DOHC MAXIM Carb Sync

[doodsaibot]

I have been riding my bike out in the hill country the last couple weekends and she has done pretty well. I typically run a 6.5k or 7k RPM steady cruise through plenty of uphill stretches. Today, when I hit around 6.5k or 7k RPM on flat roads there was a slight stumble or "hesitation." It got worse the further I rode and it even backfired once. It got to the point where the bike would not accelerate with the turn of the throttle. So I turned around and went home.
After I parked, I noticed the idle would hang around 1500 RPM and then after several seconds drop down to the normal of 1200 RPM. I slapped on my vacuum gauges and it's pulling a 6in. Hg on both sides. It was getting dark so I am going to work on it more tomorrow. I plan on building the homemade manometer for synchronization. I am not quite sure what is going on so has anyone else run into a similar scenario with their Maxim? I feel like the bike may be running lean but I am not sure. The spark plugs seem to be in a normal state.

P.S. I have not done a carb sync since last summer.

 

[doodsaibot]

I made a manometer and connected it to the bike after a solid ride today. It did not tell me anything that the vac gauge already showed. Both sides were leveled out with only about an 1/8 of an inch difference on one side. The hesitation only seems to be at a high RPM, usually 7k or more followed by a backfire. Not sure what to do next.

 

[16VGTIDave]

A lean mixture would cause surging. Sounds to me that your bike is running rich. Unless you do a plug chop when the issue occurs, reading the plugs won't help. ~7000 RPM and WOT on a DOHC is the change over from the needles to the main jets.

 

[doodsaibot]

That's interesting about the needle/jet change over. I reconnected my bike to the manometer and fiddled with the synchronizer screw again, moving it side to side to watch the vacuum fluctuate. I let it run on the manometer a bit longer this time. Turns out it was a bit rich to begin with. I also tuned in the idle mix screws (which I failed to even think of messing with the 1st time around. ) I turned them in by about one turn which probably means they were a bit rich as well? ~ Anywho, I took her out for my first group ride after all the tuning. I had no problems with higher RPMs at all during the 40 mile round trip ride. Cruised at 7000 RPMs up all of those ranch road hills, averaging about 55 mph. On the way back we hopped on the highway and I pushed it up to 9500 RPM before shifting (trying my best to keep up with that 900cc Triumph Thruxton) and it was a pretty clean acceleration all the way through. I suppose no matter how good you have it tuned you can't make up for engine displacement cuz he passed me without breaking a sweat. Although, I can say that I had no problem at all keeping pace with the Kawasaki Ninja 300.

 

[bcware]

The Ninja 300 will beat you in a straight line too unfortunately; most people just aren't able to use that revvy motor to it's maximum potential.

Get them in the corners! That's where the real talent comes in I've outpaced many bigger bikes this way. In a straight line pretty much all modern bikes are faster!

Good to hear you solved your issues.

 

[doodsaibot]

It's definitely a good feeling when you get to ride with others. That route we took is nothing but twisty turns (except the highway) so it was a good confidence builder. Just when I thought I wanted a bigger bike I found that this one is still the perfect size for me.

Theoretical question about tuning carbs; I was looking at the specs sheet for the jets and I noticed the right side jet is slightly larger. That left me thinking, would it also require slightly more vacuum on that right side rather than having them looking evened out on your manometer?

 

[bcware]

I don't believe the difference is due to vacuum, but rather differences in jet sizes are due to temperature and cooling differences.

As far as I know you should synch the carbs as normal; do not try to compensate in any way for the differences based on design. Tune it for equal vacuum on all cylinders.

There is a chance I am wrong, but I have never read anything about this sort of thing being done.

 

[doodsaibot]

Sounds like a good plan. So, if I were to balance them out evenly and then twist the throttle slightly, would the vacuum show an even balance on the manometer all the way through the acceleration? I was thinking about trying this out to see for myself.

 

[16VGTIDave]

The jetting differences are due to the crappy airbox design, nothing more that I'm aware of. Once I switched to pod filters I had to match jet sizes.

Why would you want to risk your engine ingesting what ever fluid you have in your manometers? And other than giving you some useless information that you can't act on, what would it prove? Besides, a brief twist of the throttle won't tell you anything, and riding the bike while trying to observe... Just don't.

 

[doodsaibot]

Concerning the vacuum pull on the manometer, I am simply curious as to how it would react. Nothing more.

Thanks for jet tip. I may have to go to the UNI pods next January. The air cleaner joints are hangin on but they don't look like they have but a year left on them. That will be a different project for another day.