Power

[Magua]

I've made a few of these threads before at different times in different forums, so this is a compilation and a place to brainstorm!

Earlier in the year I did a top end rebuild on my 1980 cb400n, camchain, piston rings, valves etc, however, I was still down on power after the work. It has the original exhaust system, and does not appear to be leaking.

I ordered two Keyster carb kits and they seemed to make a world of difference once installed, I could now sit at 120kph etc. From memory I did a carb balance after installing the carb kits. After several weeks commuting I decided to go on a proper ride (100kph vs 70-80kph on the commute), as soon as I was on the open road, my bike felt like it had reverted to its pre-carb refurbished ways, down on power throughout the rev range. I have not figured out why. I attempted to replace the coil to see if it would make a difference, it did not.

Carb balance?
Something inside gone askew?
Where would you look?

The only thing not to be changed in the carbs is the emulsion tubes, apparently the ones in the keystar kit are the wrong sort (according to my local mechanic).

[Ixion]

Inaccurate speedometer?

What quantifies "down on power" ? Down compared to what it once was? Down compared to what you think it *should* do? Down compared to a similar machine?

[Magua]

Down compared to what it once was? Down compared to what you think it *should* do?

Those two. It's down compared to what it was after I first switched the carb internals, and down on what I believe it should be able to do. At 100kph it wont hold speed in top gear unless you're on the flat (a good headwind will drop you back) or unless you're heading down a hill. Both contrary to the reviews I've read about the bike's performance, 'cruising at 80mph' etc. 80mph is more akin to my top speed.

[Motu]

I'd be looking at the carb diaphrams - if they don't lift the slides it's going to be gutless on the top end.If you reckon they are perfect,then there has to be some other reason they can't lift - they are lifted by vacuum,if it's down on vacuum it's not going to lift them.What causes low vacuum? - ign timing,valve timing,valve seating,blocked intake,blocked exhaust.

[browny]

are you sure your cam timing not out a tooth

[Ixion]

I take it this is a simple Simon bike? No SRAD sort of stuff, no emissions shit? You say top end and valves were done/checked. Assuming they were that should rule them out. Is the acceleration OKish? Just down on top speed? Or is it gutless generally? If we assume valves OK (dangerous things , assumptions), ignition timing isn't very likely on a modern bike (you don't adjust it any more). Cam timing wouldn't change. I think you tried a couple of exhausts? I take it that the "not as good as after I did the crabs " bit is with the same zorst? Leaves the crabs. Did the crab overhaul include new diaphragms? Is the throttle opening fully? Does it have a vacuum operated fuel tap? Do the crab slides drop back easily with a nice click when you raise them up? (have to remove the air box probably)

[Rodney007]

compression check

correct plugs (euro models fuel octane)

accelerator cable adjustment

heat bungs on head (check around head for holes)the tiny ones, can dump compression straight out...(dont think heat bung is correct name, frost plugs or something)

make sure your leads are on in order (prolly wouldnt idle nice if so)

correct JETS?, float level? (wrong size will majorly effect your power, and u cant visualy see jet size diference -- usualy-)

carb balance? flat spots?

leaks between carbs-->head??? (adapters from carb to head can sometimes not fit tightly)

weak spark on any or all cycldners?


good luck!

[R6_kid]

Do you have a fuel filter on it? If not why not?

Also is it a 5 speed? My CB650 is basically a 4 speed with 5th gear for 'overdrive' cruising - top gear is just for holding 'cruise' speed and getting better fuel economy.

[Magua]

I'd be looking at the carb diaphrams - if they don't lift the slides it's going to be gutless on the top end.If you reckon they are perfect,then there has to be some other reason they can't lift - they are lifted by vacuum,if it's down on vacuum it's not going to lift them.What causes low vacuum? - ign timing,valve timing,valve seating,blocked intake,blocked exhaust.

I don't think it has a diaphram, a rubber boot right?. Cv carbs.
Valves were lapped (intake and exhaust) when I took the head off. Come to think of it, when I last did a balance the reading was down on vacuum, but I had trouble trying to convert the units from the tester to what was in my manual. Hm, I'll get my hands on an ignition timer.


Just down on top speed? Or is it gutless generally? If we assume valves OK (dangerous things , assumptions), ignition timing isn't very likely on a modern bike (you don't adjust it any more). Cam timing wouldn't change. I think you tried a couple of exhausts? I take it that the "not as good as after I did the crabs " bit is with the same zorst? Leaves the crabs. Did the crab overhaul include new diaphragms? Is the throttle opening fully? Does it have a vacuum operated fuel tap? Do the crab slides drop back easily with a nice click when you raise them up? (have to remove the air box probably)

It's very simple simon, sohc parellel twin, no emissions things. 2 headers into a balancer and into 2 silencers (the original exhausts, I got rid of the straight through rubbish). Throttle seems to operate fine, slides seem to move easily, but I can check those again. Not sure if it's a vacuum tap. It seems to be generally gutless, though a lot more noticeable when you're at 100kph. It will pootle a lot better at eighty, but requires a lot of effort to keep it around 100.


are you sure your cam timing not out a tooth

Yep.


compression check

correct plugs (euro models fuel octane)

accelerator cable adjustment

heat bungs on head (check around head for holes)the tiny ones, can dump compression straight out...(dont think heat bung is correct name, frost plugs or something)

make sure your leads are on in order (prolly wouldnt idle nice if so)

correct JETS?, float level? (wrong size will majorly effect your power, and u cant visualy see jet size diference -- usualy-)

carb balance? flat spots?

leaks between carbs-->head??? (adapters from carb to head can sometimes not fit tightly)

weak spark on any or all cycldners?


good luck!

Compression's good.
Correct plugs.
Not sure what you mean by heat bungs.
Replacement jets are the same size, I don't think I can alter the float level.
Was balanced a few months ago. Flat-ish spot 5-6,000rpm.
No problems with missfiring, assume spark's fine.
Carb to head had new seals with the rebuild.


Do you have a fuel filter on it? If not why not?

Also is it a 5 speed? My CB650 is basically a 4 speed with 5th gear for 'overdrive' cruising - top gear is just for holding 'cruise' speed and getting better fuel economy.

There's one in the tap, but I've added one between the tap and the carb. 6 speed.

Anyone want to lend me a timing light? heh

[Motu]

Ah,Honda - Kehein or whatever carbs....they have a piston like an SU,much better than a silly rubber thing.

So,it runs better at low speeds,and runs out of puff at higher speeds and load? You have fiddled around with the carbs,and I reckon you've stuffed something up - but you'll never find it because you did it,and think you've done it right...can't find fault with your own work.Not picking on you there...it's human nature.

Otherwise it's cam or ign timing.

[roadracingoldfart]

Can i ask what gearing you are using , and what is standard for the bike.
Early Honda twins (and multis) dont hold a good steady speed in top if you over gear them .
You may have to play with a couple of teeth diff to make it better.

My old CB400/4 with taller gearing wouldnt pull top gear at just above legal speeds so i had to use 5th unless i was revving the snot out of it in top.

Paul.

[cs363]

Err also, you may be overlooking the obvious - if you've used Keyster carb kits I would assume that they were purchased from the US, probably off EBay?
These kits are for US market so may not work so well down here due to our different climatic conditions.
Did you check that jets, needles and so on that you may have replaced were replaced with the same size/number items?