Hello and thanks in advance for any help. I have a very well used moto 4 i'm in way too deep on. Just replaced the head with a Yamaha one off ebay, and amazon cylinder due to it smoking and running like crap (cam and bearing were gone). I have had the carb off count less times, cleaned, put new kit in..jets are clean including side holes. Pilot screw is backed out 2 turns.

As stated, the unit will start, doesn't have knocks or bad sounds coming from engine but it back fires, won't idle like a normal motor should, have to use the choke to keep running, or keep rpm up high to keep from stalling. I haven't put much time on it riding yet but it seems fine when the RPM are up on the short ride i did.

Two big questions.
1. What are the symptons of a cracked carb boot? Is there any way to test it easily ? The one i have does have a split in the joint of the flange, but i'm not sure if it effects it or just a normal small imperfection in a old part, it's very small to say the least. When these crack are they slits or big cracks that are obvious?

2. I have pictures of the timing marks and don't feel they are lining up as well as they should. In the manual and online i read about the T mark which is what i used. There is also a line right after the T which makes me wonder if that's the line. The Line on the top of the gear which is supposed to be lined up with the mark seems out by 1/2 link (see pics). Is this normal? Should i try setting it to the other side of the Mark to see if it makes a difference? I also took a pic of the original flywheel with the same marks as the one installed. In the manuals they show a line under the "T" mark, clearly mine don't have that. Is this close enough or should it be moved over a tooth? Pictures where taken with Tension-er Installed and set. One final note, i do realize in the pic the T looks a bit off, even when it's perfect it still appears that the top mark is still off by 1/2 a link.





Thanks again.
Brian
Toronto, Canada

 

It is the line beside the T mark that you set the timing by. The one by the F would show up if you put a strobe on it for ignition timing, don't know why they bother, as the spark has to be "right" by the flywheel because it is fired by the flywheel. You can only set the cam to the nearest tooth on the sprocket. If they are half a tooth out, I set them so as the chain wears and stretches it will get nearer to "right".

By "boot" do you mean the inlet manifold? I have never done it myself but some recommend spraying starter fluid onto the crack in the manifold, with engine idling. If revs alter until the fluid disperses, it must be getting through the crack into the engine.

Make sure the float level is correct and if it still won't run right, try altering the needle height in the throttle slide, if this has any adjustment. If this improves things, alter the idle mixture screw to get a better idle. I have done a couple of Yam 250s with the idle mixture adjuster missing altogether. The first ran fine with it missing, so by the "if it ain't broke don't fix it" maxim, I left it alone. The second one ran like crap, so I got a new adjuster needle for it, and it instantly seemed to run right with the two turns out setting.

Backfires are usually mixture well out, either rich or lean, but an air leak into the exhaust can also cause this.

 

thanks for the reply, much appreciated. By intake i do mean between carb/cylinder, will try the starter fluid. I will also try screwing "Out" the slide valve to try and richen up the carb settings. i should also have mentioned i did lap the valves on the new/used head before reassembling although i forgot to test the seal before putting it back together. I also tried a compression test which was only 60 psi, i assumed this was due to the new rings not being broken in.

I will try adjusting the slide tonight, maybe that will do the trick.

Thank Again !

 

Some Days I just want to cry......

so today i went back out, put a new air filter and spark plug in (arrived via amazon today). Buttoned it up, messed with the idle screw a bit and got it running pefect for the 10 min it did. Then it stopped. I had forgot that i had loosened off the cam gear bolt, it backed out and here is the result..... Looks like i'm gonna need to pull off the bottom covers to find the pieces.....and get a new cam For those who don't know.... there is supposed to be a hardened pin where that hole is. Now there are bits, and a pin in the bottom of the engine.

 

I thought i had made a post but i guess not, if this is a repeat sorry.
I fiddled some more after work and finally got it running perfect. Then, the engine failed becuase i had forgot i had loosened the cam gear and it backed off. This caused the gear to break off the pin in the cam shaft along with bits. I am also suspecting this is an interference engine ? I just want to cry lol

 

Thanks for the advice, I appreciate it.