Lack of power= carb issue?

[ST_SHP]

I've finished putting back together my '91 ST Universal Japanese Motorcycle project. I now have about 30 miles on it. It starts up & runs fine with a few backfires when cold, seems to make good power in the low range- getting up to around 60 in 2nd gear is quick. Once I get on the road in higher gears, it turns into a slug! If I'm riding at a constant 60-65 and roll on the throttle, it stumbles pretty badly and barely accelerates. I don't have a working tach as yet so I can't give an RPM range. On a long open road, it tops out at 95 MPH.

When I put the bike back together, I put in a new fuel pump (the mod using one from a Honda Civic that's posted here) & filter, bypassed the vacuum thing, and dipped each carb for 24 hours and used new seals from factory carb kits. The carbs on the bike (and air cleaner) had been modified by a PO. The carbs with #135 jets (stock is 128 as I recall)- the air cleaner had several holes drilled in it for more airflow. There's a writeup online somewhere about this- it was supposed to make more power. The bike has a fresh tank of premium fuel.

I need to recheck the compression, but it really seems like a carb issue. One other thing- I used a laser thermometer on the exhaust and the right side was reading a lot higher at idle than the left- running lean?

Does this sound like anything other than carbs? If they are the problem, is there a competent rebuilder I can ship them to?

Thanks for any help.

EDIT
The carbs were also synched using a flowmeter.

 

[Mark]

I know of no one that has benefited from 'improving' airflow or changing jets and quit putting premium in your bike!

If you're fine through 2nd do you think you're using higher rpms to get there?
I'm not sure how carbs are the limiting factor...

 

[v8-7]

Is the timing adjustable on the 1100's?

How do the plugs look ?

Maybe put the stock jetting back like Mark alluded to.

Then I think I'd look at a fuel or air delivery problem .

 

[ST_SHP]

@Mark- The premium was a one time thing, trust me.

It seemed like a seat of the pants 6-7K RPM @ 60 in 2nd. I figure there has to be a speed in gears chart here somewhere for these.

I'm thinking carbs due to the power increasing slightly when I let off the throttle- it's like I have to overshoot where I want to be. I'd almost say it was running out of fuel. I may try & rig a fuel pressure gauge inline and see if it drops at higher RPMs.

@v8-7- I'm not aware of any timing adjustment, haven't pulled the plugs. Off topic, does your user name mean you have a Mazda with a V8?

Here's the carb mod page involving the larger jets & opening up the air cleaner cover:
http://www.st1100.org/steinborn/

I found a place on eBay that does carb rebuilds, it's $300 for all 4 shipped. I want to exhaust everything else before I go that route.

 

[v8-7]

Pull all the plugs and have a look, that might tell you a lot especially since you said the temps were different at idle .

I would also take the temps right after after a spirited run .

Yes, I have a 1985 RX-7 with a 347.0 ford motor that's been warmed over to ~ 340 rear wheel HP

 

[sirepair]

Check vacuum lines. Did you leave the PAIR system on?

And make sure she's not "suckin air" around the boots. Pull the tank cover off and with the bike at idle, spray some WD-40 around each carb boot. If idle increases, there's a leak.

Since it idles and runs mid-range ok, I would be looking at the mains and diaphrams.

With as much wrenching as you have been doing on this thing, you can rebuild the carbs yourself for a lot less $$! As long as the diaphrams are OK.

 

[schlep1967]

If everything was stock I would guess diaphrams. With everything frankensteined, I would watch ebay for a set of carbs and a new airbox.

 

[ST_SHP]

Thanks for the advice. Forgot to add PAIR was removed and block plates added to the heads w/ new gaskets. I'll not be able to do anything with it for a couple of days but will check the areas mentioned.

@v8-7,
I've had two and sold both. So as not to spam up the board off-topic, here are links to my '84 GSL-SE w/ Gen 1 383 Chevy V8/T5 and '88 convertible w/ Gen 3 5.3L aluminum block/T56.

 

[George]

What Pat said about leaks around the boots. I've had carbs to look perfectly seated but one side not in the boots at all. It can throw ya. If right side is not seated, may 'splain the temp variation too.

Rolling off the throttle slightly increases power, implies lean at the top. If ya left the added holes in the air cleaner open, try taping over them for a quick check. Also, try running with the choke full or partially on. If either or both improve performance, proves the lean theory.

Keep us poSTed.

 

[jvick98]

QUOTE: If I'm riding at a constant 60-65 and roll on the throttle, it stumbles pretty badly and barely accelerates. I don't have a working tach as yet so I can't give an RPM range. On a long open road, it tops out at 95 MPH.

Sounds like one of our bikes when I used old Stabil. Put a good dose of Seafoam in fuel, run for 15 minutes and let set for a few days. Worked for me.

Jim

 

[wjbertrand]

Didn't see anyone mention the vacuum fuel petcock. When you roll on the throttle, manifold vacuum drops and a sketchy vacuum petcock might not be staying open well enough to meet fuel demand. Bypassing the valve is a quick procedure to try as a test.

I second the suggestion to cover the holes in the air box, at least as an experiment, I've heard of drilling the air box causing pretty severe driveability issues.

Bad carb diaphragms can cause similar issues. Couldn't tell if these had been replaced or not. Easy enough to inspect once the air box is off, just look under the chrome tops of the carbs.

Lastly, by any chance have you fitted a K&N filter with the foam sock around it? If so, remove the outer foam sock, many reports have been posted that leaving it in place causes excessive restriction and running problems with the ST1100.

 

[saaz]

I can vouch for the extra airholes giving rise to the problems you are having. I tried briefly and plugged them up as I was not willing to spend the time to fiddle around with the tuning. If I had free access to a dyno could be another matter.

 

[kirkcdl]

I have the same problem...I see references to the diaphragms,however the parts references on Bike Bandit don't show a diaphragm,what exactly is the diaphragm and where do I find it?Thanks...

Never mind,found the "vacuum tubes" in the fiche...

 

[ST_SHP]

Here's an update after a morning of testing...

WD40 around the boots- no problem indicated.
Choke on at road speed in top gear- ran worse under acceleration
Taped over all drilled holes in the air housing- seemed to run worse
Ran w/o air filter (1/4 mile or so )- No real difference
Compression check- left cylinders 155 warm engine, tester chuck fell apart before I could test right cylinders
Plugs pulled after run w/ short idle to shutdown- LF, RF, RR all soot black. LR tan no soot.

It is a K&N, but no foam pre-filter. The element is fairly clean from what I can tell. If it had been an issue, the above testing should have identified it.

My thoughts-
The left rear carb looks like it is the first one fed from the pump. I have an aftermarket clear filter on it (got it from one of our bike vendors, forget which one). I'm not seeing any fuel going through it, or there is no air in it and all I'm seeing is fuel. I pinched the hose w/ my fingers at idle and there was no difference.

I also want to try running it with one plug wire at a time disconnected to see if it may be an ignition side problem.

After the bike cools down, I'm going to pull the hose after the filter & turn on the key to see how much gas is coming out. The bike seems to get up to speed very fast in the lower gears, but then has no power in the upper ones. If the plugs weren't fouled, I'd think this was a fuel supply issue- filter or pump.

BTW, I have the tach hooked up now but need to calibrate it. I'm seeing about 2800 RPM @ 65 MPH in 5th gear. Seems kind of low.

EDIT
The vacuum thingy has been removed. I stand corrected on the fuel routing- the rear carbs are fed first.

Pulled one wire at a time & restarted. Idles about the same, but taking off in 1st gear there is a noticeable lack of power, so I don't think it is an ignition fault.

 

[Mark]

I'm seeing about 2800 RPM @ 65 MPH in 5th gear. Seems kind of low.

That is low.
I get an indicated 80mph @ 4000 rpm in 5th.
Due to the speedo's bias you're really doing 75mph at that point.

 

[Doodaa]

Compression check- left cylinders 155 warm engine, tester chuck fell apart before I could test right cylinders
Plugs pulled after run w/ short idle to shutdown- LF, RF, RR all soot black. LR tan no soot.

All but LR running rich or just maybe low spark, since the one plug looked good. If the added choke and taping over the air holes decreased performance, looks like more air is needed. Maybe start with new plugs and try it again without adding the choke.

 

[v8-7]

I have no specific st1100 experience, but based on your findings:

How much time did you pinch the fuel line closed ?

It probably takes a few minutes to run the float bowl dry .

Seems like you may have 2 different problems .

Running too rich in 3 cylinders = to much fuel . Again,we come back to the oversize jets as a possibility .

Is that Civic fuel pump for an injected motor ? If so was there anything , other than the stock fuel pressure regulator ,
used to bring the fuel pressure from ~40 lbs down to ~ 4 lbs ?

Did you check the float level heights ?

 

[saaz]

I suspect someone has been in the carbs trying to tune it, but never got there. Without knowing what jets and adjustments have been made a bit hard to say. It may have different needles, needles shimmed up, float levels changed etc. I have gone through adjusting carbs on my old bike with a modified motor, but the carbs were minutes to get off.

 

[ST_SHP]

I'm about ready to punt on the carbs. Will a set from a later model (i.e. a 1996) work on the '91?

 

[Mark]

I haven't ever heard of *any* difference in the carbs from any year ST1100.