No Go Fast

I have a 1975 sportster,Just had motor rebuilt,The problem I have,it idles good ,but when I give it full throttle it just bogs,I have a s&s shorty,It ran good last year except for the pounding noise from the worn crank pin.Any leads would help.... Running out of ideas.I've checked ignition timming and valve timming,everything looks good,but wondering about carb.

It doesn't backfire,just dogs when you accelerate.

Definitely, sounds like an carb problem.
Is this the same carb that you originally had with this motor?
Sounds like an accelerator pump, or jetting problem to me.
Good luck, let us know what’s up…

It's the same carb,the accelerator pump works,was going to try some different jets,and see what happens.

I'm assuming a couple things here. You said it ran good last year, so I'm assuming nothing has been changed since then. Agree that the accelerator pump could cause bogging, but it should recover after engine speed runs up as the pump isn't what you are running on at road speed. It sounds to me like possibly varnish in the bowl has partially clogged the jet or the advance is sticking. My $.02, that's the two places I'd be looking first.

Could also consider the fuel petcock filter. Checked it lately? Lots of nasties gather in a tank over a period of no useage.

The petcock is brand new.The accelerator pump works good.I put in smaller main jet...no change.when I accelerate I see gas shooting back out of carb,wondering about valve timing,haven't checked the timing advance yet but I set it 2000 rpm's and it was in the timing hole.I should check to make sure push rods aren't too tight also.

Could be the accelerator pump is squirting too much fuel and flooding, causing the bog down.

I turned the accelerator pump off,still did it ,I put in a smaller main jet and still no change,I'm thinking it's the ignition timming,maybe the timing advance isn't working right or there is a short somewhere.It's getting to be a pain!

Whacker, on your original post you said the timing and valves were already checked. You most likely have an mechanical advance, it could be as simple as worn springs on the advance unit, recheck your timing, make sure your advance is working. This is where its nice to have a friend (or yourself) with some KNOWN good replacements to borrow for troubleshooting problems. Do you know anyone with a spare carburetor, ignition or even an coil?
Good luck on your quest…
K-B

It is burning rich,the mechanical advance seems to be working,Kinda sounds like it just can't handle all the gas it's getting,doesn't recover when use goose it,I do have a smaller indermediate jet I will install it and see what happens.I did try a different coil,but there was no change.kinda looking at too rich or weak spark now,Wish I had a different carb to try.Thanks for all the help I've been getting!!!!!

check your timing advance and make sure your manifold is sealed good, ya might have a leak there, good luck Bro

I use the "band" plus the "o" ring,with aircraft style clamps,intake should have no leaks,and it would be a good idea to replace the plugs......But I did find the problem!!!!!!!!!!!!!!!!.I bought new drag pipes with baffles,the baffles were too restictive,I pulled them out and it run great!!!!!!!!!!!,I will modify the baffles,I like my bike a little less noisy then with just running straight pipes.....Thanks to all with your help.Live and Learn......

You can take a piece of iron pipe, clamp it GOOD and TIGHT in a vise, slide the baffle over it and use a ball pien hammer to lessen the sound damperer's height. Do them evenly though. You may need to go all the way around a few times, but do it slow and even.
It works, that's what I had to do with mine.
G/L