Plug chop questions. And a few about high speed running.

[Macmatic]

Do you really need to use new plugs for each test? I've seen plugs go from oil fouled to white and back to carbon black and back to tan again in the same engine so it seems like the same plugs should be OK for testing.

I'm interested in what the engine is doing above 4,000prm or so, does road speed matter or can I just run at in a lower gear at the engine speed I'm interested in?

With an 84 GS750 16v...shouldn't you be able to feel it "coming on the cams" at some point? My bike rides well and gets 40-48mpg and pulls from 15mph or so in 5th with no complaints, cruises all day (literally) at 80+mph with saddle bags and a crate on the back but somehow it just seems to be lacking something. ...that baggage was what gave me the 40mpg figure...

After putting 1,500 miles on it over a couple of weeks I....uh... got the throttle stuck wide open :wink: and didn't get it closed down again until I hit 105mph / 7,000rpm at which point I rolled off and let sanity catch up but not before I noted that she seemed about out of steam at that point or at least was gaining quite slowly.

She felt very stable at those speeds by the way. Solid even but I'm not going back there until I get things sorted...but it had to be done once anyway. :twisted: So what should acceration with a 16v GS750EF be like between say... 80-100mph? My friend on his Custom Classic Harley ended up at least 1/4mi ahead of me.....this isn't good! Not with him pushing that bay window around in front of him it isn't....

Well I'll start that as a new topic when I'm back in ATL with the bike...can't really figure it out from here.

/\/\ac

 

[oldschoolGS]

These bikes were set up rather peaky on power output. With stock cams and timing it should start making good power around 7000 rpm and not drop off until around 10,000rpm - about 500rpm below redline.

Under no circumstances should a 600lb Harley out run you at any speed.

I would check compression, ignition, and carburation first.

When I was hot rodding my 750 back in the late '80's, I could not get the stock carbs to give good power regardless of settings (pods, jet kit ect)

It was when I went with the flatslide late 80's GSXR CV carbs that the bike really started to open up. I found the 34mm Mikuni's to work well with various configurations from stock displacement to 802cc with .350 lift cams.

BTW, any early air/oil cooled GSXR carbs will bolt right up- the 1986 1052cc 34mm Mikuni's are best for the 750.

 

[Macmatic]

These bikes were set up rather peaky on power output. With stock cams and timing it should start making good power around 7000 rpm and not drop off until around 10,000rpm - about 500rpm below redline.

Ok, thats a good clue right there. 7k is about the fastest it've had it turning over so far and that was in 5th so I guess I was just trying to get up to speed before I hit the torque peak. It was one of those things where I'd been talking about wanting to make a triple digit run earlier in the day and when we got on the highway this time I got up to about 70, put her in 5th and noticed that he was running away so I rolled on and tucked in. I guess if I wanted to hit those speeds I should have been down a gear and swtiched up around 105-110.

I would check compression, ignition, and carburation first.

When I was hot rodding my 750 back in the late '80's, I could not get the stock carbs to give good power regardless of settings (pods, jet kit ect)

My setup is like this:

Good compression, stock cams and timing.
Stock airbox and lid with UNI foam filter.
Stock downpipes with 'busa mid pipes and canisters in place of the megaphones.

OE carbs with the following jets and needles with the clip on the 4th slot from the top.

Main jet = 120
Main air jet = 1.55
Jet needle = 5D10
Needle jet = Y-7
Pilot jet = 40

These might have been rejetted or these might be the stock jets for a Canadian bike. These carbs came on a Canadian parts bike.

I also have the jets listed below from the US spec engine.

Main jet = (117.5)
Main air jet = (1.7)
Jet needle = (5D12)
Needle jet = (Y-6)
Pilot jet = (37.5)


Thanks for the input! I'll start a new thread when I'm around the bike again.

/\/\ac

 

[snowbeard]

any chance it's just fuel starvation? I have experienced it at highway speeds before, just not quite as game as it seems in lower gears at the same RPM? we're sucking a lot more fuel at 7000...

 

[oldschoolGS]

Start at 60 mph, down shift to third and peg the throttle shifting at 10,000rpm into 4th and 5th gear.

If an air cooled pushrod V-twin is still outrunning you, you have problems.

These 750's are not good top gear roll-on performers, click down to 3rd and let her scream to really move.

BTW, you should not be anywhere near 5th gear at any less than 55mph. Thats called lugging and is not good for any motor. These engines are made to spin, don't be afraid to get the rpm up.

 

[duaneage]

Are the slides in the carbs lifting all the way? That is the question but not easy to determine. All of the boots need to be tight and leak free to provide proper vacuum for the carbs to work properly, anything else will lean it out and prevent the carbs from working right. Check for leaks with a running engine and WD-40 sprayed around the boots.

Try a run with the petcock on PRI and see if it runs better. Could be the petcock is going bad.

 

[Macmatic]

Are the slides in the carbs lifting all the way? That is the question but not easy to determine. All of the boots need to be tight and leak free to provide proper vacuum for the carbs to work properly, anything else will lean it out and prevent the carbs from working right. Check for leaks with a running engine and WD-40 sprayed around the boots.

Try a run with the petcock on PRI and see if it runs better. Could be the petcock is going bad.

Any way to check the slides in operation? I don't think thats it though, the carbs are newly cleaned (dipped) with all new o-rings from Rober Barr and the diaphragms were supple and intact. The boots where a little stiff but in good shape and I put in new viton o-rings (from myself).

I also did the "wd40" test when I was first setting up the bike, only I used propane. I does the same thing without the mess. I'm pretty sure the carbs are working properly and without vacuum leaks but I might have jetting issues or, more likely riding issues. Or both.

/\/\ac