Engine Restored - 40m run - now not working

[blakjak3000]

Hi guys,
Restored an old Sahara 150cc GY6 and got it running today. I drove it about 40m and it backfired/popped and then conked out. Now I cant get it to start.
I have checked timing, there is a spark but wont ignite, I'm completely baffled....

Thinks I'm NOT sure about

It sounds like air is being vented from the exhaust (may sound stupid but it didnt sound like that before)
The large hole on the cam sproket is aligned with the little ones and the 'T' on the flywheel.....but it seems to misalign itself after a few times of turning it over (by 180 degrees?)....Chain tensoner seems to be working (although has no spring under the screw top).

Today was supposed to be finishing day, and got just 40m out of her Hoping someone can suggest some other things to check?

Many thanks

JB

[blakjak3000]

Ok so i have tried 180 degrees the other way on the timing and still compression appears to be venting from the exhaust.

I stripped her back to the piston which seemed good and have checked the valve adjustments. The amount of times ive reattached the cam followers beggars belief.

I'm still stumped. I still have a spark (so I'm assuming - I know mother of all mistakes -but still spark should mean CDI and stator are ok).

Could a valve have bent from such a short run?

I have a compression tester, but really helpfully it doesnt work (I know this because it wasnt working just before I started her up).

The other mysterious thing is when testing fueling, I covered the back of the carb air intake, and ended up with fuel on my fingers?!

It feels like there is suction but also air blowing back on itself.

If anyone could suggest anything else to consider I would greatly appreciate it

(Piston, piston rings and cylinder seemed ok. Sprayed oil inside the cylinder to check it couldnt pass the piston which it couldnt. Are the valves the only thing remaining that are a possibility or have I missed something really obvious?

Thanks for any help

JB

[blakjak3000]

So..... after redoing everything a 4th time I took the head off. Testing the valve tension (by blowing in the exhaust port, and then the carb inlet port) it was apparent that air could flow freely with the valves 'shut'.

In my over zealous and excited state when i sat on the finished thing, i gave her some wellie, when i should have run her in for a few hours first

It appears the valves have bent from impact with the piston, and logically would make sense seeing as they have bent upwards (towards the head).

Quick final question....
whats more likely....bent valves OR poor quality/milling of the head?

JB

[ckau]

Originally Posted by blakjak3000

So..... after redoing everything a 4th time I took the head off. Testing the valve tension (by blowing in the exhaust port, and then the carb inlet port) it was apparent that air could flow freely with the valves 'shut'.

In my over zealous and excited state when i sat on the finished thing, i gave her some wellie, when i should have run her in for a few hours first

It appears the valves have bent from impact with the piston, and logically would make sense seeing as they have bent upwards (to wards the head).

Quick final question....
whats more likely....bent valves OR poor quality/milling of the head?

JB

Poor quality/ milling of the head would result in the head not seating properly, blowing gasket and leaking. If you had the stock head milled over 35 thousands then you possibly have valve clearance issues, 25 t0 30 on some. the numbers depend on the particular manufacture , some have more clearance than others.
you didn't state if you were using an aftermarket piston or cam so if they were stock, my guess is the cam chain tensioner you mentioned didn't reset properly, the cam skipped a few teeth and bam!!!! I'm afraid your back at base one. Replace the piston, head Assembly and cam tensioner or your going to keep on having trouble. You can pretty much count on everything from the connecting rod on up is trashed.The piston hit the valves with enough force to bend them so the impact transfered to the connecting pin at the crank connecting rod. that piston will wobble around in the cylinder eventually beat itself and the cylinder walls apart. Hone the cylinder for fresh rings. The cam itself is probably o-k.
When resetting the cam, DON'T use the timing marks on the flywheel, they are seldom accurate. With the head off, rotate the piston to top dead center and put your own marks on the fly wheel. rotate 180 and mark again. Use the mark closest to the factory mark. Use a dial indicator if possible or some sort of measuring device.
if your worried about valve /piston clearance, it can be checked by forming a pancake out of silly putty or clay on the top of the piston, assemble, rotate by hand a few cycles Remove the head, the valves will leave indentions in the putty that can be checked by removing pancake, slicing a cross section and measuring.

[blakjak3000]

Hi ckau,

Thanks for the info.... The timing with the piston rather than the flywheel is a great plan, so will break out the tipex to mark the flywheel (unless it aligns with one of the other marks).

The valves have only bent a tiny amount. The exhaust valve is less than 0.1mm and requires some pressure before air escapes, and the intake valve has a small opening of just over that as air will flow, but only just sneaks out (there is no clicking when the valve moves to indicate a serious bend), would there really be that much damage?
The piston appears fine, no marks on it and moves smoothly.
The cam rotates evenly also.

I have ordered new Valves, but from the US as noone in the UK stocks this stuff (well not that have anything in stock anyway).

Regarding the Tensioner, when you reattach it do you let it reset itself, or do you use a screwdriver to extend the tensioner and therefore force it to tension the chain? In the past I have just left it to reset itself, however there is more 'tensioning' that can be done by using the screwdriver when reseating it?

There is one thing that is odd (Possible design issue of the GY6, but then I'm no mechanic)....

Imagine you are looking through the centre of the head down towards the piston. When attaching the cam follower and tapet unit its a bit of a tight fit. On closer inspection the 4 long pins (that go from the main engine casing through the cylinder and head are located to the left side of the 4 holes they protrude from.
This can be overcome by releasing the 2 very long bolts on the right side of the head. Tightening these pulls the cylinder and head towards the right, but there is no bolts on the left to even the pressure....admittedly there are the 4 main head/cam follower bolts that hold it all in place, but still in my (very limited) experience, uneven pressure is never a good thing

Just wondered your thoughts on that and THANK YOU for getting back to me HERO!!!

[ckau]

Cam chain tension is released by turning the screw visible through the top of the tensioner, under a cap. Turning the screw raises the spring loaded plunger, taking pressure off the chain.
Turning the screw back opposite a couple of turns releases the plunger allowing spring pressure on the chain.
Normally you can hear or feel the plunger pop free when released putting pressure on the chain.
I can't remember which does what, turning the screw clockwise or counter clock to lock and unlock. Best way to understand is unbolt the tensioner from the case , turn the screw and observe what happens. if the tensioner is not manually released it's gonna jump timing.
You may be missing the 2 centering dowels in the head. Those are little tubes that fit diagonally in two of the four long head stud holes They align/center the head on the cylinder so it won't shift when torqueing down. Facing the head with the intake side up, upper left corner and lower right corner.
The dowel tube in the upper left corner is very important .It serves double duty of not only keeping the head centered but this stud hole also serves as the main oil feed path to lubricate the top end.
Oiil is pumped under pressure up through this stud hole to the rocker assembly then drains back down through the cam chain opening into the crank case. The dowel keeps the oil pathway aligned. You can see looking at the mating surfaces of the head and cylinder the two holes that are machined slightly larger to accept these dowels.
I don't think you actualy stated that the head has been milled. If the surface was not milled correctly, cut at a slight angle, this too can case the problen you describe.
If it were mine I wouldn't chance that piston. it doesn't take very much pressre to beat the connecting rod pin holes out of shape on an aluminum piston. Smacking a valve hard enough to bend it is enough to misshape a piston I would do anything I could to keep from going back in there again!

[SYCARMS]

First you will need to disassemble the head and inspect all components for possible cause of failure. Could have been caused by valve float due to weak springs, cracked valve guide or a dropped valve seat. When milling a head clearance should always be checked before to determine how much material can be removed.As well as after. You want minimum .100" clearance between valves and piston using the stock springs .075" with a good non Chinese performance spring. When removing a tensioner you must always remove tension first by turning release screw clockwise other wise tensioner could be damaged. If you have to manually turn screw to put tension on chain the tensioner is bad and needs replacement.

TOM

[blakjak3000]

Hi guys,
So found oil was leaking from the cylinder/engine casing gasket. So while I had it stripped I replaced the cylinder (had to order the part from china - In the UK its either super expensive or not in stock!). Rebuilt with new piston and rings, and new valves.

Used the advice from ckau and marked on fly wheel when piston is at top and bottom. Used the cam alignment marks when piston was at dead top. Valve clearance is 0.002" and she turns over smoothly (this time with the proper noises and air venting from the exhaust and sucking in the carb )

However, I still cannot get her to fire. Still have a good blue spark, all earths are good, and smell of fuel from the exhaust indicates there is fuel (at some level). Would someone mind elaborating on the timing pleeeease

I'm still stumped (but getting closer everytime I think). The woodruff key in the flywheel is still solid and so would the markings on the flywheel be that far off? Also using the cam markings (2 little, and 1 big hole) neither valve is open, but is that right if the piston is absolute top? I was guessing that the flywheel T timing marking means the valves are both closed and therefore the cam timing mark would be right?

Also clarification when I referred to the head being milled, I was referring to the factory build process.....as in what is the chance it was a cheap chinese part that wasnt made all that acurately?

One day I'll get her running again, although it has given me a chance to underseal the chasis

[blakjak3000]

ALSO MASSIVE THANKYOU FOR YOUR ADVICE SO FAR......
(I took this on as a challenge, and that it has been. I had ZERO real engine experience until I started the smaller buggy when I replaced the 50cc with a GX200 that had seized. However this engine is MUCH more interesting and although its tough it has been fun learning and tinkering) so again thank you for all your help so far...when she is running i'll post some links to vids/pics if anyone is interested............


Next stop, looking at a 450cc and upwards engine to replace the GY6 150 hahaha

[SYCARMS]

With the piston @ TDC the cam should be as follows, the 2 small holes and or lines will be parralelle with the top of the head, the large single hole will be pointed at the seat back. Every completed direction of travel of the piston is known as a stroke, so when the piston travels twice down and twice up it has completed 4 strokes. Within these 4 strokes the cam rotation will be once for every 2 rotations of the crankshaft. So the first time the piston reaches the top this is known as TDC the next time the piston reaches the top is known as BDC or bottom dead center. At TDC both valves will be closed whereas at BDC the intake is just about closed and the exhaust is about to open. Hope this helps you to understand and not confuse you. The valves should be set on a cold engine with the intake set to .003" and the exhaust @ .005". The setting you have at .002" is too close and will not let the valve rest on the seat long enough to dissipate the heat generated on combustion, and will eventually burn the valve. After trying to start the engine several rotations pull the plug and tell us if the plug is wet from fuel or dry.

TOM

[blakjak3000]

Hi TOM,

Thanks for your response.....
The spark plug was initially dry, but a friend had adjusted the screw on the carb in an attempt to improve fueling, so I left the spark plug out and adjusted the screw until i could see a fuel mist emerging and then turned the screw slightly back until I could smell fuel but not see a mist. (also didnt start with the mist of fuel setting).

So in relation to timing how do you establish TDC and BDC on an engine you suspect the flywheel is out (as with the camshaft and head off does TDC not look exactly the same as BDC), or is TDC and BDC simply the names (given to distinguish which piston stroke) based on what happens when the cam is fitted. I have rotated the flywheel until the piston is at the top (near the head) and made a tipex mark on the flywheel (however this is some way away from original mark). I have fuel, and spark, and in my VERY limited knowledge the engine 'sounds' right (unlike it did before when a valve was bent).

A number of rotations from the starter motor (after fitting a new cam chain tensioner) proves there is no movement in the cam chain and so timing stays set. Valve adjustments are per your recommendation. Still stumped

There is one last symptom that could be normal or indicative of an issue, and that is the carb intake. Essentially as per others advice on different threads, to encourage a spark I covered the carb air intake to richen the mixture, however my fingers became wet with fuel. I can however feel a suction during the engine rotation, but for some reason there was still fuel on my fingers? Would this mean a carb issue, mixture to rich, or still a potential valve issue that is allowing air pressure to escape up the intake.

I performed the 'amateur valve test' by removing the head and blowing as hard as i could (creating a tight seal around both the exhaust/intake holes) and unlike before (when I could hear a hissing) this time my cheeks expanded. To be fair I cant generate enough pressure to perform a proper test, but how do you test the valves, or is it a replacement based on assumption? The only test I could do was either a compression tester (which I bought from amazon and was broken when it arrived - crap!) or place some cling film over the intake on the head and theoretically it should dome inwards when suction (for fuel) occurs, but it should return to normal. If it domes outwards it would indicate either a timing issue or valve issue, what do you reckon?

Kind regards and thanks

Jamie

[blakjak3000]

Apologies for another post but have a question about timing/spark.....

If the flywheel timing marking is off and as above should work out when piston is TDC to the cam marking.....does that not only make sure the valves open and close with the correct piston positions.....what about when the spark happens. Surely the flyweel is designed that when it reaches a certain position (i'm assuming the larger metal bump aligned with the 6th coil of the stator) the CDI is instructed to release the voltage its has stored from the induction of the other 5 coils? If the flywheel is off, and the timing maybe wrong then could another reason for the engine not firing is that now the spark could be firing when the piston is at the bottom, or when the exhaust valve is open etc etc......If this is the case, how do you align all of the cam/piston/spark timing?

Apologies if this seems like a realllly noob question, but it just came to me that its the last part of the puzzle that may not be correctly timed

thanks again

Jamie

[SYCARMS]

The flywheel has a fixed timming. The only way for timming to be off a considerable amount is if the key on the crank stripped. The key secures the flywheel in a fixed position on the crankshaft.To check you will need a flywheel puller to pull the flywheel so as to inspect the key. If the key is good holding the flywheel then timming of the crank is good. When the piston is at TDC your pointer should line up with the T mark on flywheel. With the crank at TDC you will find 90% of the time that your cam will not line up exactly. It will be off 1/2 tooth to either side of center. This is due to advance ground into the cam anywhere from 3-5 degrees. You always want to error the cam to the advance side. To make this last sentence less complicated with flywheel T mark lined up with pointer install the cam put tension on chain then move flywheel in direction to line up marks on cam with top surface of head. Then observe the flywheel to see where the pointer lines up. If the pointer lines up before the T mark then the cam is advanced and this is where you want it to be, if the pointer is after the T mark then your cam is retarded and you will want to error in the opposite direction. The F on the flywheel when lined up with the pointer is when the plug fires. Note that in this position the pad on flywheel is directly under the trigger switch. You want the engine to fire prior to TDC. Hope this helped you and not confuse you. Feel free to call or e-mailme with any questions you may have.

TOM

[blakjak3000]

Hi TOM,
again thanks for your help....Cant believe I didnt think of that....literally EVERYTHING said flywheel not in the right position....Cam and piston match but the timing mark was off......pulled the flywheel and low and behold the woodruff key had sheered......ordering now and will report back when its back and the timing is rechecked thanks again mate

Jamie

[SYCARMS]

Some times we miss the easy things when answering. Since the buggies are not in front of us you become our senses with the info you give, but eventually we figure it out. Glad to see your on the right track now.

[blakjak3000]

So it was literally as simple as that.....The timing (valves matched to piston was spot on) but the spark timing was out. Sheered woodruff key replaced (and a little charge in the battery) ......
SHE LIVES!

Timing matched TDC when the correct flywheel position was found on re-fitting with the new woodruff key.

A little smokey to begin with (but I think that was the WD40 that was in there when I was testing the piston seal..... This time (with a new tensioner, cylinder, valves and piston + rings) the moral of the story is let her run in for a few hours before giving her any gas.....

Thanks everyone for your help, TOM (Sycarms) you are a gentleman and a scholar!

Big thanks from the UK!

[SYCARMS]

Glad to hear its runnin and you can now have the time of your life with it

TOM