Running weridly... intermittently losing rear cylinder/not pulling evenly

[k_49]

Well I got the Hawk up and running after adjusting the valves and took it for a spin, and I've got to say these bikes handle amazingly.

After riding it for a bit, I started to notice some weird behavior. It was stuttering/backfiring around the 3k RPM range, but eventually would climb to redline no problem. The carbs were just rebuilt, so I poked around the forums and decided to mess with idle mixture screws (turned from 2.5 out to 2.75). I could tell the bike was immediately way better on idle, and didn't experience this stutter/uneven pulling anymore.

However, as I was riding it and it started to get hot out, it started to lose power, almost like it was misfiring or losing the rear cylinder. As I pulled over on the road, it shut off on me. Gave it a little choke and was able to baby it home, but it had absolutely no power and I'm confident it was running on one cylinder. The idle wasn't right either; without touching the idle mixture screw, it was only idling at 1k.

Pulled the plugs just after making it back, front plug looked alright, but the rear plug was wet with fuel. I'm assuming this has something to do with ignition, as the rear cylinder is obviously getting fuel? The front header was hot, but the rear was only warm to the touch.

Followed the FSM's ignition diagnostic, and other than cleaning connectors, I found no problems:
- Secondary coils: 32.7k ohm (rear)
- Primary coils: 2.3 ohm (rear)
- Primary at 6P: 2.6 ohm (rear)
- Pulse gen at 4P: W/B and B (rear): 453 ohm

Everything seems to check out, I'm stumped with this one. Anything else I should be looking at? Going to run and grab some new spark plugs later, and will update. The bike has an aftermarket exhuast and K&N air filter with the airbox lid removed.

What's weird is that it will seemingly run fine for quite a while, and then start experiencing these problems randomly. If it was a carb issue, wouldn't the bike run like s#!t all the time?

[6]

Go old school. Pull the plug and see if it go "spark" and work your way from there.

[Captain 80s]

Classic symptom for bad Pulse Generators is that they act up and start failing when hot. 2 of them and they each control a cylinder.

Maybe carry a plug with you. Check spark pre-ride and the compare when it its starts acting up and still hot. ?????

[k_49]

Just checked for spark on the rear cylinder with new plugs, and it's there. With new plugs in the rear, just idling for maybe 5 minutes the front header is way hotter than the rear... Definitely both cylinders are firing, I doubt the rear is completely dead as it was idling with only 1 spark plug in the rear, and the other one empty.

I'll go for a ride soon today and send photos of what the rear plugs look like.

Many thanks!

[k_49]

Fresh out of a hot engine after ~15 minutes, here's what I got. Doesn't look like the rear cylinder is even doing anything. Quick spray with the garden hose showed the front header to be piping hot and the rear to be essentially cold.

Given there's spark, is there just no fuel going to that cylinder? If it is that lean, wouldn't that rear cylinder be hot as can be?

I didn't want to ride much more, it's way way down on power, although it did still hold idle and get up to freeway speeds.

Any thoughts?

[Talon]

If the rear pipe is cold, time go through carbs and make sure there are no blockages to the rear cylinder. It doesn't take much, especially if you have to use fuel with ethanol in it.

[6]

Just checked for spark on the rear cylinder with new plugs, and it's there. With new plugs in the rear, just idling for maybe 5 minutes the front header is way hotter than the rear... Definitely both cylinders are firing, I doubt the rear is completely dead as it was idling with only 1 spark plug in the rear, and the other one empty.

I'll go for a ride soon today and send photos of what the rear plugs look like.

Many thanks!

It'll run reasonably decent on one hole. Especially at idle. Drew did 10 laps around Loudon on 1 pot before we figured out he had an issue. Makes about 33hp on on, so about what a bros 400 makes.

Had a bike running on one for a bit that just had a clog feeding the carb to the rear. Took Gino and I most of a hawk rally day to figure out wtf.

[k_49]

Sounds like it's pretty much isolated to a carb issue, unfortunately. Guess I'll take apart the carbs this weekend, and give them a good clean. Will update!

[k_49]

Just rebuilt the carb today, and yep, problem solved. Firing on both cylinders now, smoked like a train on startup though. Went away after riding it for a bit. Hope this helps someone out in the future. Thanks everyone.

[squirrelman]

^^ so what problem did you find ?

[k_49]

Oop, sorry! Completely forgot to say it was a plugged pilot jet and circuit on the rear carb. Front carb was actually dirtier, at least in the bowl, nothing was clogged yet though. The float heights were also not correct on either carb, so I readjusted those tangs back to factory pos.

Wouldn't recommend doing this with the carbs on the bike, it's already a PITA. Just pull 'em and clean everything in one go.

[Zanderk]

Great to hear you’ve sorted it, these things can be ‘challenging’

[squirrelman]

photos of dirty carbs before cleaning can be useful and sometimes amusing

july 16 2021 003.jpg july 16 2021 002.jpg

[6]

Glad you got that solved.

The answer to 90% of hawk issues: Clean the carbs.