[Rob Walker]

I have a 1999 ACE 750 that I have ridden and maintained for 15 years. It has 47,000 miles on it. I did all the upgrades that I learned from this site over the years. I am completely stumped about the latest problem that I have. The bike starts fine (after I just replaced the started switch assembly) and all is okay at idle. Twisting the throttle to raise the revs while in neutral seems fine. Once I get on the bike and attempt to go for a ride, it doesn't like to accelerate, to the extent that I can't get over 50 mph, even with full throttle for a mile or so. This is an issue that happened after I had ridden 10 miles without issue and parked in my parent's driveway for an hour or so. When I went to leave there, I immediately found out that acceleration was an issue. I took the bike apart and can't find any cracked hoses. The four plugs seem to have proper spark. I can't find a way to recreate the problem at a standstill. The battery is quite new. The plugs have normal coloring. Neither cylinder has an obvious issue. This problem is consistent under load. I can't figure out what direction to go in for the next step to diagnose the problem. My dad has the exact same bike and he is also out of ideas. Any direction or suggestion would be vastly appreciated.

 

[swifty2014]

Usually it is the carbs that cause the lack of power problems. Maybe a bit of dirt is in a jet now.
Or a rubber slide diaphragm cracked on the long ride and now can't operate the slide to deliver fuel like it did.
Did all the plugs look about the same color?

Possible a coil cracked also if it is definitely one cylinder cutting off at 50.
You can spray water on the coils and wires and see if there is any arcing while sitting still.

 

["Captain D"]

When mine done that, it turned out to be that them diaphragms in the carbs were bad...
They had cracked and brittle...
You might be ahead to check them,
Dennis

Back then they were $108

 

[gdb069]

When was the last time you adjusted valves?

 

[Noble Thinker]

I'd first check the carbs (definitely check the diaphragms for proper seal & tears in the rubber and make sure the main jets are clean). I would second verify that the fuel pump is delivering the proper flow of fluid. It's possible that the carbs aren't receiving enough fuel because the fuel pump is weak.

 

[Rob Walker]

Thaks for all the responses. I'll start with the testing of the fuel pump and the coils this weekend, since they are fairly easy to rule out compared to getting to the carbs. I have never adjusted the valves and I bought the bike with 17k miles on it. The local dealer wants $700 to rebuild the carbs. If I only had unlimited time, I could do them myself!

 

["Captain D"]

Yeah @Rob Walker I`m NOT a "carb man" myself...
HOWEVER, I would do what I hafta though to save $700...

I replaced the diaphragms myself though...

And `tweren`t too bigga deal,
Dennis

 

[Bentone]

I can't find a way to recreate the problem at a standstill.

The way I' m reading this quote is, I can rev it up, yes? Because that says a clean carb, no tares in the slides.

 

[Troy Jollimore]

At speed is a little different. The RPMs are the same, but there is more air and fuel being drawn through the engine. Checking for any restriction to either is the easiest thing to do. If you can get ahold of a compression tester with the right-sized thread for your spark plug holes, that would be another big element to that equation.

 

[gdb069]

Just to clarify: the problem happened quickly ie one day the engine was good and then it was not.
No recent work done or mods?
The engine starts and runs OK and choke works OK?
The engine can run at all rpm and high rpm in neutral?
The engine runs poorly at all rpm not just high rpm but struggles to get over 50 mph?

Has fuel been replaced since the problem in case of really bad fuel?

I think you are on the right track with fuel pump/filter/restriction in tank.
Ignition may be a ICM/IPG problem
You may as well check the valves at some point and I am surprised you and your dad don't do this maintenance item.

 

["Captain D"]

It is not difficult to check valve clearance...

Although `twas Much easier on my CB750,
Dennis

 

[Rob Walker]

Bentone, that is correct. I had the bike on the lift with an aux gas tank and couldn't find any issue with reving it up, besides an occasional backfire when I let go of the throttle, which I'd expect. I was hoping that I'd find a gas or air problem.

GDB069, In the middle of last season, I rode for 10 miles to work, parked for the day, then rode a couple of miles to my parent's. When I left, I noticed the issue pulling out of their driveway. Riding just above idle is fine. Gaining speed is the issue. Changing to a different gear doesn't help. Starting and choke are fine. All rpms seem fine in neutral. Fuel filter and gas are new. Valve clearance check is always on the to do list, but I'm awfully good at procrastination when the bike is ridable.

 

[kesanders]

Make sure your not losing power on one cylinder under a load. Put an inductive timing light and check to see if both cylinders are firing.

 

[Bentone]

Bentone, that is correct. I had the bike on the lift with an aux gas tank and couldn't find any issue with reving it up

Notice the remote flow from the tank. So from line out of tank, line to fuel pump, the fuel pump, like out to the carbs, somewhere there is a kink in the line(s). Somewhere there's an imploding of the line separating from the hose?

Plastic drinking bottle, fuel hose from drilled bottle cap so there is an interference fit so no leak, because it's going to be taped to the bars upside down, a pinhole at the bottom of the bottle stabbed when mounted upside down, right?

Now go over 50mph and see. That or remote to the carbs. That's pick a tank nipple that flows gas, set that up to run remote to the carbs. Yes I know there is no petcock to shut off, but this is keeping the whole loop out of the loop running remote to find out if it's carb or flow issues.

So same issue running remote, sounds like carbs.
Runs 100mph all day, then it's fuel pump and hoses in or hoses out of the pump, or is the pump itself.