Honda XR bogs when revved

[RMX250DB]

Bike is a 1975 Honda XR75. Carb is spotless, jets are clean. Manual says it needs a 30 pilot and 100 main jet, I have a 30 pilot and 92 main. Manual says needle in 2nd clip from top, I have the clip in the top position. I havent been able to get it to run with the needle in any other position. I just adjusted the valves and visually inspected the points and the meet up flush and square, there wasnt a spec. for the point gap or else i would have measured it. The bike will start up and idle fine (never need to use choke) but as soon as you blip the throttle it will bog. If you rev it to much it will die. Sometimes if you take it real slow you can get the rpms to come up some but not much. Can't figure this one out since it has a fat blue spark and the carb is clean. Any help appreciated. Thanks

 

[yamahonky]

does it have good compression? if so, try a fresh tankfull of gas and a clean air cleaner, if not, new piston and rings, if all else fails, it sounds like you need leaner jets. What does the plug look like after you run it? good luck on getting it running :cool:

 

[reelrazor]

Where's the mixture screw set? Doesn't need choke, gotta be leaned at the needle..sounds to me like it is getting too much fuel right off the bat. Are you sure the air bleed holes are clear?

.016" on the points.

 

[COLEMANAPP]

I had one of those and had to constantly keep the filter in the gas tank cleared. Pull off the petcock and make sure your screen is clean. Sounds like what mine did about 30 years ago.

 

[Ol'89r]

Bike is a 1975 Honda XR75. If you rev it to much it will die. Sometimes if you take it real slow you can get the rpms to come up some but not much. Any help appreciated. Thanks

RMX.

The description of your problem sounds very much like worn cam journals.. On those models the cam runs in an aluminum journal that is part of the head. If the journals become worn the cam can move around in the journals. Aside from the obvious, the main problem with this is maintaining a point gap. Since the point cam is attached to the end of the camshaft, the point gap can change at different rpm levels.

The way to check it is to put the engine on tdc on the compression stroke. Piston will be at top and both valves will be closed. Then take a pair of vice grips and clamp them on the 6mm bolt on the point cam end of the camshaft. Try moving the cam up and down. There should be no movement. If the cam moves up and down noticibily the journals are worn.

If the cam is tight try setting your point gap at .012.

Good luck.

 

[SMAKme2X]

is ur choke off?, kind of stupid to say but iv done it before

 

[RMX250DB]

Well I fixed the problem for the most part. It turned out that the dreaded previous owner had switched carbs with an xr80 and never set it up right. I put in a new plug and dropped the clip to the 4th position and now it runs OK. It is still running a little rich since the choke is still never used and the plug is very dark. Thanks for the tips though.