Jemco 3-into-1 tips for 75 GT750

[jabcb]

The 75 GT750 currently has:
1) early-model GT750 carbs + pod filters
2) vintage aftermarket 3-into-3 chambers (never definitely figured out the manufacturer)
3) centerstand

Its gutless below 4k & makes good power 4.5k & up.
Its a bit annoying. Lots of hills where I live. If you pull out going up hill from a stop sign, it slowly accelerates while you wait for 4k rpm.
After 4.5k rpm its zoom zoom & ignore those square roads signs with number on them.

Was thinking about changing the gearing.
Instead decided to replace the vintage chambers with new 3-into-1 Jemcos.

Well, the Jemcos showed up this week & they do look good nickel plated.
Plan on installing them after Easter.

Any tips on installation & jetting?

Has anyone figured out how to modify the center stand so that you can keep it?

[titan performance]

Are you sure the bike's reluctance at lower revs is not simply a jetting issue?......pilot jets too small maybe?

[rngdng]

Well, a three into one will certainly kill your tendency to run high rpms. With that pipe it should have good low end and be gutless above 6K. My 750 with Jemco three into three pipes could do better on the low end, but my problem is the slides are 3.0 in my 34mm Mikunis and probably should be 2.5s. I live with it. I rather like the 8K rev limit.


Lane

[jabcb]

Are you sure the bike's reluctance at lower revs is not simply a jetting issue?......pilot jets too small maybe?

Unfortunately I know pretty much nothing about jetting.

The bike has the later, higher-output motor + early carbs.
I don't know if this introduces a pilot jetting issue that is separate of jetting changes for pod filters & chambers.

The motor work is being done at a bike shop.
( I likely would make a mess of it & do something like hole a piston.)

Any suggestions?

[simmons1]

Not sure what the difference is between old and new style carbs.

For what it worth, I just removed a 3-1 chamber and replaced it with a 3-3 Higgspeed chambers. I am using Uni-Pods and new style carbs.

I believe I am done jetting it for now. With the new 3-3 I am using 47.5 pilots, 120 mains, and needle clip in lowest groove.

With the old 3-1 I was using a 45 pilot 112.5 main jet, and needle clip two grooves up form the bottom.

The 3-1 had much better low end from idle to 4K and it stopped pulling at all at just under 6K. Conversely the 3-3 is a little weaker from idle to 4K pulls a LOT harder from 4K to 8k.

[tz375]

I have not run a Jemco 3 into 1 on a dyno but 3 into 3 pipes definitely make less power at low revs that a stock exhaust and the motor needs to run 1000 rpm higher to make the same power as stock. My Strader was as strong as stock at low revs but lost out higher up.

On mine the 3 into 1 ran a couple of jet sizes larger than stock where the Jemcos needed much larger jets.

I have not jetted my bike for early carbs though.

What I would do is to start with the same jets as the chambers and jet down from there - start rich (safe). and work down one step at a time.

[jabcb]

Interesting info, thanks.

The early carbs are VMs & the later carbs are BS (CV).

carb specs per the Service Bulletins:
72/73: VM carbs: pilot 30 & main 102.5 RL , 100 C & needle -3
74: BS (CV) carbs: pilot 47.5 & main 110 RL , 107.5 C & needle -3
75: BS (CV) carbs: pilot 45 & main 110 RL , 107.5 C & needle -3
76/77: BS (CV) carbs: pilot 45 & main 110 RL , 107.5 C & needle -4

Simmons1, with your old 3-1 & relative to 1975 spec: stock pilot, main up 2.5 & stock needle position.

Currently, my 72/73 VMs on 75 motor have main up 2.5. (Don't know other values.)

[RING_DING]

I have a 1975 M model with the Jemco 3-3s, stock airbox (with mesh screen removed).
Main jets 117.50 and Pilots 45. I'm very happy with the way she runs.
I removed the center stand.
A friend of mine has a 1976 A model with pod air filters and Jemcos and is running 122.50 mains and 47.50 pilots.
Hope this helps

[tz375]

Mine was really lean on 117.5's and now has 125s for separate JEMCo chambers, but was almost perfect with 117.5's on a 3 into 1 at 14:1 at full throttle all the way up to 6000.

With VM style carbs with Large Round main jets I would expect you to end up somewhere around 115- 117.5 but I would start with 120 or 125 just to be safe.

[dyrberg123]

[quote="tz375"]Mine was really lean on 117.5's and now has 125s for separate JEMCo chambers, but was almost perfect with 117.5's on a 3 into 1 at 14:1 at full throttle all the way up to 6000.

Hi everyone.

TZ what about pilot and clip position? Was it with pods?

Cheers from Tyra East oil platform...

[tz375]

It has 47.5 pilots and needle in the stock middle position and uses either UNI foam filters or open carbs with stacks

[Vintageman]

jabcb,

Was your issue RPM based or throttle position based. Its hard to tell the difference for typically when low rpm you are small throttle of course

That symptom I have learned, and have seen many times, be poor needle jet condition. Did you check the condition of your needle jets inside? I just changed my 1975 GT750 with CV carbs with new shinny needle jets. The ones I took out one was fair and two were a little less than fair inside, bike has little oxidation too. I have seen many worse. It is rare they are OK given how old they are and our air has moisture and Oxygen in it.

Also the newer engine has a much larger intake timing. Without factory air box blow back may be a little more of a problem at low rpm. For example if you add a 2-3 inches of tube between carb bell mouth and pods (or carb and engine) that may give enough of moving air columns to counter act blow back. I don’t think it will really hurt the peak RPM for still like only 7K rpm. and maybe be better if 3 into 1 is down at 6K RPM

Lets us know how you like the 3 into 1. video please, running up the road!