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Just rebuilt, won't start

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Freshly rebuilt - from the crank up. Won't start - just a nasty back fire every few seconds on the electric starter. (Yes, this is an R, but with an outboard starter)

Swapped in the unrebuilt engine and it starts right up. This says the carb, CDI, coil and wiring harness, and the fuel supply is all good.

That leaves the Stator, Flywheel, and then the hard parts only. Swapped the stator and no change.

Swapped back in the unrebuilt engine and it still starts right up.

The valve timing is correct - visually matched to the engine that runs. Valve adjustment is right.

What am I missing on the rebuilt engine that would cause this???

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Regardless of your comparison, I would check the timing, clearances including the auto decompressor again.

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Carb slide, smaller flat part in upside down? Hole goes on the bottom.

He used the same carb on the other engine.

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You may be off on the cam timing.

take the cam sprocket off, rotate the crank one rotation back to the timing mark, reset the cam sprocket and it should cure the problem. I had the same issue with an 02 that belonged to a buddy. he went through the same process you are describing, he brought to me and I had it going in about 45 Min with this fix.

Just my 2 Cents.

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OK, tried that took off the cam gear, turned the engine one revolution and remounted the cam gear. No joy.

I wasn't sure why that would work anyway as there have been numerous threads about the ignition not knowing compression stroke from exhaust stroke to know which TDC to fire, that there is a spark every revolution.

Anyway, I have checked things over several times, and still, swapping back to the base engine and it runs, and the rebuilt one doesn't.

The only thing I can think of that is active now that hasn't been changed is the flywheel. Or, I have two bad stator/ignition sensor assys here, one bad on the engine I bought and rebuilt, and one bad on the engine I was just running last season that tossed a rod.

What a mystery.

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OK, tried that took off the cam gear, turned the engine one revolution and remounted the cam gear. No joy.

What a mystery.

What if you are off 1 or 2 teeth?

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Understand that is a common problem. We blew the base engine (which is the currently "running" engine) twice last season due to a shop screwing up the timing before I knew much about the engine. I have since become pretty much expert at cam timing. Using a piston indicator, the left side timing marks, and the cam gear marks. Like I said in the first post, the running engine (which has not been touched, and has been raced) and the non-running engine, which was just rebuilt, side-by-side, have identical timing for the cam, versus the timing marks, and the piston position.

I just tore down the left side again, as that is the only "active component" area, to look at the flywheel. I was holding the flywheel from the engine that threw the rod in my hand, and noticed they were different.

I found the part numbers on the flywheels, and the one in my hand, from a '02 motor is part number - MEBA 0032000-9812 followed by 8CX

The one on the non-running engine ('04) is - MENA 132000-0052 followed by 9EZ

So this is a Honda part, Service Honda shows the same part is used for 02-04, 31100-MEN-000 - so I guess tmw I will be calling Honda to see if I can track this down.

Overall, they look the same as to where the timing magnet is and the timing marks and all.

The non-running engine I purchased used, as a 2004 engine, and I sent it off for rebuild without any testing. It had all of its parts and looked fine. The rebuild was to put in an aftermarket rod and of course new piston, rings, etc. We swapped onto the engine an '02 head that was on the engine that tossed the rod. It had a Hot Cam Stage 1, and kibblewhite valves. The head was not touched by the rod issue. And we took out the Hot Cam and put in a stock '02 cam.

The stator tests with a DVM the same as the one on the running engine.

So I am at a loss at this point, unless it is the flywheel.

To review the information:

Running Engine: '02 with '04 piston, stock cam

Non running engine: '04 with '04 piston, '02 cam, KW valve kit

Common between the two, what is on the kart: Carb, Coil, CDI ('04), Wiring harness ('04, with adapters plugs for '02 stator when needed, like on running engine) and Fuel pump/ fuel supply, and water system.

What is not common: Stator (which we swapped an "04 and replaced it with an '02 on hand from a previously running engine), Flywheel and hard parts. Spark plug replaced, swapped several times, etc.

The third engine I mention from time-to-time is an '02 that I bought from the Classifieds here, it had the worked head, with the Hot Cam and KW Valve kit. We ran it for about 3 races, and then it threw the rod. Looks like bottom bearing failure, or, the rod itself just let go, the bottom end of the rod that wraps around the bearing broke into three pieces, and the cage was severely mangled. The rod buried itself in the case where the counterbalance shaft runs from side to side. The piston was cocked in the bottom of the cylinder and damaged the cylinder a bit. It might even be below where the piston even goes normally. But like I said, the head wasn't touched. The valves were perfect. And that is the head on the non-running engine. The head from this engine is being worked for a valve kit. I am trying to decide between KW and RHC. But that isn't the point right now.

Thanks for all your time.

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Well, it sounds like you know what you're doing. I hope this doesn't come across the wrong way, but it happens. Are you using a known good spark plug in the rebuilt engine? Have you done the voltage and resistance checks on the other electrical parts mentioned?

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I have swapped out the plug a couple of times. I put a new plug in the running engine last time it was in, and it was fine, and then have used that plug since.

So today, I changed the flywheel, no change to the problem.

I swapped out the '02 cam, and put back in the Hot Cam that was with this head, no change, although I didn't reshim and the exhaust was about .0015 tighter than before. The intakes were unchanged at a very loose .005

That's it, I have changed everything except the entire head, and I may do that tmw after I do some talking with Honda about the flywheels, and the engine builder that did the rebuild. He has been kept up to date by email, but he is just as stumped as I am. All of the "usual suspects" are what stays on the kart and are used by both engines.

Done for tonight.

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When you were turning it over did the exhaust start smelling like it was getting fuel? For kicks you could manually verify brightness of the spark flame that is hopefully jumping the spark gap. I think the real test would be done with a scope but if there were no spark for example it would help point in a direction.

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I would make sure that when you plug the stator wires into the harness that you don't get the 2 blue wires mixed up. On the 02's they both had female bullet connectors and it was very easy to make this mistake. I would assume that you have done something to accommodate using the two different year stators with whatever harness you are using on the chassis. One of the blue wires has a solid yellow line running down it and the other blue wire does not. I learned this lesson the hard way once.

Tex Mitchell

Baja Designs

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Well, the winner is Firedude. My engine builder and I got together this afternoon, and with all of the other troubleshooting - - which to finish Sunday, swapped the flywheel, no change. Swapped back in the Hot Cam, just to see if the '02 cam I picked up was messed up, no change. Stuck the running engine back into the kart and it fired right up, as usual.

So, at the end of yesterday, and this AM, I was back to just the core engine, and so the engine builder and I agreed that I should bring it in for a leakdown test and we found an exhaust valve leaking, rather badly. This head was on the engine that tossed a rod, and although both the piston and the head/valves have no witness marks of things colliding, it would seem that there was enough collision when the rod let go to tweak at least one of the exhaust valves. So, shame on us for not checking that closer prior to reassembly.

We stuck on the spare, stock, 04 head we had removed from this motor and the leakdown was perfect. So, the engine is now all setup with that head, the '02 cam, and reshimmed and all that. We had planned to rebuild that head before using it, but we have a race this weekend, and no more time. So, we will have to hope that there is not a pending valve failure or anything, but it is properly adjusted. I will be sticking it on the kart in the AM and testing it, and breaking it in, and the doing all of the final setup and packing for the weekend.

Thanks for all the suggestions and comments.

And yes, I am very careful with the wiring - always - and, I have 2 really short jumpers with male/male connectors that adapt the 02 stator assy to the 04 ignition. Now that all of this is sorted, I will be moving to all '04 electrics as I can to eliminate this little thing.

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Good call firedude. I assumed you lapped and blued the valves when you rebuilt the motor so I didn't consider a valve leak. Glad you found the problem.

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