I'm Shuddering

Hi all, 92 Heritage.

I installed the Mikuni 42 thx DP about 2 months ago, great improvement in general performance.

Had not rode in about 2 weeks due to work. I have figured out by now that on a cold start I needed to twist the throttle once to get the bike to start on a cold start after installing the Mikuni.

Today the bike was harder to start than 2 weeks ago before this trip. The carb backfired several times taking off from stop before up to temp. I blew it off as to not being started for 2 weeks and it had been a little cool here, 30 at night for a few nights. This had not happened before since I installed the Mikuni 2 months ago.

Took the bike out and put in about a 50 miles. Cruising along fine then I stopped for a smoke. Bike is up to Temp. (I could wait a hour for a smoke).

Tried to start bike and it would not fire right up right away, it had been instant 2 weeks ago, took 3 tries to fire it up. Took off and out-of-the-blue at 50mph I felt like I was losing a wheel, shuddering effect. I pulled over and checked the wheels for play, all seemed good, went on home 25 miles. Put the bike on a jack and checked the play in the wheels and all is tight.

Any ideas what would cause this on a bike that was perfect 2 weeks ago? Plugs look good, new 8.8 mm accel wires. I have a SE ignition (non adj) that is about 14 years old and original coil. Any Help, tests or advice would be great.

Could it be the shuddering was a sudden power loss or severe stumbling?

It sounds like you might have water in the gas do you run a little alcohol in the tank or treatment every once in a while?

Have you removed the petcock and cleaned the filter lately?

Easiest way to check if it is fuel related is to make sure you have both fuel flow by removing the air cleaner cover and observing for accelerator pump squirt when you twist the throttle. If you have a good squirt you have fuel. Check your plugs for fouling and check for a good hot spark.

Get back to us...

Thanks CD​

New Petcock (DP), full tank Chevron 2 weeks old same station as always premium grade. Never used fuel conditioner before. The plugs look a little on the lean side, mostly dirty white with tan/brown on one side of Porcelain. I only checked them after pulling into the carport so may not be a good reading. Got a good fuel flow. Filter is new Mikuni K&N also.​

Best way to describe is it really feels like something is out of balance, vibration/shudder feel, all wheel weights still in place, Just felt Squirrely, I will check it out again this weekend.​

What are the symptoms of a bad coil and ignition module?​

How do I check the spark?​

It could be when you changed the carb out you may have ended up with an air leak in the intake, also check the hose that goes to your vom if it is leaking or come off it could cause the problems you are getting. Dutch

Thanks Dutch,

I will use some Carb cleaner to check for leaks. Right stuff?

I am dumb, what is the VOM?

I'm sorry hit wrong letter should be vos it senses vacuum and sends signal to ignition module. If bike is still stock it should be located under tank near choke bracket you should see small air hose running to it. Dutch

shudder with new mikuni

Couple of things.

1. Are you using the enrichener at cold startup? If not, could be cause of starting issues requiring to "twist" the throttle to get gas in the motor before starting.

2. If you use the enrichener when warm it could flood and cause no or hard start condition.

3. As far as a simple spark check, once the shudder happens pull over, get off bike and put your hand behind each of the exhaust pipe outlets and rev the bike. If you feel cooler air out of one pipe or the other you are not getting spark to that cylinder. IHowever, it's odd that it comes on all of a sudden.

4. Of course, you could pull off the spark plug wire while running. If no appreciable change in motor response then that spark circuit is faulty. If the spark is good, 40k volts should get your attention as well using this technique!

Concur with Goliath. check your vacuum system for leaks, especially sensor related....those things are a PITA enough on cages, yanno?

scoot

Thanks for the Tips...

I adjusted the primary chain today, had about an inch of play cold. Adjusted to spec. Checked torque on wheels, pipes and most other points I thought needed.

Studdering may not be a good description. The feel is more of a vibration issue. The best way to explain in words would be, The normal vibration I had at 80 mph I now have at 60 mph. This is what came on quickly.

I can feel it in the floor boards and see it in the mirrors. Does that bring on any ideas as to what I should check? I sure could use some advice.

On the 92 I do not think (I really do not know) that I have too many vacuum/sensor points to check. Do you have any Specific's I should check. I do think the problem could be a timing/air/fuel issue. I hope it is that simple.

Thanks for the help....

Did you check the top engine mount for cracks and being tight? A broken mount causes some wierd vibes.