Not your typical "Check Fred's guide" pr

Jon Dalton /

Posted under 'very puzzling moped behaviour' about my moped's high speed engine cutout. This has me baffled and not even any the good advice from Ron Brown has led me to an answer though it had narrowed it down quite a bit. From most recently, here is everything I've done:

-sprayed carb cleaner into carb during miss - engine dies completely. Repeated this several times - even the slightest shot of carb cleaner kills or almost kills the engine.

-Checked float bowl vents - found that they were clear

-took float bowl off while running, level looks perfect

-did screwdriver spark test while running on stand - spark still jumps during miss

-changed plug, coil, condenser, wire - no effect

-adjusted timing and needle settings - this affects the speed and throttle level where it will cut out

-changed points and plug - no effect

-tried different float levels - no effect

-cleaned the carburetor - no effect

-changed carb with other moped - misses a whole lot less but slower than other moped

-checked valves + cylinder head, cleaned off all the carbon - no effect

-tried putting choke on during miss - makes the engine miss more

I would conclude:

-the engine isn't losing spark

-fuel flow isn't the cause of the problem

-carburetor is most likely causing it

-the miss would be caused by too much gas rather than too little gas.

By adjusting the needle, I can make it lean enough that it will run at almost full throttle without cutting out, and go pretty fast, but it is undrivable at this state because it hesitates so much you can hardly get it moving. It appears to me that it gets too much fuel at high speed / throttle and and not enough at low speed. The moped was previously running great with the same jet and needle in it.

For the record, compression is excellent (oversize piston / rings, rebore), valves reground, ports cleaned., and yes I did put a new spark plug in, though it wasn't by any means the first thing I tried!

Re: Not your typical "Check Fred's guide&quot

Jon.. it sounds like maybe you have a leaky head gasket.

I could be wrong though.. there is an easy way.. get a small propane torch or butane and don't lit it.. just open it up while your moped is running and trace around your head.. if your engine speeds up you just found your leak.

I could be crazy.. but sounds like you checked those other things pretty damn good and it might be time to try anouther area then the carb.

Just an idea though...

Re: Not your typical "Check Fred's guide&quot

Clean your exhaust.. if you just want to check it out, try running it with your exhaust off..

Re: Not your typical "Check Fred's guide&quot

Reeperette /

Do a vaccum leak test.

Dunno how it's normally done, but since I smoke, I blow a nice exhale of nicotene-laced poisoncloud down the fuel intake and see if any comes out.

There's another way to do it, prolly...but that's what works for me...cause an intermittent vacuum leak might cause such a problem but fubar'ing yer mix at high speed.

-R

Re: Not your typical "Check Fred's guide&quot

I don't know if this is the same thing, but I had a 35cc chainsaw that did the same thing. Turns out it had a crack in the head that ran across the plug threads. It would open up a little bit at full throttle and result in a small loss in compression and then it would lean out. Had it TIGed and rethreaded and it works fine now.

Richard

Re: Not your typical "Check Fred's guide&quot

david f martin /

My scoot does this every once in awhile, too. I'll be cruising along at 35, then it will cut out till the bike slows to about 25, then it will take off again.

I just had an idea, probably a dumb one. Could the reed valves be hitting a resonant frequency with the engine rpm, causing them to "float"?

david

a few ideas

I didn't respond the first time because some things are complicated.. but you have done some good investigating.

You still need to tell make/model (Honda 4 stroke 50 ?).

A couple things...

... is there a chance the "choke" (enrichening mechanism?) has lost its seal and is essentially running 'choke-on" all the time ?

(I don't know if it has a choke or enrichener).

If its an enrichener... disassemble and take a good look at the the parts.

... are you sure that you got the cam timing correct on reassembly ?.. (one tooth off will make funny things happen)

... have you checked the ignition timing with a strobe light while running ?... it would be a good idea.

Last... you might have already explained this.. but... did it start acting funny before or after the re-build ?

Re: Not your typical "Check Fred's guide&quot

Maybe the intake valve is not closing properly?

What are your compression readings when the engine is warm? Is it in the low end of the allowable range?

Re: Not your typical "Check Fred's guide&quot

Jon Dalton /

Well, It could be possible that the intake valve isn't closing because of expansion at higher temperature and not enough slack in the rocker arm. I haven't got the engine really hot and checked the valves for a while. Compression feels very good, better than it ever was before the rebuild. I haven't used a gauge on it. Thanks for the help, I'll check that intake valve right away.

Re: a few ideas

Jon Dalton /

A response from Fred!!! Now we might be getting somewhere!

It is a '75 Honda PC-50 with carb and intake from a Honda XL-75 dirtbike. (was running great with same carb)

Choke is a simple manual choke, and does stay open.

Cam has never been fooled with at all

Haven't checked it with a timing light

Problem started after the rebuild, but not immediately after.

Ok. The latest news is, every time the engine cuts out, a white smoke or mist comes out of the carburetor, and you can feel a pressure difference while holding your hand next to it. The mist I believe to be air/fuel mixture and/or exhaust, but I really can't tell. It does this in perfect conjunction with the engine cutout. If the intake valve isn't closing all the way, that would explain stuff blowing out the carb as well as it not wanting to run. So now it's not so much what could be wrong with the carburetor, but what could prevent the intake valve from closing? Pushrod jiggling out of place? Gunk in the crankcase where the cam and lifters are? Carbon on valve or valve stem? The whole valvetrain is going to get some extra attention tomorrow

Re: Not your typical "Check Fred's guide&quot

Quick check for the intake valve. Rotate the engine till you get the piston at as close to TDC (Top Dead Center) as you can.

In this position both valves are supposed to be closed. Take out the spark plug and screw in the compression test fitting. Remove the air cleaner from the carb. Put about 100-140 psi into the compression test fitting using a air compressor hose. If you have air coimg out your carb's back side(air filter side) this could indicate a stuck open intake valve. Based on this test, you may want to do further checking of the intake valve and the valve guide to make sure it is sliding easily the full range in the guide and that it is seating properly.

quick compression test

Jon Dalton /

For someone without an air compressor or a compression gauge, how about taking the valve cover off and checking to see if the valve is out as far as the exhaust valve and if there's any slack in the rocker arm? Could it pass this test and still have a leaky valve? Valves were reground, so it should be an issue of sticking rather than leaking.

cam timing cam timing cam timing

> Cam has never been fooled with at all

Wrong....... if you had the top end off for a rebuild... then you were supposed to re-set the cam timing correctly per the manual... and you might have gotten the cam timing wrong if you didn't follow correct procedures... 4 stroke overhead cam motors are not easy like 2 strokes and pushrod 4 strokes.

That is CRITICAL... do it right.

> It is a '75 Honda PC-50 with carb and intake from a Honda

> XL-75 dirtbike. (was running great with same carb)

Ran great for how long with the different carb on it ?

Did you put this different carb on BEFORE or AFTER the rebuild ?

> Choke is a simple manual choke, and does stay open.

By that I assume you mean it is a 'choke plate' that blocks the carburetor throat ?

> Problem started after the rebuild, but not immediately after.

How soon after ? 2 hours ?

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

> Ok. The latest news is, every time the engine cuts out, a

> white smoke or mist comes out of the carburetor, and you can

> feel a pressure difference while holding your hand next to

> it. The mist I believe to be air/fuel mixture and/or

> exhaust, but I really can't tell. It does this in perfect

> conjunction with the engine cutout. If the intake valve

> isn't closing all the way, that would explain stuff blowing

> out the carb as well as it not wanting to run. So now it's

> not so much what could be wrong with the carburetor, but what

> could prevent the intake valve from closing? Pushrod

> jiggling out of place? Gunk in the crankcase where the cam

> and lifters are? Carbon on valve or valve stem? The whole

> valvetrain is going to get some extra attention tomorrow

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

That is all fine and good...

but... don't even consider anything till you check for proper cam timing... that is absolutely critical and you are wasting time looking at anything till you do check cam timing.

After you check or set the cam timing...

Do a valve clearance check... get the motor at TDC (ON THE COMPRESSION STROKE)... and set the valve/rocker clearance per the manual.

or is this a pushrod motor ??

since I don't actually know what exactly this motor is... is it a pushrod motor or not ?

Is the cam in the head with a cam-chain spinning it ??

I think by '75' they were all overhead cam... but I am not sure.

(overhead cam with cam chain means you have to set the cam timing correct)

Re: or is this a pushrod motor ??

Jon Dalton /

It is a pushrod engine. I think all the P-50's and PC-50's were except for a few rare early ones.

The moped ran well with the bigger carb for at least 3 months before the rebuild, and the engine cutout started about 3 weeks after, being driven daily. While running with the bigger carb, after the rebuild but before the engine cutout, I was clocked at 60 Km/h

Re: or is this a pushrod motor ??

Ron Brown /

Jon,

I am inclined to go with the sticky intake valve theory so far.

A couple of observations.

You never resonded to the blocked exhaust theory someone sugested. Do you know yours is OK?

Some carb cleaners will allways kill an engine which makes using it as a gas substitute, self defeating. Gasoline is the stuff to use, maybe out of a small dropper.

If your gas had water in it, every time a water droplet made its way through the carb, you would get misfiring and when the water hit the hot engine, you would get white vapor (steam). I don't really think this is the problem, just thought I would throw it in. : )

This is not a water cooled engine is it?

Ron

Re: quick compression test

Yep, do with, with what you have to get it done.

Good Luck

Re: or is this a pushrod motor ??

Jon Dalton /

It runs the same with no exhaust. I just checked the intake valve and found there is the right amount of slack in the rockers. Then I took the valve spring off and pushed it in and out and turned it with my hand. It moves freely and doesn't stick at all when doing this. Next I suppose i'll have to take the head off and check the seal very closely.

Re: or is this a pushrod motor ??

Ron Brown /

Jon,

Check the pushrod and valve to make sure they are not bent.

Ron

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