Jetting for Higgspeed chambers? GT750M

[TPS]

Hello all,
I did a bit of searching but could not find anything conclusive just yet, i've just removed the Bromlech 3-1 system off my '75 GT750M and fitted a nice shiny set of Higgspeed chambers. Ran it last night for a few minutes at idle, neighbours are going to love me. he he.

At any rate, i'm after a starting point for jetting.

Currently i'm running 47.5 pilot jets, standard needle on the bottom clip, and 115 mains. Standard air box and filter.

I dont think this is such a bad place to start, but may be a bit lean on the pilots. Can anyone let me know where they have started or ended up when playing with the jets after fitting Higgspeed chambers?

Thanks in advance.

[Craig380]

Drop an email to Higgsy, he'll be happy to help

If you're running the stock airbox and filter, you may not need to change that much. Obviously it's a different bike and different chambers, but when I fitted Allspeeds on my 380 (retaining the stock airbox and filter, I just removed the intake snorkel off the back of the airbox), it only needed a 1/4 turn on the pilot screws. If anything, it's a little rich in the mid range, but I'm happy with that.

[Madbuffalo]

TPS, I'm also running Higgspeed chambers on my GT750M so hopefully I can help steer you in the right direction. My bike, like yours, is mechanically ALL stock except the chambers. I have tried lots of different jetting and settings over the past year since buying the chambers and here's what worked for me.

Mains - 117.5 (x3)
Pilots - stock
Idle Air Screw - stock
Needle - center setting

As far as emailing Higgsy I hit a dead end with that. All he would tell me was that since all bikes are slightly different from one another I needed to find what worked best for me on my own, I assume for legal reasons. Still I think it helps to have a basis which is why you now have my settings but I agree that you need to do some checking on your own. I'd start with 120 mains and work your way down. Something else that might help you is to switch to a good synthetic injector oil. The way Higgspeed chambers are designed at the exhaust ports they seem prone to blow-by, at least in my case. I was using a cheap marine grade oil and was getting lots of blow-by all over the engine even with the oil adjustment backed all the way off. Switching to a good synthetic oil solved the problem completely.

Once you get them tuned-in I think you're really gonna like them! While I lost a bit on the bottom end the power band is noticably stronger and I can cruise along on the expressway comfortably at 1/8 to 1/4 throttle. The "popcorn" sound is a vast improvement from stock pipes too, IMO. Hope that helps.

[TPS]

Thanks heaps for the feedback Mad Buffalo, thats exactly what i was after.


I agree 120 Mains is a good spot to start, gives me an excuse to buy some more jet sizes. You can never have too many.

Interesting the pilots dont need much tweaking, i was not expecting that.

As for the blow-by, i have been using shell semi-synth 2t oil for a few years now so i will keep an eye out for any problems there. I find it hard to justify the expense of full synthetic. I have had the Bromlech system on for several years and found it also robbed me of a lot of low/midrange, but had a substantial power band above 4.5k.

I havent yet ridden it with the chambers on, due to recent knee surgery, but it's going to be interesting to compare the two setups. My neighbours all got an ear bashing last night when i set up the idle circuit.

These are not quiet pipes, are they..

[tz375]

What jets did you have in for that Bromlech system?

And did you say that the Bromlech made more top end than stock? Don't tell that to the doubters here who think that all 3 into 1 sytems are worse at the top end than anything else including OEM pipes.

When do we see the back to back dyno runs - Bromlech and Higgs? That would be interesting.

[TPS]

I had 47.5 pilots, needle clip on the bottom groove, and 115 mains.

I first drilled out some 110 mains to about 118, (#56 drill bit) but then settled for the 115's after some plug chops. I found the power band came on harder with the needle in the second from bottom position, so i ran it like that for a while, but soon figured that with the needle on the bottom clip it was actually running slightly better in the mid range.

So thats where i ended up.

The power band on the bromlech always comes on at 4 - 4.5k so it has to be an exhaust effect, it would quite happily charge straight through the redline and no doubt into self destruction if i let it. The torque increases as far as i have let it rev.

The M has a weak bottom/mid range at any rate, compared to the L i had a go on once. Even with the standard pipes on and i found that to be marginally worse with the bromlech system. But there is no doubting the top end launch.

I am looking forward to making a comparison with the higgspeed chambers, but i dont have a dyno sorry.

Wish i did.

Wonder what the Bromlech system would sell for on ebay? it's in good nick.

[TPS]

Well,
Early indications for me are that the Higgspeed chambers are already an improvement over the bromlech 3-1. I threw the Chambers on and gave it some exercise today with the same/existing jetting.

Bottom end is unchanged but there is a marginal torque increase above 3k, not huge but enough to be able to keep the the bike in top gear a bit longer between 80-100km/h. The power band now comes on earlier at 4000 rpm and pulls ever stronger all the way to redline. However, as i roll throttle on and come onto the main jet at wide open, there is a small loss of torque and a slight exhaust note change. Slightly more noticeable below the power band. This seems a bit worse as the bike warms up. No problem while on the needle though.

The bike certainly has more torque from 4000 but the power band feels more linear than it did with the Bromlech system.

It's also a lot more fun to ride. The Snap, Crackle and pop coming out of this thing now is diabolical.

I feel that correctly jetting these chambers will be easier, as there is already more feelable definition between Pilots/needle and mains, than there was with the Bromlech system. With that, it felt like i could change the mains either way without making discernable difference sometimes, like there was a lot more overlap happening between all the circuits.

I dont like how my sidestand now sits lower and scrapes the ground a bit because the LH Chamber wont let it come all the way up. That annoys me a bit, because i cant exactly remove both stands. Anyone come up with a fix for that?

[Craig380]

Good stuff

If the WOT issue feels worse when the engine's fully warm, it sounds like it's maybe a little rich at the top end.

The chambers will improve efficiency at higher rpm (better cylinder filling due to the stronger pull through the transfer ports, AND less fresh mixture escaping down the exhaust port).

Check the plug colour and shine a torch on the piston crowns through the plug holes, if they read good then it may be a case of just raising the needle one notch.

For what it's worth, when I fitted chambers to my 380 (using stock airbox but without the intake snorkel, stock foam filters & carbs), I only needed to adjust the pilot screw a little. 3,000 miles later, it's still a touch rich at the top end but not so bad that it's worth fiddling with the jetting. More importantly, it will hold prolonged WOT / high speed without problems.

Keep half an eye on the plugs and enjoy

[tz375]

Let's keep in mind that CV carbs don't work the same way as slide carbs. Wide open at less than full revs, the slide is still down a long way.

Main jet on a CV carb has a proportional effect at all engine speeds. To get the main jet right, the bike has to be WOT and full load and high speed.

Anything less than that and the slide is hanging down and the needle is the main event. At WOT and say 1/2 - 3/4 engine speed, it's all about the needle. You could try fine tuning the air bleed through the slide or springs to change how fast the slides rise, but I wouldn't.

At lower speeds - say 1500-2500, float level is the way to tweak it according to Mark Salvisberg at Factory Pro the jetting and dyno people.

[Madbuffalo]

Well,
Early indications for me are that the Higgspeed chambers are already an improvement over the bromlech 3-1.

What was that again? Richard, did you catch that?

TPS, It sounds like you have a good idea of what needs to be done in the way of jetting to get your tuning right. Did you try the 120 or 117.5 mains? 105 still sounds a bit lean and a larger main could solve your WOT issue. As far as your kick stand dragging, no I haven't found a solution yet. I put them on last year right before the annual Tail of the Dragon ride and learned fast the kickstand dragged with any significant lean. I came home from the trip with a not-so-nice dent in the left pipe. Let me know if you find a solution for that one.

[tz375]

Bromlech used claim a power increase for those pipes, but I was pointing out that no one around here believes that a 3 into 1 could make more power than stock. I would have been very disappointed if Higgsy's pipes made less power than a 3 into 1. My only issue with those pipes is that popcorn sound.

[Madbuffalo]

Ok ok, point taken. You know loads more than me about these performance mods anyway. Just Joshin ya

Josh

[ja-moo]

Bromlech used claim a power increase for those pipes, but I was pointing out that no one around here believes that a 3 into 1 could make more power than stock. I would have been very disappointed if Higgsy's pipes made less power than a 3 into 1. My only issue with those pipes is that popcorn sound.

3 onto 1 can be tunen for certain RPM ranges, just as in snowmobiles and such. But physics will always keep a 3 into one at a full power disadvantage. It seems the Bromlech was tuned for a bit higher RPM power, and the lace of any real low end power shows that. It seemed to be lacking everywhere as opposed to the Higgsby pipes.......

[tz375]

Good points all. I always thought that Piper and Bromlech were just three headers dumping into a large diameter pipe and eventually muffler. Did the Bromlech have much of a tuned shape to the main body?

[TPS]

Mad Buffalo,
I have the other mains on order at the moment, i will post my progress as i fiddle with it for you and anyone who may be interested. Since the last run, i have put the pilots back to 45's, raised the needle clip to the center setting and i will stick 120's in it to start with. Then take it from there. But the carbs are still out so im waiting for the mains first.

Thanks for the tips Tz, i had actually always thought that CV carbs were fully on the main jet at wide open throttle, at any speed. But the more i thought about it, that is the purpose of CV carbs isn't it. If the slide were to be fully up at full throttle at any speed, you may as well be using flat slide carbs.

The Bromlech doesn't exactly have much in the way of tuning shape so to speak. It is just three dumping into a larger manifold. I've got some photo's of it off the bike i will post when i get the chance, so you can see the shape of it. The bike certainly does feel like it has more torque from 5k to 7k with the Bromlech, over the standard pipes. However, like it has been suggested this may be in part to the loss of mid-range amplifying any power increase.

So without some hard Dyno testing, i could not say for certain the Bromlech system increased the total power of the bike over a standard system. But what i can say for certain, is that the Bromlech system gives a sharper power increase from 5K over the standard system. It does on the 75 model at any rate.