VM34s and 2-1 Pipes

[Punkskalar]

Ok folks, I'm not 100% at my wits end here, but I'm wondering a few things on these VM34's (all sourced from 650Central.com)

I've had the same problem on 2 engines in the past few years. I've followed Griz's tuning guides each time. Then when I was stumped I would talk to Mike at 650Central and even though my settings were just as he would have it, the engines just don't seem right

First engine:
Stock Engine, freshly rebuilt, good even compression. Pamcopete Ignition (NOT REPHASED) with VM34's and MikesXS 2-1 Pipes and muffler.

That engine would NOT run at cruising speeds at all, it would sputter and kick like crazy. Once it got done sputtering, it was like a 2 stroke in how it came to life and would simply soar to redline. But at cruising speed it would was simply no fun... Gave up on that set of VMs and swapped in a cruddy set of BS38's and it ran perfectly. Left the BS38's on it

2nd Engine:

750cc (wiseco 10:1), 175lbs in each cylinder, Shell Cam, Ported Head, VM34's, Pamcopete Ignition. Tried the 2-1 Exhaust from 650Central with the carbs he supplied (at the same time, same purchase) - then tried an old Mac 2-1 with no difference. This engine is also NOT rephased.

Same exact thing as the first engine, this thing just SUCKS at mid throttle and cruising speeds. I'm getting ready to tear into it, been doing some research.

On my searching, I see folks say "You can't just roll on the throttle, VM's are not good for that" which seems contradictory to a good tuned set of carbs. I can "creep" up on the midrange of throttle and it's slightly better, but cruising speed still sucks. I haven't put on a set of OEM carbs yet, but I'm "this" close to ditching the VM's again.

Then in my research, I see 2-1 pipes causing alot of midrange problems on these bikes with VM's and wanted to see who is running them on a Street Bike (not a race bike, i'm looking for street performance) and what their setup is. I really don't want to go back to stock carbs on this particular bike, but the way it runs is keeping us from enjoying it as much as we could. I am also debating putting lectrons on it, but hate to spend the $$$ to do that...

So, whos running VM34's with 2-1 pipes, and did you have as much tuning problems as I am having? This is the only engine/pipe/carb combo that seems to beat me to pieces with tuning

 

[Punkskalar]

I'm also going to put on a set of Pandemoniums "Yo-Mama" pipes on this last engine to see if it helps, instead of the Mac 2-1. Even though I love the way the Mac looks for this particular build...

 

[Scrambled]

Ok folks, I'm not 100% at my wits end here, but I'm wondering a few things on these VM34's (all sourced from 650Central.com)

I've had the same problem on 2 engines in the past few years. I've followed Griz's tuning guides each time. Then when I was stumped I would talk to Mike at 650Central and even though my settings were just as he would have it, the engines just don't seem right

First engine:
Stock Engine, freshly rebuilt, good even compression. Pamcopete Ignition (NOT REPHASED) with VM34's and MikesXS 2-1 Pipes and muffler.

That engine would NOT run at cruising speeds at all, it would sputter and kick like crazy. Once it got done sputtering, it was like a 2 stroke in how it came to life and would simply soar to redline. But at cruising speed it would was simply no fun... Gave up on that set of VMs and swapped in a cruddy set of BS38's and it ran perfectly. Left the BS38's on it

2nd Engine:

750cc (wiseco 10:1), 175lbs in each cylinder, Shell Cam, Ported Head, VM34's, Pamcopete Ignition. Tried the 2-1 Exhaust from 650Central with the carbs he supplied (at the same time, same purchase) - then tried an old Mac 2-1 with no difference. This engine is also NOT rephased.

Same exact thing as the first engine, this thing just SUCKS at mid throttle and cruising speeds. I'm getting ready to tear into it, been doing some research.

On my searching, I see folks say "You can't just roll on the throttle, VM's are not good for that" which seems contradictory to a good tuned set of carbs. I can "creep" up on the midrange of throttle and it's slightly better, but cruising speed still sucks. I haven't put on a set of OEM carbs yet, but I'm "this" close to ditching the VM's again.

Then in my research, I see 2-1 pipes causing alot of midrange problems on these bikes with VM's and wanted to see who is running them on a Street Bike (not a race bike, i'm looking for street performance) and what their setup is. I really don't want to go back to stock carbs on this particular bike, but the way it runs is keeping us from enjoying it as much as we could. I am also debating putting lectrons on it, but hate to spend the $$$ to do that...

So, whos running VM34's with 2-1 pipes, and did you have as much tuning problems as I am having? This is the only engine/pipe/carb combo that seems to beat me to pieces with tuning

I was t about to order a set of VMs from 650 Central to go with my 2 into 1 from Mikes (650 direct) . The Idea was to get better midrange performance, but if you can't tune them I sure as hell couldn't.

So, I guess I will send my BS 38s to Old school carbs too. I was thinking about Mikes 32mm flat slides before but there is just way too many complaints about them . Have you tried those out yet ?

 

[Punkskalar]

I had a set of the "PWK Rip-Offs" in hand, decided not to even bolt them up...

On a side not, EVERY other set of carbs I have got from Mike at 650Central have worked flawlessly with just some minor tweaking. It's just these bikes with the 2-1 pipes that seem to give me a fit...

So this is NOT a bash thread on 650Central by any means - I love getting parts from them

 

[Scrambled]

I had a set of the "PWK Rip-Offs" in hand, decided not to even bolt them up...

On a side not, EVERY other set of carbs I have got from Mike at 650Central have worked flawlessly with just some minor tweaking. It's just these bikes with the 2-1 pipes that seem to give me a fit...

So this is NOT a bash thread on 650Central by any means - I love getting parts from them

I know , I am not bashing 650 Central either ( they are fantastic) ,I just wan't carbs that work with my exhaust . I like my 2 into 1 setup and don't want to change it.

 

[osteoderm]

I don't know if this will help, but sharing info certainly doesn't hurt! Current set-up:

'74 TX650, stock engine internals except for:

VM34
25 pilots
180 mains
6F9 needles (raised one slot)
159/P6 needle jet
23mm float height

5" Uni single-stage filters
TC Bros intakes/boots

Gordon Scott 2-1 exhaust (1-3/8" primaries, 1-3/4" secondary)
Straight-through 1-3/4" cone-style muffler

HHB PMA
PAMCO

I get a tiny stutter just off-idle; my idle is a little low, my throttle is (if anything) a little too fast-acting, and I've never taken the time to really truly sync the carbs, so I'm pretty sure patience and tuning will fix that. The bike starts gasping above 7000rpm, but is smooth, torquey, and responsive from 2600-4800rpm (where, honestly, 90% of my riding happens).

Does 60mph at 3800rpm with 17-32 final drive and my Clydesdale ass aboard "Snapping" the throttle open can introduce a stumble, but I've never had a problem with roll-on; in fact, the top-gear 30-70mph roll-on is great! Between the GSE exhaust, longish intakes, and VM34's, I was looking for midrange torque, and I feel that I've gotten it. There's surely small improvements to be had through more thorough tuning, but overall I have zero complaints.

 

[Scrambled]

I don't know if this will help, but sharing info certainly doesn't hurt! Current set-up:

'74 TX650, stock engine internals except for:

VM34
25 pilots
180 mains
6F9 needles (raised one slot)
159/P6 needle jet
23mm float height

5" Uni single-stage filters
TC Bros intakes/boots

Gordon Scott 2-1 exhaust (1-3/8" primaries, 1-3/4" secondary)
Straight-through 1-3/4" cone-style muffler

HHB PMA
PAMCO

I get a tiny stutter just off-idle; my idle is a little low, my throttle is (if anything) a little too fast-acting, and I've never taken the time to really truly sync the carbs, so I'm pretty sure patience and tuning will fix that. The bike starts gasping above 7000rpm, but is smooth, torquey, and responsive from 2600-4800rpm (where, honestly, 90% of my riding happens).

Does 60mph at 3800rpm with 17-32 final drive and my Clydesdale ass aboard "Snapping" the throttle open can introduce a stumble, but I've never had a problem with roll-on; in fact, the top-gear 30-70mph roll-on is great! Between the GSE exhaust, longish intakes, and VM34's, I was looking for midrange torque, and I feel that I've gotten it. There's surely small improvements to be had through more thorough tuning, but overall I have zero complaints.

Your bike is performing the way I wan't mine to, but I think you have a bigger budget to work with than I do. I will probably just go for reliability and reasonable performance on this bike and put the big money parts into the tracker I want to build . So, I am going to send my old BS 38s to OldSchool and order me a PMA from Hughs . That should do the trick for now .

 

[Scrambled]

I had a set of the "PWK Rip-Offs" in hand, decided not to even bolt them up...

On a side not, EVERY other set of carbs I have got from Mike at 650Central have worked flawlessly with just some minor tweaking. It's just these bikes with the 2-1 pipes that seem to give me a fit...

So this is NOT a bash thread on 650Central by any means - I love getting parts from them

Now here is a good question , is there any way ( or an easy way ) to get better midrange out of a set of BS 38s on a stock engine ? Are there performance intakes and boots that work with them ?

 

[GreasyC]

On carbs where the throttle cable connects directly to the slide like that snapping the throttle open will never be great unless the carb has an accelerator pump. You've moved the slide up to allow more fuel to flow but there's not enough manifold vacuum yet to draw that extra gas up into the air so the pump adds it for a second until the air speed catches up enough to do it itself.

That's a completely different situation than steady cruising at mid rpm though. If you're having problems like that with the 2-1 pipe I would suspect something to do with the tuning/timing of the pressure pulses in the exhaust and intake. As far as I know nobody measures that kind of stuff on these old bikes so you're not going to find the answer on here. I'd experiment with different intake tract lengths. You could try the aluminum intake manifolds a lot of places sell but for a lot cheaper you could try different lengths of PVC tube or whatever you have between the carb and the pod filter.

 

[Curtis P]

Have it go on the dyno, unless you have a friend that is good with tuning, the dyno is the best option you got to get tuned.

 

[Scrambled]

OK , that is worth a try and won't cost much.

 

[Scrambled]

True, I guess I should find a shop with a dyno for the fine tuning. You are pretty close to where I live, do you know any place with a dyno that would tune an antique ?

 

[Curtis P]

This guy is well worth the drive to here in peterborough, not only does he build kick ass bikes but he races and dyno tunes.

http://www.alienracing.net/

 

[Scrambled]

Cool. Thanks.

 

[grizld1]

I don't know what kind of 2-1's you're running, Hugh, but I've tuned VM34 and VM36 carbies for both Kerker and Mac 2-1's and didn't run into anything like the grief you're describing. The usual issue with 2-1's is a midrange flat spot, and the usual fix is to sleeve down the collector a tad. Sounds like you might need to work with the needle position-pilot jet combination a bit.

 

[Black Rose]

So, I have to ask Hugh. Have you run the same carbs, jetting and pipes on a 277 deg motor ? If you have, and not had the issues described in your first post. What does that suggest ?

 

[hotdog]

I have the 34 mikunis and GS 2-1 pipes and runs perfectly. I ditched the drag pipes I was running, it ran like shit with them on.

 

[Punkskalar]

So, I have to ask Hugh. Have you run the same carbs, jetting and pipes on a 277 deg motor ? If you have, and not had the issues described in your first post. What does that suggest ?

Ive not run a 2-1 exhaust on a personal rephased engine yet with VM carbs. Just OEM configuration.

Running that Mac 2-1 Pipe, it is definitely the midrange flat spot that I am trying to work out. With some rejetting I'm getting closer, I'll keep you folks posted on when I get it running proper. I'm determined on this one

 

[Punkskalar]

Ok, I've been working on this one a bit today.

The midrange does not seem to be getting much better. Slightly better, but not much.

Started with the following:

200 Main
27.5 Pilot
6F9 Needle
159 P-5 Needle Jet
2.5 Slide

Played with needle clip at all heights. No major changes.

Talked with Michael at 650Central some more, he suggested getting richer in the pilot and coming down on the main a bit to clear out the midrange stumble.

So then:

180 Main
32.5 Pilot
6F9 needle
159 P-5 Needle Jet
2.5 Slide

I've also tried every possible clip height on the needle and it's not much different.

Then I tried a 160 Main and it seemed to get keep some of the stumble out of the midrange, but lacked on power quite noticeably.

Plugs are showing rich still though... I'm gonna put a set of Yo-Mama Pandemonium Pipes on it and see if it gets any better... I happen to have a set laying around.

 

[bojangles]

Reviving an old thread here. I keep coming across this post as I am wanting a 2-1 pipe on my 1980 XS650 with VM34's (from 650 Central). Can anyone comment if they also have problems with a MAC, or MikesXS 2-1 exhaust and VM34's?

Thanks