Spark, fuel, compression, but no start!! By mossy173 (Read 17,306 times)

[mossy173]

Alright, I'm not fooling around, this time I'm just going to throw parts at the damn thing until it runs right. New auto-choke, new carb, and new piston rings all ordered off ebay. I'm sure it's either fuel delivery or ring gap, I'm gonna corner this thing.

What you say about having to use the main jet really seems like a good idea--that's gotta be it. We'll find out when that new carb comes!

[Alleyoop]

Trick of the trade is put the philps screw driver on the screw and give the top of the screw driver a good little tap. That will usually loosen the threads inside and then lean on the screwdriver and turn.

The reason I said to check your ENRICHER is because it will only start and IDLE at 3000 rpms.
That is a No or not enough fuel for starting and idleing, if it was just the PILOT JET and passage that were clogged IT would start and IDLE for the 1st five minutes then die once the ENRICHER cut off the EXTRA FUEL.
But yes you can take the carb off and clean the PILOT JET and PASSAGE and work on getting the ENRICHER OFF while it is out.
Alleyoop

[mossy173]

So the enricher doesn't put in more gas through the idle jet then, it's actually through its own outlet into the airway? Yeah, that would seem to explain the behavior pretty well. Welp, I'm not interested in taking the carb apart anymore, so I'm going to try just plunking a new carb and enricher in there and seeing what happens. This will be interesting to see.

I was pleased to see that the new carb has steel fasteners. Hopefully I've stripped my last carb screw.

[Alleyoop]

That is another way to go about it, then you can mess with the Old carb and have it as a spare for parts.
Alleyoop

[eagleridermb]

As long as the cam was timed with the piston at TDC, and the timing marks were aligned, the valves should open at the proper time. The only way the cam can be 180 degrees out and valves can open on the wrong stroke is if the cam were timed with the piston at BDC. EagleRiderMB

[mossy173]

Well, through in the new carb, and got no start and a wet plug. I tried it at all throttle and mixture screw positions. I removed the crankcase breather tube to see if I could feel blowby out the valve cover vent, and there's some, but it doesn't seem like it's really blowing.

I took the top end apart to recheck the installation of the piston rings. The gaps are a bit big, at 0.011 and 0.012" for the two compression rings. Is that outside what's tolerable? I put everything back together after making sure the rings are in the correct order and orientation, and it runs just exactly as it has been--no start unless I crank it with supplementary starter power and starter fluid. Then it runs, but will only idle high. If I try to lower it the thing dies. The kickstarter absolutely won't start it.

What's bothering me is that I still haven't really properly tested compression--I hear that it should be 140+, and I'd imagine that it could blow my finger off the spark plug hole much, much lower than that. Could the large piston ring gap be killing compression to the point where it won't start or idle except at very high revs?

[mossy173]

It also worries me that I don't really feel resistance on the kickstarter. Back when I had a KZ750 I could stand on the kickstarter, and the compression would hold me up. I know that these smaller engines won't be able to do that, but I should feel *something*, right?? It is spinning the engine alright, however.

[rob63]

Yes there should be some resistance on the kickstart. Another way you can get a rough idea of compression is to leave the spark plug in and turn the flywheel clockwise by hand, it should get to a point where it`s at the top of the compression stroke and need extra effort, maybe two hands to get it over the point of no return. If you can turn it easily with one hand then you have a compression issue.
Might be time to invest in a compression tester buddy and do a proper check.

[mossy173]

Yyyeah a compression tester and a new set of rings are both on their way.

[mossy173]

it's the valves!!!!! THE VALVES!! On a hunch, I pulled the head and did a trick I read about elsewhere, shining a light through the ports and seeing if I can see light around the valve from the cylinder side. I could see light all the way around both valves, and after pulling the valves out, I saw on the exhaust valve (what seems to me) a casting defect that left a 1/8 x 1/32" hole in the valve seal.

my god.

So, the quality of this ebay big bore kit is terrible. I threw out the valves that came with it--both were thin, warped, and there's that giant casting defect in the exhaust valve. I pulled the valves from the stock head (same size, thank god) and lapped them in the big bore head, then installed them. Now there's no light visible around the valves at all. I'm going to put everything back together today, and I'll bet you a dollar it runs like a top.

BUYER BEWARE. those ebay kits may be cheap, but you'd better be prepared to lap valves and do whatever else is necessary to make them roadworthy!

[mossy173]

Also, it seems like the finger test for compression is out. It will only fail when the compression is so low that the engine stands no chance of running at all, which I guess is useful in extreme cases, but not when there's a less dramatic loss of compression like mine.

also: I read about another trick which would have helped here, but I never tried it. if you hold a small (dollar sized) piece of paper against the exhaust while the engine is running, it should only blow away from the exhaust. If it ever flutters toward the exhaust there's a leak in the exhaust valve--you know this because the vacuum generated by the intake stroke is leaking through to the exhaust system, sucking your piece of paper backwards.

[mossy173]

The upshot: runs like a top. Starts on the first turn-over every time, electric or kick, idles beautifully, lots of punch. Glad that mystery is solved.

Watch out for the valves on BBK's!!!

[Alleyoop]

FANTASTUS GOOD JOB Mossy,
Great find and yes your correct the dollar trick has been around for decades. And that trick will usually work good on scoots to determine a valve leak.
Alleyoop

[rob63]

Bloody hell, the gy6 I was trying to start last week had the same problem although my valves had taken a slight tap when the bottom end blew. Only found it when I held the head up to the light. Never thought about mentioning it here as you`d fitted a brand new kit.