get at the choke plungers to check them there was a BUNCH of junk in in the T-valve inlet that would have made it into the carbs if it weren't for the float seat net and I JUST cleaned that part out. I think there is some fuel starvation going on here. Part due in fact to the tank not venting and I think my petcock are going to be full of junk also (since beyond them was full of junk too).

I'm going to shotgun blast the problem this Wednesday when my plugs on in. I'll get new plugs in it, take off the petcocks, clean them, get some gas and wash out the tank to figure out what's going on inside there, and find out what to do about this venting problem.

[DGXSER]

Well, to start with clean the gas cap. I did a write up on the process some time ago. Not sure if it ever made it to the tech tips or not. HERE it is.

I would also clean your carbs back out, and the T,s and your fuel lines. Pull the petcocks out, clean them up well.

Then take some acetone to the tank, wrap and tape off your paint as acetone is a great paint stripper. Then clean everything up as best you can, reassemble and see how it goes.

[bikerphil]

The vent is in the gas cap which can be taken apart and cleaned. The hose on the back of the tank is not a vent, it is a drain for water/excess fuel that would otherwise accumulate around/under the recessed fuel lid.

[D0wn5h1ft]

The vent is in the gas cap which can be taken apart and cleaned. The hose on the back of the tank is not a vent, it is a drain for water/excess fuel that would otherwise accumulate around/under the recessed fuel lid.

Oh, well then I'm just imagining things then I guess...because I soaked that gas gap in evapo-rust and made it shine like the day it came out of the factory (not quite). That's about the only part on this whole thing I'm sure is 100% clean.

I'll just wait until the plugs get here to do anything else then.

[motoman]

Well I found part of my issue. My tank doesn't vent I'm pretty sure. Air won't come in or out of the vent hose. I popped the gas lid and ran it WOT and it definitely responded much better than before (still stock jetting, velocity stacks, and with the dual mac slip ons ON).

I have a funny feeling my tank "refurbishing" didn't go as well as I had planned. When I pulled the carb rack apart to get at the choke plungers to check them there was a BUNCH of junk in in the T-valve inlet that would have made it into the carbs if it weren't for the float seat net and I JUST cleaned that part out. I think there is some fuel starvation going on here. Part due in fact to the tank not venting and I think my petcock are going to be full of junk also (since beyond them was full of junk too).

I'm going to shotgun blast the problem this Wednesday when my plugs on in. I'll get new plugs in it, take off the petcocks, clean them, get some gas and wash out the tank to figure out what's going on inside there, and find out what to do about this venting problem.

If you haven't already, may wanna put a couple litlle inline filters with longer fuel line on filter end, criss crossing them when hooked to petcocks. Un-cross them for the extra length with tank turned backwards in seat location and turn to prime watching for any overfueling before installing intake filters or tank.

[natemoen]

Oh, well then I'm just imagining things then I guess...because I soaked that gas gap in evapo-rust and made it shine like the day it came out of the factory (not quite). That's about the only part on this whole thing I'm sure is 100% clean.

I'll just wait until the plugs get here to do anything else then.

Did you actually take the gas cap apart or not? If not then you still need to pull it apart and clean it.

[jetmechmarty]

Oh, well then I'm just imagining things then I guess...because I soaked that gas gap in evapo-rust and made it shine like the day it came out of the factory (not quite). That's about the only part on this whole thing I'm sure is 100% clean.

If you did that without complete disassembly, we have found a problem. There is a small foam filter in the cap. Likely, it's rotten. If you soaked it, the orifice is probably completely plugged. You're probably only venting through a leaking seal, if at all.

[D0wn5h1ft]

I dissembled it all the way down to where the guide suggested (which is all the way). Is there a way to test if it is venting properly?

[DGXSER]

Best test I know is to get where you have your issue at 5k and open the gas cap. Or leave it open when you take off. Then see if your 5k issue remains, improves, or disappears.

[skids]

Best test I know is to get where you have your issue at 5k and open the gas cap. Or leave it open when you take off. Then see if your 5k issue remains, improves, or disappears.

Make sure the fuel level is down a bit or you will get wet with gasoline!

[motoman]

Some of us ole geezers here at times just assume basics like clean tank/pecocks/carbs has already been gone thru using some of the tech tips, me included. Seen to find out otherwise later in the saga, which seems more and more the norm.
So, at this point, hopefully your gonna be actually starting from scratch again, going back thru ALL the before mentioned.

[WMarshy]

Have you tried a can of Seafoam in the gas?