Help! Engine trouble with first bike!

I just bought my first bike. It is a 1979 Hondamatic 400 Hawk. Had been sitting a long time, so my buddy who rides and does a bit of work on bikes said he could help me get it running. We took apart and cleaned the carbs, got some new gas and oil in there and a new battery. It started right up. Doesn't run right, though. Having 2 issues:First, even after it warms up, it will die if you try to let it idle without the choke. Second, whenever you open up the throttle, with or without choke, the engine pulses, like revving up and going quiet really quickly over and over. At this point, neither of us know what to do. Can anyone offer any help?!

It's gotta be something to do with the carbs, but I'll let someone more knowledgeable than myself try to help you figure out exactly what.

Sounds to me like your not getting quit enough gas, Did you clean out the gas line fully? Is there an in line filter that could be plugged? Also make sure that the carbs are attached nice and tight and that any gaskets are in good shape.

If it idles with the choke on then it's not an air issue so much as the choke forces more gas into the carbs, with the choke on and the engine running do you notice smoke coming out of the exhaust pipe? I would expect a little as the purpose of the choke is to make the engine run a little rich.

I am betting on a fuel delivery problem

Didn't really seem to be any smoke out the exhaust, even on choke. Tried just letting the tank drain on its own, and didn't seem clogged. Also had checked the line into the carbs and between the two carbs, which seemed fine. I'm not sure if there is an inline filter at all...I'll have to check that. One weird thing: once or twice while checking it out forgot to turn the gas petcock back on and it seemed to run the same with or without it.

When you say that you cleaned the carbs, did you totally dissemble them and clean, I can't find any manuals online but see some blurbs about Rubber Diaphrams etc, Are they in good shape? also did you Sync the carbs?

We did totally disassemble them and clean them. Rubber looked in good shape. Haven't synced the carbs...what all does that entail?

Now I am no MC mechanic, but you need to make sure that the carburetors are both in sync. This means that they both are in the same throttle position at all times. You can buy a sync "gadget" probably at any real good auto supply store. I found a website that might give you some help.
put a www in front of each site below

nighthawk750.com/mods/carbsync.html

Also the website below is for a repair/maint manual for your bike

themotorbookstore.com/40twin19.html

here is a website with a do it yourself sync kit

faq.ninja250.org/wiki/How_do_I_synchronize_the_carburetors%3F

I still personally think you may have a plugged jet since the symptom points to not enough fuel

I'll check the jets again and look around to see if maybe anything else is keeping the motor from getting enough gas. It seems like the carbs are getting enough, since it will drip out the overflow a bit when running. I'll look into it! If anyone else has any other ideas in the meantime, feel free to let me know!

As for your first issue: adjust your idle speed screw on the carburetor so the throttle blades are more open at idle. What may be happening is that your throttle blades are closing too far with the choke off, cutting off the air supply.


As far as the bike running with the fuel petcock in the 'off' position, this is because there is still fuel in the carbs and in the fuel lines from the last time it ran, so it is using the fuel FORWARD of the petcock to run. Eventually, it would cut out and die once the residual fuel in the lines runs out. I've done this a few times, gotten on the highway and realized that it was off and just flicked it back on, and it was fine. I'd say you could get a few miles before you'd stall out.

I have tried turning up the idle screw...hasn't seemed to have made any difference at all. Jets looked good and clear. Think I might end up having to take it in, since I'm all out of ideas. Anyone know about how much it should cost to have a bike checked out?