Jump to content
Sign in to follow this  

Compression Release and Starting?

Recommended Posts

I just put a Mikuni pumper on my xr4 in an attempt to get rid of the flat spot. Hasn't helped much but I'm still tuning the carb with the "advice" of XR's Only (that's another story).

The bike used to start with a couple kicks. Now, it won't start cold, warm, or hot unless I pull in the compression lever kick it a few times and then let the lever out and give it a good kick. If it doesn't fire up on the first kick, I have to do the same thing again. It's a pain in the ass. Any ideas what's going on here? 😢

Share this post


Link to post
Share on other sites

Do you twist the throttle any when you're trying to start it? When you twist the throttle, even a little bit, you shoot fuel into the carb. If you're not paying attention to what you're doing, you could be flooding it before you even get it started. Pulling the decomp lever opens the right exhaust valve and when you kick the engine over, you're flushing out this flooded condition. You need to talk to someone with a pumper carb to get their starting routine.

Share this post


Link to post
Share on other sites

Several riders with the Mikuni on the XR400 claim starting is easier.

Hard starting can be caused by these problems.

o Old spark plug

o Valves need adjusting.

o Jetting is off.

o Starting procedure is not correct.

Share this post


Link to post
Share on other sites

I have a Mikuni pumper on my XR400. it is REALLY easy to flood. The trick is to not turn the throttle at all until it is running.

Pull the comp release.

Kick 3-4 times easily.

Let go of the comp release and kick it to start. Mine starts on first or second kick.

The only time I have difficulty is when the engine is between hot and cold. No partial choke is my only complaint with the Mikuni.

Share this post


Link to post
Share on other sites

To start my 400 with the mikuni pumper I hold the kill switch, pull the decompression lever, and kick the bike through about 5 or 6 times to get a good air / fuel charge into the cylinder. Next, if it is freezing cold outside or the bike has been sitting for a few days I pull the choke out, if not, I leave it in. I slowly kick the bike thru until I reach TDC, then, with no throttle applied, I give it a good boot. Usually starts first or second kick. That Pumper really should take care of the flatspot on your machine. I have no bog at any throttle position under any conditions. What is your elevation, and what are you running for jetting / fuel screw setting? Any other engine / exhaust mods? I have not had any problems with mine, and when I installed it and rode it for the first time, it was like a completely different bike - it made all the difference in the world. Kevin

Share this post


Link to post
Share on other sites

Another item I forgot.

Compression release cable is pulling on the compression release.

Make sure the rocker arm on the side of the engine is full back and

there is some play in the cable.

Share this post


Link to post
Share on other sites

I ride at 3000 to 8000 feet. The only mod's to the bike, other than the Mikuni, are a foam Uni filter with the airbox snorkle removed, and a "Competition Baffle" purchased from Baja Designs. The carb came from XR's Only supposedly jetted by the owner for my riding altitude and for my bike specifications with a 12.5 P and a 125 M. The bike does seem to be a bit more responsive on the lower end but nothing significant. Also if I wick the throttle when the engine is at an idle it wil bog severely.

Share this post


Link to post
Share on other sites

Here in Connecticut (sea level to roughly 1200 feet) I am jetted with a 15P, 130 main, so I think that you are in the ballpark. What about your fuel screw? My machine is particularly sensitive to even the most minor fuel screw adjustments. I am at 2 turns out and my bike is pretty happy with that setting. If I go with 1 and 1/2 turns out, I will develop a slight hesitation on hills. Additionally, there is one other item that is very important to adjust on the mikuni pumper - the pumper actuator. On the rider's right side of the carb you will see a white plastic lever that depresses the pumper rod when you twist the throttle. Attached to that lever is a small phillips head screw that adjusts the timing of the pumper (there is a spring on the screw to keep tension on it so it won't back out). You need to turn that screw all the way in until the spring is neatly compressed, not all crunched up, just neatly compressed. When you do this, as an end result, you should have about a 2 or 3 mm gap between the actuator arm and the pumper rod itself. When I initially installed my pumper, I had a peculiar bog on the bottom end. I called XR's Only and they told me to check that adjustment, and sure enough, that screw had backed out and thrown things off. I adjusted it properly, and it has been prefect ever since. Additionally, I put loctite on that screw to help keep it from moving. Kev_XR is correct - that bike should take off when you twist the throttle. Keep in touch. Kevin

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×
×
  • Create New...