GT550 engine builders/tuners please help

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jaybob
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GT550 engine builders/tuners please help

Post by jaybob »

HI I just got a new set of Jemco's for my '75 gt550. I can't wait to put them on but I want to carry the concept just a little bit farther.

I'm looking for streetability with a power boost from stock. I'm building a cafe racer type bike.

With the improved chambers of the Jemcos I'm thinking I want to upgrade the carbs with a set of VM's or some sort of flat slides (PJ, PWK). 32mm I think was recommended in the smokerider performance write up. I could use some good suggestions to get the best application for me.

Also I want to shave the head a little bit, maybe 1mm. Again this info comes from smokeriders. I plan on getting a spare head to modify.

Engine will be set up with a Boyer ignition. No porting or anything for now unless blueprinting the jugs would be worthwhile. Engine has a fairly fresh rebore and set of rings already from the previous owner.

So am I on the right track here? Any other thing I should consider or re-think?

Thanks for the help
Jason
jeff g
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Post by jeff g »

Chambers and 30-32 mm carbs will give you a 10-20% increase in power over stock. Milling the head without any other porting changes will cause reliability issues.

Whatever changes you make, be sure to jet the carbs 1 size larger than "optimal". If you jet the carbs optimally, you'll have a limited altitude and weather condition that you can ride the bike. Make sure that you jet all 3 carbs the same. Stock settings had the center cylinder 1 jet size smaller than the right and left. That was because of the crossover tubes inthe stock exhaust that you won't have with the chambers.
jaybob
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Post by jaybob »

Jeff,

Thanks for the info.

Do you know if the stock carbs can be bored 2mm?

What kind of reliability issues are we talking about with the higher compression? What ype of porting is needed to work well in an overall package?

If a guy jetted the carbs for "optimal" on the coldest temps you would plane to ride in then would it be safe to say that they would be a little fat and OK in most other conditions?

Thanks again
Jason
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Post by rngdng »

What kind of reliability issues are we talking about with the higher compression? What ype of porting is needed to work well in an overall package?



Holes in pistons would be the most likely problem. If you jet optimally, as Jeff said, you're on the edge of hurting the engine. That's OK for racing, but on the street, jet it a little fat for safety and reliability.


Lane
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jaybob
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Post by jaybob »

I understand. Fuel is a major factor in keeping these engines cool.

Thanks for the help. I've got a few more questions coming later.
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Suzukidave
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Post by Suzukidave »

I know a set of flat slides would be a cool thing but for a budget carb improvement a set of the 32mm slide carbs off the 72`73 GT750 would be a good set up . Along with rich jetting with a head skim retarding the ignition a few degs is also a good thing to do when first dialing in . Over heating the plug tip and holed pistons soon follows .
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jaybob
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Post by jaybob »

Suzukidave wrote: Along with rich jetting with a head skim retarding the ignition a few degs is also a good thing to do when first dialing in . Over heating the plug tip and holed pistons soon follows .
You lost me on these two sentences together. Are you saying that a head skim when setting up new pipes and carbs will cause the hot spot?
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Post by rngdng »

Yes, the skimmed head can cause high compression, which (if too high) can cause detonation, which will cause you to burn pistons. .5mm should be fine, but be aware of the danger, and watch it closely until you get it sorted.

It would be easier on you to rejet for one major modification at a time.


Lane
Last edited by rngdng on Tue Nov 25, 2008 7:20 pm, edited 1 time in total.
If you stroke it more than twice; you're playing with it.

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tz375
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Post by tz375 »

raising compression helps to create more power all across teh rev range - that's the good news.

Bad news is that compressing a gas raises its temperature and raising compression versus stock makes the combustion chamber even hotter.

Dave is suggesting that fat jetting will tend to keep the motor cooler and less prone to overheating and detonation. Spark plug tips are a good indicator of combustion chamber temperature.

Also keep in mind that a well designed set of chambers will pull more fresh gas out of the exhausts and then stuff it back in just before the port closes - so when the motor is on the pipe, it makes more power by stuffing more gas back into the combustion chamber (making for a higher Effective compression ratio)
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Post by Suzukidave »

the older i get the faster i was
jaybob
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Post by jaybob »

I got a line on a good extra head. I'll get the pipes and carbs on and spend this next spring getting dialed in and then decide whether to ry a different head or not.

Thanks for the patience and advice!!!

Now to scare up those carby's

Jason
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Post by Barry S. »

A little port clean up would help, the 380 I built a few years ago was terrible. The area at the bottom of the cylinders where the fuel goes into the transfers was not chamfered at all, the bottom edge of the cylinder liner was still square going into the transfer ports. A little grinding to smooth it up had to help.
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jaybob
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Post by jaybob »

Thanks also Barry.

I definitely plan on chamfering all of the squared off edges. Depends on what I find when I open it up. I have no clue how far the previous guy went when the re-bore was performed.
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Post by Suzukidave »

Good to hear you got your GT750 carbs with the cables as a package deal :D you know we need to see some pictures of the progress :roll: .
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Post by Suzukidave »

Here's a custom 550 with stinger pipes and what looks like a Yamaha R6/1 tail http://www.youtube.com/watch?v=oflWMbMU ... re=related
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