Spark plugs and lean mix

[Stickysteve44]

Noticed a lot of bluing on the left side header. My ir thermometer says it's running about 30-50 degrees hotter. About to sync carbs and adjust mix screws. Fortunately a previous owner removed the plugs over my mix screws. That's pretty exciting as I was worried I'd drill in to the screw trying to remove them.

Anyways, it came with bp8 plugs which seemed to be fouled and a bit cold. The bp7's I put in seem to be too hot. Should I switch back to the bp8's or stay with 7's

Top pic is 8 bottom is 7. Left cylinder on left, right on right.

I obviously need to sync and adjust mix first.

 

[5twins]

The correct plug is the BP7ES. The "P" stands for a projected tip. Your 7's look correct. Your 8's, besides being the wrong number, also appear to be the plain "B" type plug without the projected tip.

Yes, you need to sync the carbs, that's why one side is running hotter than the other. It may also partially account for the difference in plug colors too.

 

[5twins]

Also, make sure your plugs are gapped correctly. With the stock ignition and coil, you would want them at .028" to about .030". Don't go above .032". New plugs often come gapped for cars which can be in the 40s. That's too big for your bike.

 

[Bushyeyed1]

The plugs for our bike are dirt cheap so get some new ones. Like 5twins mentioned sync you carbs that may save you from having to remove and disassemble the carbs. You could probably check your valves while its cold also before you do that as they may be causing a rich condition. If all that is good then the next thing I would check is your float level.

 

[Stickysteve44]

The plugs for our bike are dirt cheap so get some new ones. Like 5twins mentioned sync you carbs that may save you from having to remove and disassemble the carbs. You could probably check your valves while its cold also before you do that as they may be causing a rich condition. If all that is good then the next thing I would check is your float level.

I did the valves. Still ran hot on the left side. I synced the carbs just now and adjusted the mix screws back to stock. Just rode a bit. Waiting for the bike to cool down a little to check the plugs. The left header is closer to the right now. Still a bit more hot. But not much. The BP7's are new.

The correct plug is the BP7ES. The "P" stands for a projected tip. Your 7's look correct. Your 8's, besides being the wrong number, also appear to be the plain "B" type plug without the projected tip.

Yes, you need to sync the carbs, that's why one side is running hotter than the other. It may also partially account for the difference in plug colors too.

And you're right 5Twins. Those were just plain B8's. What's the difference in how they run vs the p types?

 

[5twins]

The "P" type are supposed to give a better burn because they stick out more into the combustion chamber and the firing tip isn't as blocked by the threaded part of the plug.

 

[Stickysteve44]

Here are the bp7's after everything was adjusted and another short ride.

 

[Bushyeyed1]

That sure looks better.

 

[Stickysteve44]

Thanks for the pic. Haven't seen that one yet. These plugs are brand new (probably about 10 miles) and I've only rode in the city so far. The low rpms and constant idling at stoplights probably effects my plug pulls right? I'm going to go for a long ride tomorrow and recheck.

 

[Bushyeyed1]

Pretty much three throttle ranges to check plugs. Idle to 1/4 throttle is your air fuel screw and your pilot jet. 1/4 to 3/4 throttle is your needle jet. And 3/4 to full throttle is the main jet. There could be other conditions though that add to being rich or lean though. It is hard to get your plugs perfect because even temperatures and pressure effect the conditions. I don’t think the picture of the plugs now look bad Better to be a little rich than lean. Maybe just turn in the fuel mixture screw on the left plug in an 1/8 of a turn.

 

[Stickysteve44]

The previous owner put #45 pilot jets in. He gave me the old ones which were #30.. Stock for Bs34s is #42.5 if I'm not mistaken. I'm wondering why they were so lean before and why the po put jets in with such a large difference.

But I feel like it may be a bit rich at lower throttle.

 

[Bushyeyed1]

Stock main jets are #132.5 stock pilot jets are #42.5.

 

[Stickysteve44]

Stock main jets are #132.5 stock pilot jets are #42.5.

That's what I thought. The previous owner went to #45 and #135. He swapped out the air filters however. And the bike feels great along the entire power band. But the pilot jets that he swapped out were super low.

 

[Bushyeyed1]

Try reading the plugs after running at idle then adjust the air fuel mix screw in relation to what you read. Turning in the screw leans and turning out richens. After you get that good ride it at About 1/4 Throttle cut it off and read plugs. If you don’t feel a stumble at 1/4 while your cruising at low speeds you could probably leave the 45 pilot and one size up on the main is fine with the 135. Yeah 30 isn’t stock unless those carbs were on another model Yamaha. Wonder what your needle looks like?

 

[Stickysteve44]

So the bike runs great when it's on. But cold starts now are extremely difficult. I have the mix screws set to 3 turns out. The carb guide states 3-3 1/2 for bs34s. If I turned it out more would that help a bit with the cold start?

 

[DirtyErnie]

Once the bike is hot, then start adjusting the mix screws. You're probably plenty far out on the turns, maybe too far. turn them in until the rpm picks up, and then adjust the throttle stop to slow the engine down. You want to find the peak RPM for each screw, and then maybe nudge them 1/4 turn out or somesuch to be safe.

As far as heat range goes, You're going to have to get in a good pull at wide-open throttle. Up a hill in 3rd gear is my favorite. If your heat range is right and mix is close, you should see about 1/4-1/3rd of the threads on your plugs get discolored. You'll never see that at low power levels, just riding around.

Check the fuel levels in your float bowls. Make sure that's at the bowl/body seam before you get too deep into worrying about jets. Trust me on this, I lost most of last summer and a pile of money on jets chasing foo-birds last summer when I should have been cleaning carbs and tweeking floats. Should have just spent the money at the Liquor Store (and florist, would have avoided some nasty glares from She Who Must Be Obeyed.)

 

[Bushyeyed1]

So the bike runs great when it's on. But cold starts now are extremely difficult. I have the mix screws set to 3 turns out. The carb guide states 3-3 1/2 for bs34s. If I turned it out more would that help a bit with the cold start?

It is my opinion but for your nearly stock jetting somewhere around 3 to 3 1/2 turn out would be fine. The most you can safely turn the mix screw out is 4 turns. When I have my jetting and fuel air mix close to perfect I always have to use my choke on the first cold start. I don’t have to leave it fully extended long and usually push it in mid way after it starts. If you didn’t ever have to use your choke then you would probably be on the rich side of the spectrum. From the looks of your plugs in the last picture I would almost call it good. If at idle you rev it and it doesn’t hang (lean) or bog down and back to idle (rich) and when you ride around 1/4 throttle and it doesn’t bog or jutter and finally when you open it wide open and no juttering past 4500 rpm call it good. Better to be a little rich than lean on the plugs.

 

[5twins]

If you started the bike cold using the choke, that will quickly blacken the plugs. Then it will take a few miles of riding to get them burned clean again so a reading can be taken.

The 3 to 3.5 turns out works well with stock jetting on an all stock machine but once you start modding and increase the pilot jet size, you can dial the setting in more, to like 2.5 or so. The porcelain on the plugs will stay pretty darkly colored if you don't. Ideally, you want a ring of color on the porcelain only down at it's bottom. It's difficult to see down there but your plugs are obviously colored much higher up.

I like to try to get mine coloring about the bottom quarter of the porcelain. As I mentioned, dialing your mix screws in will clean up your porcelain more.

 

[5twins]

Also, with today's modern fuels, you don't always get shades of brown on your plugs, you're dealing more with shades of gray.

 

[Stickysteve44]

Gotcha. It was starting right up first kick without the choke before I tuned everything and swapped the plugs to the bp7s. I figured out I just need a priming kick (and choke of course) now. I'm just a little dumb that's all. Haha