Topic: Missing horsepower

[SunLar]

I have just had my CB 750 on the dyno bench. I would lie if i said i wasn't disappointed. 50 hp on the rear wheel.

Problem is i run 810cc, portet head, new valves, action four ss-1 cam, Tmr32 carbs.

The guy who portet the head said he had expected around 70hp.

I ran a cr test and it was 9/ 9,5

Any suggestion to where i maybe can find the missing HP??




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[Tim2005]

Did the dyno give a printout with the air/fuel ratio values?

[SunLar]

Yes i did, ran way to rich with 150mains, ended up with 137,5 still a bit rich at top but had a lean spot around 4- 4500 rpm. Considering, maybe carbs are to big :/


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[Jerry Rxman Griffin aka MuthaF'er]

Could be the carbs. With my 810, Yoshi Daytona cam, stock F0 head, stock carbs, Kerker at 6900' I was running 115's with the needles pulled up one notch and it was a tad on the rich side.

[SunLar]

we talked about getting even smaller jets, but are nervous around getting the midrange too lean.
Think, I have to recheck valve clearence and timing

[Medyo Bastos]

or paint it red!


...Save a tree, eat a beaver...

[dragracer]

Did you verify cam timing??

[scottly]

50 RWHP is less than a stock 750? My 836 with a stock un-ported K1 head and Weber carbs with 30mm chokes put out 70 RWHP on the dyno, and I believe with a ported head would be close to 80. Can you post the dyno printout?

[SunLar]

Did you verify cam timing??

No, not yet. Hopefully i will whitin a week

[SunLar]

50 RWHP is less than a stock 750? My 836 with a stock un-ported K1 head and Weber carbs with 30mm chokes put out 70 RWHP on the dyno, and I believe with a ported head would be close to 80. Can you post the dyno printout?

I will try to get a printout

[PeWe]

My 836 with ported head, 34mm in valves same cam and carbs as you gave 80rwhp.
Jets: Main 135 (150), pilot 30 (27.5), needles 4:th (as std), fuel screws 1.5 turns out.  KN pod filters and rather open 4-1.
main 150 might be for vel stacks

Changed main to 130 with 4-4 without diffusers. This is too lean.

Ignition might be retarded which make engine powerless. I heard pinging from low so I retarded ignition and lost the pinging AND power. Too lean carbs got richer mixture (pilot) and correct ignition woke up the power.

[SunLar]

PeWe I'm glad you wrote - We tried to retard the ignition and it didnt got better. I can see i wrote 137,5 main, but that was wrong we ended up with 132,5 but he still think it's too rich


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[SunLar]

My 836 with ported head, 34mm in valves same cam and carbs as you gave 80rwhp.
Jets: Main 135 (150), pilot 30 (27.5), needles 4:th (as std), fuel screws 1.5 turns out.  KN pod filters and rather open 4-1.
main 150 might be for vel stacks

Changed main to 130 with 4-4 without diffusers. This is too lean.

Ignition might be retarded which make engine powerless. I heard pinging from low so I retarded ignition and lost the pinging AND power. Too lean carbs got richer mixture (pilot) and correct ignition woke up the power.

What kind of pistons do you run?

[PeWe]

I runned old RC 836 that have a dome that fits the head with widened chamber. Old style to open chambers and have pistons with wider domes.

- Cam timing correct? I timed my SS-1 cam IN: 22/55, EX: 56/24 @ 1mm lift and 0 lift (measured total lift was 9.18mm IN and EX). I tightened the valve adjuster screw until the valve started to lift (needle moved 0.01-0.02 mm) to remove all play.
- Really too rich with higher throttle lift?
- You have pods?
- Do you have an A/F measurement when running the Dyno? My dyno chart has it when a sensor was sticked into the exhaust. Measured at WOT.
I had 132.5 main when that dyno was done. I changed cam to RC295 copy and 135 gave more torque when twisting throttle on 5:th gear from 80-90km/h.

I reduced the acc pump stroke later on, to min setting. I checked the floats before and reduced carb 1-2. I saw this summer that cyl 1-2 ran extra lean close to damage and $$$$.
I have purchased 3 more clear tube tube tools (VM29 tools) so I can check all 4 at the same time. Wego III Air/fuel sensor kit is under shipping, here next week. I'll check all cylinders one by one next year. Sensor in a sniffer pipe to be mounted into the exhaust. Measure during normal driving, not only WOT. It will cost a lot if it runs too lean at 100-120km/h and pistons will melt. The throttles are not much opened at that range.

I secured the carbs in frame when it's not good have them hanging and bouncing up-down in carb boots only. Thick strap and rubber hose against frame secure the rack

[MRieck]

I'd do a quick leak down test to make sure everything is sealing well (valves & rings)

[Bill/BentON Racing]

I'm running 137.5-142 mains, RS34's,  1080 kit, hi compression,  327 cam ,ported head, etc, @ least 110hp. You are too rich my .02, but you gotta check all the other things mentioned,  G'luck,  Bill

[SunLar]

"leak down" ?
I have done a compressiontest a it was between 9 and 10

[Jerry Rxman Griffin aka MuthaF'er]

Keep in mind your main jets are mainly for WOT, not mid-range. Where do you have your mid-range needles set?

What is your elevation? What type of air cleaner set up? What exhaust? Stock size valves? 

I'm running 137.5-142 mains, RS34's,  1080 kit, hi compression,  327 cam ,ported head, etc, @ least 110hp. You are too rich my .02, but you gotta check all the other things mentioned,  G'luck,  Bill

     +1000 Try 120 mains. Not sure about the mid-range circuit of those carbs?? All else fails put the stock carbs back on.

[turboguzzi]

before going round and round (easy to chase your own tail) why dont you post the graph?, lots can be learned form it.

my money is on ignition issues and the fact that it runs doesnt mean you have a fully healthy one, it can make huge difference. not only timing/advance, could be actually weak spark, wrong gaps or wires,
the odd main jetting you are running doesnt make much sense to me, another pointer at ign.   

[SunLar]

Thanks guys!  You have all been very helpful, lots of good info.
Did a quick check on timing, something is not right. I'm pulling the engine out later this week and do a more precise timing.
Sorry for not posting the graph but i have to download a program before I can put it up. I will keep you posted when the engine is up and running again.


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[MRieck]

"leak down" ?
I have done a compressiontest a it was between 9 and 10

Yes .....a "leakdown test". You pressurize the a cylinder with a known psi and watch another guage to see how well the cylinder is holding the air pressure, You can hear if air is leaking past the valves or rings. A compression test is good but I have no ides what you mean by it being between 9 and 10. What pressure did you read?  180 lbs, 200lbs etc for each cylinder?
 Here is a leak down tool
  http://racetoolz.com/

[SunLar]

Thanks Mike
the tool i used for messuring is not a precise tool, just giving me a compression reading on each cylinder.
I can see this leak down tool is very helpful.
I'm almost sure now, my timing is not correct, almost 10 degree off on a quick messure.
Have to work on that


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[scottly]

Where are you measuring the 10 degree error, at the cam? There are 34 teeth on the cam sprocket, so 10.588 degrees per tooth.
Does your compression tester read in Bar? One Bar equals about 14.5 PSI, so 10 Bar is 145 PSI. Cam timing will also affect cranking compression, so verify that first.

[MRieck]

Thanks Mike
the tool i used for messuring is not a precise tool, just giving me a compression reading on each cylinder.
I can see this leak down tool is very helpful.
I'm almost sure now, my timing is not correct, almost 10 degree off on a quick messure.
Have to work on that


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Yes....bad cam timing will have a very bad effect on performance. I know through past experience. We all need experience to become better.

[SunLar]

Where are you measuring the 10 degree error, at the cam? There are 34 teeth on the cam sprocket, so 10.588 degrees per tooth.
Does your compression tester read in Bar? One Bar equals about 14.5 PSI, so 10 Bar is 145 PSI. Cam timing will also affect cranking compression, so verify that first.

Had a degree wheel on. Actually I don't know about the compression tool, have to take a look. Just read the scale