Topic: 836 Won't Break 90mph in Fifth, but 110 in Fourth

[low-side]

     Here's what I 've got: CB750K5 with an 836 kit, Kerker 4 into 1 with 8 disc spark arrestor, K&N Pods, mildly ported heads, 135 mains with 40 low-speed jet and needle set 1 richer than stock (clip second from bottom), main jet air bleed at 120, and a Megacycle 63b cam.  In first through third gear, the bike will run to redline at full throttle.  In fourth, it hits a problem spot around 6000rpm but if you back off and feather the throttle to 7200 it will take full throttle again and run to redline.  In fifth you can only run up to about 6200 and it just bucks and coughs.
     I've tried different mains:  135 feels good until it acts up; 140 is too rich and 130 is too lean.  Needle runs stupid rich any higher and too lean lower.  Enlarging the mainjet air bleeds helped because you couldn't fight through it in fourth before.  I've even enlarged some of the holes in the emulsion tube and that helped some too but not enough. 
     Has anyone run into this before?  Any tips or ideas?  Thanks for any ideas.

[KB02]

First of all, I would suggest posting this in the "SOHC/4 Bikes" section as you will get more view and better responses.

Secondly, it's an interesting problem you are having as it appears to be a vehicle speed issue and not an engine speed issue. If you have no problem running 0 to full throttle in 1-3 gear, then you engine sounds like it is running fine and I wouldn't change anything on it. From Carbs to crank, nothing is really any different to the engine regardless of what gear you are in.

The only thing that I could thing to have you check would be your pod filters. Many people have complained about the filter pods getting bad air due to turbulence at speed. I have even heard some people complain that their bike shuts down with a good cross wind (granted, that is an extreme case, but it shows the point that I'm after).

Check out this thread: Pod Mod - It Actually Works... by crystalhelix. give something like that a try and ride the bike again and report back.

If that's not the issue, then it might be a bearing somewhere thats worn out and causing issues at higher speeds (not letting the bike get up to higher speeds due to the extra drag of the wear).

[kslrr]

I believe it is a horse power issue.  More RPM=more HP.  He is able to get to the higher RPM's in the lower gears because the load on the engine is just low enough that it can get through a slump (6000 rpm?) in the power band and then on to the next HP peak.  In 5th gear, the engine is getting bogged down in the slump.

Low-side, is the final drive ratio still the same?

At what throttle position in fourth gear do you hit 6000rpm?
Possibility #1 - fuel/air mixture is not what it should be at that position.
Once you know that position, you might be able to find needles with a modified profile.

Possibility #2 - back pressure too high at 600rpm @ a given load.
You might be able to tune the exhaust.

Possibility #3 - Do you have an after market cam?
Whether you have the stock cam or not, installing a slotted cam gear so that you can change the cam timing would allow you to tune out the power band slump. IMHO.

[754]

Just for the hell of it try 120,s for a few minutes, and check your plugs. You seem to be way to big on your jets and perhaps are not reading them correctly?

Just a thought, as a motor that size often  cant use jets that size.

[bobrhix]

hi,,,, when i rebuilt carbs on my gl1000 goldwing-- i had the floats to low. it acted likw what you are describing. would run hard thru several gears then starve for gas. just a idea.  bob

[low-side]

     Thanks for the replies guys.  Float height is at 25mm so it should be fine.  I found that I had 3 heavy floats early on and swapped them out so initially I was too rich over the entire range.  It will rev higher with smaller jets, but it isn't making very good power and ran hotter than Hell with 125s.  My final drive ratio is 18/50, though it wasn't much different with 17/50.  I'm thinking about this cam's characteristics and how it is interacting with the load on the engine and am thinking along the same lines as KSLRR.  I already have 8 discs in the arrestor, but a few more might help.  I might be too advanced on my valve timing, so pulling the motor and retarding the valve timing might be in order (last resort though, I tweaked my back last time and couldn't ride the damned thing for over a week). 
     In the various stages of this process, I've always run into this problem around 5/8 to 3/4 throttle.  From the various charts I've looked at this looks like the same spot the needle setting and main jet overlap in effectiveness and (I think) is compounded by the bike coming on cam around 6000 and drawing real hard through the carbs.  I think that this combined with the load from the top gears could be my problem.  I'm tossing around the idea of grinding my spare set of slides down from a 2.5 to a 2.0 (richer), lower the needle 1 setting (leaner) and seeing how that performs.  I'll try to do that Thursday and I'll report back with results.  Thanks again.

[kslrr]

Removing material from the bottom of the slides will make it richer?  I am asking because I don't know and would like to learn more.  I have heard of modifying the needles at the trouble spots.

[low-side]

The greater the height of the cut out, the leaner the mixture between 1/8 and around 3/8 throttle.  The only way to reduce the height of the cutout (and richen the mixture) is to raise the bottom of it by removing material.  I need to look at the set-up and see if it's feasible or just going to ruin a few slides.

[kslrr]

Wow, that could be a tricky mod.

[Beemer]

Just curious, but why are you running such a high tooth count countershaft sprocket? 

This sounds like an engine load related problem to me.  When I was actively racing my RC 1170 stroker back in 1979, I had a similar issue, and I wound up finally swapping carbs out.  I'm guessing you're running around what, 10.5:1 compression?  You might want to look at that cam and see if they recommend 12:1 or better compression.  That was my problem.  I had a wild ass cam (and later dropped the CR for an ATP turbo kit, NOT a smart idea with a wild cam, but god would it pull on the top end) with the original pistons in the kit being around that same 10.5:1, I was flowing plenty of air, but I wasn't compressing it enough under a load (I never did understand the math, but I trusted Russ Collins and Mark Roberston (omaha tuner/builder that was in a hole in the wall in Omaha at that time) and raised my CR and that fixed it.  Still doesn't make sense to me 30 years later either.

The other thing you might check is to try BACKING OFF on your spark timing a bit.  Lots of advance at really HIGH rpm's can actually hurt power..retarding the high rpm advance can net you a couple of HP in some cases, especially if your advance weight springs are a bit weak (giving you full advance under too much of a load..remember your gear ratios..big difference on engine load between first gear and 5th due to gearing)

I know these are long shots, but I've been racing cars and bikes since 1974, and I've found some rather unintuitive answers to rather stubborn problems I had that nobody could figure out for me. 

I'm sure you hit all the bases with ensuring that jet orifices are REALLY what they are stamped as (I did find several jets in my selections that were NOT what they were marked as, so use a plug guage to verify the jets).  Also, if you have a coil breaking down under load, that could have an effect.  You didn't say if you had an electronic ignition or points either..dwell time under load can make a difference as well if you're running points.

I can't see increasing back pressure making the motor run any better.  However, if you try reinstalling the factory filter box/filter, you might be suprised. 

Other than these, I can't think of anything other than the ignition curve itself, under load, your final timing under load, and the cam hitting at six grand, right where your problem is in the top two gears.  Is it possible that you aren't jetted right for precisely the rpm that the cam hits in?  While we all know the circuits used under WOT, the carbs are set up for a stock cam.  Could be that under load you're actually leaning out WHEN the cam hits.

The easy and expensive fix is to just turbo the motor and run a single carb or fuel injection.  There was a really nice dual weber/solex manifold on ebay a week ago that went fairly cheap too.

I'm converting one of my super sports to E85, and managed to snag a set of cbr900RR carbs off a mini sprint car motor that were converted to pure alky for 50 bucks off ebay..square slide carbs at that!  I may need to fab up a restrictor plate to get a better vacuum signal, but you might think about swapping over to some newer carbs too..

Dave

[low-side]

     Well, the cam specs are .370"/.370" with duration of 282/282 and I can't find the literature but I believe 110 degree lobe centers.  You're right, I have 10.5 to 1 CR.  I know that's a lot of duration and a lot of overlap, but they should still be a match, but if nothing else works it's worth a shot.  I've been considering backing off on the spark advance and seeing how it does on top end, I might give that a shot today.  I'm running the 18 tooth countershaft sprocket because it lets me get to highway speeds (and a little more) before I hit the problem area.
      I don't know how to describe it without leading you guys toward what I think it is.  A few hundred RPM before it gets really bad, you get a little vibration, a little hesitation,  a coughing but you can still accelerate, though not as well as before.  Then it gets real bad.  It will start coughing and bucking and decelerating and get hotter than hell.  Spit boiling rapidly on the valve cover (if it doesn't just bounce off) hot.  When I first started trying to jet it, I had some 150s in it and it was soggy and fell on its face around 5000 rpm.  I ran down in steps to 125s and it didn't make any power but it would run to 6500 rpm in top gear.  With 135s and my other modifications I can run up to about 6200rpm (making good power) before things get stupid.  A set of carbs from a bike that came with more cam might be a good idea, but I'm going to try some other suggestions first.   
     Again, thanks for all the suggestions.  I've really gotten a bunch of good ideas and will report back when I've tried a few of them.

[Markcb750]

Sounds like intake air flow restriction causing the motor to loose power at high air flow rates. 

If it does not miss fire at high rpm, it just will not pull, my guess would be to look for plugged or undersized filtration...Have you tried running with open velocity stacks, should not hurt for a few miles.


The next thing I might suggest is a miss-timed camshaft closing your intake valves too soon for proper high speed operation.  a lot of motors, (I don't know specifiably about OHC Hondas) will run with a few degrees of missed cam timing (one tooth)...

[Honda_Nut]

Sounds to me like your gearing has 5th too far below the peak of your torque curve while at the lower range of the gear. It seems like you might be able to drop a tooth on your counter-sprocket and actually get a higher top speed and acceleration.

[low-side]

Success!  I picked up a set of larger (32mm or 33mm, don't remember) carbs cheap and it works great!  After everything i thought about it and decided that I was running out of carb.  I picked up a set of carbs off an '85 CB700SC, threw a set of 132.5 mains in them, and can run any throttle position in any gear and it just goes.  It doesn't just do better on the top end either, but on the bottom as well.  Even with a 3'' extended swingarm it pulled the front wheel off the ground in first and in top gear I ran out of room on the highway around 125mph and still pulling strong.  Thanks to everyone who responded and happy riding!

[KRONUS0100]

Hey..totally unrelated thought ...if it is getting that hot at the valve cover.......make sure the upper end is getting plenty of oil...this has been a problem on big bore kits.

[Really?]

Glad you figured it out low-side.  And, it is all in writing for the next person with this issue to see. Kewl.

[low-side]

Thanks for the heads up on the oiling issue Kronus, but now that she's running right she's running cool.  I baked the rings on the maiden voyage so she smokes like hell so she'll be coming down again this winter and I'm thinking about taking the restrictor jets up one size for insurance against top end starvation.