BBK help.

[mikeh]

Hey guys.

I've been a long time lurker and really have enjoyed reading the posts on here, and actually could use some advice.

So I recently installed a 47mm bbk with original 33mm head. The actual install went pretty decently, took my time setting the rings properly, making sure to get the timing tdc via the variator/ fan lined up at the correct mark etc. Reinstalled the head and got the valve lashing to .004 and all that jazz. Once I got the carb primed it start right up. I then started to dial in the carb, I'm running a pod air filter so i know I needed to upjet quite a bit and settled in around 92-94. I started to notice a few problems. Low to mid-range and idle were trash and even going full throttle it would only get up to20 mph, obviously way quicker but the stock 33mm piston had me topping out at 35-39mpg GPS verified so I knew something was off. I also could tell there was low range pinging or detonation going on so I was very confused. And concerned. I tried to upset my pilot jet to 35 and that maybe made it worse. I was so confused. I took out my spark plug and on a bad whim thought I should change the spark. Well I cross threaded and broke off the new spark right in the head. No excuses for that, I was in a rush and was hot and totally screwed up. I'm a city water plant mechanic and I have a large shop at work and tried to use some easy outs on the head, but that failed. So its new head time. I found a new 47mm head, which actually matches my new piston size on parts for scooters.com and I'm going to get that.

But here is the entire reason for asking this and writing my whole life story. In my haste to install my kit, I never blocked off the Pair port! My kit didn't even come with a plate. Could this have been the entire performance issue from the jump? Any advice?

Thank you guys in advance and sorry for the ramble.

-Mike

[Zino]

What Kit?
What scoot and engine ?
Pop up a sig line with your baseline so we can help If you take some pics also and post them.

Were you running open filter with your original engine ?
If so use that as your baseline for jetting and Go up 20% on the main and work down from there .

If you still have airbox those are easier to tune than Pod filters.

92-94 jetting with what type of carb?

[mikeh]

Nov 3, 2019 14:48:52 GMT -5 Zino said:

What Kit?
What scoot and engine ?
Pop up a sig line with your baseline so we can help If you take some pics also and post them.

Were you running open filter with your original engine ?
If so use that as your baseline for jetting and Go up 20% on the main and work down from there .

If you still have airbox those are easier to tune than Pod filters.

92-94 jetting with what type of carb?

Good point! Its a 2019 49.9cc gator gy6.
When it was stock and still had its old 33mm piston I was running on the stock airbox and carb, which I think typically have a 78 main.

after the BBK I tried upjetting and running stock air but it would bog down a lot, it seemed to run better with the open air filter pod.

I didn't install the PAIR system restrictor plate on the valve head because not only was one not included but I also forgot. After I replace the ruined valve head with a bigger 47mm valve head I'm going to start back at baseline with jetting.
My original carb is sealed via manufacturing screws.

Could the uncovered PAIR system vent on the have caused these low performance issues that led me on this wild goose chase?

[scooterted]

I would expect an unblocked pair port could cause poor running. I made a blockoff plate with a small chunk of mild steel.

Easy peasy with the shop you have access to.

[Zino]

Start with as close to your original and go from there .
Every time I am stuck I go back to my last working baseline and go from there .
You had 2 wild cards open filter and the open pair vent .

I am a 2 stroke guy so I am not familiar with the pair port .
Is this something that would add or subtract air from the fuel mixture being open and unregulated?

[mikeh]

Nov 3, 2019 15:30:24 GMT -5 scooterted said:

I would expect an unblocked pair port could cause poor running. I made a blockoff plate with a small chunk of mild steel.

Easy peasy with the shop you have access to.

Yeah I really slap my forehead on forgetting to block the PAIR port, I guess I was so happy that I got those devil piston rod clips in I just put it back together.

The larger 47mm valve head is PAIR free so I won't even have to deal with it.

[mikeh]

Nov 3, 2019 15:39:37 GMT -5 Zino said:

Start with as close to your original and go from there .
Every time I am stuck I go back to my last working baseline and go from there .
You had 2 wild cards open filter and the open pair vent .

I am a 2 stroke guy so I am not familiar with the pair port .
Is this something that would add or subtract air from the fuel mixture being open and unregulated?

As far as back to baseline I can go is oem box filter and down her to a 32 pilot and a 90 main jet. I think a lot of the problems originally was the open PAIR, unconnected to anything. Seems like that would be a compression nightmare. I looked at the bike today, drive is sitting TDC and the piston is resting in that slightly depressed position, leaving that nice cavity of the valve chamber, so I'm pretty sure that fine, also I triple checked and checked again the oil ring gaps when I installed it, and i put them in the correct position. I'm thinking compression lost due to the open pair and compounded by overjetting. When I order the new head and spark plug, and get everything torqued back up ill repost. Please let me know if you guys have more ideas.

[Zino]

You are on the right path for the fix.
After you get her running right with the oem box then you can move her back to open filter.

[GrumpyUnk]

The PAIR thing should have little to no effect on tuning. PAIR uses crankcase pressure pulses to pop open a valve that allows air into the exhaust port. Given the exhaust port is 'after' combustion, it should have little effect on performance. If there was any unburned fuel in the exhaust, the PAIR system will add a bit of excess air to be used to allow it to burn, and it also dilutes a bit the exhaust gas composition by adding a bit of air. That's all it does. I think the mnemonic is Pulse Air Injection R-something, or pulse-air for short.
Either way, if you feel the engine seem to lose power when you open the throttle further, say past mid-throttle, I suspect you may have a too-rich mixture. You can change the main jet, and may also be able to adjust the needle position setting.
From the postings, it was noted the adjustments were blocked by the manufacturer. You can drill the plug to gain access to the idle adjusting(mixture) screw. I got some 'broken screw extractor' tools from amazon that did the job. They have a drill on one end and a reverse-thread tapered extractor on the other. They are not the 'easy out' design(easy out is longer and looks similar to a tapered drill bit), as the reverse thread is a lot shorter.
tom

[mikeh]

So my new 47mm cylinder head is sitting at my house from partsforscooters.com, they are local to me, about an hour away, I got my head in one day via priority mail.

So I'm going to take my time, make sure I get my valve lashing to .004, and the head bolts torqued down. I'm going to drop my jetting down to 90 main and 32 pilot and reinstall the box air filter and start over. I got a huge jar of anti seize from work and I'm for sure putting that on my spark, can't believe I forgot that..

I ordered a spark plug and a tachometer from Amazon and I should get it tonight, any idea what my idle rpm and max rpm's should be?

Thanks!

[scooterted]

I try to idle around 1800-2000 rpm.

I dont want to go over 8500 rpm maximum. I try to be below 8000 rpm most of the time. I dont find much power past there anyway, and I dont want to burn through cranks like crazy.

[mikeh]

So I got the new head in, pretty sure I got the cam tdc and got the valve lash .004 intake and about .005 ext. Got everything torqued back to spec, new exhaust gasket and new chain tensoner. Got the old carb back on, swapped out the 94 main jet and went down to a 90, put the stock air box on and once I got it primed but it bogged and and wouldn't idle, like it was starving for fuel. Tried adjusting the air fuel mix screw. Nothing. If I hold the throttle and it run but will die right away.

So stock air box, 90 main, a 33 pilot I think, and no nothing. One note to make the carb is a nobody Amazon aftermarket carb, that's been opened up, jets changed out etc.

I had to run out in the middle of the rebuild job to get my wife and son dinner and came back to the fuel line dumping fuel in the garage. I about hung myself. This morning I fixed the fuel issue I think and tried starting it, it started up and wanted to idle but eventually died out.

I went to work with the stock oem carb, dremeled the sealed screws and am now able to use stock carb, I hear actually they work very well.

I'm at a loss now guys, I got a 3 day weekend coming so if you can give me some ideas please help.

All the love,

-mike.

[scooterted]

I find that if adjusting the idle mix screw and nothing, im way out.

Do you have different size pilot jets? Ive found that the pilot jet must be close to start easy. Sometimes ive been able to get it started and idle around 4000rpm and slowly start lowering the idle and swapping pilots until it gets down to where i want it.

It can be frustrating. I spent a couple hours doing it once (i was suprised how much fuel i lost constantly dumping the bowl, had an empty tank). A tach is nearly a must have, expecially for early tuning.


FWIW. I used a 31 pilot with my 47mm bbk and stock carb. A 30 worked, but the mix screw was more than 2 turns out and i kept loosing mix screws.

[dexameth]

Hey, I work at Partsforscooters.com and would love to help you get this dialed in (I'm a big fan of the 139QMB engines). Get down to Pinellas and ride with us!

Anyhoo... the jetting may be too rich. 50mm bore kit with pod filter and open exhaust would call for 95 main jet. I'd go a lil smaller especially if you are using the stock air box.

The a/f screw mix is only for idle. Dial that in first. Warm the scooter up then center stand it. Turn the a/f screw a quarter turn out, see if it idles up or down. Screwing in leans it out, screwing out richens it. If you get it to idle higher, great! now turn another quarter turn and see if it idles higher again. do this until it doesn't idle higher when you do a quarter turn, then go back a quarter turn. Now set your idle speed to around 1600rpm.

When the idle is set, you can now focus on main jet and needle clip position. Runs great at WOT but bogs getting there? That's needle positioning. Again, trial and error to find the correct spot. SOME stock carbs are not adjustable in the needle, ditch that carb and use the aftermarket.

[scooterted]

^^
That is excellent advice.

One can richen a part throttle mixture by stacking washers under a non adjustable needle. Adjustable needles are better.

This is probably the best how to article if you havent seen it.
www.49ccscoot.com/faq/carbtune.html


Edit: I like to spray out a carb when i go to tune it. Spray out the passages, especially idle passages with carb cleaner.