Jetting and sprockets

I'm all pumped to install my stroker crank and 54mm cyl. and piston tomorrow. It'll be going in the bitchin' green K-4.


Currently the bike has an 88 kit with the TB v1 race head, NHRS pipe, bigger intake, race mild cam oil cooler and high volume oil pump as well as a new clutch with heavier springs (3sp auto clutch) and VM20 Mikuni carb with a K&N pod filter.
Now the questions.... What should be my base starting point for main jets and pilot jet? Also, will the stock sprocket combination be good or is there a better combination that is recomended by those that are running a 117cc configuration? If it helps I'm at about 800-950 ft above sea level. My measured tire circumference is 55.125"
The jets I currently have start at 130 and go to 160 in #5 increments.
One last thing, weather or not it has a bearing on things. When I pulled the head a couple weeks ago, the top of the piston and combustion chamber were very dark brown to black yet my plug was bone white. Should I consider dropping down a couple heat ranges on my plug or just let sleeping dogs lay?
Looking forward to your input.

Mike.

Fuglet01 said:

When I pulled the head a couple weeks ago, the top of the piston and combustion chamber were very dark brown to black yet my plug was bone white.

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That sounds alot like what my spark plug and head looked like when it burned up/leaned out hard on 10%ethanol 91. I really hope this is not your case, but thought I'd mention it. The exhaust valve had also turned a little white-ish.
BTW, I like the bitchin green too!:4:

In ontario our Shell premium gasoline is ethanol free. That's all I run in the bike. I guess to be 100% sure of not getting ethanol I could buy a 5 gallon can of gas at the local marina. It's guaranteed ethanol free.

Fuglet01 said:

In ontario our Shell premium gasoline is ethanol free.

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No biggie then. I'm glad I was wrong. At least we can throw this question out and move on. I put a plug in a CRF70 that was one step hotter and it caused it to start misfiring at high speed. Put the factory one back in and it ran fine again. Not sure what I was thinking at the time.lol. If your plug is too hot it may do the same.

this is a guess, but an educated one, when you increase the displacement, you're gonna have to increase the fuel delivery. you want a nice even carbon coating on your parts. not LOTS of carbon buildup, but a nice, even coat. on a plug, just a tiny bit of visible carbon. there's pictures here. somewhere…. I don't have any. sorry.

Regardless, figure out the main jet first. the pilot only feeds fuel on idle. when you get close on the main, use the needle to fine tune (in the throttle piston). THEN figure out how it idles. in reality, you can probably use the air screw to get it to where it needs to be. one step in either direction on the pilot is PROBABLY all you'll need. not so for the main. if you start off close you're lucky.

and you're also lucky to have pure gasoline.

HEat ranges on plugs seems to me more like a bandaid fix than a real one. with proper tuning, the stock heat range shouldn't let you down. i've has a c7hsa (the same one) in my bike for probably 1000 or 1500 miles since i built it. i might have changed it once, but not because it was misfiring. and the old one still worked so it's in the top left drawer of my toolbox.

a carboned up plug will short out, and no longer be an insulator, therefore, no longer produce spark.

just some input.

Plug heat range is at least as widely misunderstood as fuel octane ratings. It's nothing more than a measurement of how quickly a plug dissipates heat, allowing you to match the plug to the engine to get optimal plug temps. Too cold, it'll foul; too hot, you'll get preigntion, possibly even melted electrodes...neither situation is desirable. Nearly all of these motors work best with a C7, or C6.

Not to be the contrarian here but these engines can perform very well on gasohol (E5-10), as long as the carb is jetted correctly. The drawbacks of ethanol-laced pump gas are short shelf-life and inconsistent blends. FYI, pump gas has less gasoline than it used to have...even the ethanol-free flavors are lame. You have a better chance of getting actual gasoline at a marina, or airport. The only truly stable gasoline I know of, circa 2014 is VP Hydrocarbons "storage blend"...it's basically what was available when these were new bikes.

If you want to play it safe, find the rich limit at WOT, then go 1-2 steps leaner. That could mean nothing more than raising the jet needle C-clip 1-2 notches, or dropping-down 1 main jet size.

As for gearing with an 88cc bore-up + 3-speed, it's not going to make much more torque beyond stock thus you'll probably be able to go +1 on the C/S sprocket and that's about it. By the numbers, it doesn't seem like much. However, gearing is only part of the explanation. Of the expected 10mph top speed increase, half of it will come from the engine being able to pull another 1000rpm+/- in top gear.

If you're going for a 108-117 tune based on a 51mm, or 52mm, stroke you'll need to go about 20-25% taller on your sprocket combo.

That's what I'm doing. 51mm stroke crank and 54mm bore. I see locally advertised race gas for $4.00/liter. Thoughts?? I'd like to try it once just for s#%^* and giggles.

I just mentioned the plug heat range because when I disassembled the top end, the piston and combustion chamber were quite heavily coated in carbon. I only have about 250 miles on the build yet the plug was pretty much pure white. I guess I could have looked at it and said to myself. Damn!! This thing is running way too lean. Even though judging by the piston top it was a tad rich. Like you've said before, reading plugs ain't what it used to be.

Trying to find ethanol-free gasoline? This should help:

about pure-gas.org

Getting the piston & combustion chamber hot enough to shed the fluffy black stuff takes "freeway miles". IOW, those parts don't get anywhere near as hot as the plug (at least they'd better not) and with far more thermal mass, they take longer to reach maximum operating temperature. The old saying "taking it out on the freeway to burn out the carbon" had a basis in fact.

IMO, race fuel is best avoided. Aside from being a waste of money, TEL compounds are acidic and most of what's being sold as race fuel these days has very little gasoline content...testing has shown that some aren't gasoline at all(!). Think pump gas has a brief shelf-life?...

You can try going to your local airport, I can go to mine with a certified gas can and walk right out to the gas pump. It is 101 octane, low lead av fuel.

Looks like the stroker will have to wait a little longer. Still having no spark issues with my 1980 ct70

Decided to take one more kick at the cat. Got my new piston set and the oil is draining and parts are coming off the bike.
Hope it works out this time.

After 4 hours of working on it I have it all back together. Pulled the choke and 1kick and it was running (only for a few seconds then it died. Tried for half an hour to get it going and it just sputtered. Pulled the fuel line and no gas came out. Pulled the inline petcock and gas came pouring out.
The petcock is a brass housing with a round brass insert drilled to allow fuel to pass through it. The insert has a rubber sleeve with holes that line up with it to seal it all up. I found the sleeve came loose and remained in the off position even though the petcock was turn to on. I'm thinking that's the reason I had so many problems dialing in the jetting when I installed the vm20.
I pulled out the 135 main and went up to a 155. I'm hoping that'll be a good starting point. The #20 pilot seems small as I was at close to 3 turns out from seated so I popped in a 22.5 pilot. Now the only problem is the oil cooler I got from tbolt. It seems the banjo bolts are just a little too short as I can't tighten them up they must only be catching by one thread. I'll bring one to my brother and see if he will machine me a couple with a little longer threads on them.
As it stands right now I have a 51 mm stroke crank, 5mm bore TB race head with race cam mikuni vm 20 carb, aftermarket exhaust with header wrap, all new clutch fibers and heavy springs, hi volume oil pump, 16t front and 35t rear sprockets and dual purpose tires with a measured circumference of 55 1/8" I'm hoping to get close to 60mph top speed but I mainly want to be able to cruise comfortably at 35-45 mph. Can't wait to take it out for a spin

Took it out for a quick spin this evening. All I can say is wow! I should have put a stroker crank in long ago. Going uphill in 3rd gear at 30mph it'll pull and be doing 45+ by the time I get to the top. I'm running 16t front and 35t rear sprockets. It deals like it needs a 4th gear. Any suggestions?
I did run into what may be a major issue. When I got home it was smoking like crazy. I thought maybe just the dye burning out of the header wrap. I looked down and my shoe was covered in oil (not good) took a rag started wiping the bike down and found where it was coming from. It seems I missed tightening the cam chain roller bolt. Well, it's gone and now I just hope I can get the roller back in place to get another bolt in. Good thing I have a spare. Any ideas how I may be able to do it without tearing it down?
I just hope I didn't lose too much oil and bugger something up.

I don't really see any way to get the cam roller back in place without at least pulling the head. You may be able to just disconnect the exhaust hanger bolt and slide the head up a few inches so you can get the roller back in place.

Try pulling the stator plate, you can get it in place from there.

I got really lucky on this. I pulled the cam cover, took a flashlight and looked down into the cam chain passage. The roll had only move about 3/4" towards the cam. I was able to gently push it back and get the bolt in ( with some low strength loctite). Just glad there was no damage to the roller. The bolt couldn't have come out too far from home as only a little oil was lost.

Mike.

I've been riding around for a couple days now and I'm only having some minor issues with the bike. I have a persistent oil leak at the oil cooler banjo bolt (I'll have my brother machine me another one to replace the one it came with) and my signal lights don't work very well. I guess I will have to go through and clean all my grounds. All the bulbs tested fine with the meter.
I switched out the 16 tooth cs sprocket for a 17 tooth and it made a huge difference for riding in the city. I did leave the 35t in the rear although I may try a 33 or 34 tooth later on. I'm not trying to get max top speed I just want to be able to cruise comfortably at 35-45mph. The vm20 with the 155 main seems to be the perfect match for this setup. I'm running a 22.5 pilot but I'm backed out a little more than 2 1/2 turns so I texted my parts guy to source me a #25 pilot. With the new clutch friction disks and the heavy springs there is absolutely no slippage.
If anyone is thinking about adding a stroker crank to a daily rider, I say just do it. You won't be disappointed.

Mike.

Having the bolt fall out and the roller rattling around inside the tunnel, for a short time, is unlikely to result in tears...especially with a new timing chain, tension piston cap, etc. And you won't get much oil leakage, at that location. Usually, the roller will head south but, can be fished-out with a short length of bailing wire. Glad yours decided that you were overdue for a lucky break.

Leaky banjo fittings are not common, that's one good reason why they're used as brake line fittings. You might have a nicked sealing surface, or just need another sealing washer. Those dead-soft aluminum washers not only work well, they work best with minimal torque. Over-tightening the bolt(s) can distort them and result in leaks. The only solution is a new washer. Might be possible to add a rubber O-ring to one side, as long as it doesn't contact the bolt head.

You have the pilot jetting backward. Turning the airbleed screw out (CCW) leans-out the mixture. You'll probably do better with the #20 and the airbleed adjusting screw set closer to one turn out from seated as your starting point. That's only an educated guess.

As for gearing, you've still got some R&D ahead. Stock, these were geared for ~4.78mph/1000rpm in 3rd gear. 17/35 (2.05) is only about 12% taller than than the stock 15/35(2.333), taking you to 5.35mph/1000rpm. You'd have to spin the motor 9400rpm just to reach 50mph. That's twisting it harder than is necessary, or beneficial. I'd want to start closer to 6.0mph/1000rpm, roughly 20% taller than stock...to go with that 52% displacement increase. 17/32 will put you closer to 5.9mph/1000rpm.

You'll have to factor-in your tire circumference, which is ~2.5% shorter than stock. It does make a difference, more than might seem possible, intuitively. For a street bike, dialing-in the gearing pays. IMO, you want to find the limit of what the engine can pull without being weak in 3rd gear. Yes, top speed will increase (on the flat, all else being the same) as you approach the limit of engine torque. Once you cross the line, top speed will actually decrease, in a big way, with light headwinds and minor grades. You only need to be ~3% over-geared to get the weak performance in top gear. It really doesn't matter that you're not going for maximum top speed. Optimal gearing just happens to support it. You don't have to push it any hard just because it can go faster. The benefits of optimal gearing also happen to include a usable first gear and minimal revs per mile...IOW, efficiency & relaxed cruising. This setup ought be to able to maintain 35-45mph all day without breaking a sweat. There's nothing to be gained by spinning the motor extra revs, except more cylinder & valve wear.