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Discussion Starter #1
I'm trying to get my 72 Honda SL125 to start. It hasn't run since I bought it. I changed the battery and points and now I've got a nice strong blue spark. It had low compression so I changed the piston, rings and cylinder and had the head resurfaced and lapped the valves. gas wasn't flowing into the carburetor so i sprayed carb cleaner down the fuel inlet until some gunk came out. Now fuel is flowing into the carb. After trying to kick start it a couple times it kicked back and almost sprained my ankle. now I'm wearing boots but it still isn't starting. If kick it slow, sometimes it makes a pop and smoke comes out the exhaust. Getting this bike to run has been much more difficult that I expected but I feel like I might be getting close. I'm not sure what I should try next. Any suggestions?
 

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Have you actually take the carb off and dismantled/cleaned it?
 

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Yes I took the carburetor all apart and cleaned it. There was just the one inlet where a bunch of dirt came out but everything else looked like new. I was thinking about just buying a new carb, they're only $55 shipped. Are there any tests I can do on the one I have now?
 

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Personally, I would shoot a very short shot of starter fluid into the carb throat and kick the engine over. If it starts briefly you know it's a fuel issue.

Not being there, my suspicion is some of the jets and smaller passages in the carb being plugged still..

Not a bad idea to double double check valve timing and clearance are OK. (Probably do that before the starter fluid test.)

What's the quality like on that $55 carburetor?
 

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You could also check your spark plug. After that much kicking it should be wet if it's getting fuel. I also suspect carb is still plugged.
 

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Did you check and set the ignition timing when you installed the new points? Also, I would double check that the cam timing is correct after doing a top end rebuild. Having a nice spark won't help if it is sparking at the wrong time!! Let us know...
 

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Discussion Starter #7
I tried to start it with a shot of starter fluid but didn't notice an improvement. I also checked the spark plug and it was wet and smelled like gasoline. Does that rule out the carburetor as the sole source of my problem?

I believe I had the timing and points set right but I'll double check tonight.

When I pulled the plug to double check the spark today I noticed the spark seemed kind of sporadic. It looked like it might be sparking more than once per 2 revolutions. I'm thinking that might be a problem.

I'd also like to verify my compression is good after the work I did on the cylinder and head. I've owned two compression gauges but neither of them has ever given me a good reading. Am I just unlucky or is a $25 compression gauge never going to work well?
 

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What do you mean by "a good reading"? I own both cheap and expensive set, and while the cheap isn't the same quality, it generally gets me pretty darn close.. Remember, throttle open and kick it until the PSI plateaus.. several kicks. What's it giving you?

Based on what you said about the starter fluid test, I would concentrate on things other than the carb for a bit..
 

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I also own both a Snap-On brand and a Harbor Freight generic, and the cheap one works better in my opinion.
I agree with Cstart. I would double check your electrical first. Before the plug got wet you said you had strong spark so to me it sounds like a timing issue.
 

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I tried to start it with a shot of starter fluid but didn't notice an improvement. I also checked the spark plug and it was wet and smelled like gasoline. Does that rule out the carburetor as the sole source of my problem?

I believe I had the timing and points set right but I'll double check tonight.

When I pulled the plug to double check the spark today I noticed the spark seemed kind of sporadic. It looked like it might be sparking more than once per 2 revolutions. I'm thinking that might be a problem.

I'd also like to verify my compression is good after the work I did on the cylinder and head. I've owned two compression gauges but neither of them has ever given me a good reading. Am I just unlucky or is a $25 compression gauge never going to work well?
For now... yes, I would rule out the carb. If it didn't pop over a few times on starting fluid, and the plug is wet with gas, I would assume you have either a timing issue or a compression issue.

As for your observation about it sparking too often, that's because it actual sparks at the top of the compression stroke and the top of the exhaust stroke. That is normal. What you need to determine now is if it is sparking at the right time, and is there adequate compression. Personally, I didn't know that a compression tester can be bought for $25... Nor would I buy one at that price. My cheapest one came from Harbor Freight for like $59, and my best is Matco for $250... But like Jag13 already pointed out, I trust the Harbor Freight one the most.

If the compression is at least 90psi or better, then compression is not the issue. But what a lot of newbies do on top end rebuild is that they set the cam timing 180 degrees out of phase. Meaning, the timing marks on the crankshaft match up twice during the 4 stroke cycle, but only once is it on the compression stroke. The other time they match is when the exhaust valve(s) are open. Proper valve timing is achieved when the crankshaft marks line up AND both intake and exhaust valves are closed. I fear yours is set wrong... Please check this out....

Jack
 

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Discussion Starter #11
I went back and checked the timing. The camshaft and crankshaft were set correctly but I had the points gap too wide. I guess I need to set the points every time I take apart the head. I used a battery and light bulb to check the timing and its dead on now. Its still not starting, even with starting fluid. Now I'm thinking its bad compression.

I ordered compression tester number three and I finally got a good one. My previous 2 compression testers leaked. I think the schrader valves were bad. Unfortunately I'm only getting 80 psi of compression after all my rebuilding effort. Is that too low for this engine to run? my outboard only had 50 psi and it runs like a champ but from what I've read this engine needs more compression.
 

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Discussion Starter #12
I retried the compression test after pouring a bit of oil in the cylinder and my compression went up to 115. That lead me to think there was a problem with the rings. Note: i had just installed brand new piston, rings and cylinder. When i took apart the engine to check the rings i found the top two rings were stuck in the piston groove. I was able to get the rings out with an razor blade but they're still pretty tight in the groove. Is that unusual for new rings? I think I'm going to try to sand the rings now.
 

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Rings

Sam 115 pounds of pressure is what my 41 Ford tractor has when new.
It is enough to make an engine run, but a bit down for a bike motor.
The piston can get hard black gunk in the ring grooves. This will make new rings too tight. They should float free. Snap an old ring and use it to clean out the gunk. It can be quite hard. Check with a new ring outer edge for fit, after you clean. If the fit is good then install the rings. Also check your valves for seating. Old school was one angle wide valve seats. Newer thinking is three angles with a more narrow seat. A chat with an engine re-builder might help you with this.
Pay close attention to what One Eye said about the cam timing. Crank the engine over by hand, and study the 4 stroke action of valves opening and closing, with the plug firing when both valves are closed. Pops and bangs and kick starters firing back, are good signs.
I am not a fan of starting fluids. Everything you need is on the bike.

Unkle Crusty*
 

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Discussion Starter #14
It started today!

It started today!

After sanding the rings my compression stayed up but it still wouldn't start. Then I noticed I wasn't using the correct spark plug. I ordered the correct plug and it started on the first kick. No starting fluid necessary. Thanks for all the help.
 
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