Topic: CB750 Racing

[bear]

Racing in the rain is interesting, some guys are really good and enjoy the rain, others refuse to race.

Your dead right there Brent.
I was the only bloke in our class with a dirt track and speedway background.
I loved the rain!
The rest of them......................................not so much.

Cheers,
Brian

[JohnN]

I grew up racing in the dirt and I still think a wet race in dirt is more fun than a wet race on pavement! On the other hand, I probably would have enjoyed sliding around on wet pavement 30 years ago.

[Haybus]

Had an awesome time this weekend. Felt like I was getting comfortable on the bike and the track. She ran real strong on Saturday, very happy. But practice on Sunday something went wrong, lost all the mid range power. #4 was 100 degrees cooler than the rest. That's not good. Changed plugs and ran it in the parking lot, still the same. Pulled the bowls off and there was some silt in the bottom. #4 main jet and needle jet looked like they had a little crap in them. Ran it in the parking lot again and still the same. Noticed the #4 plug wire laying along the head, whoohoo, that's why she didn't run good after the cleaning. Yeah right, didn't make any difference. But it's weird, seems like #4 is firing, just not well. Oh well, I'm here to race no matter how sick she is. Power seemed ok at WOT, so that's how I rode. Practically killed it on both starts, but once in the upper revs she was so-so, but not great.

Pull the tank off today and strain the gas. Nothing. Pulling the carbs off to clean them and I notice #4 choke is on. Hmmm, 1, 2 and 3 aren't on. Ha, the #4 choke arm broke off the shaft and the choke plate slid down. And then I kick my self for not pulling the filters at the track. What a dumbass.

Can I just pull the chokes out of all the carbs or do I need to leave them in and somehow block their motion? I don't think there's any repairing of the actuator arm to the shaft. They're the stock Keihin carbs with the sliding choke plates.

Even though the bike ran crappy for the race I met the most wonderful people and had the greatest time racing!!

Alan

[bwaller]

Glad it went well except for the choke fiasco. I've said it before, the vintage race crowd are the best people anywhere!

What part do you need. I have carb bits. 


http://www.cmsnl.com/honda-cb750k2-four-usa_model14362/partslist/E25.html#results

[Haybus]

Brent, I don't see that they ever point to it in the drawing. It's the little sheet metal arm that connects to the shaft going through the carb body to actuate the choke. The shaft and arm look like they're just pressed together, but the hole in the sheet metal arm is pretty torn up. What I'd rather do is just remove the chokes, but I'm not sure if that would cause any problems? I've never used the chokes to start.

Alan

[bwaller]

If it'll start without them go for it, but block off any air infiltration.

[Haybus]

I just snugged up all the shaft bolts. Makes them practically impossible to move, with the rubber gasket doing most of the anti rotation work. I'll be sure to check #4 once in awhile, but I don't think it's going anywhere now. Took her for a very brief ride and she's back to normal. Got straight to 5th gear without drama. Whoohoo!!

Alan

[JohnN]

Glad to hear it was a minor repair- sounds like you are ready for round 2!

[Haybus]

I'll be there rain or shine John. Hopefully I can snag last place with a little more speed this time 

Alan

[turboguzzi]

good to hear you liked the experience, expect addiction to develop soon...
would take the whole choke assy & shaft off i was you, can only improve airflow

like your fairing, where is it from?

and what's the deal at the massage girls tent BTW?

[Haybus]

TG, fairing is a Rickman half race fairing from Airtech Streamlining. My wife saw that tent and asked the same thing. Me:"What are you talking about, oh that, I never really got to that end of the track". Wife: "Uh, I can see that's you on your bike." I never saw them working on any customers, but I did see them walking out to watch a race. At first I thought they were strippers dressed like cops, with riding crops, and the shortest shorts I've seen in quite some time, pulling a prank on someone. I'll pay a little closer attention next time 

[Haybus]

I didn't get the memo about longer shocks before I ordered them. Thought I would run them this season and send them back to have them lengthened. John suggested I walk a couple pits over last weekend and take a look at some extenders on another bike. That John is always looking out for me  . So there they are. 1.5" on the bike, 1" and 2" on the table just in case. Can easily change them at the track in a few minutes. Excited to see how much they help the bike turn.

Alan

[JohnN]

Nice work; you could probably sell the unused extenders at the track! Next I might try talking you into losing a couple of chain links to shorten the wheelbase.
John

[bwaller]

Be aware that although the taller rear helps turn in by reducing rake angle, it also reduces trail. It can make the front "nervous" if something happens to upset the chassis. Tank slappers sometimes result, which is what ruined my day/season. 

Next on the list, besides John's request for wheelbase attention, will be triple clamps with less offset to bring the trail number back in line.

It's never over man! 

[voxonda]

Totally agree with Brent,

I raised my bike at the rear with 20 m/m, together with a 18 inch front made the bike nervos. I lengthen the swingarm by 40 m/m and use triples with 45 m/m off-set instead off 60m/m
Just made sets for a Davies 500 with 25 m/m off-set.

[Retro Rocket]

Totally agree with Brent,

I raised my bike at the rear with 20 m/m, together with a 18 inch front made the bike nervos. I lengthen the swingarm by 40 m/m and use triples with 45 m/m off-set instead off 60m/m
Just made sets for a Davies 500 with 25 m/m off-set.

Yep, the boys are onto it, the 750 has only around 3.75 inches of trail stock , thats not much trail at all, {most modern sport bikes are around the 4 inches of trail},as has already been said, raising the rear will reduce trail even further and make for a pretty scary ride, it also reduces the rake , the bike will will turn in better but it will want to oscillate  or "tank slap" as its more commonly known, been there, done that...!!  Rob's got the right idea with the reduced offset triple clamps, they will add trail and help stabilize the whole bike.

[JohnN]

Agreed on the offset, Alan is already talking about making up a set of triples. I ran 45mm offset last season but recently switched to 30mm and am very please with the overall improvement.
John

[Haybus]

My plan was to play with the extensions at the next race and work on triples if needed for the race in June. But if everyone thinks I need to go straight to less offset triples then maybe I'll get to work on that now. I always thought riders feel a bike's suspension a bit differently. What feels good for one person might be unrideable for another. But maybe that's more compression, rebound, stiffness, sag related than rake and trail?

Alan

[JohnN]

In the end it is the combination of settings that will make it work for you, but as noted above the stock trail is minimal. Furthermore, you already decreased trail by installing an 18" front wheel. Each incremental addition to rear shock length will reduce trail further. I think it is safe to say that as a minimum you can decrease offset to 45mm and work from there for future changes. The good news is the change to 45mm is cheap and easy as a 79 750K triple (which is 45mm) will bolt right on. You can find them on Ebay relatively cheap. You could experiment and then build your own tree to suit your needs.

Some riders like to increase wheelbase. For me, I found that reducing wheelbase made it easier to turn. I did have stability problems before but finally have that almost eliminated and have confidence in the bike that I did not have before.

John

[Retro Rocket]

I always thought riders feel a bike's suspension a bit differently. What feels good for one person might be unrideable for another. But maybe that's more compression, rebound, stiffness, sag related than rake and trail?
Alan

Your last statement is correct, rake and trail are in the physics category, if you have way to little trail, it really doesn't matter how good you are, it will buck you off at some point. You know what a shopping trolley looks like with its wheel shaking from left to right..?  Thats what a lack of trail does...   If you can accurately measure your rake with the new shock mounts fitted and give me the height to the center of the front axle from the ground and your offset, {60mm i suspect at the moment if stock} I can tell you what your trail will be currently, I have a trail calculator that makes it all relatively easy, also, if you know how much trail you are aiming to get, it will then give you an offset measurement for the triple clamps...

[Haybus]

It's currently 24 degrees and 12.125" (307.975mm) to center of axle. If 60mm is stock offset for 1975 then that's what I have. I was looking for the offset on the WWW, but the number eluded me.

John, were you running the 79 750K triples with the GL forks? I thought they were 36mm legs?

Alan

[Retro Rocket]

It's currently 24 degrees and 12.125" (307.975mm) to center of axle. If 60mm is stock offset for 1975 then that's what I have. I was looking for the offset on the WWW, but the number eluded me.

John, were you running the 79 750K triples with the GL forks? I thought they were 36mm legs?

Alan

Alan, your trail at the moment is around 71.43mm, are you sure the rake you quoted is correct..?  Thats an awfully small trail number, I wouldn't even ride that on the street like that if thats all correct. I remember someone telling me at some stage that, for every inch higher at the rear, equates to about 1 degree less in rake, stock rake is 27 degrees....

[Haybus]

Well, pretty sure. Digital inclinometer on the steering neck and fork legs. Both read 66 degrees so 24 degree rake. Stood up straight, both wheels touching the ground. However, that's with the 1.5" extensions installed. Kind of makes sense since I have an 18" front wheel and raised up the rear. I haven't ridden it, so don't worry.

I'll swap out to the 1" extensions and no extensions tomorrow and check the rake again so I have some reference numbers. I just purchased a 79 triple setup off ebay. That should bring my trail up to 3.46" (87.88mm) with the 1.5" extensions. I realize there's a lot at play here, but what's a safe trail number? Something above 3" or 76mm?

Alan

[Retro Rocket]

Well, pretty sure. Digital inclinometer on the steering neck and fork legs. Both read 66 degrees so 24 degree rake. Stood up straight, both wheels touching the ground. However, that's with the 1.5" extensions installed. Kind of makes sense since I have an 18" front wheel and raised up the rear. I haven't ridden it, so don't worry.

I'll swap out to the 1" extensions and no extensions tomorrow and check the rake again so I have some reference numbers. I just purchased a 79 triple setup off ebay. That should bring my trail up to 3.46" (87.88mm) with the 1.5" extensions. I realize there's a lot at play here, but what's a safe trail number? Something above 3" or 76mm?

Alan

Most modern race bikes are closer to 90-100mm trail, factory stock gsxr 1000 have 23.5 rake and 3.86 inches {98mm}  of trail
,if your numbers are correct, a 40mm offset triple will give you about 93.32mm of trail, a 45 mm offset gives 87.85,. 76mm is still very little trail... I would wait for the other racers here to chime in , but I think somewhere between those 2 figures i just quoted would be good, you can then adjust rear height up or down to get more or less trail..

[JohnN]

Alan:
 I used 79K triples when I was using the stock front end, prior to switching to the Goldwing front end. The GL1000 also has 45mm offset so I used that until I switched to a custom made 30mm triple for this season. I have about 24 degrees rake and 4.2" trail. Last season I had 25 degrees and 3.7" trail (w/ 45mm offset) which did not work well. Your 1" extensions with the 79K triple sounds like a safe move for now, and work from there as you get a better feel for the bike and the track. Next race at the Ridge is a smoother track; I found it more forgiving with less trail than Pacific so it is a better place to experiment with new settings. Turn 1 at Pacific was often scary when I had less trail; it is stable now!
John