Jump to content
Sign in to follow this  

Backfiring/popping after auto-decomp cam upgrade & pipe/jetting switch

Recommended Posts

I posted this in the jetting section, but I'd like to see if anyone here has any feedback.

I committed a cardinal sin and modified more than one item at the same time. I upgraded to the auto-decompression cam in a 2002 YZ 250F. At the same time switched back to the stock pipe and changed the pilot jet back to stock at 42. The bike now will not idle without the choke. With the choke it runs fine, revs and can actually be ridden in 1st without backfiring. After the choke is pushed in, the bike starts to backfire/pop and sputters out to a stall. The bike ran decent before with an FMF pipe but had a few hanging idle issues and the occasional stall; hence I was switching back to the stock pipe and stock pilot jet.

At this point I'm not sure if I have a carb issue (clogged PJ?), an exhaust leak (I put in new mid-pipe washer and tightened it up well) or messed up timing?

I counted the pins (12) when I put the cam back in and the bike is ride able in first gear with the choke, therefore I think it’s likely a jetting/exhaust issue not timing. Maybe an exhaust leak at the head pipe was created from the flex of switching the pipes.

Does anyone know what the effects of messed up timing would be? I will continue investigating, but appreciate any feedback/experiences.

Thanks,

Ian

Share this post


Link to post
Share on other sites

It sounds like its too lean. Put in a larger pilot jet (45) and see if that cures it. If it still sputters, and has a hanging idle adjust the fuel screw slightly richer.

Share this post


Link to post
Share on other sites

To start with, you don't time the cams in a YZ250F by counting pins. You use the timing marks on the cams. The only exception is when re-timing an older, non-auto decomp cam from WR to YZ or vice versa. 12 is the right number, so if everything is lined up right, that at least is OK.

If it won't run off the choke, you may have either blocked the pilot jet, not seated it correctly, or slipped up in some other way, such as putting the slide plate on upside down. Check your work

Share this post


Link to post
Share on other sites
To start with, you don't time the cams in a YZ250F by counting pins. You use the timing marks on the cams. The only exception is when re-timing an older, non-auto decomp cam from WR to YZ or vice versa. 12 is the right number, so if everything is lined up right, that at least is OK.

If it won't run off the choke, you may have either blocked the pilot jet, not seated it correctly, or slipped up in some other way, such as putting the slide plate on upside down. Check your work

Gray, why do you keep saying to NOT count the pins? :thumbsup:

The Technical Bulletin M2006-009 from YAMAHA clearly states that "there should be 12 pins between the two vertical marks on the cam sprockets". :ride:

See item 6 in this link. This is page four of the Technical Bulletin M2006-009. I don't mean to offend you since we appreciate your wealth of knowledge about engines in general but I think we should trust Yamaha and count the pins on our YZFs.

Share this post


Link to post
Share on other sites

What if you put the crankshaft at bottom dead center and put 12 pins between the two cams? Would the timing be right?

The only thing the inter-cam pin count can possibly tell you is whether the two cams are position correctly in relation ship to each other. What is important is their relationship to the crankshaft position. If each cam is timed correctly with the crank, the pin count will be correct. As such, it is useful only as a double check method. Note that in item 5, the cams are timed using the marks on the sprockets (referring to figure 2). Item 6 suggests that the pin count be used to "verify correct timing" between the cams.

Otherwise, as I said, it is only useful in situations such as altering the time of older, non-AD cams, or in the installation of a 450 cam in a 400/426, in which case the factory timing marks on the cam don't work with the older heads.

I don't mean to offend you
I'm not that touchy, and you aren't... well you know who. If you're willing to discuss the matter reasonably, you can challenge anything I post. That's part of the idea, no?

Share this post


Link to post
Share on other sites
What if you put the crankshaft at bottom dead center and put 12 pins between the two cams? Would the timing be right?

The only thing the inter-cam pin count can possibly tell you is whether the two cams are position correctly in relation ship to each other. What is important is their relationship to the crankshaft position. If each cam is timed correctly with the crank, the pin count will be correct. As such, it is useful only as a double check method. Note that in item 5, the cams are timed using the marks on the sprockets (referring to figure 2). Item 6 suggests that the pin count be used to "verify correct timing" between the cams.

100% correct :thumbsup: ,Use the 12 pin count only for double checking after You have TDC on the comporession stroke with the cams and they are timed correctly(punch marks line up),then I count the pins.

I know you know this,but this is for anyone else.You can be TDC with the flywheel position.Then you think you have the cams lined up and on the compression stroke,but both cams can be 1 tooth off, 1 tooth advanced too much on the exhaust cam and 1 tooth retarded on the intake.This will give you the 12 pin count,but the cam punch marks will be off.

Share this post


Link to post
Share on other sites

I'm not that touchy, and you aren't... well you know who

I think I know who your referring to :thumbsup:

Share this post


Link to post
Share on other sites
I'm not that touchy, and you aren't... well you know who

I think I know who your referring to :ride:

Actually I don't know who Gray is talking about... :thumbsup:

Share this post


Link to post
Share on other sites
You can be TDC with the flywheel position.Then you think you have the cams lined up and on the compression stroke,but both cams can be 1 tooth off, 1 tooth advanced too much on the exhaust cam and 1 tooth retarded on the intake.This will give you the 12 pin count,but the cam punch marks will be off.
If both cams were retarded or advanced as a set, one timing mark would be above the head, and the other below. This may be what you're thinking of. But in order for there to be 12 teeth between the two 12:00 o'clock marks, they have to both be rotated the same direction.

In the scenario you suggest, the exhaust cam is advanced one tooth, which means it is rotated one tooth too far forward relative to the crank, so that it gets where it's going in advance of the correct time. Then you have the intake cam retarded one tooth, or rotated back one from the correct spot, so it's late opening the valves. That would result in there being 14 pins between the top punch marks, not 12.

Share this post


Link to post
Share on other sites

I stand corrected,you can have the 12 pin count with both cams either retarded or advanced 1 tooth each. Not 1 cam retarded and the other advanced.I was thinking 1 tooth advanced and 1 retarded would cancel each other out.Very good. :thumbsup:

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  

×