Top end query;

[XSokieSPECIAL]

Those of you who know me know what I have. For those who don't I have an 80 special with 79 carbs, Mac 4 into 2 with side kicks, stock airbox, 18" stand alone windshield, and an 850 F/D.

Here's my question: I can't seem to get it to go any faster than about 98MPH. I can pull it up just over 100 in 4th gear but as soon as I shift into 5th it slides back down to around 98.

It seems to run all but perfectly other than that.

Do y'all think that is right with the taller gear and windshield or am I having an issue?

[DGXSER]

I know with Thunderstruck which was a bare bone stock machine with the 750/850 FD I had no problem pulling up into the 7k rpm range with it in 5th gear from about 3k on up. Now it went alot faster from 3-5k if I downshifted, but it would get there and pretty darn quick after 5k to 7k. By my calculations that should be well over 110 MPH. Now i did not do it very often, maybe three or four times total and only on very open fairly level roads.

So besides the windshield, performance wise should be very close. I doubt the windshield should be holding you back THAT badly.

[XSokieSPECIAL]

That's kinda what I've been thinking but the dang thing runs perfectly otherwise.

I've noticed a strange rattling from the ignition cover lately that I hadn't noticed before. Not bad, but different.

I'm wondering if something may have gone awry with the centrifugal advance.

[GARTHXS]

i suspect advance

Or lack of advance, pull off the left side cover and see if it is advancing under throttle. My 78 was not really pulling after 85 and my vacuum advance was not hooked up. I checked and adjusted timing and rigged up the vacuum and now she pulls like a mule. These old bikes do run out of steam and to pull over 100 you will have to keep it in fourth with the fd mod

[jmnjrpa]

I haven't done the swap yet but I mounted the turbo style shield about 2 weeks ago. If anything it seems to have less drag than without. Don't know about triple digits. Aw shucks, you caught me. I did get there (maybe 100.....OK, maybe 120... I ain't saying more without my lawyer) last weekend uhhhhh maybe a couple times--but just for a second, officer.

Kidding aside, your top end should remain consistent with either gears as the wind speed is your greatest enemy. The power it takes to exceed a certain point is proportional to the force exerted by the wind. Wind power is squared--in other words a 40 MPH wind verses a 20 MPH wind's force is not double but instead squared! Relationship wise that means if the 20 MPH wind takes 4 HP to overcome the resistance then a 40 MPH would take 16 HP. Whereas the road friction and other limitations are relatively a straight line progression.

That being said the taller gears will take a little longer to reach the point of resistance limitation. The difference in top end should be minor. With the shorter gears you will redline at 140 MPH which is also (coincidently) the wind resistance limitation of a 100 HP bike. I would venture to guess you have another issue.

Check points should be: compression, shape of final drive (and mod), shape of middle drive, possible fuel restrictions, plug conditions, coils, valve clearances, wheel bearings, possible brake drag and maybe not last but not least atmospheric heat (heat alone can rob 10% of available HP).

Also +1 on your advance.

[XSokieSPECIAL]

Yea, It's really hot and humid today and that is no doubt playing it's part.

I do need to pull that breaker plate off and take a look though.

Is that allen bolt that holds the outer timing plate on right or left hand thread?

[dbeardslee]

I've got a setup that's pretty close to yours - 750 fd, windshield, 4/1, pods. I've had it up to 135, and it still wasn't through. I think the windshield actually gives you better aerodynamics, as without it your body's cupping air. Having skydived for 12 years I can tell you that a body position like your in when riding can really catch a lot of air. The windshield tends to channel the main part of the air flow over you. Just sayin' .

[bikerphil]

Timing plate bolt is right handed thread.

[XSokieSPECIAL]

Thanks Phil

[Ivan]

First off, that MAC 4-2 isn't going to do you any favors. Seems like these scoots like the H pipe or a very low restriction 4-1. The MAC isn't eiither. The H pipes (Jardine spagetti, factory) seem to help the low end a bunch, but don't pull on the top end as hard as a 4-1. However the 4-1 looses low end but above 5k, all friggen hell breaks loose and you'll feel like you've been shot out of a rail gun. My 79 has a very long exhaust, the muffler sticks out past the axle a good 6". I think that length is what makes the low end not stumble as bad as the Mac and Cherry Bomb I had on the other bike. So, I am pretty sure that enough volume in the exhaust plays a large role in power under the curve. You work at Midas, make a good exhaust. I'd bet you could sell them for at east $100 a pop.

Second, these bikes have an 8500 redline. Use it. Following you the other day, you seemed to be lugging around the low rpm range. The power curve doesn't kick in on these until about 3500 and then it seems there is another spike around 5k. If I am going for all out speed, I don't shift out of third and 4th until I see redline. If you short shift so the scoot catches next gear at 4k, it'll never dig its way up the tach.

You're mechanically accomplished enough that you'd know if anything else were amiss. However, get it figured out by August, or you'll end up wishing you had.

[randy]

These old bikes do run out of steam and to pull over 100 you will have to keep it in fourth with the fd mod

Nope, I have an identical setup and regularly break 100 in 5th and there's WAY more to spare, but that's when I run out of nerve

[XSokieSPECIAL]

LOL.......Ivan.........That is just about the NICEST ass chewing I have ever had. Enjoyed it didn't ya?

You are definitly right about me tending to lug the engine a bit. That stems from an overwhelming desire NOT to have to tear it down every week or two because I've intercoursed up some internal part because of blatent abuse.

Some people enjoy that sort of thing, I'm not one.

I think your right on about the exhaust. My obnoxious homemade straight through mufflers seemed to add to the top end and I may just put the loud bastards back on.

Besides, if I can't outrun ya I'll just run ya off the road..........LOL...

[crazy steve]

Top speed is entirely about horsepower; gearing can have an effect, but generally speaking, if the gearing is close you'll get the same top speed irregardless. Gear too low, and you'll hit redline before you top out. Too high, and the mechanical disadvantage can limit top speed.

So it sounds like you're down on hp...

'78E original owner

[Ivan]

I don't think these engines will grenade from revs. I have killed one from stupidly not putting a new filter on after having it out of the bike, but nothing else seems to go to hell from riding them too hard. The second gear issue will accellerate from hard shifting, but that's an easy fix. I rode red pretty hard all the time with the first engine. With the cherry bomb exhaust and my infintile tuning skills, it was either all or nothing, so I kept it revved up in the twisties all the time. It was quite happy screaming along at 5k and above. However, I value what little hearing I have left and went to Jardines.

Bring it up some time and we'll figure out your speed problem, even though everyone knows it the chicken holding onto the bars.

[cywelchjr]

Top speed is entirely about horsepower; gearing can have an effect, but generally speaking, if the gearing is close you'll get the same top speed irregardless. Gear too low, and you'll hit redline before you top out. Too high, and the mechanical disadvantage can limit top speed.

So it sounds like you're down on hp...

'78E original owner

I would partially agree with you (or maybe it's fully?). It was a common problem (or is) on the mid 90's to 2002 Camaros that top speed could NOT be achieved in high gear. Because of a combination of things, you needed to be down 1 gear to reach the top speed the car could attain. This may or may not be an issue with these bikes with the 750 or 850 FD since IIRC the issue with the Camaros was that 5th and 6th were both overdrives, and 6th was just too much over in an attempt to increase gas mileage, this might be an issue with our bikes, but I would think it unlikely.