Ironhead Can't get past 4000 rpm

[Kingrabbit]

My bikes a 73 Sportster.

Issue: bike starts and idles well, accelerates fine up to about 4000 rpm then loses accceleration. Doesn't pop or fart just seems to hit a power ceiling. Same in every gear.
Seems to overheat too.
Anybody have any thoughts on this?
My carb is the Zenith Bendix 16p12 with just the adjustable low speed screw.
My Clymer says # 1 suspect for Loss of Acceleration is the carburetor is set incorrectly.
I am willing to pull this old carb off but would rather not if I don't have to. The bike was working fine awhile back and I had it on the highway for a300 km run just a month ago.

Donnie
1973 XLH 1000

[randy3934]

I'd drop that bowl, if there's junk in that bendix it'll do just that

[Hopper]

Done anything to your exhaust lately? Too restrictive baffles in drag pipes etc will do what you describe.
So will a clogged air filter.
So will fuel blockage in the carb or petcock etc.

[IronMick]

Originally Posted by Kingrabbit

... bike starts and idles well, accelerates fine up to about 4000 rpm then loses accceleration. Doesn't pop or fart just seems to hit a power ceiling. Same in every gear.
Seems to overheat too ...

My experience with lack of fuel from the fuel system [slow jet too small, float level too low, etc] is that the engine sputters; then if i release the throttle momentarily [to allow the bowl to re-fill] it picks up again. So i am wondering if it is a carb problem. With a lack of fuel for whatever reason i would expect sputtering.

A couple of years ago mine would not accelerate past 5000 RPM. After checking everything over a period of weeks i discovered that the tach was incorrect and it was reaching the 6000 RPM limit.

I would start with the basics: pushrod adjustment, intake/exhaust leaks, ignition timing; just to be certain these are good and are not messing with the symptoms. Then ... ?

[Kingrabbit]

Thanks you guys.
So can I can pull the bowl off the carb without removing the entire carb?
There is an in-line filter which I cleaned with carb cleaner -there was some crud in there.
I did put baffles in the bike recently as well. It has drag pipes on there.
Mick do you mean that after momentarily releasing the throttle,allowing the bowl to refill, that you could overcome the acceleration ceiling of 5000 rpm? My tach is suspect - it doesn't work at idle but seems 'normal' at when riding.

[IronMick]

Originally Posted by Kingrabbit

... So can I can pull the bowl off the carb without removing the entire carb?
There is an in-line filter which I cleaned with carb cleaner -there was some crud in there.
I did put baffles in the bike recently as well. It has drag pipes on there.
Mick do you mean that after momentarily releasing the throttle,allowing the bowl to refill, that you could overcome the acceleration ceiling of 5000 rpm? My tach is suspect - it doesn't work at idle but seems 'normal' at when riding.

My only carb experience is with Keihin butterfly carbs.

I never work on the carb while it is on the bike. Perhaps it is OK if you have the patience. But after removing the bowl, which is the easy part, then there are the jets etc. I prefer to use this carb stand, then i can check everything carefully; IMO this is the only way to accurately check/set the float level ...



If the filter had crud then probably so does the bowl, jets etc. A good carb cleaning may be in order ...

Cleaning a Carb
http://xlforum.net/vbportal/forums/s...d.php?t=185361

I was posting about two separate situations. When the bad tach read 5000 RPM the engine was actually running at 6000 RPM and had hit the rev limit set in the Ultima ignition module. The other situations were with float level too low, slow jet too small, or air filter clogged up.

The sputtering was similar in all of these situations.

[brucstoudt]

try taking the baffles back out. carb may have been richened to run them by,p.o.

[dr.feltersnatch]

before you go messing with things, what has changed. you said it was running fine not too long ago so something has changed. if its a clogged fuel filter and you mess with the carb settings now your chasing 2 problems. if the fuel filter was clogged get a new one, or atleast take it off and see if your problem goes away. i dont think just some carb cleaner will clean it properly. and if its clogged it will starve you of fuel.

[rivethog]

If your petcock stil has the screen on it, get rid of the inline filter.
What changes'repairs have you made since it last ran correctly?

[FearlessFrisbee]

Carb cleaner will not remove debris from inside a filter. They're cheap, replace it. Also, a lean carb increases engine heat.

I'd check some of the things noted here and if carb work is needed, remove it like Mick says. Ya just can't get good access to the jets and stuff with it on the bike.