Experiment 33mm carbs on a 500 for fuel mileage

BETHEVIPER

BETHEVIPER

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So here it goes. I have a 500 venture that we use for the kid rides and my wife from time to time. I am amazed at the poor fuel mileage. I am at the point of throwing a 600 triple in the sled. I do want to try one thing. I am never scared of a challenge(see other posts)or putting time into making something better, even if it is 20 years too late.

500cc sled has 250cc per piston, similar to the 233cc per piston of a 700 viper, srx, sx motor. If you say, that other 17cc requires you to go from 33mm to a 36mm carb, I say nay. These triples flow through these smaller carbs at 500rpm higher. Flow is not the issue. 600 triple use 31mm carbs. The 600 twin for some reason doesn't need a larger carb increasing per piston cc another 50.

My point is this, did yamaha mess up the 500 intake system but had too many parts ready to go to for the twins. They do build some things great but have been known to overlook others.
Case in point:

no grease fittings on the first mono skid
blocking off the boost port and no pantlegs on the back of the cylinders of red head
never offering a long travel srx
no use of multiangle helixes
no falling rate geometry in the rear skid
W arms
The list of the engineering nuttyness goes on and on with yamaha.

My experiment, to setup a set of 33mm carbs to run on the 500 twin. I have a friend with another 500 to have something to check it against. Right now, with some clutching issues, I am getting 5.5 mpg. I can not believe the thing doesn't foul plugs. I have yet to find anyone who gets over 10 with 8 being the norm.

Should be a good project and I don't know how long it will take as I don't have free time like i used to. If anyone has done anything similar, would love to hear it.

Also, if anyone could chime in who is into porting that would be great. maybe there is something there that could sacrifice a couple horse power for a mile or two per gallon.

Post any ideas you might have on the subject.

Time to get the bendalight out and do some jetting.
 
Sounds like a cool project.
Buddy of mine just went the opposite way (don't know why) and put a set of 38's from an Exciter SX on his 500. Says it runs well, but Im sure mileage is even worse. lol

Question.
What do you think about using a wiring harness, CDI and tps from a 600 twin ?
Do you think it would be more efficient?
 
I get 9,5 mpg with tempaflow on my 97 Venture 500 liquid. You must have som issues with the carbs i guess?
 
norwegian said:
I get 9,5 mpg with tempaflow on my 97 Venture 500 liquid. You must have som issues with the carbs i guess?
Click to expand...

Another consideration when comparing MPG is are we talking US gallons or imperial . I use imperal but I'm sure Betheviper is referring to the US standard

There's an idea, instead going threw all the trouble of changing carburetors and jetting, just use the imperal MPG!!!! Instant gains without any work!!!!! Lol
 
Last edited:
Also, food for thought:

500 has same jetting as a 600
needles are the same
except the fuel screws on a 500 are at 1 3/4
and the 600 are at 1 1/2

so the 500 is actually richer than a 600...
 
yes and no. Jetting numbers can be the same but the amount of fuel through the hole can be different based on vacuum on one side and atmosphere on the other. A 600 all things being equal would draw more vacuum on that jet.

For instance, a 600 and 700 srx are jetted almost entirely the same, other than pilots. Pilots mostly control that off idle to 1/4 throttle where the motor has it's least draw of air as rpms are the lowest. the 600 cylinder has less draw on the pilot than a 700 does which is why it needs 2 sizes larger pilots to run the same (a little of this is it coming into a slightly hotter timing curve with the different cdi box)
Less air, through can mean larger jets needed for the same fuel mixture.

sorry, thinking out loud. If I have any of this incorrect, bring on the knowledging
 
I fixed that issue with my 2001 500 venture. I sold it! and bought a 2003 RX-1 with 1,070 miles. But can't wait to see the results.
 
You nozzles are worn out.

BETHEVIPER said:
So here it goes. I have a 500 venture that we use for the kid rides and my wife from time to time. I am amazed at the poor fuel mileage. I am at the point of throwing a 600 triple in the sled. I do want to try one thing. I am never scared of a challenge(see other posts)or putting time into making something better, even if it is 20 years too late.

500cc sled has 250cc per piston, similar to the 233cc per piston of a 700 viper, srx, sx motor. If you say, that other 17cc requires you to go from 33mm to a 36mm carb, I say nay. These triples flow through these smaller carbs at 500rpm higher. Flow is not the issue. 600 triple use 31mm carbs. The 600 twin for some reason doesn't need a larger carb increasing per piston cc another 50.

My point is this, did yamaha mess up the 500 intake system but had too many parts ready to go to for the twins. They do build some things great but have been known to overlook others.
Case in point:

no grease fittings on the first mono skid
blocking off the boost port and no pantlegs on the back of the cylinders of red head
never offering a long travel srx
no use of multiangle helixes
no falling rate geometry in the rear skid
W arms
The list of the engineering nuttyness goes on and on with yamaha.

My experiment, to setup a set of 33mm carbs to run on the 500 twin. I have a friend with another 500 to have something to check it against. Right now, with some clutching issues, I am getting 5.5 mpg. I can not believe the thing doesn't foul plugs. I have yet to find anyone who gets over 10 with 8 being the norm.

Should be a good project and I don't know how long it will take as I don't have free time like i used to. If anyone has done anything similar, would love to hear it.

Also, if anyone could chime in who is into porting that would be great. maybe there is something there that could sacrifice a couple horse power for a mile or two per gallon.

Post any ideas you might have on the subject.

Time to get the bendalight out and do some jetting.
Click to expand...

Install new nozzles and your mileage will improve alot.
 
Nozzles, I have heard of that on dirtbikes. the needle pounding back and forth beats the nozzles larger causing midrange to go rich.

I havent had a chance to go out on it. building a tight primary and secondary to remove that from the equasion. I have a few different helixes that end in the 41-39 area and start in the 48ish area. any one of them should bet the clutching closer. I need the clutching close to correct so I can then tell what my gain or loss is. 15inches of snow coming to northern maine this week, riding this weekend so should try a few things.
 
Grab a couple new ones if they are still availabe. Compare the hole size -old vs new. You won't believe it.
It's not your clutching.
 
no updates, just cleaned the death out of the carbs this weekend and went for a short ride. I have two primaries, on a comet and one yamaha, both work identicaly. Sled still has dead spot in the mid range. Hard to hold a steady speed in the 6000 range. I dropped my needles a spacers and installed one size smaller main as this is a fair weather sled. No need for -10 jetting. mileage is better but I think I will try a set of nozzles in it just in case. I also set the float level as it was way off. Now that I atleast have a base line I can start making it better. also notched the reeds and found a few of them were dirty being held open, redid them so they were closed at rest.
 
Good luck on the nozzles, they're a Yamaha OEM part only and discontinued, no NOS parts that I've been able to find and Mikuni does not have anything that will cross reference to it or can be made to work with the dimensions I've given to a couple mikuni dealers.

I've half way thought about ditching the rack mount carbs and going with a set of TM's from something else, something with available parts. Other than tuning I'd think the cables would be the hardest part.
 
Taking it out again tomorrow. Going to throw a couple helices at it. Also backing the carb spring off a bit as well. That thing pulls so hard.
Changing out a wire out donut header pipe seal. Loosing pipe pressure and the head pipe sets 1/4 in shorter. Shot in the dark that it would change it much.
 
Wow, what a difference backing the carb spring off. It took 4.5lbs to pull and 4 to hold at any given place the throttle button, measured from the tip. I backed the spring off to the next setting, roughly 180deg off the original location and now it is 3 to pull and 2lbs to hold. Rode today and can't understand why yamaha didn't do this to start. My daughter was able to drive for 15-20min without stopping to complain about her thumb hurting. I know it doesn't sound like much, but that is a 25% reduction. I have one more thing to try and get it down a little more. I will change the shape of the cam arm the cable wraps around so the cable pulls from further away from the pivot around half throttle. That should get the hold pressure down some more and some of the pull pressure around the most used part of it's travel. I cold change it out for a tripple racks crank which is further from the pivot but there is exactly enough travel in the throttle button for the rotation of the carb crank from idle to wot. All I can do is change the ratio within that travel, I cant change the travel.

clutching. I have checked and fixed anything to do with the motor that could give it a dead spot, short of changing the carb nozzles. I am holding that as a last resort as there doesn't seem to be any anywhere and they are 50bucks. I dropped the needle a spacer so if it is a wore out nozzle, that should help it. Worn out nozzles from the needles beating back and forth would seem more plausable if the motor didn't have a boost bottle. Boost bottles take up that spitback pulse that rocks the needle back and forth in the nozzle and eventually wears it out along with the needle. This is a more common thing on high mileage motorcycles.
I will take it out tomorrow for a blast without a passenger and see if I do have a lean or rich condition at those rpms. I checked it for lean today with the choke. To check for rich, I need to run my carb vent tubes back outside so I can cover them while riding to lean it out.
 
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