I'm having trouble starting the quad. It turns over fine and eventually makes a loud bang from the front of the exhaust pipe and sparks come out there. The exhaust pipe is hot even tho the motor never really fires up.

The quad has been running well and starting easy and then made a noise that sounded like a branch was stuck in the chain. Was going about 40 MPH at the time and stopped, shut down, checked for branch (nothing), and haven't been able to start it since.

Any pointers would be appreciated.

Thank you.

 

Check compression if you have a gauge. If you don't have one,then check for spark.If has spark pull the carb and the reed cage and look at the piston and skirt. Back firing sounds like two strokes I've found over the years where pistons have seized and some have broken a lower part of the skirt and started back firing.

 

Thank you for the response.

Checked the spark and it was good.

I don't own a compression test guage so I borrowed on from O'Reilly Auto Parts. The guage quickly went to 25 but then stopped climbing there. Stopped trying to start and the guage stayed at 25 until I disconnected the guage. I am guessing that compression should be higher than that but I don't know and I don't know where to look. I have the Clymer manual but I haven't found a range in there yet.

Haven't checked the piston and skirt yet. Will breaks there be obvious? Hope to check that tomorrow.

Thanks again.

 

Might as well tear the top end down. 25 psi is way below the minimum 100 psi that the manual call for. Be sure to check for any trash/metal particles in the bottom end plus check the rod for any up and down play at the crankshaft.Also check for noisy or rough feeling crank bearings as you slowly roll the rod around. That is after you remove the piston. If there is any doubt on the bottom end best to have a new rod kit and crank bearings installed.

 

We finally got the head and cylinder off. We see the bottom of the piston has broken and there is a mess under it. Do you have any recommendations at this point?



We've inspected the cylinder and the walls look good: I don't see any scratches at all in that. We'll clean up all the bits we can find with a vacuum but how much of that crankcase do we need to pull apart to clean?

Thank you for your help and guidance.

 

I tried posting a reply a few hours ago but it appears that it was lost so I'm trying again.

We had some house projects get in the way of this quad and just now got this cylinder apart. I took a picture of the piston and top of the crankcase and have attached it below. There's quite a mess of bits there.

The wall of the cylinder looks good: I don't see any scratches anywhere. The cylinder head looks good. Clearly the piston and all gaskets need to be replaced and we'll take a closer look at the rod tomorrow.

Can you give me any guidance on how to clean the crankcase out? Like do we just vacuum the bits out that we can find or do we need to get it all apart and make sure every shaving it out of there? I don't know how sensitive that is down in there.

Thank you for your help.

 

Here's another update. My previous updates failed to go - perhaps because I attached pictures so I won't put in pictures this time.

We pulled the piston and cylinder off and found that the skirt of the piston was in a million little bits on, and in, the crankcase. We cleaned the crankcase out to the point that it felt like everything was moving well. The rod has no up and down play. The cylinder head was very clean. The old piston had some chips out of the top but really the only obvious damage was the skirt.

It felt like the crankcase was clean and the rod was moving okay so we put it back together. After getting the main bits attached we did a compression test and that was good. We put everything else back on and fired it up easily. There are certainly still some bits in the crankcase because the engine was purring nicely idling but there was some scratching / gravelly noise.

We're going to take it apart again but I don't know how to really get that crankcase apart and clean it out. Are there any 'bottom end' kits like the nice top end kits that have all the gaskets and everything? I've read that the crankcase is really difficult to get apart and that it should be taken to someone for that - can this bottom end work be done reasonably at home? Are there any tools you'd recommend to make it more doable?

Thank you.

 

The manual is really the first thing you need.http://visit.tradebit.com/visit.php/...75/264162958/9 if you don't have one. And yes this is a complicated engine if it has to be torn down,but if taken step by step from the manual you can do if fairly easy. The engine has 3 cases and also houses a counter balancer and internal water pump. Dirt Cheap Yamaha, Polaris, Suzuki & Kawasaki OEM Parts & Accessories – Cycle Parts Warehouse Several different full bearing and gasket kits on ebay along with just gasket kits if you want to buy what bearings you need elsewhere. New Polaris 400L 400 Complete Engine Rebuild Kit Crankshaft Bearing Seals Gasket | eBay You're doing the right thing if you have a new top end installed as any particles in the lower engine can take out a new piston and rings. Tools such as the clutch puller you can make with a 3/4x16 pitch bolt. Other tools such as the clutch spider tool are on ebay. Counter balancer puller is offered here,plus some have made their own. Counter Balancer Puller Not necessary to have the slotted nut socket as a punch will work.

 

Thank you again!

We have the Haynes and Clymer manuals for the 1994. Sometimes they will differ a little (e.g. Clymer says to use Loctite 515 on base cylinder gasket but Haynes doesn't). I think both warned not to install the clip that hold the piston pin in without a tool but we couldn't actually find an install tool anywhere (for that we ended up using youtube videos and needlenose pliers). Having the different pictures from both manuals has been helpful.

I really appreciate the guidance you've given us.

 

Don't need the piston circlip tool.. Place rags around the cylinder so it doesn't drop in the engine. You can install one piston circlip on the bench if you wish. Easiest way for me(IF you have good thumbs also) Place one end of the circlip in the groove,place your finger at the bottom of the circlip,hold pressure and as you push with your thumb,use a small pick to push the other end into the piston groove(don't distort the circlip!) and it will snap in place. Always have the open end at top or bottom,NOT sideways as rpm from engine can compress them and sling them out.. Not good for pistons or cylinder walls when they get caught.