How fast should my engine go

Sldgehmmr

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Apr 23, 2019
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Hi to all, I have a cheap ebay engine and was wondering what the average top speed is on a stock engine. Now I've done mods to go faster cause I'm a heavy guy(240lbs) and needed the extra power but my max speed is 38.8 mph with a 44 tooth sprocket. I've got a high compression head, opened up stock exhaust, 40mm intake, transfer port channels on the piston and I windowed the piston on the intake side for more flow. Should I be going faster or am I where I should be?
 
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Sldgehmmr

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Lol well I'm a newbie to the motor bike scene but I did benefit from the piston skirt mod. It's not a full window piston, I took away the assess material that hung into the intake port. I'm trying to improve my air flow to gain more power. I've done alot of research on these engines before I bought it but still learning.
 
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indian22

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I'd second that opinion on speed, but did you just forget to mention that you're also running a reed valve?

To answer your question about stock engine speed comparison.I have a 44 t stock 66 cc motor on a cruiser frame and weigh 175lbs. and my elevation is about 1200 ft. and 30 mph is about top on a cool moist morning. So I'd say some of your mods are really working, but as was stated in the earlier post the piston window modification has no effect unless you also run a reed valve setup with it.

Rick C.
 
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indian22

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I think I now follow what you did, but using the term piston window for what you were describing is an altogether different thing and that's what threw me.

Rick C.
 
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Sldgehmmr

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I think I now follow what you did, but using the term piston window for what you were describing is an altogether different thing and that's what threw me.

Rick C.
I got ya man like I said I'm new to this but very experienced engine builder, so I'm just sticking to the fundamentals of engine performance... air flow. I am running a stock carb could I benefit from getting a speed carb?
 

indian22

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I think that once you get an engine running well with a carb, any carb on these engines, it's as good as it gets at that point of tune. Your bike at this point sounds like it's running really good and I personally wouldn't screw that up.

I run Dellorto clones on all my 66 cc motors & have good luck with them once I get the jetting right, but just changing the carb didn't seem to effect the power noticeably, even after getting the correct jets in place...on a pure stock motor setup.

However when effective power modifications are made the Dellorto carb has seemed to supply adequate fuel mix to supplement the higher state of tune with a change of jet size & on some mods a plug range switch as well. I have a lot of jets, they are really inexpensive and easy to change out. Reading plugs after each ride will direct you to the proper carb setup. Don't get me wrong I'm just using the Dellorto as an example that I have a lot of experience with on my CG motors, there are other good carbs that others are using and making good power as well.

I plan each of my motor builds to reach a certain performance goal, many commercial vendors use the term stages, one thru three. I stick with stockish, modified and highly modified; all relating to street tunes that I can ride on a daily basis. Once I decide the tune I want that motor to sing I build it complete with all modifications in place before I tune the carb. Most builders seem to want to test each modification along the way & fiddle endlessly with getting the carb right at each change . I don't do it that way, I make all mods first and tune the carb last to feed those modifications.

When going for power increases that are really racing spec then a jump in carb size is definitely something to consider, but carb size increase by itself won't help performance in an engine that isn't setup to take advantage of the increased flow.

Changes that really make power do so in proper combination with one another and then the carb can be setup to run properly with all these changes. I'll give you an example. I have a 66 cc motor that has several power modifications, reed, piston window, Dellorto carb, after market aluminum head, jug modified with an extra port for the reed etc. I'd say it makes say 4.5 hp with the stock kit muffler and that's what I use day in and day out, but sometimes I want to cruise in the country and switch to a KTM expansion pipe which really brings the little motor to life, but also requires a jet & plug range change to take advantage of the expansion pipe.

One carb fits the bill from stock to highly modified once it's properly set up for the engines complete state of tune. I'd say I'm pushing the Dellorto close to it's limit with the KTM pipe & could benefit by going up in size to a small Mikuni. My choice of carbs isn't perfect, but it works well for me and at your point of tune so does yours!

Have fun as you modify and test.

Rick C.
 
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Citi-sporter

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IIRC some of the reed manifolds for these engines actually increase the intake restriction, the purpose of windowed piston/ reed boxes in 1970's DS motorcycles was to broaden the torque and flatten to power band a little for trail riding.

In especially small displacement piston ported bikes like the Hodaka Ace 100, going to a reedbox and windowed piston in the later Road Toad 100 lowered the RPM point at which the engine's powerband 'came on' while not killing the topend. It's a delicate balance and these engines are a pretty old school piston ported design. You really would need to come up with a better cylinder casting with a reedbox and 3rd port widow below it for it to be a better engine.
 
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indian22

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IIRC some of the reed manifolds for these engines actually increase the intake restriction, the purpose of windowed piston/ reed boxes in 1970's DS motorcycles was to broaden the torque and flatten to power band a little for trail riding.

In especially small displacement piston ported bikes like the Hodaka Ace 100, going to a reedbox and windowed piston in the later Road Toad 100 lowered the RPM point at which the engine's powerband 'came on' while not killing the topend. It's a delicate balance and these engines are a pretty old school piston ported design. You really would need to come up with a better cylinder casting with a reedbox and 3rd port widow below it for it to be a better engine.
Good point on reed restriction and power band observations Citi. All reeds and all reed setups are not equal, nor are they designed to provide equal benefits thru the entire power band & I've found that various expansion pipes used in conjunction with a properly designed reed induction system is also one of the keys to enjoying the benefits of the reed.

When RSE & other vendors started marketing focused on the China Girl owners; they greatly "oversold" their product and the benefits that one might expect from using their reed designs. Cutting piston windows was never mentioned, nor was machining a third port in the jug ever noted as modifications that were mandatory to taking advantage of the reeds operation. I modify both the piston and the jug. So a lot of folks got burned on reeds. It didn't take long and some, Norm at Venice was one, started working on solutions that would work and make power in properly modified motors. I use the RSE reed & a piston window along with 3rd. port jug. I machined both the piston and the jug to take advantage of the reed & two states of tune as I explained above: the kit muffler setup benefits more in the low to midrange, while the KTM expansion chamber really smooths out & comes on pipe at 7,500 rpms & will crank on out past 12,000 of course the real power lays between 7/5K and 10 K rpm's, not really gaining anything in the power sector at those higher rpm's.

So some of us have made those necessary engine changes and I for one enjoy the performance they provide. I have another traditionally modified piston port motor that really makes good power, but my reed port engines blow it away, apples for apples, both motors running KTM expansion pipes or both with stock kit muffler. In fact the traditionally tuned engine, with higher compression, ramps, ported etc. is really more cranky at low rpm's than the reed motors (I ride two with identical reed engine setups).

Two things I'll add as reed valve benefits: first blowback through the carb is minimized and secondly lubrication and cooling of both the wrist pin and the piston is vastly improved by that straight shot of fuel mix through the piston window to the piston interior.

Reed boxes I'm still working on, but haven't noticed any real improvement in performance to this point. I believe a power boost is there waiting to be found but I haven't seen it yet.

Substantial power improvements typically require work and alterations, even to the so called bolt on mods, but sometimes it's really worth the effort. I'm certainly pleased with the results I've received.

Rick C.
 

Citi-sporter

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I have a lot of 1960's~70's 2 stroke motorcycle experience, especially small bikes:

1971 Suzuki TS90cc
1969 Yamaha LT-5A 100cc (2)
1968 YG-5T 80cc
1964 MF-1 50cc
1968 Yamaha YL-1 100cc TwinJet
1975 Honda MT125cc Elsinore street (ElsySnore)
1971 Hodaka Ace 100cc
1976 Hodaka Road Toad 100cc

Of these bikes the TS90 and the Road Toad and to a lesser extent the Yamaha Trailbikes were decent midrange power, trail plonkers. Bikes that you don't have to row the shifter constantly to keep the engine on boil, being able to roll out of tricky trails while at about 3K to 5K rpm.

Both the Suzuki and Yamahas were rotary valves and that is an entirely different ball o' wax.

But for midrange pull on these bike engines, seems to be their strong suit as stock. I found that a better carburetor is a big factor for better running, at least on my 48cc. I adapted a Yamaha PW50 carb because it has such decent metering from idle to mid throttle. It loafs along at 18 to 22 mph with the engine doing slight 2/4 break, which is reminiscent of a friend's old Yamaha HT-1.
 
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Sldgehmmr

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Thanks for all the info guys. I'm considering a carb swap cause I kinda feel like I've hit my limit. I'm not looking to add more fuel, I'm looking for a bigger cfm carb to take in more air. With the 40mm intake and opened exhaust I feel like my stock carb is the bottle neck if my air flow. I forgot to mention at the beginning I also took out my base gasket (yes I did seal the bottom end and no it doesn't leak) but that also helped my power band
 
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indian22

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Yeah I'm a fan of the small 2 stroke antiques so I'll add Bultaco, Huskies, Vespas & Simplex to a short list that includes most of your list as well, but I'm a really old biker & love the dirt, but I'm just as fond of the 4 cycles and pure pedal power...now I'm learning a bit about e-bikes as well.

It's all good!

Rick C.
 

indian22

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Thanks for all the info guys. I'm considering a carb swap cause I kinda feel like I've hit my limit. I'm not looking to add more fuel, I'm looking for a bigger cfm carb to take in more air. With the 40mm intake and opened exhaust I feel like my stock carb is the bottle neck if my air flow. I forgot to mention at the beginning I also took out my base gasket (yes I did seal the bottom end and no it doesn't leak) but that also helped my power band
Could be, doesn't cost much to try parts out & if it helps performance super, if not try something else. Considering the changes you've made and the weight you're carrying your bike is running really well. From what you've posted with no photos it's really guess work on our part as to what you've done. "opened exhaust" is that port matching, piston ramping etc. or does that mean gutting the kit muffler or all the above? If you gutted the muffler that's a huge mistake and is truly robbing hp...unlike 4 stroke motors the 2 strokes demand a bit of back pressure to work well & from that point on 2 stroke exhaust theory fills entire books! If you are running an "open" expansion pipe, a good one, then you have the necessary back pressure to take advantage of the built in pipe harmonics of the expansion chamber to pull the fresh intake charge in as the burnt fuel exits & this really builds power in the purposed design band range of tune.

I understand that you feel the carb is limited, but have you read your plug immediately after a full throttle run to see if what you think is happening really is?

Increased air flow through a larger carb will demand more fuel to keep the engine from running lean...that's just the way a carb works. Also building power means more fuel consumed per mile. A gallon of fuel doesn't last half as long in my modified engines...no matter how I try to stretch it, without pedaling. No free hp lunch.

Rick C.
 

Sldgehmmr

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Apr 23, 2019
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Could be, doesn't cost much to try parts out & if it helps performance super, if not try something else. Considering the changes you've made and the weight you're carrying your bike is running really well. From what you've posted with no photos it's really guess work on our part as to what you've done. "opened exhaust" is that port matching, piston ramping etc. or does that mean gutting the kit muffler or all the above? If you gutted the muffler that's a huge mistake and is truly robbing hp...unlike 4 stroke motors the 2 strokes demand a bit of back pressure to work well & from that point on 2 stroke exhaust theory fills entire books! If you are running an "open" expansion pipe, a good one, then you have the necessary back pressure to take advantage of the built in pipe harmonics of the expansion chamber to pull the fresh intake charge in as the burnt fuel exits & this really builds power in the purposed design band range of tune.

I understand that you feel the carb is limited, but have you read your plug immediately after a full throttle run to see if what you think is happening really is?

Increased air flow through a larger carb will demand more fuel to keep the engine from running lean...that's just the way a carb works. Also building power means more fuel consumed per mile. A gallon of fuel doesn't last half as long in my modified engines...no matter how I try to stretch it, without pedaling. No free hp lunch.

Rick C.
To clarify I gutted the baffles out of the kit exhaust and welded the cap back onto it so it wouldn't be like an echo tip lol. I'd like to get a aftermarket exhaust but can't afford it. Yes I've been keeping a close eye on my plug to determine whether I'm too rich or lean. Right now it's just barely too rich but is rather it be too rich than lean. So maybe my true bottle neck is in the exhaust. I appreciate your advice and can take pics of whatever to help you understand where I'm at (build wise) and where i can benefit. Right now my engine makes good power with what is done but if like more out of it (I guess that's the drag racer in me) i daily ride my bike and drivability is good. I'd just like to have a beast 2 stroke on my bike lol
 
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javy mcdees

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6 months started the bike project, since then have like 5000 miles so far I built my own engine mostly stock but some mods iron cylinder MZ pipe had the NT carb then the RT carb now run a 19mm Dellorto way better tune now, bike went 40 with RT carb but acceleration was poor took awhile to get speed and failed with a head wind to hold speed but the 19mm Dellorto is a power house just runs good as I buy carb parts for it. solid 40mph with a serious head wind maybe faster wind at my back? I use a 7cc head the high comp head will burn you out if you drive on the street, piston will fail some way or another from heat. 7cc head run wide open all day.
 
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javy mcdees

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Could be, doesn't cost much to try parts out & if it helps performance super, if not try something else. Considering the changes you've made and the weight you're carrying your bike is running really well. From what you've posted with no photos it's really guess work on our part as to what you've done. "opened exhaust" is that port matching, piston ramping etc. or does that mean gutting the kit muffler or all the above? If you gutted the muffler that's a huge mistake and is truly robbing hp...unlike 4 stroke motors the 2 strokes demand a bit of back pressure to work well & from that point on 2 stroke exhaust theory fills entire books! If you are running an "open" expansion pipe, a good one, then you have the necessary back pressure to take advantage of the built in pipe harmonics of the expansion chamber to pull the fresh intake charge in as the burnt fuel exits & this really builds power in the purposed design band range of tune.

I understand that you feel the carb is limited, but have you read your plug immediately after a full throttle run to see if what you think is happening really is?

Increased air flow through a larger carb will demand more fuel to keep the engine from running lean...that's just the way a carb works. Also building power means more fuel consumed per mile. A gallon of fuel doesn't last half as long in my modified engines...no matter how I try to stretch it, without pedaling. No free hp lunch.

Rick C.
I was getting 40 miles to 2 quarts of gas now I get about 35 miles to the same 40 milesbasically use a quart more of gas a day
20190515_172308.jpg
 
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indian22

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To clarify I gutted the baffles out of the kit exhaust and welded the cap back onto it so it wouldn't be like an echo tip lol. I'd like to get a aftermarket exhaust but can't afford it. Yes I've been keeping a close eye on my plug to determine whether I'm too rich or lean. Right now it's just barely too rich but is rather it be too rich than lean. So maybe my true bottle neck is in the exhaust. I appreciate your advice and can take pics of whatever to help you understand where I'm at (build wise) and where i can benefit. Right now my engine makes good power with what is done but if like more out of it (I guess that's the drag racer in me) i daily ride my bike and drivability is good. I'd just like to have a beast 2 stroke on my bike lol
I hear you on the drag racer heritage, racing in general...hard to completely set aside even if you want to and I don't.

I get the cost part as well & even these little engines can eat into a limited budget. So building as dollars come available is often the only way to enjoy this hobby and I'm a big fan of having fun with this hobby. Good explanation on the stock muffler mod and getting the cap back on helps create some pressure which is a plus. The first real performance 66 cc I built used the stock kit muffler, I was so disappointed in how it ran (didn't run) speed wise. I sometimes run an induction tach and hr. meter on new builds not a very accurate tach but gets in the ball park with a bit of real time delay on rpm. This motor would run pretty good to about 6,500 rpm & and then very little occurred. I have access to a run way that is really flat and smooth & I tried everything I could think up to make it run...nothing helped. One cool morning I gave It another shot and at 6,500 it again seemed out of steam, but I held her wide open, maybe running 30 mph, for another 1/8 mile or so & it came up on the pipe like I'd hit it with a 100% shot of nitrous and screamed past 10,000 rpm's! All that motor needed was to get past 7,000 rpm's and it would really scoot.

Now here's the point to the story. 2 strokes need a good expansion pipe to really perform well; you could say it's like what a good 4 stroke feels like when it comes up on cam, only more. I've ridden two strokes since the early '60's and knew this, but blamed everything else first,especially my motor mods. If you are fighting the "pipe" the motor will never spin like it could. The only reason the stock pipe worked that day was it was a perfect day, low 60 degrees and relatively high humidity. It never ran like that again with the stock pipe, but when I added a quite average budget expansion chamber, designed to really work at high midrange it would blast through that rpm bottle neck anytime.

Pipe designs are all over the place and I've a half dozen assorted in my parts collection, plus some used motorcycle chambers I "altered" that didn't perform in the range I wanted...now just scrap. It helps if you have help selecting an expansion chamber & setting it up to run with the mods you've already made...this would include carb and settings.

Venice motor bikes can walk you through this 'cause he gets it & builds them, rides and most importantly sells what really works. Most vendors can't accurately answer your questions and only sell what they have & don't understand what they are selling. Venice really understands these little Chinese two strokes!

Rick C.
 
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indian22

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Javy with the MZ pipe where do you feel it really kick in? I'd think it would really start working at the higher mid range rpms on up I also run the 7cc aluminum head on two of my bikes with the Dellorto carb and vary the squish with copper gaskets. I'm running the thickest copper gasket with the 7cc head, I forget the actual thickness, and a thin base gasket, but it seems every engine I set up varies some on gasket requirements. The biggest takeaway from this is don't try to build the highest compression engine possible. They might be fast for a little while...

Also noticed you solved the engine offset problem so often encountered with the addition of a real motorcycle style expansion chamber.
 

Sldgehmmr

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Javy with the MZ pipe where do you feel it really kick in? I'd think it would really start working at the higher mid range rpms on up I also run the 7cc aluminum head on two of my bikes with the Dellorto carb and vary the squish with copper gaskets. I'm running the thickest copper gasket with the 7cc head, I forget the actual thickness, and a thin base gasket, but it seems every engine I set up varies some on gasket requirements. The biggest takeaway from this is don't try to build the highest compression engine possible. They might be fast for a little while...

Also noticed you solved the engine offset problem so often encountered with the addition of a real motorcycle style expansion chamber.
Off set issue? Im an experienced mechanic but only with 4 stroke engines like sbc, bbc, sbf so 2 strokes are kinda unmapped land for me but obviously I'm doing something right
6 months started the bike project, since then have like 5000 miles so far I built my own engine mostly stock but some mods iron cylinder MZ pipe had the NT carb then the RT carb now run a 19mm Dellorto way better tune now, bike went 40 with RT carb but acceleration was poor took awhile to get speed and failed with a head wind to hold speed but the 19mm Dellorto is a power house just runs good as I buy carb parts for it. solid 40mph with a serious head wind maybe faster wind at my back? I use a 7cc head the high comp head will burn you out if you drive on the street, piston will fail some way or another from heat. 7cc head run wide open all day.
I've been over 500 miles with a hi comp head WOT daily with no issues same piston since day 1
 
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FOG

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Something I use as a tuning tool is a Trail Tech head temp gauge.

https://www.ebay.com/bhp/trail-tech-temperature

I see they've gone up $10 since I bought mine, but I'd buy it again. It's battery operated 24/7 but apparently doesn't need much juice. Mine's been 5 years already. Normal operating range on my 360 is around 350 degrees. Long climbs and I mean mile or more long will get it over 400.

Another way to get it over 400 is too drop the needle one notch! That's all. And that's where it's so useful as a tuning tool. If I start walking on the leaner side it'll let me know .....

It also records and intermittently flashes the max temp as you ride. What resets the max is when the temp rises past 200 degrees. You park it. It cools off. The next time you ride as soon as it gets past 200 you get a new max.
 

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