Top end issues, time for BBK? By sledwrecker (Read 3,158 times)

[sledwrecker]

I've migrated over to this forum following the advice of a local friend who hangs here a lot. I have a 08' Gy6 BN157QMJ-3 (16" wheels.)

Last week I got stranded for the second time due to no start (first time was ebay racing CDI failure.) I got it home and once again checked the valves which were still in spec. Next I did a compression test.. 105psi.

I proceeded to get a leak down tester and definitely had a leak from my intake valve.

Long story short I lapped my valves it did not appear they were bent and no markings on the piston of a collision with the valve. I also put in new gaskets and valve seals. I put it all back together and compression raised 5psi to 110. This explains my hard starts as book says min spec is 130psi. I tested my wife's and her scoot is 155 so I know my compression gage is accurate.

Ok so now I'm looking at BBK's and I want to go as big as I can without having to bore the case. I heard there is a 61mm NCY kit that will fit in 90% of GY6 without a bore but haven't tracked one down yet. My bore ID is 62.49 I know I can do a 59mm kit without any issues. Reliability is of the up-most importance. I'll sacrifice a little power for longevity and not walking or paying for cab rides home after being stranded.

So the other issue is my BN157QMJ-3 motor. Scrappydawgs sells parts for them but I honestly think my top end is pretty GY6 standard. The intake boot is definitely different but I was able to use the stock GY6 157QMJ head and base gaskets on my scooter when I re-assembled my top end. Is there an easy way to tell which cylinder to buy or do I need to contact the seller directly and get measurements?

Is boring the case that big of a deal? I'd love to do a 63mm kit but I know that my crank is unique and to my knowledge no one makes an aftermarket stronger one for this specific engine. Doing the 63mm would be fantastic but if it would turn my stock crank or rod to mush I don't want to chance it.

Any advice would be great, Thanks!

[Bashan]

MonsterGY6 sells the 61mm kit. It's a direct bolt on with no boring. We did have a thread where there was some issues with boring. However, Monster GY6 has been doing that for years with reportedly good success. I did a 58.5 upgrade that added a lot of power and didn't require any boring. I just didn't want to complicate things since I was doing it manly for a rebuild tech thread. Rich

[sledwrecker]

Yeah I'm starting to really lean towards that NCY set, I need to do something and soon my scooter is sitting in the garage with it's guts spilled out and the nice summer days are dwindling down fast. I'm planning to try and pull the trigger by EOD today so that hopefully I can have parts in my hands early next week.

After looking at the thread you linked to I'm opting out of the 63mm kit. With my stock rod and crank I'm not taking a chance. I'd rather just keep buying top end kits then risk bending or throwing a rod like that, my bottom end is hard to find and very costly at that.

[Alleyoop]

Well my motor is a 157QMJ and I had Jim from RapidRepiar bore it out and slap in a 62mm and a 60mm stroker crank. It is now 182ccs instead of 149.6. It has lots of power and my trike is heavy weighs 350lbs plus my butt 180lbs and it cranks. At a stop light cars behind me do not have to wait for the trike to get up to speed, I am right behind the car in front of me. Just remember you will loose some high rpms it used to get to before.

Also with more displacement you can put in taller tranny gears for more top end speed.
Alleyoop

[sledwrecker]

So I've been on the phone with scrappydawgs and they are telling me my options are very limited. They said based off the numbers I told them I can buy virtually any GY6 BB cylinder but I really can only get their 4-valve head. Basically, he said that the BN157QMJ has a taller head then standard GY6 scooters and longer valves.

He directed me to a link on their site, here is an excerpt:
"GY6 Scooter Version B - This engine is found in alot of higher priced quality scooters like Zhen, Lance, Fly, etc. The engine number will be BN157QMJ, and will have a 16" rear wheel ( although some Lance Duke Tourings have the smaller wheel BN engines, which are standard parts, with some other exceptions). This engine is made by BenNeng of the Zhongneng Industry Group. This engine has alot of non-interchangable parts, like the variator, head, stator, etc. The head on these is taller than a standard GY6 head, and has longer valve stems. The stators are 12pole DC types. The belts are longer than standard, and are often 906. The cylinders however will fit from standard parts. Our big bore kits will fit -B models. The performance 4 valve head will fit these without use of the longer chain and retention studs, as this engine already has them."

My motor is stamped 157QMJ-3 (no BN prefix) I DO have 16" wheels, the much taller intake manifold, an oversized variator (122mm if I remember correctly) and run a 906 belt. However I do know that my CDI is AC so I'm not sure if I have the 12-pole DC stator. I may have a sorta hybrid 157QMJ that's not entirely a BN model.

So anyway, I went out to the garage and grabbed my calipers and started measuring.

Can someone possibly confirm measurements from a standard Gy6 (157QMJ)

Valves:
- Intake Valve length 65.87mm
- Intake Valve Stem width 5.08mm thick
- Intake Valve Seat width 27.81mm

- Exhaust Valve Length 65.39mm
- Exhaust Valve Stem width 4.96mm thick
- Exhaust Valve Seat width 22.93mm

- Overall head height is 59.98mm to 60.30mm (seems to be thicker on one side.)

See the attached picture for the bolt pattern measurements.

Attachments:

[Bashan]

This page should help, and this one also. If you need more I have a head and cylinder sitting around at the house I can measure.

[sledwrecker]

Thanks, so far so good. If I can confirm valve lengths I'm going to order up a standard 157QMJ BBK. I know there are differences in my motor but I think from what I'm seeing the top ends are GY6 "standard" if there is such a thing. <phew...!>

[Bashan]

I'll get those measurements as soon as I get some breakfast and go downstairs. The head I have downstairs was on a "P" engine and the height measures exactly like that one on the link I posted. My thought is that the port size might be different but the height is the same. We have some threads where guys had to use spacers for stroker cranks but that wasn't because of the head. Even if the head was slightly taller it might affect the cam chain but that's it as far as I can see. Sprocket seems to think the P head is different and he's certainly no dummie, you should read this thread over. Considering the manufacturing irregularities of the Chinese a few weird heads on the market wouldn't surprise me.

[sprocket]

Thanks Rich!

There are tall P heads and normal P heads...go figure...some manufcturers used a 'P' for the tall heads, like Lifan and others...others stuck a 'P' in the engine code becuse it was a THURSDAY...no friggin' reason...perhaps they would sell more scooters with the 'P' in the engine code???

So measure the head..it is only way to find true 'P' head... it will be73.25 mm high..larger valves, better porting, longer cam chain, more aggresive cam profile..

[sledwrecker]

Well I'm looking at the NCY 61mm cylinder and head. These are reportedly drop ins and will bolt up without machining the case. I called the dealer and they measured the OD bore of the cylinder at 62.5 and mine is 62.49 I'm sure it will fit I might just hit it with a light grit sand paper to get that hundredth of a mm flush.

I dug around since my last response and I'm nearly 99% positive I have standard GY6 top end parts. Head height, valves, cylinder, cylinder bolt spacing, everything is a match so far. I'm going to buy from reputable "None eBay" dealers who offer refunds if for some reason it wont fit. Wish me luck.

[sprocket]

News to me... everything over 59mm needs to be bored...from my experience.

[tvnacman]

How many bolts hold the valve cover on 2 or 4 ?

John

[sledwrecker]

Well folks I figured out what the real problem is... (Sorta)

It's my a9 cam. There's a long thread about it over at scootdawg and I don't want to link it here, might be against the rules.

Basically with my a9 MRP cam my compression dropped nearly 40psi. There's a debate about why but the fact is this is what's wrong.

I put in my stock cam and now my compression is up around 140+ I even saw it jump to 150 at one point. Before I figured all this out I ordered a 60mm BBK high compression kit from scooter elements, I have not found a performance head yet to pair with it.

If anyone thinks they might know why an a cam would lower compression I'd love to entertain your ideas. I want to note when I put the stock cam back in I had to loosen the tappets 1 1/2 turns out from the a9 cam setting. They were waaaay too tight. On the a9 cam the valves had been adjusted several times to try and eliminate tapping which was never completely resolved.

At this point I'm going to keep this scooter with the stock setup. Last week I bought a vento chasis gy6 off craigslist with a blown motor. I'll likely put the high compression kit on that once I tear it down (or sell it and go for a ceramic 63mm and stroker) but we'll see. The stocker is going to be my daily til winter hits, the vento will straight up be a mod bike.

[Bashan]

From my days of messing around with performance cams in V8s many years ago I can point out a few things in regards to this issue.

• The duration overlap on higher performance cams will give a lower overall compression reading. This is because the valves stay open longer during the rotation cycle of the engine and their open phases overlap each other longer. This allows less time for the cylinder to build compression but does not mean anything is wrong with the valves or rings. This effect is negated at the higher RPMs of running speed.
  
• The speed of rotation affects the compression reading. A slow cranking speed allows more air to escape via the unclosed valves. This turns in a lower compression reading but does not mean anything is wrong with the valves or rings.
  
• My A9 cam and high compression kit turned in a lower compression reading than when it was stock, about 120. It definitely cranked slower which was understandable with the higher compression ratio. I'm sure the A9 has a greater valve overlap than stock and I would assume the A12 has even more.
  
• My engine runs fine so I'm happy with the results so far. It burns a little oil but I'm waiting for the rings to seat.

[sledwrecker]

Sept 12, 2012 4:01:28 GMT -6 Bashan said:

From my days of messing around with performance cams in V8s many years ago I can point out a few things in regards to this issue.

• The duration overlap on higher performance cams will give a lower overall compression reading. This is because the valves stay open longer during the rotation cycle of the engine and their open phases overlap each other longer. This allows less time for the cylinder to build compression but does not mean anything is wrong with the valves or rings. This effect is negated at the higher RPMs of running speed.
  
• The speed of rotation affects the compression reading. A slow cranking speed allows more air to escape via the unclosed valves. This turns in a lower compression reading but does not mean anything is wrong with the valves or rings.
  
• My A9 cam and high compression kit turned in a lower compression reading than when it was stock, about 120. It definitely cranked slower which was understandable with the higher compression ratio. I'm sure the A9 has a greater valve overlap than stock and I would assume the A12 has even more.
  
• My engine runs fine so I'm happy with the results so far. It burns a little oil but I'm waiting for the rings to seat.

I thought that your first bullet point might be the issue and I appreciate your response, this is the first one I've seen yet that is more then just "my cam works fine, it's not your cam dummy."

After I put in my a9 cam the bike ran well up until a couple weeks ago when I got stranded. I was out on a ride and after making several stops the scooter simply would not start anymore. I had spark, I had fuel, it just didn't make sense. I got it home and checked my valves and they were still in spec. That's when I did the compression reading and got 105 with the cam. The book for my scooter says 130psi is the min in spec and anything lower could have no start / hard start issues.

I honestly haven't rode the scooter yet since putting the stock cam back in. My wife and I are going to do a ride this evening and I'll find out if my hard start problem is now resolved or if it's caused by something else entirely.