Power is only at the top

[smoketriples]

Now that Cafe hondazooki is on the road.....

Have to rev her to 4500 to get a decent launch, anything lower and it lugs horribly, too slow to leave in traffic without horns from the cages behind me. It's a rocket when launched that way and will go through the gears as fast as I can shift 'em.

Same thing in higher gears. 70mph (real) is about 4000rpm in top gear, and if I want to pass someone I have to drop a gear or it just lugs and won't accelerate much.

The chambers are from previous owner and seem very aggresive. Home builts (original suzuki downpipes welded to these fat chambers with the brand 'factory pipe' on them, connected to clamp on silencers).

Previous owner was unclear as to how much engine work had been done (I don't think he did it himself), other than he knew the pistons were oversized and something about middle cylinder being a left cyl piston (???).

Any thoughts on making her more street friendly? This one would make a great vintage drag bike but I'm not into drag racing.

=e

[Coyote]

something about middle cylinder being a left cyl piston (???).

Unlike the GT750, the GT550 pistons are all alike i.e you can't mix them up. Sounds to me you need to hunt different exhausts to ever tame it down. Any pics?

[smoketriples]

Pics are in the pics area of the board. I'm hoping it's all a pipes issue and not some overly aggressive work on the ports. I'd fit stock pipes just to see but the rearsets are in the way and it would be a lot of work to remount those just to test pipes.

Meanwhile for the 'real' GT, wife has promised to buy me a nice drill press for Father's day so I'll be able to make that puller plate for the barrels .

E

[tz375]

The PO is correct that on a GT750 there are two left pistons (L&C) and one right piston.
Factory Pipe Products made chambers for teh GT series at one time, though they scrapped the tooling years ago.

Pipes + porting tend to result in no bottom end, but I suspect that carburation and gearing are exacerbating the issue. What is the gearing and what jets are in those carbs at the moment? CV carbs don't usually work very well with pipes and ports and better results can be obtained with VM ot flatslides. You could try dropping a tooth off the front sprocket and then look at the carbs. It almost sounds like one carb dead on the pilot circuit cutting in as revs rise.

What jets are in there and how clean are the carbs? Pay particular attention to all the tiny drillings in the body and float bowl. You may be surprised how many tiny holes there are and how blocked they get especially air jet passages and the by-pass drillings.

[smoketriples]

Carbs are very clean now . Lots of soaking and high pressure air and even very thin wires to get everything like it should.

Main jets are 120's, pilots are stock. Needles two slots from bottom.

-Eric

[tz375]

Those carb settings should be close enough.

I'm going to guess that it's either pipes or ports or maybe a combo of both. We could pull the barrels off and create a port map and work out what - if anything - has been done there.

Another alternative is to raise the compression to add more bottom end but we'd need to be sure that you don't already have too much compression.

Or it might be possible to slip on a set of street pipes with the footpegs spaced out a little, just to get it around teh block for comparison purposes.

Are you 110%sure the timing is correct and that there are no tears in the diaphragms? Did you pop the air box off and watch to see that all 3 slides rise and fall together at more or less the same speed and time? If one of them is lazy(sticky), that might explain the lag.

I wonder what the dyno curve looks like compared to other bikes with say Jemco pipes. It may be that yours is the same as the others, but I suspect not. J&R pipes back in teh day had a terrible reputation for making zero power below 5k and then signing off 1500 revs later.

[smoketriples]

These pipes don't look a bit like Jemco's, Honestly the quality is poor, and the chamber isn't built in stages like the Jemco, just a huge chamber that at the end tapers quickly. Then of course it has those heavy silencers on the ends, which don't have removable baffles.

You are right about one cylinder being slower to come up than the others, middle is slightly delayed. Slide moves easily and no tears in the diaphragm, and just for grins I tossed another slide in there and no change. I haven't messed with mixture, it's set at 1/2 turn out.

Compression is around 120 so I guess I could mill an MM or two off of the head, but don't want to make it even tweakier than it already is......but that is certainly one of the cheaper fixes.

Gears are 15/52 if i'm counting the rear right. Remember the rear is honda. By GPS speed this thing redlines just below the Ton. Speedo of course is quite optimistic and thinks it's going 111 at redline.

I'd like a wider power band, and since my usual commute is cruising at 65 I don't actually need a setup that'll do the Ton.

No way to get stock pipes on there and be able to ride. My rearsets are located at the passenger pegs.

[tz375]

Those pipes app[ear to be Factory Pipe Products aka FPP. I have no experience with them on a GT but on an RZ they do exactly what you describe. They are a top end pipe on the RZ and it's possible that they work that way on a GT too.

They are a little short for a slow revving motor and probably stretch the upper end of the power band but probably don't help at low revs.

Raising compression helps all the way through the power band but has less effect at the top and more at the bottom. It just doesn't need to be too high though. If the head and barrels are stock you could safely remove 1mm without any issues and more if you also raise the exhaust port.

If the barrels have been ported, it's a whole different proposition to get back some midrange. Two things kill bottom end when porting - too much exhaust duration and too much intake. People love to lower the intake floors and that doesn't help at all.It kills the bottom end and does nothing until the motor is doing at least 5500 revs.

I would drain the water and measure the head and ports for width and dis0tance down from the deck. If it all looks more or less stock or lightly modified, take 1mm off the head and get a decent composite head gasket from Cometic.

[smoketriples]

Aight, sounds like some fun work ahead.

Really tempted to put a set of Omar's 3 into 1 or a Jemco 3 into 3 and then sell these FPP , but I do need to pull that head off and take a look, if the barrels are modified then I honestly may just put another top end on it, I have a few motors.

[Craig380]

Generally, a shorter and steeper end cone will narrow the effective RPM range, which would go a long way to explaining why it's a rocket above 4,000 with nothing below.

Just checked the pics of your bike, she looks really sweet by the way. The chambers are really short and fat, I'd say they're definitely made for top-end power to the exclusion of anything else.

UK Allspeeds look a similar short length, but they use the long-stinger-inside-the-chamber trick which gives the right tuned length while keeping the actual pipe length shorter.

As you mention, may be worth trading the FP pipes for a different set, or just keeping them for when you're feeling evil

[smoketriples]

Well, the third possibility would be finding the right pipe and then cutting and extending the fat part of the chamber. Have MIG so the welding isn't an issue, however, finding the right pipe and then the work to pound them about so they match the beating these pipes have had over the years would take a bit of time.

Then of course the question is, how much length to add.....

Eric

[tz375]

That is the easy part, as long as we have the current dimensions and the port timings, we can work out what the pipes are doing and what changes would be simple to make.

One school of thought at the time was to use pipes which are clearly not the tuned length but do stretch the powerband upwards. In other words use short fat pipes to change the shape of the power curve. Allspeeds are way too short and the long internal stinger doesn't change that, but they appear to work quite well

I don't have a set of Allspeeds to measure and the only dyno curve I have is highly suspect. If anyone has those dimensions and a curve I'd be happy to plug them in and see what the simulation says, and compare that to the real curve to calibrate the software and then take the FPP dimensions and work out what is needed.

it really comes down to what teh current ports are like, because that may be contributing to the situation

[smoketriples]

Yep a look at the ports is needed. I just have to get off my butt and do that. Only dyno around here that will do bikes is at the Harley shop and they are expensive. I had three runs done to get my electra glide dialed in after major upgrades on it (cams, headers, mufflers, intake, etc).

That said though, I'd like to still get a run done as is on the hondazooki to document how it is now, and verify what my butt dyno says the curve is.

Well, more to come. It's nice today so I may just put up the wrenches and hit the fields with the dirt bike.

=e

[tz375]

If the Hoggly Doggly shop has a decent tech who really knows how to use that dyno, it would be good to get a baseline run. They don't need to know how to tune a 2 stroke if all you want is a baseline. What they do need though is a brake on that dyno. They can't expect a 2 stroke to slow down the drum/roller the same way that a big twin does.

[smoketriples]

It's a good shop, the owner is a fairly famous racer (now his grandkids run the team I think).

Popped my trick back getting the motor out of the other '74s frame so no dirt riding. Since I could sit just fine I pulled the carbs back off hondazooki (middle is a touch slow) and cleaned out low speed circuit yet again, moved the 120's up to 130.

Idles better, middle still is lazy, lower temp at idle by pyrometer but at 2k all even out real quick. The 130's are probably too big though, not nearly as crisp at that 5k leap as it was.

Misread rear sprocket, it's a 44. Honda wheels, rear is a 130/80-17. I believe that works out to be a tad taller than the stock GT750 rear. My 15/44 (wish I knew how to calc the tire for sure) is I'd guess equiv of around 15/40 if I had stock tire and wheel? Maybe 15/42?

I have 14/46 on order. A top end of 85mph is fine with me, I don't usually get over 65. That should take care of getting off of a light without having to rev to 4500. Will leave the 130's in until test with the new gearing....since I went back to the pods a little rich isn't a bad thing, I'd rather foul a plug than hole a piston.

e