Re: Make it faster

Ron Brown /

Hi I just discovered that I could interact with this forum without being a member.

This post is a follow up to the earlier discussion on running rich at max throttle/rpm.

I recently aquired a pair of 1978 Motobecane Model 70's with about 500 miles each.

When I got the first one running, it would 4 cycle at about 23 mph. Removing the airbox and cleaner raised this to about 27 mph.

When I got the second one running, it would run 27 mph with the air cleaner and air box in place.

For some unknown reason, the first one had a 58 main jet and the second had a 54 main. Swapping jets swapped the symptoms. I used a clock bushing tool to reduce the 58 jet to slightly smaller than a 54 and the number 2 now runs a little better than the number 1.

When the weather warms up a bit I will be doing more testing as I have the advantage of two identical bikes to compare. I will post the results.

On the rich/lean thread, I agree that reducing the oil/gas ratio does not affect the mixture but it does reduce plug fouling caused by the rich mixture and I suppose if you are running that rich, less oil in the gas is ok.

Does anyone know if this rich at high rpm problem is the manufacturer's method to restrict speed to < 25 mph? It seems like such a crude way to do this, but evrything on my mopeds is esentially like new and stock, so I can't see a better reason for this problem. I just have a hard time believing that a solution resulting in bad gas mileage and fouled plugs would be the best they could come up with.

For those who are considering a smaller main jet to help with this problem, keep in mind that the tendency for the mixture to get richer, increases as the throttle is opened wider. This is because the air box and filter create a greater pressure drop as the air flow increases. I think that this may cause a too lean condition at lower rpm. I will try to test this by doing plug chops under various conditions. In any case, when installing smaller mains, check the plugs immediately for good color. Remember, 2 cycles run best just before they sieze up.

That&#039;s what I was thinking

As far as whether the manufacturers used the main jet size as a form of governor.

Back a while ago when Evan posted about his engine (Puch Maxi) "4 stroking" at top end.

Then Tony accurately described all the symptoms of too rich a main jet but said he wasn't convinced it was too rich of a main jet..... but that the same problem had cropped up several times in the last year on this forum with no clear solution.

That was when I began to believe that the European manuf. might have looked at the US market with its restrictions on speed and HP and might have decided that the best (and cheapest) solution was to simply put too big a main in their Euro spec peds and meet the US spec's.....(too big a main preventing the ped from revving out and making more HP and speed... like it should)

They probably figured that anybody would figure this out quickly (after all they are experts.... isn't everybody?) and rejet leaner to get back to correct performance and speed.

But the truth is a lot of moped dealers (especially Euro peds) have very little mechanical knowledge and just want to sell them and make money.... so they might not have any idea what to tell his customers when they come back wondering why it doesn't seem to run right.

The Jap manuf peds are sold at motorcycle dealers with good service departments... and I beleive the Jap peds are designed specifically for this market (in some cases) and are not "muzzled" when they leave the factory.

But I do know specifically of 3 Jap motorcycle models that are intended to be modified immediately if you want full performance... by removing restrictors, or switching to a different exhaust pipe (included when you buy the bike).

I have limited experience with Euro peds... that is why I am mostly guessing at this as a possibility.... but I almost posted this exact note here a couple days ago... the Ron brought it up !!

RE: That&#039;s what I was thinking

Ron Brown /


Of course! That is why one had a #54 and the other had a #58.

Although I believe these two mopeds originally came from the same owner, one has a local bike shop sticker and the other does not, indicating that they may not have been purchased at the same shop. One may have known to change the jet while the other did not.

I also find it suspicious that the air boxes have stickers with part #'s and USA printed on them. I would kill for a look at a european intake tract.

When you mention removable restrictions on jap bikes, what are you refering to? I understand european "learner" bikes use somethin easy to defeat but I do not know what this is either.

Any help?

By the way, only one Motobecane came with an tire pump so I shortened a standard bicycle pump tonight. If anyone is interested, just ask.


RE: That&#039;s what I was thinking

The Jap bikes with restrictors in them are not street legal bikes...

they are the biggest Honda 4-stroke, and the biggest Yamaha 4-stroke

these are supposed to be 'trailbikes'...(with headlight and taillight)

The other one is a now discontinued Suzuki 2-stroke trailbike,

(that was the one that came with the extra pipe)

Since they are not street legal I think the manuf's corked them up to be more

"socially responsible".... but they are also fairly easy to get street legal, and a lot of people uncork them anyway.... just like all the mopedders... everybody wants to go a little faster.

I have also heard of the UK "learner restricted" street legal bikes that they cork up so tight as to reduce a 30 HP bike down to 17HP... I believe they do that with intake and exhaust restrictors.

RE: That&#039;s what I was thinking

Of the two '78 Puch Sport Mark II's that I have that go into "four-cycle" mode at 23-25mph, thereby limiting the top speed, one of them I have had since '79 and it didn't always do that, it always use to get right up to 29-32mph, even with my 200+ pounds of dead weight. This is a problem that it kind of developed, actually associated with my having to screw around with cleaning the carb after it got gummed up while I was in Michigan for 10 months and it was in Florida. To be honest, it has never run right since then, oh by the way, another part of the problem, along with a float chamber full of varnish, was a rusted metering rod. I very, very carefully ploished the rod and I do not think that it is the problem at high speed because at WOT it is esentially completely withdrawn and all the fuel metering chores are the job of the main jet. (I left fuel in the ped on purpose, it had looked like I was getting a lot of rust inside the tank if I drained it and left it empty, so I leave it full and drain the fuel out and discard it each winter when I return, the stupid fuel shut-off valve leaked enough over time to let a drop or two into the float chamber which evaporated in the summer heat, and then another drop, and so on for 10 months, now I pull the hose off. The Puch fuel tank is integral with the frame down tube, so internal tank rust might eventially destroy the structural integrety of the frame, also bummer to clean out every time) I also have a '79 Newport II, same ped but for wheels and seat, it pulls 30mph anytime with me. So, I think there is something screwed up with the carb, although, that air cleaner suggestion is interesting, except I thought it was more of an induction noise silencer than filter. It isn't exhaust, been there, done that, got the T-shirt. Lots of noise but no faster! I am also going to screw around with the float system, my theory being that if the float needle/seat have deteriorated and can no longer seal off the flow of fuel when subjected to high speed engine vibrations, the level of fuel in the float chamber will be momentarily too high and result in an over rich mixture of fuel to air. And some day I would like someone to explain to me why a two-stroke cycle engine will fire only every other time (so called four cycling) when the mixture is too rich?

RE: That&#039;s what I was thinking

Ron Brown /


Thanks for the input. If your ped used to run ok, I would wonder if the main jet and/or the needle jet (or it"s equivalent), are corroded. I have had problems with old motorcycle jets which had a dull or etched surface causing a rich mixture. This was only visible to me under a jeweler's loupe. Replacement with shiny jets solved the problem.

I would not be suspicios of the float needle and seat if the plug does not foul at idle, when even a slight richness causes great problems. If your carb has a brass float, make sure it has nothing inside it, if it is composite, it may have absorbed fuel making your float bowl level too high. Did you poke anything through the jet to get the sludge out of it?

Having two of the same model moped, you do have the option of swapping carbs, and/or parts, provided that you check the jet #'s etc. Also, make sure that the air cleaner element is clean and dry.

On my over rich ped, even the air cleaner housing without the element was enough of a restriction to have a noticeable effect.

My understanding of the four/cycling symptom, without going back to my 2 cycle book and re-reading it, is that a rich mixture burns more slowly sothat the incoming charge has to fight a still burning mixture. This results in a non combustible mixture for that cycle. On the next cycle, the combustion chamber is flushed outand the mixture is ok.


RE: That&#039;s what I was thinking


Yes, it is an obvious exercize to swap parts between the two Puchs to aid in problem cause identification, however, I am reluctant to take a chance on screwing my sweet running Newport II to get my ratty Sport Mark II running at full speed again. But now that I've got a second bike with the same problem (I now have two slow Sport Mark II's and the faster Newport II) I am encouraged to get to the bottom of this. This summer, I picked up a pretty good looking carb from an eBay auction, and another off a junk J.C.Penney Pinto, so I have a bunch of stuff to work with without disabling the good ped. For now, the stuff (and I) are up here in Mich. and the mopeds are in Florida so I'll have to wait till I get down there to start experimenting.

Your hypothesis for why an overly rich fuel/air mixture causes a two-stroke cycle engine to fire every other cycle is an interesting one that I had not considered. My experience and knowledge in IC engine combustion theory, while fairly extensive, is very much limited to four-stroke cycle, spark ignited engines and I had been trying to think of how the residual trapped gases, for instance, could dilute the incoming charge to the extent necessary to prevent ignition. In fact, I wonder, if the mixture in the combustion chamber is still burning when the piston uncovers the transfer port between the cylinder and the crankcase, what prevents the flame front from burning back down the passage and igniting the mixture in the crankcase with disasterous results? Boy, I think that would get your attention. So, you mentioned a book on two-stroke cycle IC engine combustion theory, what do you recommend for me to read?

RE: That&#039;s what I was thinking

Ron Brown /


I can claim no extensive knowledge of the infernal combustion engine, however, being also stuck in Michigan, the winter is a good time to read. The only book I have which deals with 2 cycles extensively is "Tuning Your Two Stroke Engine" by K.G.Draper A.M.I.M.I. and published by Coles in Canada, c1977. I picked this up in a used book store, an excellent source for interesting motohead books. I would guess that you may have some success at your local library.

On the subject of firing while the inlet port is open, I am helping one of my neighbors to re-build a small 2 cyl outboard and I am fairly sure that he had this problem. However, I think it was caused by oil on the points from a leaky top seal causing it to fire at odd times. When we ran it in a tub in the driveway, it coughed and sputtered and banged and actually blew oily bubbles out of the crancase halves joint.

I think this is avoided in the average two cycle because the mixture is being blown in under some pressure and the inlet port closed fairly early in the cycle. This said, I have experiences an occasional "pop" from the carb.


2-stroke tech

Books on 2-strokes... the 3 best (that I know of) are written by these guys.

Gordon Blair (a British engineer)...don't remember title.

Graham Alexander Bell (also a Brit engineer)...Titled ::2 stroke engine theory and performance (or something like that...been a long time)

Gordon Jennings (an outstanding American engineer though he never went to college) Titled..The 2-stroke Tuners Handbook

.......Sadly , Jennings died here just a few days ago... he was an outstanding 2-stroke engine researcher and engineer. Who among other things was consulted on GM's Orbital 2-stroke engine design program at it's beginning.... he told them stuff that they didn't believe (they were all 4 stroke engineers).... 2 years later they were having all the problems he told them they would...

....He also was the designer of the small 2-stroke engine made for the US Army for their small (8 HP?) radio controlled battlefront video spy plane...which were designed to fly over enemy positions while transmitting video back to base ...(while being shot at)... they were used in Desert Storm.

There are many articles on engine tuning and theory by him at this site....... scroll to the bottom and look at the index of technical articles... Gordon Jennings.. a great mind!!

In a correctly running engine the charge burning is completed within less than 10 degrees of crank rotation (remembering from a long time ago).. which is looong before the exhaust port opens..which is well before the transfer ports open.. so the charge would never light the fresh intake mixture (in that case).

But exactly why it "four strokes" while too rich is a good question.

And Ron.. Tony was talking about the transfer ports, not the intake port.. the intake port is in fact wide open as the spark plug is lighting the mixture at TDC.

RE: That&#039;s what I was thinking

Ron Brown /


Your message was too long and did not give me an answer box so I foolishly clicked "reply to this message" several times. Please ignore any empty posts.

Thanks for the references and yes on the transfer port. I guess when you do not use the terms too often they all look the same.


RE: That&#039;s what I was thinking

Thanks for the book leads, Fred, I'll have my librarian friend see if she can borrow one or more for me. I met Gordon Jennings briefly several (6-8) years ago when he visited us at General Motors Engineering Staff, he was a neat guy. Yes, almost all the fellows and gals involved with our 2-stroke cycle program (except a couple new hires) were 4-stroke people. (fresh off the design of the SATURN engine, if I remember) One guy came from Ireland, University of Dubland, I think, and there were some with a Detroit Diesel background, but hay, where are you going to find 2-stroke talent in the heart of GM's advanced engine design community? And no, he wasn't ignored at all, the team knew that there was a universe of knowledge that they lacked when they began the program, and there was a lot of skepticism of the information that was being provided by the Austrailians. It finally ended up that, like the Wankel rotary engine program thet proceeded it by at least a decade, the two-storke engine could not be made to meet the current, proposed, and expected federal and state vehicle exhaust emissions standards and it was dropped.

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