down on power

[p_s]

'85 GS550ES

New rings & honed bore 1500 miles ago. New head gasket. Valves lapped & checked for leaks with gasoline. Compression is 150/143/150/145 hot WOT.

It is down on power, especially at high revs. It does 100 but takes a while to get there. I've checked the cam timing twice, the valve clearances three times, and the spark timing once. The plugs all look okay. I put in a new Suzuki air filter today. I have checked the jets twice to make sure I put the bigger mains in #2 and #3. The slides move very smoothly. The rubber airbox tubes are fairly new. I also put compressed air into it through the breather tube and sprayed the tubes with soap: no bubbles. I also sprayed the exhaust with soapy water: no bubbles. (I also felt for a leak.) I checked the float height last August.

I don't know what else to do. I don't want to say how I ran in the motor, because no matter which way I did it, half of you will have strong opinions that I did it wrong.

[DMPLATT]

Your compression #'s look good, so it's not how you broke in the rings.

Sounds to me like a slipping clutch.

[p_s]

Your compression #'s look good, so it's not how you broke in the rings.

Sounds to me like a slipping clutch.

But it's not getting to redline as quickly as it did before.

[tkent02]

is your exhaust clogged?

How did it run before you tore it down? Was it underpowered then? A partially clogged exhaust will slow any engine down.

[focus frenzy]

welcome to a fresh tight engine! how many miles on the rings?
smaller engines are more sensitive to friction than a bigger engine, give it some time.

I have seen a oil change knock 10mph off a marginally powered vehicles top speed.

just look at the numbers for a long term test on a car (car and driver, motor trend magazine) at 30,000 miles they more often than not are a bit quicker than they were when brand new.

[p_s]

How did it run before you tore it down? Was it underpowered then? A partially clogged exhaust will slow any engine down.

The exhaust is something I didn't think about. Something is slightly tweaked so that it's a big pain to take off the pipes, but you might be right. It ran very well 6k miles before I tore it down. In the 6k miles after that I never had a chance to run it up at sea level without luggage on a long straight road, so I can't say if it was right or not. (I tore it down because it had a stripped countershaft and I had another engine with a bad head.)

From my experience with an off-brand air filter, I know the bike is very sensitive to the right restriction in the airbox.

Can it be electrical? The electronic advance is working although I didn't measure it. The flash from the timing light never wavered. It doesn't feel like it's cutting in and out.

I thought it could be fuel flow, so I ran it up to 30 mph in 3rd at low throttle and then dropped down to 1st WOT and it didn't change.

Both wheels spin nicely with just the slightest drag from the brakes.

[duaneage]

Fuel delivery. It's running out of gas. Check the tank filter, the petcock, kinks in the fuel line, etc,

have any idea how much fuel it drinks at high rpm? a lot. Even a small bike. TRy it with the petcock on PRI first, that's an easy one.

[p_s]

Fuel delivery. It's running out of gas. Check the tank filter, the petcock, kinks in the fuel line, etc,

have any idea how much fuel it drinks at high rpm? a lot. Even a small bike. TRy it with the petcock on PRI first, that's an easy one.

I did try running with the petcock on prime. No change. I'll look at the float bowl filters & the tank filter next.

FWIW, I just peeked up the exhaust tips. There is a thin layer of soot but it doesn't look abnormal. I also put 100 psi into each combustion chamber with the exhaust valves open and it seemed to flow okay, although what happens at 1000 psi could be different.

[duaneage]

If you have a filter on it you might be getting vapor lock. The line filter , if used, should be as small as possible to limit vapor pressure buildup. large filters are no good since there is no fuel pump to overcome the inevitable vapor pressure that comes from riding and heat.

last fall Catbed was at my house syncing his bike and fuel would not flow from the tank next to the bike into it because of vapor pressure. When we removed the large filter and used a small one fuel ran again.

[Dave8338]

It sounds like a flow restriction if anything. Is the filter over oiled? You've checked the exhaust as best can be, without taking it off the bike.
Try putting the air filter in a Zip Lock bag with a few layers of paper towels in there and press out as muck oil as you can. Repeat if needed.

[p_s]

If you have a filter on it you might be getting vapor lock. The line filter , if used, should be as small as possible to limit vapor pressure buildup. large filters are no good since there is no fuel pump to overcome the inevitable vapor pressure that comes from riding and heat.

I haven't added any filters. I assume there is one inside the tank. (?)

It sounds like a flow restriction if anything. Is the filter over oiled?

It's a paper filter.

[DMPLATT]

But it's not getting to redline as quickly as it did before.

Sorry, you said down on power at High Revs. Just sounded like a clutch slip. I had that on my 750ED.
edit: If the engine is getting to red line, then I would look at the clutch.

[p_s]

Sorry, you said down on power at High Revs. Just sounded like a clutch slip. I had that on my 750ED.
edit: If the engine is getting to red line, then I would look at the clutch.

It's getting to red line slowly. It should pull hard to redline but it seems like it lets up a bit in the upper revs. The speedo, tach, and butt dyno are all showing a drop in power. I did measure the springs & clutch plates when I did the rebuild--all measured new. (Although I know with springs that is no guarantee.) I should have some time to tinker tomorrow.

[gearhead13]

You said you put bigger mains in 2&3. What was the reason for that?

[p_s]

You said you put bigger mains in 2&3. What was the reason for that?

The stock configuration has bigger mains in the middle for cooling. The engine/carbs/airbox/exhaust are 100% stock.