View Full Version : Any ATV "experts" here?

08-30-2011, 06:07 PM
I'm right in the middle of putting a timing belt and water pump on one of my cars and my son is asking me for help on his 4 wheeler. It's a 2002 Yamaha 660R Raptor and the carburetor looks Greek to me. It stopped running one day and now it cranks but won't fire up. He can roll start it and it runs fine, then after it warms up for a few minutes it will fire right up with the electric start. Sounds like a choke problem to me but when I took the caps off of the carbs I had no idea what I was looking at. When it was running, I could adjust the idle set screw and the butterflies worked properly. The choke cable runs some sort of plunger and I can see the plunger thru a little hole in one of the carb throats. I see no hole in the throat of the other carb. The plunger goes in one side of the carb and there is a spacer (item #3) between the two carbs. It looks like there is a long screw (item #4) on the opposite end of the cable.

If no one knows about these things, my only option is to remove the carbs and take them apart to figure out how they work (or have him take it to a mechanic :rolleyes:). One of his buddies said that these "racing" machines should not be able to roll start in 1st gear due to the high compression, so he must have a ring/piston problem. I don't know about all of that because it runs fine once it roll starts (in 1st gear). Does anyone know how the choke works on these things??? I can get a picture of the plunger too.

08-30-2011, 08:15 PM
sounds to be low on compression and the starter doesn't spin the engine fast enough when cold, where the push starting in 1st gear does spin it fast enough. then after it warms up, the compression levels are high enough with the expansion due to the heat. might be time to hone and replace rings (and/or pistons)... just a gut feel but I've been working on similar off-road motorcycle engines for over 40 years. another great site to see various mechanical schematics similar to the one you posted is bike bandit http://www.bikebandit.com/houseofmotorcycles/2002-yamaha-yfm660rp-raptor/o/m8701#sch251172 for future reference.

08-31-2011, 10:16 AM
Thanks for the response. I will check the compression but I guess I will need some sort of extension to get my gauge down inside the head??? I'm also going to give it a small shot of starter fluid when it's cold and see what happens.

08-31-2011, 10:51 AM
depending on the head configuration, you might need some form of specialty compression tester. Most of the later model dirt bikes requires a tester with a flexible hose to get down to the proper depth of the cavity in the head.

08-31-2011, 11:41 AM
agree that checking compression would be the first place to start. I have found some PWC forums quite useful in helping me figure out my boys jet ski. Maybe you could have some luck poking through an ATV forum?

08-31-2011, 12:54 PM
My compression tester does have a long flexible hose. I wasn't sure if I could get it screwed in tight enough to avoid any leak in compression and still be able to remove it. All I will do is check the compression and the choke for my son. We are trying to help him out by letting him move back home and save some money. He already has two 4 wheelers and last week he bought a motorcyle.:mad: But that is a whole different story.